A.S. Larkin, J. Tatar, C.W. Dolan (2011). “Parametric Study of Web Bonded CFRP Shear Reinforcement on Internal Steel Stresses”, ACI Special Publication 275, pp. 1-12.

A four-point Iosipescu shear test is used to evaluate the effects of CFRP reinforcement on crack width development and the corresponding stress of the internal steel reinforcement. The test program indicates that the steel reinforcement yields following cracking in a concrete beam test specimen containing a No. 10 (No. 3) 71 mm2 (0.11 in2)-420 MPa (60 ksi) yield strength reinforcing bar in most cases; however, some CFRP reinforced specimens have a steel strain as low as 45 percent of the yield strain prior to the failure between the CFRP and the concrete surface. The research conducted a parametric study and provides a correlation between CFRP shear reinforcement ratios, crack width, and internal steel reinforcement stress. The results are found by testing several shear friction type specimens where the CFRP varies in size while the internal steel reinforcement is held constant.

J. Tatar, H.R. Hamilton (2015). “Bond Durability Factor for Externally Bonded CFRP Systems in Concrete Structures”. ASCE Journal of Composites for Construction, 20(1).

Lack of understanding and confidence in the long-term performance of externally bonded fiber-reinforced polymer (FRP) composites in concrete structures still inhibits their application in repair of aged structures. While synergistic effects of multiple exposure conditions can be severe, it is generally agreed that the single most significant issue with externally bonded FRP composites is their susceptibility to degradation when exposed to moisture. This research utilized small-beam three-point bending test specimens to study FRP–concrete bond performance when subjected to accelerated conditioning environments (immersion in water and exposure to high humidity at elevated temperatures). Bond strength retention (Rb) was determined by dividing the conditioned strength by the average control strength. Test results from the present research were combined with other test data to form a database of over 700 test results. By utilizing an apparent analogy of FRP-concrete bonded systems to adhesive anchors, a bond durability factor (BDF) that quantifies loss in bond capacity due to accelerated conditioning is determined equivalently as a characteristic test value for adhesive anchors. Given their resistance to the environment, only carbon-fiber-reinforced polymer (CFRP) composites were examined. For the purpose of the analyses, and based on available data, it was determined that all CFRP systems may be split into three categories: wet-layup without putty, wet-layup with putty, and precured laminate. BDF corresponding to wet-layup without putty was determined to be 0.60. BDF for wet-layup with putty was not established due to the observed sensitivity to increasing conditioning temperature. CFRP laminate specimens failed prematurely by composite rupture or at the adhesive–composite interface; BDF was not determined as long term durability data corresponding to FRP–concrete bond failure mode was not available.

B.P. Blackburns, J. Tatar, E.P. Douglas, H.R. Hamilton (2015). “Effects of Hygrothermal Conditioning on Epoxy Adhesives Used in FRP Composites”. Construction and Building Materials, 96C, pp. 679-689.

Durability of FRP composites and their bond to concrete is essential to structural integrity of an FRP repair. Epoxy adhesives are used to form FRP composites, and as a bonding medium between the FRP and concrete substrate. Susceptibility of epoxy to the negative effects of water and high temperature affects the longevity of FRP repairs in hygrothermal environmental conditions. The presented work investigates the effects of such environments on the curing kinetics of epoxy. Competing effects of plasticization and post-cure during accelerated conditioning are discussed; recommendations targeting researchers, practitioners, and manufacturers are made based on the research findings.

J. Tatar, H.R. Hamilton (2015). “Implementation of Bond Durability in the Design of Flexural Members with Externally Bonded FRP”. ASCE Journal of Composites for Construction, 20(3).

Externally bonded fiber-reinforced polymer (FRP) composites are becoming a method of choice for the repair/strengthening of civil structures, some of which are situated in environments detrimental to FRP-concrete bond durability. Based on analysis of an extensive database of durability test results from notched-beam three-point bending tests, a bond durability factor (BDF) of 0.60 was identified as an appropriate estimate of bond durability for wet-layup carbon fiber-reinforced polymer (CFRP). This factor was compared to a normalized database of durability data from the literature, and it was found that a 0.60 BDF agrees well with these data. A modified procedure for determining the ultimate design strain primarily in bond-critical applications (such as external flexural and shear FRP reinforcement), is proposed. This procedure makes a distinction between durability properties associated with the FRP rupture and debonding failure modes. When used for practically feasible strengthening ratios and in conjunction with BDF of 0.60, the proposed procedure yields overall ultimate flexural-member design strengths that are 0–15% lower than those determined using current design procedures.

