In this study, an analysis of the region of the paste-aggregate interface is carried out by means of scanning electron microscopy. A microanalysis of the Ca/Si ratio in and outside this region is also performed. The images suggest that the presence of silic;r fume produces a more homogeneous interface and outlines the synergetic effect of silica fume and slag, decreasing the CH quantity in concrete or changing the crystals morphology. This influences the development of concrete strength, since the absence of the transition zone compressing the CH crystals facilitates the formation of a better connection of the paste-aggregate system.

High performance concrete is generally specified to meet special requirements such as higher compressive strength, lower permeability, higher resistance to aggressive environments and longer durability. The design of structures must be based on the knowledge of all concrete properties and the determination of creep values of paramount importance in several cases. This paper presents the influence of water-cement ratio and level of hydration for concretes with compressive strengths ranging from 20 MPa to 75 MPa. Creep of four mixtures with water-cement ratios of 0.29, 0.37, 0.52 and 0.75 with 6% of silica fume and a fixed slump was determined. Specimens were loaded at ages 3, 7, 28 and 90 days and maintained with a constant load for 90 days. Concrete testing included creep, compressive strength, modulus of elasticity, autogenous deformation and drying shrinkage. The paper presents creep coefficients, autogenous volume changes, drying shrinkage and their correlation with age and water-cement ratio. Test results showed that high performance concrete presents lesser creep if compared with concretes with lower compressive strength and that differences between specific creep values range from 12% to 43%. High performance concrete presented significantly higher values of autogenous volume changes. Tests confirmed that drying shrinkage is directly related to the water content of the mixture, whereas similar values were obtained from tests performed on several specimens representing different mixtures with various compressive strengths but containing approximately the same amount of water.

A model to explain the compressive strength of high-performance concrete was developed using multiple linear regression correlating the compressive strength and variables representing volume ratios of paste materials. The data used to develop and validate the model came from a wide range of binary and ternary silica fume, rice husk ash and fly ash concretes that were made by Isaia with the same set of materials. In such tests, the water-cementitious materials ratio by mass varied from about 0,30 to about 0,50, with rates of cement replacement by pozzolan by mass from 0 to 50 %. The 3, 7, 28, and 91 days compressive strengths of 39 different concrete mixtures were measured. High correlation indices were found in most of studied regressions and the accuracy of the predicted values of compressive strength can be considered very good, specially when obtained from a set of concretes made with the addition of just one kind of pozzolan. In this case, the mean difference the addition of just one kind of pozzolan. In this case, the mean difference between the experimental and the predicted values were 0.4 MPa at the age of 28 days.

The excellent performance of a large industrial floor, built in Buenos Aires with a Shrinkage-Compensating Concrete, made with Ordinary Portland Cement and an expansive agent, created an interest in applying this solution in other parts of Argentina. To establish whether the solution was also applicable elsewhere, it was required to investigate the effect certain relevant local conditions might have on the behaviour of the Shrinkage-Compensating Concrete. For that purpose, an experimental programme was conducted, aimed at studying the effect variables such as cement type, expansive agent dosage, temperature of the concrete and curing conditions have on several relevant properties of the Shrinkage-Compensating Concrete. The investigated properties were: restrained expansion and shrinkage, compressive strength and coefficient of air-permeability. The experimental results showed that the combination "blended cements + expansive agent" presents advantages compared to the combination "OPC + expansive agent". The results of the investigation were put into practical use for the construction of a large industrial floor in the City of Rosario, Argentina, where a Shrinkage-Compensating Concrete made with Portland-Slag Cement and an expansive agent was used with excellent technical results and with economic benefits too.

The characteristics of cement paste are very important for the workability, the mechanical properties and the durability of concrete, particularly in High Performance Concrete (HPC). Several problems of concrete such as slump loss, retardation in setting time or excessive heat of hydration, are directly related with some characteristics of the cement paste. This paper presents the results of a study made on cement pastes to select cementitious materials and chemical admixtures, and adjust mixture proportions before the concrete mixture design. Changes in the fluidity and the behavior of different super-plasticizers combined with cementitious materials including cements of different fineness, and different types or contents of mineral additions (calcareous fillers, natural pozzolans or silica fume) were comparatively studied by using the Marsh cone. It was found that the saturation point (defined as the dosage of superplasticizer over which the flow is not substantially modified) may change significantly with the type of superplasticizer, the fineness of the cementitious material and the incorporation of mineral additions (especially silica fume). Moreover, a quick and easy procedure was applied to analyze the behavior of different cement pastes regarding hydration process, setting time and heat development during the first ages.