Showing papers in "Construction and Building Materials in 2017"

Deep Convolutional Neural Networks with transfer learning for computer vision-based data-driven pavement distress detection

Abstract: Automated pavement distress detection and classification has remained one of the high-priority research areas for transportation agencies In this paper, we employed a Deep Convolutional Neural Network (DCNN) trained on the ‘big data’ ImageNet database, which contains millions of images, and transfer that learning to automatically detect cracks in Hot-Mix Asphalt (HMA) and Portland Cement Concrete (PCC) surfaced pavement images that also include a variety of non-crack anomalies and defects Apart from the common sources of false positives encountered in vision based automated pavement crack detection, a significantly higher order of complexity was introduced in this study by trying to train a classifier on combined HMA-surfaced and PCC-surfaced images that have different surface characteristics A single-layer neural network classifier (with ‘adam’ optimizer) trained on ImageNet pre-trained VGG-16 DCNN features yielded the best performance

Use of sea-sand and seawater in concrete construction: Current status and future opportunities

Abstract: This paper presents a critical review of existing studies on the effects of using sea-sand and/or seawater as raw materials of concrete on the properties of the resulting concrete, including its workability, short- and long-term strength as well as durability. It has been shown by existing research that concrete made with sea-sand and seawater develops its early strength faster than that of ordinary concrete, but the former achieves a similar long-term strength to the latter. Existing studies have also shown that the use of sea-sand and seawater may have a significant effect on chloride-induced steel corrosion but has only a negligible effect on the carbonation process of concrete. Strong evidence exists that a combination of mineral admixtures for the concrete and reinforcement with fiber reinforced polymer (FRP) can effectively solve the durability problem associated with the abundance of chloride ions in sea-sand seawater concrete (SSC). Such use of SSC also offers a good opportunity for the incorporation of recycled coarse aggregate (RCA) in concrete, particularly those that have been chloride-contaminated, as has been demonstrated by some preliminary research. The current understanding of the behavior of SSC, as summarized in the present paper, provides a solid basis for further research in the area to enable the wide use of SSC in concrete construction worldwide, particularly when combined with FRP as the reinforcing material.

Cementitious materials for construction-scale 3D printing: Laboratory testing of fresh printing mixture

Abstract: In this study, a framework for performance-based laboratory testing of cementitious mixtures for construction-scale 3D printing is developed, where workability of a fresh “printing mixture” is studied in terms of print quality, shape stability, and printability window. Print quality is described using measures of surface quality and dimensions of printed layers. Details of two proposed test methods for evaluation of shape stability, namely, “layer settlement” and “cylinder stability” are also provided. Experimental study of four different mixtures revealed that inclusion of silica fume and Nano-clay significantly enhance shape stability. The results of five conventional test methods, as well as four proposed tests are used to discuss the performance of mixtures.

A critical review and assessment for usage of recycled aggregate as sustainable construction material

Abstract: The ever increasing population and urbanization has led to construction of high rise structures and demolishing existing old low rise ones. This has become not only the cause of natural resources depletion at an alarming rate but also gradually becoming a challenge for sustainability. Concrete industry consumes a majority of natural resources especially in developing countries. In recent years the concrete industry has started using Construction and Demolition (C&D) waste in structural concrete application owing to the availability of waste from demolition of old structures and the reduction in cost of acquiring aggregates. This can allow the concrete industry to reduce its carbon footprint and thus help it to continue to grow without harming the environment. In this backdrop, this paper provides an account of properties of concrete prepared with recycled aggregate, analyses the important findings on Recycled Aggregate Concrete (RAC) in the recent time and discusses the suitability of its usage in construction. The open literature suggests that the durability and mechanical properties of RAC is slightly inferior than that of conventional concrete. However, with the use of admixtures and modified mixing approaches, the desired properties of RAC can be obtained. Collation and analysis of more than 200 research papers in this area on various facts of Recycled Aggregate Concrete, on one hand, may be considered as a step ahead for formation of design methodology and, on the other hand, a valuable stating document for further research.

