Showing papers in "Materials and Structures in 2014"

Geopolymers and other alkali activated materials: why, how, and what?

Abstract: This paper presents a review of alkali-activation technology, moving from the atomic scale and chemical reaction path modelling, towards macroscopic observables such as strength and durability of alkali-activated concretes. These properties and length scales are intrinsically interlinked, and so the chemistry of both low-calcium (‘geopolymer’) and high-calcium (blast furnace slag-derived) alkali-activated binders can be used as a starting point from which certain engineering properties may be discussed and explained. These types of materials differ in chemistry, binder properties, chemical structure and microstructure, and this leads to the specific material properties of each type of binder. The secondary binder products formed during alkali-activation (zeolites in low-Ca systems, mostly layered double hydroxides in alkali-activated slags) are of significant importance in determining the final properties of the materials, particularly in the context of durability. The production of highly durable concretes must remain the fundamental aim of research and development in the area of alkali-activation. However, to enable the term ‘highly durable’ to be defined in a satisfactory way, the underlying mechanisms of degradation—which are not always the same for alkali-activated binders as for Portland cement-based binders, and cannot always be tested in precisely the same ways—need to be further analysed and understood. The process of reviewing a topic such as this will inevitably raise just as many questions as answers, and it is the intention of this paper to present both, in appropriate context.

Cement and carbon emissions

Abstract: Because of its low cost, its ease of use and relative robustness to misuse, its versatility, and its local availability, concrete is by far the most widely used building material in the world today. Intrinsically, concrete has a very low energy and carbon footprint compared to most other materials. However, the volume of Portland cement required for concrete construction makes the cement industry a large emitter of CO2. The International Energy Agency recently proposed a global CO2 reduction plan. This plan has three main elements: long term CO2 targets, a sectorial approach based on the lowest cost to society, and technology roadmaps that demonstrate the means to achieve the CO2 reductions. For the cement industry, this plan calls for a reduction in CO2 emissions from 2 Gt in 2007 to 1.55 Gt in 2050, while over the same period cement production is projected to increase by about 50 %. The authors of the cement industry roadmap point out that the extrapolation of existing technologies (fuel efficiency, alternative fuels and biomass, and clinker substitution) will only take us half the way towards these goals. According to the roadmap, the industry will have to rely on costly and unproven carbon capture and storage technologies for the other half of the required reduction. This will result in significant additional costs for society. Most of the CO2 footprint of cement is due to the decarbonation of limestone during the clinkering process. Designing new clinkers that require less limestone is one means to significantly reduce the CO2 footprint of cement and concrete. A new class of clinkers described in this paper can reduce CO2 emissions by 20 to 30 % when compared to the manufacture of traditional PC Clinker.

Mortar-based systems for externally bonded strengthening of masonry

Abstract: Mortar-based composite materials appear particularly promising for use as externally bonded reinforcement (EBR) systems for masonry structures. Nevertheless, their mechanical performance, which may significantly differ from that of Fibre Reinforced Polymers, is still far from being fully investigated. Furthermore, standardized and reliable testing procedures have not been defined yet. The present paper provides an insight on experimental-related issues arising from campaigns on mortar-based EBRs carried out by laboratories in Italy, Portugal and Spain. The performance of three reinforcement systems made out of steel, carbon and basalt textiles embedded in inorganic matrices has been investigated by means of uniaxial tensile coupon testing and bond tests on brick and stone substrates. The experimental results contribute to the existing knowledge regarding the structural behaviour of mortar-based EBRs against tension and shear bond stress, and to the development of reliable test procedures aiming at their homogenization/standardization.

Effect of internal curing by using superabsorbent polymers (SAP) on autogenous shrinkage and other properties of a high-performance fine-grained concrete: results of a RILEM round-robin test

Abstract: The article presents the results of a round-robin test performed by 13 international research groups (representing fifteen institutions) in the framework of the activities of the RILEM Technical Committee 225-SAP “Applications of Superabsorbent Polymers in Concrete Construction”. Two commercially available SAP materials were used for internal curing of a high-performance, fine-grained concrete in combination with the addition of extra water. The concrete had the same mix composition in all laboratories involved but was composed of local materials. All found a considerable decrease in autogenous shrinkage attributable to internal curing. Also, with regard to the shrinkage-mitigating effect of both particular SAP materials, the results were consistent. This demonstrates that internal curing using SAP is a robust approach, working independently of some variations in the concretes’ raw materials, production process, or measuring technique. Furthermore, the effects of internal curing on other properties of concrete in its fresh and hardened states were investigated. These are consistent as well and expand considerably the existing data basis on properties of concrete materials containing SAP.

