National Academies Press: OpenBook
« Previous: Chapter 3 - Laboratory Testing Program
Page 61
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 61
Page 62
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 62
Page 63
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 63
Page 64
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 64
Page 65
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 65
Page 66
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 66
Page 67
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 67
Page 68
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 68
Page 69
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 69
Page 70
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 70
Page 71
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 71
Page 72
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 72
Page 73
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 73
Page 74
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 74
Page 75
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 75
Page 76
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 76
Page 77
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 77
Page 78
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 78
Page 79
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 79
Page 80
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 80
Page 81
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 81
Page 82
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 82
Page 83
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 83
Page 84
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 84
Page 85
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 85
Page 86
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 86
Page 87
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 87
Page 88
Suggested Citation:"Chaper 4 - Field Evaluations of CLSM." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
Page 88

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

61 Introduction This chapter summarizes the studies on the field perfor- mance of CLSM and the use of the data collected and experi- ence gained to validate and improve upon the test methods, specifications, and guidelines developed in the laboratory portion of the study. An overview of the research approach and objectives is provided, followed by discussions on the studies performed at six field sites throughout the United States. Lastly, some of the main findings of the overall field efforts are presented. Research Approach The key deliverables of this research project will be useful only if they are eventually applied in the field. Thus, a com- prehensive field testing program was included in the latter stages of this project, which aimed to gain insight into specific technical or practical issues regarding CLSM. The objectives of the field testing plan included the following: • Close the gap in understanding of CLSM by addressing key research needs • Assess applicability and efficiency of test methods devel- oped in laboratory • Assess appropriateness and validity of test methods, specifi- cations, and construction guidelines developed in labora- tory and through a synthesis of current practice Field Testing Plan Several technical issues were identified in the early stages of this project as requiring significant attention in the labo- ratory portion of this project; some of these issues were fur- ther deemed to be important enough to address in actual field trials: • Long-term strength gain/excavatability • Short-term strength gain/constructability • Corrosion of metals in CLSM • Productivity and cost (especially relative to compacted fill) • Resistance to freezing and thawing • Construction issues (i.e., floating of pipes) • Settlement • Use of by-product materials • Environmental issues • Permeability (for various reasons, including drainage and leak detection) After identifying the key unresolved technical issues re- garding CLSM and selecting agencies to participate in the field testing, the research team developed a field testing pro- gram, as shown in Table 4.1. This program encompasses the most important technical issues and includes the CLSM ap- plications relevant to this project. More emphasis was placed on the most common CLSM applications, such as backfill, whereas less emphasis was placed on less common applica- tions, such as bridge approaches. The test matrix shown in Table 4.1 captures the main technical issues and addresses several common interests with the field testing partners. Un- less otherwise noted, the test methods, specifications, and guidelines developed under this project were implemented in the various field tests. The researchers recognized from the onset of this field test- ing program that not all of the long-term data (e.g., corrosion) would be generated or collected during the finite duration of this project. However, they attempted to generate and synthe- size as much relevant data as was feasible; in some cases, field tests were continued beyond the completion of this project and will be monitored and evaluated through research collab- orations formed as part of this project. This information, when available, will be presented to the relevant AASHTO committees for review and possible inclusion in future CLSM construction. C H A P T E R 4 Field Evaluations of CLSM

62 Field Test at the University of Texas–Austin Introduction Significant field testing was performed at the J. J. Pickle Research Campus at the University of Texas–Austin. The main goals of these tests were to evaluate the use of CLSM as trench backfill; to establish a link between laboratory tests and field performance; and to study the impact of materials, mix- ture proportions, and curing regime on long-term strength gain and excavatability. Materials and Mixture Proportions Six CLSM mixtures were included in this study, as shown in Table 4.2. Each of the mixtures was procured from local ready- mix concrete producers, each of whom had experience with producing CLSM for various applications. The mixture pro- portions were based primarily on experience gained from the laboratory portion of this study. Mixtures were selected to span a range of materials and proportions and to generate strengths that would result in various degrees of ease of excavatability. Intentionally, no trial mixing was performed using materials similar to those used in the field test, specifically to deter- mine if prescriptive mixture proportions (e.g., cement con- tent, aggregate content, water added to achieve target flow) would result in desirable mixtures (e.g., target flow, mini- mal segregation/bleeding). On-site adjustments were available for these mixtures if they arrived at the field test either too dry or too wet in consistency, as discussed later. However, if the water added to the drier mixtures resulted in excessive bleed- ing or segregation, no further water was added. Mixture Type I Cement (kg/m3) Fly Ash Type Fly Ash (kg/m3) Concrete Sand (kg/m3) Water Content (kg/m3) Air Content (%) Flow (mm) Mixture Temperature (°C) Density (kg/m3) Flash 0 Class C 224 1672 165 4.0 190 35.2 2179 A1 30 – 0 130 130 29.5 200 33.6 1539 A2 60 – 0 130 130 28.5 220 34.5 1539 Paste 60 Class F 1195 485 485 1.0 420 42.5 1795 F1 30 Class F 180 175 175 2.25 100 36.8 2051 F2 60 Class F 180 175 175 2.5 140 35.2 2083 “–” = not used Table 4.2. Mixture proportions for excavation study. Agency or Organizationa Issue/Application UT Austinb NRMCA c Hamilton County (OH) EBMUDd Texas DOT TAMU e Technical Issue Long-term strength gain/excavatability Short-term strength gain/constructibility Corrosion of metals in CLSM Productivity and cost Resistance to freezing and thawing Construction issues (i.e., pipe floating) Settlement Use of by-product materials Environmental issues Permeability/leak detection CLSM Application Backfill Utility bedding Void fill Bridge approach aInformation on productivity and cost was also obtained from the New York DOT but is not included herein (for conciseness). Also, a field test was planned with the Florida DOT, but permitting issues prevented the field test from occurring. bUT Austin = University of Texas–Austin cNRMCA = National Ready-Mix Concrete Association dEBMUD = East Bay Municipal Utility District eTAMU = Texas A&M University Table 4.1. Matrix of field testing issues and applications.

Experimental Program Six trenches, 3 m long, 1.2 m wide and 0.9 m deep, were pre- pared side by side on the Research Campus site of The Univer- sity of Texas at Austin. The trenches were spaced 0.75 m apart. Each of the CLSM mixtures was placed in a single trench in the order listed in Table 4.2 (from Flash to F2), all on the same day. The fresh properties of CLSM mixtures were measured at the site, including flow, density, air content, and mixture temper- ature. A needle penetrometer (ASTM C 403) was used to char- acterize the setting and hardening of CLSM backfills. A Kelly ball (following ASTM D 6024) was also used to evaluate early- age hardening. Additional samples were prepared and stored in a 23°C environment, and their setting and hardening behaviors were monitored and compared to the field evaluations. For each trench, two rows of thermocouple wires (one 0.3 m from the bottom and the other 0.6 m from the bottom) were installed to monitor the temperature changes every 10 minutes. The unconfined compressive strength and splitting tensile strength of cylinders stored under standard laboratory condi- tions (23°C and 100 percent relative humidity) and outdoors (adjacent to the trenches) was measured at various ages. The excavatability of the six CLSM mixtures was evaluated at an age of 10 months. Manual tools, such as shovel and pick, were used to evaluate the excavatability of CLSM. A dynamic cone penetrometer was used to estimate the strength profile of the backfill. A proprietary device, the GeoGauge, was evalu- ated in the field, despite the relatively poor performance of the device in the laboratory phase of the project. This device was included in this field test to determine if the past poor per- formance of the device was due to size effects and boundary conditions that might be present in laboratory testing, but perhaps not in field conditions. Results and Findings Fresh Properties Table 4.2 presents the data on the fresh properties of the various CLSM mixtures. The target flow for the mixtures was 200 mm, but the mixtures as placed varied from very little ini- tial flow (mixtures F1 and F2) to a very highly fluid mixture (Paste). Water was added to the stiff mixtures to remedy the flow, and fly ash was added to the Paste mixture to reduce the flow and minimize segregation. Subsequent testing of the constituent materials used in the various mixtures confirmed that the sand was poorly graded and contributed to the poor flowability of the mixtures. Although the adding of water at the jobsite can help boost the flowability, it also can lead to bleeding and segregation, especially for mixtures that are not optimized. Thus, on-site water additions, which are common options for CLSM (or concrete) producers, can be a useful tool in adjusting flow, but the ultimate ability to achieve a flowable, segregation-resistant mixture is dependent on the other mixture components, especially aggregate gradation and quality. One option employed in this field test was increas- ing the fly ash content to reduce fluidity (and segregation), although this option is not feasible in the field for ready-mix truck-delivered CLSM. The experience gained in this field test shows that prescriptive specifications may not always be applicable for CLSM, that there exists some ability to modify CLSM mixtures with jobsite adjustments, and that the ulti- mate ability to optimize CLSM for a given application and properties would benefit from trial mixing, when applicable. The setting and hardening of CLSM backfills were moni- tored using several different approaches that had previously been studied in the laboratory phase of this project, as sum- marized in Table 4.3. There was generally a good correlation between the walk-on time and the soil penetrometer value, which suggests the latter can be used in the field practice to characterize the setting and hardening behaviors of fresh CLSM mixtures. However, the ball drop method (ASTM D 6024) seems to be too severe for CLSM mixtures. Even for mixture Flash, a hardening period of 11.6 hours was required to resist the ball drop. The use of the needle penetrometer (ASTM C 403) on the trenches and in parallel specimens stored at 23°C illustrated the significant impact that temper- ature has on setting and hardening. Using the needle pen- etrometer readings as an index, the trench mixture hardened 63 Mixture Walk-on time after placing (hours) Time for soil penetrometer value to reach 6 kPa (hours) Time for Kelly ball drop to generate dent diameter less than 76 mm (hours) Flash 0.1 – 11.6 A1 3.7 2.0 Greater than 72 hours A2 3.1 0.8 Greater than 72 hours Paste 15.4 4.3 26.3 F1 1.7 1.6 15.8 F2 1.7 1.0 13.0 “–” = too stiff for measurement Table 4.3. Setting and hardening determined by different approaches.

