Long-Term Field Performance of Ready-Mix Recycled Aggregate Concrete
Presented By: Humera Ahmed
Affiliation: The University of British Columbia
Description: Concrete waste is the most prominent and permanent burden on landfills generated by the construction industry. A growing body of literature in lab settings has already recognized the importance and sustainability of recycling this waste in the form of recycled concrete aggregates (RCA) and substituting it for natural aggregates in the production of new concrete. However, intense utilization of RCA in the large-scale industrial production of new concrete (structural and non-structural) is not widely accepted by many international standards due to its high porosity, a disparity of source concrete, and lack of territorial field studies. Keeping this in view, the present research was planned in 2013 to provide an insight into the applicability of recycled aggregate concrete (RAC) to the Southern Interior of British Columbia (BC), Canada. In this research, the long-term performance of two full-scale field studies of the foundation system and municipal sidewalk constructed with ready-mix RAC is presented. The results of this research are based on laboratory investigations and assessment of the in-place long-term performance of RAC made with different replacement levels of RCA. Long-term performance is examined for 5 years in terms of in-place compressive strength using rebound hammers. In addition to this, strength correlations developed specifically for RAC for the estimation of compressive strength based on rebound numbers are discussed. The results show that RCA has a significant potential of substituting natural aggregate in the production of new concrete of 25 and 32 MPa compressive strength. Furthermore, RAC shows a higher rate of compressive strength development after 28 days of curing and results in comparable long-term compressive strength to conventional concrete. By providing an insight into the sustainable application of RCA in the production of new concrete, the results of this study can serve as a basis for updating the current standards in Canada.
Evaluation of Quality Level of Recycled Concrete Aggregate (RCA) Coarse Aggregate Sources Using Petrographic Methods
Presented By: Mengesha Beyene
Affiliation: SES Group & Associates, LLC
Description: One of the main factors for the low usage of RCA coarse aggregate in new concrete construction in pavements and transportation structures is the absence of reliable methods to characterize the quality level of these materials. The presentation includes a list of items that can be evaluated petrographically to assess the quality levels of the RCA for use in concrete construction.
The following key factors are proposed that need to be carefully examined during RCA petrographic evaluation: (1) Relative volume of residual mortar and its physical properties including relative paste hardness, water drop absorption rate, relative degree of dust covering of the RCA aggregates,; (2) Relative density of paste microstructure/bulk-paste porosity; (3) existing damage in RCA particles by deterioration mechanisms in service such as ASR, sulfate attack, freeze-thaw, etc.; (4) Condition of the Interfacial Transition Zone (ITZ) at mortar-aggregate interfaces; and (5) Aggregate types/mineralogy which may be potentially reactive with alkalis in the new concrete. Additionally, cement paste composition, degree of hydration of cement grains (partially hydrated and/or relics of cement grains), degree of carbonation, and the variability of the particle properties need to be studied also.
The quality levels of the following RCA types, as sampled, need to be evaluated: (1) Crushed concrete pavement or structure of known quality and constituents (embedded steel, etc., removed), (2) Crushed returned concrete from a concrete production operation that has hardened and then is ripped, and crushed, (3) Crushed/mixed urban rubble of mostly concrete, but often includes other brittle materials (such as brick and tile), and (4) Returned concrete treated in a revolving mixer drum to produce hardened cement/aggregate nodules.
Experimental Analysis of Cement Mortar with Varying Replacement Levels of Crumb Rubber
Presented By: Rubaiya Rumman
Affiliation: University of British Columbia
Description: Disposal of the huge amount of rubber waste produced every year by the tire industry is a big concern. However, crumb rubber has an immense potential to be used in civil works, though mostly in non-structural forms. Crumb rubber can also be used in concrete as a replacement for fine aggregate. Since there is no available guideline on the usage of rubber crumbs in concrete, its use is still limited. Moreover, contrasting results on fresh, mechanical and durability properties of rubber crumb replaced concrete are observed in previous studies. These inconsistencies give rise to the necessity of conducting studies on crumb rubber as a constituent in concrete. In this study, mortar samples were prepared with a water to cement ratio of 0.49 having crumb rubber replacement levels of 0, 10, 30, and 50% of sand particles in two different batches with cement to sand ratio of 1: 2.5, and 1: 3.0. Fresh and hardened properties of the mortar samples were observed through various tests and their failure pattern was investigated. The results indicated crumb rubber addition decreased the overall compressive strength of the mortar specimens, reduced their workability, and delayed the initial and final setting time. However, failure was more ductile and gradual with the increasing percentage of crumb rubber in mortar samples.