Laboratory investigation of bitumen based on round robin DSC and AFM tests

Abstract: In the past years a wide discussion has been held among asphalt researchers regarding the existence and interpretation of observed microstructures on bitumen surfaces. To investigate this, the RILEM technical committee on nano bituminous materials 231-NBM has conducted a round robin study combining differential scanning calorimetry (DSC) and Atomic Force Microscopy (AFM). From this, methods for performing DSC and AFM tests on bitumen samples and determination of the influence of wax on the observed phases, taking into account thermal history, sample preparation and annealing procedure, are presented and critically discussed. DSC is used to measure various properties and phenomena that indicate physical changes such as glass transition temperature (T g) and phase transition such as melting and crystallization. In the case of existence of wax, either natural or synthetic, it can further indicate the melting point of wax, that could be used to determine wax content. The results from seven laboratories show that T g temperatures obtained from the heating scans are more repeatable and easier to obtain in comparison to the cooling scans. No significant difference was noted for T g's obtained from the first and second heating scans. AFM is an imaging tool used to characterize the microstructures on a bituminous surface. Using AFM three phases in the materials with wax could be distinguished. The changes in the phases observed with AFM for increases in temperature were correlated with the DSC curve, and it could be established that the so called "Bee" structure disappeared around the melting peak in the DSC curve. Thus, this research has confirmed the relation between the microstructures on a bitumen surface and the wax content. cop. 2013 RILEM.

Current practice and open issues in strengthening historical buildings with composites

Abstract: Modern techniques and innovative materials are often quite rapidly proposed and allowed in current practice, even for restoration of historical constructions, in which essential preservation criteria must be taken into account. The considerable variability and complexity of masonry structures and types means that choosing the most appropriate structural models and interventions is particularly difficult, since they must be based on suitable knowledge of both existing and new materials, and on their interactions in environmental and loading conditions. This paper discusses the potentials and limitations of externally bonded composite materials in masonry structures and components, in the light of knowledge acquired from research in the field, together with the requirements and recommendations of codes and restoration documents. The analysis of some case studies is presented, to highlight the advantages and constraints in the use of composites for strengthening historical buildings.

Potential and limits of FTIR methods for reclaimed asphalt characterisation

Abstract: In recent years, as a result of both environmental policy and industrial economic strategy, the amount of asphalt pavement that has been recycled has risen dramatically. As a result of this, several international research projects have shown the need for improvements in reclaimed asphalt (RA) characterisation to support these higher recycling rates. During the life of a pavement, as well as the structural damage induced by traffic, the intrinsic properties of asphaltic materials are affected through the oxidative ageing of the bituminous binder. The molecular changes associated with this oxidation can be monitored via variations in Fourier transform infrared (FTIR) spectra. Therefore, if the relationships between binder properties, such as penetration, softening point and complex shear modulus and the spectra parameters can be established, the characterisation of RA produced by the milling of old pavements can be greatly improved. However, the interpretation of oxidation parameters from these spectra is not straightforward, and many different techniques are used to calculate them in the asphalt community. This paper gives a brief overview of asphalt oxidation and the FTIR principle. It then presents the results from four different calculation methods for the oxidation parameters applied to a set of more than 100 spectra obtained during the RILEM TC-ATB-TG5 round robin test. From this study, it is obvious that the conclusions of the analysis performed on the same set of raw spectra can differ considerably according to the FTIR oxidation index calculation method adopted. Using the RILEM database, the potential links between a given ageing index and the physical properties are then evaluated. As a result of this study it can be concluded that ageing comparison through FTIR should be considered as relative and restricted for use in measurements at different ageing steps on the same type of initial mixture. For FTIR spectra to be comparable between different laboratories, harmonisation of the data acquisition procedures is needed. Also the different calculation methods of the oxidation indicators seem to reflect different physical properties. So a first step toward harmonisation would be to agree on the relevant physical properties actually needed for efficient RA characterisation. This could then lead to a common FTIR index calculation method, which would reflect the properties of interest. For this, additional research is needed to improve knowledge of the relationship between the chemical oxidative process of a binder and the evolution of its physical properties.

