International Concrete Abstracts Portal

The results of an experimental investigation on the behavior of one-way reinforced concrete slabs strengthened in flexure with a 5-cm concrete overlay are presented. Shear transfer at the concrete interface was provided in two separate ways: bond between the overlay and concrete substrate and dowel action only through shear connectors. Four series of slabs were tested: Series 2 and 3 were strengthened; Series 1 and 4 were cast at one time and had the geometry corresponded to the slabs before and after intervention, respectively. During the experiments, displacements, interlayer slips and strains were measured. The test results indicate expressively the better performance of slabs strengthened with a bonded overlay. A design procedure for evaluating the behavior of both repair procedures in service and at failure is proposed. Good correlation was also achieved.

This study was aimed at evaluating an industrial production system of rice husk ash (RHA) with low carbon level. The system, developed in partnership with a local industry, consists of a continuously rotating furnace that can reduce the carbon level of residual RHA through re-burning. Different temperature bands, retention time in the furnace interior, and grinding degree were studied. The results indicate the feasibility of producing a material with reduced carbon levels, which is highly reactive and can be commercially used in the production of high performance concrete.

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.