Kenichiro Nakarai

• Doctor of Engineering
• Professor at Hiroshima University

Restraining the expansion of expansive concrete with embedded rebars can exert chemical prestressing, which may affect the durability of concrete structures. This study aims to investigate the durability performance of expansive concrete by understanding the mechanism of air permeability changes while considering the variations in reinforcement arr...

In the presence of steel reinforcement, the expansion energy induced by the expansive agent is restrained, resulting in exerting prestress on the concrete called chemical prestressing; this can simultaneously compensate for shrinkage and improve the performance of the concrete. This paper presents a comprehensive study on the performance of chemica...

Predicting carbonation resistance via early site tests is crucial for controlling the longevity and durability of concrete structures. Therefore, this study aimed to provide a nondestructive approach for predicting carbonation resistance utilizing in situ air-permeability and surface moisture measurements. The Torrent air-permeability method, coupl...

The long-term strength development and durability of in-situ cement-treated soil were investigated by physicochemical analysis and geochemical thermodynamic modeling. The measured strength of the volcanic cohesive clay treated with high-sulfate cement continuously increased for five years and then persisted until 22 years. X-ray analysis revealed t...

The safe management and disposal of radioactive wastes are of critical importance for maintaining a sustainable society. In nuclear waste management, cement and concrete are commonly used to immobilize radioactive waste. Cementation is a common technique used for conditioning low-level and intermediate-level radioactive wastes, which should ensure...

The influence of temperature on the transport properties of cover concrete is crucial for adjusting in situ measurements. This study investigated the effects of storage room temperature on air permeability and water sorptivity measurements. Four temperature conditions (5, 20, 30, and 35 °C) with constant relative humidity (60%RH) were applied to 11...

Radioactive waste disposal is a very important issue when constructing a sustainable society with nuclear energy. To construct disposal facilities, cementitious materials are used for various purposes, such as structural elements, storage cells, tunnel plugs and engineered barriers. This technical report describes the history and current status of...

The radioactive cesium (radio-Cs) released following the Fukushima Daiichi Nuclear Power Plant accident was unevenly distributed in soil, sediment, and buildings. It is therefore crucial to evaluate the impact of radio-Cs and to prevent its spread throughout various environments. Herein, we investigated the Cs adsorption capacity of woodchip-mixing...

Rainfall is an environmental factor that can affect the properties, including air permeability and water absorption of concrete. In this study, the influence of rainfall on the air permeability and water absorption was investigated utilizing two nondestructive methods, namely, Torrent air permeability test and the water intentional spraying test (W...

Cement treatment is a popular method to enhance properties of soft soil such as mechanical properties, durability, and physical properties. Compressive strength is known as one of the most important indicators governing the quality of improvement of cement-treated soil. The type of soil and cement greatly affected the compressive strength of cement...

High durability is required of concrete structures to achieve long-term service life as important infrastructures. The high durable structures by excellent related technologies also contribute to establishing a sustainable society with carbon neutral in the view of the global trend. The authors have been working on the development of a simple techn...

High temperature or thermal curing is commonly applied to concrete fabrication but raises concerns for potential microcracks caused by temperature changes. This is particularly concerning for concrete using limestone coarse aggregate given the significant mismatch in thermal deformation between the aggregate and the surrounding mortar. This experim...

Calcium leaching from cementitious materials into bentonite is a key process for the long-term alteration of cement–clay interfaces of engineered barrier systems. Strong chemical gradients between cement and clay drive the precipitation of minerals such as calcium silicate hydrate (C–S–H) and calcite. To analyze the mineralogical and porosity evolu...

Calcium leaching from cementitious materials in contact with bentonite in nuclear waste repositories can alter the functionality of an engineered barrier system. In this study, we contribute to the fundamental understanding of calcite precipitation at cement–bentonite interfaces by adding carbonate to bentonite. In addition, we accelerate the trans...

