International Concrete Abstracts Portal

During the 70’s and 80’s, University buildings in southern Mexico were made using 20 MPa compressive strength concrete, based only on strength criteria, thus, using a very high water to cement ratio. Nowadays, some of those buildings are showing problems associated with concrete durability. In this work, two concrete buildings from the College of Economy were evaluated. The College is located in an urban zone 40 km away from the coast in a region with a hot sub humid climate. Visual inspection showed concrete cracks and delaminations. Results from the evaluation showed that steel corrosion was damaging the structure, the measured carbonation front was already beyond the concrete cover, and the superficial hardness tests suggested that low quality concrete was the main cause of the problem.

The concrete cover has different characteristics when compared to the concrete confined within the structural elements. Such differences come from the absorption produced by the formwork, from the water evaporation typically pronounced in outer concrete layers, from the wall effect, etc. On account of these factors, it is expected that the concrete cover has worse characteristics in comparison to those of the inner concrete and that this aspect influences the durability as well as the performance of concrete structures. Therefore, the present work aims to evaluate the differences existing between the concrete cover and the inner concrete, taking account the analysis of chloride effective diffusion coefficients (Def) measured in two layers of a beam prototype of reinforced concrete. The study comprises two different concretes cast with W/C 0.40 and 0.55, without curing. Def of two year old specimens were experimentally obtained by means of a migration test, by determining the coefficients in two different depths: in the concrete cover, with a test specimen of 2.0 cm thick, and in a region deeper than 6.0 cm. Contrarily to the expected, the main results showed that Def values in the concrete cover were about 10% of those measured in the inner concrete. The partial carbonation of the concrete cover was the major cause to explain the reduction in chloride movement within the cover layer, since it reduces the paste total porosity.

It is recognized that reinforcement corrosion is the main distress behind the present concern regarding concrete durability. In spite of the numerous papers published on the subject, relatively few are devoted to the development of in-situ measurement techniques in general, and even less to the measurement of corrosion. However, it is known that it is important to obtain an accurate (non-destructive) in-situ identification of the zones suffering corrosion, and it is necessary to appraise the importance of this corrosion, that is, the loss in cross section and the rate of its progress. Corrosion measurement techniques are needed for identifying corroding zones, predicting the rate of corrosion of reinforcement, predicting the residual life of the structure and monitoring the efficiency of repair systems. A review is made of existing in-situ techniques, and of recent developments of new techniques, including application examples in reinforced structures. Thus, descriptions are provided for methods based on the measurement of polarization resistance using modulated confinement of the current, the use of permanent embedded sensors and the determination of the passivity state in structures cathodically protected. Comments are also provided on the needed treatment of their results with the expected evolution and integration over time to account for changes due to climatic conditions in the environment surrounding the concrete structure.

This study was aimed at investigating the influence of chemical activators on compressive strength and chloride-ion penetrability on binary and ternary concrete compositions, whose pore solution composition, electrical conductivity and total mercury intrusion volume were also determined. The chemically activated mixtures displayed higher chloride-ion penetrability and potassium sulfide was identified as the most effective activator. The study also showed that for a given mixture the influence of total porosity on chloride-ion penetrability was greater than the influence of specific conductivity of the pore solution.

This present work presents a electrochemical chloride extraction in concrete containing chlorides. The main variables in the tests were water/cement and concrete cover, which are variables easily controlled by civil engineers. Chloride penetration in the test specimens was carried out in two distinct ways, namely in a salt spray chamber and through semi cycles of drying and partial immersion. The influence of concrete cover and water/cement on the efficiency of electrochemical method was assessed by determining the initial and the final chloride contents. The test results confirm that this method extracted chloride ions successfully. On average, this method removed 77% to 91% of the content initial chloride at a distance of 0.5 cm from the surface, and 50% to 79% of the content initial chloride near the rebars.