much faster (about 10 hours’ difference in reaching similar target penetration values) than the specimens stored in the laboratory. Despite the high ambient temperatures during this field trial, the CLSM mixtures did not generate significant heat within the trenches. All of the mixtures, with the exception of Paste, remained at temperatures below 45°C during their hydrating phase. The trench containing Paste reached a max- imum temperature of 64°C, which resulted in higher com- pressive strengths than previous laboratory testing would have suggested, as discussed in the following section. Hardened Properties Compressive and Splitting Tensile Strengths. The com- pressive and splitting tensile data for the various mixtures are shown in Tables 4.4 and 4.5, respectively. For mixtures A1 and A2 (no fly ash included), there was little difference in strengths between cylinders stored at the site and those stored in the fog room. This finding is consistent with laboratory findings from Chapter 3 that showed that straight cement mixtures were less sensitive to temperatures than mixtures containing fly ash. For CLSM mixtures containing fly ash, specimens cured at the site had much higher strengths than those in the fog room during the first 3 months. Ultimately, strengths of specimens cured in the fog room approached those of specimens cured on site at later ages (e.g., 10 months) in this study, as illustrated in Fig- ure 4.1 for Mixture F2. Two other mixtures that exhibited interesting behavior were Flash and Paste. The mixture referred to as Flash stiff- ened and gained strength rapidly, with a strength of about 600 kPa after 24 hours and a straight gain to 6 MPa after 28 days (with little increase in strength thereafter). Similar strength- gain behavior was observed for the mixtures used in the repair of bridge approaches in San Antonio, Texas, as described later in this chapter. The Paste mixture was found to have com- 64 7 days 28 days 90 days Test Site Fog Room Test Site Fog Room Test Site Fog Room Mixture Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Flash 3351 15.3 – – 6117 8.8 – – 5974 9.2 – – A1 40 9.6 – – 66 10.1 36 22.6 99 18.5 75 15.8 A2 326 9.0 303 28.2 458 19.5 446 3.2 508 10.3 504 5.5 Paste 352 5.8 73 17.3 484 30.1 222 13.7 653 30.2 391 31.3 F1 2014 8.2 680 3.1 3455 5.8 1876 6.3 3445 1.9 2898 7.2 F2 2693 3.9 1445 1.5 6573 6.8 3372 2.7 7744 3.9 7207 2.8 180 days 300 days Test Site Fog Room Test Site Fog RoomMixture Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Flash 6460 8.4 – – 7299 8.6 – – A1 112 13.5 69 5.8 86 25.6 79 5.9 A2 435 19.3 550 0.3 598 23.0 – – Paste – – 537 21.3 761 7.4 – – F1 4194 19.2 – – 3934 3.3 – – F2 7715 9.1 7961 4.2 8637 3.3 – – “–” = Not enough specimens were available for testing at this age. Table 4.4. Compressive strength of CLSM mixtures from UT-Austin field test. 7 Days 28 Days 90 Days Test Site Fog Room Test Site Fog Room Test Site Fog Room Mixture Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Average (kPa) C.O.V. (%) Flash 757 13.7 – – – – – – – – – – A1 – – – – 10 11.2 – – – – 7 17.5 A2 30 22.9 31 27.2 48 47.6 60 10.3 76 11.5 55 20.9 Paste 352 5.8 73 17.3 484 30.1 222 13.7 391 31.3 653 30.2 F1 197 30.4 80 18.2 296 3.3 170 25.4 503 10.4 350 16.9 F2 388 9.5 141 14.4 918 6.2 504 11.3 1149 11.1 981 14.5 “–” = cylinders were not available for testing Table 4.5. Splitting tensile strength of CLSM mixtures from UT–Austin field test.

pressive strengths ranging from around 4.5 to 6.5 MPA after 90 days, which was significantly higher than mixtures cast pre- viously in the laboratory (using different cement and Class F fly ash but similar proportions), which typically exhibited strengths less than 1 MPa after 90 days. The higher strengths observed in this field trial may be due to higher field temper- atures (for the site-cured cylinders), differences in fly ash reactivity, jobsite modifications to the paste mixture, or other factors. As shown in the data from Tables 4.4 and 4.5, the ratio be- tween tensile and compressive strength for a given mixture and age of testing ranged from between about 8 to 15 percent, which is comparable to ratios observed for conventional con- crete mixtures. However, this ratio did not necessarily corre- spond to the compressive strength of the mixture; that is, for conventional concrete, higher compressive strengths tend to yield lower tensile–compressive strength ratios. For CLSM, this inverse relationship does not necessarily exist, but rather, the actual ratio between tensile strength and compressive strength appears to be more related to constituent materials (e.g., presence of fine aggregate). This evaluation of tensile strength and its relation to other properties was included in this field test based on the findings from the laboratory phase, which suggested that tensile strength may be a better indica- tor of excavatability than compressive strength. For conventional concrete, cores are often extracted from field structures to check compliance with project specifica- tions. Although coring CLSM installations creates unique problems related to fragility of the material, it was attempted for this field trial as a proof of concept. The results were mixed; coring was possible only from the mixtures exhibiting quite high strength values. Three 100 × 200 mm cores were suc- cessfully extracted for subsequent strength testing from the trenches containing Flash, Paste, F1 and F2 mixtures. Of these mixtures, extracting cores from Flash, F1, and F2 was partic- ularly difficult, which may explain the lower strengths mea- sured on the cores (compared to specimens stored adjacent to the trench), as shown in Table 4.6. The cores from Paste were actually slightly higher than those cured adjacent to the trench, confirming that the higher temperatures experienced within the trench resulted in higher strength values. This exercise shows that coring is feasible for certain CLSM mix- tures, provided they are strong enough to handle the coring action. It also shows that storing cylinders near the jobsite is a reasonable indicator of actual CLSM performance in adjacent installations; storing these specimens in the same ambient environment helps to elucidate the effects of temperature on actual strength development. Excavatability. A major focus of this field test was the evaluation of excavatability as a function of materials, mixture proportions, age, and excavation method. A range of methods was used to evaluate ease of excavation, including direct methods (i.e., shovels, pick, and backhoe) and indirect index- ing methods (i.e., DCP, Kelly ball, strength, GeoGauge, and removability modulus). Some tests were performed at various ages, and, for conciseness, only the tests conducted 300 days after trench placement are summarized in Table 4.6. 65 8000 St re ng th (k Pa ) 6000 4000 2000 0 1 10 Compressive strength (field) Compressive strength (fog room) Tensile strength (field) Tensile strength (fog room) Age (days) 100 1000 Figure 4.1. Compressive and splitting tensile strength developments of mixture F2 specimens cured on site and in fog room.

Only the trenches containing mixtures A1 and A2 were able to be excavated manually (i.e., using shovels and picks). As one would expect, excavating mixtures A1 and A2 using a back- hoe was also easy. The remaining trenches ranged from diffi- cult but possible (Paste) to very difficult and nearly impossible (F2) to excavate with a backhoe. Following are discussions on indirect methods of evaluating or predicting excavatability. The DCP was found to clearly differentiate the excavatabil- ity of the six CLSM mixtures. Because this penetrometer can be forced through the whole depth of the backfill and the lowest penetration index is selected, this approach has the advantage that it is not affected by a deteriorated surface. This advantage was also demonstrated in the testing of the two trenches at the National Ready Mixed Concrete Association (NRMCA) in Maryland (discussed later in this chapter). Although the data generated in these field tests, coupled with the excavatability tests described in Chapter 3, are extensive, providing absolute guidance on DCP values that separate excavatable CLSM from non-excavatable CLSM is not possible. However, based on the data generated within this project, a DCP index of 5 mm per blow can be proposed as a general rule of thumb, below which there could be problems for manual excavation. Stiffness val- ues generated by the GeoGauge were able to differentiate A1 and A2 as being excavatable, but for the other trenches, where the stiffness of the backfill material is beyond the capacity of the equipment, the outputs seemed to be random. This phenom- enon is clearly shown by the measurements of mixtures F1 and F2, where F2 should be stiffer as indicated by the DCP index and actual excavation experience. The diameter of the dent caused by the dropping of the Kelly ball was also evaluated as an indicator of CLSM excavatability. Although this approach is acknowledged to measure only the properties of the upper layers of CLSM, the values did corre- late quite well in this field test with DCP values, successfully predicting that Paste was easier to excavate than mixture F1. Long-term compressive strength is often used as a criterion to assess the excavatability of CLSM (ACI 1999). For this field trial, cylinders were tested in compression and tension after having been stored on site for 300 days, as shown in Table 4.6. Clearly, the availability of this type data would be a luxury for an actual CLSM installation, but the data shown in Table 4.4 would often be available (particularly, the data from 28-day cylinders stored in the fog room). The two trenches that were easiest to excavate (those containing mixtures A1 and A2) also yielded low compressive strength values (for the site-cured specimens tested on the day of excavation) well below the 1 MPa value that is sometimes used in the field as a rough index of excavatability. While ease of excavation was linked to lower compressive strengths for these two trenches, the other mix- tures exhibited no clear link between strength and excavata- bility. For instance, mixture Paste had a higher strength than mixture F1, yet Paste was easier to excavate. This result can mainly be attributed to the lack of aggregates in Paste, because, in general, CLSM containing aggregates is more difficult to excavate. Thus, compressive strength by itself is shown to be an unreliable indicator of excavatability. This shortcoming is further compounded by the limited availability of strength values, which are generally available for only laboratory-cured specimens and usually for only the first month or so after cast- ing. These short-term tests do not adequately represent the long-term strength gain of field CLSM, nor do they capture the temperature-related effect that field installations experi- 66 Methodsa Flash A1 A2 Paste F1 F2 Round-head shovel Nearly impossible Easy Easy Nearly impossible Impossible Impossible Square-head shovel Impossible Easy Easy Impossible Impossible Impossible Pick Difficult Easy Easy Difficult Difficult Very difficult DCP (mm per blow) 0.2 12.5 5.6 0.3 0.05 Not penetrable GeoGauge stiffness (MN/m) 41.1 13.7 24.7 29.8 45.8 41.3 Compressive strengthb (kPa) 7299 T 8 T 6 446 7156 3934 8637 Tensile strengthb (kPa) 1297 12.4 71.1 761 454 953 Fog room REc – 0.2 0.8 2.3 2.5 3.4 Field REc 4.9 0.3 0.8 3.6 3.4 4.8 Kelly ball (cm) 4.1 12.7 11.4 4.4 3.5 No dent Backhoe Difficult Very easy Easy Difficult (but possible) Very difficult Very difficult (nearly impossible) aAll testing performed 300 days after trench placement unless otherwise noted. bCylinders stored for 300 days on site prior to testing. cRE is based on 28-day compressive strength. Table 4.6. Direct and indirect evaluation of excavatability (excavation performed 300 days after trench placement).

