JOURNAL OF ADVANCED CONCRETE TECHNOLOGY (Japan Concrete Institute)

Kinematic Model for Shear Assessment of RC Short Columns Subjected to Frost Damage

Kanazawa, T; Ushiwatari, Y

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.335-345, 2021, APR.

(https://doi.org/10.3151/jact.19.335)

Abstract

Few reports have described practice-based models assessing ultimate strengths of existing reinforced concrete (RC) members subjected to frost damage. This paper presents a kinematic model for shear assessment of damaged RC short columns based on the upper bound theorem. Without regressive functions, the developed model predicts the shear strength contribution of damaged concrete when the displacement field is divided into undamaged and damaged zones based on damage depths obtained from core sampling. The model accuracy is verified by comparison of its predictions with those of earlier test results of 14 RC columns presenting shear failure after freeze-thaw exposure. The analytical predictions show good agreement with experimentally obtained results within error of 20%. Shear strength predictions for different damage depths are presented for an existing RC bridge pier with severe frost damage. Rational shear assessment was achieved because the kinematic analysis directly correlates the damage depth with shear strength reduction.

Benchmark Finite Element Calculations for ASCET Phase III on a Reinforced-Concrete Shear Wall Affected by Alkali-Aggregate Reaction

Kojima, M; Kodama, T; Jin, CR; Maruyama, I

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.280-300, 2021, APR.

(https://doi.org/10.3151/jact.19.280)

Abstract

In this study, finite element (FE) analyses were conducted on a reinforced-concrete (RC) shear wall that is affected by an alkali-aggregate reaction (AAR), which were then applied for a benchmark studies in OECD/NEA/CNSI/ASCET (Organization for Economic Co-operation and Development/Nuclear Energy Agency/Committee on Safety of Nuclear Installations/Assessment of Structures subjected to Concrete Pathologies) Phases II and III assessments. A commercial software has been modified to account for this AAR expansion, which is affected by the stress field and change in physical properties of the concrete. The impacts of boundary conditions, modeling in two and three dimensions, and material properties on the load-displacement curve and crack patterns were carefully evaluated. Finally, although similar load-displacement curves and crack patterns were obtained, the peak load due to brittle failure of an RC shear wall affected by AAR could not be reproduced. Consequently, it was found that the rotation of the loading stub and anchoring procedure of the base stub were critical conditions for load-displacement relationship of RC shear wall, and meshing capturing the arrangement of reinforcement bars is crucial for FE analysis with two-dimensional (2D) condition, and finally, the occurrence of initial cracks and the loading capacity could not be clearly reproduced. This suggests that consideration of the placement of rebars and covering concrete in the mash setting in three-dimensional (3D) model affected the failure mode of the concrete. It is necessary to consider the possible failure mechanism and to reflect such features in numerical modeling.

Modeling and Simulation on Static and Fatigue Behaviors of Intact and Frost Damaged Concrete with Ice-strengthening Effects

Wang, Z; Gong, FY; Ueda, T

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.346-358, 2021, APR.

(https://doi.org/10.3151/jact.19.346)

Abstract

Concrete structures serving in cold and wet regions usually suffer frost damage and thus have server deterioration. Many researches have been conducted to reveal the damaging mechanism and damaged mechanical properties of concrete under the effect of frost action. It has been widely known that the strength and stiffness of frost damaged concrete without using air-entraining agent decrease under room temperature. However, there will be a different story if the frost-damaged concrete is saturated and loaded under freezing temperature. Water existing in pores and cracks will freeze into ice, which provides additional strengthening effects. This paper presents a multi-scale modeling and simulation work on the static and fatigue behaviors of frost damaged concrete with consideration of such ice-strengthening effects. The micro-mesoscale damaging and strengthening effects induced by ice formation are modeled and integrated into the mesoscale analytical approach - Rigid Body Spring Model, and the macroscale static and fatigue behaviors are simulated. It is found that the freezing temperature has a positive (strengthening) effect on the static strength, while it has a negative effect on the fatigue life for both intact and frost-damaged concrete. Test is also conducted with available experimental evidence to validate the developed approach. Satisfactory correlation is found through the comparison between simulation and experiment.

Novel Accelerated Test Method for RH Dependency of Steel Corrosion in Carbonated Mortar

Cheng, LG; Maruyama, I; Ren, YQ

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.207-215, 2021, MAR.

