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 concrete is subject to a temperature rise due to the hydration of the cement. Cube or cylinder reference specimens stored alongside a structure will not exhibit this temperature rise to the same degree and therefore will not have the same maturity as the concrete in the structure. Temperature matched curing is a method whereby cubes or cylinders of a similar maturity to the structure are produced, and is an extremely efficient method of determining minimum formwork striking and prestressing time. This paper gives a brief account of the history behind temperature matched curing in the United Kingdom, where the recent issue of a British Standard Institution document has focussed attention to the subject. Details are presented of the design and construction of a portable matching system for use on site. The system which is extremely robust, (but light enough to be carried by control cube temperatures to within -+, temperature within a structure. Results are given which show the advantages and disadvantages of such a system and illustrate the difference of maturity and hence strength between reference cubes stored in the curing bath and reference cubes placed next to the structure itself. one man), is able to 1oC of the concrete

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.

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

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.