Passive Reinforcement Research Articles

Design of prestressed concrete precast road bridges with hybrid simulated annealing

This paper describes one approach to the analysis and design of prestressed concrete precast road bridges, with double U-shaped cross-section and isostatic spans. The procedure used to solve the combinatorial problem is a variant of simulated annealing with a neighborhood move based on the mutation operator from the genetic algorithms (SAMO). This algorithm is applied to the economic cost of these structures at different stages of manufacturing, transportation and construction. The problem involved 59 discrete design variables for the geometry of the beam and the slab, materials in the two elements, as well as active and passive reinforcement. The parametric study showed a good correlation for the cost, geometric and reinforcement characteristics with the span length, which can be useful for the day-to-day design of PC precast bridges. A cost sensitivity analysis first indicates that a maximum 20% rise in steel costs leads to an 11.82% increase in the cost, while a 20% rise in concrete costs increases the cost up to 4.20%, namely 2.8 times less. The analysis also indicated that the characteristics of the cost-optimized bridges are somewhat influenced by different economic scenarios for steel and concrete costs. Finally, there is a growth in the volume of concrete when the steel cost rises; surprisingly, the variation in the volume of concrete is almost insensitive to its rising price.

Reinforcement Calculated Method Based on Disease Model of DongFu Bridge

On the basis of the static and dynamic testing of the bridge, the original structure model and the model based on stiffness reduction of Dongfu Bridge were analyzed and calculated with the finite element method. The main problem that the anti-shear area of the section in the middle pivot position is not enough is found. On the basis of combination of the passive and active reinforcement styles, a reasonable and feasible reinforcement plan on the girder beam of the main bridge is raised. These conclusions have an important reference value on the bridge reinforcement.

Experiences in Barcelona with the use of fibres in segmental linings

This paper presents the most outstanding experiences regarding the use of fibres as the main reinforcement in precast segmental linings in the metropolitan area of Barcelona. It is known that the addition of structural fibres improves, on the one hand, the mechanical behaviour of the structure during its construction, especially in cases such as the thrust of the jacks, and on the other hand it leads to a reduction of the global costs by reducing the conventional passive reinforcement. The aim of this paper consists in presenting three real experiences that are representative of the application of FRC in urban tunnels and a design methodology to take into account the structural contribution of the fibres. Two particular cases of the application of this design method are presented. In the first case, the use of 25 kg/m 3 of fibres has led to a reduction of 70% of the conventional reinforcement initially proposed in the project. In the second one, which was planned to employ fibres but without considering its structural contribution, the parametric study reflected the possibility of reducing up to a 38% of the rebars adding 25 kg/m 3 of steel fibres in the concrete mixture. In light of good results, construction companies in Spain have become aware of the advantages of using fibres in these structures and have carried out experimental stretches. This attitude has also been influenced by the approval of the new Spanish Code, which includes the FRC as a construction material with design purposes.

Pullout resistance characteristics of chain type retaining system

In this study, we carried out several experiments and implemented three different types of pullout resistive bodies to conduct tests under identical conditions in order to examine the pullout resistance mechanism of chain reinforcement and the resistive elements. Furthermore, pullout resistance characteristics are studied based on laboratory and in-situ tests when steel bar and L-type steel angle are used as passive reinforcement. Comparing theoretical and measured values, the measured yield pullout force was 1.2 - 3 times greater than theoretical values depending on the reinforcement method or normal stress. The result indicates that the yield stress is greater for a higher normal stress, a longer chain and when passive reinforcement is combined. The difference was not significant between bar and L-type steel angle in terms of the increase of the yield pullout force. Key words: Chain-type retaining system, pullout resistance, large scale pullout test.

Design of prestressed concrete precast pedestrian bridges by heuristic optimization

This paper deals with the economic optimization of prestressed concrete precast pedestrian bridges typically used in public works construction. These bridges are made of a precast concrete beam that integrates an upper reinforced concrete slab for the pedestrian traffic. The beam has a U-shape cross-section. Typical span lengths range from 20 to 40m and the width ranges from 3.00 to 6.00m. The study shows the efficiency of heuristic optimization by the simulated annealing (SA) and the threshold accepting (TA) algorithms. The evaluation of solutions follows the Spanish code for structural concrete. Stress resultants and envelopes of these structures are computed by direct calculation. Design loads are in accordance to the national IAP code for road bridges. The algorithms are applied to a typical pedestrian bridge of 40m of span length and 6.00m of width. This example has 59 discrete design variables for the geometry of the beam and the slab, materials in the two elements and active and passive reinforcement. The evaluation module includes the limit states that are commonly checked in design: flexure, shear, deflections, etc. The application of the SA and TA algorithms requires the calibration of the initial temperature and threshold, the number of variables modified in each iteration, the length of the Markov chains and the reducing coefficient. Each heuristic is run nine times so as to obtain statistical information about the minimum, average and deviation of the results. Best result has a cost of 38,[email protected]? for the SA algorithm and 38,[email protected]? for the TA algorithm. Finally, solutions and run times indicate that heuristic optimization is a forthcoming option for the design of real prestressed structures.