Durability evaluation of geopolymer and conventional concretes

Abstract: Durability of concrete strongly influences the service life of structural members. Durable concrete protects embedded reinforcing steel from corrosion and reduces the potential for concrete spalling under chemical attack. This paper evaluates the performance of geopolymer concretes manufactured using either class-F fly ash or blended fly ash and granulated lead smelter slag (GLSS). The performance of ordinary Portland cement (OPC) concrete is also investigated as a reference for evaluating the durability characteristics of geopolymer concretes. All concrete specimens were continuously immersed up to nine months in four different chemical solutions: 5% sodium chloride, 5% sodium sulphate, 5% sodium sulphate + 5% magnesium sulphate, and 3% sulphuric acid. Throughout the exposure period, the change in mass, compressive strength, splitting tensile strength, flexural strength, water absorption, sorptivity and porosity were evaluated. The influence of wetting–drying and heating–cooling cycles on the mass loss and compressive strength was also investigated. The results revealed that the OPC concrete has lower water absorption and sorptivity than the geopolymer concrete. Furthermore, it is shown that sodium sulphate has the greatest impact on geopolymer concretes, while OPC concrete is more susceptible to sulphuric acid attack. The results showed that, in general, the durability performance of geopolymer concrete is superior to that of OPC concrete within the range of the considered exposure.

Applications of using nano material in concrete: A review

Abstract: This review paper discussed on the nano materials in concrete. Nowadays, the application of nano materials has received numerous attentions to enhance the conventional concrete properties. Eventually, the introduction of nano materials in concrete is to increase its strength and durability. Nano material is defined as material that contains particle size which less than 200 nm. For the purpose of concrete study, the application of nano materials must be at least 500 nm in size. The addition of ultrafine nano material will help to reduce the cement content by partially replacing cement on weight basis to improve the binding effect. The ultrafine particles of nano material will also help reduce the formation of micro pores by acting as a filler agent, producing a very dense concrete and automatically reduce the growth of micro pores in the UHPC structures. Moreover, this paper presents on the advantages and benefits to enhance the concrete by utilizing nano materials.

Long-term durability of basalt- and glass-fibre reinforced polymer (BFRP/GFRP) bars in seawater and sea sand concrete environment

Abstract: This paper presents a study on the long-term performance of basalt- and glass-fibre reinforced polymer (BFRP/GFRP) bars in seawater and sea sand concrete (SWSSC) environment. Accelerated corrosion tests were conducted using two types of SWSSC solutions at different pH and temperatures, and for different durations. The tensile tests of pre-exposed bars suggested the GFRP bars to be more durable than BFRP bars, while the Young’s modulus of all specimens remained unchanged. Scanning electron microscopy (SEM), X-ray computed tomography (CT) and energy dispersive X-ray spectroscopy (EDS) results were utilized to explain the damage mechanism. The long-term behaviour of BFRP and GFRP bars under the service construction condition was also predicted using Arrhenius degradation theory.

A strain-hardening cementitious composites with the tensile capacity up to 8%

Abstract: Fiber reinforced concrete (FRC) has advantage in tensile ductility over the normal concrete. Among the family of FRC, engineered cementitious composites (ECCs) are known for their strain-hardening behavior and high tensile capacity. However, even the ductility of normal ECC is not sufficient to support it to be a solo structural material. To improve the tensile capacity of ECC to a higher level, a new cementitious material, ultra-high ductile cementitious composites (UHDCCs), is developed with the specially selected polyethylene (PE) fibers. The present paper introduces the mixture process and a series of mechanical tests on UHDCC with 3 different mixtures. Uniaxial tension test indicated the outstanding strain hardening and saturated multiple cracking properties of UHDCC. At ultimate state, the crack spacing of UHDCC was generally less than 2 mm with the residual crack widths less than 100 μm. The tested UHDCC exhibited the averaged tensile strain at peak stress over 8% with some mixture even exceeding 12%. UHDCC had the compressive strength ranging from 45.9 MPa to 121.5 MPa. The strain hardening behaviors were observed in the compression test of UHDCC-1 and UHDCC-2. To figure out the formation of tensile capacity, comparative studies were conducted on the pseudo-strain hardening (PSH) indexes of UHDCC. The test results demonstrated that the ultra-high ductility of UHDCC originates from the ultra-high crack bridging capacity. It implies that with sufficient bridging provided by fibers, UHDCC can maintain the tensile ductility at an amazing level, despite the high fracture toughness of matrix. Additionally, analysis demonstrates that the classic PSH criterion is still valid for quantifying the tensile capacity of UHDCC.

