Abdeldjelil Belarbi

Constitutive Laws of Softened Concrete in Biaxial Tension Compression

A softened truss model was developed over the last 10 years at the University of Houston for predicting the behavior of reinforced concrete subjected to shear and torsion. The model, based on the conditions of equilibrium and compatibility, employs realistic constitutive laws for concrete and reinforcement. The required constitutive laws for concrete and reinforcement were examined by testing 22 full-size reinforced concrete panels. These large test panels were subjected to tension in one direction and compression in the perpendicular direction. Testing showed that the strength and stiffness of concrete in compression were softened primarily by the presence of tensile strains in the perpendicular direction. The effect of other parameters, e.g., load path, percentage of steel, and spacing of reinforcing bars, was assessed. Based on these tests, improved softened stress-strain relationships of concrete in compression were developed.

Crack detection of a full-scale reinforced concrete girder with a distributed cable sensor

A new concept of designing cable sensors for health monitoring of large-scale civil infrastructure has recently been proposed by the present authors. The concept was developed based on the change in topology of the outer conductor of a coaxial cable sensor. One such sensor was fabricated with its outer conductor tightly wrapped with a commercial tin-plated steel spiral that was covered with solder. It was mounted near the surface of a 15 m long reinforced concrete (RC) girder with a 762 mm square hollow cross section and 152 mm thick walls. The girder was tested under a progressively increasing cyclic torsion creating 45° inclined cracks around and along the girder. The main objectives of this study were to implement the distributed cable sensor technology in large-scale reinforced concrete structures, to understand the performance of a sensor under cyclic loading for detecting and locating cracks, and, finally, to address implementation issues such as signal loss, non-uniformity in sensor construction, and recoverability.

TORSION OF HIGH-STRENGTH REINFORCED CONCRETE BEAMS AND MINIMUM REINFORCEMENT REQUIREMENT

To study the effect of high-strength (HS) concrete on torsional behavior of reinforced concrete (RC) beams, 9 full-size beams were tested under pure torsion. The main study parameters were concrete strength and amount of reinforcement. Concrete strength ranged from normal through all grades of HS concrete. The amount of reinforcement varied from less than the minimum to the balanced condition (when expected crushing of concrete occurs at the same time as yield of steel). With the aim of keeping the inclination of the concrete struts at roughly 45 degrees, equal percentages of reinforcement were provided in transverse and longitudinal directions. Results show that the minimum amount of reinforcement defined in ACI 318-99 is inadequate for equilibrium torsion of HS RC beams, and a new expression is proposed. It was found that the torsional capacity of under-reinforced beams is independent of concrete strength, and the amount of londitudinal reinforcement was more effective in controlling crack width than the amount of transverse reinforcement (stirrups).

Effective Bond Length of FRP Sheets Externally Bonded to Concrete

Strengthening and repair of concrete structures using externally bonded fiber reinforced polymer (FRP) composite sheets has been popular around the world during the last two decades. However, premature failure due to debonding of the FRP is one of the important issues still to be resolved. Numerous research studies have dealt with the debonding problem in terms of Effective Bond Length (EBL), however, determination of this length has not yet been completely assessed. This paper summarizes previous works on the EBL and proposes a new relationship of the EBL with the FRP stiffness based on the existing experimental data collected in this study.

Seismic Test Methods for Architectural Glazing Systems

An ongoing effort is being made at the University of Missouri-Rolla to develop standard laboratory test methods and codified design procedures for architectural glass under seismic loadings. Recent laboratory work has yielded some promising results regarding the development of an appropriate seismic test method for architectural glass, as well as identifying ultimate limit states that quantify the seismic performance and damage thresholds of various glass types. Specifically, a straightforward “crescendo-like” in-plane dynamic racking test, performed at a constant frequency, has been employed successfully. Two ultimate limit states for architectural glass have been defined: (1) a lower ultimate limit state corresponding to major glass crack pattern formation; and (2) an upper limit state corresponding to significant glass fallout. Early crescendo tests have yielded distinct and repeatable ultimate limit state data for various storefront glass types tested under dynamic racking motions. Crescendo tests will also be used to identify and quantify serviceability limit states for architectural glass and associated glazing components under dynamic loadings. These limit state data will support the development of rational design procedures for architectural glass under seismic loadings.

