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.     

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.     

Experimental investigation on thermal and mechanical behaviour of composite floors exposed to standard fire

This paper presents experimental investigations on the thermal and mechanical behavior of composite floors subjected to ISO standard fire. Four 5.2 m×3.7 m composite slabs are tested with different combinations of the presence of one unprotected secondary beam, direction of ribs, and location of the reinforcement. The experimental results show that the highest temperature in the reinforcements occurs during the cooling phase (30–50 °C increment after 10-min cooling). The temperature at the unexposed side of the slabs is below 100 °C up to 100-min heating, compared to the predicted fire resistance close to 90 mins from EC4. For the slabs without secondary beams, the cracks first occur around the boundaries of the slab, while for the slabs supported by one unprotected secondary beam, concrete cracks first occur on the top of the slab above the beam due to the negative bending moment, and later on develop around boundaries. Debonding is observed between the steel deck and concrete slab. The secondary beam significantly impacts the deformation shape of tested slabs. Although a large deflection, 1/20 of the span length, is reached in the tests, the composite slabs can still provide sufficient load-bearing capacity due to membrane action. The occurrence of tensile membrane action is confirmed by the measured tensile stress in the reinforcement and compressive stress in the concrete. A comparison between measured and predicted fire resistance of the slabs indicates that EC4 calculations might be used for the composite slabs beyond the specified geometry limit, and the prediction is conservative.     

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.     

Applicability of the Small Diameter Sampler for Niigata Sand Deposits

Applicability of tube sampling for Niigata sand deposits is discussed through bender element and cyclic triaxial tests for samples obtained from two-chambered hydraulic piston samplers (Shogaki, 1997) with inner diameters of 48 mm and 50 mm, a one-chambered 70 mm diameter sampler, a 125-mm rotary triple-tube sampler and the frozen (FS) sampling method (Yoshimi et al., 1989). The relationship between the relative density (D_r) and normalized SPT N-value (N₁) obtained from small diameter samplers with inner diameters of 45 mm and 50 mm samplers was close to that of the FS and the N₁ coefficient was greater than those of the 70-mm and other tube samplers. The stress ratio at 20 cycles (R_L20) and the initial modulus of rigidity (G_CTX) of samples obtained from the 45-mm and 50-mm samplers were greater than those of the 70-mm, 125-mm rotary triple-tube and other tube samplers. However, the R_L20 values obtained from the 45-mm and 50-mm samplers were smaller than those of the FS sampler in the area of N₁>24. The G_BE and G_CTX values obtained from the 45-mm and 50-mm samplers were close to those of the FS sampling. Therefore, the 45-mm and 50-mm samplers can take equally high quality samples for Niigata sand deposits.     

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.     

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.     

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.     

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.     

Strength and deformability of mineral wool slabs under short-term compressive,tensile and shear loads

The results obtained in the experimental study of mineral wool slabs under short-term compressive, tensile and shear loads, used for insulating flat roofs, cast-in-place floors, curtain and external basement walls, as well as for sound insulation of floors, are presented. To describe the experimental data of strength and deformability of mineral wool slabs, the regression equations of the linear form, provided with one-sided confidence intervals for predicting the resultant characteristic with probability Р = 0.90, are used. The interrelationship between strength and deformability characteristics of mineral wool slabs under tension and shear, and their strength and deformability under short-term compression are determined. Based on the experimental data, differences in deformability and strength characteristics of mineral wool slabs under shear load in mutually perpendicular planes (in the direction of slab forming and perpendicular to it) are demonstrated.     

Strength correlation between individual block,prism and basic wall panel for load bearing interlocking mortarless hollow block masonry

This paper focuses on the development of the compressive strength correlation between the individual block, prism and basic wall panel for load bearing interlocking hollow mortarless blocks. The interlocking blocks used were developed by the Housing Research Centre at Universiti Putra Malaysia. The blocks consisted of stretcher, corner and half blocks. Forty individual block units from each type were tested under compression. The compressive strengths of 10 prisms assembled by stacking two stretcher blocks and two half blocks were evaluated. In addition, four wall panels each having a dimension of 1.2 × 1.2 m were assembled and tested under axial compressive loads. The results obtained were compared with those found in bonded masonry. BS 5628 Part 1:1992 were used for predicting of the compressive capacity of the bonded masonry. The interlocking mechanism, crack patterns and failure mechanism of the interlocking masonry specimens are highlighted and discussed.The correlations between the compressive strength of the interlocking masonry individual block (f_cb), prism (f_cp) and standard panel (f_cw) found in this analysis were f_cp = 0.47f_cb, f_cw = 0.83f_cp and f_cw = 0.39f_cb. Test results indicate that the interlocking mechanism and strength of the block in the load-bearing wall was satisfactory.     

