Browsing Engineering and Informatics by Author "Yang, KeunHyeok"
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Aggregate interlock in lightweight concrete continuous deep beamsYang, KeunHyeok; Ashour, Ashraf F. (201101)There are very few, if any, available experimental investigations on aggregate interlock capacity along diagonal cracks in lightweight concrete deep beams. As a result, the shear design provisions including the modification factor of ACI 31808 and EC 2 for lightweight concrete continuous deep beams are generally developed and validated using normal weight simple deep beam specimens. This paper presents the testing of 12 continuous beams made of alllightweight, sandlightweight and normal weight concrete having maximum aggregate sizes of 4, 8, 13 and 19 mm. The load capacities of beams tested are compared with the predictions of strutandtie models recommended in ACI 31808 and EC 2 provisions including the modification factor for lightweight concrete. The beam load capacity increased with the increase of maximum aggregate size, though the aggregate interlock contribution to the load capacity of lightweight concrete deep beams was less than that of normal weight concrete deep beams. It was also shown that the lightweight concrete modification factor in EC 2 is generally unconservative, while that in ACI 31808 is conservative for alllightweight concrete but turns to be unconservative for sandlightweight concrete with a maximum aggregate size above 13 mm. The conservatism of the strutandtie models specified in ACI 31808 and EC 2 decreased with the decrease of maximum aggregate size, and was less in lightweight concrete deep beams than in normal weight concrete deep beams.

Application of Plasticity Theory to Reinforced Concrete Deep BeamsAshour, Ashraf F.; Yang, KeunHyeok (200811)This paper reviews the application of the plasticity theory to reinforced concrete deep beams. Both the truss analogy and mechanism approach were employed to predict the capacity of reinforced concrete deep beams. In addition, most current codes of practice, for example Eurocode 1992 and ACI 31805, recommend the strutandtie model for designing reinforced concrete deep beams. Compared with methods based on empirical or semiempirical equations, the strutandtie model and mechanism analyses are more rational, adequately accurate and sufficiently simple for estimating the load capacity of reinforced concrete deep beams. However, there is a problem of selecting the effectiveness factor of concrete as reflected in the wide range of values reported in the literature for deep beams.

Axial behavior of reinforced concrete short columns strengthened with wire rope and Tshaped steel plate units.Yang, KeunHyeok; Ashour, Ashraf F.; Lee, ET. (200903)This paper presents a relatively simple column strengthening procedure using unbonded wire rope and Tshaped steel plate units. Twelve strengthened columns and an unstrengthened control column were tested to failure under concentric axial load to explore the significance and shortcomings of the proposed strengthening technique. The main variables investigated were the volume ratio of wire ropes as well as geometrical size and configuration of Tshaped steel plates. Axial load capacity and ductility ratio of columns tested were compared with predictions obtained from the equation specified in ACI 31805 and models developed for conventionally tied columns, respectively. The measured axial load capacities of all strengthened columns were higher than predictions obtained from ACI 31805, indicating that the ratio of the measured and predicted values increased with the increase of volume ratio of wire ropes and flange width of Tshaped steel plates. In addition, at the same lateral reinforcement index, a much higher ductility ratio was exhibited by strengthened columns having a volume ratio of wire ropes above 0·0039 than tied columns. The ductility ratio of strengthened columns tested increased with the increase of flange width, thickness, and web height of Tshaped steel plates. A mathematical model for the prediction of stress–strain characteristics of confined concrete using the proposed strengthening technique is developed, that was in good agreement with test results.

Effectiveness of Web Reinforcement around Openings in Continuous Concrete Deep Beams.Yang, KeunHyeok; Ashour, Ashraf F. (200807)Twenty two reinforced concrete continuous deep beams with openings and two companion solid deep beams were tested to failure. The main variables investigated were the configuration of web reinforcement around openings, location of openings, and shear spantooverall depth ratio. The influence of web reinforcement on controlling diagonal crack width and load capacity of continuous deep beams with openings was significantly dependent on the location of openings. The development of diagonal crack width and load capacity of beams having openings within exterior shear spans were insensitive to the configuration of web reinforcement. However, for beams having openings within interior shear spans, inclined web reinforcement was the most effective type for controlling diagonal crack width and increasing load capacity. It has also observed that higher load and shear capacities were exhibited by beams with web reinforcement above and below openings than those with web reinforcement only above openings. The shear capacity at failed shear span of continuous beams tested is overestimated using Kong et al’s formula developed for simple deep beams with openings.

