International Concrete Abstracts Portal

This paper deals with the consistency of High-Performance Concrete. Three water-reducing admixtures and two cements were tested by casting sixteen series of silica fume concretes. The study of the consistency was investigated as a function of time by using slump test and its comparison with heat flow measurement. The W/C are 0.25 and 0.35, and the amount of silica fume varies between 0 and 30% as a partial replacement of cement mass Two cements with different C3A, contents are used. The influence of the admixture chemistry was studied using polynaphthalene, polymelamine and polyvinyl superplasticizers. The results indicate that the polynaphthalene admixture is the most efficient one to control the consistency of concrete. Nevertheless polymelamine shows better fluidification properties than polyvinyl. Concerning the effect of silica fume, the results show that a partial replacement up to 10% silica fume does not reduce concrete workability. The increase in silica fume content corresponds to a quicker slump loss with time. Correlation between slump test results and heat flow measurements indicates that silica fume has an accelerating effect on the first hydration reactions and leads to a quick modification of the consistency of concrete. About the cement type, it has been noticed that the total amount of admixture is linked to the C3A content.

Chemical admixtures play a central role in modern concrete materials and technologies. In conjunction with mineral additives such as silica fume, chemical admixtures have enabled major improvements in many of the properties of concrete, particularly, compressive strength and durability. Chemical admixtures have also assisted in developing new concrete technologies, for example, concrete pumping and self-leveling, underwater concreting and shotcreting. Chemical admixtures have further promoted the use of secondary industrial materials (blast furnace slag and fly ash) in cementitious systems, contributing to resource conservation and environmental sustainability. In the continuing quest for more cost-efficient and environmentally acceptable materials and technologies, it may thus be expected that chemical admixtures will continue to play an important role in future generations of concrete. Probing into the future, how will concrete chemical admixtures evolve in the coming decades? What trends can be anticipated in future developments and use of these admixtures ? What will be the driving influences for these developments? This paper addresses some of the issues that are considered relevant driving forces to promote changes in the use of currently available chemical admixtures, or in the development of new admixtures. The trends already apparent in cementitious materials and concrete applications provide a reasonable basis for proposing probable trends in the evolution of concrete admixtures into the 2 lSt Century.

Superplasticized high-volume fly ash concretes with 50% of portland cement replacement were made by incorporating two different chemical admixtures based on sulfonated naphthalene (SN) or acrylic polymer (AP). Portland cement with a Blaine fineness of about 400 or 500 m2/Kg was replaced by 50% of ground or un-ground fly ash. The content of the cementitious material (Portland cement + fly ash) was about 470 kg/m3. The concretes with SN were manufactured with a slump in the range of 190-200 mm, whereas the slump of the concretes with AP was in the range of 220-230 mm. Due to the different effect of the superplasticizers, the water-cementitious material ratio (w/cm) was 0.32 or 0.29 for the SN or AP admixture, respectively, although the dosage was slightly lower for the latter. Cube specimens, 150 x 150 mm in size, were cured at 5°C or 20°C and compressive strength was measured at 1 to 90 days. Due to the lower w/cm, the strength of the concretes with the acrylic polymer was significantly higher with respect to those with SN. The better performance of the AP superplasticizer, in terms of compressive strength, was obtained at early and later ages independently of the curing temperature (5 and 2OC), and the fineness of the portland cement and fly ash. Due to the lower w/cm of concrete with the AP admixture with respect to those with the SN superplasticizer, the durability of high-volume fly ash concrete can be improved in terms of lower penetration rate of CO2 or chloride ions.

The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The results obtained show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work.

Reactive Powder Concretes (RPC) - in form of superplasticized cement mixtures with silica fume, steel fibers, quartz fine sand (100-400 um) and/or limestone coarse aggregate (0.1-8 um ) - were studied in comparison with modified RPC where artificial aggregates substituted for natural aggregates. Artificial aggregates were obtained by grinding portland clinker coarsely so that fine and coarse aggregate were obtained with approximately the same particle size distribution of natural fine quartz (100-400 um and limestone gravel (O.l-8 mm) respectively. The source of clinker-aggregate was the same as that used for portland cement as binder of RPC. The idea was to improve the bond strength between cement paste and aggregate due to some hydration of the clinker-aggregate surface. RPC specimens were cured at room temperature (2OO C) or steam-cured at low or high pressure at 90°C or 160°C respectively. Compressive strengths were measured as a function of time at 1-28 days. The 28-day compressive strength level was as high as 200 MPa. Regardless of the curing temperature, compressive strength of RPC was increased by about 20 MPa when clinker-aggregate was used instead of natural aggregates. These results indicate that the bond strength of the interface between cement paste and aggregate is improved when clinker particles are used instead of natural stones. Scanning electron microscope observations of the microstructure confirmed this hypothesis and indicated that the interface between cement paste and natural aggregate is the weak point in RPC.