Over the years we have been able to overcome the inherent weaknesses of concrete thereby making it more suitable for a wide variety of applications. One major development has been reinforcement by short randomly distributed fibers that remedy weaknesses of concrete such as low crack growth resistance, high shrinkage cracking, low durability, etc.

In fiber reinforced concrete, the use of one type of fiber alone helps to eliminate or reduce the effects of only a few specific undesirable properties. It is believed that the use of two or more types of fibers in a suitable combination would not only help improve more properties of concrete, but would also help provide synergy amongst the fibers. This aspect of combining the fibers, i.e. hybridizing the fibers in a rational manner to derive maximum benefits, was studied in this thesis.

High performance fiber reinforced concrete, with a matrix strength of about 85 MPa was used. A n attempt was made to make the concrete suitable for practical use, with the required workability, air content, density, etc. This was achieved by making use of proper admixtures including silica fume, superplasticizers and an air entraining agent.

The amount and type of fibers to be used in the hybrid composites were planned such that the synergistic behavior of the fibers could be evaluated. The basic property of the hybridized material that was evaluated and analyzed extensively was the flexural toughness of the material. The various fiber types used in diverse combinations included macro and micro fibers of steel, polypropylene and carbon. Control mixes, single fiber reinforced concrete mixes, double fiber hybrids and triple fiber hybrids were investigated.

Along with flexural toughness, the size effect of micro fibers, plastic shrinkage resistance, pull-out response of a single macro fiber, impact and shear were also studied. Research clearly indicated that there was synergy associated with many fiber types. In particular 2 denier micro polypropylene fibers, when hybridized with the polypropylene macro fibers (HPP) and carbon micro fibers demonstrated maximum synergy.

Although significant synergy was observed, it is believed that the synergy is underpredicted in our tests. The minimum volume fraction of macro fibers used for any of the mixes was 0.5% and it appears that this macro fiber volume fraction is too high to observe maximized synergy in the hybrids. This amount of fiber appears to be high enough to make the post peak response of the matrix insensitive to the addition of small dosages of other fibers, such as micro fibers.