International Concrete Abstracts Portal

The hardening of concrete mixtures is caused by chemical reactions occurring in the Portland cement paste fraction of the mixture. The extent of these reactions is related to the non-evaporable water content in the reaction products, and determine hardened properties of the concrete mixture. High-performance concrete mixtures often contain cementitious materials in addition to portland cement that increase strength and /or durability. These cementations materials can modify the rate of reactions and thus the development of mechanical properties. This work studied non-evaporable water content in low w/c mixtures with and without silica fume in relation to the development of compressive strength and ultrasonic pulse velocity over time. Results were compared with those for a conventional mixture with a moderate w/c. It was observed that the long-term non-evaporable water contents were higher for the conventional mixture than for the low w/c mixtures. The addition of 10% silica fume in the low w/c mixtures caused an even lower long-term non-evaporable water content. This addition of silica fume did not affect the relationship between non-evaporable water content and ultrasonic pulse velocity, but did affect the relationship between non-evaporable water content and compressive strength beyond a certain degree of cement hydration.

The impact of a Shrinkage Reducing Admixture (SRA) on the properties and performance of conventional concrete was investigated. The SRA studied was a commercially available glycol ether blend. Tests were conducted on two structural air-entrained concretes commonly used in bridge deck construction. The SRA dosage rate was varied between 1.0% to 2.0%, by weight of cement. Influence of the SRA on fresh concrete properties, compressive strength development, unrestrained shrinkage, and freezing and thawing durability was examined. It was determined that the SRA has a slight effect on initial workability and a moderate impact on compressive strength development. Free shrinkage was reduced on the order of 50%. The freeze-thaw durability of some SRA mixtures was found to be below generally accepted limits. This behavior was traced back to problems associated with maintaining entrained air. It was discovered that the SRA mixtures tended to lose air more rapidly than the control mixtures. Further, petrographic analyses suggest that the air content in hardened SRA concrete may be noticeable less than that measured in fresh concrete. Strategies to over come this problem are proposed.

High strength concrete (HSC) typically exhibits improved surface abrasion resistance, reduced chloride penetrability, and improved resistance to freezing and thawing damage. For these reasons, HSC use in transportation structures is increasing due to the potential for increased service life. Although several potential benefits are associated with the use of HSC, these mixtures may exhibit increased sensitivity to early-age shrinkage cracking. In addition to weakening the structure, cracks increase the rate at which corrosive agents can penetrate the concrete, thereby accelerating the potential deterioration of the reinforcing steel and concrete. For this reason, it is essential that the concrete which is used to build transportation structures exhibits sufficient resistance to early-age cracking, in addition to the aforementioned benefits, to produce durable structures. The objective of this paper is to demonstrate that a holistic design approach is required to specify material composition for durable concrete structures. Experimental results and theoretical modeling predictions are used to illustrate the characteristics of higher strength concrete that result in increased cracking potential. Theoretical simulations demonstrate the role of both material properties and the surrounding structure on the tensile residual stress developemnt and cracking potential. In addition, results demonstrate that a shrinkage-reducing admixture (SRA) may be used to decrease the potential for early-age shrinkage cracking in HSC while sustaining the advantageous mechanical and durability properties associated with HSC.

The shear stress detrmined using ACI 318-95 is limited to the stress determined for 10,000 psi (69 MPa) concrete. This limit has been set because it was considred that there was not enou data to justify the extension of existing equations to higher strengths. Modification are suggested which adapt the ACI equations so thy apply to the full range of concrete strengths. The proposed equations predict the shear strength of beams more accurately than the current ACI equations. For the beams investigated, the coefficient of variation is only 2/3 of that obtained using the ACI equations. The proposed equations apply to concrte stengths as high as 18,000 psi (124 MPa). Aoart frin tgeur simplicity, the proposed equations have a number of other advantages. Unlike ACI equations 11-1 to 11-3, they correctly predict the increased strength of beams with small shear-span to depth ratios. Because the proposed equations assume the most pessimistic location of cracks, the maximum stirrup spacing requirements are satisfied sirectly so that adtitional spacing limitations appear to be extraeous. To illustrate the improvement that can be obtained using the proposed equation, published test results are compared with predicted values.

ACI Code-95 recommendations for column design are based mostly on test results on concretes with strengths up to 42 MPa. Due to differences between high-strength concrete (HSC) and normal-strength concrete (NSC) material and structural behavior, using these recommendations for HSC columns does not mean that the same level of safety as for NSC is obtained. As a consequence, the reliability that the same level of safety as for NSC is obtained. As a consequence, the reliability of HSC columns must be assessed. Since most of the variables involved in column design (material properties, geometric characteristics, loads, etc.) are random, a basic step in the reliability assessment of HSC columns is the modeling of uncertainties associated with both column strength, as well as, load effects is presented. Regarding the computation of the statistics of the HSC column strength, many issues have to be resolved: (a) the scarcity of information on the variability of the compressive strength of HSC; (b) the unavailability of a closed form solution to express column strength; and © the compatibility with the assumed failure criterion. Regarding the computation of the load effect statistics, special attention given to the case of slender HSC columns.