No abstract

To assess several quality parameters of hydraulic concrete, cubic samples of this material are commonly assembled in engineering laboratories using molds. It is ubiquitous the use of cubic metallic molds of volume 125 cm3; these molds require the application of demolding substances (such as oils) on their surfaces to avoid the adhesion of the final hardened concrete sample to the molds. Despite this common practice suitable for evaluating mechanical traits of the materials, the use of a demolding agent spoils the concrete samples for the assessment of surface and water absorption parameters. To overcome this difficulty, we propose a new anti-adherent cubic mold that requires no demolding agent. The construction of the new mold based on anti-adherent polytetrafluoroethene (PTFE, Teflon®) is described with its use. To assess the claimed advantages of the new anti-adherent molds in testing surface and water diffusion traits of concrete samples, experiments on water contact angles, rates of water absorption and drying were performed on concrete specimens obtained with the classic and the new PTFE molds. It was proved the advantage of the anti-adherent mold over common metallic molds. The use of an oil as demolding agent in the concrete samples assembled in metallic molds produced conspicuous differences of water contact angles and absorption in comparison to clean concrete samples produced with PTFE molds.

Concrete materials having decreased water-absorption traits are optimum for some construction applications. Hydrophobicity accompanied by a decreased water absorption in a concrete material can be achieved through its formulation with hydrophobic silica particles. Despite this effect of hydrophobic additions to concrete, the water absorption and hydrophobicity of the final concrete materials can be affected by the water affinity of the coarse aggregates used for their preparation. The role of these coarse aggregates was studied in concrete with added hydrophobic silica particles and prepared with three coarse aggregates having marked differences in their water absorptions: quartz rocks (low), igneous andesitic rocks (medium) and igneous basaltic rocks (high). The hydrophobicity and water-absorption traits of the concrete materials depended on the coarse aggregates used in their preparations. The materials prepared with quartz as coarse aggregates produced less water-absorbing materials. In contrast, the use of igneous basaltic rocks produced materials with greater absorption. A pretreatment involving impregnation of the igneous basaltic rocks with hydrophobic particles before their use in the preparation of concrete material caused a significantly decreased water absorption.