Showing papers in "Materials and Structures in 2017"

Optimization and performance of cost-effective ultra-high performance concrete

Abstract: This paper presents a mix design method for ultra-high performance concrete (UHPC) prepared with high-volume supplementary cementitious materials and conventional concrete sand. The method involves the optimization of binder combinations to enhance packing density, compressive strength, and rheological properties. The water-to-cementitious materials ratio is then determined for pastes prepared with the selected binders. The sand gradation is optimized using the modified Andreasen and Andersen packing model to achieve maximum packing density. The binder-to-sand volume ratio is then determined based on the void content, required lubrication paste volume, and compressive strength. The optimum fiber volume is selected based on flowability and flexural performance. The high-range water reducer dosage and w/cm are then adjusted according to the targeted mini-slump flow and compressive strength. Finally, the optimized UHPC mix designs are evaluated to determine key properties that are relevant to the intended application. This mix design approach was applied to develop cost-effective UHPC materials. The results indicate that the optimized UHPC can develop 28-days compressive strength of 125 MPa under standard curing condition and 168–178 MPa by heat curing for 1 days Such mixtures have unit cost per compressive strength at 28 days of 4.1–4.5 $/m3/MPa under standard curing.

The steel-concrete interface

Abstract: Although the steel–concrete interface (SCI) is widely recognized to influence the durability of reinforced concrete, a systematic overview and detailed documentation of the various aspects of the SCI are lacking. In this paper, we compiled a comprehensive list of possible local characteristics at the SCI and reviewed available information regarding their properties as well as their occurrence in engineering structures and in the laboratory. Given the complexity of the SCI, we suggested a systematic approach to describe it in terms of local characteristics and their physical and chemical properties. It was found that the SCI exhibits significant spatial inhomogeneity along and around as well as perpendicular to the reinforcing steel. The SCI can differ strongly between different engineering structures and also between different members within a structure; particular differences are expected between structures built before and after the 1970/1980s. A single SCI representing all on-site conditions does not exist. Additionally, SCIs in common laboratory-made specimens exhibit significant differences compared to engineering structures. Thus, results from laboratory studies and from practical experience should be applied to engineering structures with caution. Finally, recommendations for further research are made.

Repeatability and sensitivity of FTIR ATR spectral analysis methods for bituminous binders

Abstract: Bituminous binders as organic materials are prone to aging mainly by oxidation. Aging changes the viscoelastic behavior of the material over time towards higher stiffness and brittleness. FTIR has been increasingly used lately to investigate impacts of oxidative aging on the chemical structure of bitumen. Especially the carbonyl and sulfoxide bands are affected by aging and commonly used to describe changes due to evolving oxidation of a binder. However, spectra obtained from FTIR can be analyzed in fundamentally different ways and the analysis method applied to a spectrum has an impact on the gathered results and especially on the repeatability and sensitivity with regards to oxidative changes. For the presented study, two bituminous binders, a PG 64-16 and an SBS modified PG 76-22 PM were aged by RTFO and PAV and subsequently measured by FTIR. The obtained spectra (up to 90 individual spectra per aging state) were analyzed employing various methods using either the original or a normalized spectrum, a band maximum or integration based calculation of indices from an absolute or tangential baseline. By analyzing the coefficient of variation of the different analysis methods, it was found that not all analysis methods exhibit the same repeatability and sensitivity. From the findings of the study it can be recommended to work with normalized spectra, use an absolute baseline and work with integration of areas for index production in favor over band maximum based methods.

Effect of superabsorbent polymers (SAP) on the freeze–thaw resistance of concrete: results of a RILEM interlaboratory study

Abstract: This article presents the results of an interlaboratory experimental study performed by 13 international research groups within the framework of the activities of the RILEM Technical Committee 225-SAP “Applications of Superabsorbent Polymers in Concrete Construction”. Two commercially available superabsorbent polymers (SAP) were tested in terms of their influence on the freeze–thaw resistance of ordinary concrete. To test the robustness of the method, all participating laboratories used locally produced materials. Furthermore, following this aim, various accelerated methods were used to estimate the resistance of the concrete to freeze–thaw cycles. The effect of adding SAP was from insignificant to considerably positive in terms of improvement in material performance as determined by reduced mass loss after freeze–thaw cycles; only one participant observed worsening of the material behaviour. At the same time, due to the addition of SAP, a much less pronounced decrease in the dynamic Young’s modulus was observed as a result of freeze–thaw testing without deicing salt.