Flow and Compressive Strength of AlkaliActivated Mortars.Yang, KeunHyeok; Song, JK.; Lee, KS.; Ashour, Ashraf F. (20090101)Test results of thirty six ground granulated blastfurnace slag (GGBS)based mortars and eighteen fly ash (FA)based mortars activated by sodium silicate and/or sodium hydroxide powders are presented. The main variables investigated were the mixing ratio of sodium oxide (Na2O) of the activators to source materials, watertobinder ratio, and fine aggregatetobinder ratio. Test results showed that GGBS based alkaliactivated (AA) mortars exhibited much higher compressive strength but slightly less flow than FA based AA mortars for the same mixing condition. Feedforward neural networks and simplified equations developed from nonlinear multiple regression analysis were proposed to evaluate the initial flow and 28day compressive strength of AA mortars. The training and testing of neural networks, and calibration of the simplified equations were achieved using a comprehensive database of 82 test results of mortars activated by sodium silicate and sodium hydroxide powders. Compressive strength development of GGBSbased alkaliactivated mortars was also estimated using the formula specified in ACI 209 calibrated against the collected database. Predictions obtained from the trained neural network or developed simplified equations were in good agreement with test results, though early strength of GGBSbased alkaliactivated mortars was slightly overestimated by the proposed simplified equations.

Inclined reinforcement around web opening in concrete beamsYang, KeunHyeok; Ashour, Ashraf F. (2007)Twelve reinforcedconcrete continuous deep beams having web openings within interior shear spans were tested to failure. The main variables investigated were the opening size and the amount of inclined reinforcement around openings. An effective inclined reinforcement factor combining the influence of the amount of inclined web reinforcement and opening size is proposed and used to analyse the structural behaviour of continuous deep beams tested. It was observed that the end support reaction, diagonal crack width and load capacity of beams tested were significantly dependent on the proposed effective inclined reinforcement factor. As this factor increased, the end support reaction and increasing rate of diagonal crack width were closer to those of companion solid deep beams. In addition, a higher load capacity was exhibited by beams having an effective inclined reinforcement factor above 0.077 than the companion solid deep beam. A numerical procedure based on the upperbound analysis of the plasticity theory was proposed to estimate the load capacity of beams tested. Comparisons between the measured and predicted load capacities showed good agreement.

Influence of inclined web reinforcement on reinforced concrete deep beams with web openings.Yang, KeunHyeok; Chung, HS.; Ashour, Ashraf F. (200709)This paper reports the testing of fifteen reinforced concrete deep beams with openings. All beams tested had the same overall geometrical dimensions. The main variables considered were the opening size and amount of inclined reinforcement. An effective inclined reinforcement factor combining the influence of the amount of inclined reinforcement and opening size on the structural behaviour of the beams tested is proposed. It was observed that the diagonal crack width and shear strength of beams tested were significantly dependent on the effective inclined reinforcement factor that ranged from 0 to 0.318 for the test specimens. As this factor increased, the diagonal crack width and its development rate decreased, and the shear strength of beams tested improved. Beams having effective inclined reinforcement factor more than 0.15 had higher shear strength than that of the corresponding solid beams. A numerical procedure based on the upper bound analysis of the plasticity theory was proposed to estimate the shear strength and load transfer capacity of reinforcement in deep beams with openings. Predictions obtained from the proposed formulas have a consistent agreement with test results.

Influence of section depth on the structural behaviour of reinforced concrete continuous deep beamsYang, KeunHyeok; Ashour, Ashraf F. (2007)Although the depth of reinforced concrete deep beams is much higher than that of slender beams, extensive existing tests on deep beams have focused on simply supported beams with a scaled depth below 600 mm. In the present paper, test results of 12 twospan reinforced concrete deep beams are reported. The main parameters investigated were the beam depth, which is varied from 400 mm to 720 mm, concrete compressive strength and shear spantooverall depth ratio. All beams had the same longitudinal top and bottom reinforcement and no web reinforcement to assess the effect of changing the beam depth on the shear strength of such beams. All beams tested failed owing to a significant diagonal crack connecting the edges of the load and intermediate support plates. The influence of beam depth on shear strength was more pronounced on continuous deep beams than simple ones and on beams having higher concrete compressive strength. A numerical technique based on the upper bound analysis of the plasticity theory was developed to assess the load capacity of continuous deep beams. The influence of the beam depth was covered by the effectiveness factor of concrete in compression to cater for size effect. Comparisons between the total capacity from the proposed technique and that experimentally measured in the current investigation and elsewhere show good agreement, even though the section depth of beams is varied.