ence. In summary, long-term strength behavior, with cylin- ders subjected to similar time-temperature histories, can serve as a better indicator of field behavior and excavatabil- ity, but even this approach would not recognize that the spe- cific materials and proportions (e.g., presence or lack of sand) can profoundly impact excavatability. As reported in Chapter 3, the splitting tensile strength of CLSM might be a better indicator of excavatability than com- pressive strength, because the actual excavation of CLSM mim- ics a tensile failure in the material. In this field trial, the tensile strength values correlated quite well with DCP indexes and ease of manual or mechanical excavation. Although tensile strength may be a better index of excavatability, the inherent variability in tensile results is higher than that for compression testing, and therefore, precautions should be taken to lessen observed variations. The use of a removability modulus, as proposed by Hamil- ton County (Ohio), successfully predicted the excavatability of the six CLSM mixtures. As described in Chapter 3, this approach takes into account the 28-day laboratory-cured strength of a given mixture and its in-situ density to calcu- late a removability modulus. Values of RE greater than 1.0 are assumed to be non-excavatable. The RE data shown in Table 4.6 was based on 28-day laboratory-cured strength val- ues, as per the Hamilton County approach. In addition, the field-cured 28-day values were also used to calculate RE, which slightly increased the RE values for the non-excavat- able mixtures and had a negligible effect on the excavatable mixtures. Excavation Study at NRMCA (Silver Spring, Maryland) Introduction A major concern historically with using CLSM in backfill applications is related to ease of excavation, for instance, when CLSM is used in utility applications. During the course of this project, major efforts were undertaken to investigate this issue by evaluating CLSM that was cast either in the lab- oratory or field and then later excavated by various methods. However, because of the finite duration of the project, exca- vatability was assessed within a matter of months (or a year or so in some cases) after CLSM placement. This limitation was addressed in a unique way in a field test performed at the NRMCA facility in Maryland: two CLSM trenches were ex- cavated that had been placed about 6 years earlier as part of a separate CLSM research effort. Because the trenches were cast with the intention of tracking long-term CLSM properties, quite a bit of information and data were available, including earlier attempts at excavation. This section describes the excavation study and relates this experience to various engi- neering properties. Background Information In September 1996 NRMCA cast two CLSM mixtures into the two trenches evaluated in this field study. The two CLSM mixtures were cast despite the heavy rain from a hurricane. For convenience, the two trenches are referred to herein as Northeast (NE) and Northwest (NW). The CLSM mixture in NE consisted of 29.6 kg/m3 portland cement; 1406 kg/m3 high-carbon, Class F fly ash; and 292 kg/m3 water (similar in nature to the Paste mixture from the UT–Austin study). The CLSM mixture in NW was composed of 28.5 kg/m3 portland cement, 180.9 kg/m3 Class F fly ash, 1409 kg/m3 concrete sand, and 270 kg/m3 water. Testing Program A range of tests was performed on the two trenches; in ad- dition, limited testing (compression and splitting tensile) was performed on cylinders that remained in the fog room from the original mixtures. The following tests were performed (results are described later in this section): • Excavatability (square-head shovel, round-head shovel, pick, backhoe) • Needle penetrometer (field version of ASTM C 403) • Soil penetrometer (hand or pocket) • Torvane shear tester • Kelly ball (after ASTM D 6024) • GeoGauge • Dynamic cone penetrometer • Compressive strength – 75 × 150 mm and 150 × 300 mm cylinders (capped with sulfur) – 100 × 200 mm cylinders (capped with polyurethane pads) • Splitting tensile strength (150 × 300 mm cylinders) Results and Discussion Excavatability The two CLSM trenches evaluated in this study were buried approximately 0.6 m below grade, with a layer of soil above the trenches. A backhoe was first used to remove the soil and to expose the CLSM. Groundwater was found on the exposed NE trench, and a lower elevation was formed to drain the water. Both CLSM trenches were visibly in good condition, with no signs of freeze-thaw damage or other forms of distress. How- ever, the CLSM in the NW trench had segregated, especially in the upper 80 to 100 mm. Table 4.7 summarizes the results of various evaluations either directly or indirectly related to excavatability. The NE trench was quite easy to excavate manually, but the NW trench was very difficult, if not impossible, to remove manually, which 67

was somewhat surprising because manual excavation of this trench was possible 4 years earlier (2 years after placement). Both trenches were easily excavated using a backhoe (after the completion of the other tests). The Kelly ball test and Torvane shear test (which measures the shear resistance of soil as the device twists) were not able to differentiate the manual excavatability of the two trenches. This inability may be because these two tests involve near- surface measurements of CLSM, and the surfaces of these trenches were somewhat disturbed during the removal of the top soil that covered the trenches. The needle penetrometer (field version of ASTM C 403) and soil or pocket penetrometer were both used on these trenches, but as expected, the NW trench was impenetrable because of its higher strength. These devices are better for measurements of earlier CLSM properties and impact on constructability. The GeoGauge (Model H-4140), which did not perform very reliably in the laboratory trials described in Chapter 3, was able to discern the difference in excavatability between the two trenches, with the measured stiffness of the NW trench found to be almost 5 times as high as the NE trench. Figure 4.2 shows the DCP being used in the NW trench. The DCP is often used in pavement construction to evaluate the compaction or density of subgrade, subbase, and base ma- terials. One advantage of this method is it allows for evaluation of CLSM penetrability as a function of depth of placement. The DCP index is defined as the penetration per blow and it has been correlated empirically with CBR values. Based on infor- mation provided by the DCP manufacturer, the CBR values along the depth of NE and NW materials were calculated and are shown in Figures 4.3 and 4.4, respectively. CBR values of 100 (which NW surpassed) correspond to a well-compacted stone backfill, which presumably would be difficult to excavate, as was NW. One interesting observation from the NW trench was the significant difference in the DCP values (and calculated 68 Method NE Trench NW Trench Square-head shovel Easy Nearly impossible: only shallow dents were made on the surface Round-head shovel Easy Very difficult: small pieces were removed Pick Easy Difficult: pick could penetrate into the mixture Kelly ball (ASTM D 6024) Average diameter 95 mm Average diameter 87 mm; C.O.V. 4.0% Torvane shear tester Average 3.9 kg/cm2; C.O.V. 18.1% Average 3.2 kg/cm2; C.O.V. 9.9% Needle penetrometer (field version of ASTM C 403) 5.7 MPa Out of range Soil penetrometer 4.0 MPa Out of range Stiffness (using GeoGauge) Average 10.3 MN/m, C.O.V. 18.3% Average 47.4 MN/m, C.O.V. 1.8% DCP 4.5 mm per blow 1.3 mm per blow RE 0.23 1.04 Backhoe Easy Easy Table 4.7. Evaluation of excavatability of test trenches at NRMCA. Figure 4.2. The DCP being used in the NW trench. 0 5 10 15 20 25 1 10 100 CBR D EP TH , i n. 0 100 200 300 400 500 600 1 10 100 D EP TH , m m Figure 4.3. The CBR profile of NE trench.

CBR values) between the CLSM in the upper 80 to 100 mm of the trench and the CLSM below this point. To further evaluate this difference, the upper 80 to 100 mm portion of the trench was removed and found to be composed almost entirely of paste, without aggregates, clearly showing the effects of segre- gation. This segregation may have occurred as a result of the inherent segregation susceptibility of this specific mixture or because of a hurricane that occurred shortly after trench placement. The calculation of RE was found to clearly differentiate the excavatability of the two trenches. This RE value was calcu- lated based on strengths measured under the NRMCA proj- ect, and it is encouraging that the RE value was able to discern the removability of the two trenches, especially because the NW trench was originally designed to be excavatable. Compressive and Splitting Tensile Strengths Table 4.8 summarizes the strength tests performed on cylin- ders that had been stored in the fog room for about 6 years. Only a limited number of cylinders (four per mixture) were available for testing; three were tested in compression and one in tension from each set. The unbonded caps used for some of the compression tests had a Shore A durometer of about 5. By combining the strength data from this field test with previously published data (Mullarky 1998), the short- and long-term strength development can be plotted, as shown in Figure 4.5. This graph emphasizes the long-term strength gain exhibited by the CLSM in the NW trench, which ultimately resulted in difficulties in excavation. Field Test at Hamilton County, Ohio Introduction Hamilton County (Ohio) has historically been one of the most innovative and advanced users of CLSM in the United States and was selected as a partner for a field test to tap into this experience. The objectives of this test were to investigate the constructability and early-age properties of CLSM and to evaluate the long-term corrosion performance of ductile iron pipes embedded in CLSM. This section briefly summarizes the main aspects of this field test; however, the key findings from the corrosion study will ultimately be collected by long-term monitoring of the site because of the long-term nature of cor- rosion in field installations. Experimental Program Three CLSM mixtures shown in Table 4.9 were chosen by Hamilton County engineers from a list of their approved CLSM mixtures. Mixture S10 is commonly used for backfill applica- tions in the Hamilton County area. Mixture CDF1 is basically similar to S10, except the Class F fly ash used is high in carbon and is typically not allowed by state highway agencies for use in conventional concrete (mainly because of concerns with air entrainment) and sometimes not allowed by some agencies for use in CLSM. This mixture was selected to demonstrate that materials considered “off-spec” for some applications can be suitable for use in CLSM. The third mixture, designated as FF1, is a fast-setting mixture typically used for backfill applications when setting time is a critical issue. Flow and temperature were measured following ASTM D 6103, “Flow Consistency of Controlled Low Strength Material (CLSM).” Temperature was 69 0 2 4 6 8 10 1 10 100 1000 CBR D EP TH , i n. 0 25 50 75 100 125 150 175 200 225 250 1 10 100 1000 D EP TH , m m Figure 4.4. The CBR profile of NW trench. Compressive Strength Splitting Tensile Strength Mixture Dimension (mm) Load Rate (kN/min) Capping Method Strength (kPa) Dimension (mm) Load Rate (N/min) Strength (kPa) 150 x 300 13.20 Sulfur 1779 100 x 200 6.60 Sulfur 1864 NW 100 x 200 6.60 Pads 1461a 154 × 304 6.60 170 150 x 300 1.32 Sulfur 408 100 x 200 0.66 Sulfur 436 NE 100 x 200 0.66 Pads 379b 154 × 303 0.66 42 aThe specimen was unintentionally crushed by sudden loading. bLarge cavities were observed on the specimen surface. Table 4.8. Compressive and tensile strength of CLSM cylinders (stored in fog room for 6 years).