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.     

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.     

Performance of deteriorated corrugated steel culverts rehabilitated with sprayed-on cementitious liners subjected to surface loads

A variety of alternatives to rehabilitate culverts have been developed over the past decades given their advantages over conventional open-cut culvert replacement. However, the performance of many of these systems has not yet been examined through laboratory testing. The objective of the present paper is to examine the performance of deteriorated steel culverts rehabilitated with spray-on liners when subjected to surface loads. Two 1200 mm diameter corrugated steel pipelines with similar levels of deterioration in the invert-haunch area were buried to a depth of 1200 mm and tested under service load employing a load frame simulating a single axle of a Canadian design truck. The pipelines were then rehabilitated with spray on-cementitious liners (each with a different target thickness). Once rehabilitated, the pipelines were examined again under the service load employing the single axle load frame at 1200 mm of soil cover, and then tested employing a tandem axle load frame at 2100 and 1200 mm of soil cover. During all tests, changes in diameter, curvature and liner strains were monitored. The data obtained indicates that the flexible pipelines responded like semi-rigid structures after rehabilitation. It was also observed that the difference in liner thickness of 30% did influence the response of the pipelines, and that extreme fiber tensions during service loading were 7% and 13% of the tensile strength of the liner materials for the 76 mm and 51 mm liner thicknesses that were specified.     

钢-超高性能混凝土组合板冲切性能试验研究和承载力计算

钢-超高性能混凝土(UHPC)组合板是将钢板与UHPC通过连接件组合成整体,具有高强、高延性、抗开裂、施工便利等特性,可应用于桥面板、防护工程等结构中。由于钢-UHPC组合板相对较薄且往往承受集中荷载,因此需要对其抗冲切性能进行重点研究。通过变化连接件参数、UHPC厚度、钢板厚度和加载区边长,完成了14块板件在集中荷载作用下的冲切试验。研究结果表明,配筋能显著提升组合板的承载力和延性,钢板和钢纤维使板件达到极限荷载后具有一定延性,组合板承载力随栓钉连接程度、UHPC厚度、钢板厚度和加载区边长的增大而增大。通过塑性极限理论分析了钢-UHPC组合板在弯曲机制和冲切机制下的两种承载力计算方法,并在此基础上提出了考虑折减UHPC抗冲切贡献的承载力计算方法,计算结果与试验值吻合良好。     

Load transfer mechanism in pile group due to single tunnel advancement in stiff clay

Construction of tunnels in urban cities may induce excessive settlement and tilting of nearby existing pile foundations. Various studies reported in the literature have investigated the tunnel–soil–pile interaction by means of field monitoring, centrifuge and numerical modelling. However, the load transfer mechanism between piles in a group, the induced settlement and the tilting of a pile group due to tunnel advancement has not been investigated systematically and is not well understood. This study conducts three-dimensional, coupled-consolidation finite element analyses to investigate tunnelling effects on an existing 2 × 2 pile group. The construction of a 6 m diameter (D) tunnel in saturated stiff clay is simulated. Responses of the pile group located at a clear distance of 2.1 m (0.35D) from a tunnel constructed at three different cover-to-diameter-of-tunnel ratios (C/D) of 1.5, 2.5 and 3.5 are investigated. The computed results are compared to published data based on field monitoring. It is found that the most critical stage for settlement, tilting and induced bending moment of pile group due to tunnelling is when the tunnel face is close to the pile group rather than at the end of tunnel excavation. The depth of the tunnel relative to the pile group has a vital influence on the settlement, tilting of pile group and the load transfer mechanism between piles in pile group induced by tunnel excavation. Tunnelling near the mid-depth of the pile group (i.e. C/D = 1.5) induces the largest bending moment in the piles, but the settlement and tilting of the pile group are relatively small. Based on a settlement criterion, apparent loss of capacity of the pile group is 14% and 23% for tunnels constructed at depths of C/D = 1.5 and at both C/D = 2.5 and 3.5, respectively. The largest load redistribution between the front and rear piles in the group and the largest tilting of the pile cap towards the tunnel occurs when tunnel excavated at C/D = 2.5.     

