International Concrete Abstracts Portal

Evaluates cracking resistance for concretes with compressive strengths between 60 and 110 MPa, including superplasticizers and/or condensed silica fume. Two types of concrete ring with 81 cm external diameter are tested and their shrinkage is measured over time. The first ring is cast around an aluminum ring, shrinkage-induced strain is measured, and the strains are subsequently transformed into stresses based on the theory of elasticity and knowledge of the elastic constants of aluminum. After some days, the ring breaks and the rupture stress by restrained deformation of the concrete is determined. A second concrete ring is cast, but without the internal metal ring. For this ring, measurement is made of the free shrinkage of the concrete. The value of the stresses and strains, in conjunction with the compressive and flexural strength, creep, and coefficient of hygrometric permeability (measured in other test specimens) are measured. Based upon available test data, the superplasticizer raised the mechanical strength but reduced the cracking strength of the concrete. The joint introduction of the superplasticizer, together with condensed silica fume, raised the mechanical strength of the concrete even further, but also increased its cracking resistance. To explain the test results, it is necessary to resort to the coefficients of hygrometric permeability and stress gradients, responsible for a reduction in the rupture stress of the concrete, which is higher in the first case than in the second.

Fatigue properties of high-strength concrete in compression were studied. Two types of normal-density concrete and one type of lightweight aggregate concrete have been tested. The numbers indicate the planned mean strength in MPa of 100 x 100 x 100 mm cubes. The influence of different moisture conditions was studied in an introductory investigation. Three different sizes of cylinder were tested for each of the three curing and testing conditions: in air, sealed, and in water. The tests showed that the fatigue properties of both the air and water conditions were scale-dependent, while the sealed condition was hardly influenced by the sizes of the specimens. The main investigation dealt with the influence of the variation in stress levels on the fatigue life. Test conditions with constant maximum stress levels showed significantly longer lives when the stress range was reduced. If the load levels were defined relative to the static strength, there was no obvious difference between the fatigue properties of the concrete qualities included in these tests. An additional investigation was performed on ND95 cylinders exposed to different combinations of cyclic load levels. It was found that initial cycling at lower load levels was beneficial for the fatigue life at the higher load levels. Based on the results of the experimental work, a design proposal for fatigue of concrete in compression was established.

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.

The results of 18 tests on slender composite columns consisting of rectangular hollow steel sections filled with concrete are presented. The columns had a length of 3 m and a cross section of 120 x 120 mm. They were simply supported and the load was normally applied with an eccentricity of 20 mm. As a reference, the squash load was evaluated with tests on short columns (stub tests). The purpose of this study was to evaluate the possible advantages of high-strength concrete, confining effects of composite sections, and the shear transfer at the interface. Basic parameters that varied between the tests were: concrete compressive strength, steel yield stress, and thickness of the steel tube. In additional tests, the effect of load eccentricity, additional reinforcement in the column, debonded interface, and the way of load application were examined. These tests showed that the load-bearing capacity, as well as the ductility in the ultimate state, increased for these eccentrically loaded columns.

Paper summarizes results obtained as part of a recent research program on high-strength concrete (HSC). In this research, normal density concrete (mean cube strength of 65 to 115 MPa) and lightweight aggregate concrete (mean cube strength of 60 to 90 MPa)