Mechanical Properties of Rubberized Recycled Aggregate Concrete
Presented By: Kishoare Tamanna
Affiliation: University of British Columbia
Description: Recycling and utilizing waste tires and construction and demolition waste as alternatives to virgin aggregates in concrete can provide an environmentally friendly and effective solution for the exigent crises of both natural aggregate demand and environmental hazards. This study focuses on the material characterization of concrete with crumb rubber (CR) treated with NaOH solution and recycled concrete aggregate (RCA) under static compression and flexural loading. Three replacement levels of CR (0, 10, and 20% by volume) and RCA (0,50, and 100% by weight) replacing natural fine and coarse aggregates, respectively, were incorporated in this study. Rubberized recycled aggregate concrete (RRAC) demonstrated enhanced material behavior in terms of normalized strain capacity under compression and flexural load-deflection compared to both conventional concrete and recycled aggregate concrete (RAC). Overall, RRAC yielded satisfactory compressive and flexural behavior for use in structural concrete.
Effect of Shear Span-to-Depth Ratio on the Behavior of RAC Beams
Presented By: Nariman Khalil
Affiliation: University of Balamand
Description: This presentation is part on an extensive research program into the effect of several parameters on the shear behavior of beams made with recycled concrete aggregates. Eight beams were designed to fail in shear. Three values of shear span to depth ratio were considered: 2.5, 3.5, and 4. Two replacement ratios of coarse aggregates were studied: 50% and 100%. Beams made with natural aggregates were also cast to serve as control beams. Mix design and the longitudinal reinforcement ratio of 1.04% were kept the same for all test beams. Deflections, concrete compressive strains, steel strains, crack patterns, ultimate shear capacity; and failure modes were all observed and analyzed, and the relevant conclusions were drawn. ACI Code provisions for shear strength prediction were evaluated against test data.
Leaching Composition and Associated Microbial Community of Recycled Concrete Aggregates
Presented By: Matthew Adams
Affiliation: New Jersey Institute of Technology
Description: Recycled concrete aggregates (RCA) have been used as an alternative sustainable material in the construction industry, particularly as fill material and rip rap. Although RCA application has been successful, its long-term environmental impacts in are still being considered. Currently, the most important known concern is the high alkaline pH of leachates when water moves through the RCAs due to their high content of calcium hydroxide (Ca(OH)2). In this study, bacterial enrichment potential to reduce the alkalinity of two different sources of RCA were examined. One source of RCA was made from laboratory-produced concrete that was crushed approximately 90 days after being cast. A second source of RCA was from a stockpile of demolished concrete that had been in service in transportation applications in New York City; the exact source(s) of the material are unknown. Washed and un-washed lab and field RCAs were bio stimulated by being exposed to an alkaline medium in batch experiments. pH, metal composition and microbial community changes in the leachates were monitored over time. Results show that pH of field RCAs could be decreased to fewer concerning values, as low as 8, while lab RCA samples were resistant to change. Furthermore, the long-term effectiveness of this process was assessed by exposing these new bio-enriched treated RCAs to DI water during 840 h after two times washing. Even though, the pH increased to high alkaline level again, it happened significantly slower; pH increased from 9 to 11 in 264 h and 504 h for field-unwashed and field-washed samples, respectively and stayed roughly constant over 840 h compared to an increase from 11 to 12 in 24 h for both original field RCAs. Metal concentrations in the leachate were much lower than initial releases in field bioleaching treatments except aluminum and copper, which were higher.