International Concrete Abstracts Portal

SP229 This special publication contains the papers presented at the Fourth International ACI/CANMET Conference held in Olinda, Pernambuco, Brazil, on September 6-7, 2005. It contains 32 papers on recent advances in concrete materials and testing on topics of chemical admixtures; deformations, creep, and cracking control; durability; fiber concrete; fire resistance; nondestructive tests; quality control; strengthening of structures; structural behavior; supplementary cementing materials; and sustainability. Specific papers include: Self Consolidating Concrete, High-Performance and Normal Concrete Affected by Creep at Different Age, Curing, Load Level, Strength, and Water-Cement Ratio with some Interrelated Properties; Properties of Concrete with Recycled Concrete Coarse Aggregates; Application of Different Curing Procedures in High-Performance Concrete; and many more.

Six HPC mixtures were batched multiple times to assess the batch-to-batch repeatability and also the normality of the compressive strength results. The HPC mixtures had compressive strength values at 28 days between 61.0 to 94.3 MPa. The coefficient of variation (CV) is a useful measure of the batch-to-batch repeatability; a low CV is indicative of a high level of repeatability. At 28 days, the CV ranged from 2.10% to 8.30%. The repeatability of four of the mixtures was considered “excellent” or “very good” according to the ACI 363 standards, while the repeatability of the remaining two mixtures was considered “poor.” With five of the six mixtures, the CV was lower at 28 days than at 1 day. One of the issues most likely to have contributed to the CV of the compressive strength results was the innacuracy of determining the aggregate moisture contents. The HPC compressive strength results generally followed a normal distribution. However, some irregularity was observed in the relative frequency histograms.

Research on structural concrete incorporating high volumes of low-calcium (ASTM Class F) fly ash has been in progress at CANMET since 1985. In this type of concrete, the cement content is kept at about 150 kg/m3. The w/cm materials ratio is of the order of 0.30, and fly ash varies from 54 to 58 percent of the total cementitious material. A large dosage of a superplasticizer is used to achieve high workability. This paper presents the results of an investigation conducted to determine the flexural fatigue strength and the endurance limit (for four million cycles) for high-volume fly ash concrete and its corresponding plain concrete (control concrete). A total of 40 prisms, 20 prisms 75 x 100 x 400 mm in each concrete, were tested in flexural fatigue at a frequency of loading of 20 cycles per second (HZ). The prisms which survived four million cycles of fatigue loading were tested in static flexure (modulus of rupture). The high-volume fly ash concrete has slightly higher (7%) endurance limit than the plain control concrete. There was an increase (15 to 30 percent) in the static flexural strength (modulus of rupture) for both high-volume fly ash and plain concretes when the prisms had been previously subjected to four million cycles of fatigue stress that was lower than its fatigue strength.

The evaluation of the condition of concrete structures suffering reinforcement corrosion lacks proper methods in standards or codes. At present, it is made by using non systematic methodologies and not considering the proportion of concrete or steel section that are damaged. In this paper, a methodology is described that considers the real state of the structure and the loss in steel cross section as well as the loss on steel/concrete bond or the concrete cracked section. The methodology considers two levels of detail. The first, a Simplified Method, is based on the use of corrosion indicators and is applied to make a preliminary assessment of the structural condition or to classify different ratios of damage in a semi-quantitative manner. The second, a Detailed Method, is based on the calculation of the ultimate states considering the reduced section. In both methods three steps are considered in the assessment: inspection, diagnosis or evaluation of present state, and prediction of future evolution. In the inspection phase, the minimum amount of testing needed for a correct characterization is described. In the second step, the simplified method uses “indicators” to classify the damage level, while the detailed method evaluates how corrosion has affected the concrete-steel bond, how much steel cross section has been lost, and the extent cover cracking. Finally, a prediction is made through the determination of the corrosion rate to give guidance on the urgency of intervention. The detailed method verifies the behaviour from the application of the limit-states theory. The whole process is presented in the form of a manual for engineers.

Nondestructive test methods (NDT) have been used to evaluate concrete strength using curves that correlate the NDT measurements with the compressive strength of concrete established by a laboratory testing program. Usually the parameters that affect these curves are the water-cement ratio, the aggregate type, the maximum aggregate size, and the cement type of the concrete. This work presents a study on the correlations between the compressive strength of concrete and measurements from three nondestructive test methods: ultrasonic pulse velocity, probe penetration, and rebound hammer. The study included 30 different concrete mixtures made with materials used in the city of Rio de Janeiro. The compressive strength and nondestructive tests were carried out at ages of 3, 7, 14, 28, and 90 days. Analysis of variance (ANOVA) was used to investigate the importance of different parameters on the results of the compressive strength and nondestructive tests. Simple and multiple regression analyses of the obtained results lead to the proposed expressions for evaluating the compressive strength of concretes used in Rio de Janeiro from measurements using one or two nondestructive tests.