Providing excellent performance as a structural material, concrete has long been deemed essential for modern civilization and recognized as a material that will continue to maintain and support the development of human society. Now that recycling of concrete in a completely closed loop has become technically feasible, concrete is being seen in a new light. This paper reviews the background to this development referring to changes in social systems and the introduction of new technologies. In view of the fact that consideration of the global environment will be required in the future at every step of the production of concrete and concrete structures, this paper goes on to overview the methods for identifying social needs related to concrete structures, the manner in which production systems of structures should meet such social needs, lifecycle design techniques for structures, and techniques for expressing the environmental performance of structures. The authors finally discuss the nature of true recycling and a truly recycling-oriented society, based on the above-mentioned discussions.
Concrete is the most important material employed in public works and building construction projects. The large amount of concrete production causes large energy consumption, resources depletion, CO2 emissions, and other environmental impacts. This paper describes the environmental aspects of concrete and proposes environmental design as a new design paradigm for concrete structures. It also describes design examples to illustrate the rationality of environmental design.
This paper describes a model to predict the acoustic absorption of Enhanced Porosity Concrete (EPC). The acoustic absorption coefficient was determined experimentally using an impedance tube, while an electro-acoustic analogy was implemented to develop the predictive model, considering the pore structure of EPC as a series of resistors and inductors. The physical features of the pore network were experimentally characterized using image analysis and a pore volume characterization technique. A parameter termed “structure factor” was introduced to account for the increased density of air that is not displaced by the acoustic wave pressure. The maximum acoustic absorption coefficient was found to decrease linearly with an increase in the structure factor. The development of this model and its correlation with physical measurements enable the prediction of acoustic absorption in EPC based on the geometric features of the pore structure. This model enabled a parametric study to be conducted to ascertain the effects of pore size, aperture size, porosity, and specimen thickness on acoustic absorption. An optimal pore to aperture diameter ratio was observed to exist, that maximizes acoustic absorption. The parametric study is believed to be able to aid in the design of EPC for acoustic absorption by better understanding the type of pore features that should be targeted for best performance.
This paper discusses concrete structure design considering the environmental impact generated in the manufacture, construction, service, maintenance, demolition, and disposal and recycling processes of the structure. Environmental impact, including its reduction, is considered as a performance parameter of the concrete structure, similarly to serviceability, safety, and durability. Two types of design methods that respectively verify environmental performance and consider environmental performance are proposed. The design method for verifying environmental performance is applicable when environmental performance is required of a concrete structure. In this type of design, verification of environmental performance is carried out in the planning stage and is followed by inspection in the planning stage as well as the operation stage. The design method for considering environmental performance is applicable whether or not environmental performance is required of a concrete structure. In this type of design, verification or selection of environmental performance is carried out in the planning stage, and inspection is performed in the operation stage. Regarding the verification method, two types of methods are proposed according to the diversity of environmental performance requirements.
While at present mostly recycled into road subbase, the amount of demolished concrete in Japan is expected to increase rapidly and exceed the demand for road subbase in the near future. To promote the recycling of concrete, a technology to produce high-quality recycled aggregate has been developed. This technology employs the heating and rubbing method. In order to investigate a future concrete recycling system, first of all, a specific model considering indices of economic activity is established to forecast the amount of demolished concrete in the future. Furthermore, an input-output table is extended by a detailed description of concrete-related industries such as construction, aggregate, cement, and ready-mixed concrete, and several concrete recycling processes have been added. The linear programming model connected to the input-output table assumes that the technology will be introduced in 2020. A subsidy for high-quality recycled aggregate and a carbon tax are found to be effective ways to promote the early introduction of the technology. This series of analysis can be widely used in other countries for investigating suitable recycling systems focusing on the cement and concrete industry as well as the applicability of each individual concrete recycling technology.
