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.

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 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.

A study was conducted to determine if the current design formulas can adequately predict the capacity of reinforced concrete beams tested at early ages and to determine the effectiveness of stirrups in young beams. Three sets of reinforced concrete beams were tested-with the main variables being beam age and stirrup spacing. It was found that the ACI-318 ultimate strength design method provides a lower bound to flexural strength of young reinforced concrete beams. For beams failing in shear, however. the current code provisions appear unsatisfactory. For example,the contribution of shear reinforcement was observed to be less than that predicted by the code when beams failed in shear at early ages. A change in mode of failure was often observed at early ages and the ductility of young reinforced concrete beams , was less than that for the 28 day old beams.

Early age history of concrete is crucial to determining how crete will be. Knowledge of the environmental effects on concrete at early ages is needed to predict concrete deflections and crack resistance. Properties of Concrete at Early Ages offers the information needed on properties of concrete at early ages for the engineer to ensure safe construction practices. Various topics presented include: field control and monitoring of concrete strength gain, the early-age behavior of reinforced concrete members, and mechanical properties in young concrete. Other topics are: early age shear strength, effect of early age construction loads on long term behavior of slab structures, and effect of accelerated curing. 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. SP95