International Concrete Abstracts Portal

Sixteen agencies of the U.S. federal government have developed an interagency proposal for promoting the use of high-performance concrete and other materials for use in the nation's infrastructure. They are working jointly with the Civil Engineering Research Foundation (CERF) to enlist private sector support for sponsoring a research and development program aimed at getting the materials into use. CERF is drawing upon the technical community, such as that in ACI, to define the various research needs and studies that will lead to materials acceptance. Materials other than concrete are addressed in other parts of the total program. Workshops were held in the spring and fall of 1993 to develop schedules and priorities. A tentative cost for the concrete program is approximately $200 million over 10 years, which includes some technology transfer and which would be expected to be matched by some private sector funding.

The use of high-strength lightweight concrete (HSLWC) in offshore oil and gas platforms is becoming more common. The constant wave action on these structures imposes continual fatigue loading on the concrete. Paper reviews previous research on both compressive and flexural fatigue behavior of HSLWC. The fatigue behavior of HSLWC is comparable or somewhat better than high-strength normal-density concrete (HSNDC) tested under the same conditions. The cyclic strain behavior of HSLWC is significantly different than for HSNDC and there is little change in strain behavior with increasing cycles of load until failure occurs. The fatigue life is reduced when the concrete is tested in submerged conditions. There is no significant difference between the S-N curves for reinforced and nonreinforced concrete. The mechanism that causes HSLWC to have comparable or better performance than HSNDC is attributed to the improved microstructure of the matrix-aggregate interface. This improvement reduces microcracking that typically leads to fatigue damage. The effect of crack blocking by sea salt depositions is discussed.

Bond reinforcing bars and concrete under impact loading were studied for both plain and steel fiber reinforced concretes. Experiments consisted of both pullout tests and push-in tests. The design compressive strengths of the concrete were 40 MPa (normal strength) and 75 MPa (high strength) at 28 days. The impact loading induced bond stress rates ranging from 0.5 x 10 -4 to 0.5 x 10 -2 MPa/sec. The bond under stress rates ranging from 0.5 x 10 -8 to 0.5 x 10 -4 MPa/sec was also studied for comparison. Each reinforcing bar was instrumented with five pairs of strain gages to monitor the actual strains during the bond-slip process. All test data were collected by a high-speed data acquisition system at a sampling rate of 200 sec. Stress distributions in both the steel and concrete, bond stresses and slips, bond stress-slip relationships, fracture energy in bond failure, and internal crack development were investigated. It was found that compressive strengths increased the bond-resistance capacity and fracture energy in bond failure, and therefore had a great influence on bond stress-versus-slip relationship. This effect was increased by high loading rates and steel fiber additions, especially for the push-in loading mode.

In general, the structural member using high-strength concrete is accompanied by high brittleness, which may result in the unexpected dangerous failure. For economy and safety, high-strength concrete may be used for compressive members (vertical members) and low-strength concrete for flexural members (horizontal members). ACI 318-89 recommends that when the specified compressive strength of concrete in the column is greater than 1.4 times that specified for the floor system, the column concrete shall extend 600 mm into the slab from column face to avoid unexpected failure. The structural behavior of beam-column joints with two different compressive strengths of concrete for the beams and the columns has not been investigated adequately. ACI-ASCE Committee 352 recommends that for joints that are part of the primary system for resisting seismic lateral loads, the sum of nominal moment strengths of the column sections above and below the joint ( M c), calculated using the axial load, which gives the minimum column moment strength, should not be less than 1.4 times the sum of the nominal strengths of the beam sections at the joint ( M b). Thus, those recommended values should be examined before high-strength concrete can be used with confidence and convenience in structural members. The results showed that the ACI 318-89 extension distance of 600 mm is safe at least for members up to 300 mm in total depth, and the 2h (h is overall depth of the beam) extension distance was found to be safe also for members under flexural loading with a column-to-beam flexural strength ratio of 1.8.

An experimental investigation was conducted on the shear behavior of deep beams made with steel fiber reinforced high performance concrete (HPC). Twenty-six beam specimens with various shear span-effective depth ratios, steel fiber contents, amounts of vertical and horizontal web reinforcements were tested under static loads. In addition to the strength test, extensive instrumentations were designed for the measurements of average strains of reinforced concrete in the shear span and strains of web reinforcements. The web-shear cracking initiated as the first inclined shear crack. About 30% increase in the inclined shear strength and 25% increase in the ultimate shear strength can be achieved with addition of 1 .O% steel fiber for specimens having a/d= 1 .5. The strain of vertical web reinforcements became negative and the horizontal web reinforcements were stretched to yield state for specimens having a/d ratios approach 0.5. The measured load-deformation relationships of reinforced concrete and strains of web reinforcements were compared with the prediction of the softened truss model of steel fiber reinforced concrete proposed by other investigators. Good correlation was found from the comparisons.