Influence of Shear Reinforcement on Reinforced Concrete Continuous Deep BeamsYang, KeunHyeok; Chung, HS.; Ashour, Ashraf F. (2007)Test results of 24 reinforced concrete continuous deep beams are reported. The main variables studied were concrete strength, shear spantooverall depth ratio (a/h) and the amount and configuration of shear reinforcement. The results of this study show that the load transfer capacity of shear reinforcement was much more prominent in continuous deep beams than in simply supported deep beams. For beams having an a/ h of 0.5, horizontal shear reinforcement was always more effective than vertical shear reinforcement. The ratio of the load capacity measured and that predicted by the strutandtie model recommended by ACI 31805 dropped against the increase of a/h. This decrease rate was more remarkable in continuous deep beams than that in simple deep beams. The strutandtie model recommended by ACI 31805 overestimated the strength of continuous deep beams having a/ h more than 1.0.

Influence of Type and Replacement Level of Recycled Aggregates on Concrete Properties.Yang, KeunHyeok; Chung, HS.; Ashour, Ashraf F. (2008)Test results of nine recycled aggregate concretes and a control concrete using only natural aggregates are reported. The recycled aggregates used were classified into three different types according to their measured specific gravity and water absorption, namely, RG I for recycled coarse aggregate having a specific gravity of 2.53 and water absorption of 1.9%; RG III for recycled coarse aggregate having a specific gravity of 2.4 and water absorption of 6.2%; and RS II for recycled fine aggregate having a specific gravity of 2.36 and water absorption of 5.4%. The replacement levels of both recycled coarse and fine aggregates were 30, 50, and 100% in separate mixtures. Slump loss and the amount of bleeding with time were recorded for fresh concrete. Compressive and tensile strengths, moduli of rupture and elasticity, and unrestrained shrinkage strain were also measured for hardened concrete. The properties of fresh and hardened concrete tested, together with a comprehensive database reported in the literature, were evaluated with respect to the relative water absorption of aggregates combining the quality and volume of recycled aggregates used. In addition, the properties of hardened concrete with different replacement levels and quality of recycled aggregates were compared with the design equations of ACI 31805 and empirical equations proposed by Oluokun for natural aggregate concrete, whenever possible. Test results clearly showed that the properties of fresh and hardened concrete containing recycled aggregates were dependent on the relative water absorption of aggregates. In addition, the moduli of rupture and elasticity of recycled aggregate concrete were lower than the design equations specified in ACI 31805, when the relative water absorption of aggregates is above 2.5% and 3.0%, respectively.

Load capacity of reinforced concrete continuous deep beamsYang, KeunHyeok; Ashour, Ashraf F. (2008)Most codes of practice, such as EC2 and ACI 31805, recommend the use of strutandtie models for the design of reinforced concrete deep beams. However, studies on the validity of the strutandtie models for continuous deep beams are rare. This paper evaluates the strutandtie model specified by ACI 31805 and mechanism analysis of the plasticity theory in predicting the load capacity of 75 reinforced concrete continuous deep beams tested in the literature. The influence of such main parameters as compressive strength of concrete, shear spantooverall depth ratio, main longitudinal bottom reinforcement, and shear reinforcement on the load capacity is also investigated using both methods and experimental results. Experimental results were closer to the predictions obtained from the mechanism analysis than the strutandtie model. The strutandtie model highly overestimated the load capacity of continuous deep beams without shear reinforcement.

Mechanism analysis for concrete breakout capacity of single anchors in tensionYang, KeunHyeok; Ashour, Ashraf F. (2008)A numerical technique based on the theory of plasticity is developed to predict an optimum failure surface generatrix and concrete breakout capacity of single anchors away from edges under tensile loads. Concrete is regarded as a rigid, perfectly plastic material obeying a modified coulomb failure criteria with effective compressive and tensile strengths. The failure mode is idealized as an assemblage of two rigid blocks separated by failure surfaces of displacement discontinuity. Minimization of the collapse load predicted by the energy equation produces the optimum shape of the failure surface generatrix. A simplified solution is also developed by approximating the failure surface as two straight lines. The effect of different parameters on the concrete breakout capacity of anchors is reviewed using the developed mechanism analysis, ACI 31805, and test results of 501 castinplace and 442 postinstalled anchor specimens. The shape of failure surface and concrete breakout capacity of anchors predicted by the mechanism analysis are significantly affected by the ratio between effective tensile and compressive strengths of concrete. For anchors installed in concrete having a low ratio between effective tensile and compressive strengths, a much larger horizontal extent of failure planes in concrete surface is predicted by the mechanism analysis than recommended by ACI 31805, similar to test results. Experimental concrete breakout capacity of anchors is closer to the prediction obtained from the mechanism analysis than ACI 31805. ACI 31805 provisions for anchors sharply underestimate the breakout capacity of castinplace and postinstalled anchors having effective embedment depths exceeding 200 and 80 mm (7.87 to 3.15 in.), respectively, installed in concrete of compressive strength larger than 50 MPa (7250 psi).