This study proposes a new method to analyze important factors that affect the air permeability of concrete cover. By conducting non-destructive tests on mockup box culvert specimens exposed to the outdoors (without rainfall), the changes in the moisture content, air permeability, and water absorption of concrete cover were investigated over a perio...

It is a well-known fact that moisture in the pores hinders the flow of gas through them, thus having a strong effect on the gas-permeability of concrete. Contrary to laboratory tests, where specimens can be preconditioned by drying, site air-permeability tests are normally conducted under the natural moisture conditions prevailing at the moment of...

To improve the strength of cement-treated sand effectively, the use of various cement types was investigated at different curing temperatures and compared with the results obtained from similar mortars at higher cement contents. The compressive strengths of cement-treated sand specimens that contained high early-strength Portland cement (HPC) cured...

Blast furnace slag is extensively used as a supplementary cementitious material to improve the performance of concrete. However, shrinkage and early cracking are common in such slag concretes, so expansive concrete has been applied as a countermeasure. This study investigated the impact of slag types on the engineering properties of expansive concr...

Recently, fracture energy has received considerable attention owing to its importance in predicting crack propagation in concrete structures. The increase in the use of limestone aggregate concrete showing small fracture energy requires a deeper understanding of the key factors affecting fracture energy. In this study, a digital evaluation is perfo...

Deformation of reinforced concrete (RC) beams at shear tension failure is not so significant because the failure generally occurs in an elastic state of tension reinforcement. Shrinkage strain of concrete is generally comparable with elastic strain of reinforcement in terms of magnitude. This may imply that the shrinkage affects the shear tension s...

Air permeability plays as a vital durability index to evaluate the quality of the reinforced concrete structure including precast concrete box culvert. In the present study, the air permeability of a precast concrete box culvert applying a typical steam curing condition was monitored to evaluate the cover concrete quality. The Torrent air permeabil...

Apparent chloride diffusion coefficient, Dc, plays as a vital index in evaluating resistance of structures against chloride attack. Our previous study reported that the coefficients obtained from existing concrete structures were ten times smaller than those obtained in chloride immersion tests. In the existing structures, chloride ions are supplie...

The durability of concrete structures is crucial for their sustainable use; therefore, this study aims to provide a method for their service life prediction. The first stage of the study presents an in-situ air permeability measurement approach to evaluate the carbonation resistance of steam-cured concrete. The changes in properties of steam-cured...

. A primary current focus in concrete shear design is how to ensure the shear resistance of the concrete structures to withstand the possible adverse creep effects during the service life. All shear-carrying actions are supposed to depreciate due to the enhanced critical shear cracks under sustained loads. However, only a few studies that evaluated...

Unlike slender RC beams, shear failure mechanism of RC deep beams is mainly governed by shear compression; a considerable amount of the load is carried by the compression struts of concrete. For this reason, when subjected to a long-term sustained load, the shear strength of RC deep beam could be reduced as the sustained load might have negative im...

Casting direction is an external factor that can influence the properties, including air permeability of concrete structures. In the current study, the effect of this factor on the air permeability of precast box culverts was investigated. The Torrent air permeability test was used to measure coefficient of air permeability kT for two box culvert s...

Torrent air permeability tester has been widely used as one the most reliable non-destructive tests for evaluating the quality of the concrete cover. In this study, the Torrent tester was employed to evaluate the influence of rebar con-finement on the surface layer quality of expansive concrete. Blast furnace slag cement was used with the expansive...

Cement-treated soil specimens were contacted with surrounding soils having different water contents in order to investigate surface strengths affected by calcium leaching. The strength of sound parts of the treated-soils continuously increased until 2 years, the maximum material age in this study. On the other hand, the surface layer of the treated...

This study validated the effectiveness of a new, rapid, and cost-effective method to nondestructively evaluate the quality of early-age cover concrete using the water intentional spraying test (WIST). The WIST-measured water absorptivity at various ages, including promptly after demolding, was correlated with measured coefficient of air permeabilit...

