International Concrete Abstracts Portal

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.

Discusses natural and artificial carbonation of mortar and concrete. The theoretical analyses and experimental results show that in both cases the mechanism of carbonation of mortar or concrete is the same. They are comparable when CO2 diffuses in the gas phase, the carbonation coefficient equation is Q = a1(2C1 / KP)«. The experimental results also indicate that the pores with radii over 320 A have a great effect on the diffusion coefficient, and the following relation holds: ln a1 = 105.66Ec - 0.877 where Ec is the volume of these pores divided by the total volume of the system. The results point out that there are some active sites on pore walls where CaCO3 first nucleates and the Ca++ near the pore moves toward these sites as the CaCO3 crystals grow.

It has been stated that some structures failed due to the use of alumina cement. This failure was connected with the physical and chemical changes of concrete. The reason for this effect has been studied on the samples prepared from a concrete structure that collapsed suddenly after 30 years of use without any symptoms of defects. Various methods of examination were used, e.g., chemical and thermal analysis, X-ray diffractometry, scanning electron microscopy, besides mechanical tests. The failure was attributed to a combination of two main factors. First, the hydrated alumina cement was converted and then carbonated so that gibbsite and calcite, which have slight binding properties, were formed. The highly converted and carbonated concrete lost considerable strength and could not sustain the stress in the construction.

A comprehensive review of the literature about durability (D-) cracking due to frost action in portland cement concrete pavements is developed. D-cracking is defined and described and the mechanisms causing the phenomenon are discussed. The idea that D-cracking is moisture oriented is established. It is affected by freezing temperatures that cause enough volumetric change in the moisture that exists in the cement-aggregate matrix and initiates a durability line crack in the concrete. Factors such as physical characteristics of aggregates and mortar, geographic location, maximum size of coarse aggregates, source of aggregates, and use of deicing agents are found to be among the main factors that affect the development of durability cracking. Tests to indicate frost resistance in aggregate are also reviewed. These tests are of two general types: weathering tests such as unconfined and confined freeze-thaw tests, and measurements of a physical property correlated with performance such as porosity, pore size, and absorption tests. The use of petrographic analysis is an absolute necessity to identify frost-susceptible aggregates. Other tests such as ASTM "Test for Resistance of Concrete to Rapid Freezing and Thawing" (C 666), ASTM "Test for Critical Dilation of Concrete Specimens Subjected to Freezing" (C 671), the PCA method, the Iowa Pore Index Test, and particularly ASTM "Evaluation of Frost Resistance of Coarse Aggregates in Air-entrained Concrete by Critical Dilation Procedures" (C 682) are also considered satisfactory methods to predict field durability performance of concrete aggregates. Researchers such as Axon and others, Iyer and others, and Thompson and Dempsey developed some pertinent tests that could be used in this area as well.

Permeability, a prime factor related to durability of concrete, was measured by water and gas flow in plain and reinforced concrete. The results are discussed in terms of porosity and pore size characteristics as determined by mix proportions and hydration parameters. 617-387 Development of impact-resonant vibration signature for inspection of concrete structures