When reinforcement in concrete corrodes, cracks are induced in cover concrete because volume of corrosion products are much larger than that of original steel. Because these cracks can be the index of corrosion grade, many researchers have studied about the relationship between crack width and corrosion weight loss. Recently, it was reported that corrosion products have large influence on the relationship because each corrosion products have its own density and volumetric expansion ratio. However, formation process of corrosion products in alkaline environment have not been made clear and at this point it is difficult to predict corrosion products formed in concrete in various environment. Then in this study, in order to make clear formation process of corrosion products in alkaline environment, corrosion products formed in solution of various OH- and Cl- concentration were evaluated by Raman spectroscopy. And additionally, influence of dissolved oxygen on corrosion products was also investigated. Furthermore, arrangement of corrosion environment of reinforcement in concrete was tried, based on field study.
Corrosion of reinforcement is the representative deterioration of concrete structures, and it may cause the reduction in load-carrying capacity. Therefore, protecting reinforcement from corrosion is one of important subjects for concrete structures. Recently, in order to protect reinforcement, use of penetrant type inhibitor is increasing. However, evaluation method of penetration depth of these materials has not been established, and it is difficult to judge whether these materials work well or not. Then in this study, in order to develop the technique to evaluate penetration depth by surface analysis in the future, evaluation of penetration depth and active ingredient amount of 3 penetrant type inhibitors were tried by Raman spectroscopy as a basic study. To evaluate the amount, granular silica was mixed into concrete as a target of Raman spectroscopy analysis, and the amount penetrated into the silica ball was analyzed. Here we found that as the depth from coated surface became deeper, the relative amount of the detected active ingredient becomes smaller. This result indicates that the penetration depth can be predicted by evaluating the near surface by Raman spectroscopy.
Creep behavior of the epoxy resin anchor for CFRP sheet embedded in concrete was studied. Creep tests in 20°C and 40°C with the sustained loads of 3.0kN and 4.5kN were conducted for 42 days (1000 hours), where 3.0kN corresponded to the design load for the anchor. As a result, it was found that the creep displacement in the 40°C test was very large at the beginning of loading compared to the 20°C test. The glass transition temperature, Tg, was assumed to be the reason for the large creep displacement in the 40°C test. Therefore, Tg, tensile strength, and tensile modulus of elasticity were measured with various curing conditions for the epoxy resin. The measured Tg was 41°C with 23°C-7days curing and it increased up to 57°C for 7 days in case the curing temperature was increased from 23°C to 40°C. In addition, it was found that the tensile strength and the tensile modulus of elasticity showed linear relationship with Tg in the range of 41°C to 57°C, and the tensile modulus of elasticity in 40°C was 40% of that in 20°C when Tg was 45°C. The curing condition for 45°C of Tg was equivalent to the beginning of the 40°C creep test. Thus, it was concluded that the reason for the large displacement in the 40°C creep test was because Tg was relatively close to the test temperature.
Considering the durability of fly ash concrete against the chloride attack, a quantitative evaluation of the rebar corrosion rate in concrete is necessary. In this study, reinforced concrete specimens containing premixed chlorides were prepared, using concrete with cement or fine aggregate partly substituted by fly ash. The corrosion rate of rebar in the specimen was evaluated by the polarization resistance which is an electrochemical corrosion index. The specimens were stored in the wet or dry condition at the temperature of 20 or 40 ºC. As a result, the corrosion rate of rebar was greatly suppressed by admixing fly ash with the increase of the concrete resistivity at the temperature of 20 ºC, while such suppression effect decreased with the rise in temperature to 40 ºC. The suppression effect was remarkable in the cases of fly ash addition as the substitution of fine aggregates. Moreover, the influence of a variation of the chloride content in concrete on the steel corrosion rate was significant when the specimens were stored in the dry condition. The influence of the environment temperature and the chloride content in concrete on the steel corrosion rate measured in this study could be generally evaluated by using an existing equation.
“Shirasu” is a whitish sand found in pyroclastic flow deposits that includes a crystal mineral and an amorphous silicate. It has the properties of both a fine aggregate and a mineral admixture owing to its pozzolanic reaction. To utilize Shirasu as industrial resource, separation of the amorphous silicate and the crystal mineral is required. It is shown in this study that dry gravity classification is effective for dividing Shirasu into the crystal mineral and pumice, and can be used to obtain the amorphous silicate. This paper reports fundamental experiments on a high-strength concrete with a pulverized fine Shirasu powder used as a mineral admixture. In particular, fine Shirasu powder comprising the amorphous silicate pulverized to a particle size of 1μm exhibited high performance as mineral admixture equivalent to that of silica fume.
This study focuses on fundamental properties of bugholes generated from concreting. Bughole, a surface imperfection of concrete, occurs randomly and remains on hardened concrete surface. Bugholes are often observed on concrete having a negative angle, such as sidewall of tunnel lining concrete. The bughole problem may be significantly related to appearance (surface quality) of tunnel lining concrete rather than strength and durability. The authors reported characteristics and a reducing method of bugholes in our previous study. The previous investigation focused on a concrete mixture used in tunnel lining. To understand the properties of bugholes, various concretes should be tested as well as the tunnel lining concrete. The present study uses a transparent acryl form to observe bugholes generated from fresh concrete, and employs a steel form to examine bugholes on hardened concrete. The remarkable observations are (1) quantity of bugholes gradually decreases in accordance with higher slump and lower water-cement ratio; (2) unfilled-concrete (void) contributes to increase of large bugholes (2 mm or greater); (3) quantity of bugholes (2-5 mm) are related to unit weight of each components except for coarse aggregate.
