Durability, Resistance To Spalling After High Temperature And Creep Characteristics Of Cellulose Fibre Reinforced Concrete

Cellulose fiber is the third generation plant fiber used in concrete engineering following after chemical synthetic fiber. Synthesized by particular plants from alpine region, cellulose fiber exhibit very high strength/weight ratio and hydrophilic properties which make it can be easily dispersed in the concrete composite. Cellulose fiber reinforced concrete (CFRC) has good crack resistance and excellent durability performance. However, its propersities should be firstly systemic studied to promote the application of CFRC in civil engineering. In this paper, forces on the tunnel lining circumstance, the CFRC which can meet the bearing capacity and durability requirements of the were prepared through preferring raw material, mix design and optimizing molding process. Besides, the durability in bending tension force coupling with environmental factors, high temperature performance in the fire and creep properties under compression were also investigated.Firstly, the possibility of cellulose fiber used as internal-curing fiber in the concrete and the reliability as promotion measures to enhance durability were explored. The hollow structure of cellulose fiber was studied by Laser scanning confocal microscope and SEM. Meanwhile, saturated water absorption rate of fiber slice, mechanical properties of single cellulose fiber (including tensile strength and initial elasticity modulus) and effect of water absorption characteristic of cellulose fiber on hydration process were also tested. The fiber dispersion degree in matrix was quantitatively evaluated, and the morphology in hardened cement paste was observed. Results showed that cellulose fiber has a unique hollow structure and good water absorption, and could be evenly dispersed uniformly in the concrete. The free water absorbed in fiber have little affect the hydration process, and make the fiber applied as the internal curing fiber of concrete.Durability performance of CFRC, inclouding resistance to permeability of chloride ion, frost resistance, carbonation resistance and resistance to sulfate erosion, were tested under stress of 40% flexural strength coupled with single environment factor. Results revealed that under the coupling load, the durability of CFRC was better than unloaded PC. The durability of CFRC under flexural load was relatively weaken than that of unloaded because of the micro structure fracture introduced by bading. Relative dynamic elastic modulus was decreased 8.4% after 300 times of freezing-thawing cycles. Electric flux of chloride ions increased 17%. Carbonization depth bacame 0.2 mm、0.7 mm、0.6 mm and 1.3 mm deeper after 3 d、7 d、4 d and 28 d. carbonization.Secondly, the spalling resistance property of CFRC under 600 ℃,800 ℃ and 1050 ℃ and aging for 2.5h,4h and 5.5h were studied respectively. With the help SEM, MIP, thermal analysis and XRD phase analysis, the relationship between the microscopic structural degradation and attenuation law of macroscopic properties, and anti-burst mechanism under high temperature were researched. The results predicated that the cellulose fibers could significantly improve the spalling resistance performance of CFRC under high-temperature. Approaching the melting point of the fiber around 300 ℃, fibers trends to softened, leading to the volume decrease, and raised the porosity. The aging time now play the primary dominate rule to the mechanical properties of CFRC. Longer time for aging, higher pore volume will be left after fiber soften, leading to more weaken of the concrete mechanical properties. Holding time dominately affected the damage the mechanical properties and micro structure of the concrete below 600 ℃. While above 800 ℃, this was no longer the main influence factor. The test results of microstructure implied that:with the temperature raising, the concrete hydration products constantly dehydrated and decomposed, making the matrix gradually loosing the dense. Cracks in the interface of aggregate and matrix appeared and gradually developed, and finally through the body. On macro scale, this process was exhibited as physical and mechanical properties continued lost, until the concrete lost its carrying capacity. When the temperature arrived the melting point of the cellulose fibers, the fibers melted, leaving a lot of channels in the matrix. On the one hand, it reduced the strength of the concrete specimens, on the other hand, it reduced the steam pressure inside the CFRC, and reduced the possibility of the concrete to burst.Lastly, creep behavior was studied of concrete mixed with 0、0.9kg/m3、1.1kg/m3 and 1.3kg/m3 cellulose fiber under 40% compressive strength. Fiber variable was introduced to modify the CEB-FIP (2010) creep model, and long-term creep deformation properties of our series of concrete were predicted using modified model. Cellulose fiber could reduce the compressive creep deformation of concrete, and the enhancement was more efficiency with the longer curing time. The effect of fibers on the creep properties of concrete mainly depend on the fibers dispersion in the concrete, defections in the interface layer between the fiber and the matrix, and the promoting effect of releasing internal moisture for hydration in the concrete, which were closely related to the fiber content and curing period. The correlation coefficients were more than 0.98 by fitting of the correction CEB-FIP (2010) creep modelintroduced fiber variable and experimental data, so that the modified model was carried out to predict long-term creep strain of concrete.