The results presented and discussed are those of a study aimed at understanding how and why tint heterogeneities occur at the surface of concrete. In particular, the study involved laboratory tests on mortars which were aimed at examining the effect of the following parameters on the phenomenon of tint heterogeneity: W/C, super-plasticizer content, cement alkali content, mould type, and curing conditions. The results appear to show that two of the parameters aforementioned have a major influence on tint homogeneity: the homogeneity of the binder volume fraction distribution resulting from the use of super-plasticizers, and the absorption properties of the mould.

The growing use of polycarboxylate-based superplasticizing polymers can be attributed to the numerous advantages they provide to the production and quality of concrete mixtures. The ability of polycarboxylates to fluidify and maintain the workability of concrete, while having minimal impact on setting characteristics, has contributed to increased strength and durability, and has allowed for more economical completion of numerous concrete handling operations. With the increased application of polycarboxylates in concrete, more frequent opportunities exist for these polymers to be used with other chemical admixtures such as conventional water reducing, set retarding, and set accelerating agents as well as other functional admixtures. The resulting admixture combinations have resulted in a wide range of interactions. This paper discusses the highly synergistic strength increase observed between a condensed polyacrylic acid-aminated polyether-based superplasticizer and several calcium salts typically used in admixture formulations.

During 1997 the migrating corrosion inhibitors of the organic type were introduced in Denmark, mainly for the renovation of reinforced concrete structures. Before the introduction earlier tests were studied and supplied with various test series on the organic type of corrosion inhibiting admixtures. The paper deals with the review of earlier test series, the documentation tests carried out before the introduction and the conclusion drawn from these tests. It was the conclusion from this study that the organic type of corrosion inhibitor causes: Increase of the threshold value of chloride in concrete which involves that the initiation period of time can increase by appx. 100 %. Decrease of the corrosion rate after the rebars have started initiation. The rate of corrosion can be decreased to 15 - 25 % of the corrosion rate of rebars in concrete without the organic type corrosion inhibiting admixtures, all other parameters being equal. On the basis of these findings a mathematical model predicting service lifetime has been developed. This model is based upon the HETEK-model of chloride ingress into concrete exposed to marine environment, especially the marine atmosphere, cf. [l]. The model of chloride ingress applies an approximation in order to obtain simple mathematics. The application in Denmark of migrating organic corrosion inhibitor and corrosion inhibiting admixture of the organic type are mainly for repair work of RC structures degrading as a consequence of chloride exposure and carbonation.

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.

Polycarboxylate-based superplasticizers (air-entraining and high-range water-reducing agents) contain a polycarboxylate-based dispersant (PAS and a dispersion retainer. The PA adsorbs to the surface of the binding material particles. Due to steric effects, the particles are well dispersed, resulting in a high level of water reduction and high fluidity (l),(Z). It is considered that in the study of the dispersion effects of superplasticizers, it is important to understand the adsorption properties of the PA. We focused on the molecular size and adsorbed number of PA molecules. Based on the chemical structure and molecular weight of the PA, we calculated its molecular size and determined that the maximum length of the extended trunk polymer was 20 nm and that of the side chain was 7 nm. Considering the effective volume based on thermodynamics, one PA molecule is adsorbed to every 400 nm2 of particle surface. However, according to calculations based on the actual measurement of specific surface area and adsorption amounts, one PA molecule is adsorbed to every 100 nm2 of the particle surface. It is suggested that PA molecules shrink and are adsorbed on the surface of binding materials more densely than expected.