Showing papers in "Materials and Structures in 2015"

Review: optical fiber sensors for civil engineering applications

Abstract: Optical fiber sensor (OFS) technologies have developed rapidly over the last few decades, and various types of OFS have found practical applications in the field of civil engineering. In this paper, which is resulting from the work of the RILEM technical committee “Optical fiber sensors for civil engineering applications”, different kinds of sensing techniques, including change of light intensity, interferometry, fiber Bragg grating, adsorption measurement and distributed sensing, are briefly reviewed to introduce the basic sensing principles. Then, the applications of OFS in highway structures, building structures, geotechnical structures, pipelines as well as cables monitoring are described, with focus on sensor design, installation technique and sensor performance. It is believed that the State-of-the-Art review is helpful to engineers considering the use of OFS in their projects, and can facilitate the wider application of OFS technologies in construction industry.

Durability properties of high volume fly ash concrete containing nano-silica

Abstract: This paper reports the results of an experimental study on durability of ordinary concrete and high volume fly ash (HVFA) concrete containing nano silica (NS). In this study, concretes containing 2 and 4 % of NS are prepared at a constant water/binder ratio of 0.4 for testing ages of 3, 7, 28, 56 and 90 days. The concrete containing 2 % NS exhibited similar compressive strength of that containing 4 % NS and is used to evaluate its effectiveness in HVFA concrete. The compressive strength and durability tests including water sorptivity, volume of permeable voids (VPV), chloride permeability and porosity are investigated experimentally. Results show that besides improvement in compressive strength, the addition of 2 % NS significantly reduced the water sorptivity, VPV, chloride permeability and porosity of HVFA concretes. It is also revealed that the above durability properties of concretes containing 38 % class F fly ash and 2 % NS as partial replacement of cement are superior than ordinary concrete containing 100 % cement.

Cyclic behaviour of typical metal connectors for cross-laminated (CLT) structures

Abstract: An extended experimental programme on typical cross-laminated (CLT) connections was performed at IVALSA Trees and Timber Institute. The paper discusses the results of monotonic and cyclic tests in shear and tension (pull-out) carried out on hold-downs and steel angle brackets used to anchor the wall panels to foundations or to connect wall panels to floor panels. Mechanical properties such as strength, stiffness, energy dissipation, impairment of strength and ductility were evaluated and are critically discussed in the paper. Significant ductility and energy dissipation was attained in most of the tests. Nevertheless, brittle failure modes were observed in some tests, indicating the need for introduction of capacity based design principles for CLT connections. The overstrength factors, which are needed for capacity based design, were also evaluated for the different types of connection tested. A comparison between the test results and the analytical formulas provided by current codes of practice and new proposals is also provided. The approach developed by Uibel and Blas gives slightly more accurate CLT metal strength predictions compared to the existing formulas in Eurocode 5. Both approaches lead to very conservative results. However, analytical models for the prediction of CLT metal connectors’ stiffness significantly overestimate the experimental values. Therefore, it is recommended that currently only experimental strength and stiffness values of hold-downs and angle brackets be used in seismic analyses. Some proposals to improve the mechanical performance of metal connectors in terms of strength and stiffness are also given based on this experimental and analytical study.

Role of carbonates in the chemical evolution of sodium carbonate-activated slag binders

Abstract: Multi-technique characterisation of sodium carbonate-activated blast furnace slag binders was conducted in order to determine the influence of the carbonate groups on the structural and chemical evolution of these materials. At early age (<4 days) there is a preferential reaction of Ca2+ with the CO3 2− from the activator, forming calcium carbonates and gaylussite, while the aluminosilicate component of the slag reacts separately with the sodium from the activator to form zeolite NaA. These phases do not give the high degree of cohesion necessary for development of high early mechanical strength, and the reaction is relatively gradual due to the slow dissolution of the slag under the moderate pH conditions introduced by the Na2CO3 as activator. Once the CO3 2− is exhausted, the activation reaction proceeds in similar way to an NaOH-activated slag binder, forming the typical binder phases calcium aluminium silicate hydrate and hydrotalcite, along with Ca-heulandite as a further (Ca,Al)-rich product. This is consistent with the significant gain in compressive strength and reduced porosity observed after 3 days of curing. The high mechanical strength and reduced permeability developed in these materials beyond 4 days of curing elucidate that Na2CO3-activated slag can develop desirable properties for use as a building material, although the slow early strength development is likely to be an issue in some applications. These results suggest that the inclusion of additions which could control the preferential consumption of Ca2+ by the CO3 2− might accelerate the reaction kinetics of Na2CO3-activated slag at early times of curing, enhancing the use of these materials in engineering applications.

