International Concrete Abstracts Portal

SP-154 In 1995, The Canadian Centre for Mineral and Energy Technology (CANMET), in association with the American Concrete Institute and other organizations sponsored a second conference on Advances in Concrete Technology. The objectives of this conference was to bring together representatives from industry, universities, and government agencies to present the latest information and explore new areas of needed research and development. Thirty two papers from 20 countries were reviewed and accepted for inclusion in this new publication based on the symposium subject, advances in concrete technology. The range of subjects is varied due to the wide range of experts involved in this project.

Presents the results of an ongoing experimental research program on creep and shrinkage behavior of high-strength concrete loaded at an early age (16 hours) and a normal age (28 days). The experiments were carried out on high-strength concrete with three types of aggregates (crushed gravel, granite, and limestone). The concretes were dried and loaded at ages of 16 hours and 28 days after casting. Loading levels with stress/strength ratios ranging from 0.15 to 0.70 were adopted in the experiments. The creep deformations were measured for a duration ranging from 90 to 210 days. The experimental results are compared in this paper with the predictions of CEB-FIP Model Code 1990, the modified MC90 model, and the model proposed by ACI Committee 209. The aging effect (in particular, at early ages) is emphasized and the influences of various factors on the aging effect are discussed.

The resistance of rice hull ash (RHA) concrete to freezing and thawing in saline environment was studied in the laboratory, for non-air- entrained high performance and normal concrete. The Swedish standard test for concrete resistance to freezing and thawing in saline environment was used. Although the number of tests was limited, the results were very promising for the use of RHA in non-air-entrained normal or high performance concrete. The laboratory salt scaling for concrete with 15 to 30 percent replacement of portland cement with RHA indicated that RHA concrete without air entrainment would be fairly resistant to freezing and thawing in most applications except for in very severe climates. No indications on an accelerated scaling rate over time was observed for RHA concrete, as opposed to the accelerated scaling rate found for a non-air-entrained high performance silica fume concrete tested.

The intrinsic nature of lightweight concrete is to produce a material which, in addition to having high strength, can also have high performance in severe service conditions. The reason for high performance is examined in light of physical, chemical, and mechanical properties of the vesicular aggregate used to produce lightweight concrete. The manufacturing process usually involves heating the aggregate to 1200 C which, in addition to rendering it more stable than conventional aggregates when concretes made from it are exposed to fire, also results in a less stiff aggregate inclusion that more closely matches the stiffness of the cement paste matrix. The use of less stiff aggregates results in a reduction in internal stress concentrations in the concrete which, in turn, leads to reduced microcracking. The role that this plays in enhancing the performance of this type of concrete is discussed in the paper. The special nature of lightweight concrete provides opportunities for design professionals. Recommendations on how best to achieve high performance concrete using lightweight aggregate are provided.

This paper covers analytical and experimental investigation of high- strength concrete beams reinforced with high-strength prestressed concrete prisms as main reinforcement. Fiber optics technology has been developed and used in this investigation to measure the flexural crack widths developed throughout the full loading history of the specimens. Thirteen beams, 8 in. x 12 in. (200 x 300 mm) is cross section and having a 9.0 ft (2.74 m) span were tested to failure. The embedded prestressed prisms had a length of 9 ft, 6 in. (2.90 m) and cross-sectional dimensions ranging between 1.5 in. x 3.0 in. (38 mm x 76 mm) and 4.5 in. x 3.0 in. (114 mm x 76 mm). The prisms were prestressed with 7-wire, 3/8 in. (10 mm) diameter, 270 ksi (1860 MPa) tendons. Concrete strength in both the prisms and the beams was in excess of 14,000 psi (100 MPa) using silica fume as a partial cementitious replacement, as well as a high-range water reducer (superplasticizer) to attain the desired workability and compressive strength. A study of the extensive data accumulated in this research program, supported by the National Science Foundation, resulted in expressions for the evaluation of flexural crack widths in ultra-high-strength concrete composite beams. Test results also showed that the embedded prisms delayed the development of cracks, while the additional use of non-prestressing steel significantly reduced the crack spacing in the beams and limited the crack width at the onset of prism cracking.