Ali H. Al-Gadhib

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.

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.

Modeling of Shrinkage and Creep Stresses in Concrete Repair

During the repair and rehabilitation of concrete structures, mistakes in design, selection of materials, and construction practices will lead to incompatibility between the repair and the existing concrete substrate. Most of the literature reported in the field of repair reveals that dimensional incompatibility, drying shrinkage in particular, is one of the major problems of concrete repair. This paper addresses the problem of stress buildup in concrete repair in the form of a jacketed column resulting from moisture diffusion. As moisture evaporates from the external jacket into the surrounding ambience of known relative humidity, the hardened concrete column restrains free-shrinkage movement of the repair layer. As a consequence, primary tensile stresses are set up in the jacket caused by restrained shrinkage. The jacketed layer under increasing tensile stresses is also subjected to tensile creep deformation, which results in a stress field of reversed sense as that caused by restrained shrinkage. These secondary stresses caused by restrained creep serve to relieve the primary shrinkage-associated stress field, and, as a result, the net or total stress buildup is reduced. A nonlinear finite element model is used to obtain the time-dependent moisture loss profile in the jacketed column. This model is interfaced with a finite element-based two-dimensional stress analysis program called SHCPAN for computing time-dependent restrained shrinkage and creep stresses. For a typical column repair, numerical experiments are carried out to highlight the influence of both a sealed jacket/column interface and a porous interface on the stress buildup in the jacketed columns.

Experimental and Computational Modeling of Low Cycle Fatigue Damage of CFRP Strengthened Reinforced Concrete Beams

Improvement in the short-term behavior of deficient reinforced concrete beams is very well known through extensive testing of reinforced concrete beams strengthened by externally bonded carbon fiber reinforced polymer strips. However, long-term performance of such beams needs to be assessed before the method can gain full acceptance. Prominent modes of failure in strengthened reinforced concrete beams reported in literature are concrete crushing in compression, carbon fiber reinforced polymer plate rupturing in tension and peeling of the carbon fiber reinforced polymer plate due to high interfacial shear and peeling stresses at the plate cut-off point. Modeling of the overall system of strengthened reinforced concrete beams requires modeling of the individual components that ultimately leads to failure i.e., concrete, carbon fiber reinforced polymer and concrete - carbon fiber reinforced polymer. Component elasto-damage constitutive models are developed for concrete crushing, carbon fiber reinforced polymer rupture and concrete-carbon fiber reinforced polymer interface failure, calibrated by data from both suitably designed experiments and from experiments reported in literature, and are utilized to investigate the low cycle fatigue behavior of reinforced concrete beams strengthened by carbon fiber reinforced polymer strips. The predictive ability of the model is tested by comparison to data from two different beam strengthening schemes i.e., with end anchorage and without end anchorage.

CDM based finite element code for concrete in 3-D

Abstract This paper presents a three dimensional finite element code DAMAG3D for nonlinear analysis of concrete type materials modeled as elastic-damage. The CDM model adopted is the one as proposed by SUARIS W, OUYANG C, FERNANDO V. M. Damage model for cyclic loading of concrete. J Engng Mech, American Society of Civil Engineers 1990; 116(5): 1020–35. for monotonic and cyclic loading of concrete structures. Code DAMAG3D is applied to simulate response of concrete under monotonically increasing load paths of uniaxial compression, Brazilian test, strip loading and patch loading, with reasonable correlation established with experimental results and results from other nonlinear constitutive models.

Finite Element Modeling of CFRP-Strengthened Low-Strength Concrete Short Columns

Carbon fiber-reinforced polymer (CFRP) sheets and plates are now being extensively used as retrofitting/strengthening system, due to high strength, low weight, corrosion resistance, and ease and speed of application. Structures with very low-strength reinforced concrete columns, constructed during the early 1970s and 1980s, are rampant in many countries. These structures are prone to collapse during a seismic event. It is important to investigate if these low-strength concrete columns can be made safe by CFRP sheet strengthening. An experimental and numerical investigation conducted on short circular low-strength concrete column is presented. The results of this study showed that the confinement equations for concrete columns as per ACI and CSA codes give erroneous results for these columns. The CSA code equations require satisfaction of certain constrains, which are not applicable to such columns. Control specimen (unconfined specimens) results showed that both CSA and ACI equations overestimate the ultimate loads of low-strength RC columns by an order in magnitude. For CFRP-confined specimens, the ACI and CSA code equations overestimate the strength by about 13% and 24%.

Evaluation of unconfined compressive strength of carbonate sedimentary rocks in Saudi Arabia using indirect tests

Unconfined compressive strength (UCS) of rocks, determined by loading the rock specimens along their longitudinal axis without lateral restraint, is one of the important engineering properties required for assessing the mechanical behavior of the rocks. However, determining UCS using direct method requires stringent sample preparation and testing that need higher costs and longer time. Therefore, the evaluation of UCS of rocks using the correlations developed empirically between UCS and some relevant parameters obtained through indirect tests on rocks would be an alternative approach. In the present work, 40 samples of carbonate sedimentary rocks, belonging to Cenozoic deposits of the Great Ghawar Uplift geological province, near Ain Dar, Saudi Arabia, were collected. All samples of rocks were tested to determine their mass density, ρ, point load strength index, Is(50), ultrasonic pulse velocity, Vp, and UCS. The test results were analyzed and the correlations of UCS with ρ, Is(50), and Vp were obtained for indirect estimation of UCS of the rocks considered under this study. The relationship between UCS and Vp showed highest degree of fit among three relationships followed by the relationships between UCS and ρ and UCS and Is(50).