Simulation of fresh concrete flow using Discrete Element Method (DEM): theory and applications

Abstract: This article provides an overview of the development and the contemporary state of research in the field of simulating fresh concrete flow using the Discrete Element Method (DEM). First, this work originating from TC 222-SCF simulation of fresh concrete flow, covers the mathematical methodology, the identification of the model parameters and the link between the rheological properties of fresh concrete and the parameters of DEM-based models. Various examples of the estimation of model parameters and calibration of the model were demonstrated, followed by verifications by comparing the numerical results and the corresponding predictions by analytical formula and laboratory experiments. Furthermore, software used in concrete engineering and existing industrial applications of the developed particle models were described, showing the potential of DEM.

Natural carbonation of aged alkali-activated slag concretes

Abstract: Alkali-activated slag concretes stored for 7 years under atmospheric conditions are assessed, and the structural characteristics of naturally carbonated regions are determined. Concretes formulated with a 400 kg/m3 and water/binder (w/b) ratio between 0.42 and 0.48 present similar natural carbonation depths, although these concretes report different permeabilities after 28 days of curing. The inclusion of increased contents of binder leads to a substantial reduction of the CO2 penetration in these concretes, so that negligible carbonation depth values (2 mm) are identified in concretes formulated with 500 kg/m3 of binder. Calcite, vaterite, and natron are identified as the main carbonation products formed in these concretes. These observations differ from the trends which would be expected in comparable ordinary Portland cement-based concretes, which is attributable to the physical (permeability) and chemical properties of alkali-activated slag concretes promoting high long-term stability and acceptably slow carbonation progress under natural atmospheric conditions.

In situ assessment of structural timber using non-destructive techniques

Abstract: This paper summarizes the test recommendations for selected non-destructive testing (NDT) techniques as developed by members of the RILEM Technical Committee AST 215 “In situ assessment of structural timber”. The recommendations cover visual inspection, moisture content determination, species identification, digital radioscopy, and ground penetrating radar. The paper includes a matrix of common NDT to assess structural timber. The discussion of each technique is intended to provide users with sufficient information to understand the theoretical basis, typical equipment set up, and basic capabilities and limitations.

Out-of-plane behaviour of infill masonry panels strengthened with composite materials

Abstract: The performance of various composite materials applied to strengthen hollow masonry panels under out-of-plane actions is compared here. The strengthening solutions belong to three reinforcement Externally Bonded (EB) wet lay-up systems: (i) bidirectional composite meshes applied with inorganic matrices, i.e., Textile Reinforced Mortars (TRM); unidirectional composite textiles applied with (ii) inorganic matrices, i.e., Steel Reinforced Grouts (SRG), or (iii) organic (epoxy) matrices, i.e., Fibre Reinforced Polymers (FRP), Steel Reinforced Polymers (SRP) and Natural FRPs. Carbon FRP, flax and hemp NFRP, and basalt and glass-based TRM were examined. As inorganic matrix, a cement-based mortar was used for TRMs; in the case of SRGs, comparisons with a magnesia-based matrix were also carried out. Twenty-seven specimens were subjected to simplified four-point monotonic bending tests, aimed at reproducing in the laboratory the failure condition of infill masonry walls under out-of-plane actions. The results are compared in terms of failure mode and mechanical improvement, and provide an analytical evaluation of moment–curvature behaviour according to bilinear laws.

Dispersion of CaCO3 nanoparticles by sonication and surfactant treatment for application in fly ash–cement systems

Abstract: This research aims to offset the negative effects of fly ash on the early-age properties of cementitious materials with the use of calcium carbonate (CaCO3) nanoparticles. The main focus is to enhance the effect of the nanoparticles by improving dispersion through ultrasonication and use of surfactants. CaCO3 aqueous suspensions with various surfactant types and concentrations are prepared and subjected to different sonication protocols (varying duration and amplitude). Dispersion and stability are quantitatively measured by comparing their absorbance spectra through spectrophotometry and qualitatively evaluated through SEM imaging. The effectiveness of sonicated CaCO3 nanoparticle additions in accelerating setting and improving early-age compressive strength gain of fly ash–cement pastes is investigated. The sonication protocol is optimized and the most effective dispersion is achieved with polycarboxylate-based superplasticizer. Good agreement is found between the dispersion measurements and mechanical performance.

Tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC)

Abstract: The tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC) under constant amplitude fatigue cycles is presented. Three series of uniaxial tensile fatigue tests up to a maximum of 10 million cycles were conducted with the objective to determine the endurance limit of UHPFRC that was supposed to exist for this material. The fatigue tests reveal that an endurance limit exists in all three domains of UHPFRC tensile behaviour at S-ratios ranging from 0.70 to 0.45 with S being the ratio of the maximum fatigue stress to the elastic limit strength of UHPFRC. Rather large variation in local specimen deformations indicates significant stress and deformation redistribution capacity of the UHPFRC bulk material enhancing the fatigue behaviour. The fatigue fracture surface of UHPFRC shows features of the fatigue fracture surfaces of steel, i.e. fatigue crack propagation is identified by a smooth surface while final fracture leads to rather rough surface. Various fatigue damaging mechanisms due to fretting and grinding as well as tribocorrosion are identified.

Behaviour of recycled aggregate concrete filled stainless steel stub columns

Abstract: Past research indicates that recycled aggregate concrete (RAC) could be successfully used in concrete-filled steel tubular (CFST) columns Their yielded performance is almost as good as that of the traditional CFST columns In addition, as a comparatively new construction material, stainless steel can be used to replace carbon steel for enhancing the durability and ductility of CFST columns With an aim to combine the advantages of both RAC and stainless steel, RAC is proposed in this paper to be used as a filling material for stainless steel tubes A test program is introduced in this paper to investigate the behaviour of RAC-filled stainless steel stub columns For comparison purposes, reference specimens with carbon steel tubes are also tested In the end, finite element analysis is conducted to simulate the current test results and those reported in the literature

Cement paste content and water absorption of recycled concrete coarse aggregates

Abstract: In this paper, we first show that the amount of residual cement paste in recycled concrete aggregates (RCA) depends on the initial paste content, on the mechanical properties of the initial cement paste and on the mechanical quality of the transition zone at the interface between the initial natural aggregates and cement paste. Our results suggest that water absorption of RCA at 24 h can be seen as the simple sum of the capillary absorption of both residual cement paste and initial natural aggregates. The absorption kinetics is similar to the capillary absorption of traditional uncrushed concretes but, on the industrial time scale of interest, is strongly affected by the size and configuration of the residual cement paste patches at the surface of the initial natural aggregates. We finally propose a tentative frame for a classification of RCA based on water absorption rate and water absorption capacity at 24 h.

Understanding interactions between cementitious materials and microorganisms: a key to sustainable and safe concrete structures in various contexts

Abstract: Building materials can be exposed to microorganisms (mainly bacteria, fungi and algae) in almost every aqueous medium or damp environment, water being the indispensable condition for life development. The activity of microorganisms can be responsible for mineralogical, chemical and microstructural damage to the material (biodeterioration). Deleterious effects can also concern the aesthetics of a building (proliferation of colored biological stains on facades and roofs) or the quality of indoor air (presence of microorganisms in damp buildings). However, microorganisms can also have positive effects (healing of materials) and their action is explored through the development of bio-based protective systems intended for building materials. In all cases, understanding interactions between building materials and microorganisms is an indispensable step toward the development of more sustainable, better quality, safer structures in many environments. This paper presents two examples where the action of microorganisms has—or is likely to have—strong impact on the durability and safety of concrete structures. The first example concerns the biodeterioration of concrete in agricultural and agro-food environments. The second example is that of the abiotic and biotic reactivity of nitrates in repository of intermediate-level long-lived nuclear wastes. The paper presents the approaches used to explore and understand the phenomenology of bio-geo-chemical interactions in these complex environments. These studies notably comprise the development of test methods and experimental pilots to enable these explorations to be carried out. Current shortcomings in the scientific literature and in the standardization environment are also highlighted.

In situ assessment of structural timber using stress-wave measurements

Abstract: This paper summarizes the test recommendations for in situ assessment of structural timber using stress wave measurements as developed by members of the RILEM Technical Committee AST 215 “In-situ assessment of structural timber”. In the first part, the basic principles, the equipment, and the practical application of stress-wave-based testing using the time-of-flight method are described. A detailed testing procedure provides hands-on information on the execution of in-field stress wave testing. A typical example is given to demonstrate step-by-step on how to evaluate stress wave readings and the health state of the inspected timber member. The latter part of the paper gives a short overview of the use of acoustic tomography and ultrasonic echo methods.