(https://doi.org/10.3151/jact.19.207)

Abstract

Degradation of reinforced concrete (RC) structures can occur through the carbonation-induced corrosion of reinforcing bars, and this process is a major concern for the durability of RC buildings. Structures located in relatively humid inland environments are especially vulnerable. Therefore, it is important to clarify how relative humidity (RH) affects steel corrosion rates in carbonated concrete. In this study, a novel accelerated test method is presented, which shortens the experimental duration and simplifies the experimental method. A miniaturized specimen was created with 20 x 20 x 40 mm(3) dimensions and an effective carbonation depth of only 5 mm. The corrosion rate of rebar in the small mortar specimens was studied at different equilibrium RH conditions, which were controlled using saturated salt solutions. The accelerated carbonation process was found to be much faster than in traditional concrete experiments. Finally, the relationship between water content (as a function of RH) and corrosion rate showed that the corrosion rate of rebar in carbonated mortar has a strong dependency on RH. The relationship between the mortar resistance and the corrosion rate indicated that the corrosion process of rebar in carbonated mortar is under resistive control when RH above 80%, and under anodic control when RH below 80%.

41 Year Long-Term Durability of High Volume Blast-Furnace Slag Cement Concrete

Hashimoto, M; Kurata, K; Ohtsuka, Y; Dan, Y

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.248-258, 2021, MAR.

(https://doi.org/10.3151/jact.19.248)

Abstract

In this study, we investigated the durability of high-volume ground granulated blast furnace slag (GGBS) blended cement concrete containing over 70% of GGBS for possible general structural applications. The concrete specimens used were exposed to natural outdoor conditions for 41 years on a building rooftop. The following is found. The exposed top surface of concrete with 88.5% GGBS 4000 replacement, the exposed top surface and the corners of sulfated slag cement showed peel failure of the paste, but the specimens of concrete with 68.5% GGBS 4000 and GGBS 2000 replacement were in sound condition. The compressive strength of all mix proportions did not decrease significantly over 41 years. The carbonation depth of concrete specimens containing 70% GGBS was about 7 to 9 mm, and about 15 mm for specimens containing 90% GGBS. Despite the high volume of GGBS content (70%) in the concrete specimens, traces of Ca(OH)(2), which is involved in the chemical reaction of GGBS, were found in parts that remained uncarbonated. Ca(OH)(2) increases the alkalinity of the specimen and is thus considered to have a rebar corrosion-inhibiting effect. This paper is the English translation from the authors' previous work [Hashimoto, M., et al., (2019). A study on the long-term durability of high-volume bast-furnace slag cement concrete for 41 years. Concrete Research and Technology, Vol.30, pp.77-84. (in Japanese)].

Time-Dependent Deformation of a Concrete Arch Dam in Thailand - Numerical Study on Effect of Alkali Silica Reaction on Deflection of Arch

Joshi, NR; Sriprasong, T; Asamoto, S; Sancharoen, P

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.181-195, 2021, MAR.

(https://doi.org/10.3151/jact.19.181)

Abstract

A detailed inspection of a dam in Thailand is reported for its time-dependent gradual expansion towards upstream, contrary to the expected downstream creep deflection owing to hydrostatic loads. In this study, based on petrographic analysis and SEM, a sample of cored concrete from the dam was found to undergo a low to moderate level of ASR. The potential for future expansion was verified using an accelerated laboratory test. The experimental data were used to evaluate the dam performance by conducting an FEM analysis. The numerical model was calibrated with the observed deflection, and the mechanical stresses owing to the combined ASR and hydrostatic loads were estimated for various ages of the dam. In addition, stress and deflection were predicted using probabilistic methods. A sensitivity analysis was also performed to monitor the behaviour of the dam under various environmental conditions and input parameters. It was found that the gradual deterioration by ASR does not pose a high risk to the dam under normal loading conditions.

Water Uptake in OPC and FAC Mortars under Different Temperature Conditions

Kiran, R; Samouh, H; Matsuda, A; Igarashi, G; Tomita, S; Yamada, K; Maruyama, I

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.168-180, 2021, MAR.