Flexural strength and elastic modulus of ambient-cured blended low-calcium fly ash geopolymer concrete

Abstract: Fly ash geopolymer is an emerging alternative binder with low environmental impact and potential to enhance sustainability of concrete construction. Most previous works examined the properties of fly ash-based geopolymer concrete (GPC) subjected to curing at elevated temperature. To extend the use of GPC in cast-in-situ applications, this paper investigated the properties of blended low-calcium fly ash geopolymer concrete cured in ambient condition. Geopolymer concretes were produced using low-calcium fly ash with a small percentage of additive such as ground granulated blast furnace slag (GGBFS), ordinary Portland cement (OPC) or hydrated lime to enhance early age properties. Samples were cured in room environment (18–23 °C and 70 ± 10% relative humidity) until tested. The results show that, density of hardened GPC mixtures is similar to that of normal-weight OPC concrete. Inclusion of additives enhanced the mechanical strengths significantly as compared to control concrete. For similar compressive strength, flexural strength of ambient cured GPC was higher than that of OPC concrete. Modulus of elasticity of ambient cured GPC tend to be lower than that of OPC concrete of similar grade. Prediction of elastic modulus by Standards and empirical equations for OPC concrete were found not conservative for GPC. Thus, an equation for conservative prediction of elastic modulus of GPC is proposed.

Characterizing the generation and flows of construction and demolition waste in China

Abstract: Associated with the continuing increase of construction activities such as infrastructure projects, commercial buildings, and housing programs, China has been experiencing a rapid increase of construction and demolition (CD and the landfill space demands were estimated to range from 7504 million m3 (the worst scenario) to 706 million m3 (the most optimistic scenario) accordingly. Consequently, increasing the recycling rate and reducing landfill rate of C&D waste could not only improve the potential recycling economic values, but also dramatically reduce land use and potential environmental impacts.

Mechanical properties and durability of high-strength concrete containing macro-polymeric and polypropylene fibers with nano-silica and silica fume

Abstract: The current study investigates the effects of different amounts of polypropylene (PP) and macro-polymeric (MP) fibers on the mechanical properties and durability of high-strength concrete containing silica fume and nano-silica. In total, 280 concrete specimens were produced in 28 different test groups, for which the parameters of compressive strength, tensile strength, modulus of elasticity, water absorption, and porosity were evaluated. The macro-polymeric fibers in volume fractions of 0.25, 0.5, 0.75, 1.0, and 1.25%, and the polypropylene fibers in volume fractions of 0.1, 0.2, 0.3, 0.4, and 0.5% were used in this study. Furthermore, one set of specimens with the total fiber volume fraction of 1.0% of the concrete volume was tested in order to examine the effect of hybrid polypropylene-macro-polymeric fibers on the concrete properties. In addition, the nano-silica with the weight percentages of 1, 2, and 3%, and the silica fume with the weight percentages of 8, 10, and 12% were employed in the concrete mix design. In the current paper, first, the effects of macro-polymeric, polypropylene, and hybrid fibers on the physico-mechanical properties were comparatively examined, and then the influence of using nano-silica and silica fume in the high-strength concrete with no included fiber was investigated. Finally, the optimum percentages of fibers and pozzolans corresponding to the most significant increases in the tensile strength were chosen, which were subsequently used as the optimum combination of the high-strength concrete. The results of the experimental study suggest an improvement in the concrete mechanical properties and durability following the introduction of nano-silica and silica fume. In addition, incorporating macro-polymeric fibers in the concrete mixture given the volume content of fibers improves the mechanical properties of high-strength concrete. Moreover, high volume fractions of polypropylene fibers in the concrete mixture brought about negative effects on the physico-mechanical properties of the high-strength concrete.

