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To examine the size effect in direct tension, 8 ready-mixed concrete batches classified into all-lightweight concrete (ALWC) and sand-lightweight concrete (SLWC) groups were prepared. In each group, the maximum aggregate size varied between 4 mm and 19 mm, and then the lateral depth of specimen with rectangular section ranged from 100 mm to 500 mm in each concrete batch. The size effect curves based on the basic formulas proposed by Bažant (1984) [1], Kim and Eo (1990) [2], and Yang and Sim (2011) [3] were also determined using a total of 28 lightweight concrete (LWC) data of current tests and 114 normal-weight concrete (NWC) data compiled from the available literature (Carpinteri and Ferro, 1994; Hu, 2011) [4,5], though specimens with lateral depth beside 100 mm is very insufficient even in NWC. The present experimental observations and verifications by prediction models clearly showed that the size effect is more notable with the decrease of the unit weight of concrete and it is stronger in direct tension than in compression. The validity of Bažant’s model (Bažant, 1984) [1] is significantly dependent on the maximum aggregate size, while the models proposed by Kim and Eo (1990) [2] and Yang and Sim (2011) [3] closely predict the size effect trend observed from test data, confirming that the influence of the maximum aggregate size on the concrete tensile strength and the size effect is negligible, especially for LWC.

To evaluate the effects of beam specimen depth and aggregate size on the fracture energy of lightweight concrete (LWC), different beam specimens designated into 32 notations were tested under three-point bending. In each of the all-lightweight concrete and sand-lightweight concrete groups, the maximum aggregate size varied between 4 and 19 mm; the beam depth ranged from 150 to 600 mm in each ready-mixed concrete batch with the same mix proportions. Based on experimental observations and verification of prior empirical models, simple closed-form equations were proposed to generalize the influence of the concrete unit weight on the size effect for the fracture energy of concrete. Test results clearly showed that when the maximum aggregate size is larger than 8 mm, the aggregate size in LWC has an insignificant effect on fracture parameters such as the fracture energy, crack opening mouth displacement, and characteristic length due to crack propagation through the lightweight aggregate particles. The...

The present tests prepared 9 ready-mixed concrete batches to examine the influences of the concrete type and the maximum aggregate size ( d a ) on the size effect in compression. For each concrete mixing, the aspect ratios of specimens with circular or square sections were 1.0 and 2.0, while the lateral dimensions of specimens varied from 50 mm to 400 mm. A comprehensive database including the 1492 normal-weight concrete (NWC) and 363 lightweight concrete (LWC) specimens was also established in order to trace the size-effect trend in LWC through the comparisons with the modern prediction models proposed by Bažant, Neville, Kim and Eo, and Sim et al. The present study clearly showed that the size effect and the slope of the size effect became stronger with the decrease of the unit weight of concrete. The influence of d a on the size effect observed in the current NWC specimens was smaller than the predictions obtained from the models of Bažant and Kim and Eo; while it was found to be negligible for LWC. Further, the refined model of Sim et al. confirms that the gain of the compressive strength of concrete due to the increase of d a is insignificant, especially for LWC.

In this study, the size and aspect ratio effects on the compressive strength of lightweight concrete were examined using nine laboratorial concrete mixes and three ready-mixed concrete batches. At each concrete mix, the aspect ratios of specimens with circular or square sections were 1.0 and 2.0. The lateral dimension of specimens ranged between 50 and 150 mm at each laboratorial concrete mix, while it varied from 50 to 400 mm at each ready-mixed concrete batch. The present study also proposed generalized prediction models for the size effect based on the crack band theory of fracture mechanics, which can cover important influencing parameters such as the aspect ratio and lateral depth of the specimen and the unit weight of concrete. Test results showed that the crack band zone in lightweight concrete specimens was more localized with poor crack distribution than in normal-weight concrete specimens, regardless of the geometrical dimensions of the specimens. As a result, the size effect was stronger with the decrease of the concrete unit weight, and this trend was more notable in specimens with an aspect ratio of 2.0 than in those with an aspect ratio of 1.0. The compressive strength predictions of concrete obtained from the present models are in good agreement with the test results including a total of 1661 data. The trend of the size effect against different parameters as predicted by the present models has a consistent agreement with that observed from experimental results.

