High Strength Concrete Slabs Research Articles

Experimental assessment of angle shear connectors under monotonic and fully reversed cyclic loading in high strength concrete

An experimental study was performed to investigate the behaviour of angle shear connectors embedded in high strength concrete (HSC) slab. Eight push-out specimens were tested covering various geometries of angle shear connector. On top of the experimental study, the accuracy of the available equations to estimate the load capacities of angle connectors is also evaluated for the angles embedded in HSC. The results show inadequate ductility behaviour for the angle shear connectors embedded in HSC. Nonetheless, the angle connectors exhibited good behaviour in the case of strength degradation under cyclic loading. All specimens experienced the angle fracture type of failure and showed a very low strength degradation (0.1%–1.4%) when they were subjected to low cyclic fatigue loading. This study also concludes that the current available equations estimated the shear capacity of angle connectors embedded in HSC either are conservative or overestimate the ultimate capacity.

Punching-Shear Strength of Normal and High-Strength Two-Way Concrete Slabs Reinforced with GFRP Bars

AbstractThis paper investigated the punching-shear behavior of two-way concrete slabs reinforced with glass fiber–reinforced polymer (GFRP) bars of different grades. A total of 10 full-scale interior slab-column specimens measuring 2,500×2,500 mm with thicknesses of either 200 or 350 mm and 300×300 mm square column stubs were fabricated with normal and high-strength concretes. The specimens were tested under monotonic concentric loading until failure. The effects of concrete strength as well as reinforcement type and ratio were evaluated. The test results revealed that increasing the reinforcement ratio resulted in higher punching-shear capacity, lower reinforcement and concrete strains, and lower deflections. In addition, the high-strength concrete increased the punching-shear capacity, significantly reduced concrete strains, increased strains in the GFRP reinforcing bars, and reduced deflection due to the high tensile strength and modulus of elasticity. The test results and results from literature wer...

Multi-physics blast analysis of reinforced high strength concrete

The blast damage behavior of reinforcing bars embedded in a High Strength Concrete (HSC) exposed to blast loading is investigated. In order to properly evaluate the HSC slab subject to blast loading, an explicit analysis program was used in this work. The multi-physics method of coupled Eulerian-Lagrangian solver is used to improve the efficiency and reliability of numerical simulation. To enhance the accuracy of the simulation results, the equation of state, strength and failure models of materials are implemented in AUTODYN. Specifically, the implemented multi-limit strength and failure surface models included strain rate effect and tensile fracture energy for HSC. The realized and idealized 3D quarter-symmetric models are used to obtain a reasonable computational running time, and the numerical simulation of the spalling and scabbing of an HSC slab under blast loading is presented. To validate the implemented material models and analysis method, a comparison is made with reported experimental results, and the simulation results are found to be in good agreement with the experimental results.

Adhesion of Self-Compacting Overlays Applied to Different Concrete Substrates and Its Prediction by Fuzzy Logic

An experimental and analytical study was performed to evaluate the adhesion between concrete and overlays using five different substrate surfaces and six different mixtures of self-compacting concrete and mortar. Saw cut surfaces of high strength concrete slabs were used as substrate. After the application of self-compacting concrete or mortar layers to dry, saturated surface dry, saturated surface wet, dry with bonding grout, and saturated with bonding grout surfaces of concrete slabs, they were covered with wet hessian and polythene sheets for curing. At the age of 28 days, friction-transfer tests were performed to evaluate the adhesion. The results indicate that while dry and saturated wet surfaces produced the lowest bond strength, the application of cement bonding grout improved the bond strength significantly. The results also showed that despite the noticeable correlations between the adhesion and the results of different flow tests, aggregate/cement ratio, fly ash/cement ratio, compressive strength, and water/powder ratio, the effect of the other constituents of the employed self-compacting overlays on their adhesion were not so significant. In order to predict the adhesion of self-compacting mixtures applied to concrete substrates, a fuzzy logic model was also devised. The results obtained from the fuzzy logic prediction model were compared with the average results of the friction-transfer method and found to be in very close agreement. The results show that fuzzy logic can be used to predict adhesion of self-compacting overlays.

