Showing papers in "Materials and Structures in 2019"

The effect of the steel–concrete interface on chloride-induced corrosion initiation in concrete: a critical review by RILEM TC 262-SCI

Abstract: The steel–concrete interface (SCI) is known to influence corrosion of steel in concrete. However, due to the numerous factors affecting the SCI—including steel properties, concrete properties, execution, and exposure conditions—it remains unclear which factors have the most dominant impact on the susceptibility of reinforced concrete to corrosion. In this literature review, prepared by members of RILEM technical committee 262-SCI, an attempt is made to elucidate the effect of numerous SCI characteristics on chloride-induced corrosion initiation of steel in concrete. We use a method to quantify and normalize the effect of individual SCI characteristics based on different literature results, which allows comparing them in a comprehensive context. It is found that the different SCI characteristics have received highly unbalanced research attention. Parameters such as w/b ratio and cement type have been studied most extensively. Interestingly, however, literature consistently indicates that those parameters have merely a moderate effect on the corrosion susceptibility of steel in concrete. Considerably more pronounced effects were identified for (1) steel properties, including metallurgy, presence of mill scale or rust layers, and surface roughness, and (2) the moisture state. Unfortunately, however, these aspects have received comparatively little research attention. Due to their apparently strong influence, future corrosion studies as well as developments towards predicting corrosion initiation in concrete would benefit from considering those aspects. Particularly the working mechanisms related to the moisture conditions in microscopic and macroscopic voids at the SCI is complex and presents major opportunities for further research in corrosion of steel in concrete.

Influence of supplementary cementitious materials on the sustainability parameters of cements and concretes in the Indian context

Abstract: The consumption of cement in India and other emerging economies is expected to increase because of the continuing push towards development of housing and infrastructure. The increasing production of cement and utilization of concrete are bound to have a major impact on sustainability. The present work proposes a framework for sustainability assessment, in terms of the CO2 emissions and energy demand, that can be adopted in cases where suitable databases are not readily available. Case studies for cement manufacture have been considered in South India, with different system boundaries such as ground-to-gate, gate-to-gate and CSI. The assessment made using data from the plant and other sources highlights the benefits of using supplementary cementitious materials (SCMs) in terms of reducing the impact of cement and concrete. More importantly, limestone calcined clay cement shows considerable promise in terms of reduction in CO2 emissions and energy demand in both cement and concrete, with more improvement in higher grade concrete.

Prediction of dynamic modulus of asphalt mixture using micromechanical method with radial distribution functions

Abstract: Inter-particle interaction is one of the major reinforcement mechanisms for aggregates in asphalt mixture, which is a classic example of high-volume fraction particulate composites. This paper introduced the modified Ju-Chen (J-C) micromechanical method based on two types of radial distribution assumptions for inclusions in the matrix, namely the uniform distribution and Percus–Yevick (P–Y) distribution. A two-step approach was proposed and the elastic–viscoelastic correspondence principle was used to predict the effective dynamic modulus of asphalt mixture at different frequencies. The prediction results show that the uniform distribution and P–Y distribution based J-C method could generate the upper and lower bounds of dynamic modulus for asphalt mixture, respectively. As compared to the measured dynamic modulus at different temperatures and loading frequencies, the modified J-C method showed better prediction accuracy as compared to two traditional micromechanical models based on single inclusion configuration, Mori–Tanaka (M–T) and differential scheme effective medium models. The J-C method assuming P–Y distribution provided better accuracy at the low frequencies; while the J-C method assuming the uniform distribution only had good accuracy at the high frequencies. The study findings indicate that dynamic modulus of asphalt mixture can be predicted based on laboratory tests conducted at the fine aggregate mix level and the void ratio and the gradation of coarse aggregate using appropriate micromechanics methods.

Direct shear test for the assessment of rheological parameters of concrete for 3D printing applications

Abstract: Rheology of concrete plays a major role in concrete 3D printing applications, where the concrete is pumped at high pressure and extruded through a nozzle at low speed to build the structural component. The 3D printable concrete should be stiff and is different to normal or self-compacting concrete. Hence, the common testing methods used to estimate the rheological parameters are not suitable for 3D printable concrete. In this study, the direct shear test is trialled as a potential method to measure the rheological parameters of different mixes of concrete. The effects of water–cement ratio and shear rates on rheological parameters were examined. The tests were carried out with varying shear rates, ranging from 0.5 to 15 min−1, and normal stresses, ranging from 2 to 15 kPa, for mixes with water–cement ratios of 0.3, 0.4 and 0.6. Further testing was carried out on mixes with varying aggregate to cement and fine to total aggregates ratios to study the effect of binder and aggregate proportions on the rheology of mortar. It was found that the shear rates, 0.5 to 15 min−1, have little effect on the cohesion values and friction angles. Further, the behaviour of the mixes was found to be following the Mohr–Coulomb model.

Propagation of reinforcement corrosion: principles, testing and modelling

Abstract: Reinforcement corrosion is the risk most frequently cited to justify concrete durability research. The number of studies specifically devoted to corrosion propagation, once the object of most specialised papers, has declined substantially in recent years, whilst the number addressing initiation, particularly where induced by chlorides, has risen sharply. This article briefly describes the characteristics of steel corrosion in concrete that need to be stressed to dispel certain misconceptions, such as the belief that the corrosion zone is a pure anode. That is in fact seldom the case and as the zone is also affected by microcells, galvanic corrosion accounts for only a fraction of the corrosion rate. The role of oxygen in initiating corrosion, the scant amount required and why corrosion can progress in its absence are also discussed. Another feature addressed is the dependence of the chloride threshold on medium pH and the buffering capacity of the cement, since corrosion begins with acidification. Those general notions are followed by a review of the techniques for measuring corrosion, in particular polarisation resistance, which has proved to be imperative for establishing the processes involved. The inability to ascertain the area affected when an electrical signal is applied to large-scale elements is described, along with the concomitant need to use a guard ring to confine the current or deploy the potential attenuation method. The reason that measurement with contactless inductive techniques is not yet possible (because the area affected cannot be determined) is discussed. The method for integrating corrosion rate over time to find cumulative corrosion, Pcorr, is explained, together with its use to formulate the mathematical expressions for the propagation period. The article concludes with three examples of how to use corrosion rate to assess cathodic protection, new low-clinker cements or determine the chloride threshold with an integral accelerated service life method.

Relating early hydration, specific surface and flow loss of cement pastes

Abstract: Flow loss in superplasticized systems has been mainly explained in qualitative and comparative ways over the past years. This is due to the intrinsic complexity of the underlying mechanism involving a change in the agglomeration degree as a result of cement hydration. The lack of robust and reliable experimental methodologies must have additionally discouraged researchers from attempting to understand the phenomena of flow loss in quantitative terms. Thanks to new analytical methods, it was possible to prove that after the so-called onset point, yield stress increases exponentially with the increase of both heat rate measured by isothermal calorimetry and specific surface. This paper also identifies the existence of a direct proportionality between the increase of heat rate and the increase of specific surface area during the acceleration period, most likely reflecting the nucleation and growth nature at this stage of the cement hydration.

Influence of encapsulated sunflower oil on the mechanical and self-healing properties of dense-graded asphalt mixtures

Abstract: This paper re-evaluates the effect of sunflower oil capsules on the mechanical and self-healing properties of dense-graded asphalt mixtures. Different percentages of capsules (0.50 wt%, 0.75 wt% and 1.00 wt%) were mixed into dense asphalt. The influence of capsules on the properties of asphalt such as density, indirect tensile strength, particle loss, fatigue life, and self-healing, has been investigated. The distribution and integrity of the capsules has been also evaluated by means of CT Scans. It has been proven that capsules can survive the mixing and compaction process of asphalt mixture, do not decrease its mechanical properties and they rupture and release the oil under a high compression loading. Higher capsule content in the mixture resulted in higher oil release ratios. Furthermore, the oil released from the capsules significantly increased the self-healing capability of mixtures. Results from previous research were validated, where it had been found that 0.5% of capsules is the optimal content to obtain good mechanical performance, without affecting the rheological properties of dense-graded asphalt mixtures.

RILEM TC 247-DTA round robin test: mix design and reproducibility of compressive strength of alkali-activated concretes

Abstract: The aim of RILEM TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ is to identify and validate methodologies for testing the durability of alkali-activated concretes. To underpin the durability testing work of this committee, five alkali-activated concrete mixes were developed based on blast furnace slag, fly ash, and flash-calcined metakaolin. The concretes were designed with different intended performance levels, aiming to assess the capability of test methods to discriminate between concretes on this basis. A total of fifteen laboratories worldwide participated in this round robin test programme, where all concretes were produced with the same mix designs, from single-source aluminosilicate precursors and locally available aggregates. This paper reports the mix designs tested, and the compressive strength results obtained, including critical insight into reasons for the observed variability in strength within and between laboratories.

Evaluation of roughcast on the adhesion mechanisms of mortars on ceramic substrates

Abstract: Buildings frequently suffer from the low durability of external coating layers that use mortar. Low durability can be caused by a low efficiency of the adhesion between the coating mortar and the substrate, which may be ceramic, concrete, natural stone or other material. The application of the roughcast is often used empirically to increase adhesion, however, there are few studies to support this practice. The objective of this study was to analyze the adhesion mechanisms between the substrate and the mortar layer, evaluating tensile bond strength implications beyond the microstructure in the transition zone. Two conventional mortar coatings were used, applied on two ceramic substrates fired at 700 °C and 950 °C, besides the application or not of the intermediate adhesion system (roughcast). Both mortars and ceramic blocks were characterized according to the main testing standards. The tensile bond strength was performed for all the combinations proposed. The heat of hydration of samples of the roughcast was also analyzed. The results indicated that the adoption of the intermediary adhesion mechanisms between the ceramic substrate and the mortar, referred to as “Roughcast”, significantly increased the tensile bond strength and altered the type of rupture in the test. The composition of the mortar as well as the firing temperature of the ceramic substrate also influenced the strength conditions.

Curing behavior and mechanical properties of an eco-friendly cold-mixed epoxy asphalt

Abstract: Cold-mixed epoxy asphalt (CEA) is an eco-friendly material for steel bridge deck pavements. The effects of the asphalt on the curing behavior and mechanical properties of the CEA were studied. The pot life, curing characteristics, and curing kinetics of CEA blends were investigated by rotational viscosity testing, dynamic shear rheological analysis, and fluorescence microscopy. The results showed that the CEA pot life increased with asphalt content, and the curing apparent activation energy $$ E_{k} $$ of CEA-0.4 was more than two times higher than that of the epoxy resin. The curing reaction between the epoxy resin and curing agent was hindered by the asphalt. According to the FTIR and DSC results, the curing degree of the CEAs decreased with asphalt content. The elongation at break of the CEAs increased, while the tensile and adhesion strength decreased with asphalt content.

Enhanced mechanical properties of fiber reinforced concrete using closed steel fibers

Abstract: Concrete is weak in tension and fibers are added to it to make it ductile. One of the commonly used methods, for this purpose, is the addition of steel fibers of different geometries. However, failure of fiber reinforced concrete is initiated by pulling out of fibers, failing to utilize their full capacity. In this study, closed steel fibers (CSF) are proposed for addition into concrete. The objective was to avoid the pulling out of fibers in tension for full capacity utilization. When CSF are embedded into concrete, they are expected to reach the breaking point, utilizing their maximum capacity due to the presence of concrete matrix within the closed geometry of fibers. Different volumes of straight steel fibers (SSF) and CSF were used to investigate and compare their performance. Fresh and hardened concrete properties were considered. Results indicated decrease in workability of fiber reinforced concrete with increasing steel fiber content. No significant change in concrete compressive strength and modulus of elasticity was noted. There was a maximum of 46% increase in tensile strength while 36% increase in flexural strength of concrete with the use of CSF compared to that of SSF. The failure of concrete in tension was initiated by pulling out of open fibers and breaking of closed fibers. Thus, use of closed steel fibers is recommended to make full use of steel fiber tensile capacities which may revolutionize the field of fiber reinforced concrete.

Diagonal shear behavior of historic walls strengthened with composite reinforced mortar (CRM)

Abstract: Composite reinforced mortar (CRM) represents an innovative strengthening solution for existing masonry structures. CRM is comprised of a composite grid embedded within an inorganic matrix (mortar) and it is applied as externally bonded reinforcement of masonry members. The composite grid bears the tensile stresses whereas the inorganic matrix is responsible for the stress-transfer between the composite grid and the substrate. CRM showed promising results in improving the mechanical properties of different masonry members, such as walls and arches. However, a full understanding of the mechanical behavior of CRM strengthened masonry members is still missing, which hinders the formulation of reliable CRM design guidelines. In this paper, an experimental investigation of the in-plane behavior of masonry walls made by historical bricks and strengthened with a CRM comprised of a glass fiber composite grid embedded in a lime-based mortar is presented and discussed. The parameters studied are the wall dimension and type (double leaf with and without diatoni). Furthermore, the effect of steel anchors on the strengthened wall capacity is investigated. The results obtained are finally analyzed using simple analytical formulations.

Comparison of calcined illitic clays (brick clays) and low-grade kaolinitic clays as supplementary cementitious materials

Abstract: The use of calcined clays as supplementary cementitious materials (SCMs) has been identified as a viable option to decrease the CO2 emissions related to cement production. However, while extensive data is available about kaolinitic clays in this context, other clays such as illitic clays appear to be under-studied. Therefore, in the present study, two illitic clays were compared to two low-grade kaolinitic clays in terms of transformations in the calcination temperature range 650–900 °C, and performance of the calcined clays in blended cement pastes as measured by strength evolution, heat release, hydrated phase formation and portlandite consumption. The illitic clays required a higher calcination temperature for complete dehydroxylation of their illite than what is necessary for dehydroxylation of kaolinite. These higher calcination temperatures also led to particle sintering, significantly decreasing the specific surface area of the illitic clays, particularly for the clay with the higher Fe2O3 content. Nevertheless, while the kaolinitic clays generally exhibited the best performance as SCM, the illitic clay with lower Fe2O3 content performed similar to the kaolinitic clays when calcined at optimum temperature and applied at a moderate substitution rate. These findings demonstrate that several different clays have the potential to be used as SCM and indicate possible routes to identify suitable deposits for this purpose.

Prediction of autogenous shrinkage in concrete from material composition or strength calibrated by a large database, as update to model B4

Abstract: In modern concretes, the autogenous shrinkage, i.e., the shrinkage of sealed specimens, is much more important than it is in traditional concretes. It dominates the shrinkage of thick enough structural members even if exposed to drying. A database of 417 autogenous shrinkage tests, recently assembled at Northwestern University, is exploited to develop empirical predictive equations, which improve significantly those embedded in RILEM Model B4. The data scatter is high and the power law (time)0.2 is found to be optimal for times ranging from hours to several decades of years, as the test data give no hint of upper bound. Statistics of data fitting yields the approximate dependence of the power law parameters on the water-cement and aggregate-cement ratios, cement type, additives such as the blast furnace slag and silica fume, and curing type and duration. Alternatively, the power law parameters can be reasonably well predicted from the compression strength alone. Since some database entries do not report all these composition parameters and others do not report the compressive strength, and since the concrete strength is often the only material property specified in design, two types of models are formulated—composition based, and strength based. Both are verified by statistical comparisons with individual tests, and optimized by nonlinear statistical regression of the entire database, so as to minimize the coefficient of variation of deviations from the data points normalized by the overall data mean. The regression is weighted so as to compensate for the bias due to crowding of data in the short-time range. Statistical comparisons with the prediction models in the JSCE code, Eurocode and CEB MC90-99 code (identical to fib Model Code 2010) show the present model to give significantly better data fits. Finally it is emphasized that, in presence of external drying and creep, accurate predictions will require treating the autogenous shrinkage as a consequence of pore humidity drop caused jointly by self-desiccation due to hydration and by moisture diffusion, and solving the time evolution of humidity profiles. The present model is proposed as an update for the autogenous shrinkage formula in model B4, although recalibration of the whole B4 would be needed.

Biaxial bending of SFRC slabs: Is conventional reinforcement necessary?

Abstract: Fibre reinforced concrete shows enhanced performance in statistically redundant bi-dimensional structural elements that undergo biaxial bending. However, the lack of reinforcing rebars in fibre reinforced structural elements may affect the structural ductility which may further affect the overall load bearing capacity of these structures. To investigate the influence of fibres in such elements, six concrete plates of 2000 × 2000 × 150 mm reinforced with steel fibres and/or reinforcing rebars are tested under a central concentrated load. Two of the elements are reinforced with only 35 kg/m3 of steel fibres, two are reinforced with 2-way conventional reinforcing rebars (35 kg/m3, in each direction) and two are reinforced with both steel fibres and rebars. The specimens are simply supported at the middle of each side by means of a bilateral restraint; the deflection response and cracking behaviour of all the specimens are recorded and compared. Moreover, the methodology introduced in the fib Model Code 2010 for design of steel fibre reinforced concrete is implemented to predict the ultimate load bearing capacity of these elements and its reliability is determined in comparison with the experimental values. The comparison of the behaviour of the specimens reinforced only with steel fibres, with those reinforced with steel rebars, shows the higher efficiency of steel fibres in terms of load carrying capacity, but with a lower ductility. The combination of steel fibres and rebars allows for a better exploitation of the capacity of both reinforcement solutions. Finally, the reliability of the approach implemented for the ultimate load prediction is shown and the need of rebars in providing ductility in fibre reinforced concrete members is underlined.

Waste glass as binder in alkali activated slag–fly ash mortars

Abstract: This paper illustrates the application of waste glass powder as part of the binder in slag–fly ash systems activated by NaOH and NaOH/Na2CO3 activators. To evaluate the reaction kinetics, reaction products, mechanical properties, and durability performance of glass powder modified alkali activated slag–fly ash systems, calorimetry test, X-ray diffraction, FTIR, strength test, drying shrinkage tests, and carbonation test were conducted. From the isothermal calorimeter results, glass powder shows a higher reactivity compared to fly ash but still lower than slag. The reaction products of glass power modified samples exhibit an enhancement of polymerization degree of Si–O–T, observed in FTIR. As a consequence, higher drying shrinkage exists in glass modified mortars. The mechanical performance of different samples is mostly controlled by the Ca/Si of dry mixtures and activator type. After the slag–fly ash binder system was modified by the waste glass, a significant enhancement of resistance to carbonation was identified, especially for NaOH/Na2CO3 activated mortars, which show an increase of 300% on the carbonation resistance ability compared to the reference sample. The Na/(Si + Al) ratio of dry mixtures exhibits a positive correlation with carbonation resistance.

Size effect on fracture properties of concrete after sustained loading

Abstract: To investigate the size effect on the fracture properties of concrete after sustained loading, concrete beams with three heights of 100 mm, 200 mm and 300 mm were first subjected to 30% peak load over 115 days. Thereafter, they were moved out from the loading frames and tested under standard static three-point bending (TPB) loading until failure. The initial fracture toughness, unstable fracture toughness, fracture energy and evolution of the fracture process zone were then derived based on the experimental results, and the size effect on these fracture properties of concrete after sustained loading were evaluated. The experimental results indicated that compared with the specimens under the static TPB tests without pre-sustained loading, the cracking initiation resistance for the concrete after sustained loading increased, resulting in the increase of the initial cracking load and initial fracture toughness. In particular, the tendency was more significant for the larger size specimens. By contrast, the effects of sustained loading on the unstable fracture toughness, fracture energy, critical crack length and FPZ evolution could be neglected. Furthermore, the size effects on the fracture characteristics, including the fracture energy, and the FPZ evolution were obvious for the concrete specimens both under static loading and after sustained loading.

A nearly self-sufficient framework for modelling reactive-transport processes in concrete

Abstract: This paper describes a multi-species and multi-mechanism reactive-transport modelling framework for concrete. This modelling framework has the potential to be used in conjunction with performance specifications currently being developed in the US. The modelling framework is ‘nearly’ self-sufficient as it enables electrical resistivity to be used as the main physically measured input parameter in the simulations. The model uses thermodynamic calculations to predict pore solution composition, pore solution resistivity, pore volumes, and reactions between the solid and ionic components of the cementitious matrix such as chloride binding. The measured electrical resistivity is normalized by the calculated pore solution resistivity to compute the formation factor, which is used to predict transport properties of the ionic species. The framework allows the solution of reactive-transport equations with minimal input data to assess ionic movement, chloride ingress, and time to corrosion.

Transitions in unmodified and modified bitumen using FTIR spectroscopy

Abstract: The transitions in bitumen in the temperature range of 25–75 °C are normally investigated using rheological tools. Considering the complex response of the material in such temperature range, it will be helpful if the precise nature of the material is investigated at the macromolecular level also. In this study, an attempt is made to use FTIR spectroscopy to identify the thermal transitions in unmodified and three modified bitumen. The changes in peak position and intensity of the C–H stretching vibration at 2953, 2923 and 2853 cm−1 were analyzed. Transitions, predominantly solid–solid in nature were identified in the temperature range of 35–65 °C which can be attributed to the change in conformation of the crystalline fraction. While certain peaks distinguished the effect of modification and aging, few other peaks indicated multiple transitions in the solid state of the material. The degree of crystallinity, also calculated from the FTIR spectra, indicated changes in the conformation of the material in the temperature range of 45–55 °C.

Influence of service time of recycled coarse aggregate on the mechanical properties of recycled aggregate concrete

Abstract: The mechanical properties of recycled aggregate concrete (RAC) may be affected by the service time of parent concrete used to produce the recycled coarse aggregate (RCA), which has not been experimentally investigated. Current models primarily relate RAC elastic modulus to its compressive strength. These models cannot describe the difference in the mechanisms of the influence of the incorporation of RCA on the concrete elastic modulus and that on the concrete strength. This paper first investigated how the compressive strength, static modulus of elasticity and shrinkage deformation of the RAC are influenced by the service time of the RCA (1, 18 and 40 years). The RCA was adopted as 0%, 30%, 50% and 100% replacement of the natural coarse aggregate. Based on two-phase composite material theory, the elastic modulus model was then developed for RAC which is related to the elastic modulus of the companion natural aggregate concrete by accounting for the influence of residual mortar content (CRM) to achieve a better accuracy in wider parameter ranges. The model to predict the development of the compressive strength over time was also proposed for the RAC by modifying the EC2 model. Results obtained indicated that RAC compressive strength was affected by the service time of the parent concrete, and the newly proposed models showed good accuracy in predicting RAC mechanical properties.

Drying shrinkage of alkali-activated cements: effect of humidity and curing temperature

Abstract: Alkali-activated concrete (AAC) is a recycled and low-CO2 alternative to ordinary portland cement (OPC) concrete. One challenge with AACs is their propensity to shrinkage and cracking. In this work, drying shrinkage of four different structural-grade AACs was studied, namely: an activated class F fly ash, an activated slag, and two activated fly ash/slag blends. All four binders and a control OPC binder had the same (liquid/solid)vol and initial porosity. Drying shrinkage and mass loss of paste prisms were monitored as a function of time, relative humidity (RH), and the preceding moist-curing temperature. The results show that all AACs moist cured at 23 °C (73 °F) showed high shrinkage in comparison with the OPC binder and the shrinkage increased with higher proportions of fly ash. Steam curing at 60 °C (140 °F) substantially reduced the shrinkage of fly ash-rich mixtures, but it was less effective for slag-rich mixtures. Slag mixtures showed a significant time dependent response (creep), where the pastes continued to shrink after their mass loss had reached equilibrium.

Recommendation of RILEM TC249-ISC on non destructive in situ strength assessment of concrete

Abstract: This recommendation is written to improve the assessment of the in situ. Compressive strength of concrete in existing structures by combining core strength values and non-destructive measurements. Both average strength and its scatter are considered. Deriving a characteristic strength from the assessment results is not considered here. The recommendation applies for most common techniques (ultrasonic pulse velocity, rebound hammer, pull-out) but also for less common techniques (penetration test, etc.). The recommendation does not apply to situations in which no core has been taken from the existing structure and is limited to situations where NDT is combined with cores. The recommendation introduces the concept of Estimation Quality Level, corresponding to the target of assessment, and which is put in relation with the means and strategy developed for assessing concrete. The text details all steps that must be followed from the data gathering to the checking of the quality of the final estimations. For more clarity, an illustrative example is described for each step of the assessment process.

Alkali silica reaction of waste glass aggregate in alkali activated fly ash and GGBFS mortars

Abstract: This paper evaluates the alkali silica reaction (ASR) susceptibility of waste glass aggregate in alkali activated fly ash and ground granulated blast furnace slag (GGBFS) mortars as compared to that in ordinary Portland cement (OPC) mortars. In accelerated mortar bar tests, glass fine aggregate showed much lower expansions in alkali activated fly ash and GGBFS blended mortars than in OPC mortars or alkali activated neat fly ash or GGBFS mortars. Glass aggregate was classified as non-reactive with alkali activated fly ash and GGBFS blends according to 10-day and 21-day expansion limits of the Australian Standard. Microstructural studies revealed that glass aggregate produced typical ASR products in OPC mortars and alkali activated neat GGBFS mortars due to the presence of high calcium. However, alkali activated fly ash and GGBFS blended mortars produced reaction products of low Ca/Si and high Al/Si ratios that reduced the dissolution of reactive silica present in glass aggregate causing less expansions. The observed expansion of the alkali activated neat fly ash mortar is attributed to the analcime phase found in the X-Ray diffraction of this mortar.

Influence of temperature on hydration and microstructure properties of limestone-calcined clay blended cement

Abstract: In this article, the strength and microstructure development of two low clinker blended cements; limestone calcined clay cement (LC3) and slag-fly ash composite cement (CC) cured at 27 °C and 50 °C are analysed and compared to those of ordinary Portland cement (OPC). A significant difference in the 28 days strength was observed between 27 and 50 °C cured specimens in low clinker blends. The results show that the quantities of ettringite and carbo-aluminate phases significantly reduce when LC3 is cured at 50 °C. EDX analysis shows that significant quantities of aluminate are taken up in C–A–S–H, when curing is done at a higher temperature. It was seen from BSE image analysis that the C–A–S–H formed from the hydration of LC3 is much more heterogenous than the other cements due to non-uniformity in C–A–S–H density as well as intermixing of hydration products. It was also seen that although an increase in the temperature leads to a measurable increase in the density of C–S–H in OPC and CC, the influence on LC3 is not clear, the heterogeneity of the product appears to reduce. In the case of both LC3 and CC, the hydration of clinker is slowed down, especially after the first day, when cured at 50 °C. All these factors lead to a coarsening of the pore-structure and a reduction in the compressive strength of both low clinker cements, when cured at the higher temperature.

Effects of curing temperature and superabsorbent polymers on hydration of early-age cement paste containing a CaO-based expansive additive

Abstract: The early-age shrinkage can easily result in crack of concrete in the early age, which seriously affects the durability of concrete. The CaO-based expansion additive is a normally used and effective material for the shrinkage compensation and the reduction of early-age crack risk in concrete. The hydration is a factor that significantly influences the mechanical and durability properties of concrete, and the study on the hydration of the cement paste containing a CaO-based expansive additive under different curing temperature and with superabsorbent polymers addition is lacking. In this study, tests were conducted using the low-field nuclear magnetic resonance to assess the effect of curing temperature and superabsorbent polymers on the hydration of early-age cement paste containing a CaO-based expansion additive. The experimental results show that the incorporation of CaO-based expansive additive can promote the hydration of cement paste, and 2% dosage of CaO-based expansive additive has a better promotion than that of 3% dosage. High curing temperature can effectively accelerate the hydration of cement paste containing the CaO-based expansion additive. In addition, superabsorbent polymers can also effectively promote hydration.

Performance of bio-oil modified paving asphalt: chemical and rheological characterization

Abstract: Asphalt binders modified with bio-oils derived from various biomasses have been developed for addressing pavement sustainability and environmental concerns. This study evaluated the bio-binders modified with bio-oils derived from waste cooking oil, and was aimed for the chemical and rheological characterization under different oxidative aging conditions and the aging susceptibility of the bio-binders. The chemical analysis was based on saturates, aromatics, resins, and asphaltenes (SARA) fractionation, gel permeation chromatography (GPC), and thermogravimetric analysis (TGA). The rheological characterization consisted of multiple stress creep recovery, linear amplitude sweep, and elastic recovery tests. The results indicated that aging caused shift from the light components to the asphaltenes or heavy molecules. The bio-oil modification balanced the effect of aging by producing relatively well-dispersed asphalt systems in comparison to the petroleum control. According to the SARA and GPC analyses, the petroleum asphalt was less susceptible to aging. The TGA results suggested that once aged the bio-binders were less stable under high temperatures presumably due to thermal degradation of the bio-oil molecules. The addition of bio-oil lowered the rutting resistance and marginally reduced the elastic recovery potential; according to the corresponding evaluation parameters, the aging resistance of the bio-binders was similar or slightly lower as compared to the control. The bio-oil modification improved the fatigue cracking performance and also reduced the aging susceptibility within the context of fatigue resistance. The correlation between the chemical and rheological properties of the bio-binders were in line with the implications based on the colloidal model for petroleum asphalts.

Acceleration of steel corrosion in concrete by cyclic wetting and drying: effect of drying duration and concrete quality

Abstract: To investigate the influence of concrete quality and drying duration on steel corrosion rate when cyclic wetting and drying is used to accelerate corrosion propagation. To inform future similar experimental work in the area of accelerated steel corrosion in concrete. Concrete prisms (100 × 100 × 240 mm) were made using two w/b ratios (0.40 and 0.65) and three binder types. After elimination of the corrosion initiation phase using an impressed current technique, the specimens were exposed to cycles of wetting (2 days) and drying (1, 3, 5, or 7 days). Steel corrosion rate was monitored using a coulostatic technique, over a period of ca. 170 days. Both the duration of drying and concrete quality profoundly affect corrosion rate of steel in concrete in a cyclic wetting and drying regime. A general inference from the results is that with drying, the denser microstructure concretes with high resistivity exhibited resistivity corrosion control while the less dense microstructure concretes with low resistivity exhibited both cathodic and resistivity corrosion controls. In accelerated corrosion testing using cyclic wetting and drying, the combined effects of concrete quality and drying duration need to be considered in determining corrosion rate.

RILEM TC 252-CMB report: rheological modeling of asphalt binder under different short and long-term aging temperatures

Abstract: This paper evaluates whether the temperature reduction production during Warm-Mix Asphalt (WMA) leads to a beneficial effect on short- and long-term aging properties of asphalt binder. For this purpose, two 70/100 penetration grade virgin asphalt binders were used. First, the material was short term aged at three different temperature, 123 °C, 143 °C, and 163 °C, and then, long term aged. Shear complex modulus was then measured with the Dynamic Shear Rheometer and the data used to generate master curves and fit the 2 Spring 2 Parabolic 1 Dashpot model. Glover–Rowe parameter and crossover-temperature were next determined to further investigate the effect of aging temperature on the properties of the material. Significant differences were observed between 163 and 123 °C for both short- and long-term aging conditions. However, a remarkable difference between 143 and 123 °C could be detected only for the short-term aged binder. Finally, the statistical analysis confirmed the experimentally observed trend. Based on the results and analysis presented in this paper, it can be hypothesized that a reduction in the temperature of short-term aging, which is commonly experienced during WMA production, may result in an overall substantially milder aging.

Thixotropy and interfacial bond strengths of polymer-modified printed mortars

Abstract: 3D concrete printing is an emerging construction technique for building structures layer-by-layer based on digital computer models without the need of formwork. The monitoring of interlayer bonding is crucial to ensure durability and structural integrity. This investigation aims at proposing a new methodology to assess the concurrent effects of material structural build-up (thixotropy), time gap between successive filaments, and incorporation of styrene-butadiene rubber (SBR) polymers on bond strengths of deposited layers. Four mortar series prepared with 450–750 kg/m3 binder and effective water-to-binder ratio of 0.35–0.55 are tested; the resulting thixotropy rate ranged from 0.16 to 1.1 Pa/s. Results showed that mixtures exhibiting moderate thixotropy levels (i.e., about 0.48–0.64 Pa/s) yielded the best performance regarding interfacial bonding. The incorporation of SBR proved efficient to enhance the bond strength as well as attenuate its rate of drop over the time gap between successive layers. This was related to the polymer films that coalesce in the cementitious system, thus binding the cement hydrates together and causing increased tensile strength properties. The bond strengths recorded on specimens exposed to hot temperature of 45 °C were consistently lower than those determined in standard curing conditions. The methodology proposed was successfully validated using 3D laboratory printing machine.

Experimental investigation on the bond strength between sustainable road bio-binders and aggregate substrates

Abstract: Interest is growing on the application of bio-binders in road pavements. However, currently there is a lack of data concerning the adhesion between bio-binders and aggregates, which is a crucial aspect to ensure adequate performance and durability of bituminous mixtures, especially in the presence of water. In this regard, the present investigation focuses on the evaluation of the binder bond strength (BBS) between bio-binders, characterized by different percentages of a renewable wood bio-oil and different aging levels, and aggregate substrates (limestone and porphyry), in dry and wet conditions. Preliminarily, the binders were subjected to viscosity tests to determine BBS application temperatures. The main results show that the bio-binders studied exhibit a good adhesion with limestone both in dry and wet conditions as well as with porphyry in dry conditions, resulting in cohesive failures. For porphyry substrate, after wet conditioning, a progressive transition from adhesive to cohesive failures is observed as the bio-oil content increases, indicating that the bio-oil might improve the adhesion between bitumen and siliceous aggregates. Based on previous findings on the chemical characteristics of the bio-binders, the contribution of the bio-oil to the adhesion may be attributed to its high content of esters. Overall, the results suggest that the use of bio-binders in road pavements could lead to significant benefits in terms of performance and resistance to moisture damage.