Showing papers in "Cement & Concrete Composites in 2012"

Environmental impact and life cycle assessment (LCA) of traditional and ‘green’ concretes: Literature review and theoretical calculations

Abstract: With the current focus on sustainability, it is necessary to evaluate concrete’s environmental impact properly, especially when developing new ‘green’ concrete types. Therefore, we investigated the available literature on every step in the LCA of concrete. The adopted functional unit for which the environmental impact is calculated, influences the outcome significantly. When comparing different concrete compositions, this unit should incorporate differences in strength, durability and service life. Hence, a cradle-to-grave or modified cradle-to-gate approach is advised as system boundary. When using industrial by-products as cement replacing material in ‘green’ concrete, an economical allocation of impacts is recommended. Inventory data on energy use, CO2, PM10, SOx and NOx emissions were collected and assigned to the impact categories of the problem oriented CML 2002 and the damage oriented Eco-indicator 99 impact method. Compared to Portland cement, the impact of blast-furnace slag and fly ash is about an order of a magnitude lower.

Tensile behavior of Ultra High Performance Hybrid Fiber Reinforced Concrete

Abstract: The effects of blending fibers on the tensile behavior of Ultra High Performance Hybrid Fiber Reinforced Concrete (UHP-HFRC) are investigated. Four types of steel macro-fibers (of differing length or geometry) and one type of steel micro-fiber are considered. In producing the specimens, the volume content of the macro-fiber was held at 1.0%, whereas the volume content of the micro-fiber varied from 0.0% to 1.5%. The overall shape of tensile stress–strain curves of UHP-HFRC is primarily dependent upon the type of macro-fiber, although the addition of micro-fibers favorably affects the strain hardening and multiple cracking behaviors. UHP-HFRC produced from macro-fibers with twisted geometry provides the best performance with respect to post cracking strength, strain capacity and multiple micro-cracking behavior, whereas UHP-HFRC produced with long, smooth macro-fibers exhibits the worst performance.

Cement-based sensors with carbon fibers and carbon nanotubes for piezoresistive sensing

Abstract: Electrically conductive cementitious composites carrying carbon fibers and carbon nanotubes were developed and their ability to sense an applied compressive load through a measureable change in resistivity was investigated. Two types of cement-based sensors, one with carbon fibers alone and the other carrying a hybrid of both fibers and nanotubes, were considered. Direct comparisons were also made with traditional strain gauges mounted on the sensor specimens. Sensing experiments indicate that under cyclic loading, the changes in resistivity mimic both the changes in the applied load and the measured material strain with high fidelity for both sensor types. The response, however, is nonlinear and rate dependent. At an arbitrary loading rate, the hybrid sensor, containing a combination carbon fibers and nanotubes, produced the best results with better repeatability.

Hydration and properties of nano-TiO2 blended cement composites

Abstract: Two types of nano-TiO2 particles were blended into cement pastes and mortars. Their effects on the hydration and properties of the hydrated cement pastes were investigated. The addition of nano-TiO2 powders significantly accelerated the hydration rate and promoted the hydration degree of the cementitious materials at early ages. It was demonstrated that TiO2 was inert and stable during the cement hydration process. The total porosity of the cement pastes decreased and the pore size distribution were also altered. The acceleration of hydration rate and the change of microstructure also affected the physical and mechanical properties of the cement-based materials. The initial and final setting time was shortened and more water was required to maintain a standard consistence due to the addition of the nano-TiO2. The compressive strength of the mortar was enhanced, practically at early ages. It is concluded that the nano-TiO2 acted as a catalyst in the cement hydration reactions.

The influences of admixtures on the dispersion, workability, and strength of carbon nanotube-OPC paste mixtures

Abstract: Carbon nanotubes (CNT) have excellent mechanical properties and have the potential, if combined with Ordinary Portland Cement (OPC), traditionally a brittle material in tension, to become a nano-composite with superlative mechanical properties. However, highly attractive van der Waals forces between CNTs create coherent agglomerates that prove difficult to disperse within the cement matrix and reduce the fluidity of the fresh mixture. Good dispersion of CNTs, while maintaining good workability of fresh OPC–CNT paste mixtures, is a prequalification before CNT-cement nanocomposites can be considered as a future building and construction material with enhanced mechanical properties. This paper reports the results of investigations of the dispersion, workability, and strength of CNT aqueous and CNT–OPC paste mixtures, with and without several generically different dispersants/surfactants that are compatible as admixtures in the manufacture of concrete. These include an air entrainer, styrene butadiene rubber, polycarboxylates, calcium naphthalene sulfonate, and lignosulfonate formulations. Aqueous mixtures were initially assessed for dispersion of CNTs, followed by workability testing of selected OPC–CNT-dispersant/surfactant paste mixtures. A broad range of workability responses were measured and the CNT dispersion within hardened pastes was qualitatively assessed by SEM analysis.