Fire Behavior of Thin CFRP Pretensioned High-Strength Concrete Slabs

More sustainable precast concrete structural elements are emerging from the research community utilizing high-strength, self-consolidating fiber-reinforced concrete (HPSCC) reinforced with noncorroding prestressed carbon-fiber-reinforced polymer (CFRP). An example of this is a new type of precast CFRP pretensioned HPSCC panel intended as load-bearing beams or columns for use in building envelopes. Such elements have recently been applied to architectural facade elements in Europe. A key issue in the implementation of these elements as load-carrying members in buildings is demonstrating satisfactory performance in fire. It is well known that the bond between FRP reinforcing bars and concrete deteriorates at elevated temperatures. It is also known that high-strength concrete is susceptible to explosive spalling when subjected to fire. Reductions in FRP reinforcement tensile and bond strength during fire, effects on the load-bearing capacity of prestressed concrete structures, and the explosive spalling response of HPSCC during fire all remain largely unknown. This paper provides insights into the fire behavior of CFRP prestressed HPSCC slabs through an experimental study on thin slabs exposed to a standard fire while subjected to sustained service loads. It is shown that the fire resistance of these elements is governed by fire-induced spalling or, if spalling is prevented by the use of high dosages of polypropylene microfibers in the concrete, by thermal splitting-crack-induced bond failure of the CFRP tendons in their prestress transfer zone. Neither reductions in tensile strength of the tendons nor reductions in bond strength due to resin softening at high temperature appeared to play critical roles for the tests described in this paper. Key areas for future research are highlighted.

Pseudo-Plate Method of High-Strength Steel Fiber Concrete Multi-Ribbed Slab

Basing on the disadvantages of multi-ribbed slab in practice, a new high-strength steel fiber concrete (HSFC) composite multi-ribbed slab structure is introduced. Because adding steel fibers can improve the capacity of the concrete, it also can make some influence on the mechanic performance of HSFC multi-ribbed slab, such as elastic module, shear module, flexural rigidity, and torsion rigidity and so on. So by the analysis of the material and mechanic capacity and considering the influence of steel fiber, the formulas to calculate flexural rigidity, torsion rigidity, deflection and internal force of the new composite slab are given, in which the multi-ribbed slab is equivalent to orthotropic slab by the principle of stiffness coincidence. The study in this paper can provide some valuable theory references for further study and application of multi-ribbed slab.

Construction and Quality Evaluation of Prestressed High-Strength Concrete Hollow Slabs

Combined with the application of high-performance concrete in bridge engineering, the construction technics of pretensioned prestressed high-strength concrete (PPHC) hollow slabs with new section is summarized. The qualities of concrete in C80 strength grade are statistically analyzed based on the precast of 75 PPHC hollow slabs for two bridges. The results show that it is reliable to precast PPHC hollow slabs for bridge construction.

Experimental assessment of channel shear connectors under monotonic and fully reversed cyclic loading in high strength concrete

Two series of modified push-out tests under monotonic and low cycle fatigue load were performed to study the behaviour of channel shear connectors embedded in high strength concrete (HSC) slab. In each series, four push-out specimens were tested covering different sizes of channel shear connector and different strength level of HSC. Evaluation of National Building Code of Canada (NBC) for load capacities of channel shear connectors in HSC is also covered. The results show adequate ductility of channel shear connectors in HSC. All specimens have channel fracture type of failure and showed 8–17% strength degradation under low cyclic fatigue loading compared to the monotonic loading. In application of channel shear connectors embedded in HSC, achieved strength degradation rate is lower than the other concrete types (other types of concrete means those studied in previous investigations, which included plain concrete, reinforced concrete (RC), fiber reinforced concrete (FRC) engineered cementitious composite (ECC), polypropylene (PP) concrete and light weight aggregate concrete (LWAC)). Also, it was determined that the Canadian design code is quite unconservative for the prediction of strength capacity of channel shear connectors in HSC.

