Mechanical, durability and microstructural characteristics of ultra-high-strength self-compacting concrete incorporating steel fibers

Few researches are carried out in the Gulf area to study the feasibility of producing UHSC using available local materials with the inclusion of steel fibers, and investigate its properties and durability. Local available materials and the inclusion of steel fibers with different volume fractions are investigated to produce UHSC. Different mechanical properties are evaluated (compressive strength and splitting tensile strength). Durability of the concrete in high sulfate and high temperature condition (i.e. resembling Gulf environment) is evaluated. Also, chloride permeability, bulk chloride diffusion and electrical resistivity are evaluated. Test results indicate that local material can produce UHS–FRC. The ductility of the concrete is greatly improved by the incorporation of steel fibers and increases as the fiber volume increases. Chloride permeability, bulk chloride diffusion and electrical resistivity are affected by the volume fraction of steel fibers. The inclusion of steel fibers did not have significant effect on the durability of the concrete in the sulfate environment. Microstructural investigations of UHS–FRC concrete were also performed. The microstructural investigations shed some light on the nature of interfacial bond of fibers and the cement paste and its effect on its mechanical and fracture properties.     

Hardened properties of self-consolidating high performance concrete including rice husk ash

This paper mainly presents the key hardened properties of self-consolidating high performance concrete (SCHPC). Various SCHPCs were produced with different water/binder (W/B) ratios, rice husk ash (RHA) contents, and air contents. The required filling ability and air content were achieved in all freshly mixed SCHPCs. The hardened SCHPCs were tested for compressive strength, ultrasonic pulse velocity, water absorption, total porosity, and true electrical resistivity. The effects of W/B ratio, RHA content, and air content on these hardened properties were observed. Test results revealed that the compressive strength, ultrasonic pulse velocity, and electrical resistivity increased whereas the water absorption and total porosity decreased with lower W/B ratio and higher RHA content. In addition, the air content decreased the compressive strength, ultrasonic pulse velocity, water absorption, and total porosity but increased the electrical resistivity. Based on the overall effects of rice husk ash, the optimum RHA content for SCHPC has been defined.     

Influence of steam curing on the properties of concretes incorporating metakaolin and silica fume

In this paper, influence of steam curing on the compressive strength, ultrasonic pulse velocity, water sorptivity, chloride ion permeability, and electrical resistivity of metakaolin and silica fume blended concretes were investigated. A total of seven mixtures containing various combinations of Portland cement (PC), silica fume (SF), and metakaolin (MK) were produced with 400 kg/m³ of total cementitious materials content and with a constant water/binder ratio of 0.44. For each mixture, concrete samples were either standard-cured in water at 23°C or steam-cured at 70°C maximum temperature over 17 h curing period. Test results revealed that steam curing enhanced the 1-day compressive strength and ultrasonic pulse velocity while leading to reduced long term strength in line with earlier findings. At the end of the water sorptivity, chloride ion permeability, and electrical resistivity tests, it was found that the steam-cured concretes had higher water sorptivity and chloride ion permeability, and lower electrical resistivity values compared to the standard cured specimens. Use of SF and MK as cementitious materials remarkably decreased the water sorptivity and chloride ion permeability of concretes, irrespective of the curing condition.     

Fluid transport in high volume fly ash mixtures with and without internal curing

The transport of fluid and ions in concrete mixtures is central to many aspects of concrete deterioration. As a result, transport properties are frequently measured as an indication of the durability that a concrete mixture may be expected to have. This paper is the second in a series investigating the performance of high volume fly ash (HVFA) mixtures with low water-to-cementitious ratios (w/cm) that are internally cured. While the first paper focused on strength and shrinkage, this paper presents the evaluation of the transport properties of these mixtures. Specifically, the paper presents results from: rapid chloride migration (RCM), rapid chloride penetration test (RCPT), apparent chloride diffusion coefficient, surface electrical resistivity, and water absorption. The test matrix consisted of mortar samples with two levels of class C fly ash replacement (40% and 60% by volume) with and without internal curing provided with pre-wetted lightweight fine aggregates (LWA). These mixtures are compared to plain ordinary portland cement (OPC) mortars. The results indicate that HVFA mixtures with and without internal curing provide benefits in terms of reduced transport coefficients compared to the OPC mixtures.     

