International Concrete Abstracts Portal

A test method was developed to predict the risks of ASR expansion for actual field concrete compositions. Concrete prisms (7 x 7 x 28 cm) were cast, demolded, and measured for their initial length. They were then immersed in an alkaline solution at 150 C for 3 weeks in individual stainless steel containers and their lengths were monitored weekly. The concentration of the solution was adapted to match as closely as possible the composition of the interstitial pore solution by summing up the contribution of the binder constituents to the effective sodium and potassium contents, respectively. Despite this fact, the alkali balance before and after treatment of the prisms shows that the concrete is enriched in Na 2O and/or K 2O during the cure. The expansions reached at 150 C after 3 weeks were compared to those obtained at 100 percent relative humidity in air at 38 and 60 C after 12 and 4 months, respectively. Good correlations were obtained for the 67 different concrete mixes tested. Consequently, an expansion value of 0.11 percent was proposed as a provisional limit, which is rather conservative. Reaction products were studied by SEM, optical microscopy, and electron microprobe as gels, and semi-organized and crystallized compounds, and were shown to be present in cracks and pores as well as within the paste. The composition of the products formed at 150 C seems to be restricted to a narrower range of Ca, Si, Na, and K concentrations than what has been reported at lower temperatures.

Petrographic examination and the South African mortar bar test have been performed at SINTEF--Structures and Concrete during the last 2 to 3 years to evaluate the reactivity of Norwegian aggregates to be used in concrete structures. Paper presents the relationships between these two test methods. The purpose of the petrographic examination is to identify, quantify, and group different rock types in an aggregate. These groups are: reactive (with known reactive field performance), potentially reactive, and innocuous aggregates. In Norway, further testing by the mortar bar test is recommended when petrographic examination indicates 20 percent of reactive or potentially reactive rock types in the aggregates. The mortar bar expansion after 14 days of exposure is used for the evaluation of potential expansivity of the aggregates. One main conclusion from the investigation is that mortar bar expansion increases to an upper level with increasing content of reactive rocks in the aggregates. Beyond a "marginal" amount of reactive rocks in aggregates, the mortar bar expansion increases no further. A significant difference in mortar bar expansion between different reactive rock types has not been found. The established limit of 20 percent of reactive rocks in aggregates appears, in most cases, sufficient for classifying aggregates as innocuous; however, no verification of the limit has been made.

The long-term durability of a number of special types of concretes has been tested by exposing various large-scale concrete specimens to different outdoor conditions. The concretes tested include concretes made with different types of cement and incorporating various quantities of fly ash and silica fume (microsilica) as well as mixtures thereof. In one test series, full-scale precast concrete units were installed as functional members of a fish-ladder. The units, comprising concrete with up to 50 percent of silica fume (related to the cement content), are subject to the action of lake water and freezing and thawing. They have been in service for nearly 15 years, and the results demonstrate the excellent durability of concrete with silica fume. In another test series, concrete panels and slabs are exposed to Danish outdoor climates (freezing and thawing during winters) in connection with frequent use of deicing salts. The test series also comprises large-size panels, installed in a harbor at the west coast of Denmark. For this test series, 10-year results are now available.

Corrosion of reinforcing steel is a major cause of concrete deterioration and, consequently, of loss of serviceability of concrete structures. Presents the results of a laboratory investigation to assess the effects of fly ash on the resistivity and rate of corrosion of reinforcing steel in concrete. Environmental exposure conditions were simulated in the laboratory, and corrosion tests were carried out on specimens corroded naturally or under accelerated conditions. Results show that fly ash is a very effective addition to improve the resistivity of concrete and to reduce the rate of corrosion of reinforcing steel. The resistivity of fly ash concrete is approximately double that of the resistivity of an equivalent normal portland cement concrete. Results are used to propose a model relating resistivity, porosity, and permeability of concrete with the rate of corrosion of reinforcing steel.

In Australia, the Cement and Concrete Association has sponsored a number of research projects addressing aspects relating to deterioration of concrete structures caused by corrosion of the reinforcement. The overall objectives of these projects were to identify the factors influencing steel corrosion and to quantify their effect on initiating corrosion and on the rate of corrosion. The ultimate objective was to provide practicing engineers with the relevant parameters that can be used in the design and specification of concrete structures to minimize the risk of corrosion of reinforcing steel. Reviews the major corrosion research carried out in Australia. Article attempts to correlate research findings to the conditions in practice and to quantitatively predict design life of reinforced concrete structures in an environment simulating severe exposure conditions in Australia. The design life predictions presented should be considered within the context of the assumptions and approximations made. Data presented in this paper showed that the influence of binder type is more in the medium-strength concretes in terms of time to potential jump (initiation) and corrosion rate (propagation). Therefore, it is recommended to optimize concrete mixture proportioning with respect to binder type in this range of concrete strengths to utilize the benefits possible from different binders.