Concrete is a quasi-brittle material, which when subjected to loading beyond its tensile/flexural strength fractures. Indeed concrete is relatively weak in tension and may need reinforcement to withstand tensile forces. Concrete is generally reinforced with steel so as to withstand the tensile strength deficiencies. The use of steel reinforcing bars is not likely to permit full concrete protection against impact.
Dr. Alan Richardson and colleagues from Northumbria University in the UK aimed at improving impact/ballistic toughness in concrete. Performance enhancement can be achieved by adding fibers to concrete, and this composite material performed better than plain reinforced concrete when subjected to external forces, an increase in energy absorption will protect concrete. It would appear that the dispersion of fibers throughout a concrete mix affords a degree of toughness between the reinforcement bar spacing. The research work is now published in peer-reviewed journal, Construction and Building Materials.
The research team carried out investigation on the use of type 1 micro synthetic fibers (12mm x 32 micron diameter) , type 2 macro synthetic fibers (50mm x 0.9mm diameter) and 50mm hooked end steel fibers used as post crack reinforcement. The concrete samples were subject to a variety of stress induced states, the team compares the performance of these fiber mixes to that of a plain concrete mix. From the test program adopted, cube specimens were subjected to compressive strength tests, beam samples to both three point flexural bending and single point impact loading, and concrete slab sections to shot gun fire. The parameters considered under test are, compressive strength, flexural strength, load deflection analysis, energy absorption and impact performance/resistance. ANSYS finite element analysis model was used to predict the damage recorded to the concrete slabs.
The authors observed that Type 2 macro synthetic fiber samples offered the greatest impact resistance when compared to the other fiber types. They also exhibited the highest flexural strength value and the highest degree of rear face fragment containment to under shotgun fire performance test. The energy absorption and impact resistance testing, shows that the steel fibers performed better than any of the fiber types. The compressive stress wave was observed to be greater on the rear face of the FEA models. This explains why the rear face of each slab shot to failure, experienced more damage than the front face. Type 2 macro synthetic fibers provide superior concrete post crack reinforcement and also provided the greatest toughness when compared with the other fiber types. They are highly recommended for use in blast/impact design resistance.
Alan Richardson, Kathryn Coventry, Thomas Lamb, David Mackenzie, the Addition of Synthetic Fibers to Concrete to Improve Impact/Ballistic Toughness, Construction and Building Materials 121 (2016) 612–621.
Northumbria University, Newcastle upon Tyne NE1 8SA, UK.Go To Construction and Building Materials