International Concrete Abstracts Portal

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.

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

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.