Daniel Cusson

Abstract: The development of an effective strategy for the inspection and monitoring of the nation�s critical bridges has become necessary due to aging, increased traffic loads, changing environmental conditions, and advanced deterioration. This paper presents the development of a probabilistic mechanistic modeling approach supported by durability monitoring to obtain improved predictions of service life of concrete bridge decks exposed to chlorides. The application and benefits of this approach are illustrated on a case study of a reinforced concrete barrier wall of a highway bridge monitored over ten years. It is demonstrated that service life predictions using probabilistic models calibrated with selected monitored field data can provide more reliable assessments of the probabilities of reinforcement corrosion and corrosion-induced damage compared to using deterministic models based on standard data from the literature. Such calibrated probabilistic models can help decision makers optimize intervention strategies as to how and when to repair or rehabilitate a given structure, thus improving its life cycle performance, extending its service life and reducing its life-cycle cost.

Abstract: This paper investigates the impact of internal curing on the service life of high-performance concrete (HPC) bridge decks by using analytical models to predict the times to onset of corrosion, onset of corrosion- induced damage, and failure of decks. Three bridge deck design options were compared: (i) normal concrete deck; (ii) HPC deck with supplementary cementing materials (SCM); and (iii) HPC deck with SCM and internal curing. It was found that the use of internal curing can extend the service life of high-performance concrete bridge decks by more than 20 years, which is mainly due to a significant reduction in the rate of penetration of chlorides in concrete as a result of reduced early-age shrinkage cracking and reduced chloride diffusion. Compared to normal concrete, HPC with SCM and internal curing was predicted to add more than 40 years to the service life of bridge decks in severe environmental conditions. Life-cycle cost reductions of 40% and 63% were estimated when conventional HPC and internallycured HPC were used in bridge decks instead of normal concrete, respectively, despite the fact that the inplace unit cost of internally-cured HPC can be 4% higher than that of conventionally-cured HPC, which in turn can be up to 33% higher than that of normal concrete. This is due to a longer service life and less frequent maintenance activities offered by low-permeability HPC bridge decks.

Abstract: The performance of eight commercial corrosion-inhibiting systems was assessed in the field over ten years on reinforced concrete barrier walls of a highway bridge that was subjected to severe environmental conditions. These systems were composed of one or more of the following components: anticorrosion concrete admixtures, reinforcement coatings, and concrete surface coatings/sealers. The field evaluation consisted of annual surveys of corrosion potential and corrosion rate, as well as visual inspections and testing of concrete cores. After ten years, the main reinforcement of the barrier walls, at a depth of 75 mm [3 in.], was found in relatively good condition due to an initially good quality concrete. Special bars embedded at a depth of 13 mm [1/2 in.] in the barrier walls showed signs of advanced corrosion for all systems, however, no visible signs of corrosion were found on 25 mm [1 in.] deep bars. Non-destructive corrosion evaluation over the 25 mm [1 in.] deep ladder rebars indicated that the system containing the inorganic anticorrosion admixture provided consistently lower risks of corrosion, followed by systems containing organic anticorrosion admixtures, in comparison to the control system and other systems. The low concrete permeability and different stability of the protective layer forming on the bars may explain the observed differences in the effectiveness of these systems.

Abstract: This project assessed the five-year field performance of nine commercially-available, corrosion-inhibiting systems on reinforced concrete bridge barrier walls, and their laboratory performance using accelerated testing. The corrosion-inhibiting systems included concrete admixtures, reinforcing steel coatings, and/or concrete surface coatings/sealers. This paper reports on the laboratory testing that included assessment of the effect of the corrosion inhibitors on the oxidation and reduction reactions using the cyclic voltammetry method, and their effectiveness in delaying or reducing corrosion by measuring chloride thresholds and corrosion rates. Although laboratory testing could not exactly simulate field conditions, it was useful for identifying the effects of the corrosion inhibitors on the corrosion reactions, and providing supporting information for the field evaluation.

