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thawing conditions because it isn’t air entrained. Is this true? Misconception: Dry-mix doesn’t have good freezing-and-thawing durability. Fact: Dry-mix has decades of good performance in freezing-and-thawing environments, and should not be precluded from use in those exposures. Air entraining is just one aspect contributing to freezing-and-thawing resistance. Good air void spacing in the hardened concrete is the key to good performance of entrained air in concrete. Strength and to some extent permeability also affects performance. Because shotcrete generally has a lower w/cm than conventional form-and-place work, we experience faster strength gain and achieve higher strength over time. Shotcrete also often uses silica fume, fly ash, and other supplementary cementitious materials (SCMs) that increase strength and reduce permeability. Finally, air entraining admixtures are available for use in dry-mix. Inquiry: I have heard for the best performance of shotcrete, you should avoid finishing and leave as a gun finish. Is this true? Misconception: Shotcrete should not be finished because it reduces strength, serviceability, or durability of the concrete. Fact: Shotcrete can be finished in a wide variety of ways, and has little if any detrimental effect on the strength and durability. However, proper finishing techniques should be used. Cutting and finishing (floating or brooming) by experienced finishers will help to produce sections with a consistent surface and section thickness. However, overfinishing or wetting the surface of hardening concrete (in shotcrete or cast concrete) can introduce microcracks in the surface layer. Also, requiring a smooth steel trowel finish will by its inherent nature bring extra paste and water to the surface, increasing the effective w/cm and thus reducing strength of that surface layer. However, this is the result for any cast concrete, and not limited to only shotcrete. Inquiry: I have been told that I shouldn’t use shotcrete because it will have more shrinkage cracks than my cast concrete walls. Misconception: Shotcrete will have greater shrinkage cracking than form-and-place walls. Fact: Early-age plastic shrinkage and long-term drying shrinkage are aspects of all concrete work. Plastic shrinkage cracking results from early, quick evaporation of water from the surface of the plastic, hardening concrete. With shotcrete placement, we will have our finished surface exposed to the air. Low humidity and hot or windy conditions will substantially increase the rate of evaporation. Good shotcrete contractors will evaluate appropriate methods to keep the surface damp and minimize or eliminate plastic shrinkage cracks. Long-term drying shrinkage is related to the paste content, amount and size of aggregate, and the w/ cm. Shrinkage cracking is also related to the ability of the concrete to carry tension. The designer of the concrete structure also has a responsibility to design adequate movement joints to accommodate concrete shrinkage. Shotcrete tends to have a relatively high paste content so may have a slightly higher shrinkage potential. Conversely, shotcrete has a lower w/cm (0.30 to 0.42) as compared to most form-and-place (0.40 to 0.50), so would tend to have a lower shrinkage potential. Also, shotcrete tends to have earlier strength gain, and higher 28-day strengths (both compressive and tensile) than most form-and-place concrete. This reduces the shrinkage potentially causing cracks. Thus, considering the plusses and minuses, shotcrete may balance the shrinkage potential of form-andplace. More importantly, proper attention by the shotcrete contractor to the installation, through early, wet curing and keeping curing in place for at least 7 days will significantly help reduce the potential for cracking. Also, shotcrete mixtures can use shrinkage-reducing admixtures that will help limit drying shrinkage through the critical first year after placement. Charles Hanskat is the current ASA Executive Director. He received his BS and MS in civil engineering from the University of Florida, Gainesville, FL. Hanskat is a licensed professional engineer in several states. He has been involved in the design, construction, and evaluation of environmental concrete and shotcrete structures for over 35 years. Hanskat is also a member of ACI  Committees 301, Specifications for Structural Concrete; 350, Environmental Engineering Concrete Structures; 371, Elevated Tanks with Concrete Pedestals; 372, Tanks Wrapped with Wire or Strand; 376, Concrete Structures for Refrigerated Liquefied Gas Containment; 506, Shotcreting; and Joint ACI-ASCE Committee 334, Concrete Shell Design and Construction. Hanskat’s service to the American Society of Civil Engineers (ASCE), the National Society of Professional Engineers (NSPE), and the Florida Engineering Society (FES) in over 50 committee and officer positions at the national, state, and local levels was highlighted when he served as State President of FES and then as National Director of NSPE. He served as a District Director of Tau Beta Pi from 1977 to 2002. He is a Fellow of ACI, ASCE, and FES and a member of ACI, NSPE, ASTM International, and ASCC. Shotcrete • Fall 2016 15


2016FallShotcreteEMag
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