determined following ASTM C 1064, “Temperature of Freshly Mixed Portland Cement Concrete.” The test site, identified by Hamilton County engineers, was adjacent to one of their ongoing project sites on Pontius Road, Cincinnati. The site layout for the trenches is shown in Fig- ure 4.6. Two rows of three trenches were excavated by Hamil- ton County crews. The trenches were approximately 2.7 m long, 0.9 m wide, and 1.2 m deep. The 1.2 m depth was selected because Cincinnati Water Works engineers require a depth of 1.2 m for waterlines. Hamilton County is a frequent user of CLSM for various applications and specifies it for backfill used in roadway cuts. Hamilton County has had very good success in essentially eliminating problems with settlement often encountered when conventional backfill was used. However, the major util- ity in the area, Cincinnati Water Works, has expressed reluc- tance to allow CLSM to be used in direct contact with water pipes. Therefore, in County projects involving water utilities, pipes are placed on sand beddings and backfilled with sand up to 150 mm from the crown of the pipe. The rest of the trench is then typically backfilled with CLSM. The research team believes that backfilling the trenches completely with CLSM, as opposed to using primarily sand topped off with CLSM, provides a faster construction method and potentially bet- ter long-term corrosion performance of the pipe. To test this belief, the research team used and evaluated the two backfill methods shown in Figure 4.7. The first method (Fig- ure 4.7(a)) was the standard practice as just described; each of the three CLSM mixtures was placed into a trench on top of sand (row A in Figure 4.6). In the second method (Fig- ure 4.7(b)), ductile iron pipes were elevated on wood blocks to allow CLSM to flow underneath the pipe and surround it 70 Figure 4.5. Compressive strength development of cylinders curing in the fog room. 2 NE NW 1.5 1 Co m pr es siv e st re ng th (M Pa ) 0.5 0 1 10 100 Age (days) 1000 10+ Mixture Cement Content (kg/m3) Fly Ash Type (ASTM C 618) Fly Ash Content (kg/m3) Concrete Sand (kg/m3) Water (kg/m3) Flow (mm) Temperature (°C) S10 24 Class F 148 1727 273 305 26.7 CDF1 30 Class F (high carbon) 148 1727 273 127 28.3 FF1 None Class C 237 1721 Not specifieda 305 30.0 aWater is added on jobsite to obtain the flow desired by the engineer. Table 4.9. Mixture proportions of the CLSM mixtures.

completely, then each of the trenches (row B in Figure 4.6) was completely backfilled with one of the three CLSM mixtures. Construction The delivery of the ductile iron pipes to the site was arranged by Hamilton County personnel. The ductile iron pipes (152 mm diameter) had an asphalt coating on the outside and a cementitious coating on the inside. The pipes were capped to better simulate field conditions. Figure 4.8 shows ductile iron pipes being capped and wired before they were placed in the trenches. After the ductile iron pipes were placed in position, the trenches were backfilled. CLSM was delivered to the site in ready-mix trucks (S10 and CDF1) and a volumetric mobile mixer (FF1). Mixture S10 was used in trenches 1A and 1B. Mixture FF1 (flash fill) was used in trenches 2A and 2B, and mixture CDF1 containing the “off-spec” fly ash was used in trenches 3A and 3B. While there was no issue in placing CLSM into the trenches in row A, the pipes in the trenches in row B had to be held in place by a worker to keep them from falling from their sup- ports (wood blocks) during the backfilling. This incident illustrates the need for diligence in placing CLSM in utility applications, where floating or dislodging of pipes/utilities can occur. Another observation was that, as expected, the time required to backfill the trenches that were filled com- pletely with CLSM was less than the time required to backfill the trenches that had compacted sand around the pipes. Test Results Cylinders (75 mm × 150 mm) were cast for each mixture to evaluate their compressive strength at 14, 60, and 90 days. After 71 Figure 4.6. Site layout with six trenches shown. 1A 1B 2A 2B 3A 3B CLSM CLSM 6'' ductile iron pipe6'' ductile iron pipe (a) (b) Sand Figure 4.7. Trench cross sections (not to scale). Figure 4.8. Ductile iron pipes wired and capped.

casting, plastic caps were placed on the cylinders to prevent evaporation of mixing water. These cylinders were then trans- ferred, cured, and tested by Hamilton County engineers or a local testing laboratory. Compressive strength data showed that while mixtures S10 and FF1 had similar compressive strengths at 90 days, mixture CDF1 had a compressive strength more than twice the compressive strength of the other mix- tures. The data also showed that this mixture experienced a decrease in compressive strength of approximately 100 kPa between 60 and 90 days; this loss may have been due to leach- ing away of hydration products upon moist storage. Corrosion activity of the buried ductile iron pipes is being monitored using the potential difference between the pipes and a copper/copper-sulfate reference electrode. The potential dif- ference between the electrode and the pipe is an indicator of the active or passive state of the buried pipe and can be measured with a high impedance voltmeter. For this purpose, the high impedance voltmeter is electrically connected to the pipe (connected to the wire attached to the pipe) and to the copper/ copper-sulfate electrode. The copper/copper-sulfate electrode touches the ground above the buried pipe to close the electri- cal circuit between the electrode and the pipe. Potential read- ings are performed by the Hamilton County engineers using a copper/copper-sulfate electrode provided by the research team for this testing. In addition to the potential difference study, metal coupons were fabricated from a ductile iron pipe and these samples were also buried in the trenches in row B to evaluate their mass loss due to corrosion. It is anticipated that their mass loss will be determined based on ASTM G 1, “Preparing, Cleaning, and Evaluating Corrosion Test Specimens.” Ductile iron coupons were attached to 0.3 m long sample holders in groups of four and placed in the CLSM when it was still in fluid state. Fig- ure 4.9 shows a schematic of ductile iron coupons attached to a sample holder. As described in Chapter 3, CLSM is generally better than conventional fills in protecting embedded metals from corro- sion when the metals are entirely encased in CLSM. It was also shown in Chapter 3 that if a metallic pipe backfilled with CLSM is also in contact with the surrounding soil, the potential for setting up a galvanic cell exists due to the dissimilar media (CLSM and conventional fill). To investigate this corrosion issue, four extra sample holders, each with three ductile iron coupons, were prepared. These four sample holders were cou- pled by connecting their coupons embedded in CLSM with coupons embedded in soil, as illustrated in Figure 4.10. It is anticipated that Hamilton County and Cincinnati Water Works engineers, with the cooperation of the research team, will monitor the corrosion activity for the various test- ing configurations reported herein, and it is hoped that the data will prove of use to them and other users of CLSM dealing with utilities. Field Test at East Bay Municipal Utility District Introduction The long-term strength gain and excavatability of CLSM mixtures have long been a concern for engineers at the East Bay Municipal Utility District (EBMUD). A unique aspect of this concern is that, because of a shortage of fine aggregate in the Bay Area, most CLSM in the area contains coarse aggregate as well. In general, coarse aggregate is rarely used in CLSM, and this field test was sought to determine the effect on excavata- bility. Also, this field test was selected to gain the perspective of the many utilities that are using CLSM for a range of backfill applications. The objective of the test was to investigate con- structability issues related with the use of CLSM as a backfill 72 152 mm 457 mm 610 mm Sample Holder Ductile iron coupons 305 mm Sample Holders CLSM Soil Figure 4.9. Ductile iron coupons attached to sample holder. Figure 4.10. Coupled sample to evaluate galvanic corrosion (not to scale).

material. These issues include flowability, compressive strength development, setting time, subsidence, and excavatability of CLSM. Materials and Mixture Proportions For the field test, mixtures from three CLSM producers in the Oakland, California, area were selected by EBMUD engi- neers. The proportions for the three mixtures (one per pro- ducer) are shown in Table 4.10. The raw materials (cement, fly ash, aggregates) varied from producer to producer, but the general mixture proportions were quite similar. Experimental Program Six trenches (referred to as trenches 1 through 6), approxi- mately 1.2 m wide, 1.5 m deep, and 2.7 m long, were laid out by EBMUD staff. Each CLSM mixture was used to backfill two trenches. Flow and air content of each CLSM mixture were measured before placement, and adjustments were made to achieve the target flow of approximately 150 to 225 mm. Trenches 1 and 2 were filled with CLSM mixture A. The mix- ture was determined to be too stiff and 322 kg of water was added to increase its flow to 150 mm. Trenches 3 and 4 were filled with CLSM mixture B. The mixture had a good consis- tency and no water was added. Trenches 5 and 6 were filled with mixture C, with no additional water needed to achieve the target flow. Figure 4.11 shows one of the trenches being filled directly from the chute of a ready-mix truck, which was the method used for filling all the trenches. The field version of the needle penetrometer (ASTM C 403) was used to characterize the setting and hardening of CLSM mixtures in the trenches. Cylinders (75 × 150 mm) were also cast for compressive strength testing. The cylinders were capped to prevent moisture loss and were left at the site until the test date. Three days after casting the other samples were trans- ported to a curing room at EBMUD. Samples were tested at 4, 7, 28, and 63 days by EBMUD technicians using neoprene pads (as per the recommendations provided in Appendix B) The excavatability of the CLSM mixtures was investi- gated 63 days after their placement into the trenches by EBMUD engineers. Qualitative assessments were performed to determine the excavatability of the CLSM mixture with a hand shovel, a solid steel bar, and a backhoe. Test Results Fresh Properties The flow and air content of the three CLSM mixtures are shown in Table 4.11, along with the ambient temperature and relative humidity at time of placement. The flow values for the three mixtures were adequate for the trench filling (some water was added to mixture A to obtain the desired flow). The air contents were less than expected (based on the mixture pro- portions provided by the three suppliers), but no adjustments were made to the air content of the field mixtures. Setting and Hardening Figure 4.12 shows the setting time and hardening data for the three CLSM mixtures used to backfill the six trenches dur- ing the field test. The data are based on field penetrometer 73 Mixture Cement Content (kg/m3) Fly Ash Type (ASTM C 618) Fly Ash Content (kg/m3) Coarse Aggregate (kg/m3) Fine Aggregate (kg/m3) Water (kg/m3) Air Content (%) A 18 Class F 178 909 908 168 7 B 18 Class F 178 909 906 168 7 C 18 Class F 178 771 1008 197 5 Table 4.10. Mixture proportions of three CLSM mixtures provided by local producers. Figure 4.11. Backfilling trenches with CLSM. Mixture Flow(mm) Air Content (%) Ambient Temperature (°C) Relative Humidity (%) A 152 1.0 30 31 B 216 0.7 30 36 C 191 0.4 30 31 Table 4.11. Fresh properties for CLSM mixtures used in EBMUD field test.