Statistical interpretation of results of potential mapping on reinforced concrete structures

ABSTRACT This paper addresses the theory for a statistical analysis of the results of potential mapping performed on reinforced concrete structures. The basic assumption for this analysis is that the distribution of potentials for both corroding and passive steel can be adequately described by a normal density distribution. This treatment is applied to a number of situations as to exemplify the calculation procedure. Eventually, this statistical analysis will result in improved potential criteria to distinguish corroding from passive steel reinforcement. Moreover, this procedure allows to assign to any value of the measured potential the corresponding probability to deal with an active corrosion process and provides a better estimate of the amount of corroding steel present in the structure.

Full-scale testing of a fibre-reinforced concrete footbridge

The design, testing and construction of a 33·6 m pre-stressed drop-in girder made with Ductal ultra-high performance fibre-reinforced concrete is presented. This girder was part of a pedestrian bridge that required the largest known volume of this material cast in a single pour at the time of production. The girder was post-tensioned and utilised stainless steel rebar at its ends as well as non-corrosive glass fibre reinforced polymer rebar as passive reinforcement. A two-day 90°C steam cure was performed prior to testing. Embedded thermocouples, load cells and displacement transducers were used to instrument two simply supported tests. In the first test, 90% of the factored load was applied, first symmetrically over the top of the slab and secondly eccentrically over half of the top of the slab. The results show that the measured self-weight was close to predicted and displacements were smaller than predicted with full recovery upon load removal, indicating no inelastic behaviour. No cracking was observed during testing or after installation.

Experimental behavior and design of a new kinematic isolator

This paper reports on the experimental behavior and design of a rolling self-centering precast prestressed pile (PPP) isolator. This isolation device was developed as an alternative for use in light-weight and low-cost housing located in sites with poor soils. The specimens tested had a spherical and flat–spherical top and bottom end rolling surfaces, respectively, and they generate the self-centering action by a prestressed central cable. Eight full-scale PPP specimens were tested, including two with yielding reinforcement bars at the rolling interface. The axial load on the tested specimens was constant and equal to 147.2 kN, which corresponds to a typical vertical load on a PPP for the types of building considered in this research. The experimental results validate in general the predictions of the analytical model presented; however, the theoretical model is not able to predict the energy dissipation observed in specimens without passive reinforcement, and the apparent larger initial flexibility in the element force–displacement relationship. Additionally, a procedure is developed for simplified isolation design of light-weight structures, which combines the use of flat–spherical PPPs and steel–Teflon sliders in parallel. As an example, the procedure was tested on a design alternative for a low cost housing complex built under conventional fixed-to-base foundations.

Corrosion rate of steel in concrete: evaluation of confinement techniques for on-site corrosion rate measurements

Earlier on-site investigations and laboratory studies have shown that varying corrosion rates are obtained when different commercially available instruments are used. The different confinement techniques, rather than the different electrochemical techniques used in the instruments, are considered to be the main reason for the discrepancies. This paper presents a method for the quantitative assessment of confinement techniques. The assessment is based on monitoring the operation of the corrosion rate instrument and following the current distribution between the electrode assembly on the concrete surface and a segmented reinforcement bar embedded in the concrete. The applicability of the method was demonstrated for two commercially available corrosion rate instruments based on different confinement techniques. The method provided an explanation of the differences in performance of the two instruments. Correlated measurements of linear polarisation resistance and macro-cell currents allowed the determination of calibration factors. Both instruments overestimated the corrosion rate of passive reinforcement, but underestimated the corrosion rate of reinforcement with intense localised corrosion.

The Dimensioning of Dowels for the Stabilization of Potentially Unstable Rock Blocks on the Walls of Underground Chambers

The analysis of dowels (non pre-stressed passive reinforcements) for the stabilization of potentially unstable rock blocks due to sliding is one of the most interesting problems concerning the static of underground rock chambers. The stabilizing force produced by dowels is not in fact known a priori and it depends on the dowel–rock interaction. The parameters that influence the problem are not only the geometrical ones of the dowel, but also the mechanical characteristics of the rock and the orientation of the displacement vector of the block. A detailed study of the dowel–rock interaction has been carried out in this work. This analysis is able to lead to the evaluation of the axial and shear forces in the dowel in correspondence to the crossed discontinuity of the block. The vectorial composition of these two forces constitutes the stabilization force of the dowel. An extensive parametric analysis then made it possible to determine the great variability of the stabilization force of the dowel in function of the influential parameters of the problem. The graphics that were obtained can be considered a useful design instrument as they quickly allow the dimensioning of the dowels to be done to reach the required safety factor for the rock block.

