Stephan A. Durham

Beneficial Use of Recycled Materials in Controlled Low Strength Materials

This study investigated the engineering characteristics of recycled materials in a controlled low strength material (CLSM). CLSM is a self-compacting, flowable, low strength, cementitious material used primarily as backfill and void fill. CLSM is primarily used as a replacement of compacted soil in cases where the application of the later is difficult or impossible. Strength requirements are low in comparison to typical structural concrete. This enables the use of low cost, abundant, industrial by-products for the production of CLSM. The flowability and compressive strength of CLSM mixtures created using industrial by-products is the focus of this paper. The flowability of CLSM must allow efficient placement without segregation, while the compressive strength must provide structural support but allow for easy excavation. Consequently, there are minimum and maximum performance criteria for both consistency and strength. This research investigated the effects of using recycled materials in CLSM on the fresh and hardened CLSM properties. A total of six materials were used to create 18 mixtures that were batched and tested. The cementitious materials investigated were Class C fly ash and spray dryer ash; and the aggregates tested were bottom ash, crushed glass, recycled concrete fines, and crumb rubber. The results showed that CLSM with acceptable strength and flowability properties is attainable using these recycled materials.

Effect of cement content and recycled rubber particle size on the performance of rubber-modified concrete

ABSTRACT According to the Rubber Manufacturer’s Association, the United States generated 3664 thousand metric tons of scrap tires in 2015. While most waste tires are repurposed, approximately 409.5 thousand metric tons were disposed in landfills. This study investigates an alternative use of the waste tires as a replacement of natural aggregates in concrete mixtures. This study investigated fresh concrete properties and compressive strength. Different coarse and fine aggregate rubber particle sizes were evaluated: 19-mm tire chips (TCs) and 30-mesh crumb rubber (CR). TCs were used to replace coarse aggregates, while CR was used to replace fine aggregate in the concrete mixtures in increments of 10% by volume. Concrete strength loss was reduced with a fine aggregate replacement with CR as opposed to greater losses of strength exhibited by a coarse aggregate replacement with TCs. Adequate strengths were achieved at replacement levels as high as 40% by volume with CR, whereas satisfactory strengths were achieved with only a 10% replacement of coarse aggregates with TCs. Acceptable strengths were obtained from mixtures utilizing a combination of the two rubber sizes. Cement content was observed to have greater influence on rubberized concrete compressive strength at lower rubber contents than higher levels.

Assessing Benefits of Using Geogrids in Pavements Founded on Problematic Soils

Geogrids are becoming a popular alternative for soil reinforcement in highway pavement construction to achieve improved performance in regions with soft problematic soils or with a reduction in aggregate layer thickness to reduce construction costs. To examine the potential benefits of geogrids for soil improvement, measurement of permanent deformation using triaxial tests is used in practice. However, soil subgrade improvement in a reinforced pavement system is achieved by lateral distribution of vertical stresses at the reinforcing layer, through the tensile properties of the geogrid material. Therefore, it is desirable to conduct large-scale testing to more accurately monitor the behavior of soil when geogrid is present. The current study seeks to verify the behavior of geogrid reinforced pavement systems through large-scale wheel tests performed with problematic subgrade soils found in North Georgia. The large scale specimen was prepared in a 6 feet long × 6 feet wide × 2 feet deep metal box and consisted of 12 in. of aggregate base overlying 12 in. of subgrade soil. Pressure sensors were installed near the bottom of the aggregate base layer and near the top and bottom of the subgrade layer to monitor stress distributions within the pavement system. This paper presents preliminary results showing vertical stress variations obtained experimentally in aggregate base and subgrade soils under large-scale simulated traffic tire loading. The development of a bench scale system to complement the large scale loading system and allow for microstructure evolution studies is also described.