International Concrete Abstracts Portal

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.

A new conductivity method is proposed to monitor the behavior of fresh cement-based materials during the consolidation, setting, and early hardening periods. The method relies on differences in electrical conductivity measured at different depths, and as function of time, to evaluate variations in the local composition (solids, fluids) of the material. To perform these measurements in a practical and cost-effective way, simple disposable conductivity probes were designed with multiple electrodes, after initial work with similar laboratory-type probes. The approach was tested with several cement-based systems ncluding grouts, mortars, and concrete. During the dormant period, the conductivity readings reflect changes in the homogeneity of the samples as a function of time, which are qualitatively well related to the bleeding-segregation behavior of the cement-based system. From the conductivity data obtained as a function of sample depth, a stability index could be defined using the least-squared deviation from the mean conductivity at a given time. The time-dependence of such a stability index reflects the evolution of the bleeding and segregation phenomena in the material. The multi-electrode conductivity approach also yields other valuable information, namely a determination of the initial setting, and a reasonable estimate of the rate of strength development during the early stages of the hardening period.

The time dependence of the interaction between hydrating cement particles and a poly-naphthalene sulfonate (PNS) superplasticizer has been investigated using rapid response calorimetry and other physico-chemical approaches. The study focuses on the processes which occur during the first instants following the immersion of the cement particles into the solution, in the presence, or absence, of the PNS superplasticizer. Specifically, the investigation aims to elucidate the dominant phenomena in the coupled processes taking place in the water/cement/sulfate/PNS system, and the consequences of these phenomena on 1- the reaction rates and products and 2- the cement-superplasticizer compatibility The systems discussed here are cement pastes at W/C=3 containing cements having highly different alkali sulfate contents and a normal PNS super-plasticizer. The kinetics of the initial reactions (O-30 min.) are monitored by fast response adiabatic calorimetry; the superplasticizer adsorption, and variations in the ionic composition of the interstitial solution, are also determined at short time intervals during the same period. The evolution of the hydrate phases as a function of time is monitored through XRD analyis.

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.

Total and external chemical shrinkage have been followed for a number of cement pastes until 48 h. Total chemical shrinkage is believed to roughly be proportional to degree of hydration, while the differences in external chemical shrinkage give an impression on how prone the mixture may be to micro-cracking. The difference between total and external shrinkage result in contraction pores. This study is part of a larger on-going study focusing on the early volume change of binders of high performance concrete. Variables in the present part of the study have been plasticizer (sodium lignosulphonate) and super-plasticizer (sodium salts of sulphonated melamine -formaldehyde condensate and naphthalene sulphonate - formaldehyde condensate) and water-to-cement ratio (0.30, 0.35, 0.40, 0.45 and 0.50). A few mixes were also made with a combination of plasticizer/super-plasticizer, a common practical concept. The total plasticizer/super-plasticizer dosage was 1% by weight of cement, and the cement chosen was a high strength portland cement commonly used for high performance structures. Total chemical shrinkage could be used to monitor the retardation of the cement setting by for instance lignosulphonate and the acceleration of hydration rate thereafter. The hydration development of pastes with plasticizers (LS) or super-plasticizers (SNF and SMF), as measured by total chemical shrinkage, seems virtually independent of the water-to-cement ratio in the range 0.30-0.50 during the first 48 hours. Only the hydration for the paste with the lowest w/c may be slightly accelerated immediately after setting, but reached slightly lower values at 48 h. Lignosulphonate (LS) retard setting more than the super-plasticizers naphthalene sulphonate - formaldehyde condensate and sulphonated melamine - formaldehyde condensate. The flattening-out-level of the external chemical shrinkage was only marginally influenced by w/c, with non-systematic individual differences being in the order of 0.1 ml/l00 g cement. The flattening-out- level of the external chemical shrinkage was independent of admixture type at w/c = 0.40, and the same as the reference without admixtures.