This study proposes a practical strengthening technique for enhancing the in-plane shear strength and ductility of unreinforced masonry (URM) walls using the unbonded prestressed wire rope units. Six full-scale strengthened URM walls and an unstrengthened URM wall were tested under constant axial load and cyclic lateral loads to explore the significance and limitations of the developed seismic strengthening procedure. The main variables investigated were the aspect ratio of walls, and the spacing and configuration of the prestressed wire ropes. The ductility of the walls tested was evaluated using the work damage indicator calculated at each loading step from the hysteretic loops. In addition, shear transfer capacities of masonry and wire ropes were identified based on the statistical fitting of test data for URM walls compiled from the literature and 45° truss analogy, respectively, in order to propose simple equations for shear strength of the URM walls strengthened with the developed wire rope units. Test results showed that the proposed strengthening procedure was highly effective in enhancing the in-plane shear strength and ductility of URM walls and controlling crack propagation along head and bed mortar joints. The shear strength and work damage indicator increased with the decrease of the spacing of wire ropes, showing that the inclined wire ropes were more capable of enhancing ductility than vertical wire ropes. The crack resistance capacity and stiffness of URM walls were also improved due to the additional axial compression force provided from the prestressing force applied to the wire ropes. The shear capacities obtained from the proposed simple equations were in good agreement with the test results, indicating that the mean and standard deviation of the ratios between measured and predicted shear capacities were 1.14 and 0.13, respectively.

Dept. of Architectural Engineering, Kyonggi University, Suwon 443-760, KoreaABSTRACT The current study prepared 9 laboratorial concrete mixes and 3 ready-mixed concrete batches to examine the sizeand shape effects in compression failure of lightweight aggregate concrete (LWC). The concrete mixes were classified into threegroups: normal-weight, all-lightweight and sand-lightweight concrete groups. For each concrete mix, the aspect ratio of circularor square specimens was 1.0 and 2.0. The lateral dimension of specimens varied between 50 and 150 mm for each laboratorial con-crete mix, whereas it ranged from 50 to 400 mm with an incremental variation of 50 mm for each ready-mixed concrete batch. Testobservations revealed that the crack propagation and width of the localized failure zone developed in lightweight concrete spec-imens were considerably different than those of normal-weight concrete (NWC). In LWC specimens, the cracks mainly passedthrough the coarse aggregate particles and the crack distribution performance was very poor. As a result, a stronger size effect wasdeveloped in LWC than in NWC. Especially, this trend was more notable in specimens with aspect ratio of 2.0 than in specimenswith that of 1.0. The prediction model derived by Kim et al. overestimated the size effect of LWC when lateral dimension of spec-imen is above 150 mm. On the other hand, the modification factors specified in ASTM and CEB-FIP provisions, which are usedto compensate for the shape effect of specimen on compressive strength, were still conservative in LWC.Keywords : size effect, compressive strength, lightweight aggregate concrete, shape of specimen

The effect of the maximum aggregate size and substitution rate of natural sand on the mechanical properties of concrete is evaluated using 15 lightweight aggregate concrete mixes. For mechanical properties of concrete, compressive strength increase with respect to age, tensile resistance, elastic modulus, rupture modulus, and stress-strain relationship were measured. The experimental data were compared with the design equations specified in ACI 318-08, EC2, and/or CEB-FIP code provisions and empirical equations proposed by Slate et al., Yang et al., and Wang et al. The test results showed that compressive strength of lightweight concrete decreased with increase in maximum aggregate size and amount of lightweight fine aggregates. The parameters to predict the compressive strength development could be empirically formulated as a function of specific gravity of coarse aggregates and substitution rate of natural sand. The measured rupture modulus and tensile strength of concrete were commonly less than the prediction values obtained from code provisions or empirical equations, which can be attributed to the tensile resistance of lightweight aggregate concrete being significantly affected by its density as well as compressive strength.

The water content and lightweight aggregate proportions in lightweight alkali-activated (AA) concrete need to be carefully managed to control the quick slump loss of fresh concrete and to meet the mechanical properties that are required for structural concrete. This study tested 10 lightweight AA slag concrete specimens to evaluate the effect of the water content on the workability and various mechanical properties of the concrete. The source material, ground granulated blast furnace slag, was activated by using sodium silicate and calcium hydroxide powders to produce a cementless binder. The rate of development of the compressive strength and the shrinkage strain measured from the concrete specimens were compared with empirical models proposed by American Concrete Institute (ACI) 209 for normal-weight portland cement concrete. To examine the practical applicability of the lightweight AA slag concrete, the splitting tensile strength and the moduli of elasticity and rupture recorded from the concrete speci...