Abstract: This project assessed the five-year field performance of nine commercially-available, corrosion-inhibiting systems on reinforced concrete bridge barrier walls, and their laboratory performance using accelerated testing. The corrosion-inhibiting systems included concrete admixtures, reinforcing steel coatings, and/or concrete surface coatings/sealers. This paper reports on the field testing. The field evaluation consisted of annual corrosion surveys of half-cell potentials and corrosion rates, and remote monitoring of the environmental conditions using embedded instrumentation.

Abstract: The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m3 of saturated LWA in HPC, providing 27 kg/m3 of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.

Abstract: This paper presents a maturity concept and approach of applying the maturity method for predicting the development of mechanical properties of high-performance concrete over time. An examination of the activation energy and assumptions taken in the developmental process of the existing maturity method are first presented to better understand the capabilities and limitations of the maturity method, which is supported by new and existing experimental evidence. A practical approach is then proposed in which the activation energy is determined mathematically by minimizing the error between the predictions and experimental data. It is based on a new concept that the same maturity is defined only when different specimens of the same concrete have achieved the same level of a specified property under different curing temperatures. This concept makes the maturity approach more robust and more general for application to different concrete mix formulations and different types of property. The accuracy and feasibility of the proposed approach were successfully confirmed by experimental evidence.

Abstract: This paper presents a systematic approach for the experimental testing and analysis of the early-age thermo-mechanical behaviour of large prismatic high-performance concrete specimens under restrained autogenous shrinkage and realistic temperature conditions. The loading system can apply a partial degree of restraint in order to enable the characterisation of high-performance concrete specimens without premature termination of the test in case of significant restrained shrinkage. The instrumentation system can measure the strains and other parameters from the setting time with high accuracy and reliability. The analysis method takes into account the temperature effects on the measured properties and provides equations to determine the time-evolution of shrinkage, thermal expansion, stiffness and creep of the concrete. Results from the testing of 200x200x1000 mm specimens made with a 0.34 water-cement ratio concrete are presented, analysed and discussed in the paper to demonstrate the application of the proposed approach.

Abstract: Six proprietary concrete repair systems were evaluated for their performance and durability under field conditions. The fieldwork consisted of repairing corrosion-damaged reinforced concrete barrier walls of a highway bridge, and installing embedded instrumentation for their continuous monitoring over three years. The results indicated that the proprietary repair systems reduced the one risk of corrosion in the patches; however, the risk of corrosion in the existing concrete was not reduced. All concrete repair systems suffered from shrinkage cracking.

Abstract: This paper compares the mechanical performance of highly-confined columns cast with normal concrete (NC) vibrated into place to ensure proper filling and consolidation equivalent to that of identical columns cast with self-consolidating concrete (SCC). The tested columns had nominal concrete compressive strengths of 40 to 80 MPa. Two confining stirrup configurations representing different degrees of confinement were used. The confining stirrups had nominal steel yield strengths of 400 to 800 MPa. A total of 16 columns were tested in this experimental investigation: 11 were cast either with NC or SCC in reinforced sections; 5 accompanying columns were cast without reinforcement. Three of the unreinforced columns were tested in uniaxial compression to determine the overall concrete compressive strength of the large-scale columns, while two others were cored to determine the distribution of the in situ compressive strength and modulus of elasticity along column height. The test results on reinforced columns showed that SCC yielded greater ductility, although it developed slightly lower ultimate compressive strength than NC. The study also confirmed that an increase in the stirrup yield trength can generate a high degree of confinement in well confined concrete columns, provided that stirrup spacing is kept small. The coring of unreinforced concrete columns demonstrated that the distribution of in situ properties over the column height is more homogeneous in the case of SCC than NC, which was also found adequate in general.