Effect of rice-husk ash on durability of cementitious materials

In this paper the effects of partial replacements of Portland cement by rice-husk ash (RHA) on the durability of conventional and high performance cementitious materials are investigated. Different percentages of RHA replacement levels, two RHAs (amorphous and partially crystalline optimized by dry-milling) and several water–cementitious materials ratio are studied. The following durability aspects were tested: air permeability, chloride ion penetration, alkali-silica expansion, sulfate and acid resistance. The results were compared with those of cementitious materials without RHA. It is concluded from the tested properties that the incorporation of both RHAs in concretes show different behaviors for air permeability and chloride ion penetration depending on the water/cementitious materials ratio used; in mortars, it reduces the mass loss of specimens exposed to hydrochloric acid solution and decreases the expansion due to sulfate attack and the alkali-silica reaction. The results of durability aspects due to physical or pozzolanic effects after the addition of both RHAs, and its chemical composition, in general indicate an enhanced performance, proving the feasibility of its rational utilization as a supplementary cementing material.     

Mechanical properties and durability characteristics of polymer- and cement-based repair materials

This study was conducted to evaluate the mechanical properties and durability characteristics of nine polymer- and cement-based repair mortars. Mechanical properties, such as compressive, tensile and flexural strength, elastic modulus, shrinkage and thermal expansion were studied. The durability characteristics of the repair materials were evaluated by measuring: (i) chloride permeability, (ii) electrical resistivity and (iii) carbonation depth. The mechanical properties of the selected repair mortars did not vary very significantly from each other. The elastic modulus of the polymer-based repair mortars was less than that of the cement-based repair mortars. This will lead to a reduced drying shrinkage cracking in the former repair mortars compared to the latter. The electrical resistivity of polymer-based repair mortars was more than that of cement-based repair mortars. Such a trend was not noted in the chloride permeability data. The chloride permeability in all the repair materials was very low according to ASTM C 1202 criteria. Enhanced carbonation was noted in some of the polymer-based repair mortars.     

Influence of aggregate fraction in the mix on the reliability of the rapid chloride permeability test

The present paper elucidates the influence of aggregate content of the mix on the reliability of rapid chloride permeability test (RCPT) results. For this purpose, test specimens prepared with mixes varying in total aggregate content were subjected to soaking test, RCPT and electrical resistivity measurements, and the results from these tests were compared and conclusions drawn. The RCPT results indicated the plain cement concrete to be relatively more resistant against chloride penetration than the plain cement mortar, whilst the opposite was true according to the 90-day soaking test results. The above trend did not change despite the addition of silica fume (SF) to the concrete and mortar mixes. The lower aggregate content or higher paste content of plain cement mortar and the mortar with SF is shown to mislead the RCPT results. The higher paste content in the above mix promotes the conduction of higher charge as a result of lower electrical resistivity. Thus the results derived from the present investigation emphasize the need to consider the volume fraction of aggregate in the mix with and without SF while interpreting the RCPT results. Furthermore, regardless of the total aggregate and SF content in the mix, the total charge passed (from the RCPT) through the mix decreased exponentially with increasing electrical resistivity. On the other hand, for those mixes containing either SF or a high volume fraction of aggregates the linear correlation between the total charge passed and chloride penetration coefficient (K) was poor. However, for the mix with relatively lower aggregate content and with no SF the charge passed was well correlated linearly with K.     

Influence of the usage of waste oyster shell powder on mechanical properties and durability of mortar

In order to investigate feasibility of waste oyster shell powder (WOSP) as fine aggregate to produce eco-friendly mortar, workability (slump flow and slump flow loss), mechanical properties (compressive strength and flexural strength), durability (sorptivity, volume of permeability coefficient, water permeability coefficient and chloride ion diffusion coefficient) and microstructure (pore size diffusion) were studied. The effect of replacing river sand with different WOSP proportions (0%, 10%, 20% and 30%) in flowability, strength, permeability and microstructure of mortar have been revealed. The results indicate that increasing substitution ratio of WOSP could decrease the mortar slump flow. The utilization of WOSP in mortar enhanced the compressive strength, flexural strength, resistance to water penetration and chloride diffusion. The WOSP addition exhibits a positive contribution to the pore size distribution of the mortar. Furthermore, it is founded that the utilization of WOSP as construction material is a satisfactory way to reduce waste pollution. Based on its superior mechanical property, durability, eco-efficiency and cost-efficiency in mortar, it is recommended to utilize WOSP alternative to river sand at 10–30% in construction engineering.     

Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.     

A comparison of methods to evaluate mass transport in damaged mortar

The service life of reinforced concrete (RC) structures is directly influenced by the transport properties of concrete. These transport properties are adversely affected by the presence of cracks. Therefore, for accurate service life estimation of RC structures the effect of cracks on mass transport needs to be understood and quantified. To quantify the effect of cracks, different measurement methods have been developed. In this paper, we compare different mass transport measurement methods for quantifying the effect of damage, and investigate which method is more sensitive and provides the most information on the effect of damage.In this work, damage was induced by freeze-thaw in mortar specimens. Mass transport properties were measured using electrical resistivity, rapid chloride permeability, sorptivity, drying, air permeability, water permeability, and desorption isotherm. The results indicate that the measured effect of damage depends on the mechanisms of transport used in the measurement technique, and therefore, different measurement techniques do not necessarily provide the same measure of the effect of damage. The water and air permeability are comparatively more sensitive to the presence of damage.     

Influence of feeding conditions in twin-screw extrusion of PP/MWCNT composites on electrical and mechanical properties

The influence of feeding conditions of multiwalled carbon nanotube (MWCNT) materials, namely Baytubes® C150P and Nanocyl™ NC7000, into polypropylene (PP) was investigated with respect to achieving suitable nanotube dispersion, high electrical conductivity, and good mechanical properties. Both MWCNT materials were fed at selected concentrations either in the hopper of the twin-screw extruder or using a side feeder under otherwise identical extrusion conditions (rotation speed, throughput, temperature profile) using a Berstorff ZE 25 twin-screw extruder. Afterwards, injection molding was performed under identical conditions. The results indicate that the more compact Baytubes® C150P agglomerates should be added into the hopper, as the dispersion assessed by light microscopy is better, electrical resistivities measured on compression and injection molded samples are lower, and elastic modulus, yield strength and impact strength are higher as compared to side feeding. On the other hand, for the more loosely packed Nanocyl™ NC7000 agglomerates, addition using the side feeder leads to better dispersion, lower electrical resistivity, and higher mechanical properties.     

The effects of CaCl<Subscript>2</Subscript>-blended acrylic polymer emulsion on the properties of cement mortar

Although acrylic polymer emulsions have been reported to impart many desirable attributes to cement mortar; delayed hydration, excessive air entrapment and moisture induced loss of strength have been highlighted as constraints. This paper explores the utilization of hydrated calcium chloride blended-acrylic polymer emulsion (CP) as a mitigation measure to these aforementioned drawbacks. First, the effects of 0, 0.5, 1.0 and 1.5% of CP by mass of cement on the early-age cement paste hydration and mortar flow were investigated. Thereafter, the influence of CP on the hardened porosity, moist-cured compressive strength, initial rate of capillary water absorption and rapid chloride permeability (RCPT) were evaluated. Test results indicate that the addition of CP to pastes sped up the cement hydration process, accelerating the final setting time of pastes by approximately 0.5–1.5 h as the CP content of pastes increased. Moreover, CP slightly increased the flow of fresh mortar, the hardened porosity of mortar mixtures containing 0.5 and 1.0% CP were also comparable to those of the plain reference mortar. With the exception of the 1.5% CP blended mortar, the 14 days moist-cured compressive strength of 0.5–1.0% CP blended mortar mixtures were also comparable to that of the plain reference mixture. Relative to the reference mixture, the addition of CP to mortar reduced the initial rate of capillary water absorption of mortar, with the mixture containing 1.5% CP giving a maximum reduction of 23%. Conversely, RCPT results indicate that above 0.5% CP addition level, CP generally increased the electrical conductivity of mixtures.     

