Discrete element modelling of the fresh state behavior of pervious concrete

The objective of this paper is to propose a novel numerical approach to evaluate the fresh state behavior of pervious concrete (PC). A new constitutive law applied to Discrete Element Method was developed for the simulation of the interaction between aggregates connected by fresh cement paste. Several innovative aspects regarding the type of contact model, the consideration of the contact bridge and the inclusion of the rheology coefficients were proposed. A calibration was performed with data from the literature. Furthermore, an extensive experimental program considering different shapes of aggregates, grading curves and aggregate-to-cement paste ratios was conducted to evaluate the fresh state behavior and validate the numerical models. The good fit obtained between numerical and experimental results confirm the model and the constitutive law reproduce the under uniaxial compaction, thus representing a step forward in the design and application of pervious concrete mixes.Peer ReviewedPostprint (author's final draft

Effects of biaxial confinement in mortars exposed to external sulfate attack

Research on external sulfate attack (ESA) is usually performed on small scale specimens under free expanding conditions. However, most field structures cannot expand freely due to confinement induced by the ground or other elements from the structure. As a result, ESA usually develops in confined conditions. This work aims to assess the interaction of gradual biaxial compression stresses generated by confinement with the ESA. Visual appearance, mass and ultrasonic velocity were monitored to characterize the macro-scale behavior of free and restrained mortar samples. Changes on phase composition and crystal morphology induced by confinement were evaluated by X-Ray diffraction and scanning electron microscopy. The confining stresses generated were estimated during the attack. Results indicate that ESA is not developed equally in free and confined specimens. The confinement limits sulfate availability, reduce the amount of ettringite precipitated and might induce changes on crystal morphology that reduce the degradation caused by the ESA.Peer ReviewedPostprint (author's final draft

Shear behaviour of sprayed concrete

Shear in sprayed concrete (SC) may govern the design criteria in certain applications subjected to seismic loads or in bolted areas. However, it has been scarcely studied given the complexity of reproducing the production conditions in the laboratory and the lack of standardized tests. The paper focuses on the shear characterization of SC using a shear test compatible with the sample production. For that, an experimental program is performed analysing the influence of several parameters and comparing the results to those of a reference concrete. Furthermore, the outcome validates the shear test selected for the characterization of SC.Peer ReviewedPostprint (author's final draft

The role of porosity in external sulphate attack

Design codes promote a limitation of permeability (indirectly of porosity) to reduce the sulphate ingress and improve the resistance of concrete and mortar to external sulphate attack (ESA). However, porosity could also have a positive effect on durability by generating additional space to accommodate the expansive phases. The aim of this study is to evaluate the role of porosity in ESA. For that, changes at the macro-scale, phase composition and pore network are monitored for mortar compositions with different pore-size distribution. Results indicate the existence of two mechanisms: the capacity to accommodate expansive phases controls the durability during the initial stages of the attack, while at later stages durability is defined by the permeability. Results from specimens with air-entrainer suggest that the intentional increase of porosity towards maximising the capacity to accommodate expansive products might be a valid approach in order to reduce the expansive forces generated during ESA.Peer ReviewedPostprint (author's final draft

Luong test for the characterization of the shear post-cracking response of fiber reinforced concrete

The behavior of concrete subjected to shear can be significantly improved by the addition of fibers, which may partially or totally substitute the traditional steel stirrups (also known as links). Design recommendations include formulations to account for the contribution of the fibers. However, these formulations take the results of small-scale bending tests as input parameters. The main reason for that is the lack of standardized tests and the difficulty to assess direct shear in fiber reinforced concrete (FRC) both for the design and for the quality control. The present study proposes a test for the characterization of the post-cracking shear response of FRC. The test is validated through an experimental program with conventional concrete and FRC. The influence of different parameters (geometry of the specimen, the type and the content of fiber) is assessed and a comparison is made between the push-off test and the one proposed here. Results of an in-depth statistical analysis indicate that the latter is a simpler and valid alternative to evaluate the post-cracking shear response of FRC.Peer ReviewedPostprint (author's final draft

Parameters controlling early age hydration of cement pastes containing accelerators for sprayed concrete