Abstract: Results of a study of reinforcement corrosion in four repaired concrete slab sections taken from an old bridge are presented, as well as results measured on electrochemical cells. Significant evidence is provided to help the inspection engineer to interpret the corrosion survey data taking into account the specifics of the environmental conditions that prevailed during the survey. The measurements comprised half-cell potential, linear polarization, and concrete resistivity, which are known to be sensitive to the ambient environment, especially to oxygen and water in concrete. Some semi-destructive tests, including chloride concentration, chloride permeability, and carbonation depth, were also carried out to assist the analysis and to support the results of the nondestructive corrosion testing techniques. The concrete cover of the slab samples was later removed to assess the actual state of corrosion of the reinforcement and to compare it to the corrosion predicted from the corrosion surveys. This study shows that each corrosion measurement technique has its specific advantages and limitations. Better prediction of corrosion can be obtained by analyzing the data collected from various evaluation methods with careful consideration for the effects of environmental conditions.

Abstract: An assessment of newly reconstructed reinforced concrete barrier walls on a highway bridge in Montreal indicated intense transverse cracking only a few days after concreting. Subsequent inspection of other concrete bridges in urban areas confirmed that such cracking is not uncommon in rehabilitated bridges. Analytical models, together with field and estimated data, were used to study the magnitude and roles of the factors that are the probable cause of this cracking. The results indicate that a combination of factors, such as temperature gradients, differential shrinkage and vibration due to traffic circulation, generated stresses that exceeded the low tensile strength of the young concrete.

Abstract: Designers presently have no quantitative rules to evaluate which of several proposed repair designs is the most appropriate in a given situation. A numerical model that would allow us to compare, on a quantitative basis and with relatively little work, different repair scenarios would be very helpful. This paper describes the problems that need to be solved in order to develop a realistic numerical model of the mechanical behavior of thin concrete repairs. The �critical thickness� concept is defined as the superficial zone where there are steep relative humidity and temperature gradients due to daily and seasonal weather changes. Cracking of the repair layer is maximum within this �critical thickness� because this zone is subjected to significant strain cycles.

Abstract: Despite the expanding need for concrete repair, the lack of comprehensive data and suitable guidelines leaves designers with some uncertainty as to how to proceed with the design and execution of durable repairs. To achieve lasting repairs, the properties of the repair material and substrate must be properly matched. This paper first discusses the failure types frequently encountered in patching, and the repair types used to rehabilitate structures. Then, a comprehensive list of the relevant material and structural characteristics that should be considered in the design of durable repairs and the selection of compatible patching materials is described.

Abstract: The postpeak behavior of high-strength concrete (HSC) columns, loaded in axial compression, is characterized by strain localization in the concrete leading to the formation of an inclined shear failure plane separating the concrete into two wedges restrained by the cohesion of concrete and the reinforcement steel. A model capable of predicting this postpeak load-displacement behavior of confined HSC columns loaded in axial compression is presented. The proposed descending branch is consistent with the physical phenomena observed from axial compression tests on 62 confined HSC columns from three independent research projects. The proposed model shows that the pseudobrittle failure of HSC columns, loaded in compression, comes from the fact that the residual axial capacity of the column after failure is mainly controlled by the lateral load carried by the transverse reinforcement and therefore by its yield strength.

Abstract: A stress-strain model for confined high-strength concrete has been developed and calibrated against the test results from 50 large-scale high-strength concrete tied columns tested under concentric loading. The effects of the concrete compressive strength, tie yield strength, tie configuration, transverse reinforcement ratio, tie spacing, and longitudinal reinforcement ratio are accounted for in the proposed stress-strain model. The determination of the strength and ductility of confined concrete is based on the computation of the effective confinement pressure, which depends on the stress in the transverse reinforcement at maximum strength of confined concrete, and on the effectively confined concrete area. A method is proposed to compute the stress in the transverse reinforcement at maximum strength of confined concrete.

