There are concerns about the possible increase in the solubility of radionuclides due to the presence of superplasticizers proposed to be added to cementitious materials in a radioactive waste repository. The concentration and molecular weight of a polycarboxylate type SP in hardened cement pastes has been investigated experimentally in this work. The measurement of total organic carbon in both the pore water extracted from hardened cement pastes by compression and the curing water was performed. Organic substances present in pore solution were analyzed to investigate the type and molecular weight of the chemicals. It was found that most SP (more than 90%) remained in the solid phase. Organic substance of a low molecular weight was detected in the pore water by gel permeation chromatography, possibly due to selective adsorption of a high molecular weight part of SP.
Various non-standard filler materials are used in concretes to improve fresh and hardened concrete properties. This paper reports results from investigations on the effects of a coarse glass powder on the hydration and strength development of cement pastes. It is shown that the incorporation of glass powder results in enhancements in the degrees of hydration of the cement grains in the system. Using a mixing model for non-evaporable water content, a methodology to quantify the enhancement in the degree of hydration of cement as a result of the presence of glass powder is proposed. In order to quantify the relative effects of dilution (reduction in cement content, and thus of the hydration products) and increase in degree of cement hydration on the mechanical properties, a strength index, which accounts for the mass fraction of the reacting material, is proposed in this paper. Using the strength index, it is shown that the use of coarse glass powder filler is beneficial in pastes with lower water-to-cement ratio (w/c), where a portion of cement remains unhydrated. Increasing glass powder contents are observed to result in increased heat of hydration per unit mass of cement. However, beyond a certain glass powder content, the heat of hydration decreases due to the dominance of the dilution effect. The ultimate heat of hydration obtained from a three parameter model, the cement content, and the ultimate degrees of hydration are used to specify a new parameter termed the total ultimate heat of hydration. It is suggested that the total ultimate heat of hydration per unit volume of paste can be used to compare between mixtures made with different w/c and glass powder contents since this term, as defined in this paper, accounts for both the hydration enhancement and dilution effect from filler addition.
Especially during outdoor concrete production in winter, mottled dark discoloration frequently spoils the appearance of fair-face concrete surfaces. At the Centre for Building Materials (cbm) of the Technical University of Munich, the dark discoloration that occurs in practice is characterized in terms of surface structure, near-surface microstructure and chemical composition. By varying the conditions of concrete production and storage, it was possible to simulate the discoloration in the laboratory and thus determine the main factors responsible for the mottled appearance. By comparing the results of field and laboratory investigations, the transport and Crystallization processes in hardening concrete that lead to the discoloration of fair-face concrete surfaces were determined.
Following the initiation of corrosion in reinforcement, expansion of corrosion products generates corrosion-induced cracks in concrete. At the meso-scale, accumulation and coalescence of micro-cracks create the fracture process zone. Since nucleation of an individual micro-crack can be detected by acoustic emission (AE) techniques, the moment tensor analysis of AE waves is applied to quantitatively identify cracking kinematics of a location, a crack-type and a crack orientation. At the macro-scale, initiation and extension of the corrosion-induced cracks are analyzed by means of classical fracture mechanics, applying the boundary element method (BEM). The generation of corrosion-induced cracks was simulated in expansion tests on concrete specimens. Kinematical mechanisms of micro-cracks were identified by the SiGMA (Simplified Green's functions for Moment tensor Analysis) analysis. By applying the two-domain BEM analysis, extension of the corrosion-induced crack in an arbitrary direction was analyzed. With respect to the orientations of crack extension, results of the BEM analysis were compared with those of the SiGMA analysis, introducing the normalized stress intensity factors. It is demonstrated that extension of the corrosion-induced crack is governed by the mode-I failure both at the meso-scale and at the macro-scale.