Flexural strengthening of concrete beams with EB-FRP,SRP and SRCM: Experimental investigation

Innovative composite materials for flexural strengthening of concrete structural members have been recently proposed by construction market. They are able to overcome some issues related to traditional composite material, such as high cost and fire resistance. They include composite materials made of different types of organic matrix (i.e., cement-based mortar and pozzolan-reaction cementitious mortar) and reinforcement (i.e., steel fibre fabric). An experimental investigation has been carried out on prestressed-concrete beams strengthened in flexure with traditional (i.e., pultruded carbon laminate bonded with epoxy resin) and different innovative composite externally bonded systems (i.e., steel fabrics glued with different types of adhesive) in order to compare their structural performance between them and with respect to unstrengthened specimens. At this aim, a total of fifteen specimens characterized by an overall length of 2400 mm and cross-sectional dimensions of 120 by 140 mm were subjected to four-point-bending tests. Test results highlighted the high potential of the innovative composite systems for flexural strengthening applications and similar effectiveness compared with the pultruded carbon laminates. The recorded response of the specimens is presented and discussed and the measured strength and deflection of the specimens are estimated. Comparison between theoretical prediction and experimental results shows a good agreement.     

Effects of nano-kaolinite clay on the freeze–thaw resistance of concrete

This paper investigates the effects of nano-kaolinite clay (NKC) on the freezing and thawing (F–T) behavior of concrete. In our experiments, we substituted NKC for 0%, 1%, 3%, and 5% of mixtures of ordinary Portland, cement, by weight. The blended concrete was prepared using w/c ratio as 0.5. A rapid freeze–thaw Cabinet was then used to measure the resistance of ordinary Portland cement concrete, as opposed to the concrete/NKC mixture, to examine deterioration caused by repeated F–T actions. We regularly measured the properties of the concrete specimens, including the pore structure, mass, electrical resistivity, chloride diffusion coefficient, compressive strength and dynamic modulus of elasticity. A computed tomography scan test evaluated the porosity characteristics of the concrete. This paper also applied scanning electron microscopy and X-ray diffraction tests in order to investigate the micro morphology and chemical element distributions inside of the concrete. The experimental results and visual comparisons revealed that the introduction of NKC improves the F–T resistivity values, as compared to the control concrete. The samples with 5% NKC exhibited the highest compressive strength, chloride diffusion resistivity, relative dynamic modulus of elasticity, and the most electrical resistivity after 125 F–T cycles. We designated the anti-freezing durability coefficient (DF) as the index to assess the F–T resistivity of concrete. The following research discusses the relationship between the concrete’s DF and the number of F–T cycles, compressive strength, chloride diffusion coefficient, and the electrical resistivity of the concrete samples.     

The combined benefits of CPF and RHA in improving the durability of concrete structures

The work presented is a laboratory study of controlled permeability formwork (CPF) applied to concrete where cement was partially replaced (10%, 15% and 20%) with Portuguese rice husk ash (RHA). Portuguese rice husk is a by-product which may be incinerated industrially. Various tests were carried out to evaluate the durability of concrete made with RHA at 10%, 15% and 20% replacement of cement by weight and cast with both the usual formwork and CPF. Tests carried out so far, reported in this paper, concern strength, absorption by capillarity and chloride ion penetration. Results lead to the conclusion that CPF enhances concrete performance even further when using partial cement replacement by RHA.     

Effect of grinding of low-carbon rice husk ash on the microstructure and performance properties of blended cement concrete

The effectiveness of unground low-carbon rice husk ash (URHA) as a pozzolan and the effect of grinding the URHA to finer fractions for use in portland cement system were investigated. The properties investigated include the setting time and calcium hydroxide depletion of rice husk ash (RHA) pastes; microstructure and flow behavior of RHA mortars; strength and durability of RHA concretes. Results from this investigation suggested that the URHA and ground RHA (GRHA) mixtures performed better than the control mixtures in all tests conducted except water demand and setting time. The URHA mixture revealed denser microstructure compared to the control mixture. The internal porosity created by the coarse RHA grains in the matrix and their inability to completely participate in pozzolanic reaction may be the reasons for the poorer performance of the URHA mixture than compared to the GRHA mixture. The effect of grinding the RHA to finer fractions either substantially or slightly improved all properties except final setting time. With the performance of the GRHA concrete somewhat similar to that of the SF concrete, the use of ground RHA can be concluded to provide acceptable performance in portland cement systems.     

Styrene–butadiene latex modified calcium aluminate cement mortar

This paper investigates properties of calcium aluminate cement (CAC) mortar modified with the styrene–butadiene-rubber (SBR) latex. This material may be advantageously applied as a rapid repair mortar. Mortar specimens were prepared with constant water-to-cement mass ratio; polymer solid content of latex was varied from 0% to 9%, and Li₂CO₃ was investigated as an accelerator. Specimens were treated at different curing conditions: 1, 7 days and transformation of metastable hydration products at 70 °C. The heat of hydration evolution of mortar specimens was measured by means of a self adopted isoperibol calorimeter.The measurement results indicate that SBR latex improves workability of fresh state mortar and retards nucleation and growth of hydration products. Due to polymer coagulation process and co-matrix formation permeability, stiffness and compressive strength decrease while adhesion strength to old concrete substrate, and flexural strength increase with amount of added latex.     

