International Concrete Abstracts Portal

For years, the concrete industry has used ultimate compressive strength and elastic modulus as principal design and analysis tools. This can be very misleading when cracking and failure are evaluated. With modern concrete that include roller-compacted concrete (RCC) and lower strength mass applications, cracking that is serious may not occur until the concrete is strained well beyond the elastic region. Two things are needed to resolve this problem. First, a new property called the "ultimate modulus" should be determined, along with the elastic modulus. If these values are nearly the same, the concrete is brittle and may have a low strain capacity, even if it has a high strength. If the ultimate modulus is much lower than the elastic modulus, the material is "tough" and may have a high strain capacity despite a low strength. Examples are given in which deliberately designing a lower strength concrete has resulted in a much higher strain capacity. In one case with RCC, a mixture with five times less strength resulted in a tensile strain capacity (and resistance to thermal cracking) that was three times greater. Second, there should be a better understanding of the relationships between strain capacity, strength, and modulus (ultimate and elastic) in compression as compared to those material properties in tension. With the broader range of concrete mixtures possible in today's concretes (RCC being an example), the ratio between split cylinder tensile strength and compressive strength may be twice as high for a lower strength mixture than it is for a higher strength mixture. Somewhat offsetting this is the fact that the conversion factors from split tensile strength or flexural strength to direct tensile strength are substantially smaller for low strength concretes and greater (exponentially) for high-strength concretes. When only concretes in the compressive strength range of about 20 to 50 MPa are considered, the adjustment factor happens to be about one, so this phenomenon has not been obvious or very important in the past.

Examines the relationship between deterioration of concrete and the structural performance of bridge structures. Case 1: A 37-year-old, three-span concrete slab bridge was decommissioned due to heavy deterioration. Modal testing was used to detect the mos

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.

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.