Mohammed H. Baluch

Strengthening of Initially Loaded Reinforced Concrete Beams Using FRP Plates

In this study, the reinforced concrete (RC) beams are initially loaded to 85% of the ultimate flexural capacity and subsequently repaired with FRP (fiber reinforced plastic) plates, bonded to the soffit of the beam. The plate thickness is varied to assess the premature failure initiated at the plate curtailment zone due to the high concentration of shear and peeling stresses. Different repair and anchoring schemes were conducted in an effort to eliminate such failures and insure ductile behavior. The results indicated that the flexural strength of the repaired beams is increased. The ductile behavior of the repaired beams is inversely proportional to the plate thickness. The use of an I-jacket plate provided a proper anchorage system and improved the ductility of beams repaired with plates of large thickness.

Flexural behavior of precracked reinforced concrete beams strengthened externally by steel plates

This paper presents comprehensive data and their interpretation on the plate bonding repair technique in terms of effects of plate thickness and end anchorage on ductility, ultimate load, and mode of failure. Reinforced concrete (RC) beams were preloaded to 85 percent of their ultimate capacity and subsequently repaired by bonding steel plates of different thicknesses with and without end anchorages. Anchor bolts were used for end anchorages. The repaired beams showed higher strength than the original beams, provided the plates did not exceed a certain limiting thickness. Increasing the plate thickness changed the mode of failure of the repaired beams from flexural to premature failure, developed due to shear and/or tearing of the plate, causing a reduction in ductility. End anchorages to the bonded plates could not prevent the premature failure of the beams but improved ductility with decreasing significance as the plate thickness increased, and yielded a marginal improvement in ultimate strength. A procedure for designing the bonding plate to avoid premature failure is suggested.

REPAIRED REINFORCED CONCRETE BEAMS

The repair of cracks is a necessity, and a comparison between repair methods for reinforced concrete beams subjected to different levels of cracking was made experimentally. Four methods of repair were studied: epoxy injection; ferrocement; steel-plate bonding; and combined method of epoxy injection and ferrocement. Levels of damage studied range from beam cracking at service load to complete failure of the beams. Experimental data on strength and ductility characteristics of repaired beams were obtained and comparisons were made. Epoxy injection is shown to restore strength and ductility for all levels of damage studied while ferocement increases the strength and partially restores ductility, depending on the level of damage. The steel-plate bonding repair technique leads to an increse in strength, but concomitantly with considerable reduction in ductility of the repaired beams, regardless of the level of damage. The combined method of repair leads to both increase in strength and ductility. The increase in ductility will depend on the level of damage.

Simulation of Chloride Migration in Compression-Induced Damage in Concrete

The ingress of chloride ions in a harsh chloride environment leads to the corrosion of reinforcing steel bars and reduction in the service life of concrete structures. Prediction of the service life of chloride-exposed concrete structures is strongly dependent on chloride diffusion coefficient, which in turn is influenced by several factors including materials and environmental parameters, binding of chloride ions to the solid phase of concrete, and the stresses caused by external load acting on the structure. The diffusion of chloride in concrete is increased significantly by damage associated with load-induced stress, which, if not taken into account, can lead to erroneous prediction of the service life. This paper investigates the impact of compressive stress-induced damage on chloride transport in concrete. An experimental investigation involving non-steady-state migration test as per NT BUILD 492 was carried out on unloaded concrete specimens damaged under axial compressive load. Numerical simulation of the rapid chloride-migration test using multiphysics finite element software is presented in which a phenomenological damage model and a chloride-binding isotherm coupled with Nernst-Planck equation is used to evaluate chloride migration in the damaged concrete specimens.