Bond behavior between steel reinforcement and recycled concrete

Abstract: In this paper the bond behavior of recycled aggregate concrete was characterized by replacing different percentages of natural coarse aggregate with recycled coarse aggregate (20, 50 and 100 %). The results made it possible to establish the differences between the conventional concrete bond strength and the recycled concrete bond strength depending on the replacement percentage. It was thus found that bond stress decreases with the increase of the percentage of recycled coarse aggregate used. In order to define the influence of recycled aggregate content on bond behavior, normalized bond strength was calculated taking into account the reduced compressive strength of the recycled concretes. Finally, using the experimental results, a modified expression for maximum bond stress (bond strength) prediction was developed, taking into account replacement percentage and compressive strength. The obtained results show that the equation proposed provides an experimental value to theoretical prediction ratio similar to that of conventional concrete.

Are steel fibres able to mitigate or eliminate size effect in shear

Abstract: This paper reports some recent results of an experimental campaign on fibre reinforced concrete (FRC) beams under shear loading tested at the University of Brescia: nine full scale beams, having a height varying from 500 to 1,500 mm, were tested for investigating the effect of steel fibres on key-parameters influencing the shear response of concrete members, with special emphasis on size effect. All tested members contained no conventional shear reinforcement and different amounts of steel fibres: 0, 0.64 or 1 % by volume. Results show that a relatively low volume fraction of fibres can significantly increase bearing capacity and ductility. The latter determines visible deflection and prior warning of impending collapse, which is not possible in plain concrete beams (without transverse reinforcement). The size effect issue is substantially limited. Results were compared against the shear formulation incorporated in the final draft of fib Model Code, which can be considered a valuable analytical model for FRC beams under shear loading, even in the case of the three deepest elements herein tested.

Interfacial thickness and interaction between asphalt and mineral fillers

Abstract: In order to improve the road performance of asphalt mixture, interfacial structure and interaction of asphalt mastic which is the most important component of asphalt mixture were studied. The interface thicknesses of asphalt mastics were calculated by the specific heat value changes in glass transition. The critical volume fractions of asphalt mastics were determined for each asphalt-filler combination using a simple two-point projection technique based on the reciprocal relative creep compliance. The interface interaction parameter was proposed based on modified Palierne emulsion model, and its effect factors were analyzed. The results show that the interface layer between asphalt and filler is nanoscale, which becomes thicker with the increasing of filler volume fraction, and granite filler has the thickest interface layer. The higher the temperature is, the smaller the critical volume fraction is. When the temperature is lower than 45 °C, the critical volume fractions ranking of asphalt mastics is as follows: limestone > andesite > granite. When the temperature is higher than 45 °C, the distinction of critical volume fraction is not obvious. B value can represent interaction between asphalt and filler exactly. The interface interaction ranking between asphalt and different fillers is as follows: granite > andesite > limestone. The higher the temperature is (or the more fine the filler is), the stronger the interface interaction is.

Interaction mechanism of cement and asphalt emulsion in asphalt emulsion mixtures

Abstract: The interaction characteristics of cement asphalt composite mastic (CAM) and performance properties of cement asphalt emulsion mixtures (CAEM) were evaluated in this work using chemical and mechanical test methods to investigate the effect of the presence of cement on asphalt emulsion mixtures (AEM). The chemical composition of the CAM was obtained through use of X-ray diffraction, Fourier-transform infrared spectroscopy, and environmental scanning electron microscopy (ESEM) as a means to describe the interactions between the cement and asphalt in the composite materials. Test results demonstrated that cement can hydrate with the water phase of the asphalt emulsion. Asphalt droplets can simultaneously enclose cement particles and delay the hydration reaction process of cement. The interaction mechanism of cement particles or hydration products and residual asphalt is a physical compound process. The influence of these findings on asphalt emulsion mixture design and performance properties was assessed using varying mix design components and conducting laboratory-based mechanical test methods for rutting resistance and moisture susceptibility. Mix design components varied including added water content, emulsion content, and cement dosage levels. The optimum fluids content was determined based on the dry indirect tensile strength. It was found that the cement content significantly impacts the optimum fluids content for both added water and emulsion. Furthermore, the presence of cement improves the dry tensile strength, rutting resistance, and moisture susceptibility. Based on microstructural analysis of CAM and CAEM, the mechanism by which cement improves the performance of AEM is attributed to the ability of hydration products to increase both the stiffness of the asphalt binder and the adhesion at the mastic–aggregate interface. In practical applications, this study recommends a mix design method for cement-modified asphalt emulsion mixes (CAEM) based on selection of optimum cement and emulsion contents using indirect tensile strength and verification of the design through evaluation of the moisture susceptibility and rutting resistance of the CAEM mix. Threshold values of CAEM mix mechanical properties to determine the quality of the design are proposed.

