International Concrete Abstracts Portal

Some different degrees of alteration have been found in a concrete tunnel lining affected by dissolved calcium sulfate in water due to the oxidation and solubility of the pyrites contained in the rock. In some cases the damage has been so extensive that the concrete has changed into a plastic mass. Measurements taken inside the tunnel over many years showed a range of temperature from 4 to 6 C. The chemical analysis of the concrete, the diffractometric analysis, and the scanning electron microscope analysis performed on some alteration products all clearly showed the presence of thaumasite and calcite, while no ettringite was found. The recorded thermal conditions, the presence of silica due to feldspar alteration, and the free carbon dioxide in the water lead to the conclusion that the ettringite has been transformed completely into thaumasite. To understand this transformation, some laboratory tests were carried out on mortar immersed in a water mixture containing calcium sulfate, calcium carbonate, calcium hydroxide, and amorphous silica in an environment rich in carbon dioxide, at 5 and 20 C, respectively.

Results of a wide-ranging study undertaken to examine the effect of a superplasticizing admixture on the durability of normal-workability concretes are reported. The investigation was based on cement-reduced concrete mixes covering a range of 28-day strength from 20 to 65 MPa, with water-reduced concretes included to increase the data base. Corresponding normal concretes were used for comparison. The experimental program covered included measurements of air and water permeability to assess the overall durability potential; the rates of carbonation and chloride ion diffusion to assess the possible risk of steel reinforcement corrosion in concrete; and the deterioration under alternate freezing and thawing and wetting and drying to assess the resistance of concrete to frost attack and weathering. The results showed that, for a given workability and design strength, the use of a superplasticizing admixture can be expected to effect improvement in the durability of concrete.

Recent construction of a segmental, precast concrete cable-stayed bridge across the Ohio River at Huntington, W. Va., involved two specially designed concretes: one with 6000 psi minimum strength and one with 8000 psi minimum strength. Attaining these strength levels was markedly enhanced by the availability and use of high-range water reducers, even though the fear of non-durable concretes was also heightened. The contract for this bridge required that the contractor develop mix designs and conduct a field trial operation to "shake down" his production and placing process using these mix proportions. Tests of the concretes produced during this field trial indicated that air-entrained high-strength concretes containing HRWR easily met the minimum strength requirements, but would not withstand the freeze-thaw cycling of ASTM C 666. Paper discusses the efforts involved in developing suitable concrete proportions for this work and compares freeze-thaw durability of high-strength concretes with and without entrained air and with and without high-range water-reducing admixtures.

The effect of fly ash content on the air entrainment, freeze-thaw durability, abrasion resistance, strength gain, shrinkage, and creep of concrete was studied. Two different fly ashes were used to replace 0, 20, and 35 percent of a portland cement by weight. A blended cement, containing 20 percent fly ash by weight, was also tested. Three different air entraining admixtures were used. It was found that the use of fly ash in concrete could reduce the effectiveness of air-entraining admixtures depending on properties of the fly ash, such as loss on ignition (LOI). However, concrete containing fly ash exhibited freeze-thaw resistance equal to or better than that of similar concrete containing portland cement only, provided both had similar entrained air contents. Similarly, concrete containing fly ash showed equal or better resistance to abrasion when compared to concrete of equal strength containing no fly ash. The strength gain characteristics of concrete containing fly ash are different from those of concrete containing no fly ash. The creep of concrete containing fly ash was found to be less than or equal to that of portland cement concrete when subjected to equal sustained loads, even though the 28 day compressive strength of the concrete containing fly ash was lower than that of concrete containing no fly ash. The shrinkage of concrete containing fly ash is highly dependent on the curing given to the concrete and on environmental conditions, such as temperature and relative humidity. Not only is the shrinkage of concrete containing fly ash affected by the previously mentioned conditions differently than that of concrete containing no fly ash, but concrete containing Class C fly ash is affected differently than concrete containing Class F fly ash.

Experimental studies on the resistance of concretes to freezing and thawing in a saturated sodium chloride solution are described. The concretes were made of various types of cement differing in content of blast furnace slag. They were cured in water for 1 to 48 days and subsequently stored in air with and without carbon dioxide. Also, the effect of curing, carbonation, and type of cement on the structure of hydrated cement pastes was studied by mercury intrusion porosimetry. The results indicated that for air-entrained portland cement concretes a comparatively short curing period is sufficient to obtain high durability. Prolonged storage in water may reduce durability. Carbonation may have a positive effect. For concretes made of blast furnace slag cements, the required curing time increases with increasing slag content. For cements with a high slag content, air entrainment did not result in improved resistance to freezing and thawing, and carbonation substantially reduced it. The observed behavior of concrete specimens can be interpreted in terms of microstructural changes of the hydrated cement pastes.