Vincent P. Chiarito

Experimental Testing of CFRP-Strengthened Reinforced Concrete Slab Elements Loaded by Close-In Blast

Strengthening reinforced concrete slab or wall structural elements with carbon fiber-reinforced polymer (CFRP) can improve their blast resistance. However, close-in blasts (blasts with a scaled range of less than 0.4 m/kg1/3) may undermine the effectiveness of the CFRP strengthening. This paper presents an experimental testing program on CFRP-strengthened reinforced concrete slab specimens that utilized fiber anchors. Two CFRP mitigation designs were tested under blast loads with a scaled range of 0.4 and 0.6 m/kg1/3. Tests on unmitigated reinforced concrete slab specimens provided baseline comparisons. The experimental results showed that the use of CFRP strengthening improved the blast resistance of reinforced concrete slab specimens. For a larger scaled range, 0.6 m/kg1/3, the CFRP successfully prevented flying debris and reduced the overall deflections of the slab specimens. However, for the closer scaled range, 0.4 m/kg1/3, the high shock blast pressures shattered the concrete through the thickness of the slab specimen and tore through the back-face CFRP. However, back-face velocity and overall deflections were reduced by about 75% compared to the baseline test slab specimen.

Experimental Comparison of the Performance and Residual Capacity of CFFT and RC Bridge Columns Subjected to Blasts

AbstractThe blast performance of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) bridge columns was studied through a two-phase study comprised of blast and residual axial capacity experiments. Two one-fifth-scale CFFT columns and two one-fifth-scale conventional RC columns having comparable flexural capacities were subjected to distinct levels of explosive loading, causing damage but not complete failure. The blast resilience of the damaged columns was quantified by measuring the residual axial capacity of each column. The damaged CFFT columns exhibited superior strength and ductility retention compared with the damaged RC columns. Additionally, the damaged CFFT columns demonstrated a more predictable axial compressive mode of failure because the exterior FRP tube resisted the shear crack initiation observed in the damaged RC columns.

Modified Steel-Jacketed Columns for Combined Blast and Seismic Retrofit of Existing Bridge Columns

Steel jacketing has been used extensively in the United States to retrofit seismically deficient bridge columns. This procedure, which consists of encasing a RC column in a steel jacket, is effective in providing a ductile seismic response but does not enhance the blast resistance of the column. This is because a gap is typically left at the top and bottom of the jacket to prevent increased flexural strength, such as to avoid undesirable overload of the footing or cap beam. Blast tests have demonstrated that direct shear failure can develop at these gap locations. A modification to steel-jacketed columns is proposed here to provide an added blast resistance. It consists of structural steel collars placed around the gaps and tied to the adjacent elements with postinstalled anchors. Blast tests were conducted to investigate the effectiveness of this simple proposed detail. Experimental results indicated that the concept was effective in preventing direct shear failure. Severe blast load demands were applied to investigate the behavior of the retrofitted column under extreme ductility demands. All specimens exhibited satisfactory ductile behavior, except one, which uncharacteristically failed due to fracture of the tube’s vertical weld seam.

Analysis of CFRP Strengthened Reinforced Concrete Structural Components Subjected to Close-in Blasts

This paper presents a simple procedure for the analysis of strengthened and unstrengthened reinforced concrete slabs under close-in blast loadings. The simplified procedure provides a tool to quickly check a slab or verify the results of a more advanced computer analysis. The procedure divides the response of the slab into breach, direct shear, flexural, and membrane behaviors and provides equations and references on how to calculate the capacity of each behavior. The procedure first checks the most severe failure modes, then progresses to other modes. The procedure was used to analyze the response of four slabs subjected to blasts; two of the slabs were strengthened with carbon-fiber reinforced polymer (CFRP) on the back face. The procedure was also used to analyze the response of a reinforced concrete box specimen strengthened with CFRP. The simple procedure accurately predicted the peak deflection of the slabs to close-in blast loadings.

