An experimental and numerical study of reinforced ultra-high performance concrete slabs under blast loads

Ultra-high performance concrete (UHPC) which is characterized by high strength, high ductility and high toughness has been widely applied in modern structure construction. Outstanding mechanical feature of UHPC not only enables strong yet slim structure design but also highlights its potential in protective engineering against extreme loads like impact or explosion. In this research a series of reinforced concrete slabs are tested to determine their response under explosive loading conditions. Concrete materials used in the slab construction are ultra-high strength concrete (UHPC) and normal strength concrete (NSC). In total five slabs are tested including four UHPC slabs with varying reinforcement ratios and one control NSC slab with normal reinforcement. Explosive charges with TNT equivalent weights ranging from 1.0 to 14.0 kg at scaled distances ranging from 0.41 to 3.05 m/kg^1/3 are used in the current experiments. Test results verified the effectiveness of UHPC slabs against blast loads. Numerical models are established in LS-DYNA to reproduce the field blast tests on UHPC slabs. The numerical results are compared with the field test data, and the feasibility and validity of the numerical predictions of UHPC slab responses are demonstrated.     

Blast loading response of reinforced concrete panels reinforced with externally bonded GFRP laminates

The behavior of reinforced concrete panels, or slabs, retrofitted with glass fiber reinforced polymer (GFRP) composite, and subjected to blast load is investigated. Eight 1000 × 1000 × 70 mm panels were made of 40 MPa concrete and reinforced with top and bottom steel meshes. Five of the panels were used as control while the remaining four were retrofitted with adhesively bonded 500 mm wide GFRP laminate strips on both faces, one in each direction parallel to the panel edges. The panels were subjected to blast loads generated by the detonation of either 22.4 kg or 33.4 kg ANFO explosive charge located at a 3-m standoff. Blast wave characteristics, including incident and reflected pressures and impulses, as well as panel central deflection and strain in steel and on concrete/FRP surfaces were measured. The post-blast damage and mode of failure of each panel was observed, and those panels that were not completely damaged by the blast were subsequently statically tested to find their residual strength. It was determined that overall the GFRP retrofitted panels performed better than the companion control panels while one retrofitted panel experienced severe damage and could not be tested statically after the blast. The latter finding is consistent with previous reports which have shown that at relatively close range the blast pressure due to nominally similar charges and standoff distance can vary significantly, thus producing different levels of damage.     

Structural limits of FRP-balsa sandwich decks in bridge construction

The span limits of two glass fiber-reinforced polymer (GFRP) bridge concepts involving GFRP-balsa sandwich plates are discussed. The sandwich plates were either used directly as slab bridges or as decks of a hybrid sandwich-steel girder bridges. In the latter case, the potential of the sandwich decks to replace reinforced concrete (RC) decks was also evaluated. Taking the limits of manufacturing into account (800 mm slab thickness), maximum bridge spans of approximately 19 m can be reached with FRP-balsa sandwich slab bridges, if a carbon-FRP (CFRP) arch is integrated into the balsa core. Above this limit, hybrid sandwich-steel girder bridges can be used up to spans of 30 m. RC deck replacement requires timber and steel plate inserts into the balsa core above the steel girders. GFRP-balsa sandwich slabs or decks exhibit full composite action between lower and upper face sheets. Stress concentrations occur at the joints between balsa core and timber inserts which however can effectively be reduced by changing from butt to scarf joints.     

Variation of strength and stiffness of fibre reinforced polymer reinforcing bars with temperature

This paper presents the results of tensile mechanical properties of FRP reinforcement bars, used as internal reinforcement in concrete structures, at elevated temperatures. Detailed experimental studies were conducted to determine the strength and stiffness properties of FRP bars at elevated temperatures. Two types of FRP bars namely: carbon fibre reinforced polyester bars of 9.5 mm diameter and glass fibre reinforced polyester bars of 9.5 mm and 12.7 mm diameter were considered. For comparison, conventional steel reinforcement bars of 10 mm and 15 mm diameter were also tested. Data from the experiments was used to illustrate the comparative variation of tensile strength and stiffness of different types of FRP reinforcing bars with traditional steel reinforcing bars. Also, results from the strength tests were used to show that temperatures of about 325 °C and 250 °C appear to be critical (in terms of strength) for GFRP and CFRP reinforcing bars, respectively. A case study is presented to illustrate the application of critical temperatures for evaluating the fire performance of FRP-reinforced concrete slabs.     