Carbonation of concrete: the role of CO2 concentration, relative humidity and CO2 buffer capacity

Abstract: In this study, the effect of CO2 concentration and ambient relative humidity (RH) on accelerated and natural carbonation of 18 concrete mixtures produced with nine different cement types is investigated. Increasing the CO2 concentration from 0.045 to 1 and 4 % at 57 % RH does not alter the relative carbonation resistance between the concrete mixtures. The increase of RH from 57 to 70 and 80 % RH at 4 % CO2 shows a water-to-cement ratio and cement-specific effect that affects the relative carbonation resistance between the concrete mixtures. The carbonation resistance at 4 % CO2 and 57 % RH allows assessing the carbonation resistance of concrete in sheltered and with restrictions in unsheltered outdoor exposure. The carbonation resistance below 70 % RH is mainly governed by the CO2 buffer capacity. However, in the accelerated tests at 80 % RH and in the unsheltered outdoor exposure capillary condensation is of increased importance.

Composition design and performance of alkali-activated cements

Abstract: In the present work, the relationship between the composition of the SiO2–Al2O3–CaO precursor system and the setting time, microstructure and mechanical properties of the resulting alkali-activated cement (AAC) were investigated. The results showed that with the increase of metakaolin content and the modulus of activator solution, setting time of alkali-activated cements was prolonged. The compressive strength increased with the increase of CaO content to a certain extent, but had different trends as Si/Al ratio varied. Microstructural analyses revealed that CaO content had remarkable effects on the microstructure of AAC. In calcium-free system, the strength was dependent on the three-dimensional structure of N–A–S–H gels. As the CaO content increased gradually, the main activation product changed from N–A–S–H to C–(A)–S–H gel, resulting in a more compact structure. This investigation helps to build up a practical approach for the composition design of alkali-activated cements.

A review of characterisation methods for superabsorbent polymer (SAP) samples to be used in cement-based construction materials: report of the RILEM TC 260-RSC

Abstract: Superabsorbent polymers (SAPs) have proven to be a very promising admixture which can positively influence various properties of cement-based materials. SAP samples intended for such use should be pre-tested with respect to their absorptivity as well as their kinetics of ab- and desorption prior to implementation in concrete or mortar. This not only reduces workloads in concrete laboratories in pre-testing modified cement-based mixtures but in fact discloses essentials of the eventual performance of the SAP in concrete and other cementitious materials. The review at hand outlines fundamentals of the thermodynamics of polymer chemistry as a basis for the sorptivity tests. The importance of the ionic composition of the test liquids and the interplay among expansive (swelling) and collapse-causing chemical forces in the hydrogel network are highlighted. Methods of free sorptivity testing in adequate saline solutions as well as absorbency determined subject to the application of external forces are summarised. Advantages and drawbacks of these methods are discussed, including a validation of anticipatory evaluations of SAPs’ performance as admixtures in cement-based building materials. Apart from sorptivity pre-tests several methods of instrumental analytics for the chemical characterisation of SAP samples are drawn up, which represent standard approaches of polymer-chemical analytics.

Recommendation of the RILEM TC 236-BBM: characterisation testing of hemp shiv to determine the initial water content, water absorption, dry density, particle size distribution and thermal conductivity

Abstract: This recommendation is the outcome of research conducted by a working group within the RILEM Technical Committee 236-BBM ‘Bio-aggregate-based building Materials’. The work of the group related to the study of construction materials made from plant particles. The major raw material utilised being renewable, recyclable and easily available plant particles. These particles are obtained from the processing of hemp, flax, miscanthus, pine, maize, sunflower, bamboo and other plants. In this report, the outcome of the Round Robin Testing is centred on hemp because hemp shiv is the bio-aggregate that is the most widely used in building materials and the most studied in the literature. The first round robin test of the TC-BBM published in the State of The Art Report of Technical Committee 236-BBM ‘Bio-aggregate-based building Materials’ was carried out to compare the protocols in use by the different laboratories (labs) to measure initial water content, bulk density, water absorption, particle grading and thermal conductivity. The aim was to define a standardised characterisation protocol developed from those used by the different labs. The different methodologies used by 7 labs constitute a set of statistically representative data which have been analysed to develop this recommendation for the characterisation of hemp shiv.

