Abstract
The aim of this research is to evaluate how transport properties at the surface and in the core of high-performance concrete (HPC) slabs are affected by silica fume and by superplasticizer and retarding agent dosages. The transport properties were evaluated by measuring the air permeability at two depths under the surface finished with a wooden trowel. The experimental program is based on eight different high-performance concrete mixtures (77 to 93 MPa) made with a water-to-binder ratio of 0.30. An ordinary portland cement (OPC) and a blended silica-fume cement (SF) were used to compare the effect of silica fume on the air permeability of surface and core concrete. Four mixtures were produced with a retarding agent to assess the effect of set retardation on the surface properties of high-performance concrete. For each combination of cement type and retarding agent dosages, the superplasticizer dosage was selected to produce two slump levels of 100 ± 10 mm and 200 ± 20 mm.
It was found that the air permeability of the concrete located just beneath the surface of HPC slabs can be 2 to 20 times higher than the core permeability. The ratio between the surface and core permeabilities depends on mix-design parameters such as silica fume, superplasticizer (slump), and retarding agent dosages. Increasing the slump of HPC (from 100 to 200 mm) systematically increased the surface permeability. The effect of slump (or superplasticizer dosage) on the surface permeability of the HPC decreased as compressive strength increased. The use of a retarding agent in HPC without silica fume increased the air permeability of the surface concrete. This effect is particularly important for the concrete containing a high dosage of superplasticizer. Silica fume proved very effective in inhibiting increased surface permeability due to the dosages of superplasticizers and retarding agents.
