Kejin Wang

Permeability study of cracked concrete

Cracks in concrete generally interconnect flow paths and increase concrete permeability. The increase in concrete permeability due to the progression of cracks allows more water or aggressive chemical ions to penetrate into the concrete, facilitating deterioration. The present work studies the relationship between crack characteristics and concrete permeability. In this study, feedback controlled splitting tests are introduced to generate crack width-controlled concrete specimens. Sequential crack patterns with different crack widths are viewed under a microscope. The permeability of cracked concrete is evaluated by water permeability tests. The preliminary results indicate that crack openings generally accelerate water flow rate in concrete. When a specimen is loaded to have a crack opening displacement smaller than 50 microns prior to unloading, the crack opening has little effect on concrete permeability. When the crack opening displacement increases from 50 microns to about 200 microns, concrete permeability increases rapidly. After the crack opening displacement reaches 200 microns, the rate of water permeability increases steadily. The present research may provide insight into developing design criteria for a durable concrete and in predicting service life of a concrete structure.

Effects of curing conditions on properties of concrete using slag replacement

Abstract The effects of curing conditions on properties of slag cement concrete were studied. Autoclaving (175°C, 0.5 MPa) and steam curing (80°C) were compared to normal curing (28 days, 20°C, and 100% RH). Four different concrete mix designs with the same mix proportions and different cement replacements were used: 0% slag (control), 25% slag, 50% slag, and 75% slag. The effects of slag replacement and curing conditions upon concrete properties were examined. The properties examined included mechanical properties (compressive and tensile strength), transport properties (chloride permeability and chloride penetration), and microstructural properties (pore structure and phase composition). There is little effect of slag replacement up to 50% upon strength, whereas higher replacement results in a drop in compressive strength. Steam curing reduces the compressive strength compared to the other curing types considered in this study. Chloride permeability and penetrability significantly decrease with increasing slag replacement except for autoclave curing, which is the least sensitive with respect to slag replacement compared to the other curing types considered in this study. The addition of slag reduces the continuous pore diameter, which correlates well with the initial current (IC) measured by the rapid chloride permeability test (RCPT).

Low Compaction Energy Concrete for Improved Slipform Casting of Concrete Pavements

The current practice in concrete pavement construction is to use low-slump concrete, which is processed by a slipform paving machine. Extensive vibration, introduced by equally spaced internal vibrators, is required for the concrete to reach proper consolidation. It has frequently been reported that malfunctioning internal vibrators cause over-consolidation of the concrete pavement and lead to premature cracking and durability problems. The research presented in this paper is focused on optimizing the consolidation properties and shape stability of fresh concrete mixtures so that the internal vibration may be eliminated during the slipform paving process. The experimental results have shown that it is possible to design a concrete mixture that can be consolidated without internal or external vibration and at the same time can maintain its slab shape at the end of a slipform paving process. This was achieved by selectively manipulating flowability, consolidation properties, and green strength of fresh concrete through the use of chemical admixtures or the addition of small amounts of fine materials.