An improved unbonded-type column strengthening procedure using wire rope and T-shaped steel plate units was proposed. Eight strengthened columns and an unstrengthened control column were tested under concentric axial load. The main variables considered were the volume ratio of wire rope and the flange width and configuration of T-shaped steel plates. Axial load capacity and ductility ratio of columns tested were compared with predictions obtained from the equation specified in ACI 318-05 and those of conventionally tied columns tested by Chung et al., respectively. In addition, a mathematical model was proposed to evaluate the complete stress-strain relationship of concrete confined by the wire rope and T-plate units. Test results showed that the axial load capacity and ductility of columns increased with the increase of the volume ratio of wire rope and the flange width of T-plates. In particular, at the same lateral reinforcement index, a much higher ductility ratio was observed in the strengthened columns having the volume ratio of wire rope above 0.0039 than in the tied columns. A mathematical model for the stress-strain relationship of confined concrete using the proposed strengthening procedure is developed. The predicted stress-strain curves were in good agreement with test results.

A simple unbonded-type shear strengthening technique for reinforced concrete beams using wire rope units is developed. Six two-span continuous T-beams externally strengthened with wire rope units and an unstrengthened control beam were tested. The main variables investigated were the amount and prestressing force of wire rope units. All specimens had the same geometrical dimension and arrangement of internal reinforcement. Influence of the distribution of vertical stresses in beam web owing to the prestressing force of wire rope units on the diagonal shear cracking load and the ultimate shear capacity of beams tested is presented. Based on the current study, it can be concluded that the amount and initial prestress of wire rope should be limited to be above 2.5 times the minimum shear reinforcement ratio specified in ACI 318-05 and below 0.6 times its own tensile strength, respectively, to ensure the enhancement of shear capacity and ductile failure mode of the strengthened beams. A numerical analysis based on the upper-bound theorem is developed to assess the shear capacity of continuous T-beams strengthened with wire rope units. From the comparisons of measured and predicted shear capacities, a better agreement is achieved in the proposed numerical analysis than in empirical equations recommended by ACI 318-05.

Twelve reinforced concrete continuous deep beams having web openings within interior shear spans were tested to failure. All beams tested had the same geometrical dimensions. The main variables investigated were the opening size and amount of inclined reinforcement around openings. An effective inclined reinforcement factor combining the influence of the opening size and amount of inclined reinforcement on the structural behavior of the beams tested is proposed. It was observed that the load distribution, diagonal crack width, and load capacity of beams tested were greatly dependent on the effective inclined reinforcement factor which ranged from 0 to 0.171 for the test specimens. The higher this factor, the smaller the diagonal crack width and its development rate. A higher load capacity also developed in beams having effective inclined reinforcement factor above 0.077 than in the corresponding solid deep beams. A numerical technique based on the upper bound analysis of the plasticity theory is proposed to evaluate the load capacity of continuous deep beams having openings within interior shear spans. Predictions obtained from the proposed formulas are in good agreement with test results.

The present study reports a simple unbonded-type shear strengthening technique for reinforced concrete beams using wire rope units. Fifteen beams failed in shear were repaired and strengthened with wire rope units, and then retested to failure. Influence of the prestressing force, orientation and spacing of wire rope units on the shear behavior of strengthened beams having shear span-to-depth ratios of 1.5, 2.5, or 3.25 were investigated. Test results showed that beams strengthened with wire rope units exhibited a higher shear strength and a larger post-failure deformation than the corresponding original beams. Inclined wire rope units was more effective for shear strength enhancement than vertical wire rope units. The increase of the prestressing force in wire rope units causes the decrease of the principal tensile stress in concrete, as a result, the diagonal tensile cracking strength of strengthened beams was higher than that of the corresponding original beams. Shear capacity of strengthened beams is compared with predictions obtained from ACI 318-05 and EC 2. Shear capacity of strengthened beams having shear span-to-depth ratio below 2.5 is reasonably predicted using ACI 318-05 formula. On the other hand, EC 2 overestimates the shear transfer capacity of wire rope units for beams having shear span-to-depth ratio above 2.5.

