This paper describes the history and new trends in the development of chemical admixtures in Japan. PC (polycarboxylic acid)-based agents are the main products in the superplasticizer market. A low-stickiness type PC-based superplasticizer has been developed based on the conventional PC-based superplasticizer. The flowing speed of concrete with low stickiness type PC-based superplasticizer is faster than that with the conventional PC-based superplasticizer. By addition of a new viscosity agent consisting of a mix of anionic and cationic surface active agents, the three dimensional reticulation structures is formed in fresh cement paste and the viscosity of the cement paste can be increased and segregation can be prevented. The hydration of cement in sludge water is controlled by addition of a set-retarder, and the specific surface area of cement does not increase while a large amount of unreacted alite remains in the sludge water. Recycling of concrete at ready mixed concrete plants is possible without adversely influencing the properties of concrete when sludge water with the set-retarder containing gluconate salt is used. The slurry type and powder type calcium aluminate based accelerator (CA) and calciumsulfoaluminate based accelerator (CSA) for shotcrete have been developed. In the case of CA, the final setting time of the mortar is accelerated by increasing the dosage. By adding of CSA, both the initial and final setting times of mortar are shortened with increased dosages.
The effects of chemical structures of graft copolymer on cement-dispersing performance were investigated to elucidate the fluidizing mechanism of polycarboxylate-based superplasticizer containing graft copolymer with polyethylene oxide graft chains. A graft copolymer with longer graft chains showed better dispersing stability with a small amount of adsorption. The adsorption study indicated that the graft chains elongated as the adsorption density increased. The thickness of the graft chain in the copolymer that adsorbs on a cement particle also depends on the average distance between two graft copolymers and the graft chain spacing within the copolymers, depending on the geometric features of the copolymer. The modified steric stabilization model incorporating the extension of graft chains due to adsorption and the geometric restriction of the copolymer gives a useful explanation of the relationship between adsorption and the flow of paste containing different graft copolymers.
α-Allyl-ω-methoxypolyethylene glycol - maleic anhydride copolymers were synthesized with side chain lengths nEO from 0-130 and characterized by aqueous GPC. A representation of their molecular conformation (e.g. worm-, brush- or star-like polymers) was developed. The amount of polymer adsorbed on cement rapidly decreases with increasing side chain length nEO. Zeta potential measurements using the electroacoustic method in cement paste with w/c = 0.5 indicate that worm-like copolymers with nEO ≤ 7 adsorb flat (“train” type) and form a densely packed, thin polymer film. In this case, the adsorbed amount of copolymer is high. For star polymers with nEO ≥ 34, preferred orientation of the polymer main chain is perpendicular to the cement surface (“tail” type). The result is a thick polymer layer, with a second electrochemical double layer on top. For star polymers, the adsorbed amount is low because of the higher surface occupancy of horizontally layered side groups.
To achieve durable concrete structures, the first important step is the sound placing of concrete. Compatibility among concrete materials, an important issue for controlling the workability of concrete for sound placing, has been the object of much attention over the past decade. Every engineer wants to have reliable methods to examine the fluidity performance of cement and superplasticizers. In this study, the way to examine fluidity performance is discussed from the viewpoint of fundamental mechanisms. One important point is the deforming properties of superplasticized concrete. When fresh concrete deforms homogeneously, the workability of concrete mixtures can be explained by one basic theory of superplasticizer, i.e. proportional correlation of the adsorption amount of superplasticizer per surface area of hydrate to the fluidity. Moreover, the examination of the fluidity performance of cement and superplasticizers should consider the condition of real concrete with respect to the water-to-cement ratio and mixing procedure of materials.
In order to develop a method for evaluating the fluidity of high-fluidity concrete using cement paste, the relationships between the rheological properties of cement paste and the slump flow of the concrete were investigated, using several commercial Portland cements. The rheological constants of the cement paste were measured with a rotating viscometer changing the mixing procedure, dosage of superplasticizer, and the operation conditions of the viscometer. The yield values of cement pastes measured with a parallel-plate type rotating viscometer were found to show good correlation with the slump flows of concretes.
