Yail J. Kim

PREDICTIVE RESPONSE OF NOTCHED STEEL BEAMS REPAIRED WITH CFRP STRIPS INCLUDING BOND-SLIP BEHAVIOR

This paper presents the flexural behavior of notched steel beams repaired with carbon fiber-reinforced polymer (CFRP) strips. A combined experimental and computational approaches are used to examine local plasticity near the damage and the effects of CFRP-repair. A modeling approach is proposed to take into account the bond-slip behavior of CFRP-steel interface. The experimentally validated models are further used to conduct a parametric study addressing various engineering properties of CFRP composites and adhesives. The CFRP-repair is shown to restore the strength of the damaged beam. The CFRP strip relieves the stress concentration resulting from the presence of the notch, reducing the high local plasticity. The parametric study confirms the improved effectiveness of high modulus CFRP (i.e., exceeding 150 GPa) in affecting repairs of steel members. Under static loading conditions, the stiffness of the adhesive bond line influences the local behavior of the CFRP-steel interface but has little effect on the overall member behavior.

Identifying Critical Sources of Bridge Deterioration in Cold Regions through the Constructed Bridges in North Dakota

This paper presents the performance of constructed bridges in cold regions through examining the bridges in North Dakota that is one of the coldest regions in the United States. Unique approach of a combined multiple regression and geographic information system technology is employed to evaluate the performance and to identify the critical sources affecting deterioration of the 5,289 bridges sampled from the National Bridge Inventory database inspected between 2006 and 2007. Typical parameters examined include physical, material, and environmental factors associated with the existing bridges. The importance of maintenance and repair is highlighted. Traffic volume significantly influences the level of deterioration of the bridge decks. Year built is the most significant contribution to the structural deficiency of the bridges, followed by structural characteristics and traffic volumes. The presence of water particularly influences the deterioration. Concrete bridges are more durable than steel bridges. Tru...

CFRP Repair of Steel Beams with Various Initial Crack Configurations

This paper presents an experimental program to study the behavior of notched steel beams repaired with carbon-fiber-reinforced polymer (CFRP) sheets. Particular attention is paid to examining the interaction between the level of initial damage (i.e., notch depth) and CFRP repair. Multiple stages of fatigue-crack-propagation in a steel beam are simulated by various notch sizes, including a0/h ratios=0.1, 0.3, and 0.5 in which a0= notch depth; and h = beam height. A modeling approach is proposed to simultaneously take into account crack-propagation across the steel section and debonding of the CFRP. Efficacy of the CFRP repair is more pronounced when the damage level increases. CFRP sheet stabilizes the crack mouth opening displacement of the repaired beams until significant debonding takes place. The level of initial damage affects the behavior of a plastic region above the notch tip, the rate of web fracture, and the initiation of CFRP debonding. The damage, however, does not influence the failure mode of...

Flexural Strengthening of RC Beams Using Steel Reinforced Polymer (SRP) Composites

Synopsis: This paper presents the application of a new generation of externally bonded composite material in flexural strengthening of reinforced concrete beams. The steel reinforced polymer (SRP) composite consists of high-carbon steel unidirectional Hardwire® fabrics embedded in epoxy resin, and offers high strength and stiffness characteristics at a reasonable cost. In this paper, the mechanical properties of SRP are evaluated and its application in flexural strengthening of RC beams is investigated. Six beams have been tested in three-point bending to study the effect of SRP retrofitting on flexural behavior, failure modes, and crack patterns. Test parameters include variation of the width of SRP sheets and the use of SRP U-wraps at both ends to prevent premature failure caused by delamination of the longitudinal sheet. Significant increase in flexural capacity, up to 53 %, and pseudo-ductile failure modes have been observed in the SRP-strengthened beams. Failure is governed primarily by concrete cover delamination at the tips of the SRP sheets or crushing of concrete at mid-span. It is also shown that the U-wraps have improved flexural stiffness by means of controlling diagonal crack width and providing anchorages to the longitudinal SRP sheets, which reduces their slip. Shear stress concentration near the cut-off point of the SRP sheet has also been investigated. An analytical model is proposed to predict the nominal strength of the SRP-strengthened beams.

Recent Developments in Long-Term Performance of FRP Composites and FRP-Concrete Interface

This paper presents recent developments in long-term behavior of fiber reinforced polymer (FRP) composites and FRP-strengthened concrete beams with special emphasis on the FRP-concrete interface subjected to creep and fatigue loads. Although short-term behavior of FRP-strengthened structures is intensively reported thus far, their long-term performance has not been thoroughly elucidated yet. This state-of-the-art paper provides a synthesis of recent findings on the long-term performance of FRP composites and their time-dependent bond behavior for strengthening concrete structures, including externally-bonded FRP sheets and near-surface-mounted FRP strips. The review examines the bond-stress slip response, damage accumulation associated with progressive crack propagation, and failure modes, including descriptions of the predictive models. The identified research needs to further advance FRP-strengthening technologies are addressed.

