Manuel A.G. Silva

Settlement of ballasted track under traffic loading: Application to transition zones

Transition zones corresponding to the passage from railway tracks on embankment to natural ground or settlement free structures are frequently problematic for maintenance. Changing stiffness of the track and differential settlements are the main causes for the degradation of tracks and foundations at transitions. This paper presents a methodology to predict the settlement of the track due to train loading, applicable to transition zones. The methodology is based on dynamic calculations using a (non-linear) train–track interaction model and an incremental settlement model. Important non-linear aspects in dynamic models for railway transitions are the loss of contact between the sleepers and the ballast and the non-linear constitutive behaviour of the ballast. The settlements are calculated paying attention to the factors that most influence the long-term behaviour of ballast, such as the amplitude of the applied dynamic force and the number of loading cycles. The importance of the dynamic loading from the trains and of the constitutive models used for the trackbed layers is analysed.

Influence of GFRP Confinement of Reinforced Concrete Columns on the Corrosion of Reinforcing Steel in a Salt Water Environment

AbstractThis paper concentrates on the changes on the corrosion rate of the steel bars of RC columns due to their wrapping with glass fiber–reinforced polymers (GFRPs). RC cylindrical laboratorial specimens, both unwrapped and wrapped with GFRP, were immersed in a sodium chloride solution for 20 months. The changes on concrete resistivity and the corrosion process were followed by open potential measurements and electrochemical impedance. It was possible to quantify the influence of the GFRP jackets in the solution uptake and on the corrosion of the steel reinforcement. The results showed a delay on the diffusion of the solution into concrete due to the presence of the GFRP, which allowed that the bars remained in a passive state for longer immersion periods. The GFRP barrier lowered the rate of uptake of the solution and the chloride ion penetration. However, the GFRP jacket may have the properties as a physical barrier reduced due to imperfect application. The incomplete concrete saturation makes oxygen...

Composites and FRP-Strengthened Beams Subjected to Dry/Wet and Salt Fog Cycles

AbstractCarbon (CFRP) and glass (GFRP) fiber-reinforced composites of epoxy matrix are considered in the study, with greater emphasis on GFRP. Accelerated conditioning was imposed in the form of salt fog cycles, hygrothermal cycles (tap water), and tidal-like cycles (aqueous solution of NaCl, 50  g/L), both on laminate composites and on beams externally reinforced with fiber-reinforced polymers (FRP) on their soffit. Freeze/thaw cycles were also applied to the study of degradation of the laminates. Mechanical tests showed degradation due to damage to the matrices, fiber-matrix linkage or bond between FRP and concrete. Several advanced techniques were used to interpret the results and enable better understanding of the phenomenological data. Changes on the glass transition temperature (Tg) of the epoxy matrix are reported as possible indicators of decrease of the tensile strength of the GFRP composite. Attention is given to the evolution of the relative values of the tensile strength of concrete and adhesi...

CYCLIC COMPRESSION BEHAVIOUR OF POLYMER CONCRETE

Polymeric mortars or concrete are special building materials which can be used to repair or strengthen localized areas of structural elements. Following research on the behaviour of retrofitting reinforced concrete circular columns with FRP composite materials and bearing in mind the high strength of polymer concretes, it was decided to develop a solution to seismic retrofit of reinforced concrete columns with polymer concrete. The mechanical characteristics of different polymer concretes and especially their performance when subjected to cyclic axial compression, several bending tests, and monotonic and cyclic axial compression tests were studied, namely the compressive strength, the tensile strength on bending and the Youngs modulus. Columns were also tested under axial compression and cyclic horizontal loads. The results of these tests are shown and interpreted. It is concluded that the improved behaviour in monotonic compression of polymer concrete is essentially associated with better strength characteristics of resin, whereas its superior behaviour under cyclic loading is linked to a smoother aggregate grading curve.

