The effect of aggregate size distribution on fracture mechanic of concrete.

As a composite material the properties of hardened concrete, such as strength, stiffness, fracture toughness and fracture energy, are strongly influenced by the aggregate, which typically occupies 60-70% of the volume of a concrete mix. The fracture energy of normal strength concrete is known to be dependent on the compressive strength (w/c ratio), type and surface character of the aggregate phase along with its volume and maximum diameter. However, there has been relatively little investigation of the fracture characteristic of high strength concrete related to the aggregate size distribution. Three different aggregate gradings have been carried out to investigate the effect of the aggregate size distribution on fracture energy of concrete. Aggregate size distribution used in the concrete mixes examined is represented by λ, which is the coarseness of internal random grain structure of concrete. Three-point bend testing, based on RILEM TC 50-FCM Recommendation (1985), was employed to measure the fracture energy and stress intensity factor of concrete. The value of λ has been demonstrated to have a significant effect on the fracture energy of both low (w/b=0.3) and high (w/c=0.2) strength concrete. The fracture energy of the concrete was found to be independent of strength above a limiting value of 60 5 MPa. However, the fracture energy of the concrete was dependent on λ, the maximum value of fracture energy becoming limited by the strength of the aggregate particles. The critical stress intensity (KIC) of the concrete was found to be almost independent of λ. As λ shows a significant effect on fracture energy of concrete, a predicted fracture energy model was introduced. A statistical comparison between introduced models and available models was conducted. The developed model demonstrated a better prediction when compared to available models although some variability was present. The derived and existing models were then employed as the input fracture parameter for a finite element (FE) model to predict the behaviour of both normal and high reinforced concrete (RC) beams. The results of six RC beams manufactured of concrete with an average compressive strength of 40 MPa and of 95 MPa were used to validate the FE results. Comparison was on the basis of the total load-mid span deflection curve, crack pattern and crack width. The use of different fracture energy was found to influence the models predicted results in the pre-peak region