Pull-out behavior of different fibers in geopolymer mortars: effects of alkaline solution concentration and curing

Abstract: Reinforcing geopolymer materials with fibers can enhance tensile and flexural strengths and fracture toughness. The bond between fiber and geopolymer matrix is a critical factor that needs to be investigated to optimize the performance of the fiber reinforced composite. In this study, single fiber pull-out tests are conducted on steel and polypropylene fibers embedded in geopolymer matrices; in addition, OPC mortars are tested as control condition. The following parameters are investigated: fiber type (i.e. steel and polypropylene) and shape, concentration of alkali solution in the geopolymer matrix, and curing conditions. Bond-slip performance, failure modes, and slip resisting mechanisms of different matrices and fibers are compared and discussed. The fiber deformation ratio, a novel parameter, is introduced to quantitatively investigate the effect of fiber shape on the mortar performance. In case of steel fibers, the geopolymer-fiber composite performs better for lower fiber deformation ratios, where the full fiber pull-out mechanism can be exploited. For higher deformation ratios, the strong bearing forces developed, combined with the high adhesion strength of the geopolymer-steel fiber interface, lead to more brittle failure mechanisms, such as fiber breakage or matrix failure, as observed in end-deformed and length-deformed steel fibers, respectively.

Outcomes of the RILEM round robin on degree of reaction of slag and fly ash in blended cements

Abstract: Working group 2 of the RILEM TC 238-SCM undertook a comparison of laboratories and techniques for the quantification of the degree of reaction of supplementary cementitious materials in blended cements. A common set of binary pastes of Portland cement with two slags, a calcareous and a siliceous fly ash was tested in seven laboratories. The results obtained by selective dissolution produced were quite scattered and seemed to underestimated the degree of reaction. The analysis of portlandite consumption was found to significantly underestimate the reaction unless additional data from XRD and electron microscopy was gathered to complete the corrections. Despite limited access to electron microscope among the participants and thus only a small data set being collected, this technique appeared as one of the most consistent. XRD-PONKCS gave considerable scatter, due mainly to a lack of a strict protocol and excessive overlap of slag and C–S–H signals. Overall, the study indicated that the precision of determination of the degree of reaction of SCMs in cement pastes is rather low and at best ±5%.

Dynamic Brazilian test of concrete using split Hopkinson pressure bar

Abstract: To research the fracture pattern of concrete specimens, the dynamic splitting tensile tests are conducted in different arc loading angles and impact velocities. The stress state of the specimens can be calculated by analyzing the strain gauges data on the split Hopkinson pressure bar. The specimens under the lower impact velocity achieves the stress equilibrium in loading direction, and the stress-state of the specimen is similar to that of quasi-static condition, in which the initial crack occurs at the center of the specimen and propagates along the loading diameter direction. When the impact velocity increases, the stress equilibrium is difficult to attain, and multiple cracks sometimes even ribbon fracture fragments appear at the center of specimens. The impact velocity plays a significant role in the failure pattern of concrete specimens, and different angles arc loading affect the local stress distribution of the specimens. The suitable load angle can reduce the local failure and improve the failure pattern of specimens. The stress state and failure pattern of specimens simulated by LS-DYNA coincide with the test results.

Fiber orientation distribution and tensile mechanical response in UHPFRC

Abstract: In this study, the crucial effect of the fiber orientation distribution on the tensile mechanical response of ultra high performance fiber reinforced concretes (UHPFRC) is discussed. A direct tension test method was used to characterize the tensile response of a UHPFRC material as well as to assess the actual tensile response along the principal directions in a real-scale UHPFRC structural element. Moreover, the actual fiber orientation distribution was evaluated in representative sections through an image analysis technique. The experimental results validated the anisotropy in the fiber orientation distribution and, consequently, in the tensile mechanical properties as a consequence of the casting process and the flow pattern. The concept of the fiber orientation factor was discussed as well as the approaches currently adopted to implement robust and reliable safety factors accounting for the fiber orientation distribution impact on the design methodologies for UHPFRC. Finally, the need of a comprehensive design framework for UHPFRC structures was highlighted in order to allow for fully exploitation of the material properties.