High filler contents are generally added to the cementitious system for the realisation of self-compacting concrete. Inert filler materials can be used to avoid problems caused by excessive hydration generated heat during hardening. Two different filler types were considered in this research: limestone and quartzite filler, combined with different types of Portland cement. Although the above-mentioned filler materials are considered to be inert with respect to cement hydration, experimental research shows that they interfere with the hydration processes. On the one hand, the reaction speed may be influenced due to the possibility of modified nucleation. On the other hand, the reaction mechanism is also altered due to the presence of the large filler content, with the occurrence of a new hydration peak, especially in the case of limestone filler. Based on isothermal conduction calorimetry on different cement filler systems, an existing hydration model for blended cement was modified for cement filler systems. Within the hydration-based hydration model for filler rich cementitious systems, the cement powder ratio is an important parameter. The analytical model was found to allow accurate prediction of the heat of hydration during the hardening process. This was also verified by means of adiabatic hydration tests on concrete.
A fundamental study was carried out to develop a kind of smart concrete that has a self-healing system that incorporates a heating device. Self-diagnosis composite is employed as the heating device used to heat up cracked parts in the concrete. This heating device and a pipe made of heat-plasticity organic film containing a repair agent are embedded in the concrete. The film is melted through suitable heating. Selective heat around a crack can melt the film to allow the repair agent to fill up the crack and harden the repair agent in the crack. Three-dimensional thermal analysis and an experimental study were carried out to confirm the proposed method.
Tensile creep tests on high-strength concrete have been performed. Besides up-to-date creep data which have been generated a new phenomenon has been discovered, i. e. shrinkage of loaded specimens is larger than of non-loaded ones. It seems that this phenomenon is in agreement with Powers' creep theory.
Recent years have seen a considerable amount of research on the carbonation of concrete. The standard specifications for concrete structures, outlined by the Japan Society of Civil Engineers (JSCE), prescribe a method to verify the durability of concrete structures in relation to carbonation. However, as the carbonation rate is largely governed by environmental conditions, it is important to quantitatively assess the effect of these conditions on the carbonation of concrete structures by applying the method to actual structures. The factors affecting the progress of carbonation in different members were investigated based on the results of a survey on a rigid-frame railway viaduct in service since its construction several decades before. The study results indicate that the carbonation rate of columns varies with height and depends on the exposure condition to rainwater. In contrast, the dis-persion of the carbonation depth on beams and slabs is small when compared to that of the carbonation depth on columns.
An enhanced multi-chemo-physical model for the time-dependent deformation of concrete is proposed based on thermodynamic state of moisture in micro-pores. The moisture migration mechanism is divided into 1) moisture transport through CSH gel grains and 2) water in motion within the inter-particle spaces of hydrate micro-products. The new kinematic model makes it possible to simulate both long- and short-term concrete creep. An enhanced mechanistic law of stress path dependency is introduced to cope with a wide variety of stress and ambient histories as well. Time-dependency at elevated temperature is also investigated with current high-accuracy thermo-hygro dynamics. The instantaneous plasticity in direct connection with evaporating moisture from CSH crystal inter-layers is incorporated into the predictive system. Although some mechanisms remain unverified, drying shrinkage and creep at high temperature are fairly simulated.
This paper presents a computer program for implementing a refined nonlinear strut and tie model approach for the practical design and analysis of disturbed regions in structural concrete. Nonlinear techniques in the selection, analysis and verification processes of a strut and tie model are incorporated in this program to eliminate the limitations of the conventional strut and tie model relating to the behavior and strength prediction of reinforced concrete. For the verification of the proposed model, the model results are compared to the experimental results of one-quarter-scale simply supported bottom-loaded deep beams. Analytical results showed a lower bound solution that agreed well with the experimental results. It was concluded that the nonlinear strut and tie model allows more economical design than the conventional strut and tie model. It was also concluded that for higher strength concrete, the strength of struts and nodal zones given by the ACI-318 02 code is unconservative and needs refinement to account for the brittleness of high-strength concrete.
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