J. Tatar, H.R. Hamilton (2016). “Comparison of Laboratory and Field Environmental Conditioning on FRP-Concrete Bond Durability”. Construction and Building Materials, 122, pp. 525-536.

Assessment of bonded FRP durability by means of accelerated conditioning is sometimes thought to be too harsh compared to ambient environmental conditions; consequently, this may result in underestimate of the actual durability. To assess the efficacy of utilizing accelerated conditioning protocols (ACP) for FRP-concrete bond durability testing, results from laboratory and field conditions (Sunshine Skyway Bridge in Tampa, FL) were compared. Direct tension pull-off test patches were applied to the approach span girders and notched beam three-point bending test specimens were placed on the dolphins adjacent to the bridge. These results were compared to notched beam specimens exposed to ACP. Testing indicated that characteristics of the FRP-concrete bond failure modes changed in some of the field samples within 6 months of field exposure, which may be an indication of durability problems. Moreover, ACP under elevated temperatures (60 °C) of notched three-point bending test resulted in a 36% loss of strength compared to no strength degradation after 18 months of field conditioning.

J. Tatar, D. Wagner, H.R. Hamilton (2016). “Structural Testing and Dissection of Carbon Fiber-Reinforced Polymer-Repaired Bridge Girders Taken Out of Service”. ACI Structural Journal, 113(6), pp. 1357-1367.

Carbon fiber-reinforced polymer (CFRP) is becoming a method of choice for repair and strengthening of many environmentally aged structures, bridges in particular. Durability of such CFRP repairs/strengthening efforts still remains uncertain. This paper presents the findings from structural and materials testing conducted on a CFRP-wrapped bridge girder that was taken out of service from the Indian River Bridge located in Melbourne, FL. The bridge girders, originally constructed in the 1960s, experienced corrosion damage during their service life in a brackish water environment. Following the repair of corrosion damage using conventional methods, repaired regions were wrapped with CFRP. Test results showed a modest increase in strength as a result of CFRP wrapping; fiber-reinforced polymer wrap, however, may have resulted in a reduction of chloride contamination following installation of the repair system. Furthermore, comparison of test results between the laboratory and field load tests indicated that AASHTO load distribution factors are conservative for this cast-in-place beam and slab bridge.

J. Tatar, G. Subhash, C.R. Taylor, H. R. Hamilton (2018). “Characterization of Adhesive Interphase Between Epoxy and Cement Paste Via Raman Spectroscopy and Mercury Intrusion Porosimetry”. Cement and Concrete Composites, 88, pp. 187-199

When exposed to harsh environmental conditions, the adhesive bond between epoxy and concrete was found to depend almost entirely on the effectiveness of the mechanical interlock which is contingent upon the ability of the adhesive to properly wet and penetrate the substrate. The current study is an investigation of the existence of a distinct interphase region within cement paste through the combined use of mercury intrusion porosimetry and Raman spectroscopy. A positive correlation between median pore size and depth of epoxy permeation into the cement paste substrate was observed. The Lucas-Washburn equation was found to accurately represent the driving mechanisms behind permeation of curing epoxy into the porous cement paste matrix. Interconnectivity of the capillary and gel pore structures was determined to play an important role in the permeation mechanisms. Analysis of relative conversion of epoxy via Raman spectroscopy in the interfacial region showed evidence of epoxy-cement paste interactions.

J. Tatar, C. Torrence, J.J. Mecholsky, C.R. Taylor, H.R. Hamilton (2018). "Effects of Silane Surface Functionalization on Interfacial Fracture Energy and Durability of Adhesive Bond between Cement Paste and Epoxy”, International Journal of Adhesives and Adhesion, 84, pp. 132-142

Epoxy adhesives are experiencing widespread use in concrete structures. However, a common concern regarding the adhesive joints in the infrastructure is their durability when exposed to harsh environments, most particularly, high levels of moisture. This work recognizes that adhesive bond between epoxy and substrate resists applied loads by a combination of chemical (hydrogen) bonds and mechanical interlock. Given the complexity of the stress-transfer mechanism this work focused exclusively on the chemical bond component between epoxy and cement paste, while the mechanical interlock was minimized through polishing of the cement paste substrate. A beam adhesion test method with notched interface was developed to assess the durability of chemical bonds between the adherents when aged by water immersion; surface functionalization of cement paste substrate was additionally explored as means of improving the chemical bonding and adhesion along the interface. Test results indicated that interfacial fracture energies were improved in both dry and conditioned groups with silane surface treatment. Analysis of interfacial failure modes with respect to the analytical crack kink criterion revealed that interphase region between epoxy and cement paste is characterized with higher fracture toughness than the cement paste substrate. The study lays groundwork for improvement in the durability of adhesive joints in related infrastructure through bottom-up interface design.