Abstract: This paper presents an experimental study of the behavior of large-scale high-strength concrete columns confined by rectangular ties under concentric loading. Effects of key variables such as the concrete compressive strength, the tie yield strength, the tie configuration, the transverse reinforcement ratio, the tie spacing, the longitudinal reinforcement ratio, and the spalling of the concrete cover are studied in this research program. The behavior of high-strength concrete columns is characterized by the sudden separation of the concrete cover, leading to a loss of axial capacity before the lateral confinement becomes effective. After the concrete is completely spalled, important gains in strength, toughness, and ductility are recorded for the concrete core of well-confined columns.

Abstract: Superabsorbent polymers (SAP) possess a number of features that make them attractive for use in many different applications. The aim of this chapter is to present existing and foreseen opportunities for the use SAP in many different functions to improve the performance and durability of the built environment. Two case studies are also presented in this chapter: one on a thin-wall architectural structure in Germany, and another on shotcreting of wall panels in Denmark.

Abstract: The main structural/material requirements for the design of durable repairs to concrete structures are reviewed. A strategy for durability assessment of repaired concrete structures is also outlined, including examples of various instrumentation and data analysis techniques that are found particularly useful for fi eld research. Three selected case studies are presented, each featuring some of the key methods used to assess the in-service performance and durability of repaired concrete structures, along with the main fi ndings obtained in these fi eld studies. The chapter concludes by providing recommendations to achieve durable repaired concrete structures.

Abstract: Proper curing of concrete is important to ensure that it achieves its intended performance and durability. Conventionally, this is achieved through external curing, applied after mixing, placing and finishing. As demonstrated in the previous chapters of the present report, internal curing (IC) is potentially a very promising tool in providing additional moisture in concrete for a more effective hydration of the cement. This chapter summarizes the main benefits of internal curing and presents case studies in which those benefits could be observed in existing concrete structures.

Abstract: The 1990s saw a significant increase in the use of high-performance concrete for its higher compressive strength and lower permeability. This type of concrete, however, has proven to be quite sensitive to cracking at early age. A survey of 52 departments of transportation across the USA and Canada revealed that more than 100,000 bridges � about half of the bridges monitored by the respondents � had developed transverse cracking of the deck shortly after construction. Early-age cracking of high-performance concrete in bridges is therefore a growing concern for bridge owners, as durability and service life are adversely affected. Moreover, discrepancies between the observed performance of construction materials in the field and the measured performance of the same materials in the lab are often reported. The current lack of comprehensive field research data and suitable design guidelines leaves a degree of uncertainty about the field performance of new construction and rehabilitation technologies. Because of this uncertainty, designers have difficulty in selecting materials that provide good long-term in-service performance. Field trials that afford the evaluation of design methods, material selection criteria, construction procedures, in-service performance and durability are therefore needed. This contribution presents a strategy for conducting field research and outlines various instrumentation and data analysis techniques used in field monitoring of the early-age performance of concrete structures. It offers summaries of some significant publications on field monitoring, including a case study of a rehabilitated bridge on which severe cracking at early age was observed, measured and analysed. Recommendations for future field research are also provided.

Abstract: Internal curing of concrete can be achieved by soaking porous lightweight aggregate (LWA) in water before its introduction into the concrete mix as a partial replacement for normal density aggregate. This technique is particularly useful for low water-cement ratio concrete, for which self-desiccation can lead to autogenous shrinkage, tensile stresses and cracking at early ages. A research project has been undertaken to develop low-shrinkage high performance concrete for the design of concrete structures with long service life. One specific objective was to optimize the concrete mix design by introducing selected chemical admixtures into the concrete mix by using porous lightweight aggregate as a carrier. Expanded shale lightweight aggregate sand was soaked in a solution of water and given admixtures, such as a shrinkage-reducing admixture (SRA) and/or a corrosion inhibitor (CI), prior to mixing. Several fresh and hardened concrete properties were measured and compared to those of a similar concrete mix, in which the given chemical admixtures were added directly into the mix according to the manufacturer�s specifications. The results showed that this new admixture delivery method produced no adverse effects on the desired fresh and hardened concrete properties, including compressive strength and autogenous shrinkage. The addition of SRA through LWA mitigated chemical interactions between the air entraining admixture and the SRA, which was previously found to reduce the effectiveness of the air entraining admixture. For instance, when SRA was delivered through LWA, it was found that the target air content of 5% could be achieved with 10 times less air entraining admixture.

Abstract: Highway bridges and parking structures, subject to coupled effects of mechanical loads and corrosion, often show early signs of distress such as concrete cracking and rebar corrosion leading to reduced structural performance and shortened service life. One possible solution is to use low-shrinkage low-permeability high-performance concrete (HPC) for bridge decks exposed to de-icing salts and severe loading conditions. A new HPC has been formulated to achieve low shrinkage and low permeability, high early-strength, and 28-day compressive strength of up to 70 MPa. Its mechanical performance and durability have been tested both in the lab and field under severe test conditions, including restrained shrinkage, cycling loading, freezing and thawing cycles, and application of de-icing salts. Prediction models have been developed and calibrated to predict structural performance and service life of concrete bridge decks under severe exposure conditions. Prediction models indicate that bridge decks designed with low-shrinkage HPC can achieve service lives exceeding 100 years. Compared to normal concrete decks, short-to-medium span bridge decks using low-shrinkage HPC could be built at a comparable initial construction cost, but at less than 40% of the life-cycle cost over a 100-year period.

Abstract: The widespread deterioration and some recent collapses of highway bridges have highlighted the importance of developing effective bridge monitoring strategies that can help identify structural problems before they become critical and endanger public safety. A typical major urban centre may possess several hundreds of bridges, which makes it impractical to upgrade all these bridges with surface-mounted sensors to monitor their structural performance. A two-step approach may be used, in which potentially critical bridges are first identified through a screening process by remote satellite-based monitoring, and then further investigated with ground-based monitoring and detailed inspection. The capability of Canada�s RADARSAT-2 advanced synthetic aperture radar (SAR) satellite is being investigated for use in the first step of the proposed approach, which can help prioritize in-situ monitoring and maintenance of critical bridges. Radar Interferometry (InSAR) is an advanced processing technique applied to radar images of the earth�s surface that can detect very small movements from ground features such as infrastructure systems, including roadway and railway bridges and their major components. By applying InSAR processing techniques to a series of radar images over the same region, it is possible to detect movements of infrastructure systems on the ground in the millimetre range, and therefore identify abnormal or excessive movement indicating a potential problem that needs more detailed ground investigation. A major advantage of this technology is that a single radar image, which can be obtained in darkness and through clouds, can cover a major urban area of up to 100 km by 100 km, and therefore all bridges in the area could be monitored cost effectively. Preliminary results from an application of this technology to the transportation infrastructure in the Vancouver metropolitan area are also presented and discussed.

Abstract: This paper presents the preliminary results of an experimental study to investigate the shrinkage and cracking behaviour of GFRP- and steel-reinforced concrete barriers subjected to real environmental and load conditions. This study was conducted through a collaboration project between the University of Sherbrooke and Ministry of Transportation of Quebec (MTQ). The main objective of this investigation was to evaluate the restrained shrinkage cracking behaviour of median barriers (Type MTQ 202ME) reinforced with GFRP and galvanized steel bars and fabricated using high-performance concrete (Type MTQ XIII) with a compressive strength of 50 MPa after 28 days. The barrier under investigation was incorporated in a new Highway 410 Overpass Bridge, located in Sherbrooke, Quebec, which was cast in June 2010. The barrier included one section reinforced with GFRP bars while the second section was reinforced with galvanized steel bars, which, in turn, was divided into two sub-sections with different amounts of longitudinal reinforcement (8 bars and 12 bars). The barrier was instrumented with a set of sensors, including: fibre-optic strain sensors, vibrating wire strain gauges, electrical resistance strain gauges, and thermocouples to capture the strain and temperature evolutions along the barrier length. The behaviour of the barrier for the first few months is presented and discussed in this paper.