Reproducible mini-slump test procedure for measuring the yield stress of cementitious pastes

Abstract: The mini-slump test is a fast, inexpensive and widely adopted method for evaluating the workability of fresh cementitious pastes. However, this method lacks a standardised procedure for its experimental implementation, which is crucial to guarantee reproducibility and reliability of the test results. This study investigates and proposes a guideline procedure for mini-slump testing, focusing on the influence of key experimental (mixing and testing) parameters on the statistical performance of the results. The importance of preparation of always testing at the same time after mixing, testing each batch once rather than conducting multiple tests on a single batch of material, is highlighted. A set of alkali-activated fly ash-slag pastes, spanning from 1 to 75 Pa yield stresses, were used to validate the test method, by comparison of calculated yield stresses with the results obtained using a conventional vane viscometer. The proposed experimental procedure for mini-slump testing produces highly reproducible results, and the yield stress calculated from mini-slump values correlate very well with those measured by viscometer, in the case of fresh paste of pure shear flow. Mini-slump testing is a reliable method that can be utilised for the assessment of workability of cements.

Small amplitude oscillatory shear technique to evaluate structural build-up of cement paste

Abstract: Small amplitude oscillatory shear (SAOS) technique is a valuable and non-destructive test that can be employed to investigate the changes in the structure of fresh cement paste at early age. The test requires the paste to be deformed at very small strain amplitude, i.e. within the linear viscoelastic region. In this paper, the critical strain of various cement pastes was studied, such strain corresponding to the cutoff point between the linear and non-linear regions of the strain sweep. The frequency used in SAOS test was also evaluated. Results showed that the critical strain of the cement paste made without superplasticizer was on the order of 10−5, and the addition of superplasticizer increased the critical strain to about 10−4. The critical strain of cement paste decreased with testing time. The storage modulus (G′) measured by oscillatory time sweep at the strain amplitude of 10−5 was very close to that of 10−6, but much higher than that of 10−4. The G′ and the loss modulus (G″) of cement pastes were shown to continuously increase with testing time. An abrupt drop of G′ (or crossover point of G′ and G″) was observed for the pastes measured by oscillatory time sweep at the strain amplitude of 10−5, but was not observed at the strain amplitude of 10−4.

A direct characterization of interfacial interaction between asphalt binder and mineral fillers by atomic force microscopy

Abstract: The property of asphalt mastic directly affects the service performance of asphalt mixtures and pavements. Previous studies have demonstrated that the interaction between asphalt binder and mineral fillers has a significant effect on the performance of asphalt mastics. However, the interaction hasn’t been characterized by direct tests. In this study, an adsorption–separation test of asphalt binder on surface of mineral fillers was conducted to separate the structure asphalt binder and free asphalt binder. Atomic force microscope (AFM) PeakForce QNM mode was used to characterize the morphology and mechanical property of asphalt binder at different distances to filler surface. Results show that the effected thickness of binder–filler interaction was around 1 μm. Within this specific thickness, the “bee” structure of asphalt surface disappears gradually, and the modulus increases significantly when the tested samples are closer to the aggregate surface.

Fracture properties of GGBFS-blended fly ash geopolymer concrete cured in ambient temperature

Abstract: Fracture characteristics are important part of concrete design against brittle failure. Recently, fly ash geopolymer binder is gaining significant interest as a greener alternative to traditional ordinary Portland cement (OPC). Hence it is important to understand the failure behaviour of fly ash based geopolymers for safe design of structures built with such materials. This paper presents the fracture properties of ambient-cured geopolymer concrete (GPC). Notched beam specimens of GPC mixtures based mainly on fly ash and a small percentage of ground granulated blast furnace slag were subjected to three-point bending test to evaluate fracture behaviour. The effect of mixture proportions on the fracture properties were compared with control as well as OPC concrete. The results show that fracture properties are influenced by the mixture compositions. Presence of additional water affected fracture properties adversely. Fracture energy is generally governed by tensile strength which correlates with compressive strength. Critical stress intensity factor varies with the variation of flexural strength. Geopolymer concrete specimens showed similar load–deflection behaviour as OPC concrete specimens. The ambient cured GPC showed relatively more ductility than the previously reported heat cured GPC, which is comparable to the OPC specimens. Fly ash based GPC achieved relatively higher fracture energy and similar values of K IC as compared to those of OPC concrete of similar compressive strength. Thus, fly ash based GPC designed for curing in ambient condition can achieve fracture properties comparable to those of normal OPC concrete.