TC 238-SCM: hydration and microstructure of concrete with SCMs

Abstract: This paper is the work of working group 2 of the RILEM TC 238-SCM. Its purpose is to review methods to estimate the degree of reaction of supplementary cementitious materials in blended (or composite) cement pastes. We do not consider explicitly the wider issues of the influence of SCMs on hydration kinetics, nor the measurement of degree of reaction in alkali activated materials. The paper categorises the techniques into direct methods and indirect methods. Direct methods attempt to measure directly the amount of SCM remaining at a certain time, such as selective dissolution, microscopy combined with image analysis, and NMR. Indirect methods infer the amount of SCM reacted by back calculation from some other measured quantity, such as calcium hydroxide consumption. The paper first discusses the different techniques, how they operate and the advantages and limitations along with more details of case studies on different SCMs. In the second part we summarise the most suitable approaches for each SCM, and the paper finishes with conclusions and perspectives for future work.

Long term durability properties of class F fly ash geopolymer concrete

Abstract: The environmental impact from the production of cement has prompted research into the development of concretes using 100 % replacement materials activated by alkali solutions. This paper reports the assessment of a number of key durability parameters for geopolymer concrete made from fly ash activated with sodium silicate and sodium hydroxide. Properties investigated have included workability, compressive strength, water sorptivity, carbonation, chloride diffusion and rapid chloride permeability. Microstructure studies have been conducted using scanning electron microscopy and energy dispersive X-ray spectroscopy. The results showed that both the geopolymer concretes with activator modulus 1.00 and 1.25 gave durability parameters comparable to Ordinary Portland and blended cement concretes of similar strength, while the geopolymer concrete with an activator modulus of 0.75 displayed lower durability performance. However, there is a concern over the long term performance of the geopolymer concretes with activator modulus of 1.00 and 1.25 when considering chloride induced corrosion of reinforcing steel due to the initial pH and long term chloride diffusion coefficient.

RILEM draft recommendation: TC-242-MDC multi-decade creep and shrinkage of concrete: material model and structural analysis. Model B4 for creep, drying shrinkage and autogenous shrinkage of normal and high-strength concretes with multi-decade applicability

Abstract: The original publication is available at the publisher’s web site: http://link.springer.com/article/10.1617/s11527-014-0485-2/fulltext.html#copyrightInformation. This article is accepted for publication, it is copyrighted by RILEM, and readers must contact RILEM for permission to reprint or use the material in any form.

Evaluation of different recycling agents for restoring aged asphalt binder and performance of 100 % recycled asphalt

Abstract: Because of aged binder, high reclaimed asphalt pavement (RAP) content mixes are susceptible to cracking failures and are less workable than virgin mixtures. The potential of six differently originated recycling agents was evaluated in this study to restore the desired binder properties using conventional binder test methods and the results were compared with performance-related test results of 100 % RAP mixture. Binder test results showed that application of organic products require much lower dose to provide the same softening effect as petroleum products. The workability of binder and mixture was improved compared to RAP, but remained lower than that of reference virgin mixture. All rejuvenated mixtures proved to be very rut resistant. Low temperature performance of RAP, measured by creep compliance at −10 °C, was improved with the application of all recycling agents and RAP mixes rejuvenated with waste vegetable products even performed equal or better than virgin mixture. Organic oil and both waste vegetable products provided the best performance in binder and mixture fatigue resistance tests as measured by linear amplitude sweep and fracture work density respectively. Overall, the organic products outperformed the petroleum based additives in most tests, which partly can be attributed to un-optimized recycling agent dose. It was observed that penetration test may be a good indicator for initial selection of optimum dose since the results provide indication of rut resistance as well as fatigue performance of mixture and can be easily predicted using an exponential relationship that was developed in the research.

Performance of concrete made with aggregates recycled from precasting industry waste: influence of the crushing process

Abstract: The aim of this paper is to evaluate the influence of the crushing process used to obtain recycled concrete aggregates on the performance of concrete made with those aggregates. Two crushing methods were considered: primary crushing, using a jaw crusher, and primary plus secondary crushing (PSC), using a jaw crusher followed by a hammer mill. Besides natural aggregates (NA), these two processes were also used to crush three types of concrete made in laboratory (L20, L45 e L65) and three more others from the precast industry (P20, P45 e P65). The coarse natural aggregates were totally replaced by coarse recycled concrete aggregates. The recycled aggregates concrete mixes were compared with reference concrete mixes made using only NA, and the following properties related to the mechanical and durability performance were tested: compressive strength; splitting tensile strength; modulus of elasticity; carbonation resistance; chloride penetration resistance; water absorption by capillarity; water absorption by immersion; and shrinkage. The results show that the PSC process leads to better performances, especially in the durability properties.

Effects of phosphoric acid and phosphates on magnesium oxysulfate cement

Abstract: Magnesium oxysulfate (MOS) cement is a promising material for fire-proofing and insulation applications Here, we have studied the effects of phosphoric acid and phosphates (H3PO4, KH2PO4, K3PO4 and K2HPO4) on the setting time, mechanical strength and water resistance of MOS cement X-ray diffraction was used to examine phase composition, and analytical reagents were used to prepare samples of the new phase found so that it would be examined by chemical and thermogravimetric analyses Adding phosphoric acid and phosphates can extend the setting time and improve the compressive strength and water resistance of MOS cement by changing the hydration process of MgO and the phase composition A new subsulfate phase 5Mg(OH)2·MgSO4·7H2O (517 phase) is formed, which is needle-like and insoluble in water Phosphoric acid or phosphates ionize in solution to form H2PO4−, HPO42− and/or PO43−, and these anions adsorb onto [Mg(OH)(H2O)x]+ to inhibit the formation of Mg(OH)2 and further promote the generation of a new magnesium subsulfate phase, leading to the compact structure, high mechanical strength and good water resistance of MOS cement The improvement produced by adding phosphoric acid or phosphates to MOS cement follows the order of H3PO4 = KH2PO4 ≫ K2HPO4 > K3PO4

A new 3D damage model for concrete under monotonic, cyclic and dynamic loadings

Abstract: Among the “theories” applied to model concrete behavior, damage mechanics has proven to be efficient. One of the first models for concrete introduced into such a framework is Mazars’ damage model. A new formulation of this model, called the “μ model” and based on a coupling of elasticity and damage within an isotropic formulation, is proposed herein for the purpose of 3D cyclic and dynamic loadings. Unilateral behavior (i.e., crack opening and closure) is introduced by use of two internal variables. A threshold surface is then associated with each of these variables. The shape of such surfaces has been chosen to model as accurately as possible concrete behavior under various loadings, i.e., tension, compression, shear, biaxial and triaxial, in the aim of simulating a large number of loading types (monotonic, cyclic, seismic, blast, impact, etc.). Applications of this model are presented on plain or reinforced concrete elements subjected to a range of loadings (e.g., multiaxial, cyclic, dynamic). A comparison with experimental results serves to demonstrate the effectiveness of these various selected options.

Hoop strains in FRP-confined concrete columns: experimental observations

Abstract: It is now well understood that the hoop rupture strain of fiber reinforced polymer (FRP) jackets confining concrete is often lower than the ultimate tensile strain of the component fibers. This paper presents the results of an experimental study designed specifically to investigate the two newly identified material dependent factors influencing the hoop strain efficiency of FRP jackets. 36 circular FRP-confined normal and high-strength concrete (normal-strength concrete and High-strength concrete) specimens were tested under axial compression. The results indicate that the hoop rupture strains of FRP jackets decrease with either an increase in the strength of the unconfined concrete or the elastic modulus of the fiber material. These observations were verified by additional results from a large FRP-confined concrete test database assembled from the published literature. In addition, the hoop strain-axial strain relationship of FRP-confined concrete was studied and the influence of the test parameters on the behavior was established. The findings from these investigations are presented together with an expression for the prediction of the strain reduction factor and a model to describe the hoop strain-axial strain relationship.

