Durability is defined in ACI 116R as the ability of concrete to resist weathering action, chemical attack, abrasion, and other conditions of service. Concrete can certainly be used to construct durable, long-lasting pavements and structures; we see numerous examples of this behavior daily as we go about our lives. Durability remains an issue, however, because we also see some examples of concrete construction that have not been as distress-free, or lasted as long as we would have liked. In these latter instances, usually one or more aspects of the environment, materials and mix design and/or construction were not sufficiently considered for the impact they would have on the performance of the concrete used. The objective of this paper is to review the various aspects of concrete durability as considered by ACI Committee 201, particularly as to the demands placed on the concrete as a material. Current methods used to explain the durability performance of in-service concrete or predict the performance of concrete to be placed will also be reviewed. The aspects of durability that affect and interact to produce the durability performance of concrete may be placed in five broad categories: freezing and thawing; aggressive chemical attack; surface abrasion; corrosion of embedded steel; and the alkali-aggregate reaction. Tests used to estimate concrete durability include microscopic examination, chemical techniques, characterization of the concrete components, and accelerated testing to simulate the durability aspect under consideration. Interpretation of the performance of in-service concrete is made more difficult by the fact that usually the deterioration is not caused by one type of distress, but involves a combination of factors.

The Knoop microhardness method (ASTM 384) and the Rockwell microhardness method (ASTM E 18)-each show promise for estimating water-cement ratios of hardened paste. Tests of hardened pastes at water-cement ratios from 0.30 to 0.55 were completed. A good relationship of Knoop or Rockwell microhardness to water-cement ratio exists. The Rockwell microhardness method was done using automated image analysis equipment and was much faster. Further evaluations need to be done as follows: (1) the effect of indentation size, which can be controlled by varying the load weight; (2) rate of loading effect; (3) effects of inert and chemically active admixtures (e.g. limestone, ground granulated blast-furnace slag, pozzolans); (4) the effect of the degree of cement hydration; (5) effects of carbonation; (6) magnitude of spurious data resulting because of subsurface materials (e.g. residual cement, aggregate fines); and (7) effects of different surface preparation techniques. The microhardness method has promise as a means for estimating water-cement ratio of hardened concrete paste. It is hoped that the work completed to date will be continued by others.

The performance needs of the Toronto Transit Commission (TTC), for the reinforced concrete tunnel liner segments for the Toronto Sheppard Line Subway required a service life of at least 100 years. Under the contract specification for chloride diffusion, compliance with this requirement was determined by the water permeability and chloride diffusion coefficients of the concrete. The specified test procedures required up to six months for final test results, after a segment was cast. From a practical point of view, what is required is confirmation that the segments are acceptable shortly after they have been cast. An in-situ rate of absorption (sorptivity) test was modified and implemented. This paper summarizes the implementation of this simple test procedure into the QC programme to non-destructively evaluate the quality of the concrete shortly after fog curing is complete. The primary research goal of this project was to develop a service life model that utilized the initial sorptivity value as determined by the in-situ sorptivity test combined with a diffusion value established for the concrete mix. The use of service life modelling to predict performance of reinforced concrete structures exposed to chlorides is the next logical step to provide assurance of longevity for owners.

A laboratory and field test program was undertaken to determine the perfromance of a nuclear water/cement content gauge for fresh concrete. The laboratory evaluations included study of the effects such variables as air content, pozzolans, hold time, coarse aggregate, and temperature on gauge response. The laboratory testing demonstrated that the gauge is sensitive to materials compositions and other factors, and therefore must be calibrated with exactly the same materials as will be used on the job in question. With proper calibration in a laboratory setting, the cement gauge is capable of determining cement content of fresh concrete to within approximately 10 to 20 lb/yd3 (6 to 12 kg/m3). The water gauge is capable of determining water content to within approximately 2 to 4 lb/yd3 (1 to 2 kg/m3). Field tests at two locations are described. Favorable results were acheived where calibrations were carefully carried out using the same materials as to be used in actual construction. In these cases, avearge water content determinations for a series of samples using the nuclear gauge were comparable to those obtained using a microwave oven drying technique.The gauge is well-suited for use at construction sites. Technicians (having proper radiation safety training and certification) can successfully operate the gauge after a brief period of training, and the gauge can be transported in construction vehicles and set up on-site with a minimum of effort. The test period is short, requiring approximately ten minutes per sample, including consolidating of concrete into a test bucket.

This publication should be of interest to individuals involved in concrete failure investigations, particularly those related t o durability, and in quality control efforts aimed at assuring a durable structure. Academics, researchers, materials engineers, forensic engineers, and materials producers should all benefit from the information presented in this publication. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP191