Aging concrete structures: a review of mechanics and concepts

The safe and cost-efficient management of our built infrastructure is a challenging task considering the expected service life of at least 50 years. In spite of time-dependent changes in material properties, deterioration processes and changing demand by society, the structures need to satisfy many technical requirements related to serviceability, durability, sustainability and bearing capacity. This review paper summarizes the challenges associated with the safe design and maintenance of aging concrete structures and gives an overview of some concepts and approaches that are being developed to address these challenges

Model B4 : multi-decade creep and shrinkage prediction of traditional and modern concretes

To improve the sustainability of concrete infrastructure, engineers face the challenge of incorporating new concrete materials while pushing the expected design life beyond 100 years. The time-dependent creep and shrinkage response of concrete governs the serviceability and durability in this multi-decade time frame. It has been shown that current prediction equations for creep and shrinkage underestimate material deformations observed in structures outside of a laboratory environment. A new prediction model for creep and shrinkage is presented that can overcome some of the shortcomings of the current equations. The model represents an extension and systematic recalibration of model B3, a 1995 RILEM Recommendation, which derives its functional form from the phenomena of diffusion, chemical hydration, moisture sorption, and the evolution of micro-stresses in the cement structure. The model is calibrated through a joint optimization of a new enlarged laboratory test database and a new database of bridge deflection records to overcome the bias towards short-term behavior. A framework for considering effects of aggregates, admixtures, additives, and higher temperatures is also incorporated

An investigation on the 'width and size effect' in the evaluation of the fracture energy of concrete

The parameters that describe the fracture behavior of concrete are crucial to investigate numerically the response of reinforced concrete (RC) structures. Among them, the fracture energy plays a key role in all those applications that aim to simulate the behavior of RC structures. The fracture energy is a characteristic property of a material but its experimental evaluation could be difficult for quasi-brittle materials such as concrete due to the "width effect" and "size effect" that can lead to some uncertainties in the definition of this parameter. This study presents the results of an experimental campaign conducted on notched specimens to evaluate the fracture energy of concrete. Concrete prisms with different sizes were tested using a three-point bending (TPB) set-up to evaluate the influence of the width and the size on the results. The setup has been designed to become potentially part of the ACI 446 report on fracture. Digital image correlation (DIC) was used to qualitatively and quantitatively study the strain field near the crack tip. Preliminary numerical simulations were performed to investigate the "width effect" in a discrete element framework. Copyright (C) 2017 The Authors. Published by Elsevier B.V

Long-term shrinkage prediction from theoretical considerations and data analysis

Upon the release of the data from the tragic collapse in 1996 of the record-span segmental box-girder bridge in Palau, it was found that the 18-year deflection was 200 - 400% larger than the predictions based on the American, European and Japanese design codes or recommendations. This finding triggered further studies that led to a collection of deflection histories of 69 large-span segmental bridges, most of which suffered excessive, logarithmically growing, deflections with no sign of an asymptotic bound. It thus became clear that major improvements in design codes and practices are required. Data collection efforts led to a new database of laboratory concrete creep and shrinkage data. With over 3000 test curves, this database more than doubles the size of the previous RILEM database. Unfortunately, the duration of about 94% of the available lab tests is <6 years, 97% ≤12 years, and only 3% attains 30 years, while 100-year lifetimes are generally desired. So it became evident that the only way to develop a realistic multi-decade prediction model was by joint statistical optimization of the fit of the laboratory data and the multi-decade bridge data. Regrettably, most of the bridge data are insufficient for inverse FE analysis. The relative increases of multi-decade deflection after about 1,500 days could be used for calibration. The combination of incomplete multi-decade bridge data with the short-time laboratory database posed a challenge for statistical optimization of the model parameters. Nonlinear least-square regression was used to inform the information of obtained from the database with the bridge deflection measurements. The database of laboratory tests has further been extended to include high-strength concretes (up to 167 MPa strength at 28 days), as well as modern concretes with various admixtures, classified into six classes, some of which decrease and others increase the creep and shrinkage. Through correlation analyses and the incorporation of previously studied trends, new formulas for estimating the model parameters from concrete strength and composition (with admixtures) have been identified

Constitutive models for mortars of bonded anchors

The efficient and permanently safe design of anchor system requires a thorough understanding of complex load carrying mechanisms and processes. Considering the required design life time of at least 50 years in combination with the demanded small failure probability, especially for the ultimate limit case, a suitable framework for the service-life prediction and assessment is critical. In general, accurate modelling concepts for all involved materials and processes, taking into account the associated model and prediction uncertainties, should be utilized. In case of bonded anchors three basic components - steel rod, mortar and concrete – are of relevance. In this paper, constitutive models of two different types of mortars (epoxy or vinyl-ester based) are presented. According to the preliminary experiments, the investigated epoxy based system is characterized by a pronounced ductile behaviour whereas the vinyl-ester based system is quite brittle. The proposed numerical approach, formulated in the framework of discrete particle and continuum models, is utilized to capture the aforementioned characteristics. The numerical results obtained by the proposed models are compared with the available experimental data

Aggregate effect on the concrete cone capacity of an undercut anchor under quasi-static tensile load

In the last decades, fastening systems have become an essential part of the construction industry. Post-installed mechanical anchors are frequently used in concrete members to connect them with other load bearing structural members, or to attach appliances. Their performance is limited by the concrete related failure modes which are highly influenced by the concrete mix design. This paper aims at investigating the effect that different aggregates used in the concrete mix have on the capacity of an undercut anchor under tensile quasi-static loading. Three concrete batches were cast utilising three different aggregate types. For two concrete ages (28 and 70 days), anchor tensile capacity and concrete properties were obtained. Concrete compressive strength, fracture energy and elastic modulus are used to normalize and compare the undercut anchor concrete tensile capacity employing some of the most widely used prediction models. For a more insightful comparison, a statistical method that yields also scatter information is introduced. Finally, the height and shape of the concrete cones are compared by highly precise and objective photogrammetric means

Life-cycle robustness : quantification and challenges

Life-cycle robustness is achieved when a structural member or a system is designed to maintain its intended function and required safety level within its desired life-cycle. The different character of effects that each element of the system needs to undergo (damage, ageing, extreme events, changes in usage) in conjunction with the diversity in the intrinsic material properties, form a demanding problem. Further complexity emerges when one realizes that time is not simply a variable, but a factor permeating model choices and uncertainty representation approaches. Different effects in the load side, and properties in the resistance side develop differently in time. Depending on the scale of the problem, the spatial randomness of materials such as concrete may be relevant for the accurate quantification of failure probabilities, and may require careful modelling, even at a mesoscale. For a long-term analysis, where the influence of uncertainties may dominate over predictability, robust design concepts and analyses methods that are relatively insensitive to small variations in variable inputs related to secondary effects and processes can prove decisive. On the computational side, challenges are associated with the computational cost of simulations and nonlinear analyses required to determine time-variable reliability profiles, considering all likely scenarios. Furthermore, statistical characteristics of the inputs, in particular their tail behaviour and their statistical dependence, needs to be properly captured and reproduced while maintaining sufficiently small sample size, and thus acceptable computational cost. Within this contribution, a framework for the quantification of life-cycle robustness is presented in the context of fasteners subjected to sustained load and extreme events. The emerging challenges are presented and briefly discussed