Effect of pre-soaked superabsorbent polymer on shrinkage of high-strength concrete

Abstract: Pre-soaked super-absorbent polymer (SAP) was incorporated into high-strength concrete (HSC) as an internal curing agent to study its effects on early-age shrinkage and mechanical properties. On the basis of the capillary stress based model for shrinkage prediction of concrete, together with the experimental results of cement hydration kinetics, evolution of internal temperature and humidity, development of pore structure and mechanical properties, the working mechanism of SAP was discussed. Results indicate that the addition of pre-soaked SAP significantly reduces the autogenous shrinkage as well as the early-age shrinkage of HSC under drying condition. In sealed HSC specimens, the drop of internal humidity caused by the self-desiccation effect is notably postponed by addition of pre-soaked SAP. The addition of pre-soaked SAP slightly reduces the compressive strength of HSCs and this effect is more pronounced in early-age concrete. Furthermore, an insightful comparison of the behaviours of the internal curing water introduced by the pre-soaked SAP and the additional free mixing water in concrete was made. Results indicate that the internal curing water behaves differently from the additional mixing water in influencing the cement hydration kinetics, pore structure of hardened cement pastes and the mechanical strength of concrete, due to the different spatial distribution of the two types of water in the concrete bodies. The shrinkage-reducing effect on HSC due to the addition of extra internal curing water incorporated by pre-soaked SAP is much stronger than that of the additional mixing water. Besides, the internal curing water shows much less strength-reducing effect than the additional mixing water. In virtue of the shrinkage prediction model, the working mechanism of pre-soaked SAP in reducing autogenous shrinkage of HSC is proposed on the basis of the following two aspects. The participation of internal curing water in cement hydration process leads to a total volume gain of the hardening cement pastes. Meanwhile, the release of internal curing water from the pre-soaked SAP postpones the drop of internal humidity. The synergistic effect of these two factors effectively reduces the autogenous shrinkage of HSC.

Effect of ionic crosslinking on the swelling and mechanical response of model superabsorbent polymer hydrogels for internally cured concrete

Abstract: The chemical and physical structure–property relationships of model superabsorbent polymer hydrogels were characterized with respect to swelling behavior and mechanical properties in different ionic solutions (Na+, Ca2+, and Al3+). The model hydrogels were composed of poly(sodium acrylate-acrylamide) (PANa-PAM) copolymer with varying concentrations of PANa (0, 17, 33, 67, and 83 wt%) and covalent crosslinking densities of 1, 1.5, and 2 wt%. By synthesizing the hydrogels in-house, systems with independently tunable amounts of covalent crosslinking and anionic functional groups were created, allowing for the relative effects of covalent and ionic crosslinking on the properties of the hydrogels to be directly quantified. It was found that the presence of Ca2+ and Al3+ in the absorbed fluid significantly decreased the swelling capacity and altered the swelling kinetics of the PANa-PAM hydrogels. The presence of Al3+ in solution resulted in the unexpected formation of a mechanically stiff barrier layer at the hydrogel’s surface, which hindered the release of fluid and caused the overall elastic modulus of the hydrogel to increase from O(10 kPa) for hydrogels immersed in Ca2+ solutions to O(100 kPa) for hydrogels immersed in Al3+ solutions. Tensile tests performed on isolated specimens of the stiff barrier layer yielded elastic moduli in the O(50–100 MPa) range.

Determination of particle size, surface area, and shape of supplementary cementitious materials by different techniques

Abstract: The particle size distribution, surface area and shape are fundamental characteristics of supplementary cementitious materials (SCMs). Accurate measurement of these properties is required in computational efforts to model the hydration process, and the characterization of these parameters is also an important practical issue during the production and use of blended cements. Since there are no standard procedures specifically for the determination of physical properties of SCMs, the techniques that are currently used for characterizing Portland cement are applied to SCMs. Based on the fact that most of the techniques have been developed to measure cements, limitations occur when these methods are used for other materials than cement, particularly when these have lower fineness and different particle shape and mineralogical composition. Here, samples of fly ash, granulated blast furnace slag and silica fume were tested. Different results obtained using several methods for the determination of specific surface area are presented. Recommendations for testing SCMs using air permeability, sieving, laser diffraction, BET, image analysis and MIP are provided, which represent an output from the work of the RILEM Technical Committee on Hydration and Microstructure of Concrete with Supplementary Cementitious Materials (TC-238-SCM).

Static behaviour of rammed earth: experimental testing and finite element modelling

Abstract: The paper presents an experimental program aiming at assessing the mechanical performance of rammed earth walls, namely under compression and shear loading. Axial compression and diagonal compression tests were carried out for this purpose, which allowed determining important mechanical parameters, such as compressive strength, Young’s modulus, Poisson’s ratio, shear strength and shear modulus. Furthermore, it allowed assessing the level of non-linear behaviour of the respective stress–strain relationships as well as the failure modes. The experimental results were then used in the calibration of numerical models (finite element method) for simulating the non-linear behaviour of rammed earth under shear loading. Both macro- and micro modelling approaches were considered for this purpose. The total strain rotating crack model was used to simulate the behaviour of the rammed earth material, while the Mohr–Coulomb failure criterion was used to simulate the behaviour of interfaces between layers. In general, the numerical models achieved good agreement with the experimental results, but uncertainties related to the definition of the input parameters required to perform a sensitivity analysis. The compressive strength, the Poisson’s ratio, the tensile strength and the tensile fracture energy revealed to be the most important parameters in the analyses.

Performance of blended ash geopolymer concrete at elevated temperatures

Abstract: This study involved laboratory investigation of the performance of blended ash geopolymer concrete at elevated temperatures. Geopolymer concrete composite was prepared using blended ash, pulverized fuel ash, and palm oil fuel ash, obtained from agro-industrial waste along with alkaline activators. The samples were heated up to 800 °C to evaluate mass loss, strength, and microstructural changes due to thermal impact. Ordinary Portland cement (OPC) concrete was prepared as control concrete. The deterioration of concrete at elevated temperatures was examined by X-ray diffraction, fourier transformed infrared spectrometer, thermogravimetry analyser and field emission scanning electron microscope. A comparison between the performance of geopolymer and OPC concretes—the former exhibited better performance at elevated temperature.

Void structure of concrete with superabsorbent polymers and its relation to frost resistance of concrete

Abstract: Superabsorbent polymers (SAP) can be used to control air void formation in concrete. However, due to surfactant left on suspension polymerized SAP particles during production, they may not only create SAP voids but also entrain extra air. In the present investigation, a method is tested to remove surfactant prior to concrete mixing. The method comprises rinsing of the SAP with ethanol. This effectively removes the surfactant. Freeze–thaw testing of concrete with rinsed and non-rinsed SAP shows that for equal dosages of SAP, the extra air entrained due to surfactant is considerable and can make the difference between poor and satisfactory frost-resistance. Furthermore, the results indicate that voids created directly by SAP protect concrete against frost deterioration just like other air voids; if the concrete contains enough SAP voids, these alone can provide sufficient frost resistance.

Mechanical and durability performance of concrete incorporating fine recycled concrete and glass aggregates

Abstract: In this study, the effects of fine recycled concrete and glass aggregate on mechanical and durability performance of concrete were investigated. The waste concrete and glass were crushed, sieved and re-mixed to fulfill the same gradation as the available 0–4 mm crushed limestone aggregate size fraction. The recycled aggregates were substituted for fine aggregate as 0, 15, 30, 45 and 60 % by weight. 28-day mechanical and durability performance of the mixtures were determined. The maximum reduction in strength was observed in the specimens containing 60 % recycled glass aggregate. In addition, increasing the recycled concrete aggregate content caused decrease in UPV value and increase in water absorption, depth of penetration of water under pressure, chloride-ion penetration and water sorptivity of the concrete mixture. However, in recycled glass aggregate-containing mixture, UPV value increased and transport properties were improved with the increasing recycled glass aggregate content.

Prediction of lateral form pressure exerted by concrete at low casting rates

Abstract: In this work, we propose to extend the Ovarlez and Roussel model, for estimating lateral form pressure exerted by fresh concrete, to low casting rates and long duration formwork filling. In this case, the assumption of a linear increase of the concrete yield stress with time at rest is imprecise. This assumption is valid for approximately 1 h at rest after placing and may lead to an overestimation of the lateral pressure for a longer period. Based on chemical arguments and experimental observations, it has been shown that the yield stress increases exponentially over time. The same fundamental physical assumptions made by Ovarlez and Roussel are used to predict the lateral pressure exerted on formwork. Comparison of the proposed model with experimental measurements shows that, pressure can be predicted with an accuracy comparable to the Ovarlez and Roussel model for casting times less than 1 h.

Tests on superelastic Ni–Ti SMA bars under cyclic tension and direct-shear: towards practical recentring connections

Abstract: Shape memory alloys (SMAs) are smart metals featuring either superelastic effect or shape memory effect. While current emphasis on the civil engineering application of SMA is mainly given to dampers and isolators, recent research has been directed to superelastic SMA bolts or tendons for recentring connections. However, available information on the mechanical shear response of SMA bars is still inadequate. Consequently, this knowledge barrier can cause significant uncertainties when such components are intended to resist shear action. In this respect, this paper presents an experimental study on SMA bars subject to cyclic direct-shear actions, and uniaxial tensile tests were also undertaken on the bars of similar sizes for comparison. Key results including stress–strain/force–displacement hysteretic response, recentring ability, and energy dissipation are discussed in detail. The results show sound recentring and moderate energy dissipation/damping capabilities of the bars under cyclic tension below a strain level of 3 %, but beyond this the properties start to degrade. The SMA bars with a smaller diameter seem to perform better than those with a larger diameter. On the other hand, the cyclic direct-shear performance of SMA bars is generally unsatisfactory with poor recentring, energy dissipation, and damping characteristics. Therefore, the direct use of SMA bars subject to shear loading is undesirable. For both tension and shear tests, fracture of the bars within the threaded area is the most typical failure mode, which should be minimised in future applications to ensure a ductile response of seismic resistance devices. Recognising both advantages and potential shortcomings of the SMA bars, an innovative recentring connection type is proposed, and the preliminarily findings indicate that such connections are feasible.

Statistical justification of Model B4 for drying and autogenous shrinkage of concrete and comparisons to other models

Abstract: The shrinkage prediction part of Model B4 presented in the preceding paper is here statistically justified by optimal fitting of the new NU database containing 1050 test curves and by statistical compar- isons with the existing shrinkage prediction models. Rather than attempting a point-wise constitutive model for free shrinkage, Model B4 predicts the average shrinkage of cross sections of long members, which are affected by nonuniform residual stresses relaxing due to creep and microcracking. The main improvement in Model B4, which extends the 1995 Model B3 (a RILEM recommendation), is that sepa- rate formulae are given for: (1) the drying shrinkage, which represents most of the shrinkage observed in normal concretes of high water-cement ratios, and (2) for the autogenous shrinkage, which has a different physical mechanism and is important for modern high- performance concretes with admixtures, additives and low water-cement ratios. The effect of elevated temperature on the shrinkage rate is captured through an equivalent accelerated time based on activation energy. Model B4 is statistically calibrated by the new NU database of laboratory shrinkage tests through a sequential optimization procedure which isolates different physical behaviors. The new shrinkage equations are shown to match the time curves of individual shrinkage tests well, and fit the database with minimum error. Statistics of extensive compar- isons with Model B3 and with the models of various engineering societies, including those of ACI and fib, document a superior fit of the new model to the database.

