Sand + crushed coarse aggregate ( b ) Sand + gravel coarse aggregate
Self-consolidating concrete (SCC) mixtures designated for precast, prestressed applications should be highly workable to flow easily through restricted spacing and completely encapsulate reinforcements without any mechanical vibration. Key workability characteristics of SCC can be described in terms of filling ability, passing ability, and resistance to segregation. These properties are typically characterized by data that relate to specific testing methods. In general, these methods include the components required for evaluating simultaneously filling ability, passing ability, and resistance to segregation, since these properties are rather interrelated. In this investigation, 33 SCC mixtures made with various mixture proportioning parameters, including maximum size and type of aggregate, type and content of binder, and w/cm were evaluated. The mixtures were prepared using crushed aggregate and gravel of three different nominal sizes, w/cm of 0.33 and 0.38, and three binder compositions: Type MS cement, Type HE cement with 30% of slag replacement, and Type HE cement with 20% of Class F fly ash. Comparisons and correlations among various test results used in evaluating the workability responses obtained for these mixtures are established. This is done to highlight advantages and limitations of the various test methods that can be used to assess workability of SCC designated for prestressed applications. Appropriate combinations of test methods that can be used to assess workability of SCC at the precast plant are recommended, and ranges of acceptance of the various test methods are established.
Performance-Based Specifications of Self-Consolidating Concrete Used in Structural Applications
Proper selection of test methods and workability specifications are key concerns in the optimization and control testing of self-consolidating concrete (SCC). An experimental program was carried out to evaluate the suitability of various test methods for workability assessment and to propose performance specifications of such concrete used in structural applications. Various workability characteristics were determined for approximately 70 SCC mixtures made with water-cementitious material ratios (w/cm) of 0.35 and 0.42. Workability responses included the slump flow, J-Ring, V-funnel flow time, L-box, filling capacity, and surface settlement tests. Comparisons of various test methods indicate that the L-box blocking ratio (h 2 /h 1 ) and the J-Ring flow diameter can be related to filling capacity values determined using the caisson test. It is recommended that SCC used in structural applications should have slump flow values of 620 to 720 mm. To ensure proper filling capacity greater than 80%, such concrete should have high passing ability that corresponds to L-box blocking ratio (h 2 /h 1 ) ≥ 2 0.7, J-Ring flow of 600 to 700 mm, slump flow minus J-Ring flow diameter ≤ 50 mm, or V-funnel flow time ≤ 8 seconds. Such SCC should have a settlement rate of 0.16%/h at 30 minutes, corresponding to 0.5% maximum settlement.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.