values taken from each of the trenches. Note that the legend denotes the mixture, followed by the trench number (for example, A-1 denotes mixture A placed in trench 1). The set- ting times for a given mixture were quite similar from one trench to another. Mixture A hardened quicker than the other two mixtures, which may be caused by one or a combi- nation of several factors, including lower water content (as evidenced by stiffer consistency of as-received CLSM) and different cement and fly ash sources. Compressive Strength Compressive strength test results of the three CLSM mixtures are shown in Table 4.12. For each mixture, tests were conducted on cylinders stored adjacent to the trenches and in a fog room (standard curing). Cylinders were cured under two different conditions: field and moisture room. In virtually every case, the laboratory-cured cylinders exhibited lower strengths than the field-cured cylinders. This phenomenon is most likely due to the higher temperatures on site and perhaps also due to the leaching of hydration compounds from cylinders stored long- term in the fog room. The strength of mixture A, especially after 63 days, was substantially higher than the other mixtures. Excavatability The excavatability of one trench from each of the CLSM mixtures (trenches 2, 4, and 6) was evaluated 63 days after placement using manual methods (shovel and steel bar) and mechanical methods (backhoe). The difficulty of excavating the trenches was evaluated based on whether the power and time required was low, moderate, or high; the results are sum- marized in Table 4.13. As expected, trench 2 was the most difficult of the three to excavate, and when excavated with a backhoe, the chunks removed were quite large, which could be problematic when excavating around pipes. Interestingly, trench 4 was fairly difficult to remove with a shovel, even though the compressive strength was less than 0.5 MPa at the time of removal. This difficulty in removing the CLSM with a shovel is likely attributed to the coarse ag- gregates contained in this mixture. This result illustrates that strength alone is not an adequate indicator of excavatability; 74 0 2000 4000 6000 8000 10000 A-1 A-2 B-3 B-4 C-5 C-6 0 2 4 6 8 10 12 Pe ne tra tio n re sis ta nc e (kP a) m ea su re d by th e pe ne tro m et er te st Time (hr) Figure 4.12. Field penetrometer data from EBMUD field test. Mixture 4-DayStrength (kPa) 7-Day Strength (kPa) 28-Day Strength (kPa) 63-Day Strength (kPa) Field-Cured 323 348 890 >1950 A Lab-Cured 310 410 779 >1950 Field-Cured 241 241 504 497 B Lab-Cured 212 263 351 381 Field-Cured 224 280 459 602 C Lab-Cured 221 246 358 314 Table 4.12. Compressive strength of field- and laboratory-cured cylinders from EBMUD field test.

in fact, mixture C, which was removed easily from trench 6, had a higher compressive strength than mixture B. This test also illustrated how CLSM mixtures with similar proportions can behave completely different in field applications, owing to differences in raw materials, mixing action, and placement techniques. Field Evaluation of CLSM for Bridge Approach Repair (TxDOT) Introduction A fairly new application for CLSM is its use as a subbase under bridge approaches. This section discusses the use of rapid-setting CLSM for the repair of several bridge approach slabs in San Antonio, Texas, which was done in close cooper- ation with the Texas Department of Transportation (TxDOT). This section first describes an unsuccessful attempt (before the initiation of this NCHRP project) at using rapid-setting CLSM for this application. Through forensic analyses and laboratory evaluations, the probable causes of this failed application were identified. A comprehensive study was then initiated to develop appropriate guidance for successfully repairing these bridge approaches using rapid-setting CLSM. The repair of four bridge approaches in San Antonio were then performed and inspected about 2 months after construction. Research Program Investigation of Initial Field Problems Historically, the use of CLSM by the TxDOT has been mainly for repairing infrastructure. In August 2002, rapid- setting CLSM mixtures were used to repair severe settlements of bridge approaches at the intersection of I-35 and O’Conner Drive in San Antonio. Unfortunately, the setting and harden- ing of the installations were quite slow, and steel plates had to be placed to cover the backfill to accommodate the heavy traf- fic for the next morning. The steel plates were removed the next evening, and hot-mix asphalt was placed over the CLSM. However, severe rutting and settlement were observed within a few months. Because of the poor performance of rapid- setting CLSM in this application, TxDOT decided to place a temporary moratorium on the use of rapid-setting CLSM in bridge approaches in the San Antonio area. As part of NCHRP 24-12(01), efforts were initiated to investigate the cause of the poor performance and to provide guidance on future bridge approach applications. In an effort to understand the cause of the initial failed CLSM bridge approach application, cores (100 × 200 mm cylinders) from the bridge approach were obtained and the compressive strengths were found to be in the range of 1.1 to 1.5 MPa, which indicates that long-term strengths and rigidity were not the problem. Efforts were then made to reproduce the “actual” job mixture using the limited amounts of raw materials retained from the original application and information retained from the job on the mixture propor- tions (see mixture A in Table 4.14). Given the small amount of remaining materials, a Hobart mixer was used, and three 50 × 50 mm cubes were prepared for each mixture follow- ing ASTM C 305 and tested using a geotechnical compres- sion machine at the ages of 3, 8, and 24 hours. The results are summarized in Table 4.14. The flow was measured fol- lowing ASTM D 6103. The setting and hardening processes were monitored through the penetration test as per ASTM C 403. The mixture proportions provided by the contractor (mixture A) did not result in a self-leveling, fluid mixture. About 50 percent more water was needed to make the CLSM mixture fluid enough for the desired application, with dra- matic effects on the rate of setting and hardening, as sum- marized in Figure 4.13. After significant evaluation, the fine aggregate used in the initial, unsuccessful bridge approach application was determined to be a dredged sand with most of the particles falling between 0.1 and 1 mm in size and a resultant fineness modulus of 1.33, well below the typical values for sands used in conventional concrete and many CLSM mixtures. Based on this investigation, it is quite pos- sible that the use of the fine aggregate required such an increase in the water content in the field to get the desired fluidity that the early setting and hardening behavior was greatly affected. 75 Trench Method DifficultyLevel Shovel High Steel Bar Moderate 2(Mixture A) Backhoe Moderate Shovel Moderate Steel Bar Low 4(Mixture B) Backhoe Low Shovel Low Steel Bar Low 6(Mixture C) Backhoe Low Table 4.13. Excavatability of CLSM trenches. Mixture Sand(part) Ash (part) Water (part) Flow (mm) 3-Hour Strength (kPa) 8-Hour Strength (kPa) 24-Hour Strength (kPa) A 4.4 1 0.7 0 666 992 1309 B 4.4 1 0.87 130 407 723 768 C 4.4 1 1.04 270 256 513 550 Table 4.14. Mixture proportions (parts by mass) for rapid-setting CLSM.

Materials Selection and Mixture Proportioning for Bridge Approach Repair After determining the likely cause of the failed application of rapid-setting CLSM in the bridge approaches in Texas, the re- search team and TxDOT engineers decided to jointly develop a suitable mixture for the rapid repair of the approaches for two bridges in San Antonio. These two bridges at Branch Sala Trillo of Loop 1604 between I-10 and I-35 in San Antonio needed repair due to significant problems with differential settlement (i.e., the “bump at the end of the bridge”). To avoid the previ- ously discussed problems with using rapid-setting CLSM for bridge approach applications, comprehensive laboratory test- ing was performed to select and specify the materials and mix- ture proportions for the proposed repair applications. Although the hallmark of CLSM technology is the ability to efficiently and successfully use a wide range of materials that do not conform to conventional concrete specifications (e.g., ASTM C 33 for aggregates or ASTM C 618 for fly ashes), a somewhat conservative approach was taken for this applica- tion. Given that the initial problem with the bridge approach in San Antonio was likely caused by the fine aggregate that did not conform to ASTM C 33 gradation limits, the researchers and TxDOT engineers decided to specify locally available con- crete sand that met ASTM C 33 for the newly proposed CLSM. They postulated that a well-graded sand would help control the water demand and would eliminate the need to add excess water at the jobsite. The selected fine aggregate, a natural river sand, was procured for the preliminary laboratory evalua- tions. A locally available ASTM C 618 Class C fly ash was spec- ified and was also obtained by the research team for laboratory testing. Prior experience in this project revealed that the chemical and physical properties of Class C fly ash used in rapid-setting CLSM mixtures dramatically influence the fresh and hardened properties of mixtures. Therefore, a Class C fly ash was selected from the laboratory work that yielded the de- sired setting and hardening characteristics for the proposed repair application, and this fly ash source was then specified for the field work. The fly ash had a CaO content of 27.9 per- cent and was effective because of its rapid hardening in CLSM mixtures of this type. The research team believed that by spec- ifying the actual materials to be used in the field trial, a higher level of quality assurance could be attained, and the true ben- efits of using CLSM for bridge approaches could be realized. After selection of the specific sand and fly ash to be used in the field test, the mixture proportions were then developed by testing a range of sand–fly ash ratios and, for each of these ratios, studying the effects of water content on the flowability and strength gain. Sand–fly ash ratios of 5, 6, and 7 by mass were selected based on previous experience with such mix- tures, as summarized in Table 4.15. The water content was modified for each combination to obtain a target flow in the range of 175 to 250 mm. The inherently fast setting character- istics of mixtures containing the selected fly ash created some challenges in the laboratory program. The fly ash provided by the contractor often set within 4 minutes after the introduction of water, which often limited the number of test specimens or fresh property tests that could be performed. This same rapid- 76 0 10 20 30 40 50 60 70 water:ash 0.70 water:ash 0.87 water:ash 1.04 0 5 10 15 20 25 Pe ne tra tio n re sis ta nc e (M Pa ) Time (hours) Figure 4.13. Setting and hardening of rapid-setting CLSM mixtures with varying water–fly ash ratios.