Influence of acetic acid and calcium hydroxide treatments of rubber waste on the properties of rubberized mortars

Abstract: The main goal of this study consisted in improving the performance of cementitious materials containing a percentage of rubber particles as a substitution for aggregates. Rubber surface was modified: new chemical and simple treatments based on calcium hydroxide or acetic acid solutions were applied. The effect of the above treatments was compared with that produced by sulphuric acid or sodium hydroxide solutions or by the use of plain rubber particles, already studied and reported in the literature. Fourier transformed infrared spectroscopy-attenuated total reflectance (FTIR-ATR) was used in order to characterize the rubber surface and chemical modifications. The rubber particles were observed by optical microscopy (OM) with the aim of detecting the possible physical changes on their surface. Then, the modified rubber particles were included in the composition of mortar specimens and their properties in the fresh (consistency) and the hardened state (porosity, flexural and compressive strength, capillarity water absorption and ultrasonic pulse velocity, UPV) were examined. Regarding the properties of rubber particles, their surface hydrophilicity, absorption and roughness increased after acid or alkaline treatments. Considering mortars including chemically treated rubber particles, calcium hydroxide gave rise to mortars with lower porosity and acetic acid to mortars with lower density. Mechanical properties of mortars were improved in comparison with these properties in mortars containing plain rubber particles. Alkaline solutions prompted the highest values of compressive strength. A delay in the hardening time for rubberized mortars may be deduced from the mechanical properties evolution. The partial substitution of natural aggregates by chemically treated rubber particles, improves the properties, in terms of flexural and compressive strength, of rubberized mortars.

Insights into the mechanisms of nucleation and growth of C–S–H on fillers

Abstract: A complete understanding of the mechanisms upon which a filler acts in a cement-based material, e.g. as a C–S–H nucleation and/or growth-inducing factor, is of high importance. Although various studies report on accelerated cement hydration in the presence of fillers, the reason behind these observations is not completely understood yet. This work contributes to this subject, by providing an experimental evidence on the (electro) chemical aspects of the filler surface modification in the model solution, simulating the pore solution of cement paste. The nature of the various interactions with regard to the affinity of a filler surface towards C–S–H nucleation and growth was discussed in detail in this work with regard to zeta potential measurements of micronized sand and limestone particles in the model solutions. These results are further supported by microscopic observations of morphology and distribution of hydration products on the filler surfaces, together with considerations on thermodynamic principles in view of hydration products formation and distribution. The C–S–H nucleation and growth appeared to be due to the interactions between a filler surface and calcium ions in the pore solution. These interactions were determined by the chemical nature of the filler surface. The interaction mechanisms were found to be governed by relatively weak electrostatic forces in the case of micronized sand. This was reflected by a non-significant adsorption of calcium ions on the filler surface, resulting in non-uniformly distributed and less stable C–S–H nuclei. In contrast, the nucleation and growth of C–S–H on limestone particles were predominantly determined by donor–acceptor mechanisms, following moderate acid–base interactions. Consequently, a strong chemical bonding of calcium ions to a limestone surface resulted in a large amount of uniformly distributed C–S–H nuclei.

Mechanochemical synthesis of one-part alkali aluminosilicate hydraulic cement

Abstract: A novel approach was devised to synthesize a sustainable hydraulic cement based on alkali aluminosilicate chemistry via mechanochemical activation. This approach builds upon past work on activation of aluminosilicate precursors using alkaline solutions to produce inorganic binders for concrete construction. Recent efforts to develop one-part hydraulic cements based on the chemistry of alkali-activated aluminosilicates have resorted to high-temperature processing techniques which compromise the sustainability advantages of the system, and also require curing at elevated temperatures. The mechanochemical process developed here takes place at room temperature, and yields a hydraulic cement that does not require addition of caustic solutions to render binding effects via room-temperature curing. Processing of this hydraulic cement takes place at room temperature; this advantage together with extensive use of recycled raw materials yield significant sustainability benefits. The (dry) raw materials used for mechanochemical processing of hydraulic cement included coal fly ash, quick lime, sodium hydroxide and Magnesium oxide. The mechanochemically processed hydraulic cement was evaluated through performance of tests concerned with their pH, heat of hydration, chemical composition, crystallinity and microstructure. The hydraulic cement was used to produce concrete materials cured at room temperature. The resulting concrete materials were found to provide desired levels of workability in fresh state and compressive strength after curing, which were comparable to those of Portland cement concrete. Investigations were also conducted on the hydration kinetics of the hydraulic cement and the microstructure of its hydrates in order to gain insight into its hydration process and the nature of hydration products.