Many concrete deck slabs of bridges have been deteriorated by heavy traffic, and freezing and thawing action. The water through the broken waterproof layer promotes the deterioration of concrete much more. Concrete used in replacements should have high resistance against freezing and thawing attack and fatigue in water. In this study, it is shown that the resistance to freeze and thaw attack can be improved by granulated blast furnace slag sand even without AE agent. The resistance to freeze and thaw attack of non-AE concrete with granulated blast furnace slag is almost same as that of AE concrete with the ordinary crushed sand. The fatigue of concrete suffered from freeze and thaw attack depends on the type of binder. When the crushed sand is used, the fatigue of concrete with high early strength Portland cement suffered from freeze and thaw attack is lower than that with ordinary Portland cement. When blast furnace slag sand is used, the tendency is almost same as when crushed sand is used. However, when blast furnace slag sand is used, the fatigue of concrete suffered from freeze and thaw attack can be improved with water curing period. It is also shown in this paper that blast furnace slag sand reacts with water and improve the durability of concrete.
Many reinforced concrete structures in Japan are aging. A reasonable and durable design by the numerical analysis is necessary to extend the service life of concrete structures. Based on the above background, this paper is applied the macrocell corrosion rate analysis model whom authors proposed to the members with a joint. First the model is constructed. The input data is an anode polarization curve, a cathode polarization curve, a polarization resistance, and an electrical resistance of mortar. On the other hand, the output data is a total corrosion current. Next, an analytical value is compared with the experiment value while using the mortar specimen that imitated the patch repair part. As a result, it can be cleared that the model is appropriate. Finally, the effect of the macrocell countermeasures method with the spray of the silane type surface penetrant on the joint is verified by this model. Thus, it can be confirmed that when the electrical resistance at the joint is large, the corrosion rate becomes low analytically.
Protecting steel bars in concrete against corrosion through the use of cathodic protection system has been successfully used for marine structures as the most effective method in controlling corrosion current taking place in reinforced concrete. DC current flowing into steel bars in concrete is required to be uniformly distributed, which is essential in controlling the corrosion via lowered potentials along the steel bars. The design criteria commonly used for the cathodic protection is a polarization measured on the steel bars which is greater than 0.1V immediately after power supply stops (instant-off). It has been pointed out that the current distribution varies owing to the heterogeneity of composite materials in concrete including moisture distribution and chloride ion concentration, defects typically present in concrete including cold-joint and cracks and the deterioration stages such as incubation and propagation periods. This led to variations observed in the potentials which may result in unsatisfactory protection potentials. In this study, the influence of localized defects and corrosion on the current density and potentials were examined using segmented steel bars embedded in concrete, which was investigated via electro-chemical measurements. Based on findings, a design criteria required for reinforced concrete with defects in the case of chloride-induced corrosion was shown in controlling the potential on the steel bar below -0.85V vs. CSE reference electrode especially during propagation period.
This study reports on the effect of initial curing on corrosion resistance of fly ash (FA) concretes, which was examined through chloride-induced corrosion tests induced by dry and wet cycles. Detailed analysis was carried out on the resistance against ingress of harmful corrosive substances assessed via air permeability tests and cathodic polarization measurements in determining oxygen permeability on embedded steel bars. They were largely affected by the moisture content, the pore structure including pore volume, size distribution and its connectivity which were examined via mercury intrusion porosimetry. In addition, the results showed that the corrosion resistance was observed to be dependent on the availability of calcium hydroxide consumed via the pozzolanic reactions which could affect the initiation of corrosion in the incubation period and the oxygen permeability on embedded steel bars with wet conditions for the early propagation period. Higher resistance against chloride ingress in the FA concrete could be attributed to denser pore structure formed and relatively lower moisture content in the deeper zones especially when cured under dry environment.
Drying shrinkage is inevitable in concrete mixtures when exposed to dry environment. This is not only dependent on hardened cement paste comprising the types of cement and water content, but the properties of aggregates including water absorption capacity and elastic modulus. Generally, materials design with respect to enhance the resistance against drying shrinkage cracking aims at higher strength development and reduced drying shrinkage, which can be obtained by lower water content and the proper use of cement and mineral admixtures in concrete mixtures and the use of aggregates with lower water absorption capacity and higher elastic modulus. This study examined the resistance against drying shrinkage cracking in concrete mixtures mixed with two types of slag fine aggregates (copper and manganese slag) and fly ash, which was investigated using restrained reinforced concrete specimens. The results showed that the cracking resistance was enhanced in concrete mixtures mixed with copper slag fine aggregate and fly ash, which could be explained by the fact that the restrained stress was partially relaxed during the early stages of exposure to dry environment. In addition, fresh properties including slump, air content and bleeding rate were also tested for practical application.