An analysis of the action of dowels on the stabilization of rock blocks on underground excavation walls

Underground excavation in fractured rock masses can lead to instability of rock blocks on its perimeter. Among the stabilization measures that are often used, dowels or passive bolts play an important role. These passive reinforcement measures cannot, however, be easily considered in simplified calculation techniques based on the Limit Equilibrium Method (LEM). The analysis of such elements cannot be separated from rock-dowel interaction following the displacement of the block.

EIS study of passivation of austenitic and duplex stainless steels reinforcements in simulated pore solutions

Abstract Stainless steel reinforcements have proved to be one of the most effective methods to guarantee the passivity of reinforced concrete structures exposed to highly chloride-contaminated atmospheres. In the present work, the corrosion behaviour of two traditional austenitic stainless steels (AISI 304 and 316L types), and one duplex type (2205) are compared with that of a low-nickel, much more economic, austenitic type (204Cu). Ribbed and ground bars of these four materials are studied in non-carbonated and carbonated, saturated Ca(OH)2 solutions with different chloride contents, using electrochemical techniques. The low-frequency time constant represents the charge transfer resistance (Rt) in parallel with the double layer capacitance. The Rt value of the passive reinforcements seems to be related with the quality of the passive layer. Rt values increase with the immersion time in the testing solutions and decrease with the chloride content. Moreover, Rt tends to increase when the ribs of the bars are removed.

Ductal – A High-Performance Material for Resistance to Blasts and Impacts

Reactive Powder Concrete (RPC) is a cementitious material consisting of cement, sand, silica fume, silica flour, superplastiser, water and high strength steel fibres. The material, developed by Bouygues the parent company of VSL, is marketed under the brand name of Ductal®. Production in Australia commenced in early 2003. Ductal is almost self placing, has a compressive strength of 160–200 MPa and a flexural strength of 30–40 MPa. In structural applications, Ductal is generally used without any passive reinforcement. Very fine high strength steel fibres are able to withstand secondary tensile stresses. Prestressing counterbalances the main tensile stresses due to bending. The durability properties of Ductal and its resistance to aggressive agents are orders of magnitude better than conventional high performance concrete. A predicted design life in marine environments of several hundred years is achievable. The unique characteristics of Ductal, including exceptionally high energy absorption capacity and resistance to fragmentation, make it ideal for panels and components that need to perform under explosives, impact or shock loads. The flexural toughness is greater than 200 times that of conventional fibre reinforced concrete. In May 2004, the performance of seven Ductal panels was evaluated in two large explosive trials performed at Woomera (South Australia). The panels performed remarkably well, exhibiting high levels of ductility and no signs of fragmentation. In further tests, 100mm thick Ductal blast resistant panels have been shown to effectively resist impacts by blast produced fragments using fragment simulated projectile tests. These panels can also withstand impacts from the highest class of rifle projectiles in the Australian Standard AS 2343. In June 2005, the first Ductal protective panels manufactured to provide resistance to blast were supplied to the Australian Government and installed on an Australian Government building in a high risk international location.

Analysis of a kinematic self-centring seismic isolator

This investigation deals with the analytical formulation and experimental validation of a prestressed reinforced concrete seismic isolator with kinematic constraints at both ends. The kinematic isolator was proposed initially as a low-cost solution for seismic protection of low-income people housing usually placed at the periphery of big cities where regular to bad soil conditions are common. So, the isolator is also a pile foundation with a central prestressed cable and two rolling steel surfaces at the top and bottom ends. By varying the shapes of the end rolling surfaces, different force–deformation constitutive relationships for the isolator may be obtained. Energy dissipation is introduced by yielding of passive reinforcement at the rolling interphase. Apart from stating the large-deformation formulation of the element, several relevant aspects of the behaviour of these devices are studied herein, such as the increase in the tension of the central prestressed cable, responsible for the self-centring action of the isolator, the floor uplift that results from the geometry of the isolator, and the vertical stability of the system. Experimental and theoretical results obtained for a group of 9 testing specimens show an excellent agreement in the force–deformation constitutive relationship. Although not the intent of this article, the device proposed may be extended directly as a coupling beam element for shear wall systems. Copyright © 2006 John Wiley & Sons, Ltd.