Experimental and analytical study on ultimate strength behavior of steel–concrete–steel sandwich composite beam structures

Abstract: Steel–concrete–steel (SCS) sandwich composite structure is a relative new type of system that combines the advantages of steel and reinforced concrete structure Due to its excellent strength to cost performance, it exhibits versatile potential applications in building and offshore constructions In order to reduce the self-weight of the structure and achieve composite action between the steel and concrete, ultra-lightweight cement composite and novel shear connectors have been developed and applied in the SCS sandwich composite structures, respectively Meanwhile, the development of design guidelines lags behind the innovation of the structure In this paper, experimental studies on SCS sandwich composite beams with different types of concretes and novel shear connectors are presented This is followed by the development of analytical model to predict the ultimate strength of the SCS sandwich composite beams Finally, the proposed analytical model is verified against the results from a series of beams tests which include Bi-steel sandwich beams, double skin beams, sandwich beams with J-hook connectors, angle connectors, and cable connectors Through the analysis and verification, new methods to predict the ultimate strength of SCS sandwich composite beams are recommended for design purposes

Performance of hot mix epoxy asphalt binder and its concrete

Abstract: Hot mix epoxy asphalt (HMEA) binders have been widely used on the pavement of orthotropic steel bridge decks In present paper, rotational viscosity, glass transition temperature, damping properties, mechanical properties, and morphology of HMEA were investigated using Brookfield rotational viscometer, differential scanning calorimetry, dynamic mechanical analysis (DMA), universal material tester, laser scanning confocal microscopy Furthermore, the high temperature deformation resistance, rutting resistance, and fatigue cracking resistance of HMEA concretes (HMEACs) were evaluated using Marshall, wheel tracking, and three-point bending tests Results show that the addition of asphalts postpones the cure reaction of epoxy resin The rotational viscosity of HMEA binder keeps low enough to meet the demands of asphalt mixture mixing and paving at 160 °C DMA results show that HMEA exhibits excellent damping properties The addition of asphalts lowers the tensile strength and modulus of epoxy resin However, the elongation at break of HMEA increases with the increase of asphalt contents HMEACs exhibit good resistance to high temperature deformation, rutting, and fatigue cracking performances All these results show that HMEA binder exhibits excellent performance in the steel bridge pavement

Accelerated carbonation testing of alkali-activated slag/metakaolin blended concretes: effect of exposure conditions

Abstract: This paper addresses the effects of relative humidity (RH) and carbon dioxide (CO2) concentration on the rate and effects of accelerated carbonation in alkali-activated slag/metakaolin (MK) concretes. Strength and water absorption are used alongside phenolphthalein measurements to monitor carbonation, and the effects of drying at different RHs are particularly significant in controlling carbonation rates. Different trends in the carbonation rate as a function of MK content are observed when varying the CO2 concentration, further revealing that the carbonation rates of these materials under accelerated conditions are influenced strongly by the testing protocol. The standard phenolphthalein method for testing carbonation depth appears only to be capturing the change in alkalinity with pore solution carbonation, meaning that it does not correlate well with other performance parameters at high CO2 concentrations.

Influence of asphaltene content on mechanical bitumen behavior: experimental investigation and micromechanical modeling

Abstract: The description of the mechanical behavior of bitumen on the basis of its microstructure allows its improvement and moreover the development of equivalent or even more sustainable materials with similar properties For this reasons, a micromechanical model for bitumen is proposed, allowing the description of the viscoelastic bitumen behavior depending on characteristics of different material phases The definition and demarcation, respectively, of material phases is based on SARA fractions, and polarity considerations that support the assumption of asphaltene micelle structures within a contiguous matrix and the assumed interactions between them A sufficient number of static creep tests on artificially composed bitumen samples with asphaltene contents from 0 to 30 wt% served both as identification as well as validation experiments for the developed micromechanical model An excellent agreement between experimental results and model predictions indicates that the model is able to reproduce significant microstructural effects, such as interactions between asphaltenes, which strongly influence the bitumen behavior This model is therefore expected to contribute to a better understanding of the influence of the bitumen microstructure on the macroscopic mechanical behavior and subsequently be able to describe the mechanical consequences of microstructural effects like aging

Corrosion induced cover cracking studied by X-ray computed tomography, nanoindentation, and energy dispersive X-ray spectrometry (EDS)

