Advanced Cement Based Materials最新文献

In applications where high strength is not of prime importance, automobile and truck tires can be recycled by shredding them and incorporating them in concrete. When the rubber is placed in the concrete, it should have minimal strength reduction and undergo minimal degradation due to the prolonged exposure to the concrete matrix. In this preliminary study, the stability of shredded rubber in concrete was simulated by exposing the shredded rubber to highly alkaline environments for up to 4 months. The degradation of rubber immersed in different chemicals was monitored by observing the changes in mass, swelling, tensile strength, and microstructure. The results show that the shredded rubber undergoes small change in a highly alkaline medium after 4 months, which suggests that the addition of rubber to concrete will not dramatically affect the durability of concrete.

Concrete deteriorates due to many different mechanisms. Among the most important mechanisms is the reinforcement corrosion induced by deleterious substances reaching the embedded reinforcement bars. The external sources of deleterious materials may, for example, be deicing salts, sea water, and carbon dioxide. Research has sought to determine threshold values, in terms of concentration of deleterious substances in concrete, at which reinforcement corrosion will be induced, that is, at which concentration the passive condition close to the reinforcement turns to an aggressive state. To predict when this threshold value is reached, the flow properties of the pollutant in concrete must be known. Some of the most important phenomena governing the movement of pollutants in concrete are diffusion of substances in the pore water, adsorption (and desorption) of pollutants onto the pore walls, and hydrodynamic dispersion and convection of substances due to flow of the pore water. Here a set of equations will be presented based on mass and energy balance. These coupled equations cope with the above-mentioned phenomena. The migration of ions due to an electric potential is not considered as only the initiation stage of corrosion is of interest. The constituents considered in the model are a solute γ (e.g., chlorides), the pore water α, and the solid phase s of the concrete, which is restricted to be nondeformable. The governed equation system is solved using the Petrov-Galerkin scheme and finite elements (compare references 1 and 2). Some examples of the performance of the proposed model are given.

The early strength development of ordinary Portland cement (OPC)/calcium aluminate cement (CAC) pastes (92.5/7.5, 80/20, and 20/80) was studied. Conduction calorimetry, X-ray diffraction, and scanning electron microscopy methods were used to monitor the heat evolution and microstructural changes of these pastes. It is apparent that ettringite formation contributes to the early “set strength” in the 80/20 paste. The OPC hydration seems to be delayed by the presence of CAC.

The literature and site data on the efficiency of curing are controversial regarding its effect on the mechanical properties of high strength concrete. The traditional methods of curing may fail in cases of concrete with a low water/binder ratio and with addition of silica fume. Hydration proceeds quickly, the hydrated cement paste is very dense, and water from the outside cannot reach the interior of the concrete to achieve complete hydration. Replacing 25% by volume of the aggregates by prewetted lightweight aggregates creates water storage inside the concrete, which supports continuous wet curing. The purpose of this article is to introduce a new type of high performance concrete. The materials used and their properties are shown, and the mixing procedure is given. The most important mechanical properties of the concrete under various curing conditions and the microstructure of the hardened cement paste were investigated. The results show that the method of introducing a water reservoir can be successfully applied to obtain high performance concrete with improved properties while being relatively insensitive to curing.

The chemical characteristics of a calcium aluminate-phenol resin composite with very high flexural strength are discussed. The flexural strength of the composite was found to be 120 to 220 MPa, which is greatly dependent on the fabrication method. The best system of these composites is made of calcium monoaluminate and the resole type of phenol resin. The evidence of possible cement-resin interaction has been found experimentally in roll milling, during heat curing, and in the final product. We discuss the evidence of interaction of phenol moiety and calcium aluminate based on the observations in processing, experimental data of differential scanning calorimetry, electron probe microanalysis, conduction calorimetry, and X-ray diffraction analysis. Based on our findings, we propose a cross-linking mechanism assumed to occur in processing and during curing.

Concrete skin is considered the closest zone to the surface of concrete cover of reinforcements. It usually has a different composition than the internal concrete due to phenomena such as contact with molds or segregation of aggregates. In addition, environmental actions induce a gradient of moisture along the cover depth. These circumstances sometimes produce an irregular chloride profile in the cover, which either exhibits a maximum of chloride content some millimeters inside the outer surface or sometimes shows an anomalously high chloride concentration right at the concrete surface. In the present paper, analytical modeling of chloride diffusion is used to study the relative influence of the skin thickness. This theoretical analysis aims to show that there are cases where, if the diffusivity of the skin and the bulk concrete are very different, an error is introduced when the “skin effect” is not accounted for. The clarification of this error may contribute to understanding the differences found between laboratory experiments and the long-term record of chloride profiles in the same concrete.

The properties of the fiber/matrix interface are of primary significance for the overall behavior of fiber reinforced cement based composites. The present paper gives an overview of the current knowledge regarding characterization and engineering of the interface. First, different mathematical models for the characterization of interface properties are reviewed, including strength as well as toughness based models, and basic interfacial parameters are identified. Second, engineering tools are reviewed—primarily ways of increasing the fiber/matrix bond by applying various strengthening techniques, including introduction of fiber deformations, densification of the porous fiber/matrix interfacial transition zone, and fiber surface modification using plasma treatment. The strengthening mechanisms are quantified through basic interfacial parameters, and it is shown by reference to available experimental evidence that substantial improvements in the fiber/matrix bond can be achieved, opening up the field for further optimization of fiber reinforced cement based composites. Finally, gaps in the present knowledge are pointed out, identifying areas of future research in this area.