Abstract The test method was developed to evaluate portland cement systems with different air contents, and for the purpose of determining whether the apparent poor performance of concretes containing supplementary cementing materials in the ASTM C 672 deicer scaling test can be attributed to differences in finishing effort or timing of finishing, or both.

Abstract Crack surveys of bridge decks, performed over a 10-year period in northeast Kansas as part of three studies, provide strong guidance in identifying the parameters that control cracking in these structures. The surveys involve steel girder bridges—bridges that are generally agreed to exhibit the greatest amount of cracking in the concrete decks. The surveys include monolithic decks and decks with silica fume and conventional concrete overlays. The study demonstrates that crack density increases as a function of cement and water content, and concrete strength. In addition, crack density is higher in the end spans of decks that are integral with the abutments than decks with pin-ended supports. Most cracking occurs early in the life of a bridge deck, but continues to increase over time. This is true for bridges cast in both the 1980s and the 1990s. A key observation, however, is that bridge decks cast in the 1980s exhibit less cracking than those in the 1990s, even with the increase in crack density over time. Changes in materials, primarily cement fineness, and construction procedures over the past 20 years, are discussed in light of these observations. A major bright spot has been the positive effect of efforts to limit early evaporation, suggesting that the early initiation of curing procedures will help reduce cracking in bridge decks.

Abstract A very high early strength portland cement-based concrete (VHES) is used to prevent long closures of highway lanes by completing any necessary repairs overnight. This technology has been embraced by several DOT agencies, including those in areas where the concrete will be subjected to freezing and thawing cycles. Flexural strength of VHES concrete is the critical parameter to permit traffic flow without damage to the concrete. It is possible to produce air-entrained rapid repair concrete, but this places higher requirements on the mixture proportions to overcome strength reduction caused by the presence of air. This article presents data showing that it is possible to attain the necessary strength and achieve adequate resistance to freezing and thawing cycles. The evidence presented includes accelerated testing using ASTM C 666, Procedure A, and scaling resistance using ASTM C 672. Additional evidence is presented showing that VHES concrete of low w/c can be durable with larger than acceptable spacing factor levels or without air entrainment.

Abstract Self-desiccation and autogenous shrinkage of high-strength concretes is one of their drawbacks which cannot be readily accommodated by conventional curing. The concept of internal curing using lightweight soaked aggregates, to provide internal reservoirs of water which enable uniform curing of the whole cross-section, has been advanced by several groups. The present study is intended to develop this approach further, by optimizing the porosity and size of the aggregates to enable successful internal curing with only a small amount of aggregates, which could be viewed as additives rather than bulk replacement of conventional aggregates. The approach taken was to increase the porosity of the aggregates and reduce their size, to obtain a system with numerous internal reservoirs of sufficiently small spacing to allow water to be readily discharged from the aggregate and transported over the whole range of the matrix. It was shown that aggregates with porosities of about 50% by volume, a size of a few millimeters, and contents of less than 50 kg/m3 could provide full elimination of autogenous shrinkage in concretes having w/cm as low as 0.25, with only a small affect on strength. The parameters controlling the efficiency of the aggregates were assessed, indicating that their pore structure is the most important one, and that water from within the aggregates can be readily transported into the matrix to a distance of few millimeters.

Abstract A distribution of particle sizes or particle size distribution (PSD) is a fundamental characteristic of cement powder. Accurate PSDs are required in computational efforts to model the hydration process and it is an important practical issue for the cement industry. Presently, the only available standard method for measuring the PSD of cement, namely ASTM C115, is limited in scope, with a lower size detection limit of 7.5 μm. Since there are no standard procedures that adequately cover the broad particle size range associated with portland cement powder, the implementation of different measurement techniques varies widely within the industry. Two ASTM-sponsored round robin tests were performed to (1) ascertain the techniques and methods currently used in the cement industry and (2) develop and refine a standard method or methods. The results have been incorporated into a best practice method based on the technique of laser diffraction. The aim of the current paper is to summarize the findings based on the data generated during the round robins and to summarize the various approaches available to measure the PSD of cement. A summary of the statistical analysis of the test results is described.

