Concrete pressure on formwork: influence of cement combinations and superplasticising admixtures

Abstract

This paper describes a study carried out to examine the effect of cement combinations and new superplasticising admixtures in concrete on lateral pressure development in column and wall formwork. The additions, used in structural grade concretes, included ground granulated blastfurnace slag (ggbs), fly ash (FA), silica fume (SF) and metakaolin (MK). Two superplasticising admixtures were also tested to examine water reduction/variable workability effects on concrete pressure. In addition to the constituent materials, the influence of rate of rise of concrete in formwork was also investigated. The tests involved instrumenting column and wall formwork with concrete and pore water pressure measuring devices (over their height) and recording pressure development, and the head of concrete rising vertically in the element during construction. Tests were mainly carried out under controlled conditions (20 elements of 3·0 and 8·0 m height) and supplemented with measurements at construction sites where the materials of interest were being used (12 elements). The results indicate that concretes containing ggbs and FA gave slightly higher concrete pressures than that of Portland cement (PC), while the reverse was found for SF and MK concretes. The effect of superplasticiser on concrete pressure was influenced by how the admixture was used. For reduced water content/equal workability (slump) concrete, slightly lower pressures on formwork were generally obtained compared with PC, while the reverse was noted when they were used in reduced water content/high workability concrete. Similar influences on concrete pressure were found for these at slow and fast rates of rise. In general, the trends observed in the control and site test data gave agreement. The practical implications of the work were examined with respect to the guidance given in Construction Industry Research and Information Association (CIRIA) Report 108 used for evaluating concrete pressure on formwork. It is suggested that the new materials can fit within the framework of this method with minor revisions.