In order to investigate the effects of drying shrinkage of concrete on shear tension strength of normal-strength reinforced concrete (RC) beam without web reinforcement, the loading test of RC beams was carried out. Major factors were the effective depth of 250, 500 and 1000mm as well as the concrete shrinkage. Twelve RC beams were prepared where h...

To contribute towards sustainability and cleaner production, we experimentally investigated the basic physicomechanical properties of a new porous mortar containing Japanese cedar woodchips and evaluated its durability by one-year immersion tests in ion-exchanged water and sodium chloride solution. The lightweight woodchip-containing porous mortar...

This study proposes a new durability indicator for classifying cover concrete quality through the total amount of sprayed water index (rW) using the water intentional spraying test (WIST). The usability of four WIST apparatuses was first verified through strong positive correlations between the previously proposed “repetition number of water sprayi...

The Water Intentional Spraying Test (WIST) was originally proposed as a simple nondestructive method for evaluating the cover concrete quality based on the “repetition number of water spraying,” rN. The validity of the test was verified in the present study by comparing rN with the coefficient of air permeability, kT, determined by the Torrent test...

This study investigated the effect of age on transport property measurements of concrete cover. Changes in moisture contents, air permeability, and water sorptivity were monitored for three years by conducting non-destructive tests on surfaces of site-cast concrete box culverts and mock-up specimens. Furthermore, carbonation rates in the specimens...

A simplified method has been proposed as a substitute for conventional methods to evaluate the behavior of expansive concrete. In this study, an experimental program was performed to investigate the applicability of the simplified method under steam curing. Two estimation methods for measuring the expansive strains under constraint were compared ba...

This study investigated the strength development of cement-treated soils by considering not only the interactions among cement hydration and pozzolanic reaction, but also the carbonation process. Indeed, the chemical and microstructural changes in cement-treated soils are due to carbonation combined with previous reactions and no studies took into...

This study aimed to investigate the strength development of cement-treated sand using different cement types: ordinary Portland cement (OPC), high early strength Portland cement (HPC), and moderate heat Portland cement (MPC) cured at different temperatures. The cementtreated sand specimens were prepared with 8% of cement content and cured under sea...

Sustained load problems, which can cause excessive deformation and severe damage to concrete structures, have been considered in current worldwide design codes by applying reduction factors on the compressive and tensile strength of concrete. A reduction factor in the shear design may also be required due to the decrease of shear-transfer action co...

This study aims at investigating the effects of sustained loads on the shear strengths of reinforced concrete (RC) slender beams. Slow loading rates (that were 100 and 1000 times slower than the normal loading rate) were selected to reveal creep effects. The slower loading rate indicated that increased creep occurred during the loading test. Howeve...

The present study aimed at investigating the effect of internal water curing (IWC) on pozzolanic reaction of Class-F fly ash in cement paste into which deionized water was allowed to naturally seep through a syringe needle from an age of 1 month to an age of 24 months. In the experiment, a high-early-strength Portland cement was replaced with Class...

This study investigates the effect of internal water content in cement-treated sand on carbonation progress and the effect of carbonation on compressive strength. To alter the internal water content, specimens were cured under three conditions: sealed, drying, and water sprayed. The carbonation coefficient, which was determined by a phenolphthalein...

The present study aimed at investigating the degree of fly ash reaction and porosity up to 12 months of aging for cement paste specimens internally activated by natural injection of NaOH solution and those internally cured by injection of deionized water 1 month after casting, to assess the positive effect of internal alkali activation. In addition...

The aim of this study was to investigate the effect of a high volume of roof-tile waste coarse aggregate (5–13 mm) as an internal curing agent on the compressive strength, modulus of elasticity, pore structure, and hydration and pozzolanic reactions in paste of fly-ash concrete with a low water-to-binder ratio of 0.30. The fly-ash concrete specimen...

The present study focuses on degree of fly ash reaction and pore structure in cement-based paste internally alkali-activated to clarify positive effect of internal alkali activation (IAA) on pozzolanic reaction of fly ash in cement paste in a long term. A saturated Ca(OH)2 solution was injected into cement paste specimens with 0% (Fa0) and 40% (Fa4...

