Hak-Chul (Alex) Shin
Research and Innovative Technology Administration
Concrete pavements; Cracking; Heat of hydration; Lightweight aggregates; Portland cement concrete; Rheological properties; Self compacting concrete; Shrinkage; Water cement ratio
The proposed project will investigate cost-effective, rapid pavement repair techniques that can reduce cost and duration. Two types of concrete materials are proposed to be investigated in this project, including adaptive rheology concrete, and crack-free early strength concrete for rapid pavement repair. Reducing the construction duration and enhancing early age and long term performance, is the key solution for decreasing both the direct and indirect costs. The first focus of this research is rapid full-depth repair (FDR) using crack-free early strength concrete. Portland cement concrete pavements (PCCP) exhibiting severe distress such as transverse cracks and shattered slabs and corner breaks require FDR. The distress are caused by inadequate slab length and deficient slab thickness (design issues), concrete with high coefficient of thermal expansion or modulus (materials issues), or non-uniform or insufficient base support (construction issues). The full-depth repair involves removing damaged area of the slab and placing full-depth pavement with tie bar in longitudinal joints and dowel bar in transverse joints. Due to the opening requirement of the pavement to traffic in few days after placing repair concrete, it is essential to achieve high early strength in repair concrete. To promote early age strength and setting, low water to cementitious materials ratio (w/cm) with high content of type III cement are common in rapid repair. Drying shrinkage, autogeneous shrinkage, and high heat of hydration are observed in the repair materials. The gradients of shrinkage and temperature though the thickness of repair concrete with the restraints of surrounding old concrete pavement can cause premature cracking at the surface (Shin 2000). Conventional curing methods using curing compounds and cover are not sufficient to prevent the cracking in repaired pavement. In this research, several methods will be considered to minimize stresses caused by shrinkage and temperature changes. Internal curing can reduce substantial autogeneous shrinkage at early age and increase long term compressive strength in high-performance blended cement mortars (Bentz 2007). Internal curing using the light weight aggregate (LWA), recycled concrete aggregate (RCA), expanded slate (shale), and superabsorbent polymer will be investigated. The effects of absorption and deception capacity of the aggregate with the size of materials will be investigated. Using synthetic fiber and SRA of repair concrete is the next method to consider. The research findings in the first approach on the SCC will be considered to enhance compaction of repair concrete at the bottom and side of the repairing area. The second approach is to develop flowable concrete with adaptive rheological properties to be used repair patching. The main problems associated to repair work are bonding between the repair material and the substrate and differences in shrinkage or thermal changes, leading to cracks and preferential paths for water intrusion. Based on the above methodology, the use of SRAs, expansive agents and fibers in SCC can be investigated. The advantage of the expansive agents is that shrinkage is fully compensated, while the flowability of SCC result in a better bonding with the substrate, as no air gets entrapped between the two layers. The absence of consolidation can further enhance the bonding between the substrate and the repair material. Due to vibration, water is drawn near the interface, creating a weaker bond, similar to the interface transition zone (ITZ) for coarse aggregates in concrete. Due to the increasing amount of paste and the cementitious materials content in SCC mixtures, shrinkage and cracking potential will be an issue compared to the conventional concrete mixtures (Lomboy et al. 2011). It is required to focus on optimization of mix design in terms of the paste content, Portland cement content, w/cm, and incorporation of proper types and amounts of shrinkage reducing admixtures (SRAs) and fibers to decrease the shrinkage and control the cracking potential in hardened concrete. The cost-effectiveness will be achieved by optimizing the SCC mix design. One key component in this optimization procedure is the granular skeleton formed by the aggregates. Cost-effective SCC requires an appropriate aggregate grain size distribution to minimize paste content. With this appropriate grain size distribution, the cost of the concrete can be reduced, as well as the shrinkage potential. Based on developed theories on particle packing in concrete, optimized grain size distributions can be created with locally available materials.