The applications of fibre reinforced polymer (FRP) bars in buildings can provide a potential market for their use. Since buildings may be exposed to elevated temperatures, the behaviour of carbon FRP (CFRP) reinforced concrete (RC) beams was studied at high temperatures and compared with normal temperature behaviour. Steel RC beams were used as control specimens. The failure modes of beams at normal and elevated temperatures were found to be the same. The stiffness of cracked FRP RC was less than steel reinforced concrete at normal temperature whereas FRP reinforced beams were stiffer than steel RC beams at elevated temperatures.
Recently, Prestressed Reinforced Concrete (PRC) has been accepted as a reasonable structural member that permits cracking. A PRC member is a visible design alternative to either reinforced concrete (RC) or fully prestressed concrete (PC). In Japan, PRC has been widely used for bridge structures because it is economical. PRC members are generally designed to allow cracking under full service loads. Flexural cracking in PRC beams has been already studied and the flexural crack width can be accurately predicated by equations available in the present codes (ACI, CEB-FIP, JSCE, etc.). On the other hand, shear cracking behavior in PRC members is barely understood. The objective of the present study is, therefore, to experimentally explore the shear cracking behavior of PRC beams. Three I-shaped RC and four I-shaped PRC beams were tested under four-point monotonic loading. The experimental program was carried out focusing on the influence of prestressing force, side concrete cover, stirrup spacing, bond characteristics of stirrup and the amount of longitudinal reinforcement on shear crack width. The study revealed that the prestressing force significantly reduced shear crack width in PRC beams as compared to RC beams. In addition, an equation was proposed to calculate shear crack width in RC and PRC members. The proposed formula for shear crack width shows better correlation between calculated values and experimental data than the other formulae.
This study presents a computational method to estimate a global damage index of a RC construction. The method is based on the evaluation of local damage combined with an analysis of the probable collapse mechanism of the structure. A constitutive model for reinforced concrete, including global damage variables for concrete and steel elasto-plastic models, is integrated in a multilayered finite element code. The locations of pseudo plastic hinges in a structure are obtained in the areas of maximal damage through simulation by the FE method. The global index is derived from a specific formula taking into account the damage recorded in the critical zones of the structure (pseudo plastic hinges) and the damage computed in the less damaged areas. The proposed method is validated on a RC frame structure application. The diagram “global index vs. loading” shows a specific shape and gives interesting results for the discussion of the possibility of reparation.
This paper presents various models of deterioration due to carbonation taking into consideration uncertainty factors to estimate the initiation and the rate of corrosion and to analyze the structural capacity and serviceability of RC members, i.e. shear capacity, bending strength and width of severe cracking or spalling of columns and beams with corroded reinforcing bars, based on simple formulas formed through past experiments. Then, it goes on to propose a method for evaluating the failure and severe spalling/cracking probability during earthquakes and the deterioration risk of members in specified years from construction. Further, through application of an immune algorithm to the minimization of the life-cycle cost including deterioration risk, an optimal maintenance plan and semi-optimal solutions with a high diversity for reinforced concrete members can be found in a single analysis. Finally, a case study is conducted to establish the effectiveness of this system.
There are many uncertainties relating to the deterioration of reinforced concrete (RC) structure, and as a result, the actual degree of deterioration of structures is not uniform. Currently, various safety factors are considered to cover those uncertainties. This paper proposes a new method for maintenance planning of RC structures degraded by chloride attack based on probability theory. Prediction models of deterioration caused by chloride attack both before and after repair are discussed. The effects of crack and macrocell corrosion were considered to accelerate the deterioration in the prediction model. Surface coating, patching repair, cathodic protection, and combinations thereof were considered as repairing options. Moreover, the effects of partial and full repair were also considered in the deterioration of the repair system. The actual variation of structural properties and the environmental conditions obtained from the inspection program were used directly as input for prediction. By using Monte Carlo simulation, variations of deterioration degree can be predicted. Based on the prediction result, repair and failure costs are determined and used to design the maintenance planning program. Finally, applications of maintenance planning of actual case studies are given as an example.
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