Mixture Proportioning for Internal Curing

TLDR

Internal curing supplies additional water from lightweight aggregate to hydrate cement, and a prior equation predicts the required LWA amount for any concrete mix. This paper refines the equation’s parameters to give a practical method for selecting the correct LWA quantity and improving mixture proportioning. The method requires determining cement clinker composition, calculating chemical shrinkage at 25 °C (adjusted for temperature), measuring LWA desorption to relative humidity, and inserting these values into the equation to compute the needed LWA mass.

Abstract

Internal curing is the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water. An equation has been developed in a previous study for calculating how much lightweight aggregate (LWA) is needed to supply water for internal curing of any given concrete mixture. This paper presents refinements for estimating the parameters of this equation that will provide a readily recognized means of choosing the proper amount of LWA and improving mixture proportioning. The two major factors to be considered are the variation of chemical shrinkage of cement with Portland cement phase composition and curing temperature, and the relevant value for the absorption of the LWA. In order to determine the amount of LWA needed, it is recommended that the mass composition of the cement clinker be obtained from either a detailed scanning electron microscope/X-ray image analysis or the Bogue calculation. Then, the expected chemical shrinkage of the cement at 25 deg C should be calculated. If the expected average curing temperature is above 25 deg C, the calculated value should be decreased by 0.0005 per deg C above 25 deg C. If the expected average curing temperature is below 25 deg C, the calculated value should be increased by 0.0005 per deg C below 25 deg C. The desorption of the LWA from a saturated state down to a relative humidity of relevance for the internal curing of concrete should then be measured. Finally, the determined values for chemical shrinkage and absorption of LWA should be substituted in the original equation to obtain the desired mass of lightweight fine aggregate in the concrete mixture.