High-early-strength-concrete (HESC) made of Type III cement reaches approximately 50-70 % of its design compressive strength in a day in ambient conditions. Experimental investigations were made in this study to observe the effects of temperature, curing time and concrete strength on the accelerated development of compressive strength in HESC. A total of 210 HESC cylinders of 100 9 200 mm were tested for different compressive strengths (30, 40 and 50 MPa) and different curing regimes (with maximum temperatures of 20, 30, 40, 50 and 60°C) at different equivalent ages (9, 12, 18, 24, 36, 100 and 168 h). From a series of regression analyses, a generalized rate-constant model was presented for the prediction of the compressive strength of HESC at an early age for its future application in precast prestressed units with savings in steam supply. The average and standard deviation of the ratios of the predictions to the test results were 0.97 and 0.22, respectively.Keywords: high-early-strength concrete, compressive strength, curing temperature. Age at the start of strength development at the reference temperature
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A theoretical model was developed for predicting the flexural load-deflection of simply supported concrete beams internally prestressed with unbonded carbon fibre reinforced polymer (CFRP) tendons under four-point loading. At each state in the load-deflection curve (initial cracking state, initial yielding state (if any) and ultimate load state), three equivalent rectangular curvature blocks, which can closely simulate the beam rotations at the supports and the midspan beam deflection, were determined by iteratively correcting the strain value of the unbonded tendon. The validity of the model was demonstrated by comparing its predictions with the test results of four rectangular beams and two T-beams with different prestressing reinforcement ratios, initial prestressing and type of auxiliary bar. The developed model was used to examine the effects of different parameters on the first cracking and ultimate strengths of the beam and the additional strain of the unbonded tendon to the effective strain.
Experimental observations were made on eight reinforced concrete wall specimens, with all sides being exposed to an International Organization for Standardization standard heating curve. The main variables were wall thickness, level of axial load, reinforcement ratio, concrete compressive strength and curing period. During the test, temperature increases in the wall, and the heating time plotted against axial deformations, were measured. The effects of the main variables on the fire resistance and axial deformation of the wall in fire were investigated. The walls with shorter curing periods and the wall of high-strength concrete with a thin wall thickness under concentric load were more susceptible to the loss of fire resistance. Initial axial extension followed by contraction to failure was observed for relatively thin walls that were subjected to an axial load during the fire test. Based on comparisons between the experimental observations and code provisions, it was suggested that the incorporation of load levels and the exposure condition of the wall in fire could improve the accuracy of estimation of the fire resistance.
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