Karin Lundgren

Concrete in compression: a plasticity theory with a novel hardening law

This paper deals with the modelling of the behaviour of plain concrete in triaxial compression using the theory of plasticity. The aim is to model the load resistance and the deformation capacity in uniaxial, biaxial and triaxial compression by means of few parameters, which can be determined easily. A novel hardening law based on a non-associated flow rule and the volumetric plastic strain as hardening parameter is combined with a yield surface proposed by Menetrey and William (1995). The novel hardening and softening law differs from a classic strain-hardening law, as instead of the length of the plastic strain vector only the volumetric component of the latter is used as a hardening parameter. Thus, the non-linearity of the plastic potential is utilized to describe the influence of multiaxial compression on the deformation capacity and no additional ductility measure is required. The implementation and calibration of the novel hardening law are discussed. The prediction of the model is compared to results of uniaxial, biaxial and triaxial compression tests. It is shown that with one set of calibration parameters a good prediction of the load resistance and the deformation capacity for all three types of compression tests can be achieved.

Anchorage Length of Near-Surface Mounted Fiber-Reinforced Polymer Bars for Concrete Strengthening—Experimental Investigation and Numerical Modeling

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars are being increasingly recognized as a valid alternative to externally bonded FRP laminates for enhancing flexural and shear strength of deficient concrete, masonry, and timber members. The ultimate capacity and service performance of strengthened members are deeply impacted by bond characteristics of the strengthening system on which, in the case of NSM bars, limited data is available. This paper follows up on prior investigations into the mechanics of bond of NSM bars to concrete. Experimental results completing a previous test series are reported and discussed, and a global evaluation of results of 3 different test series is attempted. A 3-D finite element model for bond of NSM reinforcement is proposed and calibrated on the basis of some experimental findings.

Severely Corroded RC with Cover Cracking

It is not uncommon that cover cracking, spalling, and delamination occur in many corroding RC structures. Previous research has mainly been concerned with corrosion levels leading to cover cracking along the main reinforcement, whereas corrosion of stirrups is often overlooked. Corrosion phenomena, including stirrup corrosion, were studied in an experimental investigation presented in this paper. High levels of corrosion were reached, up to 20% of the main bars and 34% of the stirrups legs. The occurrence of crack initiation, propagation, and cover delamination were examined. The specimens had the shape of a beam end and were corroded with an accelerated method; an imposed current was used, taking care to keep the current density as low as practically possible for the duration of the laboratory testing. The effects of this process were compared with those of natural corrosion using models from the literature. The location of the bar, middle and corner placement, the amount of transverse reinforcement, and the corrosion level of longitudinal reinforcement and of transverse reinforcement were studied. The results concerning the concrete cracking in the experimental campaign are presented. The crack patterns and widths were analyzed, showing differences between specimens with or without stirrups and whether stirrups were corroding. Finally, the effect of corrosion was simulated as the expansion of corrosion products in a finite-element (FE) model, and the results, mainly the crack pattern and width, were compared with the test results. The conclusions addressed the importance of taking into consideration both high corrosion levels and corrosion of stirrups for the assessment of deteriorated structures.

Analytical model for the bond-slip behaviour of corroded ribbed reinforcement

Corrosion of reinforcement affects the bond mechanism between reinforcement and concrete, and thus the anchorage. Reliable models describing this are needed especially for assessment of the load-carrying capacity of existing structures. This paper presents an analytical one-dimensional model for bond-slip response of corroded reinforcement. The proposed model is an extension of the bond-slip model given in the CEB-FIP Model Code 1990, and is practically applicable for structural analyses to determine the load-carrying capacity of corroded structures. Furthermore, the anchorage length needed to anchor the yield force is calculated from the bond slip, using the one-dimensional bond-slip differential equation. Results of the proposed model are compared with experimental results as well as results from an advanced three-dimensional finite element model. The suggested model is shown to give results that are consistent with the physical behaviour.

Analysis of Mechanical Behavior of Corroded Reinforced Concrete Structures

This paper presents a methodology to analyze the mechanical behavior and remaining load-carrying capacity of corroded reinforced concrete (RC) structures. The methodology is used to predict the mechanical behavior for a structure with an observed amount of uniform and pitting corrosion at a given time. The effect of corrosion is modeled as a change in geometry and properties of corroded reinforcement and surrounding concrete—that is, a reduction of steel area and ductility, removal of spalled concrete, modification of concrete response due to corrosion cracks, and modification of bond-slip properties. The methodology is applied to concrete beams affected by reinforcement corrosion, using both finite element analyses and analytical methods. A comparison of the results with available experiments from the literature indicated that the changes in failure mode and failure load caused by uniform and pitting corrosion of reinforcement can be predicted reasonably well by using the proposed methodology.