Developments of nano materials and technologies on asphalt materials – A review

Abstract: Due to its nature of large surface area and small size, nano-material shows specific characteristics compared to the common material and exhibits some novel properties and incredible features which make it possible to be applied in the field of asphalt pavement as an additive. This review focuses on introducing the main nano materials and related techniques used for nano-modified asphalts and major performance characteristics at various states. Some conventional test results including viscosity, dynamic modulus, stiffness, rut depth, indirect tensile strength and so on were employed to characterize the rheological and engineering performances of nano-modified asphalts and some innovative technologies such as atomic force microscopy, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were effectively utilized to explore their micro structures and molecular components. It can be found that the addition of nanomaterials could dramatically enhance the properties of an asphalt material such as visco-elasticity, high temperature properties, and the resistances to aging, fatigue and moisture. In addition, nano-particle type, content and production process were mainly studied and other factors such as binder type and particle size also need to be considered to explore the modification mechanism and effect. The properties of both nano-material and original binder played the key roles in determining optimal production process parameters. In addition, the degree of dispersion was governed by some factors such as the mixing temperature, duration and speed.

A review on concrete surface treatment Part I: Types and mechanisms

Abstract: The application of surface treatments in concrete has been widely investigated over the past decades. Surface treatment technology has become more important in concrete structures especially in preventing deterioration and damage when exposed to extremely aggressive environments, and in further extending service life. This paper presents comprehensive details of four types of concrete treatments, including surface coating, hydrophobic impregnation, pore blocking surface treatment and multifunctional surface treatment. Additionally, the knowledge of their interaction mechanisms with cementitious substrate is presented and discussed. The advantages and drawbacks of each treatment as well as the influencing factors on the protective effects of surface treatments on concrete, such as air permeability, bonding strength and cracking resistance, are also discussed. Despite decades of study, the mechanisms of many newly developed surface treatments remain poorly understood. A deeper understanding of the chemical and physical reaction mechanisms is therefore essential, especially at micro-scale levels.

Practical recycling applications of crushed waste glass in construction materials: A review

Abstract: The disposal of waste glass in landfills is an important environmental challenge that many countries face around the world. The repurposing of waste glass into a construction material reduces the consumption of natural resources, minimizes greenhouse emissions and alleviates landfill scarcity. Over the last sixty-five years, numerous investigators have studied reusing crushed waste glass (CWG) as a construction material. However, CWG has not been widely used in concrete or asphalt construction applications across the globe. Additionally, barriers still exist that prevent CWG from being used as a fine aggregate in concrete, such as the severity of Alkali-silica reaction (ASR) expansions within concrete consisting of CWG, and the lack of understanding of these reactions. This paper presents an overview of previous studies carried out by researchers to reuse CWG as an aggregate in concrete and asphalt mixtures, an aggregate in unbound base and subbase applications, lightweight engineering material and a cementitious material. From the literature review conducted, it can be concluded that CWG has potential use as an aggregate in construction materials. More research is required to clarify contradictions regarding the properties of concrete containing CWG as fine aggregate, as well as further investigation of the properties of foamed waste glass concrete and ultra-lightweight fibre reinforced concrete containing expanded waste glass, and the use of glass powder as a filler in asphalt.

