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 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.

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.

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