Rasiah Sriravindrarajah, School of Civil & Environmental Engineering, University of Technology, Sydney and P. Whitson, Project Engineer at Leighton Contractors discuss the results of an experimental investigation of the self-curing admixture in a flowing fly ash concrete. They conclude that a self-curing admixture is a useful ingredient in concrete mixes when water curing procedure is difficult to apply and should not be considered as alternative to water curing.
Progress in the hydration of Portland cement directly influences the development of engineering properties of structure concrete, namely strengths, modulus and impermeability. It is known that the moisture content and temperature are the two important factors affecting the cement hydration. For sufficient hydration to occur, the humidity in the pores needs to be maintained above 80%. When the ambient temperature is favourable to cement hydration, it is important to minimize the loss of moisture from concrete. Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration.
Water curing of concrete is universally accepted as the most effective and simple curing method to maintain the progress of cement hydration. This is being achieved by external water-adding techniques such as ponding, water spray and wet hessian. Concrete specification recommends a minimum moist curing period depending up on the strength, durability and ambient conditions. Modern concrete mixes containing a combination of cement and one or more of the slow hydrating pozzolanic materials require an extended curing period.
Advancement in self-curing concrete has emerged in recent time as a potential alternative to traditional water curing method. Self-curing is considered as an “internal curing system” where a water soluble polymer (self-curing admixture) is added to the concrete mix. The self-curing admixture performs similar to good quality curing membrane. This has been used in tunnel linings and underground mines to provide at least partial curing when traditional methods are difficult or even impossible to employ. Other benefits of using self-curing agents are:
- Reduction in self-desiccation
- Improvements in cement hydration
- Improved resistance to water transport
General Purpose cement (Type GP) conforming to AS3972 and low calcium fly ash conforming to AS3582 were used in the concrete mixes. Crushed river gravel, having the maximum aggregate sizes of 20mm and 10mm in equal weight proportion was used as coarse aggregate. Coarse river and fine sands in equal weight proportion were used as fine aggregate. A new generation superplasticiser (polycarboxylic ether hyperplasticiser) and a water soluble polymeric glycol self-curing admixture with a solids content of 64% were used.
The water to binder ratio was 0.36 and the superplasticiser dosage was 0.52% of the binder content. The dosage of self-curing admixture was 5 litres per m3 of concrete (or 1.03% of the binder content) for self-curing concrete mix. To minimize batch to batch variation between the control concrete and the self-curing concrete, control mix was first batched in a pan-type of mixer and a half of this mix was added with self-curing admixture to produce the self-curing concrete. Fresh concrete were tested for slump flow according to BS EN 12350-8 for the compressive strength at 3, 7, 28 and 56 days and for in-direct tensile strength and static modulus of elasticity at 28 days, in accordance with in AS1012.
Compressive Strength of concrete
The compressive strength for both control and selfcuring concretes increased with age at a decreasing rate under both curing conditions. The strength improvement beyond the age of 28 days was not significant for both types of concrete.
As expected, air-stored concrete showed noticeably lower strength than the water-cured concrete, at all tested ages. The compressive strength of self-cured concrete was marginally higher than that for water-cured concrete. However, significant improvement in compressive strength for self-cured concrete was recorded under air-stored condition, compared to the water-cured concrete.
Under water-curing condition, the compressive strength for self-curing concrete improved by 14% at 3 days and 4.3% at 56 days over the corresponding compressive strengths for control concrete.
The results showed that for the control concrete, the air-cured strength at 28 days was 35% lower than that the water-cured strength. However, for the self-curing concrete, the air-cured strength at 28 days was 28% lower than the water-cured strength. At the age of 56 days, the corresponding reductions in compressive strength due to air-curing were 34% and 26%, for the control and self-curing concretes, respectively. The use of self-curing admixture in concrete mix had shown to have some advantage in improving the strength development in concrete in the absence of water-curing.
Tensile strength of concrete
Similar to the compressive strength, the tensile strength of water-stored self-cured concrete is more than that for the water-cured control concrete. The relative reductions in the tensile strength were 17.6% and 12.4% for the self-cured and control concretes, respectively. From the limited results, the effectiveness of the self-curing admixture in improving the tensile strength of self-cured concrete under air storage condition is not observed.
Drying shrinkage of concrete
The drop in 66-day shrinkage for self-cured concrete was only 3% lower than that for the control concrete. When no water curing was provided prior to drying of the shrinkage specimens, the 66-day drying shrinkage was 414 microstrain and 381 microstrain for the control and self-cured concretes, respectively. The lowest shrinkage for the self-cured concrete is probably due to increased water retention in concrete due to the presence of self-curing admixture and reduced water loss.
The research was carried out to evaluate the effectiveness of a commercially available self-curing admixture on the performance of flowing fly ash concrete. The self-cured concrete stored in an unsaturated condition showed noticeable improvement in compressive strength and modulus of elasticity compared to that for the control concrete.
The lowest drying shrinkage for self-cured concrete was most probably due to the improved water-retention in concrete. It is concluded that the self-curing admixture has limited capability to improve the compressive strength of fly ash concrete subjected to unsaturated curing condition. Hence it could be specified in concrete mixes containing fly ash when it is not possible to provide saturated curing condition, even for a limited period of time.