Experimental study of retrofit solutions for damaged concrete bridge slabs

Accurate information on the actual performance of the structural system after retrofit is an essential part of a cost-effective bridge management program. This paper summarizes the results of a thorough experimental program concerning the reinforced concrete deck of a real 40 year-old viaduct. The structure exhibited severe damage at the extrados mainly due to environmental agents, chemical attack and action of asphalt milling machines. Samples of the deck were cut and carried to the laboratory in order to assess the possibility of retrofit. The design of retrofit was aimed at increasing the load carrying capacity through replacement of the deteriorated concrete with a new concrete overlay and strengthening in flexure for both negative (hogging) and positive (sagging) bending moments. Experimental testing on small specimens and nondestructive techniques were carried out to identify the material properties and to evaluate the level of damage. The bonding between external reinforcement and the original or new (standard or polymer-modified) concrete was assessed through single-shear push–pull tests on 33 prismatic specimens of 100 × 200 × 500 mm³ strengthened with CFRP strips. The efficiency of the retrofit techniques was checked at the structural level through four-point bending tests on eight slabs of 500 × 200 × 2000 mm³. This research can contribute to guidelines for concrete patch repair of FRP-retrofitted concrete bridge decks, to ensure better long-term performance under service loads and environmental effects.     

The drilling resistance test for the assessment of fire damaged concrete

In this paper, the evaluation of the drilling resistance is regarded as a method to ascertain the thermal damage undergone by concrete members after fire. Some preliminary tests on a good quality concrete were functional in defining the test procedure, the optimal bit diameter and the effect of the drilling thrust. A further study on uniformly damaged concrete cubes (ordinary and lightweight concretes – R_cm = 50 N/mm²) allowed to ascertain the sensitivity of the method. The reliability of this technique for the assessment of the damage depth within structural members exposed to fire has then been checked by testing some concrete panels exposed to marked temperature gradients. Finally, the viability of the method for in situ applications has been confirmed by testing the members of a precast RC structure which survived a real fire.     

Impact strength of a few natural fibre reinforced cement mortar slabs: a comparative study

This paper presents the experimental investigations of the resistance to impact loading of cement mortar slabs (1:3, size: 300 mm × 300 mm × 20 mm) reinforced with four natural fibres, coir, sisal, jute, hibiscus cannebinus and subjected to impact loading using a simple projectile test. Four different fibre contents (0.5%, 1.0%, 1.5% and 2.5%—by weight of cement) and three fibre lengths (20 mm, 30 mm and 40 mm) were considered. The results obtained have shown that the addition of the above natural fibres has increased the impact resistance by 3–18 times than that of the reference (i.e. plain) mortar slab. Of the four fibres, coir fibre reinforced mortar slab specimens have shown the best performance based on the set of chosen indicators, i.e. the impact resistance (R_u), residual impact strength ratio (I_rs), impact crack-resistance ratio (C_r) and the condition of fibre at ultimate failure.     

Nonlinear transient analysis of reinforced concrete slabs subjected to blast loading and retrofitted with CFRP composites

Computational models using the finite element method for nonlinear transient analysis of reinforced concrete (RC) two-way slabs subjected to blast loading are presented. Both as-built and retrofitted slabs with carbon fiber reinforced polymer (CFRP) composite strips are analyzed. The models are used to investigate different parameters including (a) loading duration, and (b) effect of CFRP retrofit on damage accumulation. In this study, damage is globally quantified by the amount of reduction of the first two vibrational frequencies of the slabs. Local representation of damage in terms of reinforcing steel strains is also discussed. The computational models for both the as-built and the retrofitted slabs are verified using experimental results. In these experiments, a slowly increasing uniform pressure is applied to the bottom surface of large-scale RC slab specimens using high-pressure water bag. Experimental results showed that an increase up to 200% in the load carrying capacity is achieved when using the CFRP composite retrofit system. Transient nonlinear analysis results proved the efficiency of the CFRP composite retrofit in improving the slab behavior under blast loading for different loading durations, i.e. for small, medium, and large charge weights at the same applied maximum pressure. In particular, less than 50% reduction of the fundamental frequency due to concrete damage is obtained for the retrofitted slab compared to more than 85% reduction for the as-built slab. Moreover, the maximum displacement is reduced by 40–70% with the CFRP retrofit compared to the as-built slab. As for reinforcing steel strains, the application of CFRP retrofit significantly limited the spread of yielding in time and space. The improved slab behavior with CFRP is best when retrofitting is applied to both sides of the slab.     

