This paper presents our findings on the application of X-ray microtomography to characterize the internal structure of mortars that were exposed to freezing-thawing action. A microfocus X-ray CT (micro-CT) scanner was used for the nondestructive 3D imaging of internal air voids or cracks at a spatial resolution of the order of 10 microns. Four different types of mortar specimens (i.e., non-air-entrained and air-entrained Portland cement mortar, and non-air-entrained and air-entrained fly ash mortar) were scanned after being subjected to different numbers of freeze-thaw cycles. Coupled with image analysis, the void space obtained from micro-CT was characterized in three dimensions (3D) in terms of void fraction and air void size distribution, as well as, the crack width and tortuosity of the connected crack network. Results suggest that the initial air voids follow a lognormal distribution with the highest population of modal size around 30-50 μm, irrespective of the type of mortar. As the distributed air voids of non-air entrained mortars were relatively few in numbers, the fly ash mortar in particular was the least resistant against frost damage as shown by the formation of internal cracks that meander around the weaker paste-aggregate interface. Indications also suggest that these cracks are well connected and anisotropic in 3D.
The corrosion rate of steel embedded in concrete is evaluated using polarization resistance assumed on the boundary between concrete and steel. However, the previously established polarization resistance method requires the removal of the concrete cover to connect the steel with a wire lead, and it takes a long time for the measurement of corrosion. This study proposes a new method for the nondestructive testing of deterioration caused by corrosion in concrete structures by estimating the surface resistivity of steel. For this purpose, the resistivity on the surface of a steel bar is assumed to behave similarly to the polarization resistance with the progress of corrosion. The resistivity of the steel surface is estimated using the resistivity estimation model (REM), and it is used as an index of the corrosion state. The REM is a mathematical model that considers geometric factors influencing the measurement, such as the cover depth, reinforcement radius, and electrode interval, and it can be used to estimate the resistivities of the concrete and steel bar in concrete structures. In addition, the possibility of applying this resistivity estimation method is investigated by performing an experiment on the accelerated corrosion of steel.
When a residential concrete foundation is constructed on sulfate-bearing ground, scaling of the concrete surface often arises from the crystallization and hydration pressure of the sodium sulfate that crystallizes in the pores of the concrete. Similar deterioration has been confirmed in stones and rocks and so on. This deterioration phenomenon is called “salt weathering.” We have confirmed several deterioration cases of residential concrete foundation and it is clear that such cases are widely distributed across Japan. This paper describes the results of field investigations and laboratory experiments on salt weathering of concrete. The laboratory experiment using mortar specimen shows the deterioration of mortar similar to residential concrete foundations was reproduced. We focused on the influences of the dry-wet cycle and the carbonation of the cement matrix, presuming that these are one of influential factors for salt weathering. With regard to the influence of the mix proportion of mortar, we established that low water-cement ratio has good resistance against salt weathering. However, additions to the mix proportion were not found to be effective, and the decrease in pH and the decomposition of C-S-H by carbonation are thought to affect the salt weathering resistance.
Steel in concrete may corrode due to chloride or carbonation especially at cracks and joints. The objective of this study is to experimentally investigate comprehensively the pattern of corrosion cell formation (macrocell and microcell) as well as the corrosion rate using mortar specimens with defects simulating cracks and/or joints. The three important factors are listed as follows: 1) supply position of chloride ions or carbon dioxide, 2) environmental conditions, and 3) water-cement ratio of mortar. The results indicate that, in the case of chloride induced corrosion, decreasing the water-cement ratio (0.3) increased the activity of macrocell prominent corrosion. On the other hand, increasing the water-cement ratio (0.7) increased the activity of microcell prominent corrosion. Therefore, in the presence of defects, a high corrosion rate might be promoted even at a low water-cement ratio. The study also reveals that, in the case of carbonation induced corrosion, macrocell prominent corrosion occurred regardless of the water-cement ratio, and the lower the water-cement ratio, the lower the corrosion rate. Finally, the study proves that the corrosion rate induced by chloride was higher than that induced by carbonation.