Modification Factor for Shear Capacity of Lightweight Concrete BeamsYang, KeunHyeok; Ashour, Ashraf F. (201507)The validity of the modification factor specified in the ACI 31811 shear provision for concrete members to account for the reduced frictional properties along crack interfaces is examined using a comprehensive database comprised of 1716 normalweight concrete (NWC) beam specimens, 73 alllightweight concrete (ALWC) beam specimens, and 54 sandlightweight concrete (SLWC) beam specimens without shear reinforcement. Comparisons of measured and predicted shear capacities of concrete beams in the database show that ACI 31811 provisions for sheartransfer capacity of concrete are less conservative for lightweight concrete (LWC) beams than NWC beams. A rational approach based on the upperbound theorem of concrete plasticity has been developed to assess the reduced aggregate interlock along the crack interfaces and predict the sheartransfer capacity of concrete. A simplified model for the modification factor is then proposed as a function of the compressive strength and dry density of concrete and maximum aggregate size on the basis of analytical parametric studies on the ratios of sheartransfer capacity of LWC to that of the companion NWC. The proposed modification factor decreases with the decrease in the dry density of concrete, gives closer predictions to experimental results than does the ACI 31811 shear provision and, overall, improves the safety of shear capacity of LWC beams.

Neural Network Modelling for Shear Strength of Reinforced Concrete Deep BeamsYang, KeunHyeok; Ashour, Ashraf F.; Song, JK.; Lee, ET. (200802)A 9 × 18 × 1 feedforward neural network (NN) model trained using a resilient backpropagation algorithm and early stopping technique is constructed to predict the shear strength of deep reinforced concrete beams. The input layer covering geometrical and material properties of deep beams has nine neurons, and the corresponding output is the shear strength. Training, validation and testing of the developed neural network have been achieved using a comprehensive database compiled from 362 simple and 71 continuous deep beam specimens. The shear strength predictions of deep beams obtained from the developed NN are in better agreement with test results than those determined from strutandtie models. The mean and standard deviation of the ratio between predicted capacities using the NN and measured shear capacities are 1·028 and 0·154, respectively, for simple deep beams, and 1·0 and 0·122, respectively, for continuous deep beams. In addition, the trends ascertained from parametric study using the developed NN have a consistent agreement with those observed in other experimental and analytical investigations.

Neural network modelling of RC deep beam shear strengthYang, KeunHyeok; Ashour, Ashraf F.; Song, JK.; Lee, ET. (2008)A 9 x 18 x 1 feedforward neural network (NN) model trained using a resilient backpropagation algorithm and early stopping technique is constructed to predict the shear strength of deep reinforced concrete beams. The input layer covering geometrical and material properties of deep beams has nine neurons, and the corresponding output is the shear strength. Training, validation and testing of the developed neural network have been achieved using a comprehensive database compiled from 362 simple and 71 continuous deep beam specimens. The shear strength predictions of deep beams obtained from the developed NN are in better agreement with test results than those determined from strutandtie models. The mean and standard deviation of the ratio between predicted capacities using the NN and measured shear capacities are 1.028 and 0.154, respectively, for simple deep beams, and 1.0 and 0.122, respectively, for continuous deep beams. In addition, the trends ascertained from parametric study using the developed NN have a consistent agreement with those observed in other experimental and analytical investigations.

Properties of cementless mortars activated by sodium silicate.Yang, KeunHyeok; Song, JK.; Ashour, Ashraf F.; Lee, ET. (200809)The present paper reports the testing of 12 alkaliactivated mortars and a control ordinary portland cement (OPC) mortar. The main aim is to develop cementless binder activated by sodium silicate powder. An alkali quality coefficient combining the amounts of main compositions of source materials and sodium oxide (Na2O) in sodium silicate is proposed to assess the properties of alkali activated mortars, based on the hydration mechanism of alkaliactivated pastes. Fly ash (FA) and ground granulated blastfurnace slag (GGBS) were employed as source materials. The ratio of Na2Otosource material by weight for different mortars ranged between 0.038 and 0.164; as a result, alkali quality coefficient was varied from 0.0025 to 0.0365. Flow loss of fresh mortar, and shrinkage strain, compressive strength and modulus of rupture of hardened mortars were measured. The compressive strength development of alkali activated mortar was also compared with the design equations for OPC concrete specified in ACI 209 and EC 2. Test results clearly showed that the flow loss and compressive strength development of alkaliactivated mortar were significantly dependent on the proposed alkali quality coefficient. In particular, a higher rate of compressive strength development achieved at early age for GGBSbased alkaliactivated mortar and at longterm age for FAbased alkaliactivated mortar. In addition, shrinkage strain and modulus of rupture of alkaliactivated mortar were comparable to those of OPC mortar.

