Ippei Maruyama

Effect of curing temperature and type of cement on early-age shrinkage of high-performance concrete

This paper presents the results of an experimental study on the influence of curing temperature and type of cement [Portland cement and blast-furnace slag (BFS) cement] on the autogenous deformations and self-induced stresses in early-age concrete. It was found that higher temperatures do not lead to higher deformations in the observed period, but generally cause a faster shrinkage and a faster development of self-induced stresses. Another experimental finding is that, at the temperatures tested, concrete made with BFS cement shows higher shrinkage in the first days than concrete made with Portland cement.

Evaluation of Irradiation Effects on Concrete Structure: Background and Preparation of Neutron Irradiation Test

Methods for assessing the soundness of concrete exposed to irradiation are being developed within the framework of a project of the Nuclear Regulation Authority (NRA) “Japan Ageing Management Program for System Safety”. This paper presents the background of this project and recent research works. The major reason for deterioration of concrete under irradiation conditions is expansion of aggregate due to neutron and gamma-ray irradiation. Dislocation of atoms in covalent structures of aggregate minerals, and resultant lattice constant change and alteration to amorphous phase are produced by fast neutrons. In addition, through electronic exertion, some energy deposition is accumulated as permanent distortion/strain in atomic structures. Other effects are also summarized. We plan to conduct a neutron irradiation test from May 2013. In selecting mix proportions and concrete components for the irradiation test program, sample size as a function of capsule size and gamma-heating, cement type, and aggregate size and type are determined based on preliminary experiments.Copyright

EVALUATION OF IRRADIATION EFFECTS ON CONCRETE STRUCTURE -GAMMA-RAY IRRADIATION TESTS ON CEMENT PASTE-

In assessing reduction of concrete strength under irradiated conditions, reference levels are introduced: 1x10 20 n/cm 2 for fast neutrons and 2x10 10 rad (2x10 5 kGy) for gamma-rays. Concrete structures are regarded as sound as long as irradiance levels accumulated during long-term operation are less than the reference levels. Most experimental investigations of irradiation effects on concrete were performed in the 1960’s and 1970’s. However, there is no good explanation of how concrete deteriorates under neutron and gamma-ray irradiation. Hilsdorf put the primal irradiation test data together to investigate effects of irradiance levels on residual strength ratio of concrete [1]. The reference levels were obtained from his paper. However, the test conditions in which the data quoted by Hilsdorf were obtained are very different from the irradiation and heat conditions usually found in a Light Water Reactor (LWR). This paper summarizes the interactions between radiation and concrete components and presents the results of gamma-ray irradiation tests on cement paste in order to provide a better understanding of the deterioration mechanisms of concrete under irradiation and heat conditions in LWRs.

Prediction of Temperature in Ultra High-Strength Concrete Based on Temperature Dependent Hydration Model

High strength concrete, which is characterized by a low water to cement ratio, has a greater cement content than ordinary concrete. Even if the cross sectional area of reinforced concrete members is reduced on account of high strength, significant temperature at an early age can be observed because of the higher cement content per unit of mass concrete. This can jeopardize durability and the appearance of concrete members due to early age cracking. Regarding this problem, it is of vital importance to be able to predict the temperature history in concrete members. This is especially true in the case of small section members where the temperature dependency of cement hydration is difficult to predict. In this study, temperature distribution n concrete members made of ultra high-strength concrete with water to cement ratio of 0.15 is investigated. In order to predict the temperature distribution, a cement hydration model which takes into account temperature effect on the rate of cement hydration is proposed. Also, a finite element analysis considering temperature dependent heat production is conducted for evaluating temperature history and distribution in concrete. The proposed hydration model and analysis method are discussed along with experimental results.

Cracking of high strength concrete around deformed reinforcing bar due to shrinkage

High Performance Concrete is particularly sensitive to self-desiccation of the cement paste during the hydration process, which leads to autogenous shrinkage. If a restraint is present, autogenous shrinkage may lead to high self-induced stresses, possibly causing surface and even through cracks and potentially jeopardizing the durability of the concrete structure. It is a matter of vital importance to grasp the characteristic of high performance concrete, especially in the case of RC. Two different super-high-strength concretes whose water to binder ratio is 0.15 and compressive cylinder strength at 28 days is more than 120 N/mm are developed. One of these is conventional super high strength concrete that shows very large autogenous shrinkage, namely about 800μ, and the other shows about 200 μ at 28 days due to adding expansive additive and shrinkage reducing agent. Subsequently experimental results of shrinkage induced stress in RC, using two different concrete mentioned above, whose reinforcement ratio is 8.6% are reported. Additionally an evidence of crack at the vicinity of reinforcement of SHSC when the reinforcement ratio is about 8.6% is reported and its effect on mechanical behavior of RC is discussed.

Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales.

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a ‘Diffusion-growth rate cross-plot’ that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential.

Improvement of Time-Dependent Flexural Behavior in RC Members by Using Low Shrinkage-High-Strength Concrete

This study investigates the effectiveness of low shrinkage-high strength concrete (LS-HSC) using expansive additive and shrinkage-reducing agent with regards to the time-dependent structural performance of reinforced HSC flexural members. Design equation for evaluating the flexural crack width and deformation of RC beams considering the effect of shrinkage/expansion before loading are proposed on the basis of JSCE (Japan Society of Civil Engineers) Design Code of 2002. The results show that autogenous shrinkage of conventional HSC with no additives can significantly affect the time-dependent serviceability performance. In addition, time-dependent flexural crack widths of reinforced LS-/conventional HSC beam, can be evaluated by the JSCE Code Equation, which takes into account the strain change in the reinforcement bars from the state where the stress in concrete at the depth of tension reinforcement is zero. Also, time-dependent as well as instantaneous curvatures of reinforced LS-/conventional HSC beam can be accurately calculated by the proposed equation. This equation takes into account the effect of the change in curvature due to the release of restrained-shrinkage/expansion stress at cracked section.

Properties of Expansive-Ultra High-Strength Concrete

In order to decrease cross sectional area of structural member, ultra high strength concrete with compressive strength over 150 MPa is required for building structural members which needs no steam curing. In the present study, concrete is made of silica fume cement which is composed of low heat type cement and silica fume and demonstrates high compactability. Compressive strength of the concrete with water to binder ratio of 0.15 and the effect of hydration heat of binder on compressive strength are investigated experimentally. Effectiveness of expansive additive on reduction of autogenous shrinkage is also investigated. According to the experiment, compressive strength over 150 MPa is gained by adopting appropriate aggregates without steam curing at early ages, while the strength of full sized specimens decreased about 10% at the age of 91 days

Numerical Analysis of Effect of Cyclic Thermal Load on Concrete to Concrete Bonding

This paper presents a numerical simulation and preliminary experimental investigation of bonding of concrete structures using cement and polymer cement as adhesive materials. The numerical model of concrete plates bonded with the polymer cement is created and subjected to cyclic thermal loading. As a result, the critical stresses are obtained. Also all the necessary materials for carrying out high-quality, fast and inexpensive experimental investigation of the strength of cement and polymer cement bond are prepared. The experimental setup and the first results obtained are also described. The bonding quality of the polymer cement shows satisfactory results under this kind of loading, thus the use of the polymer cement for bonding of concrete structures of different properties is reasonable in the specific fields considered in this study.

INFLUENCE OF WATER ON CARBONATION OF γ-C₂S AND CARBONATION BEHAVIOR OF HARDENED PASTE CONTAINING γ-C₂S

This paper investigates the influence of water on carbonation of γ-C₂S (γ-2CaO・SiO₂) and carbonation behavior of γ-C₂S contained in hardened cement paste. From the result of accelerated carbonation (40℃, 50%RH, 20%CO₂), carbonation ratio of γ-C₂S without adding anything increased to 4% at age of 28 days. By the addition of water to γ-C₂S before accelerated carbonation, carbonation ratio of γ-C₂S reached 33%. Further, carbonation ratio of C₂S increased to 71% by continuous supply of water. It was found that carbonation ratio of γ-C₂S depended on the supply condition of water. On the other hand, carbonation ratio of γ-C₂S in hardened cement paste continued to increase without water supply under same accelerated carbonation conditions. In the surface portion, the carbonation ration of γ-C₂S contained in hardened cement paste reached 64% at age of 28 days and 75% at age of 91 days. This result suggested that, in case of hardened cement paste, the carbonation of γ-C₂S progressed effectively by water supply from micro-pore structure and carbonated cement hydrates. Based on the above result, the use of γ-C₂S as the cement admixture could be considered effective combination for carbonation progress of γ-C₂S.γ-2CaO・SiO₂（γ-C₂S）と、γ-C₂Sを混和したセメント硬化体の炭酸化挙動を比較検討した。γ-C₂Sを炭酸化する際に水を添加することで、28日における反応率は4％から33％に増加し、更に継続的に水を供給することで、71％まで増加した。一方、セメント硬化体中のγ-C₂Sは、水の供給を行っていないにも関わらず、継続的に炭酸化が進行し、反応率は促進91日で75％まで増加した。セメント硬化体の細孔中に存在する水や、セメント水和物が炭酸化して分解する際に放出される水によってγ-C₂Sの炭酸化が効果的に進行すると考察した。γ-C₂SをOPCと併用することが、γ-C₂Sの炭酸化を促進する効果的な組み合わせであることが明らかになった。