Performance of composite girders strengthened using carbon fibre reinforced polymer laminates

This paper highlights the structural performance of steel–concrete composite girders strengthened using advanced composite laminates. Nonlinear 3-D finite element models have been developed to investigate the flexural behaviour and load carrying capacity of the girders. The composite laminates comprised carbon fibre reinforced polymer (CFRP) plates and sheets as well as steel reinforced polymer (SRP) sheets. The elastic modulus and ultimate tensile strength of the laminates varied from low to high 60–300 GPa and 700–3100 MPa, respectively. The nonlinear material properties of the strengthened composite girder components comprising concrete, structural steel beam, reinforcement bars, adhesive and composite laminates were incorporated in the finite element model. The interfaces between the composite girder components were also considered allowing the contact and bond behaviour to be modelled and the different components to retain its profile during the deformation of the strengthened composite girder. Furthermore, the load-slip characteristic of headed stud shear connectors was incorporated in the finite element models based on previous experimental and numerical investigations conducted by the author. The finite element models have been validated against published tests on composite girders strengthened using different advanced composite laminates and having different cross-section geometries, lengths, layers of laminates with different elastic moduli and ultimate tensile strengths, concrete strengths and structural steel strengths. The load carrying capacity of strengthened composite girders, load–vertical displacement behaviour and failure modes were predicted from the finite element analyses and compared against test results. Parametric studies were conducted to study the effects on the load carrying capacity and structural behaviour of strengthened composite girders owing to the change in the composite laminate elastic modulus, number of laminate layers, concrete strengths and structural steel strengths. The study has shown that the increase in the load carrying capacity and ductility of strengthened composite girders due to the increase in steel beam strength is significant with high strength concrete slab. Also, it has been shown that the increase in concrete strength offers a considerable increase in the initial stiffness of strengthened composite girders, while the increase in structural steel strength offers a considerable increase in the stiffness of strengthened composite girder in the post-yielding stage.

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.

Experimental and theoretical investigation of column – flat slab joint ductility

Most modern seismic codes use ductility as one of the basic design parameters. Actually, ductility defines the ability of a structure or its elements to absorb energy by plastic deformations. Until the end of the previous century ductility was defined qualitatively. Most research works related to ductility are focused on structural elements’ sections. This study was aimed at complex experimental and theoretical investigation of flat slab–column joints ductility. It is one of the first attempts to obtain quantitative values of joint’s ductility for the case of high strength concrete columns and normal strength concrete slabs. It was shown that the flat slab–column joint is a three-dimension (3D) element and its ductility in horizontal and vertical directions are different. This is the main difference between ductility of elements and joint ductility. In case of flat slab–column joints, essential contribution to joint’s ductility can be obtained due to the slab’s confining effect. Based on experimental data, the authors demonstrate that flat slab–column joint’s ductility depends on the joint’s confining effect in two horizontal and vertical directions. Furthermore, the influence of slab load intensity and slab reinforcement ratio on the joint’s ductility is performed in this study. It is also demonstrated that the effect of the ratio between the slab thickness and the column’s section dimension on the ductility parameter is significant. Equations for obtaining a quantitative value of a flat slab–column joint’s ductility parameter were developed.

Liquidating a hard-to-reach high-level waste repository at the Kurchatov Institute

Details of organizing the work to liquidate a hard-to-reach repository for high-level waste at a special site at the Institute are described. The bulk of the waste in the pit was encased in a high-strength concrete slab and, together with low- and medium-level waste, contained a large number of metal cans of high-level waste. Special arrangements for radiation protection set up around the pit and the techniques used to break up the concrete casing and extract the waste are described. Video cameras and a gamma visualizer were used to find high-level waste and fragments in the demolished concrete casing and to guide remotely controlled robotic equipment to them. Changes in the radiation environment in the work area were monitored operationally with a gamma locator; data from this detector were fed in real time to and analyzed and processed by a personal computer for use in carrying out the work.

