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Various empirical equations have previously been suggested for estimating the modulus of elasticity of concrete from its compressive strength. In addition, a considerable number of practical and universal equations have been proposed for predicting the modulus of elasticity when the product includes coarse aggregates and various types of mineral additions. However, such equations are not applicable to the whole data for recycled aggregate concrete because its mechanical properties depend greatly on both the type of aggregates recycled and the ratio of the mix proportions within such concrete. In this study, more than 1300 experimental data obtained from studies performed between 1980 and 2012 in Japan and Korea were used to statistically analyse the relationship between compressive strength and modulus of elasticity. The compressive strength of recycled aggregate concrete was found to range between 18 and 65 MPa, and therefore a prediction equation is proposed for recycled aggregate concrete that considers the ratio and types of recycled coarse aggregates used.

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RC beam with corroded tension rebars were strengthened with carbon fiber sheets(CFS) and were subjected to static loading tests to investigate the flexural strengthening effect of CFS. The test results confirmed that CFS are effective in reducing stresses to be carried by tension rebars and increasing flexural strength. Since the deformation capacity of the strengthened specimens was increased by anchoring the CFS and since the maximum strength of the specimens was determined by detachment or rupture of CFS, the flexural strength of the strengthened specimens could be calculated from an existing formula. Experimentally determined load-deflection relationships for RC beams were reproduced accurately through a FEM analysis modeling the bond elements between the concrete and the CFS. Practical application of the CFS sheeting method to RC beams requires improvement of the strength of bond between CFS and concrete.

Recycled aggregates (RAs) production techniques are essential for the material circulation society because RAs from demolished concrete waste can sustainably be reused as a concrete material. However, RAs can bring about several performance decreases when they are used for recycled aggregate concrete (RAC) because of the low qualities (i.e., high water-absorption rate and low density) caused by the attached hydrated cement paste on the RA surface. Therefore, both the production of high-quality RAs and the surface modification of RAs are significantly important for the extension of RAC utilization. This paper focuses on the surface modification of RFA to reduce the water absorption rate and increase density. Hydrofluorosilicic acid (H 2 SiF 6 ), which is one of the by-products in phosphoric acid manufacture, is used herein for the surface modification of the RFA. The physical properties and mechanical performance of mortar using RFA were evaluated after RFA modification. Consequently, the proposed method is effective in reducing water absorption rate and increasing density of RFA. The density of RFAs was slightly increased by 0.5-2.6% after modification. On the other hand, the water absorption rate decreased by 4-18% after modification. The compressive strengths of mortar at 28 days ages showed 18.1 MPa with modified RFA and 16.2 MPa with RFA.