The purpose of this study is to estimate the compressive strength according to the age of the concrete structure using ultrasonic pulse velocity method. If the correlation between the ultrasonic pulse velocity and the compressive strength according to the age is derived, the compressive strength of the early age of the concrete structure can be estimated at the new construction site and the compressive strength of the existing structure can be estimated at the remodeling construction site. Concrete structural specimens were constructed with 123 specimens by setting 9 parameters based on the design compressive strength of 24, 30, 40 MPa at 16, 20, 24, 48, 72, 120, 168, 360, 672 h. For the calculation of the average ultrasonic velocity according to the age of concrete, it is carried out according to KS F 2731, ASTM C597 and ACI 228-2R, and the concrete compressive strength is carried out according to KS F 2405. From correlation between ultrasonic pulse velocity and compressive strength, this experiment suggests compressive strength estimation equation. The proposed estimation equation confirmed that it is possible to estimate the compressive strength of concrete according to its age using nondestructive test methods.

The purpose of this study was to recycle red mud, an industrial byproduct that generates 300,000 tons per year, into the construction industry. Red mud was prepared as a liquid, neutralized with sulfuric acid, and replaced with cement mortar. The properties of liquefied red mud (LRM) neutralized with sulfuric acid (LRM + S) were investigated as well as its effect on cement mortar’s mechanical and hydration characteristics. The pH of LRM + S stabilized at 7.6; its SO₃ content was ~4.19% higher than that of LRM. Sulfites were contributed by calcium and sodium sulfate. The flows and setting times of the mortars containing LRM and LRM + S decreased as the substitution rate increased. The compressive strength of mortar that replaced 5% of cement with LRM + S was similar to that of the plain cement mortar. Scanning electron microscopy and X-ray diffraction revealed that the hydration products of LRM + S-containing cement mortar were similar to those of the plain cement mortar. Thus, LRM + S can be used as a cement substitute.

The purpose of this study is to enable emergency recovery of damage caused by earthquakes in structures and prevent secondary damage by controlling progressive collapse. Although there have been many previous studies using reinforcing bars and meshes, there have been few studies using carbon grid fibers as a substitute for tension members. This study aims to quantitatively present disaster response seismic performance by manufacturing concrete panels with no layers, one layer, or two layers of carbon fiber grids inserted and to compare the flexural strength and energy dissipation capacity. Flexural strength increased by 7% with one layer and 15% with two layers compared with no layer. Energy dissipation capacity increased by 30 times with one layer and by 56 times with two layers compared with no layers, and showed great improvement in terms of seismic performance. Especially because of the large increase in the energy dissipation capacity, the carbon fiber grid reinforcement method is considered to be an effective method for improving seismic performance.