Principal Investigator:	Ronald S. Harichandran, Ph.D., P.E., F.ASCE
Research Assistant:	Goli Nossoni
Funding Agency:	Michigan Department of Transportation
Period:	October 2004 - September 2007

Research Objective

Concrete structures that are damaged due to cracking and spalling associated with corrosion of reinforcing bars are often repaired using shallow depth surface patches. Cracking and full or partial delamination of the patch repairs due to shrinkage and continued corrosion is generally unavoidable. The repairs typically last only for a few years in corrosive environments associated with coastal regions or where deicing salts are used. The objective of the research was to develop an improved repair method for shallow depth surface patches.


Research Approach

The use of a fiber reinforced polymer (FRP) fabric applied as an overlay on top of a traditional polymer concrete patching material was investigated as a means of improving the durability of shallow depth patches. The FRP overlay can serve as a secondary reinforcement and act as a barrier against the diffusion of moisture and chloride ions, thereby improving the performance of the patch by reducing cracking and slowing down the corrosion process. Two-dimensional finite element analysis was used to select a suitable FRP material and configuration for the overlay. The effectiveness of the proposed improved repair method was assessed using accelerated corrosion testing of rectangular concrete prism specimens with a polymer concrete patch and an FRP overlay. Reinforcement mass loss and expansive strains in concrete due to corrosion were measured on specimens with and without the FRP overlay. The measurements indicated reductions in concrete strains, crack propagation and corrosion level when the FRP overlay was used. Three-dimensional finite element analysis was conducted to understand the influence of using an FRP overlay on the concrete stress distribution and the arrest of cracking.


Research Results

Finite element (FE) simulations indicated that a single layer of bi-directional glass FRP fabric was sufficient to withstand the load due to corrosion-induced swelling. Accelerated corrosion tests (see Fig. 1) indicated that the FRP overlay delays the onset of cracking and changes the crack pattern. When an overlay is not used, corrosion induced swelling causes a main crack to initiate in the patch and propagate downward toward the bottom of the beam. When an FRP overlay is used, the main crack initiates in the concrete and propagates sideways away from the region influenced by the overlay. The FE analysis predicted the crack patterns observed in the experiments reasonably well (see Fig. 2). It indicated that, based on mechanical effects alone, the initiation of cracking is delayed in the presence of an overlay until more of the rebar corrodes. The change in the crack pattern can delay the migration of moisture, oxygen and chloride to the corroding bar since the crack does not open through the concrete cover. This effect together with the delay of cracking should increase the durability of the patch repair. The accelerated corrosion test confirmed that the corrosion rate was reduced when an FRP overlay was used, most probably due to the reduction in the diffusion of chloride ions, moisture and air through the concrete and patch. The mechanical enhancement provided by an FRP overlay, together with reduction in diffusion of moisture and chloride ions that it may cause, therefore provides considerable improvement in the durability of patched concrete beams against corrosion (Harichandran and Nossoni 2008).

FIGURE 1 Crack patterns due to corrosion: (a) Pattern 1, and (b) Pattern 2

 

(a)

(b)

FIGURE 2. TCI along different crack paths: (a) Crack Pattern 1, and (b) Crack Pattern 2

Research Implications

The enhanced durability resulting from the use of an FRP overlay on top of a traditional polymer concrete patch will reduced the maintenance cost and labor associating with repatching bridges.

References