Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction (2008) / Chapter Skim
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, recommended specifications (Appendix C) , and recommended practices (Appendix D)
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– Selected mixtures from the test matrix CLSM Application Important Properties Potentially Important Properties Backfill Flow Compressive strength Excavatability Hardening time Settlement Corrosion of metal utilities Subsidence Freeze-thaw resistance Leaching and environmental impact Utility bedding Flow Compressive strength Hardening time Corrosion of metal utilities Freeze-thaw resistance Leaching and environmental impact Thermal conductivity Void fill Flow Subsidence Settlement Unconfined compressive strength Bridge approaches Flow Compressive strength Hardening time Shear strength Resilient modulus/CBR Settlement Freeze-thaw resistance Leaching and environmental impact Table 3.1. CLSM applications and relevant properties.
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, and two aggregate types (concrete sand and bottom ash) were used in all combinations to create a total of eight mixtures.
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Lastly, this table includes non–air-entrained CLSM mixtures containing foundry sand (selected after the difficulties encountered in entraining air in mixtures containing foundry sand)
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Because of relevance to field applications and based on important findings related to corrosion of ductile iron specimens embedded in the initial 38 CLSM mixtures, an expanded and detailed long-term corrosion study was performed (Phase II)
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Mixture Cement(kg/m3) Fine Aggregate Typea Fly Ash Typeb Water (kg/m3)
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Fly Ash Typea Fine Aggregate Typeb Water (kg/m3)
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Air Content and Unit Weight ASTM C 231 (pressure method) , which is typically used for conventional concrete, was used, with slight modification, to measure the air content and unit weight of fresh CLSM mixtures.
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was calculated, using the same mathematical approach as typically 25 Mixture Cement(kg/m3) Fine Aggregate Typea, b Fly ash (kg/m3)
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large machine Loading rate Effect of drying samples Alternative capping materials Effects of drainage Compressive strength ASTM D 4832 Curing methods, conditions CLSM vs. sand ASTM G 1 Galvanic cells ASTM G 1, modified G 109 pH ASTM G 51 Resistivity ASTM G 57 Segregation, bleeding ASTM C 940 Subsidence No Standard Triaxial shear strength USACE EM 1110-2-1906 California bearing ratio AASHTO T 193 Resilient modulus AASHTO T 292 Water permeability ASTM D 5084 Drying shrinkage No standard Excavatability No standard Splitting tensile strength Chloride diffusion ASTM C 1152 Freezing and thawing ASTM D 560 Effects on permeability Direct shear strength None Thermal conductivity None Air/gas permeability None Consolidation None Leaching None Chemical and toxicity analyses Table 3.17.
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Effects of Loading Rate on Compressive Strength. ASTM D 4832 gives little guidance regarding load rate, stating only to "Apply the load at a constant rate such that the cylinder will fail in not less than 2 min." Because of the vagueness in defining the load rate, additional testing was performed to investigate the effects of loading rate on compressive strength.
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Alternative Capping Materials for Compression Testing. In preliminary testing, as well as the testing of the initial 38 mixtures included in this study, sulfur capping was found to be an effective method of obtaining repeatable compressive strength data.
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A field penetrometer (field version of ASTM C 403) was used to evaluate the strength gain of CLSM mixtures.
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Shrinkage measurements were taken daily for the first week and once a week thereafter. Durability Test Methods Corrosion A comprehensive laboratory corrosion program was performed, with the objective to characterize the corrosion performance of ductile iron and galvanized steel embedded in CLSM and to identify key parameters that significantly influ30
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For the coupled conditions, pairs of ductile iron or galvanized steel coupons were embedded in 100 × 200 mm plastic molds that were half-filled with CLSM and half with soil. In this condition, one of the metallic coupons was completely embedded in CLSM and the other coupon was completely embedded in soil and they were connected with a 10 ohm resistor at the top as shown in Figure 3.3.
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The plastic and liquid limits of the clay were 20.9 percent and 53.7 percent, respectively, and the hydraulic conductivity coefficient was 5 × 10−4 m/year. In both phases, metallic coupons were removed from the samples at the end of the exposure period and were evaluated for mass loss following ASTM G 1, "Preparing, Cleaning, and Evaluating Corrosion Test Specimens." Ductile iron coupons were cleaned using cleaning procedure C.3.5 and galvanized steel coupons were cleaned using cleaning procedure C.9.5.
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To address this issue, by-product materials evaluated in this project were tested to determine their chemical composition and potential for leaching from CLSM. For each of the by-products included in the initial laboratory study (three fly ashes, one bottom ash, and one foundry sand)
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calculations identified significant variables as fly ash type, fine aggregate type, and the interactions between cement content and fly ash type. Bleeding and Segregation Bleeding and segregation affect the subsidence and the uniformity of the placed CLSM mixtures.
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Detailed investigations on load rate, curing and conditioning of cylinders, effects of drainage on strength, and the use of alternative capping materials were performed. The findings of the initial broad study and the later detailed studies were used to refine and improve existing methods of measuring the unconfined compressive strength of CLSM.
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Unconfined compressive strength of original 38 CLSM mixtures. Mixture 3-day fc(MPa)
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The model predicting the compressive strength of air-entrained CLSM mixtures is shown in Equation 3.2 (Du et al.
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The chemical reactivity of fly ash was found to be critical, because the strength of CLSM mixtures containing Class C fly ash was higher than similar mixtures containing Class F or high-carbon fly ash. Class C fly ash has a higher CaO content than Class F fly ash (and the high-carbon fly ash used in this study)
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A more comprehensive study on capping materials, including neoprene pads with significantly lower durometer values, was subsequently performed, as described later in this chapter. In a study focusing on the effects of cylinder size (75 × 100 mm vs.
