Behaviour of precast concrete floor slabs exposed to standardised fires

This paper quantifies the thermal movements of 14 simply supported precast reinforced concrete floor slabs of 4.5 m span and 900 mm width exposed to two standardised heating regimes used in fire resistance furnace tests. The tests were designed to show the effect of varying the slab thickness, type of concrete, imposed load, soffit protection and nature of fire exposure on the mid-span flexural deflection and axial movements of the slab ends. Measured deflections showed that during the 90 min design period of fire resistance thermal bowing was dominant and the effect of the 1.5 kN/m² design imposed load was small. The NPD hydrocarbon fire exposure caused a doubling of the flexural deflections achieved using the standard BS 476: Part 8 (now Part 20) fire exposure in the first 20 min of exposure.     

Testing of full-scale concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars

This paper presents an experimental study investigating the behavior of FRP-reinforced concrete bridge deck slabs under concentrated loads. A total of eight full-scale deck slabs measuring 3000-mm long by 2500-mm wide were constructed. The test parameters were: (i) slab thickness (200, 175 and 150 mm); (ii) concrete compressive strength (35–65 MPa); (iii) bottom transverse reinforcement ratio (1.2–0.35%); and (iv) type of reinforcement (GFRP, CFRP, and steel). The slabs were supported on two parallel steel girders and were tested up to failure under monotonic single concentrated load acting on the center of each slab over a contact area of 600 × 250 mm to simulate the footprint of sustained truck wheel load (87.5 kN CL-625 truck). All deck slabs failed in punching shear. The punching capacity of the tested deck slabs ranged from 1.74 to 3.52 times the factored load (P_f) specified by the Canadian Highway Bridge Design Code (CHBDC) CAN/CSA S6-06. Besides, the ACI 440.1R-06 punching strength equation greatly underestimated the capacity of the tested slabs with an average experimental-to-predicted punching capacity ratio (V_exp/V_pred) of 3.17.     

Repair of heat-damaged reinforced concrete slabs using fibrous composite materials

Sixteen under-reinforced high strength concrete one-way slabs were cast, heated at 600 °C for 2 h, repaired, and then tested under four-point loading to investigate the coupling effect of water recuring and repairing with advance composite materials on increasing the flexural capacity of heat-damaged slabs. The composites used included high strength fiber reinforced concrete layers; and carbon and glass fiber reinforced polymer (CFRP and GFRP) sheets. Upon heating then cooling, the reinforced concrete (RC) slabs experienced extensive map cracking, and upward cambering without spalling. Recuring the heat-damaged slabs for 28 days allowed recovering the original stiffness without achieving the original load carrying capacity. Other slabs, recured then repaired with steel fiber reinforced concrete (SFRC) layers, regained from 79% to 84% of the original load capacity with a corresponding increase in stiffness from 382% to 503%, whereas those recured then repaired with CFRP and GFRP sheets, regained up to 158% and 125% of the original load capacity with a corresponding increase in stiffness of up to 319% and 197%, respectively. Control, heat-damaged, and water recured slabs showed a typical flexural failure mode with very fine and well distributed hairline cracks, propagated from the repair layers to concrete compression zone. RC slabs repaired with SFRC layers failed in flexural through a single crack, propagated throughout the compression zone, whereas those repaired with CFRP and GFRP experience yielding failure of steel prior to the composites failure.     

Cracking behaviour of cast in situ reinforced concrete slabs with control joints

This paper presents the results of a study on the cracking behaviour of cast in situ reinforced concrete slabs with control joints. Material property tests were conducted to establish time-dependent concrete mechanical properties. Cast in situ reinforced concrete slabs with control joints were tested for up to 6 months to study the distribution of concrete cracks and development of concrete strains. A finite element method based model was developed, verified and used for a parametric study. It was found that using control joints can successfully regulate the concrete cracks. The slabs with longer joints suffered less cracks at the edges. The influence of the joint depth was insignificant; however, the joints cut all the way through the slabs caused higher tensile strains than those of shallower cuts. It was also found that the reinforcement ratio and concrete proportion had a significant effect on the cracking of the jointed concrete slabs.     