Abstract The potential for drying shrinkage cracking in concrete is related, in part, to the development of a moisture gradient across the cross-section of the concrete element. A model has been developed that incorporates experimental measurements of internal relative humidity to investigate drying shrinkage stress gradients in concrete specimens. The experimental and analytical procedure for using the model is outlined. Initial research involving application of the model has indicated that there is a relationship between drying stress gradient severity and the time to cracking under full restraint. Further development of the model could involve combination with fracture models to investigate microcrack formation and propagation.

Abstract Concrete specimens of different sizes and shapes were made with various reactive aggregates and stored under conditions favorable to the development of alkali-silica reactivity (ASR), with their expansion measured with time along the three directions. They have been cast vertically (cylinders and prisms) or horizontally (prisms and larger blocks), using a vibrating table, a vibrating needle, or rodding. The expansion due to ASR was always greater in the direction perpendicular to the casting plane. The higher the number of flat and elongated particles in the reactive aggregate, the higher the coefficient of anisotropy, defined as the ratio between the expansions perpendicular and parallel to the casting plane. This coefficient was constant through the course of the expansion. It was generally higher for the cylinders than for the prisms, and still less for larger blocks. Consolidation by rodding induced anisotropy coefficients distinctly smaller than consolidation using a vibrating table, while a vibrating needle induced intermediate values; however, all methods gave constant volumetric expansion at least up to an important expansion level. For prisms cast horizontally and measured axially in accordance with the concrete test CSA A23.2-14A or ASTM C 1293, consolidation using rodding induced long-term (axial) expansions greater by 71% compared with consolidation using a vibrating table. In order to reduce the experimental variability of the test, only one method of consolidation should be allowed. When evaluating field concrete affected by ASR, it appears important to consider the orientation with respect to the casting plane of the core samples subjected to mechanical or residual expansion tests.

Abstract During the last few years, polycarboxylate based polymers have become very popular. They contain a polycarboxylic backbone onto which ethyleneoxide side chains have been grafted. However its dispersing and slump keeping properties vary significantly, depending on the chemical structure. The dispersing properties of two polymers having a different number of polyoxyethlene (PEO) graft chains relative to the length of the backbone were evaluated in cement paste by measuring adsorption on cement paste and flow properties in different mortar compositions. The effects of the polymers on the fluidity and the retention of flow of the cementitious systems are significantly different. While one polymer gives a very strong initial water reduction, the second one shows excellent flow retaining properties. When combined, the two polymers are well suited for the production of RMC and self-consolidating concrete mixes with different cements and in different climate zones.

Abstract This study highlights the notion of robustness of combinations of cements and superplasticizers. Tests done with various cements and different families of superplasticizers showed that although a combination of a cement and a superplasticizer could be compatible, it is not necessarily robust. Sometimes a little variation in the dosage of the admixture could lead to detrimental side effects, such as segregation, excessive set retardation, or excess air content in the concrete. Results showed that the chemical composition of the cement is critical to ensure good compatibility and adequate robustness of various combinations of cements and superplasticizers. The chemical nature of the superplasticizer also plays a role in the behavior of such combinations. The C3A content, the soluble (alkali) sulphate content, and the fineness of the cement, which influence the adsorption rate of the superplasticizers on the cement particles, are among the key factors that control the compatibility and the robustness of cement-superplasticizer combinations, especially for polysulfonated admixtures. Based on the results of this study, a rough prediction of the compatibility and robustness of cements and superplasticizers could be made by analyzing the chemical composition of the cement and the chemical nature of the superplasticizer.

Abstract The Center of Building Materials (cbm) at the Technische Universitaet Muenchen has many years of experience in the early-age cracking risk of concrete. So far most of the investigations have been focused onmass concrete with regard to its semi-adiabatic thermal behavior—the development of uniaxial restraint stress during hardening has been determined in the cracking frame, simulating the centre of an approx. 50 cm-thick concrete. New results on the early-age cracking risk of special concretes with regard to their specific conditions or composition that is different to that of ordinary mass concrete will be shown in this paper. These special concretes are self-compacting concrete, pavement concrete, high-strength concrete and in particular ultrahigh-performance concrete with a compressive strength of up to 200 MPa at an age of 28 days. Furthermore, a test-based calculation method using the finite element method (FEM) was developed to predict, for example, the thermal restraint stress in concrete structures under various weather and curing conditions. The restraint stress distribution at any location in the concrete can be calculated realistically by applying appropriate rheological models for which “true values” of particular early-age concrete properties are required, which have to be determined experimentally.