EFFECT OF ANTIWASHOUT ADMIXTURES ON FRESH CONCRETE PROPERTIES

Most antiwashout admixtures are water-soluble polymers that modify the rheological properties of fresh concrete. Such admixtures have been i ncorporated into concrete intended for underwater placements and repai rs, and implemented in production of extremely workable and flowing co ncrete. They have also been used to enhance resistance to sagging of s hotcrete and produce bleed-free cement grouts for filling post-tension ing ducts. This paper presents results from a study aimed at better un derstanding of the effects of antiwashout admixtures on concrete prope rties. It will highlight benefits and limitations of employing such ad mixtures in concrete. Fresh properties of low-, medium-, and high-stre ngth concretes made using two types of antiwashout admixtures, a micro bial polysaccharide and cellulose derivative, were evaluated. Fresh pr operties are compared with those of similar concretes made without ant iwashout admixtures. Among the parameters evaluated are fluidity, blee ding, water dilution, segregation, setting time, and air content. Test results show that incorporation of an antiwashout admixture can great ly reduce external bleeding and significantly enhance resistance of co ncrete to water dilution and segregation. However, there is a signific ant increase in water demand, and a high-range water-reducing admixtur e (HRWRA) is needed to maintain a desired level of fluidity without ex cess addition of water. The combined additions of an antiwashout admix ture and HRWRA delay setting time, especially at high concentrations o f HRWRA. Viscous concretes containing antiwashout admixtures show a gr eater demand for air-entraining admixture. However, once enough air is entrained, proper air-void parameters needed to insure good freeze-th aw resistance can be obtained.