EDM performance of TiC/copper-based sintered electrodes

This paper presents a study of the effect of titanium carbide (TiC) on the performance of sintered copper-based materials as electrical discharge machining (EDM) electrodes. The aim of this study was to provide a preliminary evaluation of EDM electrodes fabricated by laser-based sintering using rapid prototyping technology (RP). Six batches of titanium carbide with content from 5% to 45% were fabricated by mixing, ball milling, pressing, and liquid phase sintering with copper-tungsten (Cu–W) and copper (Cu), respectively. The performance of the newly formed material is compared with commercial electrodes. The densification of TiC/Cu–W system was improved by the addition of nickel (Ni), as Ni shows good solubility in both Cu and W. The distribution of particle size becomes narrow as the proportion of TiC is increased. A uniform dispersion of small TiC particles in the Cu–W system and a narrow particle size distribution provide the possibility of obtaining dense electrodes. With increasing TiC, the relative density first increased and then decreased, whereas the electrical resistivity first decreased and then increased. EDM electrodes, with the addition of TiC, show good performance in surface finishing. This is an important characteristic as RP-sintered EDM electrodes are expected to be used as finishing electrodes. The surface roughness of most specimens is less than those machined using commercial electrodes. Electrodes with 15% TiC show the highest relative density, lowest electrical resistivity, and good EDM performance, i.e. lowest tool wear ratio (TWR) and highest material removal rate (MRR) at low current, and the best surface finish not only at low current, but also at high current.     

The influence of alkali content on the electrical resistivity and transport properties of cementitious materials

This paper examines the role of alkali content on the transport properties of cementitious materials. While cement with high alkali content may be a concern due to the potential for alkali silica reaction (ASR), the alkali content also influences the electrical properties of pore solution and the bulk cementitious system. Higher concentrations of alkalis decrease the resistivity of the pore solution. However, the alkalis may also impact the microstructure formed during hydration. This paper examines the role of alkali content on both the pore solution and on the microstructure that forms. Water vapor desorption measurements indicate that systems with a higher alkali content have a lower overall porosity. This dense microstructure is not caused simply by a higher degree of hydration (DOH) (i.e., an increase in reacted products), as suggested by loss on ignition (LOI), but rather possibly from the improved morphology and distribution of calcium hydroxide, as observed with scanning electron microscope (SEM).These improvements in the microstructure alter the electrical resistivity and the results of steady and non-steady state migration tests. This indicates that with higher alkali content, cement may have a beneficial influence on reducing chloride ion transport. Furthermore, alkali leaching during saturated curing is observed, quantified, and discussed as it relates to the interpretation of electrical measurements and microstructure formed.     

石墨烯表面性质对水泥砂浆复合材料力学性能和微观结构的影响

近年来,利用石墨烯及其衍生物改善水泥基复合材料性能受到了广泛关注。但是,关于石墨烯表面性质对水泥基材料的性能影响却鲜有报道。为此,采用不同浓度的L-抗坏血酸(10wt%、20wt%、30wt%、50wt%和70wt%)和还原时间(15 min、30 min、45 min和60 min)将氧化石墨烯(GO)转化为还原氧化石墨烯(rGO),然后以相同剂量(水泥质量的0.05%)加入到水泥砂浆复合材料中,研究了不同还原程度的rGO对水泥砂浆力学性能的影响。测试结果表明,通过50wt%L-抗坏血酸还原30 min制备的rGO的加入使水泥砂浆28天抗压强度和抗折强度相比于普通试样分别提高了36.84%和43.24%。SEM等分析表明,GO和不同还原程度的rGO均可促进Ca(OH)₂的结晶和水化硅酸钙凝胶(C-S-H)中二氧化硅四面体的形成,形成致密的微观结构。但存在一个最佳阈值(即通过50wt%的L-抗坏血酸还原30 min),在该阈值下,有利于rGO表面官能团与水化产物的结合。