Workability retention of high strength/superplasticized concrete

One of the methods of producing high strength concrete (HSC) is by reducing the water/cement (w/c) ratio to achieve a compressive cylinder strength of over 41 N/mm² as defined by the American Concrete Institute or over 60 N/mm² as stipulated by FIP/CEB. The consequent low workability is then overcome by adding superplasticizer to achieve a specified 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 because of its cost, the reduced plasticizing efficiency above an optimum dosage level, and the risk of loss of strength caused by segregation and heavy air entrainment, as well as possible retardation in setting and hardening. For superplasticized concrete, the rapid loss of workability presents a major technical problem for in situ construction using conventional pumping and casting methods. The present1y adopted repeated dosing method imposes manpower, quality control and cost penalties as well as the risk of superplasticizer over-dosing. This paper studies the slump loss behaviour of HSC incorporating a variety of superplasticizers. To retain the workability of HSC, a recently developed and patented admixture, carrier fluidifying agent (CFA) was used in a series of concrete mixes. It was found that CFA maintained the workability of HSC mixes for a period of up to 2 hours. The effects of CFA on the setting, hardening, air content and compressive strength development of the concrete were also studied and no adverse effect was observed. Laboratory trials using a repeated dosing method were also carried out, and apart from being labour intensive, this method was found to be less effective and more expensive.