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

Reports the results of an investigation of the effect of using ground granulated blast furnace slag to prevent alkali-aggregate reaction damage to concrete. The authors discuss the effectiveness of the blast furnace slag on the dilution, stabilization, and fixation of alkali. The relationship between the replacement ratio of blast furnace slag and prevention of the expansion due to the alkali-aggregate reaction in concrete is reported.

Disintegration of many concrete pavements (D-cracking, popouts, etc.) exposed to freezing and thawing is often connected with poor physical quality of aggregates used in the concrete. Inability to differentiate between good and poor quality aggregates is due to the lack of appropriate laboratory techniques for aggregate evaluation. A growing shortage of easily available sources of good quality aggregates highlights the need for aggregate classification. A new rapid laboratory test, called RAO-Method, as well as a new pore size distribution index based on the mercury intrusion porosimetry (MIP) analysis, has been proposed to meet engineers' expectations in the field of aggregate classification. An analysis of some research data of the RAO and MIP tests is presented in this paper to illustrate practical usefulness of the techniques. Results of long-term observations of concrete blocks subjected to outdoor conditions and the results of the new laboratory tests of the aggregates previously used in the blocks were compared. The new tests seem to provide means for more successful evaluation of coarse aggregates for purposes of diagnostics, design, and prediction of service life of concrete.

Economic solutions for production of high-strength concrete in Vietnam require use of locally available indigenous resources. Proportioning of gap graded mixtures in Northern Vietnam is therefore based on broken rock and very fine Red River sand. Additionally, normal portland cement blended with fine-grained rice husk ash (RHA) is employed in combination with a superplasticizer. This paper discusses results obtained in a Dutch-Vietnamese research cooperation program. RHA was incinerated in a specially constructed oven under temperatures up to 750 C, yielding amorphous silica with a relatively high carbon content (23 percent). Ash was ground for 18 hours in a laboratory ball mill in combination with or without use of a naphthalene type of superplasticizer. Dutch sand and gravel were used, simulating as closely as possible the Vietnamese aggregate. Particularly promising data was obtained for 19 percent sand content in the aggregate and a paste content of 500 kgf/m 3, in which the RHA content amounted to 100 and 200 kg/m 3 with a corresponding water to paste ratio of 0.3 and 0.35, respectively. RHA ground with the superplasticizer was used in such cases (yielding 75 percent of particles to be smaller that 5 micro-m). Compressive strength was found to exceed 50 MPa at seven days and 70 MPa at 28 days.