Description
To transfer solid waste to landfills, or other waste processing or disposal endpoints, waste collection vehicles deposit the collected waste at waste transfer stations (WTSs). In the majority of the WTSs, the waste is deposited on a concrete floor, from which waste handling equipment is used to transfer the waste to compaction equipment and/or large transfer vehicles. Premature deterioration of concrete floors in WTSs is often reported and is a major concern for the owners and operators of these facilities. Overlay replacement in these facilities has significant economic impacts including direct costs, operational delays, and planning hurdles. Unfortunately, published data on the contributing factors to the deterioration of concrete overlays in transfer stations and the mechanisms involved are virtually nonexistent.
The overall objectives of this research are therefore (i) to identify the contributing factors to the premature deterioration of concrete overlays in transfer stations, and (ii) to establish material design guidelines for the concrete overlay based on the observed degradation mechanism and operational conditions.
At the beginning of the project, the research team collected data through site visits and a survey. Results indicate that the service life of concrete floors in WTSs is significantly shorter than the service life expected by owners and operators; the findings support that the main factors are mechanical abrasion by waste handling equipment (e.g., loaders) as well as organic acid attack from waste leachate. Furthermore, a literature survey indicated that (i) simultaneous mechanical abrasion and organic acid attack has not been studied; (ii) existing abrasion resistance test methods are not suitable for investigating the simultaneous mechanical abrasion and organic acid attack mainly because of high variability of these test methods; therefore, there was a need to develop a new abrasion resistance test method for this project. Based on these findings, the research team (i) characterized the organic acid content of fresh leachate samples from the floors of WTSs and used the chemical concentration data to develop a synthetic leachate to simulate the conditions to which concrete is exposed at WTSs, (ii) developed a new abrasion resistance test method and supporting equipment, (iii) conducted abrasion resistance tests on cement paste and concrete in the presence and absence of exposure to leachate, and (iv) developed material design guidelines and recommendations for WTS floors.
The developed abrasion test method and equipment can be applied to cement paste, mortar and concrete materials with a maximum coefficient of variation of 4.4%, which is significantly lower than that of the existing abrasion test methods.
The results of abrasion tests on cement paste and concrete materials in the absence of exposure to synthetic leachate indicate that (i) the abrasion resistance of concrete is primarily controlled by the hardness and the volume fraction of the coarse aggregates, (ii) decreasing the water-to-3
cement ratio (w/c) of a mixture may increase the abrasion resistance of concrete but its effect is, in general, smaller than the effects of coarse aggregate hardness and volume fraction, (iii) no general correlation exits between w/c and the abrasion resistance; and (iv) no correlation exists between compressive strength and abrasion resistance of concrete.
The results of abrasion tests on cement paste and concrete materials in the presence of exposure to synthetic leachate indicate that (i) the abrasion resistance of concrete is mainly a function of hardness of the coarse aggregates; (ii) w/c has a significant effect on the abrasion resistance of concrete exposed to organic acids, especially, when soft aggregates (limestone herein) that are reactive with organic acids are used; (iii) limestone coarse aggregates provide only a short-term buffer against organic acid attack and the abrasion resistance of high w/c concrete materials containing limestone aggregates decreases very rapidly in long-term exposure to organic acids; (iv) while the use of silica fume, polymeric fibers, and latex increases the abrasion resistance of concrete in the absence of exposure to leachate, their use decreases the abrasion resistance of concrete exposed to leachate.
These findings were used to develop guidelines and suggestions for the material design of WTS concrete floors including: (i) the use of a w/c lower than 0.36; (ii) the use of a minimum of 70% total aggregate by volume; (iii) the use granite coarse aggregates which do not react with organic acids; (iv) the avoidance of pozzolanic materials including fly ash, silica fume, slag, or other pozzolanic materials that reduce the amount of calcium hydroxide in concrete; and (v) the avoidance of polymeric fibers and latex modified mixtures.
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