Enhancement of durability of concrete composites containing natural pozzolans blended cement through the use of Polypropylene fibers

A parametric experimental study of the effects of Polypropylene fibers (PPF) on early cracking due to drying shrinkage in concretes with Natural Pozzolan Cement (NPC) and its influence on its durability is presented.Concrete composites containing natural pozzolan (toba trachite), point shaped crushed aggregates and different fiber volume fractions of PPF (0.03%, 0.06%, 0.09%, and 0.12%) were studied. This mixture is characterized by high permeability and carbonation ranges.Early age cracking control ability and drying shrinkage of PPF in NPC concretes were measured under dry setting conditions. The cracked area due to drying shrinkage was measured to establish the positive effect of the different PPF volume fractions in concretes. Besides, bulk density and water permeability depth were determined. Finally these specimens were stored for two years and tested afterwards, measuring the natural carbonation depth. Then the cracking area was again measured to assess the control ability of PPF on this parameter after long time.Finally a relationship of these indicators is analyzed to understand future lifespan of these concrete composites.The results indicate that a volume fraction of 0.07% of PPF reduces cracking area due to drying shrinkage of NPC a 66%, but larger volume fractions did not increased linearly this effect, and even worse results were obtained. The increment of PPF volume fraction reduces the water permeability depth and even the carbonation depth. A reduction of the cracked area, due to early shrinkage, of a 66% in NPC concretes may reach in these concretes a 32% lower water permeability indicators (enclosed wet area) and a 43% shorter minimum carbonation depth attending to the results obtained.So it can be considered that the cracking control ability of fibers on the exposed concrete surface reduced water permeability and CO2 diffusion. However the use of fiber increased porosity and reduced bulk density and ultrasonic modulus of NPC concretes.In conclusion NPC concretes with low amounts of PP fibers (upper to 0.07% volume fraction) are less permeable and the CO2 diffusion is slower in time due to early age cracking control, producing more durable concretes.