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75 Table 4.13. Excavatability of next evening, and hot-mix asphalt was placed over the CLSM. CLSM trenches. However, severe rutting and settlement were observed within a few months. Because of the poor performance of rapid- Difficulty Trench Method setting CLSM in this application, TxDOT decided to place a Level 2 Shovel High temporary moratorium on the use of rapid-setting CLSM in Steel Bar Moderate bridge approaches in the San Antonio area. As part of NCHRP (Mixture A) Backhoe Moderate Shovel Moderate 24-12(01), efforts were initiated to investigate the cause of the 4 poor performance and to provide guidance on future bridge Steel Bar Low (Mixture B) Backhoe Low approach applications. Shovel Low In an effort to understand the cause of the initial failed 6 Steel Bar Low (Mixture C) Backhoe Low 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 in fact, mixture C, which was removed easily from trench 6, to 1.5 MPa, which indicates that long-term strengths and had a higher compressive strength than mixture B. This test rigidity were not the problem. Efforts were then made to also illustrated how CLSM mixtures with similar proportions reproduce the "actual" job mixture using the limited amounts can behave completely different in field applications, owing of raw materials retained from the original application and to differences in raw materials, mixing action, and placement information retained from the job on the mixture propor- techniques. tions (see mixture A in Table 4.14). Given the small amount of remaining materials, a Hobart mixer was used, and three Field Evaluation of CLSM for Bridge 50 × 50 mm cubes were prepared for each mixture follow- Approach Repair (TxDOT) ing ASTM C 305 and tested using a geotechnical compres- sion machine at the ages of 3, 8, and 24 hours. The results Introduction are summarized in Table 4.14. The flow was measured fol- lowing ASTM D 6103. The setting and hardening processes A fairly new application for CLSM is its use as a subbase were monitored through the penetration test as per ASTM under bridge approaches. This section discusses the use of C 403. rapid-setting CLSM for the repair of several bridge approach The mixture proportions provided by the contractor slabs in San Antonio, Texas, which was done in close cooper- (mixture A) did not result in a self-leveling, fluid mixture. ation with the Texas Department of Transportation (TxDOT). About 50 percent more water was needed to make the CLSM This section first describes an unsuccessful attempt (before mixture fluid enough for the desired application, with dra- the initiation of this NCHRP project) at using rapid-setting matic effects on the rate of setting and hardening, as sum- CLSM for this application. Through forensic analyses and marized in Figure 4.13. After significant evaluation, the fine laboratory evaluations, the probable causes of this failed aggregate used in the initial, unsuccessful bridge approach application were identified. A comprehensive study was then application was determined to be a dredged sand with most initiated to develop appropriate guidance for successfully of the particles falling between 0.1 and 1 mm in size and a repairing these bridge approaches using rapid-setting resultant fineness modulus of 1.33, well below the typical CLSM. The repair of four bridge approaches in San Antonio values for sands used in conventional concrete and many were then performed and inspected about 2 months after CLSM mixtures. Based on this investigation, it is quite pos- construction. sible that the use of the fine aggregate required such an increase in the water content in the field to get the desired fluidity Research Program that the early setting and hardening behavior was greatly affected. Investigation of Initial Field Problems Historically, the use of CLSM by the TxDOT has been mainly for repairing infrastructure. In August 2002, rapid- Table 4.14. Mixture proportions (parts by mass) for setting CLSM mixtures were used to repair severe settlements rapid-setting CLSM. of bridge approaches at the intersection of I-35 and O'Conner 3-Hour 8-Hour 24-Hour Sand Ash Water Flow Drive in San Antonio. Unfortunately, the setting and harden- Mixture (part) (part) (part) (mm) Strength Strength Strength (kPa) (kPa) (kPa) ing of the installations were quite slow, and steel plates had to A 4.4 1 0.7 0 666 992 1309 be placed to cover the backfill to accommodate the heavy traf- B 4.4 1 0.87 130 407 723 768 fic for the next morning. The steel plates were removed the C 4.4 1 1.04 270 256 513 550
