International Concrete Abstracts Portal

This CD contains 33 papers presented at the Tenth International Conference of Superplasticizers and Other Chemical Admixtures in Concrete held in Prague, Czech Republic, in October 2012. Topics include Synthesis, Characterization, and Dispersing Performance of a Novel Cycloaliphatic Superplasticizer; Compatibility between Polycarboxylate and Viscosity-Modifying Admixtures in Cement Pastes; Aspects of Gypsum-Free Portland Cement; A Novel Type of PCE Possessing Silyl Functionalities; and much more. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-288

While the drying shrinkage of concrete is due mainly to the shrinkage of cement paste, the shrinkage of rock aggregate also plays a significant role. Hence, it should be possible to reduce the shrinkage of concrete by hampering the moisture movement across the rock aggregate so that the aggregate would not shrink after casting. Since rock is porous in nature, the moisture movement across rock can only be hampered by blocking or sealing the pores. For this purpose, the use of polymer latex or water repellent to treat the aggregate before casting is proposed. To investigate the effectiveness of such aggregate treatment, a testing program was launched, in which the water absorptions of untreated and treated aggregates were tested and the shrinkage strains of concrete made with the untreated and treated aggregates were measured up to over 600 days. The results demonstrated that the proposed aggregate treatment with polymer latex or water repellent can significantly reduce the concrete shrinkage.

The improved fluidity of self-consolidating concrete by using chemical admix¬tures sometimes causes stability problems. In this paper some fundamentals of particle settling theory, such as Stokes’ law, are used to investigate particle sedimentation. Some preliminary calculations of stresses from particles (>0.125 mm [4.93E-4 in.]) on matrix (liquid and powder < 0.125 mm) with viscous fluid properties, indicate that yield stress is more important than plastic viscosity for the stability of aggregates when settling in matrix. The Kozeny-Carman equation (KCE) is used to analyse bleeding rate of fresh matrix with three main factors involved: solid fraction, specific surface area and liquid viscosity. Different stabilizing mechanisms of viscosity modifying agents (VMA) and fillers are also reviewed. Finally a settlement model is set up for further investigation.

The working mechanism for cement dispersion of an isoprenyloxy ether-based polycarboxylate superplasticizer (PCE-A) and of several PCEs based on MPEG-ester (PCE-B0, PCE-B1, PCE-B2) or APEG-ether monomers (PCE-C) was investigated via cement paste flow testing and measurement of hydrodynamic radius of solved PCE mole¬cules, adsorption isotherm, adsorption rate and zeta potential. Using “mini slump” test method it was found that the isoprenyloxy ether – acrylic acid copolymer (PCE-A) exhibits the highest water reduction, followed by the methacrylic acid – MPEG – 1100 – methac¬rylate copolymer (PCE-B2), whereas the methacrylic acid – MPEG – 250 – methacrylate copolymer (PCE-B0) required a very high dosage. Further experiments revealed that in cement pore solution, the conformation of PCE-A is star-like; while PCE-B0, PCE-B1 and PCE-B2 show a worm like conformation with flexible backbone and PCE-C exhibits a rod like structure possessing high rigidity. PCE-B2 disperses cement mainly via strong steric repulsion instigated by the long side chain. Based on adsorption and zeta potential data obtained it is proposed that PCE-A, PCE-B0, PCE-B1 and PCE-C disperse cement not only by steric repulsion originating from adsorbed PCE, but also by electrostatic and/or steric repulsion owed to non-adsorbed PCE molecules present in the pore solution. Especially PCE-B0 was found to require a dosage substantially beyond its saturation adsorption. This suggests that free PCE dissolved in the pore solution may provide the main contribution to cement dispersion. Consequently, a new type of dispersion mechanism which originates from dissolved PCE molecules exercising a “lubricating” effect between cement particles is postulated for PCE-B0.

Metakaolin (MK) comes from the calcination of kaolinite at temperatures ranging from 700°C (1292°F) to 850°C (1562°F). The resulting dehydroxylation of kaolinite produces an amorphous material (metakaolin) that is usually very reactive in alkaline media such as cement-based materials. However, the reactivity generally takes effect after a few days, so poor short-term performance is obtained, which is a disadvantage in cases where the concrete needs to be rapidly stripped of its formwork. The aim of this work was to test different chemical activators for metakaolin, in order to obtain short-term strength similar to that obtained without metakaolin. Twelve activators were selected and tested in mortars at different concentrations, using two modes of intro¬duction (in solution and in solid form). Four cements were used in the experiments (2 CEM I and 2 CEM II). Workability and compressive strength tests were carried out (at 16 h, 24 h, 48 h, 7 d, and 28 d). The results show disparities in the efficiency of the activation. Products very active at early ages often led to a decrease in strength at 28 days of age, compared to activator-free mixtures.