Fracture Toughness Investigations of Micro and Nano Cellulose Fiber Reinforced Ultra High Performance Concrete

Ultra high-strength concrete (\u3e200 MPa) is marked by brittle behavior. In this work, we are reinforcing ultra high performance concrete with a combination of micro- and nano- scale cellulose fibers to increase the toughness of the brittle material. These fiber reinforcements can potentially provide the benefit of other micro and nano fiber reinforcement systems at a fraction of the cost. The goal of this study was to measure the effects of micro and nano cellulose reinforcements on a particular ultra high strength concrete (Cor-Tuf) developed by U.S Army Corp of Engineers Engineer Research and Development Center (ERDC) researchers, on both processing parameters and fracture toughness. Micro and nano-cellulose fibers were used both separately and in combination, at volume fractions of 0, 1%, 3% and 5%. Fracture energy was measured using both notched beam and split cylinder configurations. The incorporation of cellulose fibers into ultra high performance concrete in this study presented several processing challenges. Due to the low water to cement ratio used in the mix design, the mix is extremely sensitive to changes in water and super plasticizer. Slight increases in either water or super plasticizer caused the mix to behave differently. In trial batches, water to cement ratio and super plasticizer were varied to determine requirements for the addition of fibers. In our work, we determined that the addition of micro cellulose fibers to Cor-Tuf resulted in a linearly increasing water to cement ratio and super plasticizer. Several iterations of trial batches were necessary to obtain the final mix design. This work also highlighted the sensitivity of the mix to processing order. During the trial batches, we discovered that adding additional water and super plasticizer late in the mixing stage did not work. Introducing nano cellulose fibers led to somewhat unexpected results. The nano cellulose fibers at all reinforcement levels used the same water to cement ratio. This was notable, since the micro cellulose fibers required an increase in water to cement ratio with increasing fiber content. This characteristic of the material allowed for successful mixes at higher levels of fiber reinforcement than when using the micro cellulose fiber. Results from split cylinder and three-point bend notched beam tests indicate that a 3% micro cellulose reinforcement is most effective in increasing the fracture energy performance of the material. The three-point notched beam tests indicated that the inclusion of micro fibers into the concrete increased the fracture energy by 53%. A hybrid mix consisting of both micro and nano cellulose fibers increased the fracture energy response, but to a lesser degree, 26% greater than the unreinforced material. Based on our investigations of cellulose fiber reinforced ultra high performance concrete, incorporating cellulose fibers is both feasible and effective in increasing the fracture toughness of the material. Further work is necessary to identify the mechanisms for crack and void bridging at both the nano and micro length scales. Identification of mechanisms will lead to rational optimization for specific applications