(https://doi.org/10.3151/jact.19.168)

Abstract

This experimental and numerical study aims to evaluate the penetration depth of contaminated water in the concrete structures involved in the Fukushima Daiichi nuclear powerplant. The influence of the mortar mixture on water absorption was investigated by varying the composition: mortars containing aggregates from river sand and crushed limestone sand were compared, and 15% of the cement in the mixture was substituted with fly ash. The effect of temperature in nuclear conditions is also significant; therefore, water uptake at temperatures of 20 and 60 degrees C was considered. Finally, pre-drying conditions were studied by drying the sample at two different conditions: at 105 degrees C and at 40% RH (relative humidity) and 20 degrees C. Water uptake was monitored using x-ray computed radiography in combination with mass measurements. In all cases, anomalous sorption, or a nonlinear relationship between penetration depth and the square root of exposure time was observed, with the sorption curves showing bimodal behavior. The aggregate type had no significant effect on the water uptake results. However, the samples containing fly ash clearly had lower water uptake rates, which can be explained by the differences in the calcium silicate hydrate (C-S-H) structures. With increasing temperature, the penetration was slightly accelerated at the beginning of the experiment, with the rate of penetration then decreasing rapidly. The densification of C-S-H at higher temperatures could contribute to this phenomenon. Microstructural rearrangements can also explain why the highest uptake rates occurred for samples that were exposed to severe drying conditions (105 degrees C). The experimental results were consistent when the microstructural rearrangement was considered, further confirming these conclusions.

Improvement of Concrete Properties using Granulated Blast Furnace Slag Sand

Ayano, T; Fujii, T

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.118-132, 2021, FEB.

(https://doi.org/10.3151/jact.19.118)

Abstract

A high volume of ground granulated blast furnace slag (GGBS) or granulated blast furnace slag (BFS) can enhance the resistance of concrete to freezing and thawing without the use of air-entraining (AE) agents. Furthermore, it can also enhance the resistance of concrete to chloride ion penetration and sulfuric acid attack, although the mechanism of improvement differs. In particular, BFS can reduce time-dependent strains, such as drying shrinkage strain and creep strain. The use of granulated blast furnace slag, either GGBS or BFS, promotes the durability of concrete structures by improving the mechanical properties of cementitious materials. Some of the concrete properties that are improved by the incorporation of BFS are presented in this paper. The detailed improvement mechanism of BFS has not yet been clarified. However, it is clear that it depends on the chemical reactions involving BFS and thus a critical time is required for BFS to hydrate in order to improve concrete properties. It takes four weeks to achieve high resistance to freezing and thawing by using BFS without the addition of an AE agent; use of a thickening agent can further shorten this curing period to one week. This paper is an English translation from a previous work by the authors [Ayano et al., (2014). Resistance to freezing and thawing attack of concrete with blast furnace slag fine aggregate. Journal of Japan Society of Civil Engineers, Ser. E2 (Materials and Concrete Structures), 70(4), 417-427 (in Japanese)] and [Jariyathitipong et al., (2013). Improvement of resistance to sulfuric acid attack of concrete by use of blast furnace slag sand. Journal of Japan Society of Civil Engineers, Ser. E2 (Materials and Concrete Structures), 69(4), 337-347 (in Japanese)].

Accelerated Moisture Transport through Local Weakness of High-Strength Concrete Exposed to High Temperature

Iwama, K; Kato, Y; Baba, S; Higuchi, K; Maekawa, K

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.106-117, 2021, FEB.

(https://doi.org/10.3151/jact.19.106)

Abstract

Spilling out of condensed liquid water from needle-like holes in high-strength concrete was experimentally observed under fire attack. The presence of these holes was found to prevent explosive spalling effectively in the vicinity of the holes during fire exposure tests. This spilling out occurred at about 10 to 30 minutes after the start of high temperature heating. These needle-like holes are defined herein as local weaknesses that may act as rapid paths of water permeation to reduce the risk of explosive spalling of cover concrete. The phase change of moisture from CSH solids to condensed liquid as well as free water in micro-pores was simulated by a multi-phase chemo-physics analysis of ultra-high-strength concrete. The prediction of the high-rate phenomena was experimentally proved by using embedded moisture sensor, and the high-rate discharge of condensed water though local weaknesses was analytically simulated.

Effect of Chloride and Sulfate in the Immobilization of Cs-137 in C-S-H Gel

Duque-Redondo, E; Yamada, K; Manzano, H

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.95-105, 2021, JAN.

(https://doi.org/10.3151/jact.19.95)

Abstract

Cementitious materials are commonly used in nuclear repository sites to immobilize intermediate-level radioactive wastes. This is due to the large surface area of the calcium silicate hydrate (C-S-H) gel, the main hydration product of ordinary Portland cement, which provides many sorption sites in which the contaminants can be adsorbed. The retention capacity of these materials is strongly dependent on the composition, the water content, the pH or the presence of additives. Likewise, it is also known that the durability and performance of cement and concrete are adversely affected in chloride and/or sulfate environments. In this work, atomistic simulations have been employed to analyze the effect of the presence of chlorides and sulfates in the retention and transport of Cs-137, one of the most hazardous radioisotopes, in calcium silicate hydrate. The simulations suggest that the presence of a moderate amount of chlorides does not alter significantly the Cs uptake in C-S-H gel, while a moderate content of sulfates enhances substantially the retention of Cs ions and reduces their migration throughout the pore. This behavior is attributed to the ability of the sulfates to pull Ca out the high-affinity sites from the C-S-H surface, allowing Cs ions to occupy them.