Enhancing Performance of Slab-Column Connections

The influence of concentrated reinforcement in the immediate column region, the use of high-strength concrete slabs, as well as using puddled fiber-reinforced high-strength concrete in the slab in a region close to the column were investigated. The slab concrete strengths of the specimens were 30, 35, 40, 55, 65, and 90 MPa. The performance of the slab-column specimens tested was investigated, including the punching shear capacity, cracking on the top of the slabs around the columns, and the stiffness of the slab-column connections. Predictions using different code equations are compared to the experimental results obtained from the slab-column connections tested in this study and those tested in other studies. The beneficial effects of using concentrated flexural reinforcement, high-strength concrete and puddled fiber-reinforced high-strength concrete are demonstrated.

Nonlinear finite element analysis of high strength concrete slabs

A rational three-dimensional nonlinear finite element model is described and implemented for evaluating the behavior of high strength concrete slabs under transverse load. The concrete was idealized by using twenty-nodded isoparametric brick elements with embedded reinforcements. The concrete material modeling allows for normal (NSC) and high strength concrete (HSC), which was calibrated based on experimental data. The behavior of concrete in compression is simulated by an elastoplastic work-hardening model, and in tension a suitable post-cracking model based on tension stiffening and shear retention models are employed. The nonlinear equations have been solved using the incremental iterative technique based on the modified Newton-Raphson method. The FE formulation and material modeling is implemented into a finite element code in order to carry out the numerical study and to predict the behavior up to ultimate conditions of various slabs under transverse loads. The validity of the theoretical formulations and the program used was verified through comparison with available experimental data, and the agreement has proven to be very good. A parametric study has been also carried out to investigate the influence of different material and geometric properties on the behavior of HSC slabs. Influencing factors, such as concrete strength, steel ratio, aspect ratio, and support conditions on the load-deflection characteristics, concrete and steel stresses and strains were investigated.

Behavior of high-strength fibrous concrete slab–column connections under gravity and lateral loads

Ten slab–column connections were tested under combinations of gravity and lateral loads to investigate the effect of adding steel fibers to concrete mix on the structural behavior of normal- and high-strength slab–column connections. The variables selected for this study were the strength of concrete, volumetric ratio of steel fibers, type of steel fibers, and moment to shear ratio. The test specimens consisted of two identical series. The first series constructed with normal-strength concrete with a cube compressive strength of about 28 MPa, and designated as NSC. The second series constructed with high-strength concrete with cube compressive strength of about 75 MPa were designated as HSC. Each series consisted of three specimens with steel fibers and were tested under gravity load and unbalanced moment, and two control specimens; one was tested under gravity load only, while the other was tested under pure moment. The addition of steel fibers does significantly enhance the performance of the tested slab; it improves the shear strength, increases the ductility due to deflection and rotation, yields greater stiffness, and smaller cracks widths. Further improvement was also obtained when larger aspect ratio was used. Test results showed that the high-strength concrete specimens had larger shear strength, displacement and rotation ductility ratios, and corresponding energy absorption, than the normal-strength specimens by about 7–21%, 11–64%, 106–123%, 48–150%, and 93–246%, respectively. Incorporating steel fibers with high-strength concrete improved the overall deformation characteristics of the tested specimens and resulted in less sudden and more gradual failure mode.

Chloride induced reinforcement corrosion in lightweight aggregate high-strength fly ash concrete

Reinforced lightweight aggregate high-strength concrete slabs that incorporated fly ash were exposed to 2% chloride solution for over 15 months. Chloride ion ingress, corrosion potentials, corrosion current density and electrical resistivity were determined. These slabs were compared with slabs from normal weight concrete of medium and high-strength. The results indicated that lightweight high-strength concrete slabs with fly ash in the concrete mixture showed the least amount of chloride concentration. Values of corrosion current density were very low and values of electrical resistivity were very high and indicative of extremely low corrosion current. The dense matrix of the lightweight high-strength concrete is believed to restrict continuous pores that may carry chloride ions. The effect of fly ash in lowering the chloride diffusivity further contributed to reduce harmful chloride ions. In addition, the porous sintered fly ash aggregates are believed to have acted as buffer reservoirs for the chloride laden solution and thus prevented the chloride ions from reaching the steel surface.