The effects of protected beams and their connections on the fire resistance of composite buildings

According to full-scale fire tests, it is noticed that tensile membrane action within the concrete floor slabs plays an important role in affecting the fire resistance of composite buildings. It is well known that the development of tensile membrane actions relies on the vertical support along the edges of the slab panel. However, there is at present a lack of research into the influence of vertical supports on the tensile membrane actions of the floor slabs. In this paper, the performances of a generic three dimensional 45 m×45 m composite floor subjected to ISO834 Fire and Natural Fire are investigated. Different vertical support conditions and three steel meshes are applied in order to assess the impact of vertical supports on tensile membrane action of floor slabs. Unlike other existing large scale modelling which assumes the connections behave as pinned or rigid for simplicity, two robust 2-node connection element models developed by the authors are used to model the behaviour of end-plate and partial end-plate connections of composite structures under fire conditions. The impact of connections on the 3D behaviour of composite floor is taken into consideration. The load-transfer mechanisms of composite floor when connections fail due to axial tension, vertical shear and bending are investigated. Based on the results obtained, some design recommendations are proposed to enhance the fire resistance of composite buildings.     

Experimental investigation of ultimate capacity of wired mesh-reinforced cementitious slabs

Experimental tests conducted on 27 square cementitious slabs of 490 × 490 mm simply supported on four edges and subjected to patch load are presented. The slabs had a clear span of 400 × 400 mm and provided with a 445 × 445 mm closed frame of 8 mm diameter steel bar to hold the reinforcement in place and to act as a line support. The test variables were the wire mesh volume fraction: four expanded and two square types; slab thickness: 40, 45, 50 and 60 mm; and the patch load pattern: square and rectangular. The test results showed that as the volume fraction increased the punching strength of the slabs was also increased. Adding a wire mesh to ordinary reinforcement increases significantly the punching resistance at column stub. Moreover, as the loaded area size increases both ductility and stiffness increases and the bridging effect due to the difference in the reinforcement ratio in orthogonal directions was clearly noticed. More research was needed to identify the volume fraction ratio at which the mode of failure alter from flexure to punching.     

Analytical and experimental studies on composite slabs utilising palm oil clinker concrete

The utilisation of waste materials in the construction industry is an effective way to sanitise the environment and reduces the cost of construction. In this research, palm oil clinker (POC) aggregates was used to fully replace normal aggregates to produce structural lightweight concrete. This concrete was used in the construction of composite slabs with profiled steel sheet. A total of eight full scale composite slabs, six palm oil clinker concrete (POCC) slabs and two conventional concrete slabs were constructed and tested in accordance to Eurocode 4: Part 1.1 and BS 5950: Part 4: 1994. Two shear spans were used, 450 mm for short shear span and 900 mm for long shear span. The structural behaviour of the slabs was investigated and compared. The horizontal shear-bond strength between the concrete and the steel was determined according to two methods; m–k and partial shear connection methods. Test results show that the structural behaviour and the horizontal shear-bond strength of the POCC slabs are nearly similar to the conventional concrete slabs. The mechanical interlock (m) and the friction (k) between the steel and the concrete are 117.67 N/mm² and 0.0973 N/mm², respectively and the design horizontal shear-bond strength using m-k and PSC methods is 0.248 N/mm² and 0.215 N/mm², respectively. The difference between the two methods is 13.3%. POCC is therefore suitable to be used for structural applications with a reduction in weight of 18.3% compared to conventional concrete composite slabs.     

Blast performance of reinforced concrete hollow core slabs in combination with fire: Numerical and experimental assessment

This paper deals with the analysis of the blast bearing capacity of reinforced concrete hollow core slabs when they are subjected first to fire and then to a blast load. The paper first analyzes the blast behavior of a hollow core slab without fire, for two charges, one that leads to elastic dynamic response and the other that causes plastic behavior and severe concrete cracking. The same blast analysis is repeated for slabs that had been subjected to fire. The paper is limited to temperatures up to 450 °C at the concrete surface. A discussion of the experimental setup for full scale experiments is presented and the experimental results are compared with simplified numerical models solved with the software LS-DYNA. The paper discusses many difficulties in obtaining a reliable numerical model. The software typically permits to analyze transient phenomena such as an explosion, where material properties change very rapidly (on the level of milliseconds). On the other hand, fire does not change material and structural properties that fast. The solution of this difficulty is offered by the preliminary analysis of the changes of material properties due to fire, which allows determining the proper input for the numerical blast analysis by means of LS-DYNA. The most important conclusion of the analysis is that crack patterns and blast load dynamic responses are indeed altered by fire with temperatures up to 450 °C; the eigenfrequency is reduced and the maximum dynamic deformation increases. Yet, within the limitations of the assumptions concerning boundary conditions, the examined slabs keep their blast bearing capacity after blast load scenarios up to 1.5 kg C4 with at 1 m stand off distance.     

