Use of advanced composite materials for innovative building design solutions/

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2009.Includes bibliographical references (leaves 90-98).Advanced composite materials become popular in construction industry for the innovative building design solutions including strengthening and retrofitting of existing structures. The interface between different materials is a key issue of such design solutions as the structural integrity relies much on the bond. Knowledge on durability of concrete/epoxy interface is becoming essential as the use of these systems in applications such as FRP strengthening and retrofitting of concrete structures is becoming increasingly popular. Prior research studies in this area have indicated that moisture affected debonding in a FRP-bonded concrete system is a complex phenomenon that may often involve a distinctive dry-to-wet debonding mode shift from material decohesion (concrete delamination) to interface separation (concrete/epoxy interface) in which concrete/epoxy interface becomes the critical region of failure. Such premature failures may occur regardless of the durability of the individual constituent materials forming the material systems. Thus, the durability of FRP-bonded concrete is governed by the microstructure of the concrete/epoxy interface as affected by moisture ingress. In this work, fracture toughness of concrete/epoxy interfaces as affected by combinations of various degrees of moisture ingress and temperature levels is quantified. For this purpose, sandwich beam specimens containing concrete/epoxy interfaces are tested and analyzed using the concepts of fracture mechanics.(cont.) Experimental results have shown a significant decrease in the interfacial fracture toughness of concrete/epoxy bond with selected levels of moisture and temperature conditioning of the specimens. The strength of adhesive joint degrades as implied by the failure mode shift from concrete decohesion in controlled specimens to interface separation in conditioned specimens. In this thesis, primary data on the mixed mode fracture toughness of concrete/epoxy interfaces are presented as a basis for use in the design improvement of material systems containing such interfaces for better system durability, and issues related to the structural implications are also discussed.by Tak Bun Denvid Lau.S.M

Strengthening Shear Deficient Thin-Walled Steel Beams by Bonding Pultruded GFRP Sections

Rehabilitation and retrofitting methods offer economical and feasible alternatives for upgrading aged and deficient structures. Structural strengthening using Fiber Reinforced Polymer (FRP) composites have been widely investigated by researchers and used in field applications. The main advantage of FRP composites is the superior mechanical properties they offer over traditional structural materials. One novel alternative of these retrofitting methods was developed at Louisiana State University and called “Strengthening-by-Stiffening” (SBS). In SBS, the external strengthening of shear deficient thin-walled steel structures is achieved by bonding pultruded FRP sections to buckling prone web panels. Contrary to the commonly used uniaxial tension resistance of fibers, here, the geometric properties of pultruded FRP sections play the most important role in stiffening vulnerable thin plates. The research started by testing a series of full size steel beams before and after introducing SBS. The first web panel between the bearing and transverse steel stiffeners was selected as a control panel, and a point load was chosen in an asymmetric three-point loading setup. The experimental investigation was conducted considering different web panel aspect ratios (1.0:1.0; 1.5:1.0), web thicknesses (1/8; 5/32 inch), epoxy types (brittle; ductile), Glass FRP (GFRP) configuration (geometry and orientation). For comparison purposes, one conventionally strengthened beam (by welding additional steel stiffeners) and one beam strengthened by bonding Carbon FRP (CFRP) sheets to the critical web panel were also tested. The experimental tests showed that the global failure mechanism was mainly controlled by the debonding of adhesive layer. Therefore, failure modes and phase angles were investigated for the GFRP/steel interface. Local traction-separation laws for Mode I and Mode II failure modes were determined by conducting single leg bending (SLB) tests, in which digital image capturing and processing techniques were used to determine crack tip displacement fields. Delamination failure within the pultruded GFRP stiffeners was also simulated following Hashin’s failure criteria. Finally, effective SBS design parameters were investigated using an FE model that takes the adhesive’s mixed mode fracture into account using a cohesive zone model (CZM), which was validated using experimental results. Possible extension of SBS to new construction was studied to explore creating beams free from transverse steel stiffeners by fully bonding the GFRP stiffeners as a substitute for welding of transverse and bearing steel stiffeners as a means for improving the fatigue

A Review on Strengthening Steel Beams Using FRP under Fatigue

In recent decades, the application of fibre-reinforced polymer (FRP) composites for strengthening structural elements has become an efficient option to meet the increased cyclic loads or repair due to corrosion or fatigue cracking. Hence, the objective of this study is to explore the existing FRP reinforcing techniques to care for fatigue damaged structural steel elements. This study covers the surface treatment techniques, adhesive curing, and support conditions under cyclic loading including fatigue performance, crack propagation, and failure modes with finite element (FE) simulation of the steel bridge girders and structural elements. FRP strengthening composites delay initial cracking, reduce the crack growth rate, extend the fatigue life, and decrease the stiffness decay with residual deflection. Prestressed carbon fibre-reinforced polymer (CFRP) is the best strengthening option. End anchorage prevents debonding of the CRRP strips at the beam ends by reducing the local interfacial shear and peel stresses. Hybrid-joint, nanoadhesive, and carbon-flex can also be attractive for strengthening systems

