Bars (25 mm square) of normal-consistency paste made using Type I and Type II portland cements and pastes consisting of 70 percent cement and 30 percent of a Class F fly ash, by solid volume, at the same water-to-solids ratio, were stored under four temperature regimes: the three accelerated curing regimes given in ASTM: C 684 (warm-water, boiling-water, and autogenous) and at 23 + 1.70~. Modified-cube compression tests were made and samples of the paste examined by X-ray diffraction and scanning electron microscopy at the end of accelerated curing and at 3, 7, 28, 91, 180, and 365 days. Coefficients of determination for the regression equations average approximately 0.9 for the warm-water, 0.8 for the boiling-water regimes, and were about 0.6 for the autogenous regime. The boiling-water method affects the nature of the hydration products that are present especially by degrading the crystallinity of the ettringite. The autogenous method does not provide uniform acceleration. It was concluded that the warm-water method was to be preferred. It is now used by the Corps of Engineers. The degradation of the ettringite in the boiling water regime was accompanied by the production of hydrogarnet. The micrographs of one-day old pastes cured by all relevant regimes show very large amounts of empty space in spite of the pastes having water-to-solids ratios of 0.23 and 0.25 by mass.

The construction of the Faroe Bridges between Sealand and Falster in Denmark was completed early 1985. The project inclu-ded the placement of about 42,000 m3 of concrete for the substruc-tures of the bridges. Monitoring the curing of the concrete was an essential part of the efforts invested to assure satisfactory dura-bility of the structures. The operations included: I. Pre-calculations of the heat and strength (maturity) develop-ment after placing concrete in critical sections. The require-ments to be complied with were (1) maximum temperature, (2) maximum temperature differences in the concrete, and (3) at-tainment of sufficient maturity before form-removal. II. Site-control with continuous recording of the temperature de-velopment at essential positions in the concrete, including assessment of the strength development. Means for moderating the predetermined course of the curing, in case of unacceptab-le deviations, were used. The expenses for monitoring curing technology were deemed reason-able in view of the directly documented assurance that the requi-red quality of the concrete in the structures was obtained.

Thermal analysis was made of the effects of hydration of cylindrical configuration specimens of a slightly expansive cementitious mixture. This simulated a cylindrical borehole plug, and with modification, may be applied to other cylindrical configurations. Inputs were made to the computer program ATHENAN developed to assess the thermal history of cylindrical domains with symmetrical thermal loads. Inputs to the program were the experimental isothermal rates of heat-evolution of the cementitious mixture measured at several temperatures. Using the temperature history from ATHENAN and experimental data from the volume change and mechanical properties of the cementitious mixture in the computer program SAPIV, a stress analysis was performed, which showed that tensile stresses at the interface may arise at the early stages of hydration when the cementitious mixture tends to shrink, while small compressive stresses are present at long times in such materials when the cementitious mixture tends to expand.

The effects of the following variables on the elastic modulus versus compressive strength relationship were investigated: (w/c) ratio, duration of curing, water-to-cement paste content, silica fume content, and crushed limestone versus river gravel. From a set of empirical equations the effect of paste and silica fume content may be used to predict this relationship. The results are compared to the current ACI equation ACI 318-83). The proposed equations are applicable over a strength range from approximately 500 psi to 11,000 psi. Further it was found that for the concretes investigated the static modulus can be predicted from the dynamic modulus using the resonant frequency technique. In addition the entire static modulus Venus strength relationship for each concrete can be predicted from early age (up to 3 days) measurements of dynamic modulus and compressive strength.

The paper presents results concerning the correlations between on the one hand the compressive strength at early ages of hydrothermally treated concrete - steam cured, cured in heating moulds, autoclaved - and that hardened under standard conditions (20° C and 65 % R.H.) and on the other hand the rebound number (R), ultrasonic pulse velocity (V) and ultrasonic pulse attenuation (A). The importance in practice is due to the fact that on European construction sites some building components may be subjected to a hydrothermal treatment while others may not. In such a case, dif-ferent values of the compressive strength of concrete correspond to the same values of the nondestructively measured R, V and A. This leads to the conclusion that, in the absence of information on the conditions of curing and hardening of the concrete under investigation, the results of nondestructive tests can be very misleading. The paper discusses also the influence of concrete age on the coef-ficients of variation of the compressive strength, deduced both in a destructive and nondestructive way. Examples of the practice are used to illustrate the laboratory results.