Experimental and numerical study on mechanical properties of Ultra High Performance Concrete (UHPC)

Abstract: Ultra-High Performance Concrete (UHPC) is an advanced technology in concrete industry with superior characteristics such as high strength in compression and tension, ductility, and durability. This paper determines the tensile and compressive behavior of UHPC and a comparison is made with Normal Strength Concrete (NC) for the development of a numerical model to simulate the behavior of UHPC using the Finite Element (FE). The experimental tests including a cylinder and cube compressive test, flexural, briquette and splitting tension tests to evaluate the ultimate capacity of the material in compression and tension and its modulus of elasticity. The primary focus of this research, however, was to simulate material properties of UHPC through commercial FE software allowing the study of the structures including UHPC. The numerical analysis provides the mechanical properties of UHPC that can be used in FE software using Concrete Damage Plasticity model (CDP) to define ductal UHPC in the absence of sufficient experimental data. The numerical and the experimental results were generally in good agreement.

Bacteria based self healing concrete – A review

Abstract: This paper reviews the types of bacteria used in concrete and the ways it can be applied as a healing agents. This paper also gives a brief description of the various properties of concrete which vary with the addition of bacteria. Micro-cracks are inherently present in concrete. This causes degradation of concrete leading to ingress of deleterious substances into concrete, resulting in deterioration of structures. Due to this concrete needs to be rehabilitated. To surmount these situations self-healing techniques are adopted. By the addition of urease engendering bacteria along with calcium source results in calcite precipitation in concrete. Bio-mineralization techniques give promising results in sealing the micro-cracks in concrete. The freshly composed micro-cracks can be sealed up by perpetual hydration process in concrete. The ureolytic bacteria which include Bacillus Pasteurii, Bacillus Subtilis which can engender urea are integrated along with the calcium source to seal the freshly composed micro cracks by CaCO3 precipitation. For the amelioration of pore structure in concrete, the bacterial concentrations were optimized for better results. The literature shows that Encapsulation method will give better results than direct application method and also shows that the use of bacteria can increase the strength and durability properties of concrete.

Autogenous shrinkage of high performance concrete: A review

Abstract: Autogenous shrinkage is a major concern in early age cracking of high performance concrete (HPC) Low water-to-binder ratio and incorporation of supplementary cementitious materials (SCMs) can remarkably affect the pore structure, relative humidity, self-stress, degree of hydration, and interface structure; hence, increase the shrinkage in the matrix In this paper, the mechanism of autogenous shrinkage of HPC and influential factors in its development are discussed In general, autogenous shrinkage is more pronounced in HPC, albeit, using low heat cement, fly ash, shrinkage reducing agents, lightweight aggregates, and fibers can effectively reduce it The effects of SCMs on autogenous shrinkage, relationship between different types of shrinkage and autogenous shrinkage as well as the effect of internal curing on autogenous shrinkage need to be further studied

Effects of graphene oxide agglomerates on workability, hydration, microstructure and compressive strength of cement paste

Abstract: In this study, the effects of graphene oxide (GO) agglomerates on the workability, hydration, microstructure, and compressive strength of cement paste were addressed. The workability of cement paste was reduced because of the presence of GO agglomerates, which entrap a large amount of water. The mini-slump diameter was reduced by 21% with the incorporation of 0.03% by weight GO in cement paste. Hydration of the cement paste was accelerated due to nucleation sites provided by GO agglomerates serving as seeding material in the cement paste. The incorporation of GO refined the pore structure of the cement paste. The incorporation of GO was found to have much greater impact on macropores than on large and small mesopores. At 28 days, the incorporation of 0.04% by weight GO produced a 14% improvement in the compressive strength of cement paste. Below 0.03%, the incorporation of GO had no positive effects on compressive strength.

