International Concrete Abstracts Portal

The intrinsic nature of lightweight concrete is to produce a material which, in addition to having high strength, can also have high performance in severe service conditions. The reason for high performance is examined in light of physical, chemical, and mechanical properties of the vesicular aggregate used to produce lightweight concrete. The manufacturing process usually involves heating the aggregate to 1200 C which, in addition to rendering it more stable than conventional aggregates when concretes made from it are exposed to fire, also results in a less stiff aggregate inclusion that more closely matches the stiffness of the cement paste matrix. The use of less stiff aggregates results in a reduction in internal stress concentrations in the concrete which, in turn, leads to reduced microcracking. The role that this plays in enhancing the performance of this type of concrete is discussed in the paper. The special nature of lightweight concrete provides opportunities for design professionals. Recommendations on how best to achieve high performance concrete using lightweight aggregate are provided.

Cement-based composites are used in the construction of a wide range of structures. During their service life, many of these structures are exposed to various types of aggression; their durability is generally controlled by their diffusivity and permeability of the cement-based composite. Since the assessment of these two properties by laboratory or in situ tests is often difficult and generally time-consuming, a great deal of effort has been made towards developing microstructure-based models to predict them. A critical review of the most recent developments in this field is presented in this paper. The report begins with a survey of the various mathematical concepts developed to characterize the structure of porous media. Empirical and physical models are reviewed in separate sections. Special emphasis is placed on recent innovations in the field of numerical and digital image analysis based modeling. Each model is evaluated on the basis of its ability to predict the mass transport properties of a wide range of cement-based composites and its potential application to the study of other micro- and macro-structural properties.

Presents basic information on a newly developed electrically conductive concrete. The concrete differs from previous inventions in that both high conductivity and mechanical strength are simultaneously achieved. The electrical and mechanical properties of the conductive concrete developed at Institute for Research in Construction, National Research Council of Canada are given. The material has superior electrical conductivity values and excellent mechanical strength. Experimental results of a laboratory-scale study on the application of conductive concrete to deicing and/or snow melting are presented in this paper. The results indicate that heat can be uniformly produced by the conductive concrete heating element when the element is activated by an external electric power supply. The new method is effective for deicing purposes. Power output of the conductive concrete heating element is stable over a wide range of temperature. The minimum heater power output required for deicing at various air temperatures was determined. This value is linearly dependent on the air temperature, ranging from 150 to 855 W/m 2 as air temperature varies from -5 C to -30 C.

This paper covers analytical and experimental investigation of high- strength concrete beams reinforced with high-strength prestressed concrete prisms as main reinforcement. Fiber optics technology has been developed and used in this investigation to measure the flexural crack widths developed throughout the full loading history of the specimens. Thirteen beams, 8 in. x 12 in. (200 x 300 mm) is cross section and having a 9.0 ft (2.74 m) span were tested to failure. The embedded prestressed prisms had a length of 9 ft, 6 in. (2.90 m) and cross-sectional dimensions ranging between 1.5 in. x 3.0 in. (38 mm x 76 mm) and 4.5 in. x 3.0 in. (114 mm x 76 mm). The prisms were prestressed with 7-wire, 3/8 in. (10 mm) diameter, 270 ksi (1860 MPa) tendons. Concrete strength in both the prisms and the beams was in excess of 14,000 psi (100 MPa) using silica fume as a partial cementitious replacement, as well as a high-range water reducer (superplasticizer) to attain the desired workability and compressive strength. A study of the extensive data accumulated in this research program, supported by the National Science Foundation, resulted in expressions for the evaluation of flexural crack widths in ultra-high-strength concrete composite beams. Test results also showed that the embedded prisms delayed the development of cracks, while the additional use of non-prestressing steel significantly reduced the crack spacing in the beams and limited the crack width at the onset of prism cracking.

The durability of reinforced concrete structures can be improved by resorting to methods which insure a better resistance of concrete to various aggressive environments. Some commonly used methods include subjecting concrete to a better curing practice, the use of modified concretes, and the application of surface treatments on concrete surfaces. In addition to these, efforts have been made in the recent past to develop new techniques by which the water- cement ratio in the near surface region can be lowered and a dense matrix achieved. One way of achieving this is to use a controlled permeability formwork system (CPF), in which the surplus mixing water and entrapped air are removed from the fresh concrete via a fiber liner. This produces a surface layer of concrete with a very low permeability which is likely to be highly resistant to various forms of environmental attack. Relatively little information is available at present on the efficiency of CPF in improving the protection of the concrete against various mechanisms of deterioration and on how it compares with other techniques, such as the application of better curing practices. Therefore, an experimental investigation was carried out with three water-cement ratios, five different curing regimes (air curing, wet hessian curing, and the use of three different curing compounds), and the application of a CPF liner system. Measurements of gas permeability, sorptivity, chloride diffusivity, surface tensile strength, freezing and thawing resistance, and carbonation resistance have indicated that the use of CPF can enhance the durability of concrete and that the extent of this improvement is significantly more than that obtained for the various curing regimes. This paper details the experimental program and presents results which are used to evaluate critically the use of CPF for normal concrete.