Flexural behavior of timber-high-performance concrete experimental slabs

Circular tubes are widely used in daily life and manufacture under bending load. The structural parameters of a circular tube, such as its wall thickness, number and shapes of ribs, and supporting flanges, are closely related to the tube’s bending rigidity. In this study, a tube with eight ribs and a flange was optimized, in order to obtain the lowest weight, through comprehensive structural optimization. We obtained the optimal structural parameters of the tube and the influence of the structural parameters on the tube’s weight. The structural parameters of tubes with different numbers of ribs were optimized. The tube with different number of ribs had the same inner diameter, bending load, and length as the tube with eight ribs. We conducted an experiment to verify the structural optimization simulation. Different tube sizes were subsequently optimized. The optimized tube with four trapezoidal ribs and a flange reduced the weight by more than 73% while maintaining the same deformation. The weight of the optimized tube with a flange reached a stable value after four trapezoidal ribs were added. When the number of ribs was two, the weight was the largest. The analysis results were consistent with the numerical results. A new AWATR (appropriate width and thickness of ribs can improve the bending rigidity of the tubes) formula was proposed, which can effectively improve the bending rigidity of tubes. Different shapes of tubes were optimized and compared. The optimized tube with four trapezoidal ribs and a flange was the lightest and easy to manufacture.

A new concept for designing concrete pavements by optimising the slab geometry in order to reduce the slab thickness as well as to minimise the mechanical load transfer devices has recently been proposed. Theoretically, the reduced slab size lowers the load and curling-induced tensile stresses and concomitantly a thinner concrete slab can be constructed. Full-scale test sections were constructed and tested under accelerated pavement loading conditions to validate this design concept hypothesis. The design and concrete material factors studied in this research were concrete thickness of 9, 15 or 20 cm; granular or asphalt concrete base layer; and plain or fibre-reinforced concrete (FRC). A methodology was presented to convert the channelised traffic loading to equivalent single axle loads (ESALs) so that comparisons could be made between the various test sections. The accelerated pavement testing showed that shorter slab sizes can sustain a significant number of overloads and greater number of ESALs before developing cracking relative to standard jointed concrete pavements. The most prevalent distress observed was corner cracking which occurred twice as much as longitudinal cracks, whereas only 3 out of 46 cracking distresses were transverse cracks. The 20 cm concrete slabs on granular base did not experience fatigue cracking for trafficking up to 51 million ESALs. The 15 cm concrete slabs on granular base began cracking on an average of 11 million ESALs. As expected, the concrete slabs on asphalt base resisted a significant larger number of ESALs relative to the same concrete thickness on granular base. The cracking performance of the 9 cm concrete slabs on granular base varied with the stiffness of the soil. For the 9 cm slab thickness, structural fibres provided a longer fatigue life and extended service life relative to the plain concrete slabs. Finally, the smaller slab sizes maintained a medium-to-high load transfer efficiency over the accelerated loading period for all slab thicknesses without the development of any faulting. As expected, these slab systems resulted in higher deflections, and, therefore, the granular base and subgrade layers as well as lateral drainage system must be designed and specified to reduce the rate of permanent deformation and minimise the possibility of support erosion.

