Wael W. El-Dakhakhni

Coplanar capacitance sensors for detecting water intrusion in composite structures

Composite materials are becoming more affordable and widely used for retrofitting, rehabilitating and repairing reinforced concrete structures designed and constructed under older specifications. However, the mechanical properties and long-term durability of composite materials may degrade severely in the presence of water intrusion. This study presents a new non-destructive evaluation (NDE) technique for detecting the water intrusion in composite structures by evaluating the dielectric properties of different composite system constituent materials. The variation in the dielectric signatures was employed to design a coplanar capacitance sensor with high sensitivity to detect such defects. An analytical model was used to study the effect of the sensor geometry on the output signal and to optimize sensor design. A finite element model was developed to validate analytical results and to evaluate other sensor design-related parameters. Experimental testing of a concrete specimen wrapped with composite laminate and containing a series of pre-induced water intrusion defects was conducted in order to validate the concept of the new technique. Experimental data showed excellent agreement with the finite element model predictions and confirmed sensor performance.

Alternative Strategies to Enhance the Seismic Performance of Reinforced Concrete-Block Shear Wall Systems

In this paper, seven reinforced concrete-block shear walls with aspect ratios of 1.5 and 2.2 (two- and three-storey high) were tested under displacement-controlled cyclic loading. The response of rectangular, flanged, and end-confined walls, designed to have the same lateral resistance when subjected to the same axial load, is discussed. In general, high levels of ductility accompanied by relatively small strength degradation were observed in all walls with a significant increase in ductility and displacement capabilities for the flanged and end-confined walls compared to the rectangular ones. For both aspect ratios evaluated, the drift levels at 20% strength degradation were 1.0, 1.5, and 2.2% corresponding to the rectangular, the flanged, and the end-confined walls, respectively. The ductility values of the proposed flanged and end-confined walls were, respectively, 1.5 and 2 times those of their rectangular wall counterparts (with the same overall length and aspect ratio). In addition to the enhanced d...

Non-destructive evaluation of laminated composite plates using dielectrometry sensors

The use of composite materials in marine, aerospace and automotive applications is increasing; however, several kinds of damages of composite materials may influence its durability and future applications. In this paper, a methodology was presented for damage detection of laminated composite plates using dielectrometry sensors. The presence of damage in the laminated composite plate leads to changes in its dielectric characteristics, causing variation in the measured capacitance by the sensors. An analytical model was used to analyse the influence of different sensor parameters on the output signals and to optimize sensor design. Two-dimensional finite element (FE) simulations were performed to assess the validity of the analytical results and to evaluate other sensor design-related parameters. To experimentally verify the model, the dielectric permittivity of the composite plate was measured. In addition, a glass fibre reinforced polymer (GFRP) laminated plate containing pre-fabricated slots through its thickness to simulate delamination and water intrusion defects was inspected in a laboratory setting. Excellent agreements were found between the experimental capacitance response signals and those predicated from the FE simulations. This cost-effective technique can be used for rapid damage screening, regular scheduled inspection, or as a permanent sensor network within the composite system.

Force- and Displacement-Based Seismic Performance Parameters for Reinforced Masonry Structural Walls with Boundary Elements

AbstractThere is a need to evaluate existing, and introduce new, masonry construction techniques within a performance-based seismic design context to advance their adoption in the next generation of seismic design codes in North America. In this regard, a reinforced masonry (RM) structural wall system that incorporates confining boundary elements in the plastic hinge region presently lacks specific design requirements in the Masonry Standards Joint Committee and Canadian Standards Association S304.1 design codes. In addition, RM structural wall systems are omitted completely from the plan to develop a new performance-based seismic design methodology laid out by FEMA 445. This paper presents experimental results of four specially detailed RM structural walls subjected to fully reversed cycles of displacement-controlled loading as tested by the authors. In addition, the resulting analysis also includes one wall from a previous study for comparison purposes. All walls were detailed with lateral reinforcing t...

Seismic Performance Parameters for Reinforced Concrete-Block Shear Wall Construction

The focus of the current study is to analyze previously reported test results to evaluate equivalent plastic hinge lengths for concrete-block shear walls and to extract related seismic performance parameters. Inelastic curvatures at the base of the walls are the main source of plastic deformation for flexurally dominated walls. With adequate estimation of the plastic hinge length and more realistic values for inelastic curvatures at the base of the wall, top wall displacement can be predicted more accurately. For the walls analyzed, measured compressive strains close to the base of the wall at maximum load were significantly higher than the strains specified by North American codes. The strains, although may not alter the wall strength, significantly affect the displacement ductility at maximum load. The analysis showed that the equivalent plastic hinge length varied between approximately 30 and 60% of the wall length at a drift of 1%. The product of the seismic force modification factors (for ductility a...