hardening behavior leads practitioners to use volumetric, on- site mixers for these types of mixtures that contain Class C fly ash as the only binder. Also, unlike many CLSM mixtures, the rapid-setting CLSM mixtures evaluated in this study evidenced little, if any, bleeding water. This phenomenon is mainly at- tributed to the rapid setting and hardening behavior and early formation of ettringite and other hydration products that tied up much of the available water. The setting and hardening behavior of the various mixtures was evaluated using needle penetration (measured by ASTM C 403), unconfined compressive strength, and Young’s modulus using the Spectrum Analysis of Surface Wave (SASW) method. The SASW testing was performed on three 100 × 200 mm cylinders. There was generally a good correlation between modulus development and penetration resistance for the various mixtures, with a rapid increase in both properties for the first 3 hours, followed by little change thereafter. For conciseness, only the data for mixtures with sand–fly ash ratio of 5 are shown in Figure 4.14, but this trend was evident for all the mix- tures tested. An important observation was that the early-age properties of rapid-setting CLSM were significantly influ- enced by the water–fly ash ratios. For instance, the penetra- tion resistance of 3A after 30 minutes was higher than the cor- responding value of 3B at 24 hours, while the water–fly ash ratios were different by only 0.14. The variations of water– fly ash mass ratios were greatly magnified in the different setting/hardening rates. The modulus of the 2C specimens (100 × 200 mm) was more than 3 times that of 2B specimens at 30 minutes even though the water–fly ash ratios differed by only 0.23. These observations suggest that the selected mixture not only should yield the target flow and hardening rate, but also should be fairly robust, that is, not very sensitive to small changes in water content. This extreme sensitivity deviates from the typical behavior of other common CLSM mixtures that do no exhibit rapid hardening at early ages. The unconfined compressive strength of the various rapid- setting CLSM mixtures is plotted in Figure 4.15. For almost every mixture, the strength values were mainly determined by 77 Mixture Sand(part by mass) Fly Asha (part by mass) Water (part by mass) Flow (mm) 2A 5 1 0.78 220 2B 5 1 0.90 260 2C 5 1 0.67 160 3A 6 1 0.81 130 3B 6 1 0.95 230 3C 6 1 1.09 270 4A 7 1 1.25 270 4B 7 1 1.00 130 4C 7 1 1.12 220 aASTM C 618 Class C fly ash (CaO = 27.9%) Table 4.15. Rapid-setting CLSM mixtures evaluated for bridge approach construction. Note: All mixtures had a sand–fly ash mass ratio of 5. 0 1000 2000 3000 4000 5000 2A modulus 2B modulus 2C modulus 2A penetration 2C penetration 2B penetration 0 500 1000 1500 M od ul us (M Pa ) Time (hours) 0 5 10 20 30 15 25 35 40 Pe ne tra tio n re sis ta nc e (M Pa ) Figure 4.14. Comparison between needle penetration and modulus (using SASW).

the water–fly ash ratios. The higher the water–fly ash ratios, the lower the strengths at different ages. For unknown reason(s), mixture 6:1:0.81 demonstrated abnormal strength variations with time. Using the information obtained from the laboratory- prepared rapid-setting CLSM mixtures and keeping in mind the key attributes of the mixture (including rapid hardening and robustness of properties, as a function of water content), the research team selected two mixtures with sand:ash:water mass ratios of 5:1:0.75 and 6:1:0.91. The proportions and tar- get modulus and velocity values (from SASW) of the two mixtures are shown in Table 4.16. The first mixture (5:1:0.75) was estimated to reach its target stiffness (ample for continu- ation of constructing bridge approach) in about 1 hour, with the second mixture estimated to require about 3 hours to reach a similar rigidity. These mixtures, referred to as “1-hour set” and “3-hour set,” were put forward as viable options for the field test, with the decision to be made by the contractor as to which of the two mixtures to use for the bridge approach repair. Repair of Bridge Approaches The two candidate CLSM mixtures selected by the research team were approved by TxDOT for the repair of the bridge approaches on Loop 1604. However, the contractor opted to use only one of the mixtures for the actual repair. The 1-hour set mixture was selected based on the faster setting time and increased speed of construction. The repair of the bridge approaches was performed over a 10-day period, with construction taking place on 4 nights during this time period. The construction involved two sep- arate bridges, each of which is a two-lane bridge. Each night, one lane on one of the bridges was closed from 8:00 p.m. to 6:00 a.m., allowing a total of 10 hours to excavate the original 78 0 500 1000 1500 5:1:0.78 5:1:0.90 5:1:0.67 6:1:0.81 6:1:0.95 6:1:1.09 7:1:1.25 7:1:1.00 7:1:1.12 0 1 2 3 4 5 6 Co m pr es siv e str en gt h (kP a) Time (days in square root) Figure 4.15. Strength development with time (shown in square root values) for mixtures with varying sand–fly ash–water ratios (by mass). Mixture Sand:Ash:Water(by mass) Sand (kg/m3) Ash (kg/m3) Water (kg/m3) Target Modulus (MPa) Target Velocity (m/s) 1-hour set 5:1:0.75 1627 325.4 244 2529 1071 3-hour set 6:1:0.91 1658 276.4 252 1633 848 Table 4.16. Mixture proportions recommended for bridge approach application and corresponding target modulus values.

approach backfill, cast the new CLSM section, and pave over the newly cast CLSM section with hot-mix asphalt. For a given night, a single CLSM placement on one side of the bridge consisted of a 1.2 m deep section, 3.3 m in the longitudinal direction (in the direction of travel), and 6 m in the transverse direction. There was no difficulty removing the original backfill from the bridge approaches. Figure 4.16(a) shows a typical section that was cleared of the original backfill, with the right side of the photo showing the repaired section from the previous evening. Careful examination of the CLSM placed 24 hours earlier did not reveal any visible cracks, large air voids, or “cold joints” in this massive block. A good bond appeared to have formed between the CLSM backfill and the hot-mix asphalt placed above it. The CLSM cast 24 hours prior was still warm to the touch, mainly attributed to the slow dissipation of the hydra- tion heat in such a massive unit. Figure 4.16(b) shows the plac- ing of the rapid-setting CLSM mixture into the bridge approach area. Note that the backfill was built up as thin layers. The mix- ture exhibited a flow value of 270 mm, which was sufficiently fluid for this application. The surface was bull-floated to achieve a horizontal surface to facilitate the paving with hot- mix asphalt. The research team visited the construction site on two sep- arate nights to observe the CLSM placement and to obtain test cylinders (thirty 75 × 150 mm and six 150 × 300 mm) for sub- sequent testing. Specimens were also cast and tested on site for setting time following ASTM C 403. On each night, three trucks were sampled from the middle of each load. The penetration resistance results are shown in Figure 4.17. The setting characteristics from the different truck loads varied considerably, although a penetration resistance of approx- imately 7.0 MPa was obtained in about an hour for all sam- ples. The differences in setting times between laboratory and actual field samples are mainly attributed to differences in temperature history, as well as differences in mixing action, moisture corrections, etc. The Kelly ball (ASTM D 6024) was also used as a simple index to determine the early hardening characteristics of the CLSM for the bridge approach repair and to estimate when hot-mix paving could commence. Figure 4.18 shows the use of the Kelly ball on the surface of the finished backfill; typical results for two approaches are shown in Table 4.17. It should be emphasized that the ball drop method measures only prop- erties of the surface layer of the CLSM fill. Even though the diameter of the indentation of the Kelly ball on the north- bound approach was about 90 mm, the CLSM was deemed to be sufficiently stiff to accommodate the asphalt paving. The reason for proceeding with paving despite a relatively large indentation was that the top surface of the placement was moved around significantly to obtain the required grade. The research team’s prior experience with the CLSM had shown that if the initial hydration is disturbed, the strength can be severely affected. Thus, the surface property of the fill likely did not truly represent the characteristics at deeper depths. In fact, except for the upper portion, the material beneath was in place for more than 2 hours because of the reloading of the volumetric mixers. In addition, this ball drop method is likely 79 (a) (b) Figure 4.16. Opening (a) and backfill (b) of bridge approaches.

quite severe for CLSM due to the 13.62 kg mass of the steel ball. When the CLSM was deemed to be strong enough to sup- port heavy equipment, the asphalt paving commenced. The two bridge approach backfills were instrumented with temperature-measuring devices (i-buttons) to monitor the temperature history on the second observation night. The i-buttons were placed near the center of the backfill. The read- ing was taken every 5 minutes for 7 days. The results are plot- ted in Figure 4.19. The southbound bridge approach reached its peak of 47°C 24 hours after placement, while the north- bound peak temperature of 54°C was reached about 2 days after placement. The measured temperature rise for each field section was due to the massive volume of the backfill. These high temperatures were not detected in the 150 × 300 mm cylinders prepared in the laboratory. Monitoring of Backfill Materials As previously described, compressive cylinders (75 × 150 mm and 150 × 300 mm) were cast during the placement of the rapid-setting CLSM. Because of logistical challenges in secur- ing and storing the cylinders in the field, they were transported back to the laboratory after an age of at least 3 hours, by which time the cylinders were strong enough to resist damage due to transport. The cylinders were then stored in a standard curing room (23°C and 100 percent RH) until the time of testing. The 75 × 150 mm cylinders were tested using unbonded pads (based on recommendations from Chapter 3) at ages of 1, 3, 7, 28, 90, and 180 days. Figure 4.20 shows the strength development of cylinders sampled from different batches at the jobsite. The variation of strengths was relatively high, up to 20 percent, which can be at- tributed to the cylinders being obtained from different batches, variations in moisture content in sand, and inherent variabil- ity in site-cast mixtures. In addition, visual inspection of several cylinders after being tested to failure in compression revealed the presence of deposits or lumps of white powder in the mixture. This white powder was not analyzed, but the pres- ence of this impurity, which may have been lime, likely had some effect on the setting and hardening properties of the mix- tures. Two of the sampled CLSM batches exhibited strengths greater than 5.0 MPa at an age of 6 months, which may make future excavation quite difficult. However, the intention of this 80 Note: Samples 1 through 3 were from one night’s construction, and samples 4 through 6 were from construction one week later, also at night. 0 5 15 25 10 20 30 35 Laboratory mixture Sample #6, 01:49 AM Sample #5, 12:33 AM Sample #4, 10:43 PM Sample #3, 12:54 AM Sample #2, 12:34 AM Sample #1, 12:05 AM 0 50 100 200 300150 250 Time (hours) Pe ne tra tio n re sis ta nc e (M Pa ) 0 2000 1000 3000 4000 5000 Penetration resistance (psi) Figure 4.17. Needle penetration values (ASTM C 403) for rapid-setting CLSM used in bridge approach repair. Figure 4.18. Use of Kelly ball to determine the proper timing of hot-mix asphalt paving.