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76 70 60 water:ash 0.70 water:ash 0.87 water:ash 1.04 50 Penetration resistance (MPa) 40 30 20 10 0 0 5 10 15 20 25 Time (hours) Figure 4.13. Setting and hardening of rapid-setting CLSM mixtures with varying waterfly ash ratios. Materials Selection and Mixture Proportioning tions. A locally available ASTM C 618 Class C fly ash was spec- for Bridge Approach Repair ified and was also obtained by the research team for laboratory testing. Prior experience in this project revealed that the After determining the likely cause of the failed application of chemical and physical properties of Class C fly ash used in rapid-setting CLSM in the bridge approaches in Texas, the re- rapid-setting CLSM mixtures dramatically influence the fresh search team and TxDOT engineers decided to jointly develop and hardened properties of mixtures. Therefore, a Class C fly a suitable mixture for the rapid repair of the approaches for two ash was selected from the laboratory work that yielded the de- bridges in San Antonio. These two bridges at Branch Sala Trillo sired setting and hardening characteristics for the proposed of Loop 1604 between I-10 and I-35 in San Antonio needed repair application, and this fly ash source was then specified repair due to significant problems with differential settlement for the field work. The fly ash had a CaO content of 27.9 per- (i.e., the "bump at the end of the bridge"). To avoid the previ- cent and was effective because of its rapid hardening in CLSM ously discussed problems with using rapid-setting CLSM for mixtures of this type. The research team believed that by spec- bridge approach applications, comprehensive laboratory test- ifying the actual materials to be used in the field trial, a higher ing was performed to select and specify the materials and mix- level of quality assurance could be attained, and the true ben- ture proportions for the proposed repair applications. efits of using CLSM for bridge approaches could be realized. Although the hallmark of CLSM technology is the ability to After selection of the specific sand and fly ash to be used in efficiently and successfully use a wide range of materials that the field test, the mixture proportions were then developed by do not conform to conventional concrete specifications (e.g., testing a range of sandfly ash ratios and, for each of these ASTM C 33 for aggregates or ASTM C 618 for fly ashes), a ratios, studying the effects of water content on the flowability somewhat conservative approach was taken for this applica- and strength gain. Sandfly ash ratios of 5, 6, and 7 by mass tion. Given that the initial problem with the bridge approach were selected based on previous experience with such mix- in San Antonio was likely caused by the fine aggregate that did tures, as summarized in Table 4.15. The water content was not conform to ASTM C 33 gradation limits, the researchers modified for each combination to obtain a target flow in the and TxDOT engineers decided to specify locally available con- range of 175 to 250 mm. The inherently fast setting character- crete sand that met ASTM C 33 for the newly proposed CLSM. istics of mixtures containing the selected fly ash created some They postulated that a well-graded sand would help control challenges in the laboratory program. The fly ash provided by the water demand and would eliminate the need to add excess the contractor often set within 4 minutes after the introduction water at the jobsite. The selected fine aggregate, a natural river of water, which often limited the number of test specimens or sand, was procured for the preliminary laboratory evalua- fresh property tests that could be performed. This same rapid-
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77 Table 4.15. Rapid-setting CLSM mixtures evaluated The SASW testing was performed on three 100 × 200 mm for bridge approach construction. cylinders. a There was generally a good correlation between modulus Sand Fly Ash Water Flow development and penetration resistance for the various Mixture (part by mass) (part by mass) (part by mass) (mm) 2A 5 1 0.78 220 mixtures, with a rapid increase in both properties for the first 2B 5 1 0.90 260 3 hours, followed by little change thereafter. For conciseness, 2C 5 1 0.67 160 only the data for mixtures with sandfly ash ratio of 5 are 3A 6 1 0.81 130 shown in Figure 4.14, but this trend was evident for all the mix- 3B 6 1 0.95 230 tures tested. An important observation was that the early-age 3C 6 1 1.09 270 4A 7 1 1.25 270 properties of rapid-setting CLSM were significantly influ- 4B 7 1 1.00 130 enced by the waterfly ash ratios. For instance, the penetra- 4C 7 1 1.12 220 tion resistance of 3A after 30 minutes was higher than the cor- a ASTM C 618 Class C fly ash (CaO = 27.9%) responding value of 3B at 24 hours, while the waterfly ash ratios were different by only 0.14. The variations of water fly ash mass ratios were greatly magnified in the different hardening behavior leads practitioners to use volumetric, on- setting/hardening rates. The modulus of the 2C specimens site mixers for these types of mixtures that contain Class C fly (100 × 200 mm) was more than 3 times that of 2B specimens at ash as the only binder. Also, unlike many CLSM mixtures, the 30 minutes even though the waterfly ash ratios differed by only rapid-setting