Validity of SDOF Models for Analyzing Two-Way Reinforced Concrete Panels under Blast Loading

The combined manual TM 5-1300/NAVFAC P-397/AFR 88-22, Structures to Resist the Effects of Accidental Explosions, published by the joint departments of the Army, the Navy, and the Air Force, has been used in all NATO countries for the past 50 years for protective design applications. The manual was recently reformatted to meet the Department of Defense Unified Facility Criteria (UFC). As a first step, the current production of the new document, UFC 3-340-02, focused on making the original TM 5-1300 available in a more functional format so that future technical updates can be facilitated. In this study, a single-degree-of-freedom (SDOF) model, based on the guidelines of the UFC 3-340-02, was used to formulate a FORTRAN code to predict the response of SDOF systems under blast. The code was used to generate pressure-impulse ( P-I ) diagrams for a series of two-way reinforced concrete (RC) panels with different dimensions, aspect and reinforcement ratios, and support conditions. The P-I diagram predictions wer...

Seismic Performance Parameter Quantification of Shear-Critical Reinforced Concrete Masonry Squat Walls

AbstractIn North America, the design of reinforced masonry (RM) shear walls as a seismic force-resisting system (SFRS) is very conservative in terms of quantifying the wall’s shear strength and ductility capacity. In the current study, eight full-scale squat RM walls were tested at McMaster University under quasi-static reversed cyclic loading to quantify their shear strength, idealized displacement ductility, drift-damage relationships, lateral stiffness degradation, and hysteretic damping. The test walls demonstrated shear-strength capacities up to 200% of those predicted by the Canadian Standards Association’s (CSA) masonry design code. In addition, the results demonstrated that the Canadian shear strength expression was, on average, 32 and 34% more conservative than the Masonry Standards Joint Committee (MSJC) and the New Zealand Standards (NZS) design codes for masonry, respectively. Moreover, the simplified modified compression field theory (SMCFT), adopted directly from the Canadian concrete design...

Characteristics of Rectangular, Flanged, and End-Confined Reinforced Concrete Masonry Shear Walls for Seismic Design

This paper contains detailed analyses of an experimental study conducted to evaluate the ductility, stiffness degradation and energy dissipation characteristics of rectangular, flanged, and end-confined reinforced masonry (RM) shear walls failing in flexure. The test program consisted of seven two- and three-story RM shear walls, with aspect ratios of 1.5 and 2.2, tested under reversed cyclic lateral displacements simulating seismic loading effects. Documentation of the compressive strains at the wall toes, wall base curvatures, and ductility levels attained are presented. The paper focuses on determining the extent of plasticity over the wall height, evaluating the contribution of flexure and shear deformations to the overall wall lateral displacements, identifying the trend of stiffness degradation, and quantifying the amount of energy dissipation. The rectangular walls displacement predictions at ultimate loads using Canadian Standards Association (CSA) S304.1 were in better agreement with the experimental results compared to the Masonry Standards Joint Committee (MSJC) code predictions. However, both the MSJC code and the CSA S304.1 significantly overestimated the test results for the flanged and end-confined walls. Analysis of the measured displacements showed that the contribution of shear displacement to the overall wall displacement was, on average, 21 and 25% of the total displacement for the walls with aspect ratio of 2.2 and 1.5, respectively. The relationship between the energy dissipation and the ratio of the postyield to the yield displacements was found to be almost linear for the test walls. In addition, the wall stiffnesses degraded rapidly to about 60% of their gross stiffness at very low drift levels (0.1% drift). Measured compressive strain at the wall toes were almost double those specified in both North American codes. Extent of plasticity over the wall height was about 75% of the wall length. The data presented in this paper is expected to facilitate better understanding of RM wall behavior under in-plane load to researchers, practicing engineers, and code developers. This study aimed at presenting the flanged and end-confined categories as cost-effective alternatives to enhance the seismic performance of midrise RM construction in North America.

Plastic Hinge Model and Displacement-Based Seismic Design Parameter Quantifications for Reinforced Concrete Block Structural Walls

AbstractA practical alternative to the traditional rectangular cross sections of reinforced masonry structural wall systems is to alter the wall ends to allow for smaller compression zone depths, and thus higher curvatures to develop under increased seismic lateral loads. Despite the significantly enhanced seismic performance of flanged and end-confined masonry structural walls compared with their rectangular counterparts, seismic design parameters related to the former two types of walls have not been widely investigated. In addition, prescriptive design requirements for rectangular walls are under continuous development to meet the ongoing research findings in this area. The focus of the current study is to extract specific seismic design parameters of these three types of masonry walls having different end configurations for different aspect ratios when tested under reversed cyclic loads. The parameters investigated include the equivalent plastic hinge lengths, lp, the hysteretic damping levels and the...

Influence of Slenderness on the Behavior of a FRP-Encased Steel-Concrete Composite Column

AbstractThe compressive behavior of a steel-concrete composite column encased in a fiber reinforced polymer (FRP) tube is evaluated experimentally for columns with various slenderness ratios. The composite column consists of a FRP tube surrounding a steel I-section that is subsequently filled with concrete. A total of nine column specimens were tested ranging between 500xa0and 3,000xa0mm in height. Confinement and composite action resulted in enhanced compressive behavior of the composite columns. Maximum confinement occurred in the short column (slenderness ratio less than 0.2). Confinement action reduced with increased height of the column specimens. The column load- carrying capacity, ultimate axial strain, and compressive strength of the confined concrete core in the longest specimen (slenderness ratio of 0.9) were reduced to approximately 59, 14, and 51% of the short column values, respectively. A buckling strength curve of the composite columns was developed on the basis of the experimental results.