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 dry shake hardeners (60% of quartz-based sand and 40% of portland cement), applied on the top of concrete industrial floors to improve their abrasion resistance, the alkali-content in terms of Na2O eq. can be as high as 6 g/L corresponding to 6 kg per cubic meter against 1.5-2.0 kg/m3 of Na2O considered to be the threshold value for the alkali-silica reaction. Due to this situation, coarse aggregates of the concrete substratum, in direct contact with the top layer of the cement-based hardener, are exposed to a higher risk of alkali-silica reaction at the transition zone due to the very high content of alkali in the hardener top layer. In order to prevent this type of ASR in concrete industrial floors, a special binder, containing 50% of ground slag or fly ash and 50% of portland cement, was used in combination with a quartz-based crushed sand on the top layer (40% weight of binder and 60% of quartz). Due to the presence of GGBFS or fly ash in the cement-based hardener, the ASR of the coarse aggregate of the sub-strate in direct contact with the top-layer was really prevented. However, this technique of manufacturing durable concrete industrial floors, was not accepted by the workers because they should wait too much more time for the hardening of the surface. A special mix was adopted with improved performance in terms of setting properties: a very fine ground slag, with a specific surface area of about 650 m2/kg, was combined with an accelerating admixture, and was succesfully adopted for both a durable concrete industrial floor and a trouble-free for the workers.

The purpose of this research work was to make a drying shrinkage-free concrete (SFC) ,even in non-wet curing conditions. This concrete was produced by the combined use of: a) a water-reducing admixture, based on polycarboxylate (PA), in order to reduce both the mixing water and cement, and increase the amount of aggregate;b) a special polycarboxylate (PA/SRA) including, in its molecular structure, a shrinkage-reducing admixtures (SRA) based on polyethylene glycol capable of reducing the surface tension of liquid water filling the capillary pores; c) an expansive agent based on a special calcium oxide (CaO) manufactured in a kiln at relatively high temperatures (about 1000 °C). Traditional shrinkage-compensating concretes are theoretically based on the restrained expansion produced by portland-cement products containing either calcium sulfo-aluminate or free CaO as expansive agent. However, in practice this effect is cumbersome to achieve because these concretes must be wet-cured, for at least 3-7 days after the final set. On the other hand, with the concrete described in this paper, drying shrinkage is completely compensated even in the absence of wet curing. The concrete is demolded at 3 days and then exposed to air curing. Compressive strength and restrained expansion of laboratory specimens as well field cured concrete are given.