Green concrete: Prospects and challenges

Abstract: Utilization of green concrete in construction is increasingly adopted by the construction industry owing to the drawbacks of conventional concrete and the numerous inherent benefits of green concrete. The increasing demand for green concrete has been spurred by demand for high quality concrete products, desire of nations to reduce green-house gas emission, need for conservation of natural resources and limited landfill spaces. Green concrete comes in various forms such as high-volume fly ash concrete, ultra-high performance concrete, geopolymer concrete, lightweight concrete to mention a few. Green concrete offers numerous environmental, technical benefits and economic benefits such as high strength, increased durability, improved workability and pumpability, reduced permeability, controlled bleeding, superior resistance to acid attack, and reduction of plastic shrinkage cracking. These characteristics promotes faster concrete production, reduction of curing waiting time, reduction of construction costs, early project completion, reduction of maintenance costs and increased service life of construction projects. Green concrete promotes sustainable and innovative use of waste materials and unconventional alternative materials in concrete. Suitable standards, more demonstration projects, as well as adequate training, public awareness, cross-disciplinary collaborations and further research and developments are required to promote global adoption of green concrete in large-scale infrastructure projects.

Enhancement of the properties of fly ash based geopolymer paste by incorporating ground granulated blast furnace slag

Abstract: Research efforts have been made continuously to establish fly ash based geopolymer as an alternative binder material for the production of fresh concrete because production of Ordinary Portland Cement degrades the environment by huge emissions of carbon-di-oxide and also by consuming lot of natural resources. But most of the study reveals, fly ash based geopolymer paste needs more time to get set when it is cured at ambient temperature. As a result, it is quite impractical to use fly ash based geopolymer paste as an alternative to Ordinary Portland Cement in faster construction. In this study, an effort has been made to enhance the properties of fly ash based geopolymer paste by incorporating ground granulated blast furnace slag at various percentage levels. Microstructure of the geopolymer paste is studied using Scanning Electron Microscopy. Result of this investigation shows that significant improvement on setting time and compressive strength can be obtained by adding ground granulated blast furnace slag in the mixes.

Design of geopolymer concrete with GGBFS at ambient curing condition using Taguchi method

Abstract: In this paper, the Taguchi method has been used to design optimum mix proportions for geopolymer concrete with ground granulated blast furnace slag (GGBFS) as aluminosilicate source at ambient curing condition. The influences of binder content, alkaline activator to binder content (Al/Bi) ratio, sodium silicate to sodium hydroxide (SS/SH) ratio, and sodium hydroxide (SH) concentration on the geopolymer concrete were investigated. A total of nine mix designs were evaluated. It was found that specimens with a binder content of 450 kg/m3, Al/Bi ratio of 0.35, SS/SH ratio of 2.5, and SH concentration of 14 M produced the highest 7-day compressive strength (60.4 MPa). However, the setting time was found to be short. Hence, fly ash (FA), metakaolin (MK), and silica fume (SF) were used as partial replacement of GGBFS in different proportions to increase the setting time. It was found that the setting time improved for the partial replacement of GGBFS with FA, MK, and SF.

Evaluation of mechanical and durability properties of crumb rubber concrete

Abstract: Due to rapid growth of automobile sector, disposal of waste rubber is becoming a major issue In this study, effort has been made to reduce this problem by utilizing waste rubber in the form of crumb rubber in Portland pozzolana cement concrete as a substitute of fine aggregates in varied percentages Experimental work has been conducted to evaluate compressive strength, flexural strength, density and durability properties like water absorption and abrasion resistance for the different proportions (0%, 4%, 45%, 5% and 55%) of crumb rubber in concrete Micro-structural study using XRD, SEM and optical microscopy have also been carried out in the present study It has been observed that with an increment of crumb rubber, workability of concrete decreases The output of compressive and flexural strength show slight decrease with 4% replacement of fine aggregates by crumb rubber Water absorption and abrasion resistance were also marginally affected at the same substitution level of crumb rubber in concrete Hence, it can be concluded that 4% of fine aggregates can be replaced by crumb rubber to manufacture concrete for non-structural elements

Effects of Graphene Oxide On Early-age Hydration And Electrical Resistivity Of Portland Cement Paste