There have been few reports on the shear behavior of reinforced concrete (RC) beams with corroded longitudinal bars, and the influence of longitudinal bar corrosion has yet to be quantified. Given this background, experimental investigations were conducted to investigate the shear behavior of RC beams with corroded longitudinal bars, in which parameters such as corrosion level and shear-span-to-effective-depth-ratio were taken into consideration. Analytical investigations were also performed to evaluate the load-carrying mechanism of these specimens. The investigational results indicate that the shear behavior of RC beams is influenced not only by the corrosion level of the longitudinal bars but also by the shear-span-to-effective-depth-ratio (a/d). Based on these experimental results, a modified equation capable of calculating the shear capacity of RC beams with corroded longitudinal bars was proposed and its validity was proved.
It is reasonable that when the service life in terms of structural safety of a deteriorating reinforced concrete (RC) building does not match the original target set by the user or owner, life-cycle maintenance strategies should be implemented. Although many systems for finding optimal maintenance strategies for RC buildings have been proposed recently, few have discussed the life prediction assessment for a deteriorating RC building and the effect assessment models for repair and retrofit. For these purposes, this paper was focused on the probabilistic assessment method of service life and life-cycle maintenance strategies. In the service life assessment method proposed in this study, a reliability function of structural safety performance was built on the basis of the hazard rate or hazard function of a deterioration RC building during a rare earthquake. For selecting optimal maintenance strategies, probabilistic effect assessment models for repair/retrofit works (five repair/retrofit works were selected) that consider the recurrence of deterioration in repaired areas and the deterioration proceeding in unrepaired areas were developed in this research. These models reflect the effects of the maintenance strategies on the failure and spalling probability directly and can be used to estimate the life-cycle performance and cost of RC buildings. Finally, on the basis of the effectiveness of the maintenance strategies in reducing the life-cycle cost or the minimal life-cycle cost, the optimal life-cycle maintenance strategy can also be identified by using this system; case studies were used to discuss the applicability of this system.
This study aims to develop and apply self-healing concrete as a new method for crack control and enhanced service life in concrete structure. This concept is one of the maintenance-free methods which, apart from saving direct costs for maintenance and repair, reduces the indirect costs - a saving generally welcomed by contractors. In this research, the self-healing phenomenon of autogenous healing concrete using geo-materials for practical industrial application was investigated. Moreover, a self-healing concrete was fabricated by ready-mixed car in a ready-mixed concrete factory, then used for the construction of artificial water-retaining structures and actual tunnel structures. The results show that the crack of concrete was significantly self-healed up to 28 days re-curing. Crack-width of 0.15mm was self-healed after re-curing for 3 days and the crack width decreased from 0.22 mm to 0.16 mm after re-curing for 7 days. Furthermore, it was almost completely self-healed at 33 days. It was founded that this phenomenon occurred mainly due to the swelling effect, expansion effect and re-crystallization. From these results, it is considered that the utilization of appropriate dosages of geo-materials has a high potential for one of new repairing methods of cracked concrete under the water leakage of underground civil infrastructure such as tunnels.
The driving force of drying shrinkage of hardened cement paste has been attributed to RTln(h)/v with external relative humidity h and water molar volume v in theories of capillary tension and disjoining pressure. However, these theories fail to explain the considerable hysteresis observed in length-change isotherms. In this study, the sorption isotherm and length-change isotherm of cement pastes were determined with different water-to-cement ratios and cement types, and internal pressure of shrinkage ∏ was calculated using the measured strain and elastic modulus of the skeleton. This internal pressure is a kind of disjoining pressure originated from hydration force and built up within adsorbed water films as a result of interactions between the hydrophilic solid surface and water molecules. Changes in internal surface energies of hardened cement pastes due to hydration, drying and temperature history result in a different statistical thickness of the adsorbed water layer under the same equilibrium relative humidity. The proposed model gives a rational explanation for the hysteresis in length-change isotherm.
