International Concrete Abstracts Portal

The effect of an alkaline set accelerator on the setting of the cement paste without and with a superplasticizer is compared. The results show that the presence of the superplasticizer weakens the effect of the set accelerator. In order to attain the demanded rapid setting a higher content of the accelerator is needed. The early hydration process of the cement is largely changed by the two admixtures. It can be shown that the superplasticizer restrained the formation of the calcium aluminate hydrate phases whereas the accelerator increases the rate of formation of the calcium aluminate phases in the paste. Furthermore, due to superplasticizer-accelerator interaction the morphology of the calcium aluminate hydrate phases is changed. The calcium aluminate hydrate phases precipitate as clusters with a spheroid shape. We state that the cluster formation of the calcium aluminate hydrate phases is the reason for the reduced acceleration of the setting of the superplasticizer/accelerator containing paste.

A test apparatus called "dilatometer" has been developed to predict the dosage of lithium nitrate (LiNO3) required to control ASR expansion as a function of the alkali level and aggregate reactivity. The dilatometer is instrumented so as to monitor within a short period of time the volumetric expansion of the siliceous gel produced by a siliceous aggregate. The rationality of this test procedure was explored from comprehensive laboratory experiments related to the effects of temperature, normality of NaOH test solution, and LiNO3 dosage. Determination of the level of expansion within 30 hours, using this method enables one to predict the dosage of LiNO3, which is now being used as an ASR mitigating agent, needed for a particular aggregate to control expansion due to ASR. Based on the test results, it is anticipated that this test method will be useful for predicting the optimum dosage of LiNO3 required for a particular aggregate type and source.

This paper describes the properties of lightweight aggregate concrete consisting of high performance artificial lightweight aggregate. The concrete was developed in order to be used on a rigid frame prestressed concrete bridge with a length of 256m. The concrete had good pumpability, resistance to freezing and thawing action, and high strength. The maximum equivalent horizontal pumping distance of the concrete was more than 150m with no prewetting of the lightweight aggregate. The compressive strength of the concrete was about 6OMPa and the elastic modulus was more than 21GPa.

A substantial research program has been undertaken at the Australian Centre for Construction Innovation of the University of New South Wales to determine the benefits resulting from the use of permeability reducing admixtures as integral components of concrete required to demonstrate superior durability in aggressive environments. This program used commercial concretes which contained conventional water reducing admixture, different types of supplementary cementitious materials and permeability reducing admixtures at various dose rates. The program included testing of both plastic state and hardened state properties of these concretes to assess the compatability and impact on performance properties. Both concrete and mortar testing have been undertaken in order to determine a range of properties including setting time, strength, drying shrinkage, chloride resistance, and sulphate resistance. Assessment of these tests indicates that these permeability reducing admixtures can positively influence the key performance properties indicative of improved durability.

Polymers with a shorter graft length or longer intervals of graft chains are adsorbed more easily even though the adsorption sites are decreased. The content of carboxyl groups in polymer increased with the decrease in graft chain length. The intervals between graft chains are prolonged, with changed composition of the copolymer where methoxypolyethylene is replaced by styrene or allyl sulfonates. When a larger amount of carboxyl groups was introduced on the main chain or when the distance between the graft chains was extended, the influence of inorganic electrolytes on the fluidity of the suspension with the superplasticizer was decreased. These superplasticizers can be adsorbed on limestone powder even though the adsorption sites were decreased when inorganic electrolytes were added. In contrast to the case of C032-, the addition of S042may possibly cause shrinking of the polymer chain as well as hindering the adsorption of the superplasticizer. Superplasticizers containing graft chains with longer chain length are effective for preventing the fluidity decrease when S042- is added.