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.

The DITEX computer program was developed to predict the temperature distribution in a concrete structural element during manufacture and to evaluate the resulting stresses. This program takes account of several types of boundary conditions : variable ambient temperatures, and various kinds of formwork and heat treatment. A simple and inexpensive specific test has been developed to determine the heat of hydration of the cement, the kinetics of which are a function of position. The test consists of recording the temperature during setting of a concrete sample subjected to adiabatic conditions. These tools have recently been applied to actual cases (bridges, tunnels, pressure vessels, etc.) and have demonstrated the variety of their applications: - optimization of the temperature cycle, making possible a significant reduction in fuel consumption; - better distribution of heating resistors to avoid cracks completely; - selection of the type of cement based on quantitative analysis - a judicious staging of the casting of large structures.

When the shore/reshore method of construction is used high early age short duration loads are imposed upon the supporting slabs. These loads can be calculated to be of comparable magnitude to the design service loads and are applied to concrete slabs which have not achieved their specified concrete strength. Due to the slab concrete being immature with a reduced modulus of elasticity, the immediate deflections due to the construction loads are relatively large. Creep effects are dependent upon the magnitude of the applied stress relative to the concrete developed strength. Hence creep deflections due to construction loads should be large. Deflection due to concrete shrinkage also must be considered. Five, nominally identical, one-way slabs were fabricated and subjected to different load histories to compare immediate and time dependent deformations. The load histories were modelled to represent different construction methods. All slabs were designed for a live load/dead load ratio of 0.5. Taking account of the actual moduli of elasticity, the immediate deflections were consistent. The time dependent deflections were significant and of the order of 5 to 7 times the immediate deflections. Shrinkage deflections were also greater than the immediate deflections. A method is given to determine the total long term deflection of one-way slabs in terms of the peak construction load relative to the slab strength.

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.

This paper describes an experimental investigation that was conducted to study the behavior of mortar in flexure between three and eleven hours after mixing. A total of 32 flexural tests were performed using a test apparatus that permitted specimens to be handled before final set of the mortar. The specimens consisted of mortar beams 6-inches wide x 2-inches thick x 19-inches long, which were cast and tested in flexible molds, and cured at a controlled room temperature. The tests were conducted at ages of 3, 5, 7, 9 and 11 hours, age being reckoned from the time mixing was completed. The test specimen was loaded at the middle and rested at its ends on roller supports to simulate the action of a simple beam. Load versus mid-span deflection curves were automatically recorded and then ana 1 ysed. The tolerable curvature at first cracking, the modulus of elasticity, and the flexural strength of the material at early ages were determined. The test results are summarized and presented in various forms to provide information on the behavior of mortar at early ages.