N. Brenkus, J. Tatar, H.R. Hamilton, G.R. Consolazio (2019). “Simplified Finite Element Modeling of Post-Tensioned Concrete Members with Mixed Bonded and Unbonded Tendons”. Engineering Structures, 179, 387-397.

Tendon filler materials serve as a critical layer, protecting a post-tensioned bridge’s steel reinforcement against corrosion and subsequent strength loss. Recent adoption of flexible filler materials in post-tensioning tendons in bridge construction – an alternative to the more typical cementitious grout which is currently common in the U.S. – has implications on member flexural behavior. This paper describes the development and experimental validation of a simplified, computationally inexpensive finite element modeling (FEM) approach for unbonded tendons. Flexural members containing both unbonded post-tensioned steel and bonded pretensioned steel, or “mixed tendons”, were modeled. FEM results were then validated against experimentally obtained ultimate strength, tendon stress, and plastic hinge length data. The developed analytical approach described herein was found to predict the ultimate flexural strength, failure mode and tendon stress with good accuracy, within 5%. Prediction of the ultimate displacement was fairly accurately modeled (within 15%) for the mixed tendon beams.

S.A. Morshed, T.J. Young, W.M. Chirdon, Q. Zhang, J. Tatar (2018). “Durability of Wet lay-up FRP Bonded to Concrete with Nanomodified Epoxy Adhesives”. The Journal of Adhesion.

Externally bonded fiber-reinforced polymer (FRP) composites represent a simple and economical solution for repairing and strengthening concrete structures. However, the potential for debonding failure in such strengthening systems becomes prominent when FRP-concrete bonds undergo environmental degradation induced by moisture. Low-viscosity Bisphenol A diglycidyl ether (DGEBA)-based epoxy adhesives are most commonly utilized in the engineering practice to bond wet-layup FRP to the concrete surface. This study evaluated the effects of the addition of commercial surface-modified nanosilica (SMNS), core-shell rubber (CSR) nanoparticles, and multi-walled carbon nanotubes (MWCNT) to neat epoxy on the mechanical properties of the adhesive, and strength and durability of FRP-concrete adhesively bonded joints. To determine the effects of environmental degradation, all specimens were subjected to the following environments: control – 23°C at RH 50 ± 10% for 18 weeks; and accelerated conditioning protocol (ACP) – water immersion at 45 ± 1°C for 18 weeks. Under control conditions, nanomodified epoxy exhibited enhanced mechanical properties compared to the neat epoxy. Following ACP, strength, elongation and modulus of elasticity of neat epoxy deteriorated significantly more than that of nanomodified adhesives. The bond strength of neat epoxy adhesive joints degraded most significantly (15%) following ACP, while nanomodified adhesive joints experienced minimal degradation of bond strength.

J. Tatar, C.R. Taylor, H.R. Hamilton (2019). “A Multiscale Micromechanical Model of Adhesive Interphase between Cement Paste and Epoxy Supported by Nanomechanical Evidence”. Composites Part B, 172, 679-689.

Even though epoxy adhesives are used extensively in concrete structures, little fundamental research has been conducted regarding bond formation mechanisms between the adhesive and concrete substrate, which obscures our understanding of degradation mechanisms in the bonded joints. When epoxy adhesive is applied to the concrete substrate, a transition region – termed interphase – is formed between the bulk epoxy and bulk cement paste/aggregate. Properties of interphase are thought to govern the macroscale behavior and durability of epoxy-concrete bonded systems. This work proposes an elastic multiscale model of the interphase region that is based on the existing body of knowledge on the topic. Site-specific statistical nanoindentation was used to experimentally evaluate the interphase region and verify the micromechanical model. Test results indicate that mechanical properties of the interphase region are different from those of bulk epoxy and cement paste. Experimental findings agreed with the postulated multiscale interphase model.

S.A. Morshed, A. Sinha, Q. Zhang, J. Tatar (2019). “Hygrothermal Conditioning of Wet-layup CFRP/Concrete Adhesive Joints Modified with Silane Coupling Agent and Core-shell Rubber Nanoparticles”. Construction and Building Materials.