Classification of the damage condition of preloaded reinforced concrete slabs using parameter-based acoustic emission analysis

A parameter-based acoustic emission (AE) technique is applied to AE signals acquired in physical experiments carried out on a series of predamaged reinforced concrete slabs. Three reinforced concrete slabs without shear reinforcement with dimensions of 1.50 × 1.50 × 0.23 m are subjected to cycles of a concentrated centric load with increasing peak values up to failure. The slabs had been previously exposed to impact loads in rockfall experiments and exhibit an unknown damage condition yet to be determined. Acoustic emissions are recorded during the loading and unloading cycles and evaluated. An analysis of load ratio and calm ratio associated with the Kaiser effect is performed. Damage classification is carried out successfully. Definitions of load ratio and calm ratio are reconsidered and specified. A static preloading of the slabs is approximated. The relationship between cracking process, failure mechanism and the acoustic emissions that occur is described and discussed.     

Hybrid concrete and pultruded-plank slabs for highway and pedestrian bridges

Studies on two novel uses of hybrid structural members consisting of commercially produced glass reinforced pultruded ribbed fiber reinforced polymer (FRP) planks and concrete are discussed in this paper. Pultruded planks are produced by all the major pultruders in the world and are utilized primarily as decking for platforms. These highly optimized panels have the potential to be used in many other infrastructure applications, but their flexural stiffnesses have generally been too low to be used in highway and pedestrian bridges due to current span requirements. However, when used “compositely” with concrete or cementitious materials in a hybrid form they have the potential to be much more widely used. Two research studies conducted on two possible hybrid systems of different structural depths are discussed in this paper. The first study describes the use of pultruded planks as permanent formwork in highway bridge decks where the plank is used with concrete to produce a solid slab of 200 mm depth that is typical of slabs seen in highway bridge decks. The second study describes the use of pultruded planks in pedestrian bridge decks where the pultruded plank is used with a cement-board or a cast-in-place concrete panel to produce a hollow slab of 75 mm depth that is typical of timber decking used in FRP pedestrian bridges. Tests were conducted on beam-type specimens of the hybrid slabs to investigate the load transfer mechanisms between the pultruded plank and the cementitious “overlays” for both the 75 mm and 200 mm depths. From analysis of the load-carrying capacity and failure mechanisms of the hybrid slabs it was concluded that such hybrid slabs are viable systems for both highway and pedestrian bridge decks. A bridge deck using the 200 mm deep hybrid slab system was recently constructed on a highway in Wisconsin, USA.     

Experimental and finite element analysis on the steel fiber-reinforced concrete (SFRC) beams ultimate behavior

Steel fiber-added reinforced concrete (SFRC) applications have become widespread in areas such as higher upper layers, tunnel shells, concrete sewer pipes, and slabs of large industrial buildings. Usage of SFRC in load-carrying members of buildings having conventional reinforced concrete (RC) frames is also gaining popularity recently because of its positive contribution to both energy absorption capacity and concrete strength.This paper presents experimental and finite element analysis of three SFRC beams. For this purpose, three SFRC beams with 250 × 350 × 2000 mm dimensions are produced using a concrete class of C20 with 30 kg/m³ dosage of steel fibers and steel class S420 with shear stirrups. SFRC beams are subjected to bending by a four-point loading setup in certified beam-loading frame, exactly after having been moist-cured for 28 days. The tests are with control of loads. The beams are loaded until they are broken and the loadings are stopped when the tensile steel bars are broken into two pieces. Applied loads and mid-section deflections are carefully recorded at every 5 kN load increment from the beginning till the ultimate failure.One of the SFRC beams modeled by using nonlinear material properties adopted from experimental study is analyzed till the ultimate failure cracks by ANSYS. Eight-noded solid brick elements are used to model the concrete. Internal reinforcement is modeled by using 3D spar elements. A quarter of the full beam is taken into account in the modeling process.The results obtained from the finite element and experimental analyses are compared to each other. It is seen from the results that the finite element failure behavior indicates a good agreement with the experimental failure behavior.     

CFRP strengthened openings in two-way concrete slabs – An experimental and numerical study

Rehabilitation and strengthening of concrete structures with externally bonded fibre reinforced polymers (FRPs) has been a viable technique for at least a decade. An interesting and useful application is strengthening of slabs or walls where openings are introduced. In these situations, FRP sheets are very suitable; not only because of their strength, but also due to that they are easy to apply in comparison to traditional steel girders or other lintel systems. Even though many benefits have been shown by strengthening openings with FRPs not much research have been presented in the literature.In this paper, laboratory tests on 11 slabs with openings, loaded with a distributed load are presented together with analytical and numerical evaluations. Six slabs with openings have been strengthened with carbon fibre reinforced polymers (CFRPs) sheets. These slabs are compared with traditionally steel reinforced slabs, both with (four slabs) and without openings (one slab). The slabs are quadratic with a side length of 2.6 m and a thickness of 100 mm. Two different sizes of openings are used, 0.85 × 0.85 m and 1.2 × 1.2 m.The results from the tests show that slabs with openings can be strengthened with externally bonded CFRP sheets. The performance is even better than for traditionally steel reinforced slabs. The numerical and analytical evaluations show good agreement with the experimental results.     

