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

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64 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 C.O.V. Average C.O.V. Average C.O.V. Average C.O.V. Average C.O.V. Average C.O.V. (kPa) (%) (kPa) (%) (kPa) (%) (kPa) (%) (kPa) (%) (kPa) (%) 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 Mixture Test Site Fog Room Test Site Fog Room Average C.O.V. Average C.O.V. Average C.O.V. Average C.O.V. (kPa) (%) (kPa) (%) (kPa) (%) (kPa) (%) 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. much faster (about 10 hours' difference in reaching similar between cylinders stored at the site and those stored in the fog target penetration values) than the specimens stored in the room. This finding is consistent with laboratory findings from laboratory. Chapter 3 that showed that straight cement mixtures were less Despite the high ambient temperatures during this field sensitive to temperatures than mixtures containing fly ash. For trial, the CLSM mixtures did not generate significant heat CLSM mixtures containing fly ash, specimens cured at the site within the trenches. All of the mixtures, with the exception had much higher strengths than those in the fog room during of Paste, remained at temperatures below 45C during their the first 3 months. Ultimately, strengths of specimens cured in hydrating phase. The trench containing Paste reached a max- the fog room approached those of specimens cured on site at imum temperature of 64C, which resulted in higher com- later ages (e.g., 10 months) in this study, as illustrated in Fig- pressive strengths than previous laboratory testing would ure 4.1 for Mixture F2. have suggested, as discussed in the following section. 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 Hardened Properties 600 kPa after 24 hours and a straight gain to 6 MPa after 28 days Compressive and Splitting Tensile Strengths. The com- (with little increase in strength thereafter). Similar strength- pressive and splitting tensile data for the various mixtures are gain behavior was observed for the mixtures used in the shown in Tables 4.4 and 4.5, respectively. For mixtures A1 and repair of bridge approaches in San Antonio, Texas, as described A2 (no fly ash included), there was little difference in strengths later in this chapter. The Paste mixture was found to have com- Table 4.5. Splitting tensile strength of CLSM mixtures from UTAustin field test. 7 Days 28 Days 90 Days Test Site Fog Room Test Site Fog Room Test Site Fog Room Mixture Average C.O.V. Average C.O.V. Average C.O.V. Average C.O.V. Average C.O.V. Average C.O.V. (kPa) (%) (kPa) (%) (kPa) (%) (kPa) (%) (kPa) (%) (kPa) (%) 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

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

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66 Table 4.6. Direct and indirect evaluation of excavatability (excavation performed 300 days after trench placement). 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 41.1 13.7 24.7 29.8 45.8 41.3 (MN/m) Compressive strengthb 7299 86 446 7156 3934 8637 (kPa) T T Tensile strengthb (kPa) 1297 12.4 71.1 761 454 953 Fog room RE c 0.2 0.8 2.3 2.5 3.4 c Field RE 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 Very difficult Difficult (but Backhoe Difficult Very easy Easy Very difficult (nearly possible) impossible) a All testing performed 300 days after trench placement unless otherwise noted. b Cylinders stored for 300 days on site prior to testing. c RE is based on 28-day compressive strength. Only the trenches containing mixtures A1 and A2 were able Although this approach is acknowledged to measure only the to be excavated manually (i.e., using shovels and picks). As one properties of the upper layers of CLSM, the values did corre- would expect, excavating mixtures A1 and A2 using a back- late quite well in this field test with DCP values, successfully hoe was also easy. The remaining trenches ranged from diffi- predicting that Paste was easier to excavate than mixture F1. cult but possible (Paste) to very difficult and nearly impossible Long-term compressive strength is often used as a criterion (F2) to excavate with a backhoe. Following are discussions on to assess the excavatability of CLSM (ACI 1999). For this field indirect methods of evaluating or predicting excavatability. trial, cylinders were tested in compression and tension after The DCP was found to clearly differentiate the excavatabil- having been stored on site for 300 days, as shown in Table 4.6. ity of the six CLSM mixtures. Because this penetrometer can be Clearly, the availability of this type data would be a luxury for forced through the whole depth of the backfill and the lowest an actual CLSM installation, but the data shown in Table 4.4 penetration index is selected, this approach has the advantage would often be available (particularly, the data from 28-day that it is not affected by a deteriorated surface. This advantage cylinders stored in the fog room). The two trenches that were was also demonstrated in the testing of the two trenches at easiest to excavate (those containing mixtures A1 and A2) also the National Ready Mixed Concrete Association (NRMCA) in yielded low compressive strength values (for the site-cured Maryland (discussed later in this chapter). Although the data specimens tested on the day of excavation) well below the generated in these field tests, coupled with the excavatability 1 MPa value that is sometimes used in the field as a rough index tests described in Chapter 3, are extensive, providing absolute of excavatability. While ease of excavation was linked to lower guidance on DCP values that separate excavatable CLSM from compressive strengths for these two trenches, the other mix- non-excavatable CLSM is not possible. However, based on the tures exhibited no clear link between strength and excavata- data generated within this project, a DCP index of 5 mm per bility. For instance, mixture Paste had a higher strength than blow can be proposed as a general rule of thumb, below which mixture F1, yet Paste was easier to excavate. This result can there could be problems for manual excavation. Stiffness val- mainly be attributed to the lack of aggregates in Paste, because, ues generated by the GeoGauge were able to differentiate A1 in general, CLSM containing aggregates is more difficult to and A2 as being excavatable, but for the other trenches, where excavate. Thus, compressive strength by itself is shown to be the stiffness of the backfill material is beyond the capacity of the an unreliable indicator of excavatability. This shortcoming is equipment, the outputs seemed to be random. This phenom- further compounded by the limited availability of strength enon is clearly shown by the measurements of mixtures F1 and values, which are generally available for only laboratory-cured F2, where F2 should be stiffer as indicated by the DCP index specimens and usually for only the first month or so after cast- and actual excavation experience. ing. These short-term tests do not adequately represent the The diameter of the dent caused by the dropping of the Kelly long-term strength gain of field CLSM, nor do they capture ball was also evaluated as an indicator of CLSM excavatability. the temperature-related effect that field installations experi-