Influence of Temperature Cycles on Bond between Glass Fiber-Reinforced Polymer and Concrete

Reinforced concrete (RC) beams externally strengthened with glass fiber-reinforced polymer (GFRP) strips bonded to the soffit may see their load-carrying capacity reduced due to environmental conditions-especially due to the deterioration of bond between the adhesively bonded laminates and concrete, causing premature failure. More research has been published on the detachment of the laminate progressing from the anchorage zone than on failure induced by the formation of flexural or shear-flexural cracks in the midspan followed by fiber-reinforced polymer (FRP) separation and failure designated as intermediate crack (IC) debonding. An experimental program to study degradation of the GFRP laminate beam specimens after accelerated temperature cycles, namely: 1) freezing-and-thawing type; and 2) cycles of the same amplitude (40°C [104°F]) and an upper limit approximately 70% of the glass vitreous transition temperature of the resin, Tg, is described. Effects on the bond stress and ultimate capacity are reported. Substantial differences between shear and bending-induced failure and a decrease of bond stresses and engagement of the laminates on the structural response are analyzed.

Influence of External Compressive Stresses on the Performance of GFRP-to-Concrete Interfaces Subjected to Aggressive Environments: An Experimental Analysis

AbstractDespite the fact that fiber reinforced polymer (FRP) composites are a reliable structural material with reasonable durability performance, the environment to which the strengthened structure is exposed can make the strengthening system vulnerable. In this study, the effectiveness of externally bonded reinforcement (EBR) systems when external compressive stresses are applied to glass fiber reinforced polymers (GFRP)-to-concrete interfaces in several aggressive environments is analyzed. The compressive stress imposed on the GFRP-to-concrete interface intends to simulate, for instance, the effect produced by a mechanical anchorage system applied to the EBR system. The design and the region to set those mechanical anchorage systems are not yet well understood and are mostly applied without really knowing how they will behave. This work shows an exhaustive experimental program based on several double shear tests subjected to salt fog cycles, dry/wet cycles and two distinct temperature cycles: from −10°...

Words avoiding repetitions in arithmetic progressions

Carpi constructed an infinite word over a 4-letter alphabet that avoids squares in all subsequences indexed by arithmetic progressions of odd difference. We show a connection between Carpis construction and the paperfolding words. We extend Carpis result by constructing uncountably many words that avoid squares in arithmetic progressions of odd difference. We also construct infinite words avoiding overlaps and infinite words avoiding all sufficiently large squares in arithmetic progressions of odd difference. We use these words to construct labelings of the 2-dimensional integer lattice such that any line through the lattice encounters a squarefree (resp. overlapfree) sequence of labels.

Anti-powers in infinite words

In combinatorics of words, a concatenation of k consecutive equal blocks is called a power of order k. In this paper we take a different point of view and define an anti-power of order k as a concatenation of k consecutive pairwise distinct blocks of the same length. As a main result, we show that every infinite word contains powers of any order or anti-powers of any order. That is, the existence of powers or anti-powers is an unavoidable regularity. Indeed, we prove a stronger result, which relates the density of anti-powers to the existence of a factor that occurs with arbitrary exponent. From these results, we derive that at every position of an aperiodic uniformly recurrent word start anti-powers of any order. We further show that any infinite word avoiding anti-powers of order 3 is ultimately periodic, and that there exist aperiodic words avoiding anti-powers of order 4. We also show that there exist aperiodic recurrent words avoiding anti-powers of order 6, and leave open the question whether there exist aperiodic recurrent words avoiding anti-powers of order k for k=4,5.

Monochromatic paths for the integers

Recall that van der Waerdens theorem states that any finite coloring of the naturals has arbitrarily long monochromatic arithmetic sequences. We explore questions about the set of differences of those sequences.

Rainbow connection for some families of hypergraphs

Abstract An edge-coloured path in a graph is rainbow if its edges have distinct colours. The rainbow connection number of a connected graph G , denoted by r c ( G ) , is the minimum number of colours required to colour the edges of G so that any two vertices of G are connected by a rainbow path. The function r c ( G ) was first introduced by Chartrand et al. (2008), and has since attracted considerable interest. In this paper, we introduce two extensions of the rainbow connection number to hypergraphs. We study these two extensions of the rainbow connection number in minimally connected hypergraphs, hypergraph cycles and complete multipartite hypergraphs.