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Technical Tip (a) (b) (c) Fig. 1: (a) Compressive strength of mortar cubes; (b) cylinder coring; and (c) flexural strength testing of shotcrete beam with fibers erators. Also, high early strengths require the use (a) of large proportions of portland cement in the concrete mixture, which can lead to high heat evolution, excessive shrinkage of the material, and cracking. The cost also increases substantially with increasing cement content. Alternatives to portland cement are also capable of rapid strength development. These include cal- (b) cium sulfate hemihydrate, and calcium sulfoalumi- nate (CSA) cements. Unlike portland cement, these rapid-setting cements can gain 75 to 80% of their strength within 1 day, which means less cement can be used in the mixture to achieve comparable early strength. CSA cement and calcium sulfate hemihy- Fig. 2: (a) Tekcrete Fast bonding to OPC; and drates can also be fabricated, for the most part, from (b) showing no visible laminations after curing coal combustion by-products (CCBs). These CCBs include fluidized bed combustion spent bed mate- In addition, the high w/c of CSA cement shot- rials and forced air oxidation flue gas desulfurization crete, coupled with the low heat of hydration of by-products—that is, synthetic gypsum, which plaster cement, offers a capacity to manipulate the potentially represents both a cost advantage, as well heat of reaction of these materials within a wide as an environmental advantage. band of strength and set parameters. Heat generation is critical in the rapid placement of masses of highly Development and Testing of reactive cementitious materials. These cements also offer the potential of lower overall costs. CSA-Based Shotcrete Once the CSA-based materials to be used in A primary consideration of this project was the the shotcrete were developed and tested, they were rate of strength development (compressive and used to fabricate shotcrete mortars and concretes. tensile), bonding strength to the damaged surfaces, After an initial round of screening, specimens and short-term dimensional stability. Other con- prepared from selected mixtures were tested for siderations include heat generation, pumpability, strength and dimensional stability. When deter- ease of use, stiffness of the set material, and cost. mining what tests to use to evaluate the chosen CSA cements are of interest mainly because mixtures, it was important to keep in mind that they gain strength very rapidly. They also require the sprayed-concrete material must provide struc- lower energy to produce, with significantly lower tural strength within an hour, and bond sufficiently CO emissions than portland cement. CSA-based to any substrate or surface under any conditions 2 shotcrete materials can be formulated so that they long enough to provide the necessary assistance have lower cement content than portland-based to first responders. Therefore, in addition to the shotcrete, a higher water-cement ratio (w/c), low standard cement/concrete testing—that is, com- viscosity, and yet still achieve very high early pression and stability testing of ASTM standard strength. This is due to the nature of its principal cubes, cylinders, bars, and cores; flexural strength cementitious hydration product: ettringite. These beam testing; tensile testing; rapid freezing and properties are difficult to achieve with portland- thawing testing; resistance to carbonation testing, cement-based rapid-setting materials. and so on, as seen in Fig. 1 and 2—the variations Shotcrete • Fall 2015 39


2015FallShotcreteEMag
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