field test was not to produce excavatable CLSM but rather to develop a mixture that hardens quickly, allows for rapid con- struction and paving, and performs well over time, with little or no settlement. Monitoring of Field Performance Approximately 2 months after construction of the bridge approaches, a visual survey of the approaches was performed. No differential settlement of the bridge approach sections was visible and the sections were performing very well. The “bump at the end of the bridge” was essentially non-existent, which was a significant improvement over the condition of the bridge approach sections prior to repair. At the time of this inspection, a seismic pavement analyzer (SPA) was used to evaluate the in-situ moduli of the completed bridge approach sections. The SPA is an automated non-destructive device for conducting the SASW tests in less than 1 minute (Nazarian et al. 1993). In analyzing the SASW results, a thickness of 80 mm was assigned to the hot-mix asphalt layer. The upper CLSM fill was assumed to have a thickness of 600 mm. The remaining backfill above the native soil was the lower fill. Measurements were performed parallel with and perpendicu- lar to the direction of travel on the roadway. The average mod- uli measured along the profile are shown in Figure 4.21. The modulus of the asphalt pavement was quite uniform. How- ever, the modulus of the CLSM backfill varied significantly. This observation agrees with the measured variations in com- pressive strength quite well and supports the empirical rela- tion typically used for conventional concrete, whereby the 81 Southbound Northbound Time after placement (h:m) Diameter of Kelly ball indentation (mm) Time after placement (h:m) Diameter of Kelly ball indentation (mm) 0:10 114 0:09 122 0:22 99 0:19 108 0:33 97 0:30 95 0:42 89 1:07 95 0:56 76 1:30 89 Table 4.17. Typical results of Kelly ball tests (ASTM D 6024) in two bridge approach repairs. 40 30 20 10 0 50 60 Southbound Northbound 0 20 40 60 80 100 120 140 160 Te m pe ra tu re (C ) Time (hours) Figure 4.19. Heat generation in the center of rapid-setting CLSM bridge approach sections.

elastic modulus is proportional to the square root of the com- pressive strength. One interesting result is that the upper 350 mm of the rapid-setting CLSM at bridge approach 2B was significantly softer than its lower fill, perhaps because of the addition of more water in the upper fill. This section summarized a laboratory- and field-based eval- uation of rapid-setting CLSM for use in bridge approach applications. The study was unique in that it first involved diagnosing the cause of a failed field application of the ma- terial and then used the information and experience gained in this exercise to design a mixture that was successfully used in an extensive field application. The overall experience shows that rapid-setting CLSM can be an extremely useful and ver- satile material for rapid construction, but also that care should be taken in designing and constructing field installations, with suitable quality control/quality assurance, to ensure long-term performance. Field Test at Texas A&M University Introduction A comprehensive, long-term field test was performed at the National Geotechnical Experimentation Site located at the Texas A&M University Riverside Campus, which is about 12 km west of the main university campus in College Station, Texas. The objective of the field testing was to investigate the CLSM strength gain, excavatability of CLSM, and the long- term corrosion performance of ductile iron pipes and gal- vanized corrugated steel culverts backfilled with CLSM. It was recognized from the onset that because of the long-term nature of corrosion, long-term field data would be needed. Because the site is strategic to Texas A&M University (where all the laboratory corrosion testing was done, as described in Chapter 3) and the research team has unlimited access to the test location, the research team anticipates that it will continue to monitor the corrosion studies long after this NCHRP proj- ect has concluded. This site also was unique in that it allows for the controlled (and measured) application of chlorides to 82 0 3000 5000 7000 2000 4000 1000 6000 1A 1B 1C 2A 2B 2C 0 50 100 150 200 Co m pr es siv e str en gt h (kP a) Time (days) Figure 4.20. Unconfined compressive strength of cylinders from one night’s construction (1A, 1B, and 1C) and from one week later (2A, 2B, and 2C). 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 Bridge-1A Bridge-1B Bridge-2A Bridge-2B M od ul us (G Pa ) Asphalt concrete Upper Fill Lower Fill Figure 4.21. Average moduli of backfill and asphalt pavement measured in situ using SASW.

the site, which aims to replicate field conditions that lead to corrosion and to accelerate the rate of deterioration. Site Layout and Construction Two sites, a clay site and a sand site, were selected to observe the corrosion performance of embedded ductile iron pipes and galvanized corrugated steel culverts. Because the sites are part of the National Geotechnical Experimentation Site, the types of soils and clays at the sites have been well documented. For this project, test pits on both sites were excavated using a backhoe-mounted auger to collect and analyze the soil and clay. The clay site is underlain by four distinct layers. The sur- face layer is mottled red and gray clay. This clay layer is very uniform in thickness down to about 1.83 m below the surface. The plastic and liquid limits of the clay were 20.9 and 53.7 per- cent, respectively. The hydraulic conductivity coefficient was 4.99 × 10−3 m/year. The surface layer at the sand site is mot- tled red and tan silty sand. The percentage of fine particles was 17.4 percent, and the hydraulic conductivity coefficient was 5 × 10−2 m/year. Metal pipes were placed in six trenches on each site using three different trench conditions. The trenches were 12.19 m long, 0.76 m deep, and 0.46 m wide. Figure 4.22 shows the three trench conditions used in the test: • Condition I: Metallic pipes are completely embedded in CLSM (Figure 4.22(a)). • Condition II: Metallic pipes are placed on a CLSM bedding and backfilled with soil from the site (Figure 4.22(b)). • Condition III: Metallic pipes are completely embedded in soil (Figure 4.22(c)). Commercially available ductile iron pipes and corrugated steel culverts were delivered to the site. Both types of pipe were cut into 0.76 m long pieces. The ductile iron pipes had an asphalt coating that was removed by sandblasting after soaking in lacquer thinner. Copper wires (2.32 mm diameter) were attached to each ductile iron and culvert sample to be used for corrosion observations later. After drilling and tapping the ductile iron pipe pieces, screws and washers were used to attach the wires as shown in Figure 4.23(a). Exposed wires and screws were coated with epoxy to prevent corrosion. Grounding clips 83 (a) CLSM (b) CLSM SOIL SOIL (c) Figure 4.22. Three trench conditions used for the field test. (a) (b) Figure 4.23. Wiring of ductile iron pipes and corrugated steel culverts.

were used to attach wires to corrugated steel culvert pieces. After the wires were connected, the exposed sections of the wires and clips were coated with enamel. Epoxy was applied after the enamel was cured as shown in Figure 4.23(b). Twelve of the cut ductile iron pipe samples and twelve of the cut corrugated culvert samples were painted with epoxy inside and outside leaving only a 0.15 m diameter circular area ex- posed. Counter electrodes for polarization studies were 0.15 × 0.15 m nickel-chromium wire mesh. Copper wires (2.32 mm diameter) with alligator clips were attached and soldered to the meshes. The alligator clips, the solder area, and exposed wires were coated with epoxy. These 24 pieces of pipes with limited exposure areas together with the counter electrodes can be used later for long-term corrosion rate measurements. Figure 4.24 shows three of the samples prepared for polarization testing. The six trenches were excavated on each site in a 6 × 30.5 m rectangular area using a backhoe with a 0.46 m wide bucket. After the bottoms of the trenches were cleared, pipes were placed with 0.6 m space between them. Four ductile iron pipe samples and four corrugated steel culvert samples, including one of each with limited exposure areas, were placed in each trench. The pipe pieces were placed on steel chairs to allow free flow of CLSM mixture underneath the pipes. Each piece of pipe was also secured using four stakes, driven 1 foot into the ground to prevent lateral and vertical movement. Figure 4.25 shows a trench in the clay site with pipes. The pipes prepared for polarization testing were placed at the two ends of the trenches. These pipes were placed with the exposed areas facing sideways so that when the trenches were backfilled they would be exposed to a CLSM and soil environ- ment. Counter electrodes were placed adjacent to the exposed areas. After the placement of the pipes, trenches were backfilled with a CLSM mixture provided by a local concrete supplier. The CLSM mixture contained 1483 kg/m3 sand, 34 kg/m3 cement, and 135 kg/m3 fly ash. The water–cementitious ma- terial ratio was 0.8. The CLSM was delivered in ready-mixed concrete trucks and was placed into the trenches using a chute as shown in Figure 4.26. The average time to fill the condition I (completely filled with CLSM) trenches was 8 minutes. The truck was placed at one end at the trench and the trench was completely filled from this point (Figure 4.26(a)). The average time to fill the condi- tion II (CLSM bedding and soil backfill) trenches was 11 min- utes, and the truck had to be moved three times to ensure uniform thickness of bedding layer. Condition II and III trenches were filled with native soil after the setting of the CLSM backfill. The soil was placed into the trenches in layers with a backhoe and compacted with an average of three passes of a jumping jack compactor. The average time to compact an approximately 0.25 m thick by 12 m long layer of soil was approximately 5 minutes. This speed equals approximately 30 minutes for sample sized trenches filled with CLSM (3 to 4 times longer). The wires connected to the pipe pieces in each trench ran along the bottom of the trenches and were collected in PVC boxes at the surface of each trench. The wires entered into the boxes through an inverted U-shaped conduit to prevent rain- water from entering into the boxes. A smaller box was placed inside each PVC box and wires were soldered to female con- 84 Figure 4.24. Metal pipes with 0.15 m diameter exposed surface and a counter electrode. Figure 4.25. Clay site trench with pipes.