Characterization of supplementary cementitious materials by thermal analysis

Abstract: Working Group 1 of RILEM TC 238-SCM ‘Hydration and microstructure of concrete with supplementary cementitious materials (SCMs)’ is defining best practices for the physical and chemical characterization of SCMs, and this paper focusses on their thermal analysis. Thermogravimetric analysis (TGA) can provide valuable data on the chemical and mineralogical composition of SCMs. Loss-on-ignition (LOI) testing is a commonly used, standardized, but less sophisticated version of TGA that measures mass at endpoints only, with heating generally in air. In this paper we describe the use of TGA and LOI to characterize Portland cement with limestone additions, coal combustion fly ashes, ground-granulated blast furnace slag, calcined clays, and natural pozzolans. This paper outlines the value and limitations of TGA and LOI (in the formats defined in different standards regimes) for material characterization, and describes testing methods and analysis. TGA testing parameters affect the mass loss recorded at temperatures relevant for LOI measurements (700–1000 °C) of slags and fly ashes, mainly associated with oxidation reactions taking place upon heating. TGA of clays and natural pozzolans is utilized to identify optimal calcination conditions leading to dehydroxylation and consequent structural amorphization, particularly for kaolinite. However, dehydroxylation and amorphization do not occur at similar temperatures for all clays, limiting the applicability of TGA for this purpose. Although TGA is widely utilized for characterization of SCMs, the testing parameters significantly affect the results obtained, and TGA results require careful interpretation. Therefore, standardization of TGA testing conditions, particularly for LOI determination of slags and fly ashes, is required.

Test methods to determine durability of concrete under combined environmental actions and mechanical load: final report of RILEM TC 246-TDC

Abstract: At present several methods are available to predict the durability of reinforced concrete structures. In most cases, one dominant deterioration process such as carbonation or chloride penetration is taken into consideration. Experimental results as well as observations in practice show that this is not a realistic and certainly not a conservative approach. In order to test more realistically, RILEM TC 246-TDC, founded in 2011, has developed a method to determine the durability of concrete exposed to the combined action of chloride penetration and mechanical load. In this report, a test method is presented which allows determination of realistic diffusion coefficients for chloride ions in concrete under compressive or tensile stress. Comparative test results from five different laboratories showed that the combination of mechanical and environmental loads may be much more severe than a single environmental load without mechanical loading. Modelling and probabilistic analysis also showed that the obvious synergetic effects cannot be neglected in realistic service life prediction.

Preparation of blended geopolymer from red mud and coal gangue with mechanical co-grinding preactivation

Abstract: In preparing geopolymer based on red mud (RM), preactivation processes that are generally energy-intensive are employed, including two indispensable steps, namely, calcination and fine grinding. A novel co-grinding preactivation method without calcination was proposed in this study. RM was pretreated by mix grinding with blended coal gangue (CG) (weight ratio of 8:2) for 20 min. The powdered mixture was characterized by alumina silicate dissolution efficiency test, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy. The corresponding geopolymer was also synthesized and characterized by XRD and scanning electron microscopy to assess the mechanical properties and polymerization reaction. Results showed that under co-grinding effect, the main alumina silicates of the RM/CG mixture were transformed into low-coordinated or low-polymerized phases. The phases were then dissolved in alkaline solution and repolymerized. Alkali activation with high compressive strengths completely formed dense amorphous geopolymer matrices. This work demonstrated that CG, as a milled conditioner, could efficiently expedite the preactivation of RM by grinding without calcinations. Moreover, an RM/CG-based geopolymer was successfully synthesized from the preactivated mixture.

