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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

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 paper describes a method for determining the water to cement ratio (w/c) of hardened concrete using optical fluorescence microscopy. The method is well established and has been used for many years. In Denmark the method is used for quality control of hardened concrete. The method is based on vacuum impregnation of concrete using a yellow fluorescent epoxy. During impregnation the capillary porosity, cracks, voids, and defects in the concrete are filled with epoxy. The amount of fluorescent dye entering the cement paste depends on the capillary porosity, which is determined by the w/c and the degree of hydration. After impregnation and hardening of the epoxy a thin section of concrete with a thickness of 0.020 mm (20 µm) is prepared. The thin section is analyzed under an optical microscope using a combination of a blue excitation filter and a yellow blocking filter. This is the fluorescent light mode in which epoxy filling air voids and cracks appears yellow, cement paste as shades of green, and aggregate black. The shade of green of the cement paste depends on the capillary porosity. A sample with low w/c appears dark green, i.e. has less fluorescence intensity due to a low amount of epoxy within the paste. A sample with high w/c appears light green, i.e. has high fluorescence intensity. These shades of green (fluorescence intensity) are used to determine the w/c by comparing the fluorescence intensity of the cement paste with the standards of known w/c. This paper describes the fluorescent impregnation technique, the thin section preparation, the visual determination of w/c and discusses the pitfalls in the w/c determination. Furthermore, the paper presents data from a quality assurance project and damage analysis and data of Round Robin Testing.

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 water/cementitious ratio of concrete is an important indicator of quality of concrete in the numerous ways that have been chosen to measure it. Low water/cementitious ratio concretes are high strength, low permeability, high durability and permanent concretes. It is also a property which is not frequently measured at the time of construction or on concretes which undergo distress during their service life due to the lack of standardized test methods and cheap test procedures for determining this property. In the present study, the use of electrical resistance measurements to estimate the water cementitious ratio and chloride ion diffusivity in terms of water cementitious ratio, compressive strength of concrete is explored. Both hardened and plastic concretes were studied, over a wide range of water/cementitious ratios. Supplementary cementing materials, paste volume fractions and admixture chemistries were varied as well. The results indicate that a rapid, field portable test can be used to estimate the water/cementitious ratio of plastic concrete delivered to the site, as well as samples of hardened concrete removed from structures. Use of the formation factor analogy to describe the pore system and measurements of the total pore volume allow an estimate of the transport properties of the concrete, such as diffusivity and permeability. Properties estimated by this technique are compared to those determined using the standard determination methods. The implications of using assumed pore solution characteristics are discussed. The method developed is potentially useful in both Quality Assurance and Quality Control testing of high performance structures. Additional work is required to develop a field test.