Ballistic impact and explosive blast resistance of stitched composites

The effectiveness of stitching in increasing the damage resistance of polymer composites against ballistic projectiles and explosive blasts is determined. Glass-reinforced vinyl ester composites stitched in the through-thickness direction with thin Kevlar®-49 yarn were impacted with a bullet travelling at 0.9 km s⁻¹ or an underwater explosive shock wave moving at 1.5 km s⁻¹. The amount of delamination damage to the composite caused by a ballistic projectile was reduced slightly with stitching. Stitching was highly effective in increasing the damage resistance against explosive blast loading. The increased damage resistance was due to the stitching raising the Mode I interlaminar fracture toughness of the composite. While the stitched composites experienced slightly less damage, their flexural modulus and strength was similar to the properties of the unstitched composite after ballistic impact testing. The post-blast flexural properties of the stitched composites, on the other hand, were degraded less than the properties of the unstitched material.     

高强钢丝绳网片-聚合物砂浆加固RC板抗爆性能试验研究

为了研究高强钢丝绳网片-聚合物砂浆对钢筋混凝土(RC)板的抗爆加固效果,对5块加固RC板和1块未加固RC板进行了野外现场爆炸试验,研究了砂浆强度、钢丝绳间距、钢丝绳预应力和界面增设销钉等因素对试件的破坏形态、裂缝分布及发展、跨中位移、钢筋应变等影响规律,并对爆炸试验后的试件进行了剩余承载力试验和爆炸损伤评估。研究表明:高强钢丝绳网片-聚合物砂浆加固能显著提高RC板的抗爆性能,相比于未加固板,加固板的裂缝宽度,板底跨中的峰值位移、残余位移和钢筋应变均大幅减小;加固后,构件剩余承载力大幅增加,其损伤程度大为降低。     

Load and resistance factor design of fiber reinforced polymer composite bridge deck

The majority of our bridges were constructed with conventional civil engineering materials of steel and concrete in a typical slab on girder or truss construction. Reinforced concrete bridge decks have approximately 40% life of the steel girders that support these structures. In order to support the use of alternative materials to replace deteriorating concrete decks, this paper outlines the Load and Resistance Factor Design (LRFD) of Fiber Reinforced Polymer composite (FRP) panel highway bridge deck. The deck would be of a sandwich construction where 152.4 mm × 152.4 mm × 9.5 mm square pultruded glass FRP (GFRP) tubes are joined and sandwiched between two 9.5 mm GFRP plates. The deck would be designed by Allowable Stress Design (ASD) and LRFD to support AASHTO design truckload HL-93. There are currently no US standards and specifications for the design of FRP pultruded shapes including a deck panel therefore international codes and references related to FRP profiles will be examined and AASHTO-LRFD specifications will be used as the basis for the final design. Overall, years of research and laboratory and field tests have proven FRP decks to be a viable alternative to conventional concrete deck. Therefore, conceptualizing the design of FRP bridge decks using basic structural analysis and mechanics would increase awareness and engineering confidence in the use of this innovative material.     

Experiments and failure analysis of SHCC and reinforced concrete composite slabs

For all types of concrete structures, controlling of cracking, as well as the enhancement of serviceability and ultimate flexural capacity are important issues for deck slabs. This study presents an experimental campaign and accompanying nonlinear analysis of a series of Strain Hardening Cementitious Composite (SHCC) and reinforced concrete slab systems, simply-supported and subjected to four-point loading. In order to improve flexural performance both at the service and ultimate limit states, an SHCC layer with thickness of 150–400 mm was placed on the soffit of the composite slab; the SHCC was manufactured using two different processes, namely cast-in-situ SHCCs and extruded precast SHCC panel. Nonlinear analysis of SHCC and reinforced concrete slabs was also carried out to predict moment and curvature as well as deflections of the slab systems. The developed slab systems were found to have enhanced performance with regard to both at serviceability and flexural capacity, compared to the conventional reinforced concrete slab.     

Scaling aspects of quadrangular plates subjected to localised blast loads—experiments and predictions

A series of experimental results on clamped mild steel quadrangular plates of different thicknesses (1.6, 2.0, 3.0 and 4.0 mm) and varying length-to-width ratios (1.0–2.4) subjected to localised blast loads of varying sizes is reported. Disc shaped explosive charges of varying charge diameter-to-plate width ratios (0.2–0.37) and charge heights (1.8–14 mm) are centrally positioned on quadrangular plates to provide impulses resulting in mid-point deflections in the range from two plate thicknesses to central plate tearing. The effects of varying both the loading conditions and the plate geometries on the deformation are described. A modified dimensionless number is presented for the quadrangular plate response when subjected to localised circular blast loading. In addition, numerical predictions are carried out and compared with experiments for a limited selection of plate geometries.     

