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Since the size effect law announced currently has been based on the normal weight concrete, for light weight concrete having different fracture characteristics, its application is questionable. Accordingly, in this study, a model equation to predict the effect of dried unit weight of the concrete on size effect of its compressive strength was developed and a database using existing research results was created. After determining the experimental constants of prediction models of Ba?ant based on nonlinear fracture mechanics, Kim and Eo, and this study using the database, their results are mutually compared. Finally, it was found that the prediction model of this study considered dried unit weight of concrete predicted well the test results for light weight concrete than that of the models of Ba?ant and Kim and Eo.

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.

The present study evaluates the validity of different equations specified in code provisions and proposed by the existing researchers to predict the concrete tensile capacities (direct tensile strength, splitting tensile strength and modulus of rupture) using a comprehensible database including 361 lightweight concrete (LWC), 1,335 normal-weight concrete (NWC) and 221 heavy-weight concrete (HWC) specimens. Most of the equations express the concrete tensile strengths as a function of its compressive strength based on the limited NWC concrete test data. However, the present database shows that the concrete tensile capacities are significantly affected by its unit weight as well. As a result, the inconsistency between experiments and predictions by the different models increases when the concrete unit weight is below 2,100 kg/ and concrete compressive strength is above 50 MPa. On the other hand, new models proposed by the present study considering the concrete unit weight predict the tensile strengths of concrete with more accuracy.

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...

Sixteen alkali-activated (AA) mortars were mixed and cured at water or air-dried conditions to explore the effectiveness and limitation for the application of barium hydroxide (Ba(OH)2) as an additional activator in calcium hydroxide (Ca(OH)2)-activated ground granulated blast-furnace slag (GGBS) system. In order to determine the optimum amount of Ba(OH)2 that is to be added, test specimens were classified into two series. For series I, the Ca(OH)2-to-GGBS ratio by weight was 0% and 7.5%, and the Ba(OH)2-to-GGBS ratio by weight varied from 0% to 10% at a spacing of 2.5% for each Ca(OH)2-to-GGBS ratio. For series II, the Ca(OH)2-to-GGBS ratio was 7.5% and 10%, and the Ba(OH)2-to-GGBS ratio varied up to 3.5%. The flow loss of fresh mortars and the compressive strength development of hardened mortars were measured. In addition, X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) analyses combined with a scanning electron microscope (SEM) image were carried out to ascertain the effect of alkali activators used and the curing condition on the hydration products and the microstructural characteristics of the AA mortars. Test results clearly showed that the water curing condition was more effective than the air-dried curing condition for the formation of the denser calcium silicate hydrate (C–S–H) gels that had a higher molar Si/Ca ratio, resulting in a higher strength development. The addition of 1% Ba(OH)2 also contributed to the stabilization of C–S–H gels at long-term age, which resulted in a further increase in strength with the increase of age. At the same time, the introduction of Ba(OH)2 led to the formation of 2CaO·Al2O3·SiO2·8H2O (C2ASH8) hydrates with higher molar Si/Al and Ca/Al ratios. This would also result in a higher strength development of AA mortars tested.

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.