Abstract: Alkali-aggregate reaction (AAR) is a slow expansive reaction that occurs in concrete between the alkali in cement and reactive aggregates, leading to spalling and strength loss of the concrete. This reaction takes years to develop and the speed of reaction depends on several material and environmental parameters. In order to develop and test corrective solutions to this problem in a reasonable laboratory timeframe, it is possible to accelerate AAR by using extremely reactive materials under carefully controlled environments. This paper presents a new approach that was developed to simulate AAR-like expansion and random surface cracking in just a few days without the need for a controlled environment. After testing different approaches, the successful approach consisted of the combined use of pervious concrete and expansive cement paste. It was found that the cracking patterns obtained on cylindrical and prismatic concrete specimens were very similar to those found in real structures affected by AAR.

Abstract: The main goal of this study is to develop low-shrinkage high-performance concrete in order to reduce early age cracking and premature reinforcement corrosion, and to provide long service lives to concrete structures. This paper presents a summary of the results re-lated to the evaluation of the chemical, mechanical, and durability properties of different formulations of normal-density, air-entrained high-performance concrete with a water-cement ratio of 0.35. The effectiveness of two shrinkage reduction measures is evaluated: internal curing with saturated lightweight aggregate, and a shrinkage-reducing admixture. All high-performance concretes developed in this study that used internal curing or a shrinkage reducing admixture (separately or in combination) successfully met all performance requirements in terms of workability, hydration characteristics, volume stability, strength and potential durability under severe exposure conditions simulated in the laboratory.

Abstract: This paper presents an approach for a multi-objective-based management of aging critical highway bridges to improve their life cycle performance with emphasis on improving public safety and security. The proposed multi-objective optimization framework is presented as an effective approach that overcomes some of the limitations and complexities of cost-benefit or risk analyses, where all consequences need to be expressed in monetary terms. The framework prioritizes first the critical bridges and then identifies the risk mitigation measures that can be used to satisfy several possible management objectives such as maximizing public safety and public security, minimizing traffic disruption and minimizing costs. Risk mitigation strategies can include more frequent and/or more in-depth inspections, load rating, monitoring of the structural performance and security of bridges, rehabilitation and strengthening of damaged elements, and protection of weak and vulnerable components against extreme shocks due to natural hazards or intentional attacks. A multi-objective criticality index is proposed as a prioritization criterion that achieves an adequate best trade-off between all identified and conflicting objectives. The implementation of the proposed approach is demonstrated on three examples that illustrate the prioritization process on a hypothetical network of ten critical bridges and the use of different risk mitigation measures, such as health monitoring and deterioration prediction models on bridge structures.

Abstract: This study investigated the simultaneous development of hydration enthalpy and autogenous shrinkage of cement-based systems, with the long-term goal to develop shrinkage and creep models based on thermodynamics. It was found that both autogenous shrinkage and enthalpy change of sealed cement paste systems increased with an increase in hydration time and a decrease in water-cement ratio, which is explained by the fact that both properties depend on the quantity of reaction products formed during hydration. It is anticipated that the change in hydration enthalpy could be used as an index to estimate autogenous shrinkage in cement-based systems.

Abstract: The widespread deterioration and some recent failures of highway bridges have highlighted the importance of developing and implementing effective inspection strategies, including structural health monitoring, which can identify structural problems before they become critical and endanger public safety. Continuous monitoring is becoming necessary due to ageing of bridges, increased traffic loads, changing environmental conditions, and reduced capacities, especially for medium and long-span bridges given the severe consequences of failure. The implementation of monitoring programs can assist in optimizing the in-depth inspection, maintenance, rehabilitation and replacement of bridge structures. The continuous measurements at critical discrete points of a bridge system will allow the assessment of performance with respect to different limit states, including safety and serviceability. Prediction models, updated from such monitoring data, can optimize intervention strategies as to how and when to repair or rehabilitate, thus extending service life and reducing life-cycle costs.