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Overall, these findings regarding curing temperature and cylinder storage led the research team to initiate a final investigation on the effects of curing temperature and humidity on compressive strength, as discussed in the next section. The effect of drying time on the compressive strength of CLSM cylinders was evaluated; the results are shown in Table 3.26.
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Researchers observed that, similar to drying concrete, drying CLSM samples could increase the measured compressive strength values as much as 17 percent. This observation indicates that it is not necessary to air-dry CLSM cylinders for 4 to 8 hours before capping as required by the ASTM D 4832.
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Air drying of CLSM cylinders from the third day of curing generally increased their 7-day strength, compared to the samples that were kept continuously in molds for 7 days. However, the 91-day compressive strength of air-dried cylinders was generally lower compared to the samples that were kept in molds.
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Compressive strength results obtained using durometer 20 neoprene pads performed better, especially with weaker cylinders, and exhibited only slightly larger variations than the results obtained using sulfur capping. ASTM D 4832 states that capping systems are acceptable when the average strength obtained is not less than 80 percent of the average strength of companion cylinders capped with sulfur capping compound.
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As noted, for different capping methods to be acceptable, the ASTM D 4832 standard requires the obtained compressive strength values to be not less than 80 percent of the corresponding values obtained using sulfur caps. Based on these results, the following recommendations can be made with regard to generating acceptable strength data using unbonded pads: • CLSM with compressive strength lower than 1.0 MPa should be tested using unbonded polyurethane pads (Shore OO 50, equal to Shore A durometer 5)
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• Stiffness gauge (GeoGauge) • RE • Splitting tensile strength Table 3.31 summarizes additional results from the excavatability study, including DCP values, stiffness values (using GeoGauge)
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For the E-series CLSM mixtures, the splitting tensile strength to compressive strength ratio ranged from 9 percent to 17 percent, which is higher than those typically observed for conventional concrete. Unlike concrete, this ratio did not substantially decrease with an increase in compressive strength.
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Additional information on the effect of freeze-thaw damages on water permeability of CLSM samples is provided in the section "Freezing and Thawing." Triaxial Shear Strength Using the same materials and mixture proportions as the water permeability study, the triaxial shear strength of several CLSM mixtures was measured. The results, shown in 47 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 0 0.5 1 1.5 2 2.5 3 3.5 D CP In de x (m m pe r b low )
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fst/fc (%) 10°C, dry 28.8 12.3 6.8 56.1 67.5 8.4 10°C, wet 133.0 7.5 9.8 84.5 22.8 9.2 21°C, dry 25.6 16.6 7.7 64.1 5.3 17.2 21°C, wet 127.8 8.6 8.5 142.8 7.3 15.4 38°C, dry 106.8 9.1 8.0 114.1 16.6 14.6 38°C, wet 198.2 6.0 7.5 125.3 12.5 12.6 f'st = splitting tensile strength f'c = compressive strength "–" = Not enough specimens were available for testing at this age.
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To evaluate the potential influence of resistivity, pH, fly ash type, fine aggregate type, water–cementitious materials ratio (w/cm) , and cement content on the corrosion activity of ductile iron coupons embedded completely in CLSM or sand, the percent mass loss of coupons embedded in thirty different CLSM mixtures (and eight duplicated mixtures)
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0 1 2 3 4 5 All CLSM samples Pe rc en t m as s l os s Mixture 23 Mixture 21 Figure 3.13. Box plot of percent mass loss values.
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As such, the pH of the pore solution alone does not seem to reliably estimate the corrosion performance of ductile iron coupons embedded in CLSM. Statistical analysis of the data indicated that the mean logarithm of percent mass loss values for mixtures containing bottom ash, concrete sand, and foundry sand were statistically not different from each other.
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Phase II, Uncoupled Samples. Figure 3.15 shows the box plot showing the distribution and the median of the percent mass loss data of the 361 galvanized steel and ductile iron coupons embedded in CLSM mixtures exposed to distilled water and chloride solution.
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The effects of different fly ash types and fine aggregate types were more important for samples with ductile iron coupons. Samples that contained a fine aggregate exhibited lower LPML values compared to the samples without fine aggregates regardless of the type of the fine aggregate.
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The significantly higher mean percent mass loss values exhibited by the metallic coupons in the soil section indicate that these coupons were anodes and the coupons in the CLSM section were cathodes. Because the metallic coupons embedded in the soil sections of coupled samples represent the critical anodic areas of pipes for corrosion damage, further statistical analysis was performed on the percent mass loss data of these coupons.
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• The samples exposed to a chloride solution exhibited significantly higher LPML values compared to the samples exposed to the distilled water. • The effect of environment for galvanized steel coupons was larger compared to the ductile iron coupons.
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Service Life of Ductile Iron and Galvanized Steel Coupons Completely Embedded in CLSM ASTM G 1 provides a formula to predict the corrosion rate of metallic samples. By placing the LPML values obtained from the statistical model shown in Equation 3.5 into the formula given in ASTM G 1, a service life model for ductile iron and galvanized steel pipes completely embedded in CLSM can be derived.
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Number of Cycles Figure 3.18. Mass losses vs.
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In general, the results of the freeze-thaw testing indicated that CLSM mixtures can be efficiently tested for freeze-thaw resistance following the modified ASTM D 560 with 12 cycles. Results also indicated that CLSM mixtures with high air content and high compressive strength exhibited good freezethaw resistance.
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test method to increase its accuracy and improve its user-friendliness. • The effects of temperature on strength gain of CLSM mixtures can be very pronounced, especially when using Class C fly ash.
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• The by-product materials tested in this study were found to be non-toxic. However, a testing program was proposed to evaluate other by-product materials that might be more of a concern with regard to leaching and environmental impact.
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