Controlled retempering : a mix design method

Abstract

One of the major problems in concrete construction is maintaining the required workability of a mix. This problem becomes evident in ready mixed concrete which may have to be kept in the mixer for a long time due to delays in casting. Typical examples being, long transportation distances or heavy traffic situations, which may be further aggravated by the adverse conditions of hot weather. The most common remedy to this situation is Retempering, which consists of the delayed adding of water to a mix until the required workability is regained. Retempering will increase the water/cement ratio of the mix and will therefore reduce the concrete strength. Although there are recommended limits on retempering and mixing time, this practice occurs regularly and without control, leading to uncertainties in the concrete strength. However, past research has shown that it is possible to estimate this loss in strength when the volume of retempering water is known. The aim of this dissertation is to analyse past research in an attempt to provide a mix design method for mixes which are to experience prolonged agitation before placing. The mix design method developed involves controlled retempering. In practice this would mean that just before placing, an established amount of retempering water is added to a specifically designed concrete mix. In this way one may ensure the required workability while maintaining control over the concrete ingredients. In the following sections of this dissertation the Retempered Mix Design Method shall be described. Experimental observations, used to develop the method, have been recorded and analysed. This is followed by experimental confirmation of the validity of the mix design method.