International Concrete Abstracts Portal

Calcium chloride is an excellent accelerator for concrete, improving both setting time and hardening rate. However, it can not be used for reinforced concrete in efficient dosages because of initiation of rebar corrosion. In the search for other bulk chemicals working as accelerators, calcium nitrate has proven to be a good candidate as set accelerator; however it is not a hardening accelerator (e.g. improving 1 day strength). Efforts have been made to combine calcium nitrate with other admixtures in order to make the mix work as a combined chloride-free setting and hardening accelerator. This paper sums up research over the last 10 years regarding this matter and also compares performance with other chemicals.

Some protective techniques, such as anti-carbonation coating, provide the concrete a protection due to the creation of a surface barrier that prevents the penetration of CO2 and moisture. However, these procedures are not always so effective when the carbonation process moves as a front through the concrete pores, reaching the reinforcement. In this case, the procedure often adopted is the repair technique, which can include the anchorage of the structure, the entire removal of the carbonated layer and localized or general repair. The purpose of this research was to study the non-destructive repair technique referred as realkalisation by the absorption and diffusion of alkaline solutions. In order to provide evidence for further analyses of the technical viability, besides the main realkalisation testing, some additional tests such as compressive strength, capillary absorption, electrochemical techniques and mortar adherence on realkalised substrates, were performed. The results obtained show that realkalisation through direct contact of the alkaline solution on the concrete surface is effective in re-establishing the high pH of the concrete, although it presented a slight decrease in the compressive strength. The realkalisation process does not interfere in the mortar adherence to realkalised substrates.

Lightweight concrete could be producing by incorporating expanded polystyrene in the conventional concrete. This paper discusses the results of an experimental investigation into the effects of binder materials on the engineering properties of polystyrene aggregate concrete (PAC), having the nominal density of 1800 kg/m3. Four types of binders, namely, general purpose cement, shrinkage limited cement, a combination of 60% general purpose cement and 40% low calcium fly ash, and blended cement with 62% granulated blast-furnace slag, were used. The results showed that as expected, the use of supplementary cementitious materials such as fly ash and slag reduced the early-age strength of the polystyrene aggregate concrete. The strength development of PAC was also affected by the curing condition. Although the use of shrinkage limited cement reduced the shrinkage of PAC by 12%, it increased the creep potential by 76%. The paper also discusses the relationships between: shrinkage and moisture loss; strength and rebound number; and strength and pulse velocity.

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.