Empirical modelling of CO2 uptake by recycled concrete aggregates under accelerated carbonation conditions

Abstract: In order to assess the potential CO2 capture ability of recycled concrete aggregates (RCAs) subjected to accelerated carbonation, an empirical prediction model has been developed in relation to carbonation conditions and the characteristics of RCAs. In this study, two sources of RCAs were used: RCAs from a designed concrete mixture and RCAs obtained from crushing of old laboratory concrete cubes. Two types of carbonation approaches were employed: (A) pressurized carbonation in a chamber with 100% CO2 concentration and (B) flow-through carbonation at ambient pressure with different CO2 concentrations. Four groups of RCAs particles with sizes of 20–10, 5–10, 2.36–5 and 5 L/min were optimal to accelerate the RCAs carbonation. Moreover, the CO2 uptake of fine RCAs particles was faster than that of large RCAs particles. The developed model was able to predict the CO2 uptake in relation to relative humidity, particle size, carbonation duration and cement content of the RCA under the tested carbonation conditions.

Effect of multi-walled carbon nanotubes on the performance of styrene–butadiene–styrene copolymer modified asphalt

Abstract: The performance of styrene–butadiene–styrene block copolymers modified asphalt (SBS PMA) is heavily degraded in pavement engineering due to the weak interphase and the aging of SBS. In this paper, a type of functionalized multi-walled carbon nanotubes (CNTs) with hydroxyl groups was selected to address the two problems. Brookfield viscometer, dynamic shear rheology, bending beam rheometer, atomic force microscopy, and scanning electron microscopy were used to capture the performance and micro-morphologies of asphalt samples. Results of the performance investigation showed that CNTs decreased the viscosity (100–135 °C) of SBS PMA, performing a better workability. CNTs acted as a barrier to reduce oxygen-uptake amount of SBS, providing a better anti-aging property. The anti-rutting and fatigue resistance of SBS PMA were enhanced by adding a suitable amount of CNTs. All the binders with CNTs had similar stiffness, but that difference between them and the SBS PMA was higher at temperatures above −24 °C. Therefore, CNTs do not affect the anti-cracking property of SBS PMA. Results of morphologies analyses showed that CNTs enriched the interphase of SBS PMA not matrix. The reinforced phase (SBS-phase) of the binder with suitable amount of CNTs performed uniform distribution, and an interlocked structure. A significantly pull-out behavior of CNTs reinforced the interface, formed a dense network, and changed the behaviors of molecular motion in asphalt. Thus, functionalized CNTs with finer dispersion in the matrix provided SBS PMA with better mechanical properties.

Seismic behavior of steel reinforced ECC columns under constant axial loading and reversed cyclic lateral loading

Abstract: The seismic performance of steel reinforced engineered cementitious composite (RECC) short columns was investigated in this study. RECC columns with various shear span-to-depth ratios, axial load levels and transverse reinforcement ratios, together with one control reinforced concrete (RC) short column, were tested under the combined action of constant axial loading and reversed cyclic lateral loading. Test results indicate that RECC columns are superior to RC columns in terms of ductility, energy dissipation capacity and damage tolerance. The control RC column and the RECC column with the smallest shear span-to-depth ratio (of 1.42) were found to fail in shear. All other RECC columns, with higher shear span-to-depth ratios, including one RECC column without stirrups, failed in a flexure-dominated manner. Furthermore, theoretical flexural strength and shear strength expressions of RECC columns were derived and validated by the test results.

Experimental study of strain rate effects on normal weight concrete after exposure to elevated temperature

Abstract: The effects of strain rate ranging from 10−4 to 300 s−1 on normal weight concrete after exposure to elevated temperature up to 1000 °C were experimentally investigated using a servo-hydraulic testing machine and a split Hopkinson pressure bar. The casted cylinder concrete specimens were firstly heated in a microwave oven, and then cooled down to the ambient temperature with control. Experimental results proved that the normal weight concrete after high temperature exposure still showed significant strain rate dependency. The dynamic increase factor(DIF) for compressive strength decreased with the exposed elevated temperature from 600 to 800 °C, and increased from 800 to 1000 °C. The DIF of concrete after exposure to elevated temperature is smaller than that at the ambient temperature according to CEB code. The larger the compressive strength is, the smaller the DIF of normal weight concrete after high temperature exposure will be. In addition, further comparison showed that the DIF after high temperature exposure is larger than that exactly at the same high temperature. An empirical model of DIF for normal weight concrete after elevated temperature exposure was proposed based on the experimental data. It obviously benefits the assessment of blast resistant capacity of post-fired concrete structures, as well as referred retrofitting techniques.