Effects of internal alkali activation (IAA) using roof-tile waste aggregate (RTWA) as an alkali solution reservoir on cement-based pastes with 0% and 40% replacements with fly ash, denoted as Fa0 and Fa40, respectively were investigated microscopically. Micro-hardness and porosity, and Ca(OH)2 content of pastes in the vicinity of RTWA were measured...

Use of limestone aggregate has been recently increasing to improve material properties such as shrinkage reduction. However, limestone aggregate may cause brittle failure of concrete and concrete structures. Therefore, this study compared the fracture properties of normal-strength concrete and the shear strength of reinforced concrete (RC) beams wi...

Long-term pore structure of fly ash cement paste injected with water through a needle from a 1-mL syringe was investigated, to assess the effects of internal curing on long-term mechanical properties and durability. The injection of water at 1 month to simulate internal curing evidently affected pore structure of cement paste having 40% replacement...

Internal water curing (IWC) is applied to fly ash concrete with a low water to cementitious materials ratio to increase strength and reduce autogenous shrinkage. On the other hand, internal alkali activation (IAA) accelerates pozzolanic reaction of low-calcium fly ash in the cement paste in the short and long periods of injection of alkali solution...

This study investigates the effects of carbonation, water content, and pozzolanic reaction under the drying condition on the strength development of cement-treated soils. Two soil specimen types (i.e., sand mixture and sand–loam mixture) were cured under sealed, drying, and changing water content conditions. The measured compressive strength of the...

In order to investigate the effects of drying shrinkage of concrete on shear strength of reinforced concrete (RC) beam without shear reinforcement, the loading test of the RC beams was carried out. Major factors were water-to-binder ratio of 50% and 35%, effective depth of 250,500 and 1000mm as well as the amount of concrete shrinkage. Test results...

In this study, the change in surface quality of concrete with aging was studied using concretes with different mix designs, curing conditions and exposure conditions. We used three sets of specimens. The first set was composed of rectangular blocks. The second set was specimens that simulate a part of box culvert. The third set was column specimens...

In order to evaluate the expansive properties of expansive concrete, uni-axial restraint test method is stipulated in JIS A 6202 and it have been used widely as the only expansive test method. But, actually, bi-axial or tri-axial restraint condition is usual in structures and multi-axial restrained test method such as using concentric double pipe h...

Cement-treated soils are used for improving properties of soil foundations. Short-term strength development of cement-treated soils until 7 or 28 days is one of the typical key parameters for material design. In order to discuss the possibility for developing a new method to promote short-term strength development by carbonation, this study evaluat...

This paper presents the effects of internal curing with 40% replacement with roof tile waste as coarse aggregate on compressive strength at the ages of 7, 28 and 91 days, and porosity at the age of 28 days of fly ash concrete cured in sealed condition. A water to binder ratio was 30% and the replacement percentages of ordinary Portland cement by fl...

Fly ash is often added to concrete as a filler and to improve strength and durability. The pozzolanic reaction between the silica and alumina in low-calcium fly ash and the Ca(OH)2 from hydrated cement proceeds slowly after an initial rapid reaction so unreacted fly ash particles remain in the paste even after aging. This limits the improvement in...

In recent years, alkali activation on fly ash particles has been suggested for accelerating their pozzolanic reaction, which enhances the durability of concrete. It was reported that the alkali activations become most effective when high temperature curing is applied, but it is limited for many applications. Therefore, alkali activation at normal t...

Based on calibration specimens, internal relative humidity (IRH) and moisture transfer (MT) in mortar through water absorbing and drying processes were assessed by measuring electrical resistance using stainless steel rods placed at intervals of 4 mm in the specimens. As a result, for mortar exposed to water, the initial change in IRH could be main...

Volumetric changes in concrete may affect the structural performance of reinforced concrete structures and their durability. It has been revealed that autogenous shrinkage of high-strength concrete decreases reinforced concrete beam’s shear strength compared with low-shrinkage concrete. High-drying shrinkage of concrete may also decrease the shear...

