International Concrete Abstracts Portal

SP-228CD This CD-ROM of Special Publication 228 contains the papers presented at the Seventh International Symposium on the Utilization of High-Strength/High- Performance Concrete that was held in Washington, D.C., USA, June 20-24, 2005. The symposium continued the success of previous symposia held in Stavanger, Norway, (1987); Berkeley, California (1990); Lillehammer, Norway, (1993); Paris, France, (1996); Sandefjord, Norway, (1999); and Leipzig, Germany, (2002). The symposium brought together engineers and material scientists from around the world to discuss topics ranging from the latest applications to the most recent research on high-strength and high-performance concrete. In the years since the first symposium was held in Stavanger, there has been worldwide growth in the use of both high-strength and high-performance concrete. In addition to more research and applications of traditional types of high-performance concrete, the use of self-consolidating concrete and ultra-high-performance concrete has moved from the laboratory to practical applications. This publication offers the opportunity to learn the latest about these developments.

The water/cementitious material ratio (w/cm) remains an essential, descriptive statistic for today’s increasingly complex HPC mixtures. The water/cement ratio (w/c) is also useful. A sample of 125 high performance concrete (HPC) mixtures of various materials and proportions was fitted with linear regression models relating compressive strength at ages of 1, 28 and 56 days to the w/cm and/or w/c. It was observed that strength generally increased as the w/cm or w/c was lowered. But linear regression models using a single independent variable, either the w/cm or w/c, failed to return a coefficient of determination, R2, more than 0.535. It was learned that the w/c provides a stronger indication of strength at 1 day. By 28 and 56 days, because of pozzolanic activity, the w/cm becomes a better indication of strength. Multiple linear regression models using both the w/cm and w/c capture more of the variability in the data.

The effect of air entrainment on the compressive strength of high performance concrete is presented in this paper. Generally, an increase in the total air content of one percent decreases the compressive strength of concrete two to five percent. This rule of thumb was developed from research on normal strength concrete, but there is little data on the strength reduction due to entrained air in high performance concrete. The paper presents compressive strength test results of several high performance concrete mixtures with total air contents ranging from two to six percent. The compressive strength of the mixtures varied from 42.4 MPa (6150 psi) to 95.9 MPa (13,900 psi). The results of the study support the use of this rule of thumb for high performance concrete. Data are also presented on the increased dosage rate of air entraining agents required in low water to cementitious material ratio concrete.

As with other mineral admixtures, the use of rice-husk ash leads to an improvement of the concrete internal structure, reducing the pore size and particularly an improvement in the interface bond. In this sense it can be assumed that the failure mechanism can be modified, and the concrete will exhibit a more brittle behavior. That has a special interest in high-strength concrete and in the design of large concrete structures. This paper focuses on the fracture behavior of rice-husk ash concrete. A wide range of concrete strengths are analyzed including normal and high-strength mixtures. The flexural behavior was analyzed following the general guidelines of the RILEM 50-FMC using a center-point loading arrangement on notched beams of 400 mm span, measuring deflections and the crack mouth opening displacement (CMOD). In addition, the compressive strength and the elastic modulus were measured on standard cylinders. The effects of water-cementitious material ratio and the age of testing on the strength, energy of fracture and the characteristic length on concretes with and without rice-husk ash incorporation are discussed.

The prediction of concrete properties using computer modeling is becoming widely used, and many models utilize the concept of maturity. The original maturity concept was developed empirically from the study of the temperature effect on the compressive strength of normal-strength concrete. The use of the maturity concept for estimating the development of properties other than compressive strength has not yet been sufficiently validated with experimental data, especially for high-performance concrete. This paper presents a study of the maturity method for predicting the development of key properties of high-performance concrete, such as compressive strength, splitting tensile strength, modulus of elasticity, and level of cement hydration. The derivation of the maturity method is explained and experimental evidence, collected under different temperature conditions, is presented and discussed. The results were used to study the activation energy, which is a governing parameter of the Arrhenius maturity formulation, for predicting the key properties of high-performance concrete. Recognizing the need for a more accurate determination of the activation energy for each concrete property, a new practical approach for calculating the Arrhenius maturity index is proposed.