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.     

Fire extinguishing properties of novel ferrocene/surfynol 465 dispersions

This paper reports a novel fire-extinguishing agent: an aqueous dispersion of fine ferrocene particles. In this study, the ferrocene–water–surfactant dispersions were prepared to optimize the gas-phase concentration of ferrocene, and their ability to extinguish heptane fires was examined. The fire-extinguishing efficiency was characterized by three parameters: the ferrocene concentration in the dispersion (0–175 ppm), the surfactant used, and the ferrocene particle size (d₅₀=10.4, 11.4, 21.5, and 68.8 μm). The results indicated that (1) the ferrocene (d₅₀=10.4 mm)–water–surfynol 465 dispersion is the most stable among the dispersions tested, (2) the ferrocene–water–surfynol 465 dispersions have an optimal value of ferrocene concentration regarding the extinguishing time, and (3) in the ferrocene particle size range of 10.4–21.5 μm, the minimum extinguishing time of the ferrocene–water–surfynol 465 dispersions is remarkably shorter (1.2 s) than those observed when using a conventional wet chemical agent (45 wt% aqueous solution of potassium carbonate, 12.9 s).     

Comparison of fibrous insulations – Cellulose and stone wool in terms of moisture properties resulting from condensation and ice formation

Cellulose fibres are often used as thermal insulation in buildings. The organic nature of cellulose fibres, however, makes the insulation sensitive to high moisture content. This study investigates the moisture performance of cellulose insulation when exposed to a subzero environment. The paper is focused on the condensation and freezing in the material and includes comparison with the authors previous studies on stone-wool insulation. While the used stone-wool samples were water-repellent due to resin binders, cellulose is a typical representative for hydrophilic thermal insulation to which any contact with water condensate can be crucial.Test specimens of loose-fill cellulose were placed in a special laboratory device providing high moisture load. During a period of 100 h the specimens were subjected to a continuous load of moisture at atmospheric conditions on one side while the other side of the specimen faced a surrounding temperature of 0, −10 and −20 °C and the laboratory tests were repeated three times for each set of the specific thermal conditions (T_i = +20 °C, T_e = 0, −10 and −20 °C). The results indicate that there are minor changes in the water vapour permeability of the specimens. The experimental data from the investigation is compared with a mathematical model that simulates moisture diffusivity of cellulose together with accumulation due to sorption and freezing, using the actual climatic data.     

Daylighting control performance of a thin-film ceramic electrochromic window: Field study results

Control system development and lighting energy monitoring of ceramic thin-film electrochromic (EC) windows were initiated at the new full-scale window systems testbed facility at the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, CA. The new facility consists of three identically configured side-by-side private offices with large-area windows that face due south. In one room, an array of EC windows with a center-of-glass visible transmittance T_v range of 0.05–0.60 was installed. In the two other rooms, unshaded windows with a T_v = 0.50 or 0.15 were used as reference. The same dimmable fluorescent lighting system was used in all three rooms. This study explains the design and commissioning of an integrated EC window-lighting control system, and then illustrates its performance in the testbed under clear, partly cloudy, and overcast sky conditions during the equinox period. The performance of an early prototype EC window controller is also analyzed. Lighting energy savings data are presented. Daily lighting energy savings were 44–59% compared to the reference window of T_v = 0.15 and 8–23% compared to the reference window of T_v = 0.50. The integrated window-lighting control system maintained interior illuminance levels to within ±10% of the setpoint range of 510–700 lx for 89–99% of the day. Further work is planned to refine the control algorithms and monitor cooling load, visual comfort, and human factor impacts of this emerging technology.