Abstract: The effects of graphene oxide (GO) on the early-age hydration process and mechanical properties of Portland cement paste were experimentally investigated in this study. Based on an isothermal calorimeter measurement, the hydration rate of cement was observed to increase with the increase of GO content by nucleation effect. On the other hand, the electrical resistivity development of GO-cement paste was monitored using a non-contact electrical resistivity device. The result showed that electrical the resistivity of GO-cement paste was evidently higher than that of plain cement paste. However, cement paste with excessive amounts of GO exhibited a decreased electrical resistivity due to the massive ion diffusion caused by GO. Compared to plain cement paste, the GO-cement paste exhibited obviously higher compressive and flexural strengths, but the enhancements in compressive strength began to decline when the GO amount was greater than 0.04%. The microstructure characterization indicated that GO can apparently densify the cement pastes with less porosity and hydrates networking, which is consistent with the results of hydration acceleration and strength enhancement.

Improving the mechanical properties of recycled concrete aggregate using chopped basalt fibers and acid treatment

Abstract: This paper presents the results of a study that investigated the improvement of the mechanical properties of recycled concrete aggregate (RCA) produced by adding chopped basalt fibers (BF) with contents of 0.1%, 0.3%, 0.5%, 1%, and 1.5% by total volume of the mix to treated and untreated recycled aggregates. The recycled aggregates were surface treated by pre-soaking them in a 0.1 M hydrochloric acid (HCl) solution for 24 h to remove the adhered mortars to improve the bond between the recycled aggregate and the cement. In addition, chopped BF was added to normal concrete (NA) mixes as a control for comparison. The results showed that using chopped BF minimally enhanced the compressive strength of the concrete mix but significantly improved its flexural and splitting tensile strength. Furthermore, the optimum BF content that produced the same splitting tensile and compressive strength as NA was 0.5% for untreated RCA and 0.3% for treated RCA, while the flexural strength was 0.3% for untreated RCA and 0.1% for treated RCA.

New formulations for mechanical properties of recycled aggregate concrete using gene expression programming

Abstract: This paper presents new empirical models for prediction of the mechanical properties of recycled aggregate concrete (RAC) using gene expression programming (GEP) technique. A large and reliable test database containing the results of 650 compressive strength, 421 elastic modulus, 346 splitting tensile strength, and 152 flexural strength, tests of RACs containing no pozzolanic admixtures is collated through an extensive review of the literature. The performance of existing mechanical property models of RACs is then assessed using the database, and the results of this assessment are presented using selected statistical indicators. New expressions for the predictions of 28-day compressive strength, elastic modulus, flexural strength, and splitting tensile strength of RACs are developed based on the database. The assessment results indicate that the predictions of the proposed models are in close agreement with the test results, and the new models provide improved estimates of the mechanical properties of RACs compared to the existing models.

Sulfuric acid resistance of fly ash based geopolymer concrete

Abstract: The sulfuric acid resistance of fly ash based geopolymer concrete blended with an additional calcium source is presented in this paper. Ordinary Portland cement (OPC) was added as additional calcium in the geopolymer system as fly ash replacement (0, 10, 20 & 30%). The specimens were exposed to 2% sulfuric acid solution up to the age of 365 days, and the deterioration was identified in terms of mass loss and compressive strength retained. Microstructural analysis; SEM, XRD, and EDS was also carried out. The results indicate that the inclusion of OPC (as fly ash replacement) improves the compressive strength of fly ash based geopolymer concrete specimens significantly whereas it did not have a similar effect on its resistance to sulfuric acid. The increase in compressive strength for the unexposed geopolymer concrete specimens was due to the additional calcium hydrated products which co-existed with alumina-silicate polymer structures. On the other hand, for the specimens exposed to sulfuric acid for 365 days, the inclusion of OPC at 10% showed the maximum retained compressive strength of around 52% of the strength value achieved for unexposed specimens at the same age. However, OPC inclusion beyond 10% decreases the ability of geopolymer concrete specimens to retain compressive strength. Maximum deterioration was observed when fly ash was replaced by OPC at 30%. This was due to the formation of additional calcium sulfate which increased with the increase in calcium products in the mixture. Microstructural changes were also observed for the exposed specimens at 365 days and confirmed the presence of sulfur compounds as a major cause for the deterioration.

