UCLA

To further efforts toward improvement, an innovative and durable High Performance Fiber Reinforced Cementitious Composites (HPFRCC) was developed, using hybrid steel macro-fibers with designed hook-ends, and polyvinyl alcohol micro-fibers for optimal fiber synergistic effects, crack width control, durability, and reduced maintenance and life-cycle costs for bridges. For functional performance improvements, an off-the-shelf phase change material (PCM) was utilized, optimized and incorporated into the HPFRCC as a bridge slab warmer, to improve freeze-thaw cycling durability, to reduce the use of de-icing salts, to provide improved skid resistance, and to improve safety in cold climates and to reduce traffic congestions.

The goal for developing and deploying HPFRCC with controllable functional performance is to utilize new, durable cementitious composites resistant to stringent climate demands compromised of freeze-thaw cycles, de-icing salts, plastic shrinkage and drying shrinkage cracks, chloride and sulfate attacks, corrosion and scaling, and excessive abrasion/wear due to tire chains.

This thesis utilized both numerical modeling and experimental. First, mechanical properties after incorporating PCM were discussed. Subsequently, destructive tests were performed in order to study the effect of adding PCM. In addition, thermal performance after incorporating PCM was also addressed as an important topic. As a result, freeze-thaw testing was performed in order to study PCM performance. Numerical modeling regarding material mechanical properties was proposed and compared with experimental data. Numerical modeling regarding concrete composite thermal performance was also studied. Lastly, concrete interior temperature, mechanical properties and concrete composite residual capacity were discussed.

In chapter 3, several experimental tests were performed in order to study the behavior of hybrid fiber reinforced concrete with PCM and to verify the validity of the theoretical model. Experimental tests can be divided into two categories. One is a destructive test; where concrete composite compressive strength, tensile strength and ductile capacity can be determined. The other category is a freeze-thaw test where concrete composite freeze-thaw resistance can be studied.

In chapter 4, a new crack bridging model accounting for slip-softening interfacial shear stress was proposed for randomly distributed and randomly oriented fibers after PCM were added, based on a micromechanics analysis of single fiber pull-out. The concrete composite bridging stress versus a crack mouth opening displacement (CMOP) curve and associated fracture energy were theoretically determined. In addition, a constant interfacial shear stress model was also proposed in order to compare this with a slip softening interfacial shear stress model. By applying the proposed model on various concrete composites, including 5% PCM and 7% PCM hybrid fiber reinforced concrete, the present model can well describe the slip-softening behavior during fiber pull-out.

In chapter 5, the new proposed slip-softening model was used to predict the ultimate tensile stress of a single fiber. Maximum fiber debonding stress and fiber pull-out stress was determined based on slip softening interfacial shear stress. By applying the rule of mixture, maximum fiber debonding and pull-out stress, the maximum tensile stress of a concrete composite was able to be predicted when subjected to three point bending test.

In chapter 6, PCM concrete composite interior temperature was modeled and compared with concrete without PCM after being subjected to freeze-thaw cycle. With PCM inside of concrete, interior temperature can be controlled. In preceding chapters, microcracks would be generated inside of the concrete and eventually become larger cracks by going through the freeze-thaw process. The aim of this chapter was to find a temperature gradient inside of concrete using an enthalpy method and specific heat capacity method to solve moving boundary problems. Numerical efficiency from both the enthalpy method and specific heat capacity method were also compared. Two different layouts of how PCM were incorporated into a concrete mix and were discussed in order to determine the efficiency of each design.

In chapter 7, concrete mechanical properties after being subjected to freeze-thaw cycle were modeled. In addition, concrete composite residual capacity after freeze-thaw process was also determined based on a stress-strain relationship. With PCM inside of concrete, interior temperature can be controlled. However, the relationship between concrete structure mechanical properties, number of freeze-thaw cycles and freeze-thaw temperature differences also needs to be determined.

After a correlation is found between concrete mechanical properties, number of freeze-thaw cycles and temperature difference, the stress-strain relationship can then be determined by using damaged concrete mechanical properties. A Constitutive relationship can be derived based on thermodynamic theory. Elastic damage and plastic damage were both evaluated. Once the stress-strain relationship is obtained, concrete residual life and residual durability can be stimated after going through a freeze-thaw action. Normal concrete was also compared with PCM concrete.

The aim of this chapter was to develop a damage model that account for concrete structure strength, number of freeze-thaw cycles and freeze-thaw temperature differences. Concrete composite residual capacity was also estimated and derived from free energy potential function.