Punching resistance of RC slabs with rectangular columns

Fifteen tests on high strength concrete slabs with rectangular supports and three different load patterns are reported. The results show that current code preovisions can overestimate punching resistance in some cases. An investigation, made using the finite element method, shows the influences of the shape of a support and the pattern of loading on the distribution of shear. Factors are proposed to allow for these effects while using the control perimeter and basic shear resistance of the CEB Model Code 90, and it is demonstrated that this approach provides strength estimates better than those of MC 90, BS 8110 and ACI 318. There remains a problem of the punching capacity of slabs almost failing in flexure and this is discussed.

Punching resistance of RC slabs with rectangular columns

Fifteen tests on high strength concrete slabs with rectangular supports and three different load patterns are reported. The results show that current code preovisions can overestimate punching resistance in some cases. An investigation, made using the finite element method, shows the influences of the shape of a support and the pattern of loading on the distribution of shear. Factors are proposed to allow for these effects while using the control perimeter and basic shear resistance of the CEB Model Code 90, and it is demonstrated that this approach provides strength estimates better than those of MC 90, BS 8110 and ACI 318. There remains a problem of the punching capacity of slabs almost failing in flexure and this is discussed.

A simplified method for assessing the ductile behaviour of stud connectors in composite beams with high strength concrete slab

Local connection shear failure in the absence of adequate connection ductility can reduce steel and concrete composite element bending resistance. For these elements, Eurocode 4 contains design rules and criteria for checking that implicitly prevent early connection rupture, by limiting the value of the parameters influencing maximum slip capacity. Such indirect ductility checks can, however, only be applied within the constraints of their formulation hypotheses, such as material strength as specified in the document for instance. The use of high strength concrete substantially changes the ratio between maximum slip requirement and connection deformation capacity and demands direct connection ductility checking. This paper identifies the magnitudes affecting maximum slip requirement and suggests a simplified assessment method. The method, having general application, enables assessing connection efficiency to bending collapse of the composite beams and is particularly useful at the initial structure design stage as no demanding sophisticated analysis is required.

Non-destructive testing of high strength concrete using spectral analysis of surface waves

Non-destructive testing is essential in the inspection of alteration, repair and new construction in construction industry. Researchers are exploring the performance of non-destructive testing method using spectral analysis of surface waves (SASW) in concrete structures. The method consists of the generation, measurement and processing of dispersive surface waves. In SASW test, the surface of the media under consideration is subject to an impact using, for example, a 12-mm steel ball, to generate surface wave energy at various frequencies. Two vertical accelerometer receivers detect the energy transmitted through the testing media. By recording signals in digitized form using a data acquisition system and processing them, surface wave velocity can be obtained using a dispersion curve. This study is to focus on understanding of the applicability and limitations of the SASW method in a high strength concrete. This study is also to characterize the material property of early age high strength concrete emphasizing compressive strength using non-destructive testing methods. Three high strength concrete slabs of 600, 850 and 1100 kg/cm 2 compressive strengths were prepared together with cylinders from same batches. Cylinder tests were performed at the ages of 7, 14, 21 and 28 days after pouring concrete. Using the impact-echo method, the compression wave velocities were obtained based on different high strength concrete ages and compressive strengths. The equation to obtain the compressive strengths of high strength concrete has been developed using the obtained compression wave velocities ( f′ c=1.083 V p−3816.1143). Using the method, the equation has also been developed to obtain the compressive strengths of high strength concrete based on the surface wave velocities ( f′ c=0.253 V p+16.271). This study can be utilized in examining structural elements of high strength concrete structures and be applied in the integrity analysis of high strength concrete structures with a finite thickness.