The Addition of Synthetic Fibers to Concrete to Improve Impact/Ballistic Toughness

Highlights

•

This paper examines the impact performance of fibre concrete compared to plain.

•

Type 2 fibre concrete has superior fragment containment.

•

Finite Element Analysis was reliable to predict the damage of the concrete slabs.

t the damage of the concrete slabs.

---

Abstract

Concrete is relatively weak in tension and may require some form of reinforcement to cope with tensile forces. Steel reinforcing bar is often used to cater for tensile and compressive forces. However, current research shows that the use of steel reinforcing bar does not afford concrete protection against impact. Alternatively it has been shown that where fibres are added to concrete mixes protection is afforded through increased energy absorption. It would appear that the dispersion of fibres throughout a concrete mix affords a degree of toughness between the reinforcement bar spacing.

This research investigates the use of Type 1 micro synthetic fibres, Type 2 macro synthetic fibres and steel fibres used as post crack reinforcement in concrete samples when subject to a variety of stress induced states and compares the performance of these fibre mixes to that of a plain concrete mix. The test programme adopted, subjected cube specimens 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 investigated under test were: compressive strength, flexural strength, load deflection analysis, energy absorption and impact performance/resistance. Modelling the impact of shot fire on the test specimens was carried out using Finite Element Analysis, to inform slab design. The results of this investigation are of particular significance to the resilience of concrete structures under terrorist attack.

The results show that the adoption of Type 2 macro synthetic fibres as concrete post crack reinforcement provide the greatest toughness when compared with the other fibre types and offer the greatest protection from spalling of the back face of the concrete slab, being the main consideration with regard to the performance of the slab on testing with shotgun fire. Damage containment after ballistic testing was also noted where Type 2 fibres were used. The Finite Element Analysis models were successful at predicting the damage recorded to the concrete slabs, when subject to the shotgun fire performance test.

Keywords

• Impact;
• Flexural strength;
• Toughness;
• Ballistic;
• Spall;
• Containment

• Significance Statement

  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.
  

  Figure 1. Testing fibre concrete using a 12 bore gun with a single slug projectile.
  

  Figure 2. Test micro fibre slab of concrete after 3 shots.
   

About The Author

Alan Richardson’s research spans three distinct area of fibre concrete with regard to durability and impact resistance, Self healing biological cementitious materials and sustainable reuse of materials. He has authored over 100 publications and the research is ongoing.

A current project  has between 2 to 3 years remaining through Réunion Internationale des Laboratoires et Experts des Matériaux, systèmes de construction et ouvrages (RILEM). Developing a RILEM European standard for testing of self healing cementitious materials

During 26 years as a managing director Alan has displayed project management attributes at a high level . He was acknowledged as being the most successful director in the UK operating a guarantee scheme for client protection and assured quality. This involved producing quality work to price and time and managing the people in the production process.

Journal Reference

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.