International Concrete Abstracts Portal

The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The results obtained show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work.

Reactive Powder Concretes (RPC) - in form of superplasticized cement mixtures with silica fume, steel fibers, quartz fine sand (100-400 um) and/or limestone coarse aggregate (0.1-8 um ) - were studied in comparison with modified RPC where artificial aggregates substituted for natural aggregates. Artificial aggregates were obtained by grinding portland clinker coarsely so that fine and coarse aggregate were obtained with approximately the same particle size distribution of natural fine quartz (100-400 um and limestone gravel (O.l-8 mm) respectively. The source of clinker-aggregate was the same as that used for portland cement as binder of RPC. The idea was to improve the bond strength between cement paste and aggregate due to some hydration of the clinker-aggregate surface. RPC specimens were cured at room temperature (2OO C) or steam-cured at low or high pressure at 90°C or 160°C respectively. Compressive strengths were measured as a function of time at 1-28 days. The 28-day compressive strength level was as high as 200 MPa. Regardless of the curing temperature, compressive strength of RPC was increased by about 20 MPa when clinker-aggregate was used instead of natural aggregates. These results indicate that the bond strength of the interface between cement paste and aggregate is improved when clinker particles are used instead of natural stones. Scanning electron microscope observations of the microstructure confirmed this hypothesis and indicated that the interface between cement paste and natural aggregate is the weak point in RPC.

There is presently a strong drive to minimize the amount of Portland cement used in cementitious systems replacing it to the extent possible with supplementary cementitious materials, such as silica fume, blast furnace slag or fly ash. Such an approach would enable the use of more and more environmentally friendly concrete. With respect to applications, construction practices continue to evolve towards minimum-labor technologies which are more reliable, and usually more cost-effective. Most of these applications require the use of chemical admixtures, such as water reducers, high-range water reducers, and in some cases viscosity-enhancing additives. With the increase in the replacement value of portland cement with supplementary cementitious materials and the growing trend towards using flowing concrete with high workability, it is critical to assess the risk of such mixtures to bleeding and segregation. This paper presents the results focusing the evaluation of the influence of dosage rates of high range water reducer, set retarder, and viscosity-enhancing admixture on the changes in stability of highly flowable cement-based materials. This characterization is undertaken using a newly developed method to quantitatively measure continuously and in-situ changes in the homogeneity of cementitious materials with time during their consolidation period. The method is based on monitoring the variation of electrical conductivity with respect to depth of the test sample and time using electrokinetic probe with multiple electrode pairs embedded at various heights and operating using a low voltage pulsating current at 1 kHz.

The effects of Na2S04 addition to cement pastes containing PNS superplasticizer with different molecular weight were investigated in different parameters of cement pastes, such as the rheological properties, the adsorption of PNS superplaticizer, and the development of the heat of cement hydration. Na2S04 addition significantly improves the fluidity of cement pastes made with a low-alkali-cement and a high-molecular-weight PNS. On the contrary, Na2S04 addition to a high-alkali-cement has a negative effect on the fluidity of cement pastes in the presence of PNS superplasticizer. The effect of Na2S04 addition on the fluidity of cement pastes made with a low-molecular-weight PNS is relatively small. When sodium sulfate is added to the low-alkali cements in order to adjust the soluble alkali content, it is confirmed that the cement pastes containing the high-molecular-weight PNS have better fluidity than those containing the low- molecular-weight PNS. The Na2S04 addition reduces the amount of PNS adsorbed on cement particles and the effectiveness of Na2S04 in reducing the amount of PNS adsorbed is independent to the molecular weight of the PNS. The Na2S04 addition to low-alkali-cements retards cement hydration during induction period in the presence of a high-molecular-weight PNS and then accelerates it during the acceleration period. However, the Na2S04 addition to high-alkali-cements just accelerates cement hydration and this acceleration effect is not dependent to the molecular weight of PNS super-plasticizer.

Carboxylic acid ester superplasticizers (CAE) consist of polymers in which hydrophilic polyoxyethylene ester chains (CE) are grafted onto a main chain bearing carboxylic groups (CA). In the present work, CAE copolymers characterized by different molar carboxylic acid - carboxylic ester ratios (CAKE) were synthesized and evaluated as super-plasticizers by using two different cements. The efficiency of CAE copolymers as superplasticizers was found to be dependent on the carboxylic acid - carboxylic ester ratio (CAKE) and the optimum CA/CE value in order to attain the best flowability was different for the two cements. Adsorption measurements indicated an increase of adsorption onto both the cements by increasing CAKE. On the other hand, zeta potential of cement pastes was not substantially influenced by the addition of the different superplasticizers. The results of the present work seem to indicate that both adsorption and steric stabilization are the main factors which determine the performances of CAE as superplasticizers and that CA/CE is an important parameter influencing the cement/CAE superplasticizer compatibility.