A new approach for restoring the slump of concrete mixes using admixtures was examined. A number of admixtures (i.e. a water reducer, a water reducer and retarder, a superplasticizer and a self compacting admixture) were individually added to OPC, 10% silica fume and 30% fly ash concrete batches after 1 or 2 hours from being originally mixed. The original mixes did not contain any admixtures. As a result of this process a slump increase usually occurs, but then disappears within an hour giving enough time for casting the concrete. The compressive strength, initial surface absorption and porosity of the concrete treated in this manner were evaluated and compared to the control concrete, which did not have any slump restoration, or to concrete in which water was used to restore its slump. It was found that the use of water to restore slump had an adverse effect on the properties of concrete especially the mixes with pozzolanic materials. The application of the new approach (i.e. slump restoration by admixtures) did not adversely affect, or in some cases improved the properties of the different concrete mixes. The admixture, which gave the best results in that respect, was the self compacting admixture. The recommendation to the ready mixed concrete industry is to add this admixture to the concrete on arrival at the site in order to restore the lost slump. |