The experimental results obtained on steel rebars with coatings of Si-based particles are described in this paper. It is well known that adding Si to metals generally increases their corrosion, oxidation, and erosion resistance. In this study, it is chosen to use Si-based coatings because of their proven capacity for corrosion protection in Cl- environments. In addition, this paper presents impedance spectroscopy as an alternate technique in determining the corrosion activity of coated steel rebars. Impedance spectroscopy is an electrochemical measurement technique, which measures the response of a system to an applied alternating signal. A major benefit of the technique is its ability to monitor both the bulk and interfacial responses. This research was conducted to characterize the impedance response of several coatings on steel rebars, and in so doing, further understanding of the corrosion susceptibility of the systems. The results suggest that impedance spectroscopy may be useful for monitoring corrosion activity on coated rebar.
Chloride binding behaviors of cement, which is important for estimating the chloride penetration into concrete, are investigated. Firstly chloride binding isotherms of major cement hydrates are examined and then a chloride binding iso-therm of cements is estimated from the binding isotherms of these hydrates. The results show that the monosulfate hydrate and C-S-H phases have significant chloride binding capacities, whereas ettringite and portlandite have no capacity to bind chlorides. However, there is a possibility that ettringite in cement paste dissolves at high chloride concentration to produce Friedel's salt. The chloride binding by monosulfate hydrate is attributed to the formation of Friedel's salt and the binding isotherm shows a good fit to a Freundlich-type adsorption. The chloride-binding isotherm of C-S-H shows a good fit to a Langmuir-type adsorption. The binding capacity of C-S-H is saturated at about 0.6 mmol/g at high chloride concentration over than 2 M. Based on the chloride binding isotherm of monosulfate hydrate and C-S-H, the chloride binding isotherms of cements are shown to be realistically predicted.
Chloride-induced deterioration is the most important deterioration phenomenon in reinforced concrete structures in marine environments. When a crack occurs in cover concrete, it may initiate and accelerate corrosion of steel reinforcements embedded in the concrete. The performance of the reinforced concrete structure may subsequently decrease even in the early stage of its service life. With the aim to clarify the mechanism of chloride-induced deterioration, this paper reports the results of experimental investigation on chloride ion transportation in cracked concrete and proposes a simulation model for chloride ion transportation in cracked concrete. The zone affected by cracking was treated as the exposed surface of concrete in the proposed model, where chloride transportation was assumed to be governed by the concentration of the chloride ion solution in the crack. In addition, the effects of the crack width and an apparent diffusion coefficient through the cracks on chloride ion transportation were numerically investigated and the applicability of the proposed model was discussed.
The seismic retrofit of existing RC buildings, which are vulnerable to seismic excitation, remains an active area of research. Emergency retrofit is necessary for rehabilitation of damaged RC buildings immediately after earthquake occurrences. The emergency retrofit technique is a laborsaving, quick and dry process. From such viewpoints, an emergency retrofit of RC columns damaged in earthquakes is proposed in this study. The pre-tensioned aramid fiber belt acts as a shear strengthening element as well as an axial capacity recovery method since the belt provides transverse confinement for the column. The retrofit method sufficiently recovers the earthquake performance as long as the damage caused by the earthquake is within the moderate level and the column can sustain the vertical load.
A multi-mechanical model that explicitly takes into account corrosion cracks in structural safety performance is presented to deal with materialized corrosive substances around steel bars and equilibrated damage in structural concrete. The multi-mechanics of corrosive product and cracked concrete are integrated with nonlinear multi-directional fixed crack modeling so that corrosion cracks in structural concrete can be simulated in a unified manner. Structural analysis of corroded RC beams is carried out for experimental verification of the multi-mechanical model in terms of shear capacity and ductility. RC beams, which primarily fail in shear or flexure, are discussed and special attention is addressed to the conversion of failure modes and the absolute capacity. Consideration of inherent cracking on corroded RC members is proven to be crucial for structural performance assessment and the anchorage failure of longitudinal reinforce-ment is found to cause considerable decay of member capacity.
The mechanical effect of fractured web reinforcement on structural safety was experimentally investigated by intentionally avoiding hooks or anchorage devices at the extreme ends of stirrups, which were replicas of web steel damaged by corrosion or alkali-aggregate reaction of concrete. Significant reduction in shear capacity was experimentally found without yielding of web reinforcement when the web steel anchorage was incomplete. A marked difference in failure crack patterns was also observed compared with the sound RC specimen. Longitudinal cracks were ultimately formed along the main reinforcement where the unprocessed edges of the stirrups lie. Non-linear finite element analysis was employed to investigate and simulate failure processes and static capacity. The bond deterioration zone of the web reinforcement was computationally assumed to be ten times the diameter of steel bars from the cut-off trimming of steel bars. This simple assumption in nonlinear computation was verified to be acceptable for performance assessment of damaged reinforced concrete with fractured stirrups.
