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2014SprShotcreteEMag

Following is the approved shotcrete mixture in fact flash-setting was experienced while design that met the QCS 2010 requirements for attempting to shoot out of the truck. C40 strength and durability. To meet the strict To combat the inconsistent slumps of the durability requirements, silica fume was added to mixture and flash setting, only 8 yd3 (7 m3) loads the shotcrete mixture and to provide workability, were ordered, and a concrete triage was set up. the maximum allowable plasticizer was added. When trucks arrived on site, they were either sent The high-range water reducer (HRWR), in addition immediately to a pump (delivered at just the right to providing workability, also retarded the set. slump) or held so the slump could be adjusted. Many times, a small addition of plasticizer would lb/yd3 kg/m3 make a drastic change in slump only to be fol- lowed by a flash set. With as many as four crews Cement 660 390 shooting at one time, the triage required signifi- Micro silica 50 30 cant resources to ensure that the trucks were released as soon as possible. Many trucks, Fly ash 304 180 because of frequent traffic problems or weather 0-1/4 aggregate 2395 1420 delays, were rejected, causing the delivery loop to break down and resulting in long days to meet Water 262 155 the production schedule. To deal with the delays, Glenium 183 (HRWR) 7 4.2 shooting was kept to relatively small areas and Fiber 1.1 0.7 completed in vertical layers, which minimized moving the pumps and made curing easier. To Total 3679 2180 minimize plugs, the entire pump/shotcrete system would be cleaned out as soon as each truck was Acceptance of a concrete mixture design was emptied. Finishers would begin their work almost a lengthy, involved process. as soon as the nozzleman began placing shotcrete • The first step was submittal of a proposed and oftentimes, extra finishers were placed on mixture design along with the concrete supplier each lift to speed up the finishing operation ahead qualification, yearly government approval, and of the quick set caused by sudden loss of the current batching equipment certifications. retarding. • After initial acceptance, a trial mixture was arranged in the presence of the owner’s Equipment inspector, general contractor, and requesting A desert environment is particularly hard on contractor. The trial mixture was batched into equipment. The ultrafine wind-blown sand that a truck. Every 30 minutes, concrete samples, causes the “white-outs” penetrates every pore of along with slump and temperature measure- the equipment, requiring air filters to be changed ment, are taken until 90 minutes elapse. daily. To prevent overheating, water was run over Samples were taken for strength, chloride, and the radiators of pumps and compressors during durability tests. high-temperature months. Sticky concrete • Samples obtained at the trial batch were sent required that extra time needed to be spent for laboratory testing. It took over 56 days to cleaning and maintaining equipment. Although receive all the results. pumps were rotated and the maintenance program Due to the extremely hot weather conditions was stepped up, pumps would still break down and the silica fume in the mixture, a maximum almost every other day due to the sand, heat, and dosage of HRWR was required by the supplier sticky concrete. The number of spare parts ordered to attain workability. With the significant tem- more than tripled our normal experience in the perature changes throughout the day, the concrete United States. supplier needed to vary the amount of HRWR, which led to constant slump inconsistencies at Curing the time of delivery. Poor equipment mainte- The availability of a continuous water supply nance, driver inexperience, and delivery delays impacted both curing and cleanup. Water was all ensured that no two trucks arrived with the delivered by truck and stored in tanks on site. same slump. To compound inconsistent deliv- Truck delivery of water, however, was inconsistent. eries, the inspectors enforced a rule that “not one In addition, electrical power for the water pumps drop of water can be added on-site.” This rule was also intermittent. This combination of factors was required because it was not possible to ensure often meant that there were times the crew ran out that the water carried onboard the trucks was of water while shooting, making cleanup a challenge. clean and free of chlorides. At times, adding Curing was accomplished using a variety of additional HRWR to the mixture on site led to a methods: evaporation retarder, plastic sheets, and false sense that the mixture was acceptable, when soaked burlap. 10 Shotcrete • Spring 2014


2014SprShotcreteEMag
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