Effect of calcium–silicon ratio on microstructure and nanostructure of calcium silicate hydrate synthesized by reaction of fumed silica and calcium oxide at room temperature

Abstract: A C–S–H series with calcium–silicon ratio 0.6–3.0 was synthesized by pozzolanic reaction. Phase composition, nanostructural and morphological characteristics were determined using XRD, XRF, SEM and 29Si NMR. Most of the samples were phase-pure, poorly crystalline C–S–H. Significant changes in the nanostructure of the C–S–H samples were observed when the calcium–silicon ratio reached values of 0.8, 1.0 and 1.5. At calcium–silicon ratio 0.8 the basal XRD peak began to develop, crosslinking between layers was seen below this ratio but not above, and there was a substantial decrease in mean silica chain length at this ratio. At calcium–silicon ratio 1.0 there was a pronounced microstructural change from granular to reticular and another substantial decrease in mean chain length (indicated by an abrupt increase in the Q1 peak intensity and decrease in the Q2 peak intensity). At calcium–silicon ratio 1.5 the basal XRD peak began to diminish again, the mean silica chain length decreased further, and isolated tetrahedra (Q0) were observed.

Fiber distribution and orientation in UHP-FRC beams and their effect on backward analysis

Abstract: In this study 23 ultra-high performance fiber reinforced concrete (UHP-FRC) notched beams were tested in order to investigate the effect of fiber orientation and distribution on the bending behavior. Due to the small fiber diameter and high fiber volume fraction used in UHP-FRC, the number of fibers crossing a 150 × 150 mm (6 × 6 in.) beam section can be as high as 26,000. An opto-analytical method was used to investigate the number and orientation of the fibers near the critical crack in each test beam. For the casting method used, which was in accordance with the recommendation of Association Francaise de Genie Civil (AFGC) and Service d’etudes techniques des routes et autoroutes (SETRA), a high degree of uniformity in fiber distribution and orientation was found over the beam cross section. Based on the analytically determined fiber orientation and distribution, shape functions were derived that take into account variations over the beam height. The proposed shape functions were incorporated in a backward analysis of the results from bending tests to estimate the uniaxial tensile behavior of the UHP-FRC used. The sensitivity of the backwardly calculated tensile behavior of UHP-FRC to variations in fiber density and orientation was evaluated. Based on the bending test results of the notched beam specimens, and for the casting method used in this research, the influence of these variations on the backwardly calculated tensile strength was found to be small, with a maximum stress variation of ~5 % for any given crack width.

RC columns externally strengthened with RC jackets

Abstract: In this paper the behaviour in compression of RC columns externally strengthened with concrete jacketing is analysed and a cross-section analysis of the jacketed member under axial load and bending moment is developed. The focus was to study the effect of confinement of concrete jacket on concrete core and the behaviour of compressed bars with buckling effects. Some other important aspects such as shrinkage, creep, old to new concrete surfaces and bond split effects were not included in the model because: the use of thick non-shrink grout jacket and a well-roughened surface of old-to-new concrete was supposed; long term effects were included though corrective coefficients for monolithic behaviour proposed in the literature. A preliminary validation of the model adopted was made referring to the short-term experimental data available in the literature on the compressive behaviour of RC columns strengthened with concrete jacketing. Good agreement was obtained with the available data in terms of moment-axial force domains and moment–curvature diagrams. The analysis showed the effectiveness of this reinforcing technique in improving both the strength and the ductility of RC cross-sections of columns, highlighting the importance of an accurate choice of strengthened materials (concrete grade, thickness of jacket, space and diameter of stirrups, etc.).

