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

As a result of the very low water-cement ratio in a high-performance concrete, the rate of cement hydration at early ages is significantly different from that in a normal strength concrete. The ultimate degree of cement hydration is lower in a high-performance concrete; the hydration process will terminate earlier because of the rapidly diminishing water supply. Another characteristic of high-performance concrete is caused by the relatively high dosage of superplasticizer which delays the onset of the cement hydration. This paper presents the extension of the research on temperature and strength development in hardening concrete from normal strength concrete to high- performance concrete. It models the development of heat of hydration in high-performance concrete, taking into account the effects of water-cement ratio, superplasticizers, and temperature changes. General formulations of the rate of heat of hydration as functions of concrete maturity (hydration stage) and current temperature are provided. Comparison with some test results verifies the theoretical model.

Presents results of expansion tests carried out on concretes immersed in 1/10-M and 1-M sodium chloride solutions. The concretes were prepared using two reactive aggregates, cristobalite and a natural aggregate from the southwest of the U. K. Tests were carried out both at alkali levels which were known to induce expansion due to alkali-silica reaction (ASR) and alkali levels which would not normally induce expansion due to ASR. The concretes were, at the ages of one, three, and six months, immersed in a sodium chloride solution. The concretes were stored at 38 C, 20 C, and externally. For the concretes containing the natural aggregate, it was shown that immersion in a 1-M salt solution had no major adverse effects upon long-term expansion. This is attributed to the low available reactive silica content within the concretes. In the case of concretes containing cristobalite, it was shown that the immersion in 1-M salt solution had an adverse effect upon long-term expansion. This is attributed to the high available reactive silica content of the concrete.

Presents the results of a study on the effect of curing conditions on the air-entrained, superplasticized high-volume fly ash concrete made with ASTM Types I and III cements and silica fume. For the four concrete mixtures made, the total cementitious materials content was about 370 kg/m 3, and the water- cementitious materials ratio was kept at 0.310.01. The proportion of ASTM Class F fly ash in all the mixtures was 58 percent by weight of the (cement + fly ash) content. Two mixtures incorporated silica fume at the dosage rate of 8.5 percent of the total cementitious materials content. The properties of fresh concrete including the time-of-set and autogenous temperature rise were determined. Specimens were cast and moist cured for the determination of compressive and flexural strengths, resistance to chloride ion penetration, and freezing and thawing cycling at various ages. Compressive strength and chloride ion penetration measurements were also performed at various ages on the specimens that were subjected to laboratory air curing after three days of moist curing. The use of ASTM Type III cement instead of Type I increased the early age strength significantly without affecting the long term strength development under moist curing conditions. Under air curing conditions, the concretes incorporating the Type III cement achieved significantly higher strengths at every test age up to one year. The use of silica fume resulted in only marginal improvement in the strength properties under the two curing regimes. The air curing resulted in a significantly lower resistance to chloride ion penetration (RCP) of all the concretes, but the drop in RCP was greater for those made with ASTM Type I cement. The use of silica fume increased the resistance to chloride ion penetration of concretes significantly under both curing conditions.