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A 4-year study, conducted by a consortium of three universities, on the use of high-performance concrete in highway applications was recently completed. A major goal of this research project was to determine if high-performance concrete mixes could be successfully produced in the field. In addition, an evaluation was to be made of the long-term performance of this concrete under field service conditions. Field installations were constructed in five states for this purpose. Paper provides potential users of high-performance concrete with general recommendations and guidelines for production and placement.

Experimental results on the bond behavior of high-strength concrete are presented. A total of 36 specimens was tested. The variables were the concrete compressive strength, the bar diameter, and the embedded length. The concrete compressive strength varied from 42 to 78 MPa (6000 to 11,000 psi). The bar diameters were 14, 16, 18, and 20 mm. The bond tests were conducted using a modified version of the Danish Standard DS 2082 pullout test in which the concrete surrounding the bar was in uniform tension. The test results indicate that the average bond stress at failure increases with the increase in the concrete compressive strength and decreases with the increase in the embedded length. The embedded length calculated using the ACI Building Code 318-89 equation caused a steel yielding failure. The predominant type of failure was the splitting of concrete; however, yielding of the embedded steel preceded the splitting failure in more than half of the specimens. It was observed that the ACI Building Code equation underestimates average bond stress for high-strength concrete. A model is developed to predict the bond strength of high-strength concrete in terms of the concrete cover, bar diameter, embedded length, and concrete compressive strength as variables. The proposed equation gave good prediction to the bond stress at failure of the pullout specimens tested in this investigation. 260-594

A full factorial experimental design was used to investigate the effects of the following variables on cylinder strength: end preparation (sulfur capping versus grinding), cylinder size (100 versus 150 mm diameter), type of testing machine (1.33-MN capacity versus 4.45-MN capacity), and nominal stress rate (0.14 versus 0.34 MPa/sec). Two levels of strength were used (45 and 90 Mpa), and three replicates were tested for each run. Specific gravities were measured to check on the consistency of cylinder fabrication. Statistical analyses indicated that all the factors had significant effects on the measured compressive strength. On average, the 100-mm cylinders resulted in about 1.3 percent greater strength, the faster stress rate produced about 2.6 percent greater strength, the ground cylinders were 2.1 percent stronger, and the 1.33-MN testing machine resulted in about 2.3 percent greater strength. There were significant interactions among the factors, so that the effects were greater than the average values for particular factor settings. For example, the effect of end preparation depended on the strength level. For the 45-Mpa concrete, there was no strength difference due to the method of end preparation, but for the 90-MPa concrete, grinding resulted in as much as 6 percent greater strength in certain cases. Analysis of dispersion indicated that the 100-mm cylinders had higher within-test variability, but the differences were not statistically significant. Recommendations for modifications to testing standards are provided.

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.

The interaction between aggregates and mortar matrix is an important factor for improving the strength and toughness of high-strength concrete. Experimental investigations were carried out on the influence of aggregate properties on fracture energy G F of high-strength concrete. The influence of mortar matrix strength and volume fraction of coarse aggregate on fracture energy has been tested by three-point bending in Mode I loading. The interaction between aggregates and mortar matrix was estimated by the energy balance concept on the fracture energy of aggregate inclusion G Fi and that of mortar matrix G Fm. According to the test results, G F is strongly influenced by strength properties and volume fraction of aggregates. It was clear that the improvement of toughness of high-strength concrete is obtained by using high-strength aggregate and by designing a large volume fraction of coarse aggregate.