Abstract: High-strength concrete (HSC) columns can achieve a ductile behaviour when sufficiently confined by high-strength lateral steel reinforcement. The full potential of high-strength steel, however, may not be achieved in lightly confined HSC columns, since the stress in the confining steel may not reach the expected yield stress usually assumed in design. Different approaches that determine the actual stress in the confining steel at peak stress of confined concrete for prismatic or cylindrical columns have been developed, namely (i) an iterative approach, and (ii) a direct approach assuming an equivalent circular column concept. In order to eliminate some of the complexities and assumptions used in these models, a new direct and simple approach is proposed. It is based on the compatibility of strains and equilibrium of forces in the column cross-section, and on actual deformations measured in the confining steel of 50 large-scale HSC columns made with widely different concrete strengths, steel yield stresses and confinement pressures. The proposed model shows that the effective stress in the confining steel does not exceed 400 MPa in lightly confined HSC columns. In this case, the use of high-strength steel for the confining reinforcement is not necessary. On the other hand, in sufficiently confined HSC columns, much higher lateral stresses can be achieved if high-strength steel is used with yield stresses up to 800 MPa, ensuring a very ductile load-carrying behaviour. Predictions of the peak stress and peak strain of 50 large-scale confined HSC columns showed a very good agreement with the experimental results.

Abstract: A new analytical approach to determine the ageing creep coefficient of high performance con-crete under restrained autogenous deformations is proposed. The results show that changes in creep coeffi-cient under variable stress conditions can be determined from time of setting. These changes were very high shortly after the setting time and decreased to very small changes by the age of two days. The early-age vis-coelastic behavior was also found to vary with the quantity of internal curing water provided in concrete through the use of presoaked lightweight aggregate for the prevention of self-desiccation. The early-age creep coefficients determined for high performance concrete under variable stress conditions were found to be more than 50% smaller than those predicted by existing creep models mainly developed from standard compressive creep tests on normal strength concrete. Predictions of developing concrete stresses using the creep coeffi-cients determined experimentally with the proposed approach agreed very well with measured stresses.

Abstract: This paper presents an experimental study of the behaviour of fibre-reinforced high-strength concrete columns under concentric loading. Twelve cylindrical columns having a length of 1400 mm and a diameter of 300 mm were built using concrete containing different volumes of synthetic fibres. The effects of the amount of synthetic fibres, amount of transverse reinforcement, and concrete compressive strength on the load-carrying behaviour of confined concrete columns were investigated. The results showed that the use of synthetic fibres can help the confining reinforcement to increase the peak strength and deformation of confined concrete. In particular, the use of up to 1% synthetic fibres by volume of concrete increased the load and strain at concrete spalling by 3% and 17%, respectively, when compared to identical concrete columns without fibres. This same addition of synthetic fibres resulted in an increase in the maximum confined concrete strength and corresponding strain by 16% and 29%, respectively. Under very large concrete deformations, however, the synthetic fibres did not significantly influence the post-peak behaviour of these concrete columns. It is concluded that the use of synthetic fibres in high-strength concrete columns allows the columns to reach their ultimate load capacities with lower risks of premature spalling of the concrete cover, thus improving column durability and helping the confining steel to further increase the strength and ductility of confined concrete, especially in seismic areas.

Abstract: The effects of internal curing, type of blended cement and coarse aggregate size on early-age expansion, autogenous shrinkage and strength of high-performance concrete were investigated. To do so, twelve high-performance concrete mixtures were developed and tested under sealed and room temperature conditions. The results were statistically analyzed using the paired comparison design method. It was shown that internal curing of HPC with pre-saturated porous lightweight aggregate allowed significant autogenous expansion and resulted in considerable reduction in net autogenous shrinkage. The type of cement used in concrete, which was either ordinary Portland cement, silica fume blended cement, or slag/silica fume blended cement, had a strong effect on early-age expansion, autogenous shrinkage, and the effectiveness of internal curing. For instance, the concrete specimens made with silica fume blended cement, which yielded the largest autogenous shrinkage strains under sealed conditions, obtained the best reductions in autogenous shrinkage when tested under an internal curing condition.