Durability of reactive powder concrete under chloride-salt freeze–thaw cycling

Abstract: The corrosion of reinforcements that results from chloride iron erosion and freeze–thaw damage is the key cause of concrete material failure. Indoor accelerated freeze–thaw cycling tests on reactive powder concrete are carried out in this study, and the mass-loss rate (MLR), relative dynamic-elastic modulus (RDEM), and compressive strength are measured in order to evaluate its durability. In addition, the microstructure and composition of the reactive powder concrete during the chloride-salt freeze–thaw cycles were analyzed using scanning electron microscopy, energy-dispersive spectroscopy, and mercury-intrusion porosimetry. The results show that the reactive powder concrete was durable under chloride freeze–thaw cycling. Furthermore, the values of MLR and RDEM indicated that the actual method is not suitable for reactive powder concrete and should be modified, and that ettringite-type erosion products are generated during the freeze–thaw cycles. The conclusions in this study are valid for specific reactive powder concrete mix.

Evaluation of hybrid steel fiber reinforcement in high performance geopolymer composites

Abstract: In this paper, cement free high performance alkali activated slag-fly ash composites are designed by applying the modified Andreasen and Andersen particle packing model, and steel fibers of two lengths are applied for strength reinforcement and shrinkage compensation. The influence of the fiber length, dosage and hybrid on the fresh behavior, compressive strength, stress–stain behavior under flexural load, porosity and drying shrinkage are evaluated, and the gel structure of this blended alkali system is also identified. The results show that by applying the hybrid fiber together with the particle packing design approach, a compressive strength of around 100 MPa can be achieved with a w/p ratio of 0.4. The addition of steel fiber slightly decreases the slump flow and increases the porosity, but effectively inhibits the drying shrinkage and improves the stress–stain behavior. The hybrid usage of long and short fiber shows a synergetic effect and leads to the optimum strength. The steel fiber reinforcement is beneficial for the application of alkali activated materials.

Stability and bond properties of polymer-modified self-consolidating concrete for repair applications

Abstract: Latexes including polyvinyl acetate (PVA) and styrene-butadiene rubber (SBR) are widely used to improve adhesion and bond properties of cementitious-based repair materials. The main objective of this paper is to evaluate the effect of such polymers on stability of highly flowable self-consolidating concrete (SCC) during placement and until onset of hardening. Also, the bond properties to existing concrete substrate and steel bars are investigated. Two series of mixtures prepared with relatively low to high water-to-binder ratio and incorporating 5–15 % polymers were tested. Special emphasis was placed to highlight the altered stability responses including flowability, viscosity, passing ability, and segregation resistance with respect to the European Guidelines for SCC. Remarkable improvements in the concrete-bar bond stresses were noticed with PVA and SBR additions. This was attributed to improved concrete elasticity and tensile splitting strength that increased contribution of material bearing strength around the steel bars.

Experimental investigation of bond between glass textile reinforced mortar overlays and masonry: the effect of bond length

Abstract: In this paper, an experimental program involving shear bond tests on double-lap/double-prism specimens is presented aiming to assess the bond characteristics of interfaces comprising uncoated alkali-resistant glass fiber textile reinforced cementitious mortars (TRM) applied as overlays on unreinforced masonry substrates. In total, 19 test results are presented with the bond length being the parameter under investigation. Bond lengths studied were equal to 50, 100, 125, 150, 200 and 250 mm, whereas at least two specimens per bond length were tested. According to the experimental findings, the effective bond length for the specific type of TRM overlay applied on this type of masonry substrate was found to be approximately equal to 130 mm. Failure of the bonded TRM strips was due to textile slippage within the mortar and simultaneous fiber rupture. Slippage of the entire TRM strip in relation to the masonry was practically zero as confirmed by application of a digital image correlation (DIC) method. The experimentally derived load-relative displacement curves can be approximated as bilinear up to the maximum load with a linear post-peak descending branch; a residual shear bond capacity was present only for bond lengths lower that the effective one. Finally, data on DIC-enabled crack detection and crack width evolution throughout the load response is also presented.