The objective of this work is to parametrize the early age hydration behavior of accelerated cement pastes based on the chemical properties of cement and accelerators. Eight cements, three alkali-free and one alkaline accelerators were evaluated. Isothermal calorimetry, in situ XRD and SEM imaging were performed to characterize kinetics and mechanisms of hydration and the microstructure development. The reactivity of all accelerators is directly proportional to their aluminum and sulfate concentrations and to the amount and solubility of the setting regulator contained in cement. Alite hydration is enhanced if a proper C3A/SO3 ratio (between 0.67 and 0.90) remains after accelerator addition and if limestone filler is employed, because undersulfated C3A reactions are avoided. Combinations of compatible materials are recommended to enhance the performance of the matrix and to prevent an undesirable hydration behavior and its consequences in mechanical strength development.Peer ReviewedPostprint (author's final draft

Case study of failure of long prestressed precast concrete girder during lifting

Improvements in concrete technology, reinforcing systems and manufacturing enable the use of longer reinforced precast concrete girders, contributing to the competitiveness of this solution for bridge decks. The weight of the girders should be limited in order to achieve an optimum between span length, transportation costs and lifting costs. The current tendency in design is to minimize the width of the flanges of the girder. This makes the element more flexible to lateral deformation and increases the risk of lateral instability in temporary situations without lateral supports. This is reflected in an increment of the number of accidents and damages associated with such phenomenon. The main objective of this study is to describe a real case study of lateral instability of a 46¿m long prestressed concrete girder during lifting operations as well as to perform a parametric study to understand the limits of the problem observed. In addition to presenting the case study, simplified formulations and numerical simulations are used to assess the safety margins against lateral instability. Special attention is paid to the evaluation of the provisions gathered in codes and guidelines regarding the lateral stability since they might not cover extreme cases.Peer ReviewedPostprint (author's final draft

Maturity method to predict the evolution of the properties of sprayed concrete

The maturity method provides a simple approach for assessing the strength evolution of concrete. Although it is already used in the precast industry, no reported applications with sprayed concrete may be found in the literature. Such concrete presents singular characteristics due to the spraying process and, in some cases, due to the introduction of accelerators that modify the kinetics of cement hydration. Consequently, the traditional equations that relate the evolution of mechanical properties and the maturity index might not apply in this case. The objective of this study is to adapt the maturity method to sprayed concrete. An experimental program was conducted with 24 concrete mixes sprayed in laboratory and tested for the evolution of temperature and compressive strength. An alternative equation was proposed to relate the maturity index and the mechanical properties. Subsequently, finite element models were developed to generalize the maturity curves considering the local design parameters.Peer ReviewedPostprint (author's final draft

Design procedure and experimental study on fibre reinforced concrete segmental rings for vertical shafts

Structural fibres are used to replace partially or totally the passive reinforcement in precast concrete segments for tunnel linings constructed with TBM, showing several advantages. Fibre reinforced concrete (FRC) could also be applied with similar benefits to vertical shafts. However, to the author's knowledge, this material has not been used in such application yet. The Model Code 2010 gathers an approach for the design of FRC structural elements. This approach should be adapted according to the structural needs of precast segment, for which the transient load stages are often the most critical and specific ductility requirements should be established. The objective of this paper is twofold: propose a general analytical formulation to assess the minimum mechanical requirements that FRC must fulfil in case of partial or complete substitution of the steel rebars and confirm that it is possible to replace the rebars by using fibres in vertical shaft linings. First, the general analytical formulation is proposed. Then, the segments of the Montcada vertical shaft (Barcelona) are redesigned considering the total substitution of the traditional reinforcement by fibres. Finally, two full-scale tests of the FRC precast segments were performed to verify the suitability of the analytical formulation proposed.Peer ReviewedPostprint (author's final draft

Bond strength of whitetoppings and bonded overlays constructed with self-compacting high-performance concrete

A bonded concrete overlay consists of a concrete layer poured over a deteriorated pavement. Its mechanical performance depends on the quality of the bond between the lower and the uppermost layers. This paper reports an extensive experimental program to evaluate bond strength between Conventional Concrete (CC) and Asphalt Concrete (AC) substrates and Self-Compacting High-Performance Concrete (SCHPC) overlays. In all, 8 interface treatments are tested under Direct Tension, pure shear “LCB”, and compressive Slant Shear tests. The results show that direct pouring of the SCHPC overlay over CC and AC substrates produces similar or higher strengths than the other treatments analyzed.Peer ReviewedPostprint (author's final draft