FATIGUE EXAMINATION OF REINFORCED-CONCRETE BRIDGE SLABS USING ACOUSTIC EMISSION AND STRAIN MONITORING DATA

The examination of bridges under service loading is key for understanding their condition and thus for their sustainable and efficient use. The need to extend the service duration of existing bridges is increasing, and therefore, efficient tools for examining and maintaining them are required. Monitoring of structures is recognised as an efficient method for investigating structural behaviour and evaluating structural performance. However, there is a lack of methods for choosing suitable sensors and performing monitoring on-site. The main challenges associated with monitoring include environmental conditions, complex loading, big data storage, and measurement calibration. Thus far, no research has developed and implemented a complete strain-acoustic-emission monitoring method, from instrumentation to data interpretation, to evaluate long-term changes in the internal state of reinforced-concrete road bridges. The general objective of this thesis is to characterise fatigue-related damage in the reinforced concrete of bridge-deck slabs using nondestructive methods. The influence of low fatigue stress on reinforced-concrete behaviour was studied at a microscale structural level, and this research was then used to define two indices that quantify existing damage and fatigue-damage increase over time. For this aim, an extensive long-term monitoring campaign was conducted on a bridge in service to understand the fatigue behaviour of reinforced-concrete slabs under operational loadings and to evaluate the effectiveness of different nondestructive methods. The instrumentation and calibration of the used nondestructive measurements produced recommendations for the examination of reinforced-concrete bridge-deck slabs. The methods provide a solid understanding of how to quantify the influence of traffic loading and environmental changes on structural behaviour, such as fatigue behaviour, and structural characteristics such as the connection between bridge components. The results derived from the monitoring data indicate that the instrumented bridge is safe with respect to the fatigue limit despite the potential difference between the present and original design conditions. The developed inverse method identifies the probabilistic distribution of the position and the load of vehicles crossing the instrumented slab. The results of this method define the link between the fatigue-damage distribution and the position and the load of vehicles, and reveal the sensitivity of fatigue damage to different sources of uncertainties associated with the compressive strength of concrete, the level of stresses, and the magnitude of annual traffic. Combining information from monitoring data and structural analysis models allows for accurate data interpretation and model development. The results reveal that the identification of structural parameters and the prediction of fatigue endurance of the noninstrumented structural elements can be performed accurately. Integrating acoustic emission and strain measurement results is a comprehensive method for analysing the fatigue of reinforced-concrete members by which microcracks in the concrete volume can be localised and visualised under operational loading. The stationary movement of existing microcracks and low microcrack growth govern the concreteâs behaviour under low fatigue stresses. Reinforced concrete was found to be in the stage of new-microcrack nucleation and stable microcrack growth.

On the compressive and bond strength of reinforced concrete as structural properties

Francesco Rafael Alberto Moccia

Traditionally, the concrete strength is measured on cubes or cylinders having normalized dimensions, suitable vibration and curing conditions and their strength is assessed in laboratory under fast loading rates. However, the in-situ strength of structural elements can considerably differ from that of a small and homogenous control specimen due to a number of issues. Notably, phenomena taking place during the consolidation process of fresh concrete may affect the compressive resistance of tall members as well as the bond strength of reinforcing bars located in top layers. During concrete consolidation, water migrates in the direction of the free surface while concrete settles downwards, phenomena referred as concrete bleeding and plastic settlement respectively. Under these circumstances, a decrease in the concrete properties near the upper surface is observed as well as a development of cracks and voids surrounding horizontal reinforcing bars, causing potential disturbances on the compressive stresses and affecting the mechanical engagement between bars and concrete. In addition, the response of structural concrete may differ from that of material samples due to non-uniform stress states, the material brittleness, the cracking induced by imposed strains, the rheological response of concrete and the presence of embedded disturbances. As a result, the strength measured in material samples needs to be corrected with strength reduction factors to ensure suitable structural analysis. In this thesis, an in-depth investigation is performed on the different phenomena affecting the compressive and bond strength of structural members. These aspects are assessed by means of several testing programmes instrumented with refined measurements techniques such as tomography and Digital Image Correlation. An extensive experimental programme comprising 76 column and prism tests was carried out to evaluate the influence of casting position, loading direction and bar disturbances on the compressive resistance of structural elements. The detailed measurements performed at the fresh and hardened state resulted in the proposal of consistent design rules accounting for the investigated phenomena. Focus was also given on the influence of material brittleness and the implications of internal stress redistributions on the structural response of reinforced concrete columns and compression zones of members in bending. The pertinence of the investigations were validated based on more than 400 column tests collected from the literature. The implications of casting conditions on pull-out and spalling failures were also assessed by means of 137 pull-out tests on reinforcing bars presenting variable diameter, concrete cover, casting height and embedded length. The investigations resulted in the proposal for a physically-consistent approach, evaluating the pull-out resistance as function of casting conditions and reinforcement characteristics. Finally, the phenomenon of cover spalling was examined with respect to the action of a radial inner pressure, as originated by bond engagement or associated to the volumetric expansion of corroded reinforcement. The mechanisms inducing spalling were analysed by means of a comprehensive experimental programme comprising 56 specimens instrumented with Digital Image Correlation. A mechanical model is eventually proposed to evaluate bond-related cases of cover spalling.