Shear Capacity of Monolithic Concrete Joints without Transverse Reinforcement.Yang, KeunHyeok; Sim, JI.; Kang, JH.; Ashour, Ashraf F. (201209)A mechanism analysis based on the upperbound theorem of concrete plasticity for monolithic concrete joints without transverse reinforcement is presented. Concrete is modelled as a rigid–perfectly plastic material obeying modified Coulomb failure criteria. Existing stress–strain relationships of concrete in compression and tension are comprehensively modified using the crack band theory to allow for concrete type and maximum aggregate size. Simple equations for the effectiveness factor for compression, ratio of effective tensile strength to compressive strength and angle of concrete friction are then mathematically developed using the modified stress–strain relationships of concrete. In addition, 12 pushoff specimens made of alllightweight, sand–lightweight and normalweight concrete having maximum aggregate size between 4 and 19 mm were physically tested. Test results and mechanism analysis clearly showed that the shear capacity of monolithic concrete joints increased with the increase of the maximum aggregate size and dry density of concrete. The mean and standard deviation of the ratio between experimentally measured and predicted (by the mechanism analysis shear capacities) are 1·01 and 0·16 respectively, showing a closer prediction and less variation than Vecchio and Collins' equation, regardless of concrete type and maximum aggregate size.

Shear capacity of reinforced concrete beams using neural networkYang, KeunHyeok; Ashour, Ashraf F.; Song, JK. (2007)Optimum multilayered feedforward neural network (NN) models using a resilient backpropagation algorithm and early stopping technique are built to predict the shear capacity of reinforced concrete deep and slender beams. The input layer neurons represent geometrical and material properties of reinforced concrete beams and the output layer produces the beam shear capacity. Training, validation and testing of the developed neural network have been achieved using 50%, 25%, and 25%, respectively, of a comprehensive database compiled from 631 deep and 549 slender beam specimens. The predictions obtained from the developed neural network models are in much better agreement with test results than those determined from shear provisions of different codes, such as KBCS, ACI 31805, and EC2. The mean and standard deviation of the ratio between predicted using the neural network models and measured shear capacities are 1.02 and 0.18, respectively, for deep beams, and 1.04 and 0.17, respectively, for slender beams. In addition, the influence of different parameters on the shear capacity of reinforced concrete beams predicted by the developed neural network shows consistent agreement with those experimentally observed.

Shear capacity of reinforced concrete corbels using mechanism analysisYang, KeunHyeok; Ashour, Ashraf F. (2012)A mechanism analysis is developed to predict the shear capacity of reinforced concrete corbels. Based on shear failure observed in experimental tests, kinematically admissible failure mechanisms are idealised as an assemblage of two rigid blocks separated by a failure plane of displacement discontinuity. Shear capacity predictions obtained from the developed mechanism analysis are in better agreement with corbel test results of a comprehensive database compiled from the available literature than other existing models for corbels. The developed mechanism model shows that the shear capacity of corbels generally decreases with the increase of shear spantodepth ratio, increases with the increase of main longitudinal reinforcement up to a certain limit beyond which it remains constant, and decreases with the increase of horizontal applied loads. It also demonstrates that the smaller the shear spantooverall depth ratio of corbels, the more effective the horizontal shear reinforcement.

Shear friction strength of monolithic concrete interfacesKwon, SJ.; Yang, KeunHyeok; Hwang, YH.; Ashour, Ashraf F. (201703)This paper presents an integrated model for shear friction strength of monolithic concrete interfaces derived from the upperbound theorem of concrete plasticity. The model accounts for the effects of applied axial stresses and transverse reinforcement on the shear friction action at interfacial shear cracks. Simple equations were also developed to generalize the effectiveness factor for compression, ratio of effective tensile to compressive strengths and angle of concrete friction. The reliability of the proposed model was then verified through comparisons with previous empirical equations and 103 pushoff test specimens compiled from different sources in the literature. The previous equations considerably underestimate the concrete shear transfer capacity and the underestimation is notable for the interfaces subjected to additional axial stresses. The proposed model provides superior accuracy in predicting the shear friction strength, resulting in a mean between experimental and predicted friction strengths of 0.97 and least scatter. Moreover, the proposed model has consistent trends with test results in evaluating the effect of various parameters on the shear friction strength.