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Confinement in concrete column has been well known to improve its strength and ductility. Up to present, traditional transverse steel is still used as internal confinement in concrete column. Many researchers have proposed various stress-strain relationships for confined concrete under different conditions. In general, all these models suggested increases both in compressive strength and ductility. The improved ductility is characterized by the increase of ultimate compressive strain and the flatter post-peak descending branch of the curve. Recently, research on concrete confinement has been developed further from internal to external techniques. The increasing demand of reinforced concrete column retrofits is the main reason to develop such techniques. Some external techniques have been proven to be successful in retrofitting circular concrete columns. Experimental programs as well as confining models for externally confined circular columns have been developed. However, it is hard to provide an effective confining stress by external retrofit on square or rectangular concrete column. The non-uniform confining stress on column was due to high stress concentration at the column corners. Only a few experimental and analytical studies addressed these issues. This paper proposes an analytical model for predicting the peak strength of square concrete columns confined by external retrofit as well as the shape of the improved stress-strain curve. Analytical study is also presented to provide a brief idea on the proposed analytical model in comparison with some developed models. The proposed model is also compared with the experimental results by other researchers. It is shown that the proposed analytical model can predict the behavior of externally retrofitted square concrete columns reasonably well.

The era of Construction 4.0 is characterized by technological advances used in the construction industry. One of the advancements is the use of 3D concrete printing in construction. However, until now, the development of 3D concrete printing in Indonesia is still minimal. The main challenge is to determine the composition of the material mixtures for making the mortar, having good extrudability but still has sufficient strength. The rapid initial setting time required was also different for the concrete for typical construction. Our previous mixture composition incorporating calcium oxide to accelerate the initial setting time was adequate. However, the extrusion process was still not satisfactory. In this study, the effect of cement to sand ratio, sand particle size, and the addition of synthetic micro-fiber was investigated on the main properties of 3D printing materials, i.e., initial setting time, flowability, extrudability, and compressive strength. It was found that using smaller maximum particle size sand increases the initial setting time. The addition of synthetic microfiber reduces the strength and the workability of the mortar. However, fiber inclusion has advantages as it reduces the possibility of cracking in the printed concrete. The extruded concrete specimens were shown to have significant strength reduction due to lack of compaction, and it was affected by the direction of printing showing orthotropic properties of the 3D printed concrete.

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