Chloride threshold dependence of pitting potential of reinforcements

Chloride threshold is one of the key parameters needed for service life prediction of concrete structures. The main difficulty that the determination of this parameter encounters is that the chloride threshold inducing active corrosion is not a unique value and depends on several variables. Among them the electrical potential exhibited by the rebar surface is the one less experimentally studied, even though it was identified some decades ago as the most comprehensive controlling parameter for the disruption of passivity. In the present paper the influence of the potential of reinforcements embedded in mortar on the chloride threshold has been studied for the first time in a region from +250 to −650 mV with respect to SCE. Identical samples have been tested in order to perform a statistical treatment and so mean values are given. The chloride thresholds found are expressed as total and free chloride as well as chloride/hydroxide (Cl −/OH −) ratio. The results show that chloride threshold is independent of the potential for values more anodic than −200±50 mV (SCE) being this region the typical potentials exhibited by passive reinforcements in atmospheric conditions. For potentials more cathodic than −200±50 mV (SCE), the threshold progressively increases and a linear fitting is possible. The dependence of chloride threshold with the potential approaches to the understanding why in previous works researchers have found significant differences.

Zum Einfluss der kathodischen Reaktion auf die chloridinduzierte Bewehrungskorrosion

Regarding the mechanisms of reinforcement corrosion there are still contradictions with respect to the controlling/rate-determining factors of the corrosion process. It is often discussed, that the electrolytic resistance of the concrete is the controlling factor and that the corrosion rates can subsequently be calculated from concrete resistivity. However, extensive research carried out by the authors has clearly demonstrated, that instead of concrete resistivity the resistance to cathodic polarisation normally is the controlling factor in the case of chloride-induced macrocell-corrosion. Generally, cathodic control can be related to restricted oxygen diffusion or activation control. In the present paper, these relationships are discussed in detail by results of numerous tests on the cathodic polarisation behaviour of passive reinforcement. For simple defined geometrical conditions simulating practical cases it is shown by a numerical analysis that the resistance to activation is normally the controlling factor for the corrosion rate and that oxygen diffusion has only to be taken into account, when the concrete is permanently water saturated or extremely dense. To verify, whether it is correct to estimate corrosion rates from resistivity data, tests should be carried out to check the influencing parameters on concrete resistivity and cathodic activation of passive steel surface areas.

Linear polarisation resistance measurements using a potentiostatically controlled guard ring

The use of a sensor controlled guard ring has been developed in recent years to enhance the accuracy of linear polarisation corrosion rate measurements on reinforced concrete structures. The established method of measurement uses a galvanostatically controlled guard ring device. In the method reported in this article the reinforcing steel is polarised potentiostatically by an inner auxiliary electrode and the real time plot of the current response is displayed on a laptop computer which controls the guard ring device. The area of steel polarised is confined by a current applied from an outer guard ring electrode which is controlled by two sensor electrodes positioned between the inner auxiliary and outer guard ring electrode. The potential between the two sensor electrodes is continually monitored, and the current output from the guard ring electrode varied to maintain a constant potential difference between the two sensor electrodes. The method has been validated on an electrical test circuit simulating active and passive reinforcement corrosion. Further testing has been conducted on a reinforced concrete specimen and the results compared with those of commercially available galvanostatically controlled equipment and those of standard unconfined linear polarisation resistance measurements.

Composition of reactive powder concretes

Development of an ultra-high strength ductile concrete designated RPC (Reactive Powder Concrete), was made possible by the application of a certain number of basic principles relating to the composition, mixing and post-set heat curing of the concrete. RPC 200, which can be used under job site conditions similar to those for conventional high performance concretes, can be used in the construction of prestressed structures incorporating no passive reinforcement. RPC800 is suitable for precasting, and can achieve compressive strength values exceeding 600MPa. A value of 810MPa has been obtained with a mixture incorporating steel aggregate.

Creep and shrinkage effects in prestressed beams

The results of a series of tests on prestressed concrete beams show that the addition of longitudinal passive reinforcement can have some adverse effects on the behaviour under serviceability conditions. These arise from the restraint that reinforcement provides to creep and shrinkage deformations in the concrete. High compression stresses can be induced in the longitudinal passive reinforcement by creep and shrinkage in the concrete, while in prestressed reinforcement the initial prestress tensile stresses are reduced by these actions. As a result, when the initial flexural cracks form, longitudinal passive reinforcement may sustain significant compressive stress. Experimental observations are compared with predictions from three methods of analysis. A modified form of the effective modulus method, suitable for use in a design office, was found to give predictions comparable in accuracy to the more sophisticated methods.

Effect of the Bar Diameter on the Bond Strength of Passive Reinforcement in High-Performance Concrete

The paper discusses the bond strength between a high-performance concrete (HPC having a 28-day strength of 95 MPa) and reinforcing bars. This was measured in a series of beam tests in which the concrete type (HPC and ordinary concrete), reinforcement diameter (between 10 and 25 mm), and reinforcement surface condition (smooth and deformed bars) were varied. In general, concrete-reinforcement bond strength increases with the tensile strength of the concrete, and at a hgher rate with smaller reinforcement (a more marked scale effect).