Flexural Response of Concrete Beams Prestressed with AFRP Tendons: Numerical Investigation

This paper presents the flexure of concrete beams prestressed with aramid fiber-reinforced polymer (AFRP) tendons. Three-dimensional nonlinear finite-element analysis and iterative sectional analysis are conducted to predict the behavior of AFRP-prestressed members, including experimental validation. The beams are simply supported and monotonically loaded until failure occurs. The sectional properties of the beams include a reinforcement ratio of 0.15% to 0.36% and an Ig / Icr ratio of 25 to 77, where Ig and Icr are the gross and cracked moment of inertia, respectively. Various prestressing levels are applied to the beams to evaluate the load versus displacement response, variation of neutral axis depth, effective moment of inertia, and deformability of the beams. The applicability of code provisions and existing predictive equations are examined. The prestress level in the AFRP tendons significantly influences the flexural behavior of the beams, namely, cracking load, strain development of AFRP, and neut...

Bond and Short-term Prestress Losses of Prestressed Composites for Strengthening PC Beams with Integrated Anchorage

This article presents modeling of bond performance and short-term prestress losses of prestressed carbon fiber-reinforced polymer (CFRP) composite sheets for strengthening prestressed concrete beams, using an integrated anchor system that consists of steel plates bonded with CFRP sheets. A simple fracture mechanics model, validated with the experiment, is developed to examine the bond performance of the CFRP sheets for the plate-type anchor system. To predict the short-term prestress losses of the prestressed CFRP sheets used in the integrated anchor system, a closed-form solution is developed. Seven prestressed concrete beams strengthened with prestressed CFRP sheets are used to validate the proposed model. Fracture energy and tensile modulus of the CFRP sheets are the most critical factors affecting debonding failure of the plate-type anchor system. For design purposes, it is recommended that the short-term prestress loss be 10% of the applied prestress using the proposed anchor system.

Punching Shear of Two-way Slabs Retrofitted with Prestressed or Non-prestressed CFRP Sheets

This study presents punching shear behavior of two-way slabs strengthened with prestressed or non-prestressed carbon fiber reinforced polymer (CFRP) sheets. Four two-way slabs (2360 × 2360 × 150 mm3) with a steel reinforcement ratio of 1.44% are tested under concentric load. All slabs exhibit a punching shear failure mode. The strengthened slabs show an increase of up to approximately 20% in load-carrying capacity and an increase of up to 25% in cracking load with respect to the unstrengthened control slab. A detailed stress analysis in reinforcement is conducted along the loading span of the slabs, including the critical shear perimeter surrounding the column stub. The effective strain zone near the slab—column connection, where a sudden increase of strains in the reinforcement is observed, is also studied. The development of shear stresses in the vicinity of the slab—column connection is examined. A non-linear 3D finite element analysis is conducted and analytical predictive models for the punching shear failure are evaluated as well.

Load Configuration and Lateral Distribution of NATO Wheeled Military Trucks for Steel I-Girder Bridges

This paper presents the lateral load distribution of various North Atlantic Treaty Organization (NATO) wheeled military trucks on a simple-span steel I-girder bridge (L=36 m). The military trucks are classified into the military load classification (MLC) system. The MLC trucks demonstrate different load configurations when compared to the standard HS20 truck in terms of wheel-line spacing, number of axles, and weight. A calibrated three-dimensional finite-element analysis is conducted to examine the MLC load effects. The applicability of the AASHTO LRFD provisions is evaluated using 72 different load models. The wheel-line spacing and weight of the MLC trucks cause different flexural behavior and load distributions of the bridge when compared to those of HS20. The current AASHTO LRFD approach to determine live load distribution factors may be reasonably applicable to the MLC trucks, including approximately 20% of conservative predictions.

Moving-Wheel Fatigue for Bridge Decks Strengthened with CFRP Strips Subject to Negative Bending

This paper presents the negative bending of reinforced concrete slabs strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. Six slab specimens, three of which are strengthened with CFRP strips, are tested in static and fatigue loads. A wheel-running fatigue test machine is used to simulate vehicular loads on a bridge deck. The effectiveness of CFRP strengthening for bridge decks in cantilever and pseudonegative bending is examined based on moment-carrying capacity and cyclic behavior under the wheel-running fatigue loads, including crack patterns and damage accumulation. The moment-carrying capacity (static) of the cantilever slab strengthened with the NSM CFRP strips is improved by 68.4% when compared to that of an unstrengthened slab. The damage accumulation rate of the strengthened cantilever slab owing to the fatigue load is significantly lower than that of the unstrengthened slab. The damage accumulation of the strengthened slab gradually increases and is irrever...