Experimental and Numerical Study of Structural Effects of Anisotropic Frost Damage on Reinforced Concrete Beams

Kanazawa, T; Sato, Y; Takahashi, R

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.14-25, 2021, JAN.

(https://doi.org/10.3151/jact.19.14)

Abstract

Unlike plain concrete, frost damaged reinforced concrete (RC) exhibits anisotropy because of the presence of reinforcing bars. The resultant mechanical responses are influenced strongly by the loading direction. Therefore, to ascertain the mechanical behavior up to failure of RC beams subjected to freeze-thaw action and subsequent mechanical loading, anisotropic damage models of frost-damaged RC elements were assessed in this study using three-dimensional nonlinear finite element analysis (3D-NLFEA), which revealed that the anisotropic damage affected load-deflection responses and played a key role in failure modes that differed from those of an undamaged RC beam. This paper is the English translation from the authors' previous work [Kanazawa, T., Sato, Y. and Takahashi, R., (2019). Frost damage of reinforced concrete beams and analytical evaluation of its static failure behavior. Journal of Japan Society of Civil Engineers. Ser. E2 (Materials and Concrete Structures), 75(4), 293-307. (in Japanese)].

Experimental Study on the Effect of Different Shear Reinforcement Shapes on Shear Failure Behavior and Internal Crack Pattern of RC Beams

Kawamura, K; Takemura, M; Nakamura, H; Miura, T

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.82-94, 2021, JAN.

(https://doi.org/10.3151/jact.19.82)

Abstract

Various types of shear reinforcement are used for example general closed stirrup and reinforcement bar with mechanical anchor. However, most standards and specifications take only the cross-sectional area of the vertical components of the reinforcement components into account when determining their effect, such as on shear crack development and shear strength. Consequently, the full effect of different shear reinforcement shapes on the shear failure behavior of reinforced concrete (RC) beams is not clear. In this study, differences in shear failure behavior of RC beams using three types of shear reinforcement (closed stirrups, U-shaped stirrups, and rod-shaped reinforcements with mechanical anchor) were investigated by carrying out loading experiments. The three-dimensional displacement distribution on the side faces of each beam and the internal crack patterns were obtained. It was clarified that there is a clear difference in internal crack pattern and spreading deformation behavior according to shear reinforcement shape, and this influences the shear strength of the RC beam.

Pull-Out Performance of Eccentrically Spliced Longitudinal Headed Bars for Precast Beam-Footing Connections

Ousalem, H; Takatsu, H

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.1-13, 2021, JAN.

(https://doi.org/10.3151/jact.19.1)

Abstract

For common detailing of footings in steel or precast concrete structures, longitudinal reinforcement of foundation beams is bent horizontally and spliced with reinforcement of cast-in-place footings to insure an adequate juncture for load transfer. In this study, instead of bending longitudinal reinforcement bars of both, beams, and footings, headed reinforcement bars are adopted. By doing so, a discontinuity region is created where longitudinal bars of footings become eccentric to those of beams that are embedded in the footings. To allow developing forces in longitudinal bars of beams flow to longitudinal bars of footings, a set of reinforcing ties is provided between them. As such setting of headed reinforcement bars is not common, thorough investigations have been carried out. In this paper, the pull-out performance of eccentrically spliced longitudinal headed bars with different detailing of transverse reinforcement, proposed for precast beam-cast-in-place footing connection, is discussed based on an experimental investigation. A method, based on the friction shear theory, for the strength evaluation of such arrangement is suggested.

Synergetic Effect of Expansive Agent (KEA) and Superabsorbent Polymers (SAP) on the Shrinkage, Strength and Pore Structures of Mortars

Zhang, M; Aba, M; Sakoi, Y; Tsukinaga, Y; Shimomukai, K; Kuang, YH

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.26-39, 2021, JAN.