Seismic-infrasound-acoustic-meteorological sensors to dynamically monitor the natural frequencies of concrete dams

The U.S. Army Engineer Research and Development Center (ERDC) is leading research using seismic-infrasound-acoustic-meteorological (SIAM) arrays to determine structural characteristics of critical infrastructure. Large infrastructure, such as dams, emit infrasound (acoustic energy below that of human perception) at their natural modes of vibration, which are related to their structural condition. To validate the concept and its potential use for monitoring flood control structures, a structural evaluation was conducted at the Portuguese Dam in Ponce, Puerto Rico. The dam’s dynamic properties were studied prior to the deployment of SIAM arrays using detailed finite element (FE) models assembled in COMSOL Multiphysics software. The principal natural frequencies of the dam were identified and the fundamental modes were confirmed in the infrasound pass-band. The natural frequencies of 4.8 Hz, 6.7 Hz, and 10.2 Hz, respectively, were determined for the lower modes of vibrations. A total of three SIAM arrays wer...

Dual-Hazard Blast and Seismic Behavior of Concrete-Filled Double-Skin Steel Tubes Bridge Pier

AbstractThe dual-hazard inelastic behavior of concrete-filled double-skin steel tubes (CFDSTs) is experimentally investigated as a substitute to reinforced concrete columns for bridge piers in mult...

Blast and Earthquake Resistant Bridge Pier Concept: Retrofit and Alternative Design Options

This paper summarizes a research program undertaken to develop cost-effective solutions aimed at protecting new and existing bridge column bents against multiple natural and manmade treats such as earthquakes and blast. Concrete-Filled Double Skin Tube (CFDST) column and Modified Steel Jacketed Column (MSJC) designs were proposed and tested for new bent construction and retrofit, respectively. While CFDSTs maintained their cross-section geometry in dissipating energy under cyclic loading, under severe blast loading, they deformed locally to dissipate the applied impulse and have enough residual strength to prevent collapse. MSJC, on the other hand, were developed to eliminate a deficiency observed in the form of direct shear failure near the top and/or the bottom of the jacket for steel jacketed column (SJC) subjected to blast loading. The proposed modification increases direct shear resistance in the shear-deficient areas and does not interfere with the ability of the jacketed columns to resist earthquake loading.

Evaluation of Mechanical Anchoring System to Improve Performance of CFRP Mitigated Concrete Slabs under Close-in Blasts

This research seeks to determine realistic mitigation strategies to improve the performance of reinforced concrete slabs under close-in blast events. These extreme events can create pressures high enough to powderize concrete and leave an empty rebar cage. This paper will cover the design, construction, and testing of a reinforced concrete beam specimen. The specimen was strengthened with steel plating on the compression face and carbon fiber reinforced polymer pultruded plates on the tension face. Bolts were placed through the specimen to anchor each face. The static test results show that the mechanical anchorage system used was able to fully develop the CFRP plates. However, there was significant slipping of the CFRP leading to increased deflection.

Economical and Consistent Test Methodology for Energy-Absorbing Materials Used for Mitigation of Explosive Effects on Structures

This paper discusses the use of an economical ballistic pendulum device to study the effectiveness of energy-absorbing materials to mitigate explosive detonation effects. Besides closing a structure, engineers must develop innovative measures to mitigate the effects of the extreme blast environments expected from very close-in (near-contact) detonations against key structural components. Beyond conventional structural hardening, using energy-absorbing materials to protect structures from blast effects is an option considered but not fully understood. As new materials have evolved in recent years, interest has increased in their potential use as energy absorbers for near-contact blast mitigation. For these new materials, constitutive models are either nonexistent or not well developed for predicting high-pressure and high-strain rate responses. Experiments using the ballistic pendulum were conducted rapidly and with little logistical burden; for one series of tests, over 30 experiments were conducted in 1 week. This fundamental experimental setup produced a wealth of previously nonexistent data on a variety of energy-absorbing materials, such as an elastomer (rubberlike compound), dense foams, a dilatant compound, and liquids. The ballistic pendulum was used to measure the total momentum imparted to the bob from a close-in detonation as a function of absorber materials and distance from the bob. These results provided a comparison of impulse delivered to the bob among the absorber materials with the control measurement of no absorber. Results indicate that, for near-contact detonations, the addition of energy-absorbing materials increased the impulse loading to the pendulum for selected scaled ranges of simulated threats.