Plastic deformation of polyurea coated composite aluminium plates subjected to low velocity impact

The demand for protective measures for structures is on the rise due to the increasing possibility of structural damage due to threats such as natural disasters, collision of vehicles, and blast and ballistic impacts. Application of an elastomer as a composite material with other base materials such as aluminium, steel and concrete has been considered as one of the measures to mitigate such threats. However, very limited work has been conducted in this area, especially on the feasibility of polyurea (elastomer) as a composite material against low velocity impacts. The focus of this research is to investigate the behaviour of polyurea coated composite aluminium plates subjected to rigid blunt-nosed projectile impact. AA5083-H116 aluminium alloy plates with polyurea coatings of 6 mm and 12 mm thickness were investigated. A blunt cylindrical projectile of high strength steel travelling in the velocity range of 5–15 m/s impacted at the centre of the 300 mm × 300 mm square plates. A polyurea coating was used to absorb part of the impact energy and provide protection to the plates as an energy damping material through application on the impact side of the plates. In addition, uncoated aluminium plates of the same thickness were used in the test program. A gas gun mechanism was used to fire a 5 kg projectile, and laser displacement monitoring equipment was used to record the out-of-plane deformation history of the plate during the impact. The complete test setup has been modelled numerically using the advanced finite element (FE) code LS-DYNA. The models were validated with the experimental results. Deformation time histories obtained from both the experimental and numerical studies for the plates were used to compare the ability of polyurea to effectively mitigate the damage resulting from low velocity impact. The polyurea coated plates showed a considerable reduction in out-of-plane deformation when compared to the uncoated plates. These findings indicate that polyurea can be utilised as an efficient energy absorbing/damping material against low velocity impact damage.     

近爆作用下钢筋混凝土板动态破坏的数值模拟研究

当爆炸在结构构件表面发生时,产生的冲击波将会对结构构件造成损伤和破坏,而准确预测潜在的爆炸对结构构件造成的损伤是进行重要建筑物和防护结构抗爆设计的基础。为研究近爆作用下钢筋混凝土板的抗爆性能,采用AUTODYN软件建立了混凝土和钢筋的三维分离式实体模型,数值模型考虑了应变率对钢筋和混凝土材料动力本构特性的影响以及炸药-空气-结构之间的流固耦合相互作用,分析了不同炸药量作用下钢筋混凝土板的损伤机理和破坏特征,合理展现了钢筋混凝土板从混凝土开裂、碎片形成、部分钢筋屈服断裂到板局部震塌的动态演变过程。随着炸药量的增大,钢筋混凝土板的破坏模式逐渐由整体弯曲破坏转变为局部的冲切破坏     

Slenderness effect of circular concrete specimens confined with SFRP sheets

Experimental investigation of Fibre Reinforced Polymer (FRP) confined concrete is normally conducted on relatively small-scale specimens, where the scaling effects of the specimen size are usually ignored. Few researchers investigated the scaling effects of confined concrete with Carbon FRP (CFRP), Glass FRP (GFRP) and Aramid FRP (AFRP) sheets. However, based on the authors’ knowledge, there is no information available in the literature on the slenderness effects of confined concrete with Steel FRP (SFRP) sheet. The SFRP sheet is a new type of material recently introduced for strengthening applications of concrete structures. Thus, the main aim of this investigation is to quantify and access the axial strength, axial strain, hoop strain, dilation and ductility performance of SFRP confined concrete with the increase in the slenderness of the specimens. The experimental program included eighteen specimens with varying slenderness ratios (height-to-diameter ratio) of 2 (150 mm × 300 mm), 4 (150 mm × 600 mm), and 6 (150 mm × 900 mm). Six specimens were constructed in each size, where three specimens were left unwrapped as control specimens and three specimens were wrapped with SFRP sheets. All specimens were loaded in uniaxial compression until failure. The specimens were also instrumented with a photogrammetric method termed Digital Image Correlation Technique to measure the hoop strains from the surface of the SFRP confined concrete specimens. The experimental investigation showed that the effectiveness of the SFRP sheets, measured in terms of the percentage increase in the ultimate axial strength, axial and hoop strains, and the ductility was significantly enhanced compared to the unwrapped specimens. The results also indicate that the overall performance of the SFRP wrapped concrete specimens was reduced with the increase in the slenderness of the specimens, when compared to the standard size cylinders. The study of three major design codes/guidelines to predict the ultimate SFRP-confined concrete compressive strength revealed that the FRP Building Code has the best confinement model when compared with the experimental results.     