Parametrical static analysis on group studs with typical push-out tests

Group studs are known as shear connectors in steel and concrete composite structures. By now, many composite bridges have been characterized by long lateral cantilevers. The shear studs are actually under biaxial action consisting of shear force and action in light of lateral bending moment on concrete slab induced by long cantilever and passing by moving loads. Moreover, lateral bending moment may even lead to the initiation of bending-induced concrete cracks. These two situations can both affect mechanical performance of group studs. Thus, a parametrical FEM analysis was carried out, in which damage plasticity was introduced to simulate material nonlinear behavior. In the analysis, lateral bending moments respectively inducing maximum concrete crack widths of 0.1 mm and 0.2 mm, shank diameters of 13 mm, 16 mm, 19 mm and 22 mm and stud heights including 80 mm and 100 mm were parameters. It was found that mechanical behavior of group studs with large shank diameter would be less affected by biaxial action and initial bending-induced concrete cracks seemed unfavorable to stud shear stiffness. On the other hand, typical push-out tests were executed to investigate reductions of shear stiffness and shear capacity of group studs. The reliability of FEM analysis was also verified based on the tests. In addition, stud shear capacity evaluations according to several design specifications were presented. It indicated shear capacity evaluation of Eurocode 4 got a relatively large safety factor. Moreover, the applicability of these specifications for group studs on shear capacity evaluation was also discussed.     

Experimental Study on Fire Behavior of Steel–Concrete Composite Cellular Beams with Large Opening Ratio

The aim of this study is to examine the behavior of protected and unprotected steel–concrete composite I-beams with large cell diameters (D₀/H?=?0.7) and closely spaced cell configurations under the ISO 834 fire curve. Previous studies on experimental full-scale fire performances of cellular beams have been somewhat limited under vertical service loads and different insulation properties. To address this limitation, a total of four composite beams, two unprotected (one beam with a solid web, and one cellular beam) and two protected cellular beams (60 min fire resistance with implementation of water and solvent-based intumescent coatings) were tested. As outputs of the tests, the failure modes observed, such as web buckling, the Vierendeel effect, the slab behavior, including the mechanism of concrete cracking, the overall displacement behavior (i.e., deflected shapes) up to collapse at very large deflections, and temperature changes in the steel elements are discussed. It was concluded that the quality of the intumescent coating applied is crucial in achieving the desired fire resistance. Experiments showed that unprotected trapezoid deck voids did not have a decisive influence on the behavior of the beams for up to 60 min of fire testing. At high temperatures, similar crack patterns occurred in the composite slabs of the protected and unprotected steel–concrete composite cellular beams. In the protected beams, the behavior of reactive coatings was significant and resulted in a non-uniform temperature distribution in these beams’ web and flanges.

     

Composite tubes as an alternative to steel spirals for concrete members in bending and shear

Glass fibre-reinforced polymer (GFRP) tubes are compared to steel spiral reinforcement in circular concrete members with longitudinal reinforcement and prestressing, using six beam tests. Two 324 mm diameter and 4.2 m long prestressed specimens were tested in bending. Four 219 mm diameter reinforced specimens were also tested, including two 2.43 m long beams tested in bending and two 0.6 m long beams tested in shear. In each set, one specimen was essentially a concrete-filled GFRP tube, while the other control specimen included steel spiral reinforcement of comparable hoop stiffness to that of GFRP tube. The strength of control specimens was governed by crushing and spalling of concrete cover. Unlike spiral reinforcement, GFRP tubes confined larger concrete areas and also contributed as longitudinal reinforcement, leading to increases in flexural and shear strengths, up to 113% and 69%, respectively.     

Punching shear tests on flat concrete slabs exposed to fire

Underground parking structures often consist of flat slabs connected by columns, for which punching shear is often the most critical design criterion. In fire conditions, the punching load can increase due to restraint of the thermal curvature of the slab or due to the expansion of the columns. This increase of the punching load is discussed in the paper by means of a literature review. On the other hand, during fire the punching resistance of the slab decreases due to a gradual reduction of the material properties. This reduction in bearing capacity is studied by means of real scale fire tests, consisting of 6 slabs measuring 3.2×3.5×0.25 m with a connected column stub and tested for punching shear with a specially designed loading frame. Two reference tests are executed at ambient temperature conditions and four slabs are submitted to ISO 834 curve for 120 min. Comparison of the test data with the expected increased axial load due to thermal restraint found in the literature, shows a potential danger for premature punching failure of flat slab-column connections exposed to fire.     