It is important to investigate the unique behavior of long-term strength development of cement-treated soils. The large long-term strength development is usually explained by pozzolanic reaction. Additionally, we reveled that carbonation could contribute the strength development based on the experimental study using cement treated sand. This study...

This paper covers required functions and performance with regard to the performance evaluation of cementitious materials over an ultra-long term on the order of 10,000 years, discusses the need for leaching modeling and aqueous alteration evaluation of cementitious materials, and presents concepts. The positioning of leaching modeling and the state...

Elevating the alkalinity of pore solution in fly ash cement paste at the late age is essential in order to accelerate the pozzolanic reaction of fly ash particles and enhance the long-term performance of concrete. In this research, the effect of an internal alkali activation on the pozzolanic reaction from the late age (i.e. the age of 6 months) wa...

Utilization of fly ash as a supplementary cementitious material is necessary for a sustainable society. When sufficient wet curing is conducted, fly ash is well known to make concrete dense with time, resulting in enhanced strength and durability of fly ash concrete in the long term. On the other hand, porous ceramic coarse aggregate derived from r...

In the conventional repair method of honeycombing of RC structures, the repair boundary with sound concrete may cause delamination. In this study, we proposed a new repair method which injects the epoxy resin without removing the honeycombing and then performed cyclic loading tests of RC beam specimens repaired by the proposed and conventional meth...

The purpose of this study is to investigate the pozzolanic reaction of the fly ash cement paste by an injection of an alkali solution after hardening. Cement pastes with 0%, 20%, and 40% of fly ash replacement ratios were used. An NaOH or saturated Ca(OH)2 solution was injected into the paste through a syringe 1 month after casting at normal temper...

Long-term excessive deformation of underground RC box culverts in service was monitored over 20 years and its mechanism is analytically discussed in this study. The long-term excessive deformation possibly attributes to synergy effects accompanying delayed shear failure of RC slabs subjected to vertical soil pressures and the time-dependent creep-s...

Three 200x320x2300mm RC beams were tested under three different loading conditions to investigate the effects of loading rate on the shear performance of RC beam. Tested at loading rate 10mm/1hr, RC Beam 1 represents specimen under normal loading while RC Beam 2 represents specimen under slow loading at loading rate 10mm/100hr from 60kN. Tested at...

In this study, we investigated the drying shrinkage and fracture energy of concrete with lime stone and other aggregates and their effect on the shear strength of RC beams. The results of material experiments showed that the use of the lime stone aggregate having large aggregate crushing value decreased the fracture energy and drying shrinkage comp...

A new concept for creep coefficient defined as effective creep coefficient was proposed. This concept was derived based on strain in prestressing strand directly measured in a prestressed concrete slab-girder as well as shrinkage strain of concrete specimen, which were made of ultra high strength concretes containing a molten slag fine aggregate an...

It is well known that the strength of cement-treated soil increases over the course of several years. In general, the long-term increase in strength is attributed to the pozzolanic reaction between the cement paste and the soil. The carbonation of cementitious material can also increase its strength. In this study, the effect of carbonation of ceme...

In this study, the effects of types and activation time of alkali activators on pozzolanic reaction of the paste with 0% and 40% replacement of fly ash were investigated by examining the Ca(OH)2 content and the pore volume. The results showed the presence of alkali activator decreased the Ca(OH)2 content, total and 20-330 nm ranged diameter pore vo...

In the radioactive waste disposal facility, sodium-type bentonite will be used as an artificial barrier material. They are expected to delay the infiltration of groundwater and fill the crack with the surrounding ground because they have high swelling performance. However, during the tens of thousands of years, the bentonite can be deteriorated and...

The behavior of heavy metals in cement paste is often investigated using a single species of heavy metal of which relatively soluble reagent is dosed in cement paste. In fact, however, several kinds of heavy metals could be incorporated in concrete where each heavy metal forms different kinds of compounds. In this study, incorporation of heavy meta...