Simulation of shrinkage distress and creep relief in concrete repair

Abstract This paper addresses the problem of stress buildup in the repair layer of a concrete patch repair system resulting from moisture diffusion. As moisture evaporates from the repair layer into the surrounding ambience of known relative humidity, the hardened concrete substrate restrains free shrinkage movement of the repair layer. As a consequence, primary tensile stresses are set up in the repair layer together with shear and peeling stresses at the interface of the repair layer-concrete substrate. The repair layer under non-uniformly increasing tensile shrinkage stresses undergoes restrained creep in tension, which results in the development of secondary stresses in the system. The secondary stresses due to restrained creep being of opposite sign to that of restrained shrinkage serve to relieve the primary shrinkage stress field and the net or combined stress buildup as a result is reduced. A finite element based computer program used for computing the time dependent moisture loss profile in the repair system is interfaced with a finite element based 2-D stress analysis program for computing the time dependent restrained shrinkage and creep stresses. Variation of normal and shear stresses across depth and width at critical locations in the patch repair and temporal variation of these stresses are presented. Influence of ultimate free shrinkage strain e sh ∞ and the buildup of tensile stresses versus the evolution of tensile strength capacity f ′ t of the repair is highlighted. Also, possible zones of failure are identified in the repair layer and at the interface of the patch repair system.

Elasto-damage Model for High Strength Concrete Subjected to Multiaxial Loading:

An effective compliance matrix Č is proposed to model the behavior of concrete based on phenomenological evidence and physical insight. Three parameters α, β, and γ are introduced in the effective compliance matrix Č. α and β are introduced to model the different behavior of concrete in tension and compression, while the third parameter γ is introduced to account for volumetric change. The predictive capability of the proposed elasto-damage model for uniaxial and multiaxial stress paths is investigated for uniaxial compression, biaxial compression, triaxial compression, uniaxial tension, and tension—compression— compression. The simulative capability of the model to capture the phenomenological behavior of concrete such as strain softening, stiffness degradation, biaxial strength envelope, volumetric dilatation, different behavior in tension and compression, and gain in strength under increasing confinement is reflected. The predicted results correlate well with the available experimental data.

Damage Model for Monotonic and Fatigue Response of High Strength Concrete

An anisotropic elasto-damage model for predicting the response of concrete subject to montonic and fatigue loading is presented in this study. The model utilizes a concrete appropriate damage-effect tensor M in constructing the constitutive equations. The concept of multiple bounding surfaces is used, with a varying size limit fracture surface defining fatigue loading in contrast to a fixed size limit fracture surface for monotonic loading. The model after calibration is shown to predict the monotonic, compressive uniaxial stress-strain path for concretes of various strengths as well as S-N curves depicting the fatigue response of concrete.

The effect of thermal cycling on the durability of concrete made from local materials in the Arabian Gulf countries

Abstract This paper presents results of a study on the effect of heating/cooling cycles on the durability of concrete made from available limestone aggregates in the Arabian Gulf countries. The coefficients of thermal expansion of three different limestone rocks, mortar and concrete were determined. Specimens of limestone rocks, mortar and concrete were heated in a temperature controlled oven to 80°C for 24 hours and then cooled in the oven to room temperature for another 24 hours. After 30, 60 and 90 cycles, specimens were tested for compressive and flexural strengths, pulse velocity and permeability. The test results indicate that the experimentally determined values of the coefficient of thermal expansion vary from 5.07 to 9.99 × 10 −6 /°C and that the limestone rocks tested are thermally anisotropic. Also, it is indicated that heating/cooling of concrete affects its durability performance as it loses up to 27 and 32% of its compressive and flexural strength, respectively. For all tested specimens, the greatest loss of strength is observed after 30 cycles.

CDM model for residual strength of concrete under cyclic compression

Abstract An elasto-damage model developed recently for predicting response of concrete subjected to fatigue loading (Int. J. Damage Mech. 9(1) (2000) 57), is extended to predict the residual strength of concrete subjected to initial damage resulting from the application of a known number of stress cycles. Experimental corroboration of results is established by subjecting 75×150 mm cylinders of a high quality, pre-packaged repair concrete to damage due to a specified number of cycles in axial compression followed by loading to failure to record the residual strength.

Towards a canonical elastoplastic damage model

Abstract Fundamental aspects of elastoplastic damage are outlined. Time-independent isotropic damage is considered in order to study material degradation. By splitting the total strain tensor into its components of elastic damage and plastic damage and using recoverable energy equivalence, three distinct modes of behavior are particularized. For each mode of behavior, a suitable damage variable is culled. An in-depth analysis of this formulation reveals a certain incongruity in the assumptions postulated in some of the previously proposed models. The suggested generalized concepts are supported by experimental evidence.