Abstract: As part of a major rehabilitation project on a multi-story concrete parking structure, a portion of the floor was rebuilt using a concrete containing a hydrophobic admixture, with the aim to delay corrosion of the embedded steel reinforcement. Two sections of an elevated slab and two interior ramps, rebuilt with this concrete, were instrumented for remote monitoring. Two other sections of the elevated slab, rebuilt with normal concrete, were also instrumented and used as references. The field performance of these sections has been monitored for two years since reconstruction in November 2004. In all test sections, the measurements and calculations show that the concrete tensile stresses due to restrained drying shrinkage reached the tensile strengths at early ages, regardless of the type of concrete in place. A visual inspection confirmed the presence of deep cracks in all monitored concrete slabs and ramps. This cracking is more problematic for the floor areas treated with the hydrophobic admixture, where no waterproofing membrane was applied.

Abstract: This paper presents a new experimental approach to determine the coefficient of thermal expansion of concrete at early age, in which sealed concrete prisms (75x75x295mm) are subjected to temperature cycles from 25 to 30°C in an environmental chamber from the time of setting to seven days. The entire test apparatus, including moulds and sensors, was carefully temperature-calibrated to obtain highly accurate measurements. A new calculation method is proposed to eliminate autogenous shrinkage from the measurements before the coefficient of thermal expansion is determined from the thermal deformation obtained as a function of time. For the high-performance concretes tested in this study (with and without internal curing), it was found that the coefficient of thermal expansion decreased towards a minimum value of 8 x 10-6/°C one day after the setting of concrete, and increased gradually up to a value of 10.5 x 10-6/°C at 7 days. The increased concrete moisture due to internal curing did not appear to affect the coefficient of thermal expansion.

Abstract: This paper presents an evaluation of the effectiveness and the benefits of using high-performance concrete (HPC) containing corrosion inhibitors in the construction and repair of concrete bridges. Numerical models for predicting the early-age and in-service performance of HPC bridge structures are presented. The main parameters of these models, originally conceived for normal concrete, are revised and adapted for use with HPC containing corrosion inhibiting systems. The proposed models focus on the problems of early-age cracking, chloride ingress into concrete and corrosion of the conventional reinforcing steel. A case study of reinforced concrete (RC) barriers walls is used to illustrate the predictive capabilities of the models and the benefits of using HPC in extending the service life of bridge structures. Cost-effective mitigation measures for the prevention and control of cracking of HPC bridge structures are also suggested.

Abstract: Early-age cracking of high-performance concrete (HPC) is a major concern for bridge owners, since it results in premature reinforcement corrosion, concrete spalling, high maintenance cost and reduced service life. Current design codes do not completely address early-age behavior of concrete and therefore leave a degree of uncertainty about field performance and cracking of concrete bridges at early age. The paper presents a sensitivity analysis in which a finite element model of a bridge barrier wall constructed over an existing slab is used with actual field data to assess the sensitivity of relevant concrete properties to the development of total stress in the concrete barrier wall at early age. The parameters considered in the analysis include tensile strength, modulus of elasticity, maturity, thermal effects, shrinkage and creep. Limitations of existing models and research needs are suggested.

Abstract: This paper compares the structural performances of highly-confined columns made of two types of special concretes, a self-consolidating concrete and a fibre-reinforced concrete, with those of companion columns made of normal high-strength concrete. Self-consolidating concrete is a high-fluid concrete which can easily be cast without consolidation into forms containing congested reinforcing cases. Fibre-reinforced concrete is used in this project to prevent the loss of axial capacity due to the early spalling of the concrete cover. The test results indicate that the structural behaviour of self-levelling concrete is comparable to that of fluid mixtures, while fibre-reinforced concrete is effective in preventing the early spalling of the concrete cover as well as increasing the ductility and toughness of the confined material.