(https://doi.org/10.3151/jact.19.26)

Abstract

In this work, the effects of the individual or hybrid addition of superabsorbent polymers (SAP) with varying dosages (0.1%, 0.2%, 0.3%, and 0.6%) and the lime-type expansive agent (KEA) on the length and mass change, compressive strength, and pore structures (MIP) of mortars were investigated. The results showed that the incorporation of SAP can effectively mitigate its autogenous shrinkage and the length change value of the mortar with SAP smaller than reference until 49 days, regardless of the presence of KEA. The hybrid addition of SAP and KEA increases the initial expansion of the specimens as compared with individual addition of SAP, which is a beneficial effect on compensating for the shrinkage of the mortar under drying conditions. Moreover, the addition of SAP seems to delay cement hydration and increase the volume of macropores (greater than 100 nm), thereby reducing the compressive strength of the mortars. The introduction of KEA slightly promoted the formation of micropores, resulting in a slight increase in compressive strength compared with the samples without KEA. Furthermore, in our view, it promotes pore refinement, so as to reduce moisture evaporation.

Undersampling Strategy for Machine-learned Deterioration Regression Model in Concrete Bridges

Okazaki, Y; Okazaki, S; Asamoto, S; Chun, P

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.18, pp.753-766, 2020, DEC.

(https://doi.org/10.3151/jact.18.753)

Abstract

Inspection data of actual concrete structures should be analyzed to elucidate the deterioration mechanism and construct a regression model. Although machine learning can be applied to this problem, inspection data are not suitable because machine learning targets big data with a uniform density and a balanced distribution. This study applies machine learning to a regression model of the crack damage grade in concrete bridges, using imbalanced inspection data. The model performance is improved by analyzing the influence of undersampling. Undersampling is conducted step-wise, and the models are constructed by learning all the undersampled data. The cross-validation of these models yielded the regression errors on each crack damage grade to evaluate the model performance considering the bias of data imbalance. Based on the results, the effect of undersampling on the model performance is analyzed, and the appropriate model is selected. Additionally, the influence of the model difference on the evaluation is investigated via historical change or factor analysis to confirm the effect of undersampling. This article not only presents a case study of a regression task for crack damage grades in concrete bridges, but also describes a strategy to maximize the use of imbalanced data for regression problems.

Drop-Weight Impact Loading of Polypropylene Fiber Reinforced Concrete Wall after One-Year Drying Shrinkage

Sato, Y; Naganuma, K; Ko, H; Kaneko, Y

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.18, pp.794-807, 2020, DEC.

(https://doi.org/10.3151/jact.18.794)

Abstract

Recent progress in finite element analysis aids the simulation of seismic vibration of an entire reinforced concrete (RC) building structure and indicates that drying shrinkage cracks affect seismic resistance performance. Polypropylene fiber-reinforced concrete (PFRC) is a promising material since the fibers will reduce the cracks and strains under drying shrinkage. This paper attempts to quantify the vibration characteristics of PFRC walls by means of a drop-weight test and finite element analyses. Four wall specimens having the same geometry and bar arrangement are prepared. After a one-year drying shrinkage period, the walls are subjected to impact loading of a constant collision velocity of 5 m/s, using a steel drop weight of 398.8 kg. Shear cracks are observed in the restrained wall made of plain concrete, while cracks are insignificant in the PFRC wall. Three-dimensional (3D) nonlinear finite element analyses are conducted to simulate all behaviors from drying shrinkage cracking up to the time of impact loading, and to estimate the vibration characteristics. The analysis results indicate that the polypropylene fiber content reduces the elongation of the natural period by an average of 13.7%.

Shear Bifurcation and Gravelization of Low-Strength Concrete

Yamanoi, Y; Maekawa, K

JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.18, pp.767-777, 2020, DEC.

(https://doi.org/10.3151/jact.18.767)

Abstract

Shear failure experiments of concrete beams containing a weak layer were conducted with a focus on the bifurcation of shear localization appearing at the boundary between structure and soil foundation. Low-strength concrete, which is analogous to artificial soft rocks and strengthened foundation, was used to create a weak layer that caused dispersal and bifurcation of the shear localization area, resulting in ductile fracturing of members. Pulverization of hardened cement paste and gravelization (the loss of aggregate particle's cementation) were observed in shear planes appearing in the weak layer. This confirmed the difficulty of simulating bifurcating shear localization solely by the constitutive law of concrete, which assumes firm cementation by hardened cement paste. In reference to the simulation of the disintegrated concrete slabs for bridge decks under fatigue loads, the transient model from hardened concrete to gravelized assembly was proposed, and it was successfully applied to the bifurcating shear localization of weak layers of low-strength concrete.