Thermodynamics

The degradation of polymer composites in moist environments is a limiting
factor in the advancement of composite technology. The key to mitigate this
degradation is to maintain the integrity of the fiber/matrix (F/M) interface. In this
study, the F/M interface of carbon/vinyl ester composites has been modified by
treating the carbon fiber with polyhedral oligomeric silsesquioxane (POSS). Two
POSS systems, namely octaisobutyl and trisilanolphenyl, have been
investigated. A set of chemical and mechanical procedures has been developed
to coat carbon fibers with POSS, and fabricate layered composites using vinyl
ester resin. lnterlaminar shear, transverse tension, and low velocity impact tests
on composites have indicated around 10-38% improvement in mechanical
properties with respect to control samples. Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) tests have also shown
significant improvement in glass transition temperature (T9). Hygrothermal tests,
under various environments, have demonstrated that POSS reduces water
absorption by 20-30%.

One of the major causes of structural repairs worldwide is the corrosion of reinforced
concrete structures, such as residential buildings and piers, which are exposed to harsh
marine environments. This investigation aims to provide experimental evidence of the
fire resistance of corroded high strength reinforced concrete. For this, 14 reinforced
concrete beams of three different concrete mix designs (different strengths) were
prepared along with concrete cylinders for compression strength testing (ASTM C39).
After proper moist curing, all beams were corroded, in two phases, with impressed
current, then “crack scored ”for corrosion evaluation, after which half were exposed to
fire, also in two phases, following the ASTM E-119-12 time-temperature curve, using a
gas kiln. The fire damage was evaluated and compared between phases by using
Ultrasonic Pulse Velocity technology. Finally, all specimens were tested for flexural strength by using the third-point loading method (ASTM C78) and the effects of fire on the corroded beams were analyzed according to the level of corrosion.

A dynamical phase transition between a frozen and a chaotic state has been found in one-dimensional lattice systems with annealed disorder. An exponential decay to a homogeneous state is observed below the transition point. Above the transition point damage spreading and damage decaying phases are observed. A time dependent order parameter is suggested for describing the observed behavior.

This thesis presents an experimental investigation for evaluating effects of fire exposure on properties of structural elements retrofitted by carbon fiber reinforced polymers. Mechanical properties of CFRP-strengthened reinforced concrete members, protected with secondary insulation, were investigated, before and after (residual) direct fire exposure. Direct fire contact resulted in a reduction in capacity of 9-20% for CFRP-strengthened RC beams, and 15-34% for CFRP-strengthened RC columns. Furthermore, a dimensional analysis was developed for a heat transfer relationship between full and small-scale specimens, allowing a ¼ exposure time reduction for the latter. Results from experimental investigations demonstrate benefits of employing secondary fire protection to CFRP-strengthened structures, in spite of the glass transition temperature being exceeded in the early stages of the elevated-temperature exposure. Therefore, it is suggested, that fire protection is necessary for a CFRP-strengthened structural member to retain integrity throughout the duration of the fire exposure, and upon return to ambient temperature.

In recent years, high strength concrete (HSC) is becoming an attractive alternative to traditional normal strength concrete (NSC), and is used in a wide range of applications. With the increased use of HSC, concern has developed regarding the behavior of such concrete in fire. Until now, the fire performance of HSC is not fully understood and more research is needed. Full-scale fire testing is time consuming and expensive, and the real fire scenario is different from the standard fire. Performance-based assessment methods, including numerical analysis and simplified method, are being accepted in an increasing number of countries. In this dissertation, the fire testing results both of HSC and NSC are presented, performance-based numerical models are developed to study the fire performance of reinforced concrete (RC) members, and simplified calculation methods are proposed to estimate the load capacity of fire-damaged RC columns/beams. A detailed and comprehensive literature review is presented that provides background information on the high temperature behavior of concrete materials and RC members, as well as information on fire performance assessment procedures and objectives. The fire testing results of seven batches of HSC and NSC are presented and discussed. The test results indicated that the post-fire re-curing results in substantial strength and durability recovery, and its extent depends upon the types of concrete, temperature level, and re-curing age. The fire tests also showed that violent explosive reduced the risk of HSC explosive spalling. The surface crack widths were also reduced during the re-curing process, and in most cases, they were found within the maximum limits specified by the American Concrete Institute (ACI) building code.