CLSM mixtures evaluated in this study evidenced 0.23. These observations suggest that the selected mixture not little, if any, bleeding water. This phenomenon is mainly at- only should yield the target flow and hardening rate, but also tributed to the rapid setting and hardening behavior and early should be fairly robust, that is, not very sensitive to small formation of ettringite and other hydration products that tied changes in water content. This extreme sensitivity deviates up much of the available water. from the typical behavior of other common CLSM mixtures The setting and hardening behavior of the various mixtures that do no exhibit rapid hardening at early ages. was evaluated using needle penetration (measured by ASTM C The unconfined compressive strength of the various rapid- 403), unconfined compressive strength, and Young's modulus setting CLSM mixtures is plotted in Figure 4.15. For almost using the Spectrum Analysis of Surface Wave (SASW) method. every mixture, the strength values were mainly determined by 5000 40 2A penetration 2C penetration 35 4000 2B penetration 30 Penetration resistance (MPa) 25 3000 Modulus (MPa) 20 2000 15 10 1000 2A modulus 2B modulus 5 2C modulus 0 0 0 500 1000 1500 Time (hours) Note: All mixtures had a sandfly ash mass ratio of 5. Figure 4.14. Comparison between needle penetration and modulus (using SASW).
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78 1500 5:1:0.78 5:1:0.90 5:1:0.67 6:1:0.81 Compressive strength (kPa) 1000 6:1:0.95 6:1:1.09 7:1:1.25 7:1:1.00 7:1:1.12 500 0 0 1 2 3 4 5 6 Time (days in square root) Figure 4.15. Strength development with time (shown in square root values) for mixtures with varying sandfly ashwater ratios (by mass). the waterfly ash ratios. The higher the waterfly ash ratios, the as to which of the two mixtures to use for the bridge approach lower the strengths at different ages. For unknown reason(s), repair. mixture 6:1:0.81 demonstrated abnormal strength variations with time. Using the information obtained from the laboratory- Repair of Bridge Approaches prepared rapid-setting CLSM mixtures and keeping in mind The two candidate CLSM mixtures selected by the research the key attributes of the mixture (including rapid hardening and robustness of properties, as a function of water content), team were approved by TxDOT for the repair of the bridge the research team selected two mixtures with sand:ash:water approaches on Loop 1604. However, the contractor opted to mass ratios of 5:1:0.75 and 6:1:0.91. The proportions and tar- use only one of the mixtures for the actual repair. The 1-hour get modulus and velocity values (from SASW) of the two set mixture was selected based on the faster setting time and mixtures are shown in Table 4.16. The first mixture (5:1:0.75) increased speed of construction. was estimated to reach its target stiffness (ample for continu- The repair of the bridge approaches was performed over a ation of constructing bridge approach) in about 1 hour, with 10-day period, with construction taking place on 4 nights the second mixture estimated to require about 3 hours to during this time period. The construction involved two sep- reach a similar rigidity. These mixtures, referred to as "1-hour arate bridges, each of which is a two-lane bridge. Each night, set" and "3-hour set," were put forward as viable options for one lane on one of the bridges was closed from 8:00 p.m. to the field test, with the decision to be made by the contractor 6:00 a.m., allowing a total of 10 hours to excavate the original Table 4.16. Mixture proportions recommended for bridge approach application and corresponding target modulus values. Target Target Sand:Ash:Water Sand Ash Water Mixture 3 3 3 Modulus Velocity (by mass) (kg/m ) (kg/m ) (kg/m ) (MPa) (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
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79 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- (a) 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 (b) of the Kelly ball on the surface of the finished backfill; typical results for two approaches are shown in Table 4.17. It should Figure 4.16. Opening (a) and backfill (b) of bridge be emphasized that the ball drop method measures only prop- approaches. 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 that if the initial hydration is disturbed, the strength can be to be sufficiently stiff to accommodate the asphalt paving. The severely affected. Thus, the surface property of the fill likely reason for proceeding with paving despite a relatively large did not truly represent the characteristics at deeper depths. In indentation was that the top surface of the placement was fact, except for the upper portion, the material beneath was in moved around significantly to obtain the required grade. The place for more than 2 hours because of the reloading of the research team's prior experience with the CLSM had shown volumetric mixers. In addition, this ball drop method is likely