In concrete production, as mixing water, curing water and washing water, huge volume of water is consumed all over the world. For suppressing pure water consumption, one effective method is to use sea water as replacement of pure water. If the use of seawater as mixing water in concrete is possible, it would be very beneficial and economical in construction, especially in coastal works or isolated island. This study includes several investigations on durability of concrete mixed with seawater and pure water. The specimens were exposed to tidal pool utilizing natural seawater for 32years. In this study, the effect of seawater mixing on concrete deterioration and steel corrosion of RC beam under marine condition is investigated. The results indicate the possibility of concrete production mixed with seawater.

This paper deals with the applicability of low-carbon concrete with high-volume mineral admixtures to non-reinforced blocks in ports, mainly from the perspective of strength development. Also, the applicability of low-carbon, heavy-weight concrete incorporating high-volume copper slag fine aggregate (CUS) was investigated. The CO₂ emissions during the construction in the case that low-carbon concretes are applied to wave-dissipating blocks of a port breakwater were then calculated. The lower water-to-binder ratio required for the low-carbon concretes results in a beneficial effect for fresh properties in the case high-volume CUS is incorporated. When the low-carbon heavy-weight concretes are applied to the wave-dissipating block of a breakwater, it is possible to select a smaller block to keep the required weight of the block. Consequently, although the low-carbon heavy-weight concretes require a lower water-to-binder ratio to maintain their strength, the CO₂ emissions associated with the material production can be possibly reduced.

In this study, the effect of relative humidity on the carbonation reaction of cement (OPC) samples mixed with a CO₂ fixing admixture (CFA) consisting mainly of γ-C₂S (γ-2CaO・SiO₂) was investigated. The samples used for enforced carbonation consisted of raw CFA (hereinafter referred to as “CFA_powder”), cement paste made solely of OPC (“OPC_paste”), and a mixture of OPC and CFA (“OPC+CFA_paste”). There was no significant difference in the carbonation ratio between OPC_paste and OPC+CFA_paste, but both the carbonation ratio of CFA_powder was lower than that of OPC+CFA_paste. In addition, the amount of water adsorbed by OPC+CFA_paste was greater than that by CFA_powder, confirming that the carbonation reaction proceeds when moisture in the air is adsorbed onto the particles. Furthermore, CFA_powder contains both fine and coarse particles, including cracks due to γ-C₂S dusting, and the carbonation reaction proceeds not only on the surface of the particles but also within the cracks. Both OPC+CFA_paste and CFA_powder showed higher carbonation rates with increasing relative humidity. The reaction ratio of γ-C₂S after 7 days of carbonation is higher in the OPC+CFA_paste than in the CFA_powder. This shows that CFA reacts more with CO₂ in a mixture with OPC. XRD results show that CFA_powder has lower calcite content and higher aragonite content than OPC+CFA_paste. The FT-IR results showed a peak of calcium carbonate at 1400-1600 cm^-1. The calcite peak at 1420 cm^-1 is particularly large in OPC+CFA_paste, while CFA_powder has a broad peak at 1400-1600 cm^-1. Therefore, the CaCO₃ produced by OPC+CFA_paste has a high calcite content, which is consistent with the XRD results.

As one of the efforts to reduce CO₂ emissions, concrete containing high-volume mineral admixtures is being studied, and the authors are also developing high-strength concrete without Portland cement. This paper reports on the results of the study on fundamental properties and the effect of temperature on strength development of high-strength concrete without Portland cement. As a result, the following were obtained; 1) The influence of powder and curing method on the relationship between compressive strength and Young’s modulus is small, and the influence of aggregate used is dominant, as in the case of ordinary concrete. 2) The use of ground granulated blast-furnace slag, with Blaine 6000, increases the development of compressive strength and also facilitates freeze-thaw resistance. 3) The compressive strength development of high-strength concrete without Portland cement is more temperature-dependent than that of ordinary concrete, and the apparent activation energy must be increased when evaluated at the effective age of the concrete.

In the quest for sustainable solutions to environmental challenges, the exploration of alternative materials derived from by-products has gained considerable momentum. In the engineering industry, rice husk ash (RHA) stands out as a promising candidate to replace cement. It is an amorphous powder rich in silica that possesses pozzolanic properties similar to ordinary cement. Therefore, using RHA in concrete not only promises a means of waste management, but it also can potentially increase the performance of concrete. This study investigated the mechanical properties of specimens of mortar incorporating RHA as a partial replacement for cement. To properly assess the suitability of RHA for use in pavement, the hardness and slip resistance of specimens were also tested. The results showed that the particle size and loss on ignition of the RHA significantly influences the workability of mortar incorporating RHA. Some of the hardened specimens could achieve higher strength than the control specimen. However, increasing the RHA content from 10 to 20% led to a significant decrease in the strength and slip resistance of the specimens. Nevertheless, all RHA types used in the study developed sufficient flexural strength, satisfactory slip resistance and excellent hardness values, which makes them potentially safe and comfortable, both as road and pedestrian pavements. Finally, our analysis of the CO₂ emissions during production demonstrated that the use of RHA is less damaging to the environment than is cement.

