International Concrete Abstracts Portal

Compressive strength and water penetration of three grades of high-strength concretes with cement contents ranging from 400 to 500 kg/m3 and a proprietary superplasticizer are reported. The control mixes were redesigned by adding a Class F-type fly ash at fly ash/cementitious ratios of 0.15 and 0.35. All concretes were designed for a similar workability. The strength development was monitored in three curing regimes. It is concluded that the superplasticized concrete developed a higher strength than that predicted from a reduction in the water/cement ratio. The curing conditions significantly influenced the strength development and the water penetration of the concretes. An optimum fly ash/cementitious ratio of 0.15 was found to be appropriate for the concretes; larger amounts of fly ash were found undesirable for higher strength development.

Six reinforced concrete beams and four slabs with different reinforcement ratios were tested to failure. The behavior of specimens manufactured with normal strength concrete (fc = 36 MPa) and high-strength concrete (fc = 83 MPa) was compared with respect to cracking and deflections. It was found that crack widths and crack spacings were fairly comparable for both concrete types in the region of stabilized cracking. Deflections decreased by using high-strength concrete due to the increased modulus of elasticity and cracking moment. However, for the beams, this gain diminishes at higher load levels.

A series of 28 reinforced concrete beams without shear reinforcement have been tested in shear by two-point loading. The main test parameters were: longitudinal reinforcement ratio (1.8 and 3.2 percent); shear span ratio (2.3, 3.0, and 4.0); size (b/h = 150/250 and b/h = 300/500 mm); and concrete type (normal density concrete of cylinder strength 54, 78, and 98 MPa and lightweight aggregate concrete, 58 MPa). The results are compared with other test results and concrete codes. For members made of normal density concrete of compressive cylinder strength exceeding 80 MPa, the diagonal cracking strength remained constant or showed a minor decrease in spite of the increasing tensile splitting strength of the concrete. A more significant decrease in ultimate shear strength was observed. A probable explanation is the increasing brittleness of the material with increasing strength. The new Norwegian Concrete Code, which includes provisions for high-strength concrete, predicts the influence of concrete compressive strength and aggregate types on the diagonal cracking shear strength fairly well. The influence of dimensional scale was, however, larger than expected. The shear strength formula in CEB-FIP Model Code generally overestimates the diagonal cracking strength of high-strength concrete slabs or beams with moderate longitudinal reinforcement ratios. An improved shear strength prediction formula for high-strength concrete has been adopted by the Norwegian Code. The lightweight aggregate concrete beams had relatively low diagonal cracking strength, as expected, but high ultimate shear strength. The tests confirm the results (except for one test series) found by Ahmad et al.

An equation is proposed for predicting the ultimate shear capacity of reinforced concrete columns and beams composed of high-strength concrete having a compressive strength of up to 90 MPa, and high-strength reinforcing bars having a tensile strength of 1000 MPa. Six beams and ten columns with and without shear reinforcement were tested to determine their diagonal cracking strengths and ultimate shear capacities. The shear span-depth ratio was 1.0 for the beams and 1.14 for the columns. The quantity pw åy (pw: shear reinforcement ratio; åy: yield strength of shear reinforcement) was varied from 0 to 11.2 MPa. The axial stress in the columns was varied at 0, 18.4, and 36.8 MPa. The current ACI Building Code equation for predicting shear capacity of deep beams was found to be applicable to the beams fabricated with high-strength concrete. However, it cannot be applied to the members with high axial load stress. The equation proposed in this paper accurately predicts the ultimate shear capacity of reinforced concrete columns as well as the beams made with high-strength concrete and high-strength steel bars.

A total of 21 test specimens with lapped reinforcing bar splices were tested using concretes with compressive strengths in the range of 21 to 99 MPa. For each test specimen, the concrete compressive strength, splitting strength, fracture energy Gf was determined. It was found that fracture energy of concrete appears to have a strong influence on the strength of lapped tensile splices. A comparison of the experimental results and computed values using the regression analysis equation of Orangun et al. based on a large number of tests from USA showed that the equation may be unconservative in cases of lapped splices in high-strength concrete.