A Universal Panel Tester

A universal panel tester was built at the University of Houston to test reinforced concrete panel elements subjected to any combination of in-plane and out-of-plane forces. The forces are applied by 40 in-plane hydraulic jacks of 250-kip (1120-kN) capacity each and 20 out-of-plane hydraulic jacks of 140-kip (630-kN) capacity. The sophisticated hydraulic system makes it possible to simulate any complex stress condition in a panel.

Flexural Behavior of Fiber-Reinforced-Concrete Beams Reinforced with FRP Rebars

Synopsis: The main objective of this study was to develop a nonferrous hybrid reinforcement system for concrete bridge decks by using continuous fiber-reinforcedpolymer (FRP) rebars and discrete randomly distributed polypropylene fibers. This hybrid system has the potential to eliminate problems related to corrosion of steel reinforcement while providing requisite strength, stiffness, and desired ductility, which are shortcomings of the FRP reinforcement system in reinforced concrete structures.

Smart aggregate based damage detection of circular RC columns under cyclic combined loading

Structural health monitoring is an important issue for the maintenance of large-scale civil infrastructures, especially for bridge columns. In this paper, an innovative piezoceramic-based approach is developed for the structural health monitoring of reinforced concrete columns. An innovative piezoceramic-based device, the smart aggregate, is utilized as a transducer for the purpose of health monitoring. To investigate the seismic behavior of reinforced concrete (RC) bridge columns, structural health monitoring tests were performed on two bridge columns under combined reversed cyclic loading at the Missouri University of Science and Technology. The proposed smart aggregate based approach successfully evaluated the health status of concrete columns during the loading procedure. Sensor energy plots and 3D normalized sensor energy plots demonstrated that the damage inside attenuated the transmitted energy. The wavelet packet based damage index and sensor history damage index evaluate the damage development in concrete columns under cyclic loading.

Behavior of RC T-Beams Strengthened in Shear with CFRP under Cyclic Loading

This study investigated the shear performance of an RC beam strengthened in shear with externally bonded carbon fiber-reinforced polymer (CFRP) strips, subjected to a cyclic loading for 2 million cycles at 1 Hz. The stress level in the stirrups caused by the cyclic loading used in this study was higher than those typically used in fatigue studies, which could have caused the yielding of some stirrups from the beginning of cyclic loading. This was done to reflect the fact that many RC beams in need of shear strengthening do not meet the minimum stirrup requirement for the new and increased shear demand. The experimental results obtained in this study and a comprehensive review of the existing literature showed that RC beams strengthened in shear with externally bonded CFRP could survive 2 million cycles of cyclic loading without failure. Furthermore, the residual shear strength of the FRP-strengthened beam appeared to be greater, albeit slightly, than the static shear strength of the unstrengthened control beam. This study’s results also suggested that limiting the interfacial stress in CFRP strips to less than 1.5 MPa or 25% of its ultimate interfacial strength would increase fatigue life by avoiding debonding of CFRP strips.

Design of FRP Systems for Strengthening Concrete Girders in Shear

Fiber-Reinforced Polymer (FRP) systems have been used on a project-specific basis for the last two decades. They are now becoming a widely accepted method of strengthening concrete structures. The acceptance and utilization of these new strengthening techniques depend on the availability of clear design guidelines, installation procedures and construction specifications. Standard specifications exist for all commonly used traditional materials in civil engineering structures. At this time, design specifications for FRP use are still under development. The results of several experimental investigations have shown that FRP systems can be effective for increasing ductility and strength to structural members such as columns and girders. As most of the research focused on strengthening of axial members of flexural members, there are less experimental and analytical data on the use of FRP systems for shear strengthening of girders. Shear strengthening with FRP is still under investigation and the results obtained thus far are scarce and sometimes controversial. Even in traditional reinforced concrete members without FRP, the shear design is a complex challenge and uses more empirical methods as compared to axial and flexural design methods. Adding FRP to the equation, with its specific design issues, would bring another level of complication in the design. These FRP-related shear design issues and lack of comprehensive analytical and experimental models are the main motivation for this research project. Thus, a thorough understanding of the shear design problem along with the development of an American Association of State Highway and Transportation Officials (AASHTO) design method for FRP shear strengthening of concrete girders is needed. As such, the objective of this project is to develop design methods, specifications, and examples for design of FRP systems for strengthening concrete girders in shear. The proposed specifications will be in Load and Resistance Factor Design (LRFD) format and will be suitable for recommendation to the AASHTO Highway Subcommittee on Bridges and Structures for adoption.