Repair of shear-deficient normal weight concrete beams damaged by thermal shock using advanced composite materials

The use of advanced composite materials such as Fiber Reinforced Polymers (FRPs) in repairing and strengthening reinforced concrete structural elements has been increased in the last two decades. Repairing and strengthening damage structures is a relatively new technique. The aims of this study was to investigate the efficiency and effectiveness of using Carbon Fiber Reinforced Polymer (CFRP) to regain shear capacity of shear-deficient normal weight high strength RC beams after being damaged by thermal shock. Sixteen high strength normal weight RC beams (100 × 150 × 1400 mm) were cast, heated at 500 °C for 2 h and then cooled rapidly by immersion in water, repaired, and then tested under four-point loading until failure. The composite materials used are carbon fiber reinforced polymer plates and sheets. The experimental results indicated that upon heating then cooling rapidly, the reinforced concrete (RC) beams exhibited extensive map cracking without spalling. Load carrying capacity and stiffness of RC beams decreased about 68% and 64%, respectively, as compared with reference beams. Repairing the thermal damaged RC beams allowed recovering the original load carrying without achieving the original stiffness. Repaired beams with CFRP plates with 90° and 45° regained from 90% to 99% of the original load capacity with a corresponding stiffness from 79% to 95%, whereas those repaired with CFRP sheet on the web sides and a combination of CFRP plates and sheet regained from 102% to 107% of the original load capacity with a corresponding stiffness from 81% to 93%, respectively. Finally, finite element analysis model is developed and validated with the experimental results. The finite element analysis showed good agreement as compared with the experimental results in terms of load–deflection and load–CFRP strain curves.     

An experimental study on the effect of environmental exposures and corrosion on RC columns with FRP composite jackets

The objective of this study was to evaluate the effects of various environmental conditions on the long-term behavior of reinforced concrete (RC) columns strengthened with carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) sheets. Small-scale RC columns were manufactured in the laboratory and conditioned under accelerated environmental cycling and accelerated corrosion process of reinforcing bars. Then, uni-axial compressive failure tests were conducted in order to evaluate the change of mechanical properties of the test columns due to the environmental effects. The results revealed that the mechanical properties of RC column system (RC + FRP) were altered due to the environmental conditioning and the corrosion of steel reinforcement, and each type of environmental conditions had its unique effects and features.     

Numerical prediction of concrete slab response to blast loading

In this paper, a dynamic plastic damage model for concrete material has been employed to estimate responses of both an ordinary reinforced concrete slab and a high strength steel fibre concrete slab subjected to blast loading. In the concrete material model, the strength envelope is a damage-based modified piece-wise Drucker–Prager model; the strain rate effect on tension and compression are considered separately; the damage variable is based on Mazars’ damage model, which is a combination of tensile and compressive damage. The equation of state (EOS) is also a combination of the porous and solid EOS of concrete with different forms for tension and compression states. The interaction between the blast wave and the concrete slab is considered in the 3D simulation. In the first stage, the initial detonation and blast wave propagation is modelled in a 2D simulation before the blast wave reaches the concrete slab, then the results obtained from the 2D calculation are remapped to a 3D model. The calculated blast load is compared with that obtained from TM5-1300. The numerical results of the concrete slab response are compared with the explosive tests carried out in the Weapons System Division, Defence Science and Technology Organisation, Department of Defence, Australia. Repetitive applications of blast loading on slabs are also simulated and the results compared with test data.     

Performance of reinforced concrete slabs strengthened with different types and configurations of CFRP

A total of eight reinforced concrete slabs, 2440 × 600 × 125 mm strengthened with different layers and configurations of CFRP sheets were fabricated and tested. In addition, nonlinear finite element analysis (NLFEA) using ANSYS package was used to simulate the behavior of the test specimens. After reasonable validation of NLFEA with the experimental test results of companion slabs, NLFEA was expanded to provide a parametric study of eighteen slabs. The load–deflection, load strain, and failure modes obtained from the experimental test results and the NLFEA evidently confirmed that strengthening of under-reinforced concrete slabs with CFRP improves the flexural strength capacity and reduce the ductility. This was observed for both types of CFRP. The increase in the flexural strength and the reduction in the ductility increased with the increase in the number of CFRP layers. It was concluded that CFRP strengthening of slabs could be categorized as effective, economical, and successful only if substantial increase in the flexural strength capacity is achieved without changing the failure mode to a shear failure mode at the face of the supports or to a compression failure mode. Comparison between the two CFRP types, for almost equivalent applied area of CFRP, showed that the type of CFRP has significant influence on the behavior of the strengthened slabs. The difference is attributed to the difference in the mechanical properties and the bonding quality of the CFRP material.