Microcracks developed considerably in concrete subjected to elevated temperature up to around 60°C at early ages, especially in low water-to-binder ratio (0.3) concrete. Microcracking was attributed to the stresses induced by the incompatibility in deformation between mortar and aggregate. Differences of coefficients of thermal expansion (CTE) between mortar and coarse aggregate, autogenous shrinkage of mortar and size of coarse aggregate were important factors influencing deterioration. The tensile strength of concrete was severely affected by the extent of microcracks. Concrete using ground granulated blast furnace slag (GGBFS) suffered worse damage than concrete prepared from ordinary Portland cement alone. Attempts were made to apply the acoustic emission (AE) technique to study the process and mechanism of microcracking. The skills required to practice the AE technique at early ages and at high temperature were carefully considered. AE hits agreed with the test results for deformation and tensile strength. Most of the microcracks occurred within the descending period of temperature and were classified into tensile mode. The use of coarse aggregate with larger CTE, saturated fine lightweight aggregate, and the reduction of the maximum size of the aggregate were greatly effective in reducing microcracking and improving the tensile strength of concrete made with GGBFS. Direct tensile strength was more adversely affected by microcracking than splitting tensile strength.
This paper proposes a wood interlocking block infill to prevent vulnerable R/C buildings from collapsing, and discusses its viability through experimental and analytical investigations. A series of structural tests were performed using one-story, one-bay R/C frame specimens with/without installing the proposed infill to verify its contributions. As a result, the infill significantly improved the seismic performance, particularly axial resistance and ductility, of the overall frame. Moreover, an analytical study was also conducted to clarify the distinctive characteristics of the proposed infill. Probabilistic analyses were carried out focusing on a vulnerable R/C building damaged during the 2007 Sumatra, Indonesia earthquakes, which was investigated after the earthquakes by the authors. Comparing the probabilities of collapse for three cases-without infills, with typical brick infills, and with the proposed infills-an alternative concept to prevent building collapses and to save human lives was introduced.
Despite extensive research on behavior of reinforced concrete in shear, there is still considerable disagreement among researchers in proposing and using a rational way of calculating the shear strength of reinforced concrete members. Due to lack of a universally accepted model for shear, shear design provisions still generally consist of empirical relationships that differ from code-to-code. In this study, a simple rational formula for calculating the shear strength of reinforced concrete elements is proposed by conducting a systematic parametric study on the response of RC elements in shear. This is done by using a computational model which simulates the post cracking behavior of RC membrane element on the basis of local stress transferring mechanism between adjacent cracks and microscopic stress transfer across cracks. Through a comparison with extensive available experimental tests conducted on RC elements, it is shown that the proposed rational relation can appropriately predict the shear strength of reinforced concrete elements under different in-plane stress conditions. Then the proposed relation is extended to the shear design of RC members and again it is shown that the predicted shear strengths are in good agreement with the experimental test results under different combination of axial, bending and shear forces.
This paper describes an attempt to predict the response of shear-critical ECC members that exhibit strong anisotropic stress and strain fields. The ECC members investigated include pre-cracked ECC plates under stress field rotation, orthogonally-reinforced ECC (R/ECC) panel under pure shear, and shear-critical R/ECC beams under reversed cyclic loading. To achieve a simple yet accurate prediction, the mechanics of the ECC are represented by smeared models using a fixed crack approach. The applicability of these models is demonstrated through a simulation of ECC plates and R/ECC panel responses. This demonstrates the importance of an appropriate shear transfer model in representing essential behaviors of ECC in an anisotropic field. Predictions of these models were then compared against experimental results of shear-critical R/ECC beams with a M/Vd ratio of 1.0 and 0.5. For beams with a M/Vd ratio of 1.0, a good agreement is observed in terms of hysteretic response, crack pattern, and failure mechanisms. For beams with a 0.5 M/Vd ratio, the analysis somewhat underestimates the beam capacity, although it does predict a correct failure mechanism. Overall, this paper demonstrates that practical application of nonlinear finite-element analysis to ECC structural members is possible.
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