WORKABILITY RETENTION OF HIGH STRENGTH SUPERPLASTICIZED CONCRETE/

One of the methods of producing high strength concrete (HSC) is by red ucing the water/cement (w/c) ratio to achieve a compressive cylinder s trength of over 41 N/mm(2) as defined by the American Concrete Institu te or over 60 N/mm(2) as stipulated by FIP/CEB. The consequent low wor kability is then overcome by adding super-plasticizer to achieve a spe cified workability, preferably without segregation, set retardation or excessive air entrainment. For a given workability, the lower the w/c ratio, the higher is the superplasticizer dosage requirement. However , there is a practical upper limit on the dosage of superplasticizer b ecause of its cost, the reduced plasticizing efficiency above an optim um dosage level, and the risk of loss of strength caused by segregatio n and heavy air entrainment, as well as possible retardation in settin g and hardening. For superplasticized concrete, the rapid loss of work ability presents a major technical problem for in situ construction us ing conventional pumping and casting methods. The presently adopted re peated dosing method imposes manpower, quality control and cost penalt ies as well as the risk of superplasticizer over-dosing. This paper st udies the slump loss behaviour of HSC incorporating a variety of super plasticizers. To retain the workability of HSC, a recently developed a nd patented admixture, carrier fluidifying agent (CFA) was used in a s eries of concrete mixes. It was found that CFA maintained the workabil ity of HSC mixes for a period of up to 2 hours. The effects of CFA on the setting, hardening, air content and compressive strength developme nt of the concrete were also studied and no adverse effect was observe d. Laboratory trials using a repeated dosing method were also carried out, and apart from being labour intensive, this method was found to b e less effective and more expensive.