International Concrete Abstracts Portal

This paper reports the results of a study on the resistance of lightweight aggregate concrete to the penetration of chloride ions. Concrete specimens were fabricated with a blended silica fume cement at a water-cementitious materials ratio of W/CM = 0.40 or 0.30 and with combinations of aggregate as follows: (i) limestone coarse aggregate and river sand, (ii) expanded slate coarse aggregate and river sand, or (iii) expanded slate coarse and fine aggregate. A further series of mixes was made using the latter combination of aggregates with the blended cement being partially replaced with 25, 40 or 56% fly ash. Concrete specimens were subjected to a series of tests including “rapid chloride permeability” (ASTM C 1202), and non-steady-state diffusion (bulk diffusion test). Tests were conducted at 28 and 56 days, and 1 and 3 years. The results up to one year clearly show the benefits of incorporating expanded slate in the concrete, with permeability and diffusion coefficients being reduced significantly. The improvements attributed to the presence of the lightweight aggregate appeared to increase with the maturity of the concrete and, after 3 years continuous curing, the reduction in the apparent chloride diffusion coefficient was observed to be as much as 70%. As expected, the addition of fly ash produced further reductions in permeability and diffusion. The data developed in this study were used as input parameters for service life predictions. Although, there are insufficient data to allow firm conclusions to be drawn from these analyses, it is clear that the incorporation of lightweight aggregate will lead to a significant extension of the service of life.

This paper deals with a numerical modeling of concrete carbonation, based upon durability indicators (DIs), within the framework of a durability approach. Firstly, the methodology and the selected panel of universal DIs concerning carbonation are presented. Secondly, with the purpose of protecting structures against carbonation-induced corrosion, a model accounting for the coupled CO2-H2O-ionic transports, the carbonation reactions of Ca(OH)2 and C-S-H, the pH decrease, and the microstructure evolution is described. In this model, the DIs porosity, initial Ca(OH)2 content, and liquid water permeability are introduced as major input data. Complementary parameters are also used: Ca(OH)2 crystal size, C-S-H content and capillary pressure curve. The main numerical outputs are the carbonation kinetics, the residual Ca(OH)2 and newly formed CaCO3 content profiles, and the pH value. As a first step, the model is validated with accelerated carbonation data obtained on a cement paste and on three porous concretes. The carbonation depth and profiles, measured by means of phenolphthalein spray test and thermal analysis respectively, are in good agreement with the numerical simulations. The study is completed by a sensitivity analysis. The model, together with the test methods required for the assessment of the relevant DIs, could be included in a toolkit for durability evaluation and prediction of carbonation-induced corrosion of real structures.

The dimensional stability of cement based materials (mortar or concrete) may be improved through the use of shrinkage reducers or expansive agents. In this study the combined use of a propylene glycol ether based shrinkage reducer (SRA) and a calcium oxide based expansive admixture has been investigated. Mortar and concrete specimens (prepared without admixtures or with SRA or EXP or SRA and EXP) has been compared through compressive strength determinations, free drying shrinkage, restrained shrinkage and restrained expansion measurements. A synergistic effect on the shrinkage reduction has been observed when the shrinkage reducing admixture and the expansive agent have been used together. In order to elucidate such phenomenon, the hydration of cement pastes containing these kinds of admixtures has been followed by ESEM-FEG (Environmental Scanning Electron Microscopy – Field Emission Gun) and specific surface area measurements.

In the current work, an extensive chloride-profiling programme was taken over a seven year period on a series of nine concrete monoliths placed at a marine location. These monoliths were 2.0m high and octagonal in plan with each vertical face 0.66m wide. The monoliths were placed at predefined locations to represent environmental exposure conditions of XS1 (exposed to airborne salt and not in direct contact with sea water) and XS3 (tidal, splash and spray zones) as defined with European Standard EN206-1. The concrete monoliths were constructed in groups of three (one each at the locations defined above): one group, which was used as a benchmark, represented normal portland cement concrete; the second group of monoliths was treated with waterproofing agent (caltite) added at the time off mixing and the third was treated with silane. Chloride profiles were taken at a number of positions on each monolith which were subsequently used to evaluate the performance of the concrete to chloride ingress for different exposure conditions.

Different types of carbonaceous materials have been added to concrete mixes and their effect on the mechanical properties and the corrosion of embedded steel have been studied. Using a constant water/cement ratio of 0.42 the flexural and compression strengths of concrete with different amounts of carbonaceous materials and different curing periods have been determined. Also, the effect of adding some amount of silica fume to the mix formulation has been considered. The addition of small quantities of carbonaceous materials to the mix produces an increase of mechanical strengths and a reduction of the concrete permeability. Due to this smaller permeability the corrosion levels of embedded steel are lower as compared to the ones in an admixture-free mix, in spite of the higher electrical conductivity of the composite.