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2016SummerShotcreteEMag

Goin’ Underground from time-temperature histories. Out of the various available functions, the Arrhenius equation-based maturity function is the most widely accepted. This relationship between rate of hydration, temperature and degree of hydration was first demonstrated to be appropriate for concrete in works of Freiesleben Hansen and Pedersen (1977) and is formulated as shown in Equation (1), where Ã(ξ) is normalised affinity (s–1), Ea is activation energy (J.mol–1), R is the ideal gas constant (= 8.314 J.mol–1.K–1), and T is absolute temperature (K). This function is useful while the activation energy is not varying. For cement hydration, it is applicable while the reaction is propelled through exothermic heat and is not diffusion based. This means this relationship is best applied in the ranges of 0.05 < ξ < 0.5 (Kada-Benameur et al. 2000). The activation energy and normalised affinity are dependent on the cement type, the chemical admixtures and the supplementary cementitious materials. Therefore, they must be determined for each shotcrete mixture used on site. d dt à Ea RT = − ( ) exp (1) Early Age Strength Determination for Shotcrete Currently accepted early age strength tests include needle penetrometer and stud driving and are conducted on site as described in BS EN 14488-2 (2006). At very early ages, these tests cannot be directly performed on the lining due to the danger of freshly sprayed shotcrete falling down. For this reason, shotcrete panels are used for these tests and are sprayed immediately after the lining has been sprayed. Assuming that the shotcrete for both the lining and the panels is placed in identical conditions, the lining strength development may be assessed. This indirect assessment approach, though widely accepted, does not present a complete picture since the panel and the lining may have a very different temperature history due to the different size, time of spraying and environmental conditions. New Testing Approach The proposed approach is based on developing temperature histories for the shotcrete lining using on-site thermal imaging. These histories can be applied to the maturity function, as shown in Equation (1), and a stepwise calculation can help determine degree of hydration and, in turn, the Fig. 1: Representation of linear relationship between concrete compressive strength and degree of hydration Fig. 2: Representation of change in rate of hydration versus degree of hydration development compressive strength development. Currently, this patented approach is under further development at the University of Warwick and is being referred to as Strength Monitoring Using Thermal Imaging™ (SMUTI). Jones and Li (2013) and Jones et al. (2014) discuss various aspects of this approach in detail. Before using Equation (1), input parameters, such as Ã(ξ) and Ea, are needed. Since these parameters are unique to a concrete mixture, they need to be re-evaluated if any major change is made, through lab testing such as isothermal calorimetry. Similarly, the linear relationship between fc and ξ is also unique to a given mixture and must be determined independently for each mixture type. Due to the method of application, it is not realistic to conduct Shotcrete • Summer 2016 49


2016SummerShotcreteEMag
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