nectors attached to the lid of the smaller boxes for extra pro- tection and ease of measurement. Connectors in each box are labeled with numbers 1 to 8. The connectors from 1 to 4 were attached to the galvanized corrugated steel culverts, starting from the closer end of the trench to the box and connectors 5 to 8 were connected to the ductile iron pipe samples. Connec- tors were attached to the counter electrodes of pipes 1 and 8. Figure 4.27 shows one of the PVC boxes and connectors. Before the placement of the pipes, the research team planned to expose three of the six trenches on each site to chlorides. Therefore, to prevent the chlorides from flowing with rain- water to the trenches that were not supposed to be exposed to chlorides, the sites were graded with a motor grader to have a slope of 2 percent. Figure 4.28 shows the general site layout for one of the test sites. Testing Program The flow (ASTM D 6103) and air content (ASTM C 231) of the CLSM delivered to the test site were measured prior to backfilling the trenches. Cylinders (100 × 150 mm) were cast to measure the compressive strength at 4, 7, and 28 days. Samples were capped with plastic lids after casting. One day after casting, cylinders were transported to a fog room (22°C and 98 percent RH) to be held until testing. Compressive strength testing was performed using neoprene pads and displacement- controlled testing equipment. The setting times of the CLSM mixture in the condition I trenches were measured using a needle penetrometer with a 6.45 mm2 needle tip. After backfilling was complete, the location of each piece of pipe was marked using flags. An average of 10.75 kg/m2 sodium chloride was applied to the chloride sections of each site. After the application of chlorides to the backfilled testing sites, half- cell potentials were collected as an indicator of corrosion of the embedded metallic pipes. Half-cell potentials were collected using a copper–copper sulfate (Cu-CuSO4) electrode and a high impedance multimeter. To measure the half-cell poten- tial of a piece of embedded pipe, the electrode was placed on the soil or CLSM surface above the pipe sample, after the multimeter was connected to the corresponding connector in the PVC box. To be consistent, the electrode was placed next to the identifier flags on the surface identifying the location of 85 (a) (b) Figure 4.26. Placement of CLSM into the trenches. Figure 4.27. PVC connection box and connectors.

pipe samples every time data was collected. If the surface was too dry, the surface was wetted to obtain a better electrical con- nection. Figure 4.29 shows the schematic of the half-cell potential test setup. Test Results Two CLSM samples were taken from each of the two ready- mix concrete trucks that delivered the material to the trenches, and the flow and air contents were quite uniform, with flows ranging from 225 to 240 mm and air contents ranging from 14 to 19 percent. The field penetrometer could not register any readings for about the first 11 to 12 hours after trench place- ment. The field penetrometer data are shown in Table 4.18. The particular CLSM mixture used in this study exhibited a sufficient set to support the weight of an average person about 15 hours after the completion of placement. The average strength of the CLSM, 24 hours after the placement, was about 345 kPa, measured by the penetrometer test. Figure 4.30 shows the compressive strength of the CLSM mixtures sampled from the two trucks at 4, 7, and 28 days. 86 PVC box 2% slope 20 ft 100 ft 40 ft1.5 ft Chloride Section Condition I Condition II Condition III Figure 4.28. General site layout (not to scale). Connector box Multimeter Metallic Pipe Reference electrode Ground Surface Figure 4.29. Half-cell potential reading connection. Penetrometer Results (kPa) Time after placement (hours) 1 2 3 4 5 6 0 0 0 0 0 0 0 11 0 0 0 0 0 0 12 14 21 0 21 14 0 15 34 48 34 55 41 48 19 269 290 276 255 269 283 21 331 338 359 241 345 324 24 359 345 331 338 345 345 Table 4.18. Field penetrometer results.

Long-Term Corrosion Testing As mentioned earlier in this section, a major thrust of this field test is to generate field data on the corrosion of metals imbedded in CLSM in the field. As was expected, the rate of corrosion under these field conditions has been quite low, and after more than 2 years of monitoring, little active corrosion has been measured. For completeness, a brief summary of the corrosion data (half-cell potential) is provided in Tables 4.19 and 4.20. These tables show the average half-cell potential measurement against the Cu-CuSO4 reference electrode of the four galvanized steel or the four ductile iron pipes exposed to the same conditions. The half-cell potentials shown are time- weighted averages for the conditions shown. The research team will continue to monitor this unique long-term corrosion site and hope that these data will prove useful in developing information about the service life of met- als embedded in CLSM. Long-term excavatability studies also will be performed as part of these ongoing efforts. Summary of Key Findings from Field Tests This chapter has summarized the findings from six CLSM field tests performed throughout the United States. As previ- ously mentioned, the main goals of this field testing program were to fill in the gaps in understanding CLSM behavior and 87 0 500 1000 1500 2000 0 5 10 15 20 25 30 Truck 1 Truck 2 Co m pr es siv e St re ng th (k Pa ) Time (days) Figure 4.30. Compressive strength of laboratory-cured cylinders from TAMU field test. Soil Type Environment Condition Pipe Type Weighted Half-Cell Potential (V) Galvanized –0.7672CLSM Ductile –1.0719 Galvanized –0.7018CLSM/Soil Ductile –1.0030 Galvanized –0.6850 Chloride Soil Ductile –1.0135 Galvanized –0.6997CLSM Ductile –0.8849 Galvanized –0.6165CLSM/Soil Ductile –0.8923 Galvanized –0.5585 Clay Non-chloride Soil Ductile –0.9236 Table 4.19. Time-weighted average half-cell potentials for the clay site. Soil Type Environment Condition Pipe Type Weighted Half-Cell Potential (V) Galvanized –0.7006CLSM Ductile –0.6169 Galvanized –0.6340CLSM/Soil Ductile –0.9711 Galvanized –0.4242 Chloride Soil Ductile –0.5911 Galvanized –0.8330CLSM Ductile –0.8708 Galvanized –0.4646CLSM/Soil Ductile –0.9200 Galvanized –0.3537 Sand Non-chloride Soil Ductile –0.9078 Table 4.20. Time-weighted average half-cell potentials for the sand site.

performance and to validate the test methods, specifications, and guidelines developed in the earlier stages of this project. In general, the field tests proved to be quite successful and enlightening. For the most part, the test methods, specifications, and guidelines developed under this project were found to be appropriate and effective. Several specific technical issues were addressed in the course of these field tests, with an emphasis on aspects of CLSM behavior that could not be adequately evaluated in the laboratory, such issues as excavatability and corrosion. Although the relevant data from some long-term corrosion field tests were not collected under this project (because of the slow rate of corrosion in field installations), it is anticipated that these data will be collected in the future and presented to the appropriate AASHTO committees for consideration. Some of the specific findings from this field testing pro- gram are briefly summarized below: • The basic tests for CLSM, such as flow, air content, and unit weight, were found to be effective and easy to implement, and most jurisdictions involved in the field tests were already routinely using the tests in practice. • The compressive strength of CLSM was measured in each field test using the testing methodology developed under this project. The approach for proper handling, curing, cap- ping, and testing was validated throughout the process. • The tests showed that strength measured on standard-cured cylinders can vary significantly from actual CLSM strength in field applications, mainly because of differences in time- temperature histories. This disconnect appears to be great- est when fly ash is used, and as such, users should be aware of this issue when considering long-term excavatability. • There is no single property of CLSM (e.g., compressive strength) that can be used as a definitive index for excavata- bility. Compressive strength is the most commonly mea- sured and reported CLSM property, and can be a reasonable index of excavatability in some cases. However, the discon- nect between the strength of laboratory-cured cylinders and the actual long-term strength of CLSM in trenches, etc. makes full reliance on laboratory strengths when predicting excavatability difficult. Testing cylinders in the laboratory under conditions expected in field installations is recom- mended when excavatability is a concern. • The removability modulus, originally developed by Hamil- ton County (Ohio) engineers, is a useful tool in attempt- ing to predict excavatability. This method is an empirical approach to predicting excavatability using an equation featuring the unit weight and 28-day strength value of CLSM. This approach can be further improved upon by using field-cured strengths, thereby minimizing the dis- connect with laboratory-cured cylinders, in the equation. The inclusion of unit weight is actually quite helpful as a parameter used in predicting excavatability because it tends to pick up the aggregate-related effects associated with excavatability. Specifically, in some field tests, the lack of aggregates in CLSM (e.g., mixtures with 95 percent Class F fly ash, 5 percent portland cement, and water added for desired fluidity) resulted in a mixture that was easier to excavate than would have been expected, based solely on the strength of the mixture. • The DCP was found to be a particularly useful tool in mon- itoring early-strength gain of CLSM, as well as the long- term strength and excavatability of installations. This method is unique in that it allows for measuring the prop- erties of CLSM as a function of depth, thereby avoiding a shortcoming of surface penetration tests (needle pen- etrometer, soil penetrometer) that only assess the near- surface behavior. • Due to the high fluidity of CLSM mixtures, floating of pipes or unintentional shifting of utilities may result, and users should take precautions to avoid this behavior. Such precautions are addressed in the specifications and guide- lines developed under this project for backfill applications. • More long-term monitoring is essential for a true assess- ment of corrosion of metals in CLSM. Tests initiated under this project will continue to be monitored, and the relevant findings will be communicated to the appropriate AASHTO committees. 88

Next: Chapter 5 - Conclusions and Suggested Research »
Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 597: Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction explores the use of controlled low-strength material (CLSM) in highway construction applications, in particular, as backfill, utility bedding, and void fill and in bridge approaches. The report also examines a recommended practice for the use of CLSM that was developed through a series of full-scale field experiments.

This report presents the full text of the contractor’s final report of the project and three of the five appendices, which present the test methods (Appendix B), specifications (Appendix C), and practice (Appendix D) recommended for implementation. The corrosion study (Appendix A) and implementation plan (Appendix E) are available online as NCHRP Web-Only Document 116.

There is a summary document, Paths to Practice, available.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!