Hybrid timber-concrete (HTC) floor systems have gained wide acceptance within the structural engineering community as an alternative for reinforced concrete (RC) floor systems. They consist of a concrete layer and a timber layer acting predominantly in compression and tension, respectively. HTC floor systems are lightweight and with less embodied energy compared to RC floor systems. However, perfect composite action is difficult to achieve in HTC floor system resulting in part of the concrete layer acting in tension, thus reducing the material efficiency.In this thesis, an HTC floor system with a novel hollow core (called “HTCHC floor system” hereafter) is proposed. This novel floor system consists of a top concrete layer, bottom timber layer, and a fibre reinforced polymer (FRP)-timber hollow core sandwiched between them. Thickness of each layer is designed so that the bending neutral axis of the HTCHC floor panels is within the core, resulting in concrete and timber acting predominantly in compression and tension respectively. Use of the hollow core near the section neutral axis to replace the solid timber or concrete in the HTC section is the key innovation in this study, as it significantly reduces the self-weight and increases the efficiency in material utilization without sacrificing the structural performance. Two types of FRP-timber cores are designed and tested in this study: (a) a corrugated-shaped core, and (b) a waffle-shaped core. The fabrication method and flexural behaviour of the HTCHC floor panels with each type of core are presented and discussed.First a corrugated core system (called “HTCCC” hereafter) was proposed. Utilising the corrugated core helps to reduce the concrete volume by at least 40% compared with a conventional HTC section with the same total height and timber thickness. Corrugated core also provides spacing for building services to be run through the floor. Two core configurations were developed: with core orientates parallel to the span for maximum one-way spanning capacity and with core orientation transverse to the span for generation of additional transverse spanning capacity without compromising the longitudinal span capacity. In total, eight HTCCC floor panels were prepared and tested, with the flexural capacities and critical failure modes analysed for each. Effects of different core geometries, shear force transfer methods, and manifested composite action were also closely studied. Longitudinal specimens achieved the best composite action and correspondingly the highest panel performance, with a 73% ultimate moment carrying capacity and an 85% stiffness efficiency at serviceability limit state (SLS), compared to an idealised HTC section with full composite action. In the transverse pattern specimens, shear bolt reinforcement was found to be essential for maintaining the high panel performance. It was also shown that with adequate enhancement of interfacial shear transfer and proper geometry, transverse patterned panels could closely match the one-way spanning capacity of the longitudinal core HTCCC panels.Based on the findings of the HTCCC floor panels, a new waffle core system (called “HTCWC” hereafter) consisting of a rectangular shape core with waffle-grids was designed. This core design allowed a uniform concrete layer, which can eliminate the undesirable stress concentrations within concrete which were observed in the HTCCC panels, and reduced concrete volume by 67% compared with a conventional HTC section. Employing digital fabrication techniques and computer-numerical-controlled (CNC) machines, integral connections were used in this prototype for interlayer shear transfer. The highly accurate and efficient manufacture process gave excellent quality control of the integral connectors, which simplified the manufacturing process, and improved the composite action and the structural performance of the system. Four specimens consisting of two different types of interfacial connections were fabricated and tested under four-point bending. Test results showed that the HTCWC floor system achieved better interlayer behaviour and significantly higher load capacity than the HTCCC system. They had 80-86% ultimate moment capacity and 90-93% stiffness efficiency at SLS, compared to an idealised HTC section. They also had a 44% higher weight-specific load capacity than equivalent RC floor system, and 11%-29% higher capacity than the HTC floors.Finally, a simple numerical model capable of considering the slip between concrete and timber layers was proposed to predict the behaviour of HTCWC floor panels. In this model, a new two-layer element was proposed, with the concrete and tensile timber parts modelled as the two sublayers, and the core was modelled using interlayer springs. A stiffness matrix was proposed for this new element, and then nonlinearities of the core and sublayer materials were introduced by explicitly updating the stiffness matrix at each step. Results showed that this model provides very similar prediction as the finite element software ABAQUS for elastic composite beams while using only about 10% of the elements. It also showed an accurate prediction of the HTCWC specimens regarding the overall stiffness at the linear range, the nonlinear response, and the ultimate capacity.

Protective potential of high-contrast mineral-bonded layers on reinforced concrete slabs subjected to uniform shock waves

Creep, as an intrinsic property of concrete material, will inevitably affect the performance of concrete pavement slabs in the field. However, the creep effect on the performances of concrete pavement slabs is far from being fully investigated. In this study, a test set-up is designed to measure the flexural creep of concrete beams exposed to both sealed and drying conditions. The measured flexural creep results are then modeled by the microprestress–solidification theory-based creep model which is incorporated into finite element analysis to evaluate numerically the creep effect on the moisture warping deformation, warping stress, and the total stress under traffic load in concrete slabs. Parameters including slab size, slab thickness, and subgrade modulus are considered. It is found that concrete creep has a significant effect on slab performance. Based on the measured creep properties in this study, the warping deformation of slabs can be reduced by 8–62%, and the warping stress and the total stress can be relaxed by at least 50%. Therefore, it is of importance to incorporate creep effect in analyzing warping deformation and stress generated in concrete pavement slabs. This study also provides a numerical methodology to the current performance evaluation of concrete slabs in the field.

Knowledge of the condition of an existing concrete pavement is vital to the success of any rehabilitation and maintenance project. Accurate mapping of the voids under concrete slabs provides critical information for roadway designers to derive optimum rehabilitation and maintenance strategies. The Falling Weight Deflectometer (FWD) has been used successfully for many years as a forensic engineering tool. However, the literature on applying FWD to detect voids under concrete slabs is quite limited. The FWD defections are influenced by many factors such as the load transfer across slabs, size of void area, slab thickness, and layer moduli of the concrete and supporting layers. To identify and characterize voids under concrete slabs, pavement engineers need computation methods or algorithms for interpretation of FWD deflection basin. In this study, a Finite Element Model (FEM) was applied to address the load transfer between the joints of concrete slabs. Results from sensitivity analysis indicate that there is an exponential relationship between the slab size and the load transfer efficiency. To simply the analysis without conducting FEM analysis each time, a regression equation is established to estimate the effect of slab size on load transfer efficiency. In addition, the proposed method considers the effects of slab size, thickness, and layer moduli. The proposed method has been calibrated and applied to one highway project, where the voids under the concrete slabs have been mapped accurately. It is concluded that the proposed method is very reliable and can accurately detect voids under concrete slabs using the FWD deflection basin.