Abstract: In this study, several experimental techniques are utilized to study different aspects of cracking of the protective cover due to reinforcing steel corrosion. Firstly, micro-computed X-ray tomography technique (CT-scanning) is used for monitoring rust formation during accelerated corrosion of reinforcement, and subsequent cover cracking. Secondly, the nanoindentation technique is employed to determine mechanical properties of the rust layer, which is an important input parameter for numerical models. Finally, energy dispersive X-ray spectrometry is used for elemental mapping around the steel–cement paste interface. Also, as a part of the study, the resistance of a strain hardening cementitious composite (SHCC) to corrosion induced cover cracking is examined. It was found that CT-scanning can be successfully utilized in non-destructive monitoring of the corrosion process in reinforced specimens. The nanoindentation study showed that the Young modulus of rust is highly dependent on the level of confinement provided to the rust layer by the surrounding cement paste. And, finally, SHCC proved to be an excellent alternative to brittle cementitious materials when corrosion induced cracking of the cover is a concern.

A review on alkali-aggregate reactions in alkali-activated mortars/concretes made with alkali-reactive aggregates

Abstract: Alkali-activated binders are attracting more and more attention as they could potentially be substitutes for traditional Portland cement. Alkali-aggregate reaction (AAR) has been intensively studied for Portland cement-based materials, but much less investigated for the alkali-activated binders. Thereby, precautions should still be made when the alkali-activated binders are used to produce concrete with alkali-reactive aggregates, though many studies have reported that alkali-activated cement-based materials demonstrated better performance than conventional cement-based materials. This paper reviews the research progresses on AAR in mortars and concretes made with alkali-activated binders and alkali reactive aggregates. A variety of factors, such as binders, activators, reactive aggregates are discussed. The applicability of existing testing methods for AAR in alkali-activated binders is also discussed.

Creep-recovery behavior of bituminous binders and its relation to asphalt mixture rutting

Abstract: Rutting is one of the main distress modes of asphalt pavements, especially after prolonged warm periods, and thus a great deal of research has been focused on the development of a rheological parameter that would address the rutting susceptibility of both unmodified and modified bituminous binders. In this study the Multiple Stress Creep Recovery (MSCR) test method is used to investigate the creep-recovery behavior of various bituminous binders and its relation to asphalt mixture rutting. Frequency sweep and MSCR tests were conducted on three unmodified and six elastomer and/or wax modified binders, and the resulting data were used to calculate the values of various binder rutting parameters. These binders were also used to manufacture asphalt slabs for mixture rutting simulations in the LPC wheel tracking device. It was found that the non-recoverable creep compliance parameter (J nr3200) and the accumulated strain at the end of the MSCR test (γ acc) correlate very strongly with each other and that they both have a superior capability of predicting asphalt mixture rutting compared to other rheological binder rutting indicators. An effort was made to explain the manifested nonlinear viscoelastic properties of the modified binders with their expected microstructural characteristics. The use of the MSCR test in the rutting characterization of bituminous binders is highly recommended based on the results of this research.

Synthesis, characterization and mechanisms of one-part geopolymeric cement by calcining low-quality kaolin with alkali

Abstract: Precursors for the preparation of one-part geopolymers were synthesized by calcining low-quality kaolin, which containing a lot of quartz, with NaOH or Na2CO3, then the cement pastes of ground precursor powders were synthesized by only adding water and cured at 80 °C for 3 days followed by curing at ambient environment for 25 days. The geopolymeric cement pastes obtained reasonable dry compressive strengths, though they were not quite resistant to water. Alkalis seriously alter the thermal decomposition course of kaolin, and the octahedral-coordinated Al in kaolin also turns into tetrahedral-coordinated Al according to Fourier transform infrared spectroscopy analysis. By using X-ray diffractions, it is discovered that nearly all minerals convert into X-ray amorphous substances and nepheline when the low-quality kaolin is thermally treated with alkalis at 950 °C though most kinds of minerals except kaolinite are stable after the thermal treatment without alkali at 750 °C. P-zeolite is produced during the hydration of the precursors, and the optimized precursor shows partly dissolving characteristics in water under scanning electronic microscope. It is supposed that the hydrolysis of sodium silicates and glassy substances induces the crystallization of P-zeolite and facilitates the hydrated pastes cementing, on the other hand, the delayed hydrolysis of sodium silicates with high SiO2/Na2O ratio after solidification might make the one-part geopolymers expand and reduce the softening coefficients.