Deterioration of reinforced concrete caused by corrosion of reinforcing steel under chloride environment is experimentally studied. Onset of corrosion and nucleation of cracking are estimated by acoustic emission (AE), comparing with the chloride content. Corroded areas are evaluated by the half-cell potential and the polarization resistance. To compensate the potentials, the inverse boundary element method (IBEM) is applied. Results show that AE parameters can effectively evaluate the onset of corrosion and the nucleation of cracking, which are remarkably comparable to chloride concentration. The decrease in the half-cell potential is observed, following high AE activities. It is concluded that AE technique give an earlier warning of corrosion than the half-cell potential measurement. In addition, crack types at onset of corrosion and nucleation of cracking are identified from AE parameters. Corroded areas estimated by IBEM solutions are in good remarkable agreement with those of visual inspection, although results of electrochemical techniques are marginally successful.
The anchorage zones of post-tensioned concrete members can be divided into local and general zones. The present study estimated the ultimate strengths of post-tensioned beams tested to anchorage failure using the AASHTO LRFD approximate stress analysis/design method, the critical section concept in which the strengths of the node-strut interface and local zone-general zone interface are examined, the bearing strength equation which considers the confinement effect due to reinforcing bars, and the nonlinear strut-tie model approach which incorporates nonlinear techniques in the selection, analysis, and verification processes of a strut-tie model, thereby evaluating their respective validity in the analysis and design of post-tensioned anchorage zones. The ultimate strengths of the post-tensioned beams in the nonlinear strut-tie model approach were estimated by checking the occurrence of a nodal zone failure mechanism, the structural instability of the selected strut-tie model due to the strength reduction of the struts and ties during the incremental loading steps, and conformity to the strut-tie model's geometric compatibility condition.
A large number of reinforced concrete buildings collapsed or were heavily damaged during the 6.8 magnitude Zemmouri earthquake that struck northern Algeria on 21 May 2003. A technical survey campaign was launched to assess damages and losses in the most affected areas in Algiers and Boumerdes prefectures. Reinforced concrete frame structures and apartment buildings were the most damaged categories. Buildings with shear walls or steel frames performed well. Analyses of collected data showed that damage was randomly distributed in different localities. Results and observations highlighted the following deficiencies: poor quality of materials, poor quality of construction, insufficient element sizes and dimensions, lack of good detailing and poor design. This paper also reviews the seismic hazard preparedness and disaster response, including emergency relief, administrative, technical and scientific issues, and social issues. Finally, lessons from the catastrophic event and future needs are summarized.
Soon after the 21 May 2003 Zemmouri (Algeria) earthquake that had stricken mainly Boumerdes and Algiers prefectures, inventory investigations were conducted on the existing constructions and facilities in order to evaluate losses and damages that affected these areas. This paper presents observations and analyses concerning a locality judged to be typical and might be considered as a case for the entire affected prefectures. A recently urbanized small zone of approximately 4 km2 located in Algiers Prefecture and near the border of Boumerdes Prefecture was investigated in detail. A total of 725 buildings were evaluated. Analysis of the collected data showed that damage was randomly distributed in the area. This result strengthens the assumption of low-quality materials and inadequate design, in relation to other assumptions, such as the effect of soil conditions. Reinforced concrete constructions were the most affected among the various other types, with approximately 18% of constructions of this type having suffered moderate to heavy damage. Data analyses showed that the most affected constructions were those in the range from 5 to 6 stories while buildings with only one story did not suffer any damage. Typical observed damages were, for the most part due to poor longitudinal and/or transversal reinforcements in the columns, very poor concrete quality, lack of shear reinforcements at beamcolumn joints, soft stories, pounding and the formation of short columns. Restoration of facilities and retrofitting practice on damaged buildings are reviewed in some cases.