The frictional force between concrete slab and subbase is accompanied by horizontal slab movements induced by variation of temperature and moisture in the concrete slabs. The frictional force is exerted in the opposite direction from the horizontal slab movement and causes stress in the slab. Rational evaluation of subbase friction is important in configuring joint sealing, slab thickness, and reinforced steel. Determination of the subbase friction is also required as an input for the recently developed concrete-pavement-construction program HIPERPAV. Lean concrete has been widely used as the typical subbase for jointed concrete pavement in Korea. Generally, polythene sheet is placed between the lean concrete subbase and the concrete pavement slab as a friction reducer. In addition, an asphalt bond breaker may be used as an alternative friction reducer in some cases. Three series of push-off tests were conducted to study the characteristics of subbase friction for this typical Korean jointed concrete pavement system under three different subbase conditions (I, test slab directly cast on lean concrete subbase; II, polythene sheet placed between test slab and lean concrete subbase; and III, 4-cm asphalt bond breaker placed between test slab and lean concrete subbase). For each series, tests were performed under various conditions (rate of movement, slab thickness, number of movement cycles) to investigate the influence of these potential factors on the development of subbase friction.

The New Classroom of Galuh University is a 2-storey building that uses concrete floor panel slab construction. The use of the concrete floor panel slabs is due to insufficient road access to the location to use ready mix cast concrete and to streamline project implementation time. The purpose of this study is to find out how much the cost and time comparison of the work of conventional concrete floor slabs and concrete floor panels is. This research was conducted in the new classroom building of Galuh University. The results of data analysis show that the budget required for conventional concrete floor slab work in Galuh University's New Classroom Building is Rp. 368,160,000.00,- with the price for 1 m3 is Rp. 4,613,524.65,-. And the budget needed for the floor panel slab work on the Galuh University New Classroom Building is Rp. 359,100,000.00,- with the price for 1 m3 is Rp. 4,500,000,000.00,-. Based on the price comparison, concrete floor slabs are about 2.46% more efficient than conventional concrete slabs. This is because there is a price difference of Rp. 9,060,000.00,-. The work time needed to complete the Conventional Floor Plate work is 27 days and the work time needed to complete the Floor Panel Plate work is 21 Days. Based on the comparison of work time, Floor panel plates are 22.22% more efficient for work time.

In the structural design of composite pavement with a concrete pavement slab overlaid with an asphalt surface course, it is very important to estimate the temperature gradient in the concrete slab. An asphalt surface course reduces the temperature gradient in an underlaid concrete slab, resulting in the reduction of thermal stress of the concrete slab. This effect was investigated by temperature measurement in model pavements and by thermal conductivity analysis. Thermal properties were estimated by a backanalysis by using measured temperatures over 1 year. From the numerical simulations varying the thickness of asphalt surface and concrete slab, the relationship between the reduction effect and the asphalt thickness was derived as a function of the thickness of asphalt surface course, which can be used in the structural design of the composite pavement.

Thermal stress caused by temperature distribution in pavement slab is important in the structural design of airport concrete pavements. Airport concrete pavement slab is thick; therefore, temperature in the slab varies nonlinearly throughout its depth. Thermal stress caused by nonlinear temperature distribution differs from that of linear temperature distribution. In this study, thermal stress caused by nonlinear temperature distribution was calculated by using temperatures and restraint strains measured in an experimental concrete pavement with 460-mm-thick slabs. The calculated thermal stress differed from curling stress because of the linear temperature distribution. Temperature distributions in concrete slabs of various thicknesses were computed by solving the heat transfer equation with the control volume method. This calculation revealed that temperature distributions in thick concrete slabs were highly nonlinear. Thermal stress was analyzed to compute the nonlinear thermal stress distributions in concrete slabs from the computed temperature distributions by using a three-dimensional finite element model. On the basis of the analysis, effects of slab thickness and nonlinear temperature distribution on thermal stress were discussed. The effect of nonlinearity was found to be pronounced at a slab thickness greater than 300 mm. In this range, the reduction factor for taking into account the effect of nonlinear distribution was larger than 0.7. It was also found that thermal stresses in summer and spring were relatively large.

There are a few related researches that provide well-defined models to predict the damage size under close-in explosions. This paper presents a prediction of the damage size experienced by reinforced concrete (RC) slabs subjected to close-in detonations using numerical data and a neural network–based model. To train and validate the artificial neural network (ANN), a data base is developed through a series of measurements of the damage diameter (crater/spalling) size induced in reinforced concrete two-way slabs under blast loads. The data was obtained by performing various simulations using the dynamic explicit finite element code LS-DYNA. The principle parameters controlling the breaching size are charge weight, standoff distance, and slab thickness, which were used to develop the ANN training input data set. The trained and validated neural network was used to develop an ANN model capable of predicting the breach size of concrete slabs under close-in detonations.