Study on water sorptivity of the surface layer of concrete

Abstract: This paper presents the results from a study of water sorptivity of concrete surface layer. The sorptivity is characterized by a surface sorptivity index as measured by Autoclam. In this study, different types of concrete were immersed in ultrapure water and NaCl solution prior to the sorptivity test. The influences of several factors on the value and evolution of concrete surface sorptivity index are discussed. It is found that: concrete surface sorptivity is a function of the pore structure, higher porosity and lower tortuosity lead to higher surface sorptivity; as cured in moist condition for 1 month, the surface sorptivity is an increasing function of w/c in plain cement concretes, and an increasing function of fly ash replacement if w/b is kept constant; surface sorptivity increases as immersed in ultrapure water in the first month of immersion due to leaching, and decreases thereafter as the continuous hydration of cementitious materials makes the pore structure finer and finer; the immersion in NaCl solution limits the effect of leaching because of the formation of calcium oxychloride compounds, and results in lower long-term surface sorptivity index as compared with the ultrapure water immersion, due to the formation of Friedel’s salt which reduces the pore volume and blocks the pore network.

Influence of ring size on the restrained shrinkage behavior of ultra high performance fiber reinforced concrete

Abstract: In order to evaluate the restrained shrinkage behavior of ultra high performance fiber reinforced concrete (UHPFRC), ring-tests with three different wall thicknesses and two different diameters of inner steel ring were performed. Partially exposed free shrinkage and tensile tests were carried out simultaneously to assess the theoretical elastic stress, stress relaxation, degree of restraint and potential for cracking in the concrete. Test results indicated that the UHPFRC ring specimen with a thicker steel ring demonstrated a faster theoretical cracking time, higher stress relaxation and degree of restraint than that of a thinner steel ring, whereas those factors were rarely affected by the diameter of the inner steel ring. About 39–65 % of the theoretical elastic stress was relaxed by the sustained interface pressure. Since the actual residual tensile stress of all specimens was less than the tensile strength, the computed cracking potential varied from 0.43 to 0.7, and thus no shrinkage crack was observed. Finally, the degree of restraint provided a linear relationship with the ratio of steel and concrete wall thickness.

Influence of soil grading on the characteristics of cement stabilised soil compacts

Abstract: The paper deals with experimental investigations aiming at specifying optimum soil grading limits for the production of cement stabilised soil bricks (CSSB). Wide range of soil grading curves encompassing both fine and coarse grained soils were considered. Strength, durability and absorption characteristics of CSSB were examined considering 14 different types of soil grading curves and three cement contents. The investigations show that there is optimum clay content for the soil mix which yields maximum compressive strength for CSSB and the optimum clay content is about 10 and 14 % for fine grained and coarse grained soils respectively. Void ratio of the compacted specimens is the lowest at the optimum clay content and therefore possesses maximum strength at that point. CSSB using fine grained soils shows higher strength and better durability characteristics when compared to the bricks using coarse grained soils.

Testing and modeling of a traditional timber mortise and tenon joint

Abstract: The structural safety and behaviour of traditional timber structures depends significantly on the performance of their connections. The behaviour of a traditional mortise and tenon timber joint is addressed using physical testing of full-scale specimens. New chestnut wood and old chestnut wood obtained from structural elements belonging to ancient buildings is used. In addition, the performance of different semi and non-destructive techniques for assessing global strength is also evaluated. For this purpose, ultrasonic testing, micro-drilling and surface penetration are considered, and the possibility of their application is discussed based on the application of simple linear regression models. Finally, nonlinear finite element analysis is used to better understand the behaviour observed in the full-scale experiments, in terms of failure mode and ultimate load. The results show that the ultrasonic pulse velocity through the joint provides a reasonable estimate for the effectiveness of the assembly between the rafter and brace and novel linear regressions are proposed. The failure mechanism and load–displacement diagrams observed in the experiments are well captured by the proposed non-linear finite element analysis, and the parameters that affect mostly the ultimate load of the timber joint are the compressive strength of wood perpendicular to the grain and the normal stiffness of the interface elements representing the contact between rafter and brace.

Digital image correlation techniques to investigate strain fields and cracking phenomena in asphalt materials

Abstract: This paper is the outcome of a specific task group of the RILEM Technical Committee 241-MCD “Mechanisms of Cracking and Debonding in Asphalt and Composite Pavements”. The group on “Advanced Measurement Techniques” was established in 2011 to investigate DIC applications for non-destructive and non-contact measurements of strain fields during laboratory testing. The paper illustrates different DIC/optical flow applications in measuring strain distribution during laboratory testing. Specific applications of DIC for evaluating crack initiation and crack propagation in asphalt materials are presented.