Durability of alkali-activated materials in aggressive environments: A review on recent studies

Abstract: Corrosion of reinforcement and concrete deterioration induced by aggressive media could severely reduce the bearing capacity of structures. The durability of alkali-activated materials (AAMs) in aggressive environments, such as carbonation, chloride penetration and sulfate attack, have been a research focus worldwide. Reaction products and microstructures of AAMs are different from ordinary Portland cement (OPC), therefore the corrosion mechanisms and assessments are different. This paper reviews factors influencing water absorption and permeability of AAMs, effect of gel composition and exposure environments on carbonation, chloride penetration and chloride migration test methods, and sulfate resistance in high-calcium and low-calcium alkali-activated systems. There remains a large space in these aspects to completely understand the deterioration of AAMs, as pointed out in the end. The perspectives suggested in this paper will be useful for future study on long-term durability of AAMs.

Immobilizing bacteria in expanded perlite for the crack self-healing in concrete

Abstract: Immobilization has been reported to be an efficient approach for bacteria-based self-healing concrete to maintain the high-efficiency mineral-forming capacity of incorporated bacteria over a period of time However, the relatively high-cost, local unavailability, and low adsorption capacity of the current bacteria carriers make them impractical for potential implementation in large-scale concrete structures In this study, the feasibility of expanded perlite (EP) as a novel bacteria carrier on quantifying cracks-healing in concrete via immobilization of Bacillus cohnii was demonstrated The effects of two other self-healing techniques, ie, direct introduction of bacteria and expanded clay (EC) immobilized bacteria, on the efficiency of crack-healing were also investigated Experimental results showed that specimens incorporated with EP-immobilized bacteria exhibited the most efficient crack-healing after each healing time The values of completely healed crack widths were up to 079 mm after 28 days of healing, which is larger than the value of 045 mm for specimens incorporated with EC-immobilized bacteria Field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) analysis confirmed that mineral precipitations on their crack surfaces are calcite crystals The findings obtained in this study may provide a scientific basis for the potential implementation of expanded perlite, as a new microorganism carrier, in bacteria-based self-healing concrete

Incorporation of Al in C-A-S-H gels with various Ca/Si and Al/Si ratio: Microstructural and structural characteristics with DTA/TG, XRD, FTIR and TEM analysis

Abstract: Systems of different Ca/Si and Al/Si molar ratios were investigated. Incorporation of Al increases main basal spacing, amount of bounded water and decreases crystallinity of C-(A)-S-H (calcium (aluminium) silicate hydrate). Transmission electron microscope observations showed that aluminium results in formation of more compacted, foil-like microstructure. FTIR revealed the presence of rings within the structure of C-(A)-S-H. Low Ca/Si ratio promotes Al incorporation into C-(A)-S-H, while in case of high Ca/Si ratio aluminium is also incorporated into AFm. The results show, that Ca/Si ratio is of key significance deciding on Al incorporation into C-(A)-S-H in hydrating SCMs bearing blended systems.

Hydration development of mineral additives blended cement using thermogravimetric analysis (TGA): Methodology of calculating the degree of hydration

Abstract: Reducing environmental pollution is becoming an important issue that must be taken into consideration, particularly, when producing concrete. Mineral additives used as cement replacement should help in achieving economic performance and ecological pollution reduction. A key question concerning the performance of mineral additives blended cement is the asses s ment of the contribution of mineral additives to the hydration kinetics. In this paper, the hydration degree of cement pastes containing blast furnace slag and limestone filler was investigated. The investigation was conducted first by using classical thermogravimetric analysis (TGA) methods, which are based on ultimate chemically bound water estimation. A modified TG method based on the direct estimation of the amount of mineral additives contributing to hydration reactions, was then suggested. The accuracy of the suggested method was verified using an isothermal calorimetry test on paste specimens and a compressive strength test on mortar specimens.