Fatigue Behaviour of CFRP Strengthened Reinforced Concrete Beams

The performance of reinforced concrete (RC) structures, such as bridges in the heavy haul industry, may be severely impacted by fatigue when subjected to repeated cyclic loading. Fatigue not only reduces load carrying capacity and serviceability limit states (SLS), but it can cause structural failure even when the components are subjected to low stress range cyclic loading. Corrosion damage exacerbates fatigue related problems as chloride induced pitting corrosion facilitates the formation and gradual propagation of cracks under cyclic loading. A common rehabilitation and retrofitting approach that involves patch repairing and fibre reinforced polymer (FRP) strengthening has proven effective to not only restore structural capacity, but also to enhance infrastructure service life. The structural repair process involves the replacement of deteriorated cover concrete with a less permeable patch repair mortar. The patch repair only restores durability of the structure; to restore or enhance structural capacity the repair process further involves bonding of FRP laminates. Particularly in the case of FRP's with a low elastic modulus, the design is often guided by serviceability limit states as opposed to ultimate limit states (ULS), resulting in an over-reinforced structural member. In addition, the reinforcement area of commercially available FRP strengthening may exceed the design requirements, especially at low levels of corrosion damage. In both the abovementioned considerations the design may result in an over-reinforced section. At the time when this researched was proposed, the effect of increasing damage extent on fatigue behaviour of over-reinforced RC beams was not clear and merited further investigation. A scientific experimental approach was developed to investigate the long-term performance of fifteen (15) full-scale 40MPa RC beams with dimensions 155x254x2000mm and ultimate capacity of 62.3kNm. Accelerated corrosion damage was induced in varied extents which included 450mm, 800mm, 1300mm and 1800mm length to a constant degree of 10% on all specimens. Specimens from each damage extent were patch repaired using SikaCrete214 and subsequently strengthened with externally bonded with SikaCarboDurS512 carbon fibre reinforced polymer (CFRP) laminates. Four-point bending monotonic loading tests were conducted on one (1) specimen from each damage extent. The results obtained from the quasi-static tests were used to determine two (2) cyclic loading stress ranges at which the remaining 2 specimens from each damage extent would be tested under. Under the 40% and 60% stress ranges four-point bending cyclic loading tests were carried out at a test frequency of 4Hz. Information was acquired on key performance indicators that included fatigue life, crack development, failure mode and stiffness degradation, where stiffness was assessed in terms of midspan deflection, composite material strains and neutral axis shift. Information on these parameters were collected using strain gauges, linear variable differential transducers (LVDT), DEMEC strain targets and digital image correlation (DIC). Ultimate failure loads under monotonic loading showed that despite having the highest degree of corrosion, the 450mm damage extent specimen had the highest failure load of 325kN. The failure load gradually reduced to 290kN as the damage extent was increased to 1800mm and the 0mm (control) specimen failed at the lowest load of 274kN. In contrast to the static behaviour, the specimen fatigue life enhanced by 106.3% as the damage extent was increased from 450mm to 1800mm. As expected, the 40% stress range tests yielded much longer fatigue lives than their 60% stress range counterparts. Furthermore, the experimentally obtained fatigue lives were compared to three fatigue life prediction models and the Helgason and Hanson model yielded the closest correlation with the experimental results. IV ABSTRACT Crack densities were found to increase with a longer fatigue life. An increase in damage extent was found to positively affect crack development and overall stiffness of the specimen during longterm fatigue testing. This finding was further substantiated by an assessment of midspan deflection, compression concrete strain and carbon fibre strain results, which all suggested a lower neutral axis and a lower stiffness reduction rate under fatigue loading as the damage extent was increased from 450mm to 1800mm. Furthermore, the tension concrete cracks propagated gradually during longer fatigue tests periods, while the tension steel and carbon fibre were comparably less affected by the resultant internal forces. Unfortunately, the neutral axis strain measurements using DEMEC targets were unable to assess the relative effect of an increase in damage extent as well as the compression concrete and carbon fibre strains were able to. During this experimental period, it was established that the laboratory layout was not conducive for carrying out the DIC process of long-term cyclic loading tests. The area in which testing took place did not adequately protect the camera against the environment and therefore required daily storage of the equipment. Regular movement of the camera for storage purposes introduced measurement inaccuracies which accumulated over longer test periods of up 20 days. However, for the short-term tests that did not require movement of the camera, the DIC process yielded favourable results. It was possible to capture the crack patterns early in the test period when the crack growth rate and development of new cracks was high using DIC. It was found that the high strain cracks coincided with the points of maximum vertical deflection (obtained through DIC) and eventual failure location of the specimen. The points of maximum deflection obtained from the DIC process were often not at midspan, which in the absence of the DIC process, would not have been possible to predict accurately. The results have shown that the specimens with the longer damage extents exhibit improved fatigue performance than their shorter counterparts. This revealed a stark contrast to their monotonic loading performance which favoured shorter damage extents. Furthermore, DIC holds potential to predict failure location more accurately than conventional approaches used for structural health monitoring (SHS)

Functionally graded plate fracture analysis using the field boundary element method

This paper describes the Field Boundary Element Method (FBEM) applied to the fracture analysis of a 2D rectangular plate made of Functionally Graded Material (FGM) to calculate Mode I Stress Intensity Factor (SIF). The case study of this Field Boundary Element Method is the transversely isotropic plane plate. Its material presents an exponential variation of the elasticity tensor depending on a scalar function of position, i.e., the elastic tensor results from multiplying a scalar function by a constant taken as a reference. Several examples using a parametric representation of the structural response show the suitability of the method that constitutes a Stress Intensity Factor evaluation of Functionally Graded Materials plane plates even in the case of more complex geometries