Interfacial fracture toughness of composite concrete beams

Deterioration of concrete structures is a reality that building owners have to contend with. Concrete deterioration can occur through scaling, disintegration, erosion, corrosion of reinforcement, delamination, spalling, alkali-aggregate reactions and cracking of concrete. Consequently, a vast number of concrete structures, specifically concrete pavements, are in need of rehabilitation and strengthening around the world every year.

For pavements, concrete overlays bonded on old concrete pavements to take advantage of residual strengths and improve their structural capacity and safety are increasingly becoming popular. This practice results in a thicker composite section; consequently, a much stiffer pavement whose success depends on its response to vehicular and thermal stresses of the two layers. However, interfacial delamination may set in which in due time dilapidates the pavement leading to reduction in load bearing capacity coupled with poor durability and compromise of safety. Therefore, it is important to assess accurately the interfacial bond quality.

It is well documented that the strength-based criterion method for assessing delamination at interface becomes inapplicable as stress distribution in the crack line at the vicinity of the crack tip varies greatly due to stress singularity and ‘‘oscillation” at the crack tip. Therefore, researchers have been pursuing numerical approaches, such as: the finite element analysis and/or the boundary element analysis, in the quest to extract fracture parameters for interfacial cracks due to the lack of analytical solutions.

In fact, three methods have been reported; amongst which the J-integral method which calculates the ERR (Energy release rate) has yielded promising results. Unfortunately, thorough review on this approach reveals that an appropriate test specimen and loading configuration for testing the interfacial fracture toughness of the concrete overlay on worn pavements is not well defined. In this view, Dr. Yougui Lin from the Guangxi Transport Technology Co. Ltd in China together with Dr. John Karadelis at the Coventry University in the UK employed the theory of elasticity to calculate the ERR at the bi-material interface. Their work is currently published in the research journal, Construction and Building Materials.

Technically, the researchers setup a test specimen and loading configuration and used it to measure the interfacial fracture toughness for concrete overlay pavements. Ideally, they measured the interfacial fracture toughness of SFR.RC.PMC-to-OPCC (steel fiber reinforced, roller-compacted, polymer modified concrete–to–ordinary Portland cement concrete), under

3PB (Point-Bending) tests. Finally, they predicted the crack’s trajectory in a composite beam under 4PB based on both, the measured interfacial fracture toughness, and the ERR calculation approach.

The authors reported the interfacial fracture toughness of SFR.RC.PMC-to-OPCC to being 52.0 J/m² and 22.6 J/m² for rough and smooth interfaces respectively. Moreover, validation showed that the analytical prediction they presented was in line with the experimental results. Essentially, it was emphasized that composite beams will fail by fracturing (cracking) through the top layer, without suffering any interfacial delamination through their roughened interface.

In summary, Dr. Yougui Lin and Dr. John Karadelis presented a simple numerical approach for the determination of the interfacial energy release rate, based on the theory of elasticity and using crack closure and the nodal force technique. Generally, their approach was assisted by finite element analysis in conjunction with experimentation and verified by comparing the calculated results with experimental data available. Overall, in a recent interview with Advances in Engineering, Dr. John N. Karadelis commented that it was his hope that the simplicity of their method would be useful to practicing engineers.