EPFL2021

Reinforced concrete bridges under increased railway traffic loads

Andrin Herwig

The aim of this thesis is to provide rational engineering methods for the fatigue safety examination of existing railway bridges. Increasing railway traffic loading and enhanced structural analysis allow for higher exploitation of the capacity of bridge elements. This implies that increasing action effects due to higher service loads may become fatigue relevant in the future. The application of the results allows for more precise fatigue examinations. As beneficial consequence, most railway bridges may be operated safely in the future without costly strengthening measures. Calculated stress ranges in the reinforcement that governs the fatigue resistance of structural elements are too high when using conventional resistance models. The load carrying contribution of non-structural elements and a not entirely developed crack pattern lead to fatigue relevant reduction of the stress range in the rebars. A theoretical study shows that reductions of 5 – 15 % may be obtained by considering the composite effect of the continuous rail grid. Measurements of the dynamic traffic effects were conducted on a single-track bridge. The effects of passenger and freight trains at different velocities on the structural response were measured. The results are interpreted based on the observed vertical shape of the railway track. The observed dynamic behaviour could be correctly represented by simulations using simple dynamic models. The effect of other velocities and in-creased carriage loading degrees are investigated in a parametric study. The findings of previous investigations for road bridges and the investigations of this research work for railway bridges show that the Dynamic Amplification Factor (DAF) decreases for increasing loadings. Thus, smaller DAFs are applied for fatigue relevant causing heavy vehicles. Fatigue stress ranges may be reduced by supplementary 3 – 6 %. A method based on Linear Elastic Fracture Mechanics theory is presented for fatigue life determination. Fatigue life typically is very sensitive to different calculation parameters, rendering the calculation of a reliable fatigue life difficult. The refined analysis of fatigue tests reported in literature allows better understanding of the fatigue behaviour of bridge elements. Thus, a fatigue safety concept is established that compensates the inconvenience of the high sensitivity of fatigue life calculations. This fatigue safety concept is based on the observed load carrying behaviour of inspected bridge elements. The specific fatigue behaviour with visible signs allows for the examination of fatigue safety in a pragmatic way. Specific inspection of bridge elements becomes pertinent as soon as general examinations exhibit an insufficient fatigue resistance. Fatigue behaviour of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) is characterised by increasing deformations during cyclic tensile loading. Under a maximum tensile stress level of 4 MPa the deformation of the UHPFRC layer becomes stable during cyclic loading. For slightly higher stresses, the deformation increases and the UHPFRC layer fails due to steel fibre pull out. Application of the UHPFRC layer in the flexural tension zone considerably reduces the fatigue stresses in the steel reinforcement. As the UHPFRC layer deforms, the stress range in the reinforcement is effectively and permanently reduced and may remain below the nominal fatigue limit. For design of members with a layer of UHPFRC for waterproofing and fatigue strengthening, the maximum allowed strain for stable behaviour under tensile fatigue loading should be verified.

EPFL2008

Probabilistic reliability framework for assessment of concrete fatigue of existing RC bridge deck slabs using data from monitoring

Imane Bayane, Eugen Brühwiler, Amol Mankar

Assessment of existing bridge structures for inherent safety level or for lifetime extension purposes is often more challenging than designing new ones. With increasing magnitude and frequency of axle loads, reinforced concrete bridge decks are susceptible to fatigue failure for which they have not been initially designed. Fatigue verification and prediction of remaining service duration may turn out to be critical for civil infrastructure satisfying the required reliability. These structures are exposed to stochastic loading (e.g. vehicle loads, temperature loads); on the resistance side, reinforced concrete also behaves in a stochastic way. This paper presents a probabilistic reliability framework for assessment of future service duration, which includes probabilistic modelling of actions based on large monitoring data and probabilistic modelling of fatigue resistance based on test data. A case study for the steel - reinforced concrete slab of the Cret de l'Anneau Viaduct is presented along with calibration of resistance partial safety factors for lifetime extension.