Following the accident at the Fukushima Daiichi nuclear power plant in the Great East Japan Earthquake, a large amount of water contaminated with radioactive materials has built up on the premises. The processing, temporary storage and leakage prevention of this contaminated water has become a major issue. Processing, disposal and recycling technol...

The mechanism of the long-term strength development of carbonated cement-treated soils was experimentally investigated. The unconfined uniaxial compressive strength of the carbonated specimen of the cement-treated soil was larger than that of the non-carbonated specimen cured in the sealed condition. In the carbonated specimens, the strength of the...

The accident at the Fukushima Daiichi Nuclear Power Plant resulted in spreading a huge amount of the radioactive cesium (cesium-137) widely. It is important to decontaminate the radioactive cesium and dispose the radioactive waste safely as soon as possible. This study aims to investigate the basic behaviors of the cesium-133 in mortar such as adso...

An effective flexural stiffness equation for long-term deflection of prestressed concrete with and without cracks is proposed. The equation is derived by assuming a linear strain distribution through the section, as well as considering creep, axial shrinkage and shrinkage gradient of concrete. The effect of creep is considered by applying the effec...

Here, we developed a woodchip-mixed porous mortar (WPM) by mixing wood chips obtained from thinned Japanese cedar wood with porous mortar aggregates and evaluated the adsorption properties of the heavy metal ions CuII, CdII, and PbII on the WPM. The cation-exchange capacity (CEC) of cup-shaped WPM was approximately twice as large as that for wood c...

Subcommittee on Post-Construction Quality Verification of In-Place Reinforced Concrete (JSCE 335) under Concrete Committee of Japan Society of Civil Engineers (JSCE) published the state-of-the-art report on the non-destructive inspection technique for measuring the quality of cover concrete of completed structures and the system for verifying durab...

The accident at the Fukushima Daiichi Nuclear Power Plant resulted in a huge amount of radioactive cesium spreading widely. It is important to decontaminate the radioactive cesium and dispose the radioactive waste safely as soon as possible. The sorption properties for cement materials have already been investigated by several researchers. However,...

The low alkalinity cement based on sulphoaluminate clinker was produced and the physical properties of mortar and concrete were examined using this cement. The development of compressive strength was superior, but the early stage of strength tended to reduce when there existed a greater dosage of retarder. In addition, the compressive strength deve...

The preloaded and prestressed (PLPS) geotextile-reinforced soil structure can have substantially high stiffness and stability in the vertical direction when subjected to cyclic vertical load. The struture has a high damping ratio, flexibility and ductility when subjected to cyclic horizontal load during earthquakes. The structure can also exhibit a...

A series of shaking table tests were performed to validate a high dynamic stability of preloaded and prestressed geosynthetic-reinforced soil (PLPS GRS) abutment. Three-dimensional models, which were free from the friction between the backfill and the sidewalls of the sand box, were used. The test results revealed that the dynamic stability increas...

本JCIのセメント系材料の時間依存性挙動に関する研究委員会（JCI-TC 061 A）に設置された耐久性力学WG（WG 3）は，2001年にCoussyとUlmによって提案された「耐久性力学」を，種々の劣化現象を連成作用の観点から，より体系的に整理するために再定義した。さらに，この概念に沿って，既存の劣化現象の耐久性力学の体系の中での位置づけを例示するとともに，代表的な耐久性力学的な研究事例について引用を行い，耐久性力学の意義および体系化への今後の方向性について示した。

This paper gives a summary of the activities of the Working Group on Durability Mechanics (WG3) within the Japan Concrete Institute (JCI) Technical Committee on Time Dependent Behavior of Ce- ment-Based Materials (JCI-TC061A). A re-definition of and new approach to durability mechanics is pro- posed for establishing systematic prediction and evalua...

In present paper the applicability of resistivity of concrete for evaluating chloride penetration is discussed. The resistivity of concrete was measured using an AC power source with 15 voltages and 950Hz. Effective diffusion coefficient of chloride ions in concrete was also determined by chloride migration test that is specified by JSCE G5712003....