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.

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.

At present, there are no standardized test methods for the determination of water-cement ratio (w/c) of a specimen of hardened concrete. Various methods that have been used include absorption of a water drop on a concrete surface, resistance of cement paste to scratching, polarized-light microscopy, and optical fluorescence microscopy. In these determinations, usually experience and judgment are heavily relied upon. In the absence of a reliable standardized technique, it is prudent to estimate the w/c by using specimens with known w/c and a combination of several techniques, rather than relying on a single technique. The objective of this paper is to demonstrate the successes achieved by following this approach. Concrete specimens with w/c of 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70 were prepared and analyzed by visual examination, resistance to needle scratching, water absorption, and blue-dyed thin section techniques. This combined technique approach was then used to analyze concrete specimens from the field.

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

The ability of concrete to resist the ingress of chlorides, whether from deicing salts or from marine exposure, is an important factor in determining the durability of a structure. Currently in North America, there are two standardized tests for determining the ability of concrete to resist chloride penetration: the 90-Day Salt Ponding Test (AASHTO T259) and the Rapid Chloride Ion Permeability Test (AASHTO T277 or ASTM C1202). These methods have significant limitations however, mainly related to their duration and their limits of applicability. A variety of other test methods have been developed over the past decade. With the support of the U.S Department of Transportation Federal Highway Administration (FHWA), a study was undertaken to compare the more promising of these test methods and to examine their possible application as a short term test for chloride ingress, for the purpose of evaluating new mixtures and existing structures and for use as a quality control measure. Four short-term tests were conducted on eight separate concrete mixes of various qualities. The results were compared to a bulk diffusion test.