Erdogan Ozbay

Effect of Fly Ash and PVA Fiber on Microstructural Damage and Residual Properties of Engineered Cementitious Composites Exposed to High Temperatures

This paper discusses the influence of high volumes of fly ash and micro polyvinyl alcohol (PVA) fibers on the fire resistance and microstructure of engineered cementitious composites (ECC). Composites containing two different contents of fly ash as a replacement for cement (55 and 70% by weight of total cementitious materials) are examined. To determine the effects of microfibers and ultrahigh ductility of ECC, ECC matrix mixtures of similar composition except PVA fiber are also produced and tested for the fire resistance. The mixtures are exposed to temperatures up to 800°C for one hour. Fire resistances of the mixtures are then quantified in terms of the residual mechanical properties (strength, stress-strain curve, deflection, and stiffness) and mass loss. The role of PVA fibers and fly ash is discussed through the analysis of microstructure and fiber-matrix interactions as a function of heat treatment. The microstructural characterization is examined before and after exposure to fire deterioration by ...

Self-Healing of Microcracks in High-Volume Fly-Ash- Incorporated Engineered Cementitious Composites

This paper presents the self-healing ability of engineered cementitious composites (ECCs) containing high-volume fly ash (HVFA). Composites containing two different contents of FA (55 and 70% by weight of total cementitious material) are examined. A splitting tensile strength test was applied to generate microcracks in ECC mixtures, where cylindrical specimens were preloaded up to their 85% maximum deformation capacity at 28 days. These specimens were then exposed to further continuous wet (CW), continuous air (CA), and wet/dry (W/D) cycle curing regimes up to 60 days. The extent of damage was determined by using the rapid chloride permeability test (RCPT), splitting tensile tests, and microscopic observation. In terms of permeation properties, microcracks induced by mechanical preloading significantly increase the RCPT values of ECC mixtures. Moreover, increasing FA content is shown to have a negative effect, especially on the permeation properties of virgin ECC specimens at an early age. Without self-healing, however, the effect of mechanical preloading on the chloride-ion penetration resistance of ECC with 70% FA is lower compared to ECC with 55% FA. The test results also indicate that CW and W/D cycle curing contribute and speed up the healing process of the cracks, significantly improve mechanical properties, and drastically decrease the RCPT of ECC. The use of HVFA in ECC production is likely to promote self-healing behavior due to tighter crack width and a higher amount of unhydrated cementitious material available for further hydration. Therefore, it appears that the curing conditions and ECC composition significantly influence self-healing ability.

Effect of Microcracking on Frost Durability of High-Volume- Fly-Ash- and Slag-Incorporated Engineered Cementitious Composites

This paper reports the durability performance of high-volume-fly-ash (FA)- and slag (S)-incorporated engineered cementitious composites (ECCs) when subjected to mechanical loading and freezing-and-thawing cycles. Composites containing two different contents of FA and slag as a replacement of cement (55 and 70% by weight of total cementitious materials) are examined. To find out the effect of mechanical preloading on the frost durability of ECCs, prism specimens were preloaded up to a certain deformation level under four-point bending loading to generate microcracks. Then, the preloaded and pristine (sound) specimens were subjected to the freezing-and-thawing test in accordance with ASTM C666/C666M. Experimental tests consisted of measuring the change in mass and ultrasonic pulse velocity (UPV) and residual flexural properties of ECC specimens exposed to the freezing-and-thawing cycles up to 300. Test results revealed that the frost resistance of ECCs was significantly influenced by the mineral admixture type and amount and preloading deformation. The deterioration with an increasing number of freezing-and-thawing cycles was relatively more for ECC mixtures with FA than for slag mixtures at the same replacement level. In addition, an increase in the FA replacement rate was observed to exacerbate the deterioration caused by freezing-and-thawing cycles. Apart from some reduction in flexural properties and UPV and an increase in mass loss and residual crack width, the results presented in this study, however, confirm the durability performance of ECC material under freezing-and-thawing cycles, even in cases where the material experiences mechanical loading that deforms it into the strain hardening stage prior to exposure. It is important to note that this durability of ECCs under freezing and thawing was achieved without deliberate air entrainment, and contrary to conventional concrete, no relationship of frost resistance was found to the air-void structure of the ECC mixtures.