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FEATURE TRB MEETING certain supplementary cementitious materials (SCMs), the authors affirm, adding, "Time of setting and strength development benefits are reported, generally in proportion to limestone fineness. It appears possible to fully develop potential for these performance synergies in mill-ground PLCs, in which limestone comprises the majority of the finest particles." For this research, performance trends observed in concrete with PLC were investigated using separately proportioned, commercially ground limestone and ordinary portland cement, as well as cement mill-ground PLC samples. Influences of variables such as SCM type and limestone fineness were evaluated using laboratory paste mixtures. "Set acceleration increased with limestone fineness for all combinations, including mixtures without SCMs," say Cost, Howard and Shannon. "Strength improvements were clearly evident with all SCMs, more significantly with Class C ash and slag cement than with Class F ash. All strength trends improved as limestone fineness was increased." Consistently enhanced setting and strength performance appear achievable with PLCs, they say, adding, "Optimizing particle fineness will be a key factor in achieving these benefits. Performance contributions of SCMs in combinations with PLCs may exceed those of similar mixtures with traditional portland cements, thus SCM use can be maximized and related sustainability impacts further extended." The authors conclude: • Portland-limestone cements produced at up to 15 percent limestone have the potential to significantly improve concrete sustainability, with performance at least equal to—and often superior to that—of conventional cements. • Properly optimized, portland-limestone cements clearly hydrate with synergies contributed by limestone that enable enhanced setting and strength performance, especially in combination with SCMs. • The extent of PLC performance benefits relates to limestone fineness; clinker fineness approaching that of conventional cements must generally be maintained in the composite PLC. • The particle size distribution of PLC produced to optimum overall fineness in finish grinding ball mills appears well suited for synergy-driven performance enhancement. WWW.CONCRETEPRODUCTS.COM • SCM use in concrete with PLC, to the maximum levels that are allowed with conventional cement, must be allowed and encouraged in order for the greatest possible sustainability and performance benefits of PLC to be achieved. • Higher-than-traditional replacement rates with some SCMs appear possible without loss of performance, further extending the sustainability benefits of PLCs. SCC SUITS DRILLED SHAFTS Tests show high-slump, self-consolidating concrete (SCC) is ideal for special foundations, note Celik Ozyildirim, Ph.D., P.E., and Stephen R. Sharp, Ph.D., P.E., Virginia Center for Transportation Innovation and Research, in their paper, Evaluation of Drilled Shafts with SelfConsolidating Concrete. In Virginia SCC-in-drilled shafts study, wedges hold transducers utilizing an acoustic pulse to measure concrete thickness. "A drilled shaft is a deep foundation that is constructed by placing fresh concrete in a drilled hole," the authors state. "These massive shafts have diameters ranging from 18 in. to 12 ft. or more and contain a high concentration of reinforcement bars. Drilled shafts offer economic advantages because pile caps are not necessary, and the shafts tie directly into columns. A typical shaft is easily adaptable to varying site conditions and can carry a high load." Drilled shaft excavation requires either cased or uncased methods, they say. If soils are not prone to caving, as with stiff clay, shale or limestone, a casing is not required. "When concrete is placed under water or slurry, a tremie or a pump line is used," they say. "The discharge end is placed near the bottom of the hole and the concrete flow is started. The concrete fills the shaft from the bottom and dis- places the sediments. The placement continues until fresh concrete overflows the top of the shaft." Generally, conventional concretes with high slump values indicative of high workability are placed, which can lead to bleeding and segregation. Drilled shafts are tested to make sure proper placement has been accomplished, the concrete uniform, and structural integrity maintained. "For the integrity testing, sonic tests are common," Ozyildirim and Sharp state. "In the sonic-echo test, the top of the drilled shaft is struck with a handheld hammer. A sonic wave is generated that travels down the drilled shaft. The wave is reflected from the bottom of the shaft or from a defect within the shaft and is picked up by a transducer at the head of the shaft." Consolidation of deep shafts is not practical, and the presence of large air pockets because of the lack of consolidation has been a concern, they write. Placement-related defects in the shafts are common because of lack of consolidation. But SCC provides very high workability, indicated by high slump flow values, compared to conventional concrete. "SCC has been used in Japan and Europe advantageously since the early 1990s," the authors note. "SCC easily fills congested spaces between the reinforcement and the formwork under the influence of its own mass, and without additional consolidation energy. Easy flowing SCC would permit convenient and fast placement of concrete in drilled shafts. Eliminating the consolidation problem would enhance the strength and reduce the permeability of concretes." For this study, two adjacent bridges on Route 28 over Broad Run in Bristow, in Prince William County, Va., were selected. The bridge carrying the northbound traffic had 24 drilled shafts using conventional concrete with high slump values, and the bridge carrying the southbound traffic also had 24 shafts, 12 with conventional concrete, and 12 with SCC. During placement, properties of the fresh concrete were tested and specimens were prepared for the determination of the hardened properties. Comparisons of concrete properties and placement operations between the SCC and conventional concretes were made. The integrity of the shafts within the reinforcing cage was determined using cross-hole sonic logging (CSL). MARCH 2013 | 33