Fully supported concrete slabs (FSS) were constructed and tested under static and cyclic loading in the laboratory. Deflection, strain, and load data were acquired for each slab test. The results of the FSS fatigue tests were plotted as the stress ratio (bending stress in the slab divided by the concrete modulus of rupture) versus the number of cycles to failure. FSS had a resistance to fatigue 30 percent higher than simply supported beams (SSB). For the same number of cycles to failure, concrete slabs could sustain a stress level 30 percent higher relative to beams. The concrete modulus of rupture from an SSB was found not representative of the true strength of an FSS. FSS tests under static load had a flexural strength 30 percent higher than the concrete modulus of rupture from a SSB test configuration. When the actual slab strength under static loading was accounted for in the stress ratio, the fatigue curves for concrete beams and slabs were essentially identical. Strain gauges indicated that plastic deformation in the slab occurred almost immediately with cyclic loading. Strain gauges also showed that partially cracked slabs resisted the load through cantilever action. Deflection measurements showed that crack initiation and propagation occurred in the slab before fatigue failure. Crack propagation through the slab thickness was found to consume most of the slab’s fatigue life.

Prediction of punching shear capacities of two-way concrete slabs reinforced with FRP bars

The design to resist blast loading is required in many private and governmental buildings. The research presented in this thesis characterizes the response of high strength concrete panels, reinforced with high strength vanadium steel, subjected to blast loading under controlled conditions. This work is intended to provide valuable data to study numerical models such as the commonly used single-degree-of-freedom (SDOF) models. The experimental procedure used and data collected from high-strength reinforced concrete (RC) slabs, having two different high-strength reinforcement ratios subjected to shockwave loadings using a blast load simulator are presented in this thesis. The pressure, impulse, and deflection time histories generated from the experiments along with the predicted panel deflection and damage responses are presented. The pressure impulse (PI) curves developed using a SDOF model are compared with the experimental data. Damage assessment generated from the blast load simulator experiments and a comparison of experimental behavior of high strength RC slabs with regular strength RC slabs, having two different Grade 60 regular-strength reinforcement ratios, are also presented. These results showed that while the regular strength slabs with regular strength reinforcing steel experienced slightly higher experimental deflections that the high strength slabs with high strength reinforcing steel, the reinforcement spacing or reinforcement ratio, played a more significant role in both experimental and numerical maximum peak deflections for both the regular strength concrete slabs reinforced with regular strength steel and the high strength concrete slabs reinforced with high strength steel. Experimental quantification of the dynamic resistance curves showed that the slabs with smaller longitudinal reinforcement spacing had greater ductility and post-yield behavior. Furthermore, a parametric study was performed, using the same SDOF model, comparing various high-strength concrete slab thicknesses with varying highstrength reinforcement ratios for maximum numerical deflection. The results from this study showed that the thicker slabs with larger reinforcement ratios yielded smaller maximum numerical deflections than those of the thinner slabs with smaller reinforcement ratios. Finally, the concrete damage patterns of the panels are shown and described.

This paper presents the results of a parametric analysis of structural response of concrete pavements under simultaneous action of traffic loads and temperature variations in the slab. The parameters studied include (1) temperature differential in the slab, (2) slab length, (3) modulus of subgrade reaction, (4) concrete elastic modulus, (5) slab thickness, (6) edge stiffness, and (7) joint stiffness. The effects of using a bonded interface in a composite pavement slab were also analyzed under the thermal-load condition. The finite element computer program FEACONS, developed at the University of Florida, was used to compute the maximum thennal-load-induced stresses in the concrete slabs. Temperature was assumed to vary linearly along the slab depth. The results were also compared with those obtained by the Westergaard equations and the influence charts developed by Pickett and Ray. The results of the analysis show that the temperature differential in the slab has significant effects on the structural response of concrete pavements. Also, the effects of these pavement parameters under critical thermal­ loading conditions are different from those under full subgrade support condition. It was observed that, with the presence of temperature differential in the slab, the effects of modulus of subgrade reaction can not be determined without considering other pavement parameters. For example, with a shorter slab length, the computed maximum thermal-load induced stress increases as the modulus of subgrade increases. The reverse is true for a pavement with longer slab lengths. The use of a bonded interface in a composite pavement slab is most effective in reducing the maximum stresses in the slab when a thinner layer is placed on top of a thicker layer. These observations point to the importance of considering thermal curling effects in concrete pavement analysis and design.