In this study, powered superplasticizer(PSP) agents to improve the slump loss rate of recycled aggregate concrete were developed. To evaluate the variation of fluidity against elapsed time and the mechanical properties, twenty four specimens whose main variables had the mixing condition of aggregates, such as natural and recycled gravels, and natural and recycled fine aggregates, were tested. The concrete slump with a liquid superplasticizer greatly decreased against the elapsed time and dropped by less than 50% of initial slump after two hours. However the concrete slump with the PSP agents hardly varied until after half an hour and maintained more than 85% of initial slump even after an hour. Also the PSP agents made the compressive, splitting tensile, and flexural strength of concrete increased and the shrinkage strain decreased. Considering the properties improvement of concrete, it can be recommended that optimum mixing amount of the PSP agents should be 5% of the amount of cement.

In this study, a generalized equivalent Stress Block Model has been proposed that is applicable to all types of concrete, including lightweight and high-strength concrete (HSC). First, available stress-strain models for concrete were verified through an extensive database. Then, the coefficients used in the proposed stress blocks were formulated based on a nonlinear regression analysis of the values determined from the layer-by-layer integral evaluation approach. The hypothesis underlying this integral approach is that the magnitude and location of the resultant force in the equivalent stress distribution are the same as those in the actual distribution. The extreme compressive fiber strain of 0.003 and the factor of 0.85 used to compensate for the difference between the in-place strength and the cylinder strength were assumed, as was done for normalweight and/or normal-strength concrete (NSC). The reliability and safety of the proposed stress blocks were confirmed through comparisons with the flexural capacities measured from approximately 175 normalweight and 80 lightweight concrete (LWC) beams, and approximately 100 normalweight concrete (NWC) columns.

In this paper twelve continuous slender beams were tested to ascertain the effect of the maximum aggregate size on the shear behavior of concrete beams. The typical characteristics of the failure surface along the inclined cracks of the beams tested were compared according to the maximum aggregate size and the type of concrete by using a microphotograph. The test results showed that the shear strength of light weight concrete (LWC) continuous beams increased with the maximum aggregate size, though the increasing rate was lower than that of normal weight concrete (NWC) continuous beams. The microphotograph showed that the inclined crack of mortar beams with an aggregate size of 4 mm (0.16 in.) had a near-linear shape and a smooth failure surface, whereas that of the concrete beams was emboss-shaped with a failure plane partially formed along the pastes around the aggregate particles, regardless of the type of concrete. These characteristics of the failure plane contributed to the enhancement of the shear strength of LWC beams, though the shear force transferred by the aggregate interlock was much lower than that in NWC beams. The increasing rate of shear strength of LWC beams against aggregate size is similar to that predicted from the simplified compression field theory or the empirical formulas proposed by Bazant and Sun. The modification factor for shear strength of LWC specified in ACI 318-08 and EC2 is unconservative in the continuous beams tested, showing an increase of the unconservatism with the maximum aggregate size.

Unbonded techniques to strengthen reinforced concrete columns have become increasingly popular. This study evaluates the flexural behavior of reinforced concrete columns strengthened with unbonded wire rope and T-shaped steel plate units. Seven strengthened columns and an unstrengthened column were tested to failure under constant axial load and cyclic lateral loads to explore the significance and limitations of the strengthening procedure developed for resistance against earthquakes. The main variables investigated were the volume ratio of wire rope, axial load level, and the presence of mortar cover for strengthening steel elements. In addition, the theoretical monotonic lateral load-displacement curve for strengthened columns is simply derived using the combination of section laminae method and the idealized curvature-displacement relationship. The findings show that wire rope and T-shaped steel plate units were highly effective in preventing spalling of concrete cover and buckling of longitudinal reinforcement. The flexural capacity of columns strengthened without mortar cover was slightly higher than that of the unstrengthened column, but the flexural capacity of strengthened columns with a 60 mm (2.36 in.) thick mortar cover was at least 2.5 times higher than that of the comparable strengthened columns without mortar cover. The developed strengthening procedure was particularly effective in enhancing the ductility of the columns, showing that the displacement ductility ratios and work damage indicators in the strengthened columns were much higher than in the unstrengthened column. The monotonic lateral load-displacement relationship of the column specimens predicted from the proposed numerical analysis is in good agreement with backbone curves obtained from measured cyclic lateral load-displacement relationships.