The adhesive bond between externally bonded fiber-reinforced polymer (FRP) repairs and the concrete substrate can significantly deteriorate under hygrothermal conditions. The present study evaluated the epoxy adhesive toughening with core–shell rubber (CSR) nanoparticles and concrete surface functionalization with an epoxy-functional silane coupling agent as a means of improving the bond durability under hygrothermal exposure. To determine the effect of environmental degradation, beam bond test specimens were subjected to control conditions (standard laboratory conditions: 23 ± 2 °C and RH 50 ± 10%) and a hygrothermal accelerated conditioning protocol (ACP) (water immersion at 45 ± 1 °C) for 8 weeks. Bond test results indicate that CSR toughening and silane coupling agent can improve FRP-concrete adhesive bond strength retention following accelerated conditioning by up to 15% over that of neat epoxy. Following accelerated conditioning, CFRP coupons prepared with CSR-modified epoxy retain their mechanical properties, while the CFRP prepared with the neat epoxy exhibited a significant reduction in strength (40%) and elongation (54%). CSR nanoparticles demonstrated good compatibility with the base epoxy resin, as evidenced by differential scanning calorimetry (DSC) glass transition temperature measurements.

K. Baral, J. Tatar, Q. Zhang (2019). “A High-performance Impact-resistant Concrete Mixture for Transportation Infrastructure Applications”. Transportation Research Record Journal of the Transportation Research Board.

Engineered cementitious composites (ECC) is a class of high-performance fiber-reinforced cementitious composites featuring metal-like strain-hardening behavior under tension and high ductility. The highly ductile behavior of ECC often results in high impact resistance and energy absorption capacity, which make ECC suitable for applications in structures that are prone to impact damages, like exterior bridge girders, bridge piers, and crash barriers. In a recent study, a new ECC mixture has been developed using domestically available polyvinyl alcohol (PVA) fibers and regular river sand in replacement of imported PVA fibers and fine silica sand that are normally used in other ECC mixtures. The newly developed mixture, with improved local accessibility of raw materials, enables structural-scale applications of ECC in transportation infrastructures. To evaluate the suitability of the mixture for impact-resistant structures, in this paper, the tensile and flexural behavior of the newly developed material were characterized under pseudo-static loading and high strain-rate loadings up to 10−1 s−1. Direct drop-weight impact test was also conducted to assess the impact resistance and energy absorption capacity of the material. It was ensured that the ECC mixture maintains high tensile strain capacity above 1.8% under all tested strain rates. Regarding the damage characteristics, energy absorption capacity and load-bearing capacity during repeated impact loadings, ECC was found to have 75% higher energy dissipation capacity compared with regular reinforced concrete specimens and superior damage tolerance. The research results demonstrated that the newly developed ECC has a great potential to improve the impact resistance of transportation infrastructures.

A. Siha, N. Tatar, J. Tatar (2020). “Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiTiNb shape-memory alloy wires”. Materials and Structures, 53(4), 1-15.

Concrete girders can suffer from serviceability issues due to excessive cracking and deflection. In response to this problem, a novel heat-induced post-tensioning technique utilizing unbonded near-surface mounted nickel–titanium–niobium (NiTiNb) shape-memory alloy (SMA) wires was proposed and evaluated. SMAs are a class of smart materials that can recover seemingly permanent plastic deformation when heated. The post-tensioning forces, thus, can be generated by restrained heat-induced shape recovery of SMA wires. Material characterization tests showed that 3.92-mm diameter NiTiNb wires with 2.5% prestrain can generate recovery stress of approximately 500 MPa when actuated via Ohmic heating in a restrained condition. The proposed post-tensioning system was experimentally evaluated in reinforced concrete girders measuring 2.3 m in length and 23 × 41 cm in cross-sectional dimensions. The girders were initially cracked to simulate typical girder damage. NiTiNb wires were then installed in the bottom cover of the girders and anchored at both ends. Subsequently, the wires were actuated via Ohmic heating, which triggered shape-recovery and generated post-tensioning stresses in the girder. The post-tensioning technique reduced the crack widths by up to 74% (370 μm) and recovered the residual midspan deflection by up to 49% (1.52 mm) in the cracked girders. Following post-tensioning, flexural loading up to failure showed that the cracked stiffness and ultimate moment capacity of the girders had increased by up to 31% and 45%, respectively, with a relatively small NiTiNb reinforcement ratio of up to 0.17%.

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