Modelling the structural behaviour of frost-damaged reinforced concrete structures

A methodology is introduced to predict the mechanical behaviour of reinforced concrete structures with an observed amount of frost damage at a given time. It is proposed that the effects of internal frost damage and surface scaling can be modelled as changes of material and bond properties, and geometry, respectively. These effects were studied and suggestions were made to relate the compressive strength and dynamic modulus of elasticity, as the indicators of damage, to the response of the damaged concrete in compression and tension, and to the bond behaviour. The methodology was applied to concrete beams affected by internal frost damage, using non-linear finite element analyses. A comparison of the results with available experimental data indicated that the changes in failure mode and, to a rather large extent, the effect on failure load caused by internal frost damage can be predicted. However, an uncertainty was the extension and distribution of the damaged region which affected the prediction of the load capacity.

Three-dimensional modelling of structural effects of corroding steel reinforcement in concrete

The effect of corrosion products flowing through cracks becomes significant when large corrosion penetrations take place in reinforced concrete structures and wide cracks develop; this is favourable, as it decreases the splitting stress around the bar. The effect becomes more important when the corrosion rate is low, such as for natural corrosion. Acorrosion model describing the expansion due to voluminous corrosive products was previously developed. The model is here extended to include the flow of corrosion products through cracks. The volume flow of corrosion products through a crack is assumed to depend on the splitting stress and the crack width. The splitting stress is evaluated from the strain in the corrosion products, and the crack width is computed from the displacements across the crack. A one-dimensional flow model is used to formulate the flow phenomenon and to estimate the volume flow of corrosion products. The extended corrosion model, applied in detailed three-dimensional non-linear finite element analyses of highly corroded eccentric pull-out specimens, resulted in more corrosion cracks with smaller crack openings, which better corresponded to measurements of the tested specimens. Moreover, the results indicated the important effect of the flow phenomenon on the bond strength.

A multi-level structural assessment strategy for reinforced concrete bridge deck slabs

Abstract This paper proposes a multi-level assessment strategy for reinforced concrete bridge deck slabs. The strategy is based on the principle of successively improved evaluation in structural assessment. It provides a structured approach to the use of simplified as well as advanced non-linear analysis methods. Such advanced methods have proven to possess great possibilities of achieving better understanding of the structural response and of revealing higher load-carrying capacity of existing structures. The proposed methods were used for the analysis of previously tested two-way slabs subjected to bending failure and a cantilever slab subjected to a shear type of failure, in both cases loaded with concentrated loads. As expected, the results show that more advanced methods yield an improved understanding of the structural response and are capable of demonstrating higher, yet conservative, predictions of the load-carrying capacity. Nevertheless, the proposed strategy clearly provides the engineering community a framework for using successively improved structural analysis methods for enhanced assessment in a straightforward manner.

Sustainable Potential of Textile-Reinforced Concrete

The building construction industry is in need of sustainable materials and solutions. A novel building material, such as textile-reinforced concrete (TRC), could be used to meet this demand. TRC is a combination of fine-grained concrete and multi-axial textile fabrics, which has been fundamentally researched over the past decade. TRC-based research has explored various facets of this composite material, such as its structural functionality, production, applicability and design. One key aspect that is still missing, however, is a comprehensive review of the sustainable potential of this material in terms of its input-output and durability which suitably answers to requirement no.7 of EU’s Construction Products Regulation. This article provides qualitative and quantitative evaluation of the sustainable potential and prospective development of TRC particularly reinforced by alkali-resistant (AR) glass, carbon or basalt fibers. Based on the outcome of this evaluation, carbon textile fibers were observed to hold the optimal potential mechanical behavior; additionally, it was revealed through the conducted Life Cycle Assessment (LCA), that basalt had the least cumulative energy demand while carbon had the least environmental impact.

Investigating correlations between crack width, corrosion level and anchorage capacity

Abstract In assessing existing structures, inspection results need to be linked to the effects on load-carrying capacity; to provide such information, this study has investigated the correlation between splitting crack width, corrosion level and anchorage capacity. The study was based on 13 reinforced concrete beams that had been exposed to natural corrosion for 32 years, 11 beams with splitting cracks and 2 without. The crack pattern and widths were documented before undergoing structural testing of anchorage capacity. Thereafter, the reinforcement bars were extracted and their corrosion levels measured using two methods, gravimetric weight loss and 3D scanning. The corrosion level from the weight loss method was approximately twice as large; possible reasons are horizontal or subsurface corrosion pits, and the cleaning method. Further, for the same corrosion level, the specimens in this study had much larger crack widths and slightly lower bond capacity than the artificially corroded tests in the literature; a possible reason is that these specimens had been subjected to combined corrosion and freezing. However, the corrosion level and reduction in bond capacity related to crack width were both lower in the present than in previous studies in the literature. Thus, by formulating a damage indicator from the damage visible in the form of crack widths from artificial test data, the structural capacity is estimated to be on the safe side.