Influence of thermal insulation position in building envelope on the space cooling of high-rise residential buildings in Hong Kong

At the present time, thermal insulation is almost not used in fabric of tall residential buildings in Hong Kong, as their fabric slabs usually comprise concrete layer covered on each side by plaster layers. This study investigates into the influence of an existence of the thermal insulation layer in the outside walls on the yearly cooling load and yearly maximum cooling demand in two typical residential flats in a high-rise residential building by employing HTB2, detailed building heat transfer simulation software. During the investigations, the thermal insulation layer up to 15 cm thick was placed either at the inside, or at the outside, or at the middle part of the outside wall structure. Then, the concrete layer was up to 40 cm thick. The simulation predictions indicate that the highest decrease in the yearly cooling load of up to 6.8% is obtained when a 5 cm thick thermal insulation layer faces the inside of the residential flat. The highest decrease in the yearly maximum cooling demand of 7.3% is recorded when a 5 cm thermal insulation layer faces either the outside or the inside of the flat; this depends on the flat orientation. However, much weaker reductions in the yearly cooling load and yearly maximum cooling demand are found when the thickness of thermal insulation is increased above 5 cm and the thickness of concrete is increased above 10 cm.     

Structural behavior and m–k value of composite slab utilizing concrete containing crumb rubber

Many research works have been conducted to study the fresh and hardened properties of concrete containing crumb rubber as replacement to fine aggregate. The outcome of these researches indicated that though the compressive and flexural strength of crumb rubber concrete (CRC) decreased as percentage of fine aggregate replacement increased; the CRC has lower unit weight, better slump values, better toughness and absorb more energy before failure. In view of the fact that the main strength of composite floor slab lies within the bond between the concrete and the profiled steel sheeting, therefore the using of more ductile concrete such as CRC to toping the profiled steel sheeting could produce a new composite slab system. Two sets of slabs; each set comprising three CRC composite slabs and one conventional concrete slab has been tested with two shear span (450 and 900 mm). The results showed that the CRC slabs behavior could be characterized as ductile, while the m–k value has been found to be 80.7 and 0.037, respectively.     

Improvement of residual compressive strength and spalling resistance of high-strength RC columns subjected to fire

This paper investigates the spalling properties of high-strength concrete in order to improve the residual compressive strength and spalling resistance in specimens subjected to 3 h of unloading fire conditions. This study consists of three series of experiments with eighteen different specimens varying in fiber type and content, finishing material and simultaneous fiber content and lateral confinement. They were fabricated to a 300 × 300 × 600 mm mock-up size. Results of the fire test showed that the control concrete was explosive, while the specimens that contained more than 0.1 vol% of polypropylene (PP) and polyvinylalcohol (PVA) fibers were prevented from spalling. One specimen, finished by a fire endurance spray, exhibited even more severe spalling than the control concrete. The specimen containing 0.1 vol% of PP fiber and using a confining metal fabric at the same time, showed the most effective spalling resistance; in particular, the residual compressive strength ratio was even higher than that of the control concrete before the fire test. It was demonstrated that adding fibers in concrete prevented the spalling occurrence and confining metal fabric around the main bars of concrete specimens can secure the strength of structures during the conditions of elevated temperature.     

Fire response of exterior reinforced concrete beam-column subassemblages

Investigating the structural response of reinforced concrete beam-column sub-assemblies at elevated temperatures is the purpose of this paper. This goal was achieved by conducting the ISO-834 standard fire test on two identical one-third scaled reinforced concrete beam-column subassemblage test specimens. The test specimens, which each consisted of one reinforced concrete cantilever beam anchored at the mid-height of a reinforced concrete column, were installed together in a full scale furnace and subjected to downward and upward service loads, respectively. The fire compartment fully engulfed the cantilever beams (except the beams’ top face and the loading points), the beam-column connections and the lower columns. The fire test terminated after 74 min as soon as the tensile longitudinal steel bars of the upward-loaded cantilever beam attained the predefined critical temperature 530 °C. The lower columns exhibited partial concrete spalling and typical diagonal cracks appeared at the beam-column connections. Based on the recorded internal temperature distributions at the joint cores it was found that the material strength loss in the fire had insignificant impact on the load bearing mechanism of the joints. On the other hand, the gradual decrease in rotation capacity of the beam ends during the fire course considerably influenced the load-deflection relationship. A detailed numerical work has been carried out to simulate the response of the test specimens and will be published elsewhere.