Haejin Kim

Sustainable Concrete Through Reuse of Crushed Returned Concrete

Every year an estimated 2% to 10% (average of 5%) of the estimated 455 million yd3 of ready-mixed concrete produced in the United States (2006 estimate) is returned to the concrete plant. The returned concrete can be handled in several different ways. A common approach is to discharge the returned concrete in the concrete plant and crush the hardened concrete. The coarser material can be reused as base for pavements or fill for other construction. However, it is not easy to utilize the material finer than 2 in. A research project was undertaken by the National Ready Mixed Concrete Association Research Laboratory to study the use of crushed returned concrete, referred to as crushed concrete aggregate (CCA), as a portion of the aggregate component in new concrete. Demolishing old concrete structures, crushing the concrete, and using the crushed materials as aggregates, referred to as recycled concrete aggregate (RCA), have been researched to some extent. RCA is different from CCA because construction debris tends to have a high level of contamination (rebar, deicing salts, etc.). CCA, however, is prepared from concrete that has never been in service and thus is likely to contain much lower levels of contamination. The main objective of the research project was to develop technical data and provide guidance on a methodology for appropriate use of the material. Such a step can help the ready-mixed concrete industry achieve lower operating costs, reduce substantial landfill space, and support sustainable development.

Viscosity Modifiers to Enhance Concrete Performance

The hazard rate function for concrete structures is often portrayed as a “bathtub”-shaped curve, with a finite ever-decreasing probability of early-age failures being followed by a life with a relatively low constant probability of failure that ultimately increases dramatically as the end of service is reached. Ideally, new concrete technologies should reduce the failures occurring at both ends of this service-life spectrum. VERDiCT (viscosity enhancers reducing diffusion in concrete technology) is one such strategy based on increasing the pore solution viscosity. This approach has the potential to reduce the propensity for early-age cracking while also reducing long-term transport coefficients of deleterious ions such as chlorides. In this paper, the performance of a typical VERDiCT admixture—a viscosity modifier/shrinkage-reducing admixture— is investigated in mortar and concrete, both as an addition to the mixing water and as a concentrated solution used to pre-wet fine lightweight aggregates. A reduction in early-age cracking is achieved by eliminating autogenous shrinkage stresses that typically develop in lower water-cementitious material ratio (w/cm) concrete. By substantially increasing the viscosity of the pore solution in the concrete, the resistance to ionic diffusion is proportionally increased relative to a control concrete without the VERDiCT admixture. Herein, chloride ion diffusion coefficients are evaluated for two types of concrete containing typical substitution levels of supplementary cementitious material —namely, either 25% fly ash or 40% slag by mass. For the eight concrete mixtures investigated, the effective diffusion coefficient was reduced by approximately 33% by adding the VERDiCT admixture which, in practice, may imply a 50% increase in their service life, while the autogenous shrinkage was virtually eliminated. However, these benefits in early-age cracking resistance and long-term durability are tempered by up to a 20% reduction in compressive strength that may need to be accounted for at the design stage.

Criteria for Freeze-Thaw Resistant Concrete Mixtures

Concrete, especially for improved durability, is typically specified with prescriptive provisions. More recently there has been increasing interest in evolving towards performance-based specifications, both within state highway agencies and industry (FHWA, 2014, “Guide to Developing Performance-Related Specifications, FHWA-RD-98-155, FHWA-RD-98-156, FHWA-RD-98-171, Vol. III, Appendix C,” http://www.fhwa.dot.gov/publications/research/infrastructure/pavements/pccp/pavespec/ , last accessed July 28, 2014; ACI Committee 329, Report on Performance-Based Requirements for Concrete, American Concrete Institute, Farmington Hills, 2010; The P2P Initiative, 2014, “National Ready Mixed Concrete Association, Silver Spring,” http://www.nrmca.org/p2p/ , last accessed July 28, 2014). One of the challenges in successfully implementing performance-based specifications is the existence and use of reliable test methods and specification criteria that can measure the potential durability of concrete mixtures and provide the expected service life. A state pooled fund research project (TPF-5 (179), 2014, “Evaluation of Test Methods for Permeability (Transport) and Development of Performance Guidelines for Durability,” http://www.pooledfund.org/Details/Study/406 , last accessed July 28, 2014) was developed with an objective to propose performance criteria for concrete that will be resistant to penetration of chlorides, cycles of freezing and thawing, and sulfate attack. This paper summarized results pertaining to freeze-thaw resistance. Concrete freeze-thaw (F-T) performance was evaluated by ASTM C666/C666M-15 (AASHTO T161-08 (Standard Method of Test for Resistance of Concrete to Rapid Freezing and Thawing, Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Part 2A: Tests, AASHTO, Washington DC, 2013)) and deicer salt scaling resistance was evaluated by ASTM C672/C672M-12. It was examined whether F-T performance of concrete correlated with results of rapid index tests for fluid transport characteristics of concrete. These tests included the rapid chloride permeability, absorption, and initial and secondary sorptivity. The impact of degree of saturation on the F-T resistance of concrete was also explored. Criteria for F-T resistant concrete mixtures depending on type of exposure were suggested.

Tests and Criteria for Concrete Resistant to Chloride Ion Penetration

This paper presents a portion of a state highway agency pooled fund research project to develop performance criteria for concrete that will be resistant to penetration of chlorides, cycles of freezing and thawing, and sulfate attack. This paper presents the portion of the study pertaining to penetration of chlorides. To simulate standard and service conditions, specimens were subjected to either immersion or to a cyclic wetting and drying exposure in chloride solution. Measured apparent chloride diffusion coefficients, determined in accordance with American Society for Testing and Materials (ASTM) C1556, were correlated with results of rapid index test methods that provide an indication of the transport characteristics of concrete. Rapid index test methods included were rapid chloride permeability, rapid migration, conductivity, absorption, and initial and secondary sorptivity. A set of rapid index test methods and specification criteria that can reliably classify mixtures based on their resistance to chloride ion penetration are proposed.

Increased Use of Fly Ash in Hydraulic Cement Concrete (HCC) for Pavement Layers and Transportation Structures

Fly ash is commonly used as a supplementary cementitious material (SCM) in the production of portland cement concrete. Concrete produced with high fly ash replacement levels is considered high volume fly ash (HVFA) concrete. HVFA concrete has many benefits, including reduced concrete production cost, reduced greenhouse gas emissions, and improved sustainability. Despite the advantages, there are several barriers that limit the use of HVFA concrete. One of the main limitations to the increased usage of HVFA concrete is the lack of contractor and transportation agency familiarity with the setting time and strength development of these concrete mixtures. For this research, a laboratory-testing program was developed to examine the effect of fly ash type, fly ash dosage, cement chemical composition, and environmental conditions on the hydration development, setting times, and compressive strength development of HVFA concrete. Results from semi-adiabatic calorimetry were used to develop a hydration model for HVFA concrete. Finally, the ConcreteWorks software program was used to predict the in-place performance of selected HVFA concrete mixtures when placed in various transportation structures. It is concluded that HVFA concrete may be produced to have comparable setting times and earlyage compressive strength development to conventional portland cement concrete when used for transportation infrastructure.