The quality classification of recycled aggregate is based on the water absorption rate and the density of the adhered mortar. However, the quality of recycled aggregate is affected by the adhesion characteristics between the original aggregate and mortar (Interfacial Transition Zone: ITZ). It has been reported that carbonation of recycled aggregate improves mortar quality, but the effect on the above adhesion interface is not clearly evaluated. In this study, laser-induced vibration method is conducted on recycled aggregate before and after carbonation to evaluate the quality of the adhesion interface between the aggregate and mortar. A simple spring model, in which the adhesion strength of the interface is expressed as a spring constant, is applied to the quality evaluation of the recycled aggregate. As a result, it is found that there is a correlation with Vickers hardness, which can measure the strength of the interface.

In prestressed concrete (PC) structures, inadequate grouting of tendon ducts can substantially impair the durability of the structures. This study introduces a novel approach named alternating magnetic excitation method for evaluating grouting conditions in PC tendon ducts. Using a sinusoidal alternating current to activate an excitation coil, this novel approach induces vibrations in magnetic materials within concrete, generating elastic waves detectable by sensors on the surface of concrete. The study develops a robust experimental setup and procedure, optimizes noise reduction in detected signals, and explores the method's effectiveness on prestressed concrete specimens with 50 mm concrete covers. Results validate its capability to distinguish grouted and ungrouted conditions, supported by frequency response and impedance measurements confirming mechanical resonance. Experiments on lift-off distances demonstrate effectiveness of evaluation up to 100 mm lift-off distance, showing reproducibility in identifying resonance peaks across depths. This research suggests the possibility of non-destructive evaluation in prestressed concrete structures using alternating magnetic excitation method.

Corrosion of reinforcing steel bars poses a serious challenge in concrete structure maintenance. This corrosion causes not only surface cracks but also internal cracks in the concrete owing to the expanding pressure caused by corrosion. Several studies have focused on the relationship between surface cracks and the mass loss of steel bars, seeking to establish practical inspection methods that can predict the mass loss of steel on the basis of surface cracks. However, the effects of internal cracks, or the fragile portions previously formed in concrete, on the surface cracks and corrosion of steel bars have not been sufficiently investigated. This study evaluated these effects by using X-ray technology. Mortar specimens with both axial and transverse steel bars were prepared, and one of the steel bars were corroded using the impressed current technique. X-ray imaging was used to visualize the internal cracks in the specimens. Consequently, it was revealed that the propagation of surface cracks may be diminished because of internal cracks and fragile portions in the concrete.

The main reinforcement ratio of piers constructed before 1980, when the standards older than the 1980 specifications for road bridges were applied, is often around 1.0%, and at termination point of main reinforcement is less than 1/2, the main reinforcement ratio is less than 0.5%. In such piers, cracks due to ASR expansion are more likely to occur, and the effect on loading capacity is a concern. In this study, to clarify the seismic performance of piers with ASR deterioration, RC column specimens with ASR deterioration and non- deteriorated RC column specimens for comparison were fabricated to simulate the piers at that time, and the progress observation and loading tests were conducted on one non- deteriorated RC column specimen and two RC column specimens with ASR deterioration. The loading test results showed that the maximum load of the specimen with ASR deterioration increased due to chemical prestress compared to the specimen without ASR deterioration. However, the specimen with ASR deterioration decreased toughness compared with the specimen.

It is known that macro-cell corrosion occurs at the interface between the base material and the repair material in the repair method using the patch repair. This paper focuses on a repair method that combines the patch repair method with the impressed current cathodic protection method applied to the base concrete to prevent such re-deterioration. The method was applied to RC beam specimens severely deteriorated by chloride damage in a marine environment, and monitoring was conducted over three years. The corrosion protection state of the rebar was evaluated using electrochemical methods, including potential monitoring and polarization measurements. The results showed that while the rebar in the base concrete was sufficiently polarized by the impressed current anode, the rebar in the patch repaired with polymer cement mortar exhibited low polarization. This is attributed to the high electrical resistivity of the polymer cement mortar. On the other hand, it was confirmed that the polymer cement mortar provided sufficient corrosion protection even without polarization due to its high electrical resistivity. Additionally, it was confirmed that the rebar at the interface of the repair patch was adequately protected by the cathodic protection method.