Dispersion of carbon nanotubes and its influence on the mechanical properties of the cement matrix

Abstract: An appropriate dispersion of carbon nanotubes (CNTs) is a prerequisite for their use in improving the mechanical properties of cement-based composites In this study two types of carbon nanotubes (CNTs) having different morphologies were investigated To obtain a uniform distribution of CNTs in the cement matrix, the effect of sonication on the deagglomeration of CNTs in combination with anionic and nonionic surfactants in varying concentrations was quantitatively investigated when preparing aqueous dispersions of CNTs for the subsequent use in cement paste The relationships between the quality of CNT-dispersion on the one hand and the sonication time and surfactant concentration on the other were determined using UV–vis spectroscopy After dispersion, nitrogen-doped CNTs were found mostly as individual, broken CNTs In contrast, after the treatment of the mixture of single-, double-, and multi-walled CNTs, a net-like distribution was observed where destruction of the CNTs due to sonication could not be distinguished Modification of the cement pastes with dispersions of CNTs led to a pronounced increase, up to 40%, in compressive strength and, in some cases, to a moderate increase in tensile strength under high strain-rate loading However, no significant improvement in strength was observed for quasi-static loading Microscopic examination revealed that the bridging of the C–S–H phases differed depending on the type of CNT This explained, at least partly, the observed effects of CNT-addition on the mechanical properties of hardened cement pastes

Efflorescence control in geopolymer binders based on natural pozzolan

Abstract: This paper addresses methods to reduce efflorescence in a geopolymer binder based on a pumice-type natural pozzolanic material from Taftan, Iran. Geopolymer pastes samples are analyzed for compressive strength and efflorescence formation after curing at 95% humidity for 28 days. To reduce the extent of efflorescence, Al-rich mineral admixtures such as metakaolin, ground granulated blast-furnace slag, and three types of calcium aluminate cements are incorporated into the dry binder at a range of concentrations. Hydrothermal curing at elevated temperatures also shows a positive effect in efflorescence reduction. Calcium aluminate cements show the greatest effect in efflorescence reduction, which is attributed to their dissolution in alkaline media releasing high amounts of alumina into the aluminosilicate geopolymer gel. These results confirm that it is possible to develop a more reliable geopolymer binder with improved properties either by adding a suitable amount of active alumina to precursors such as natural pozzolan, or by manipulating the curing conditions to enhance alumina release from less-reactive precursor phases.

Resistance of lignite bottom ash geopolymer mortar to sulfate and sulfuric acid attack

Abstract: This paper presents an investigation of the compressive strength and the durability of lignite bottom ash geopolymer mortars in 3% sulfuric acid and 5% sodium sulfate solutions. Three finenesses of ground bottom ash viz., fine, medium and coarse bottom ash were used to make geopolymer mortars. Sodium silicate, sodium hydroxide and curing temperature of 75 °C for 48 h were used to activate the geopolymerization. The results were compared to those of Portland cement and high volume fly ash mortars. It was found that the fine bottom ash was more reactive and gave geopolymer mortars with higher compressive strengths than those of the coarser fly ashes. All bottom ash geopolymer mortars were less susceptible to the attack by sodium sulfate and sulfuric acid solutions than the traditional Portland cement mortars.

Self-healing of surface cracks in mortars with expansive additive and crystalline additive

Abstract: This research studies the self-healing potential of cement-based materials incorporating calcium sulfoaluminate based expansive additive (CSA) and crystalline additive (CA). Mortar specimens were used throughout the study. At the age of 28 days, specimens were pre-cracked to introduce a surface crack width of between 100 and 400 μm. Thereafter, the specimens were submerged in water to create a self-healing process. The experimental results indicated that the mixtures with CSA and CA showed favorable surface crack closing ability. The optimal mix design was found to be a ternary blend of Portland cement, 10 wt.% CSA and 1.5 wt.% CA, by which a surface crack width up to about 400 μm was completely closed, and the rate of water passing was dropped to zero within 28 days. It was hypothesized that the amount of leached Ca 2+ from the matrix plays an important role on the precipitation of calcium carbonate which is the major healing product. The analyses showed that those specimens with CSA/CA additions released more Ca 2+ than that control specimen. Moreover, those specimens with additives had higher pH value which would favor calcium carbonate precipitation.

Use of nano-silica to increase early strength and reduce setting time of concretes with high volumes of slag

Abstract: The effects of nano-silica (NS) on setting time and early strengths of high volume slag mortar and concrete have been experimentally studied. Effects of NS dosages, size and dispersion methods on strength development of high volume slag mortars were also investigated. A constant water-to-cementitious materials ratio (w/cm) 0.45 was used for all mixtures. The results indicate that the incorporation of a small amount of NS reduced setting times, and increased 3- and 7-day compressive strengths of high-volume slag concrete, significantly, in comparison to the reference slag concrete with no silica inclusion. Compressive strength of the slag mortars were increased with the increase in NS dosages from 0.5% to 2.0% by mass of cementitious materials at various ages up to 91 days. The strengths of the slag mortars were generally increased with the decrease in the particles size of silica inclusions at early age. Ultra-sonication of nano-silica with water is probably a better method for proper dispersion of nano-silica than mechanical mixing method.

Preparation of Ultra-High Performance Concrete with common technology and materials

Abstract: The technology development of concrete and demand for high strength construction materials give momentum to the development of Ultra-High Performance Concrete (UHPC). Current UHPC preparation methods require costly materials and relatively sophisticated technology. To overcome these weaknesses, this paper focused on the preparation of UHPC with common technology and ordinary raw materials. Influence of binder content, water/binder ratio, ground granulated blastfurnace slag (GGBS) content, and limestone powder (LP) replacement on fluidity and compressive strength of concrete were researched, respectively. The test results show that the addition of superplasticizer and fine mineral additives enabled the UHPC to be produced at an extremely low water/binder ratio of 0.14–0.18, achieving excellent workability with a maximum slump of 268 mm and compressive strengths of 175.8 MPa at 90 d and 182.9 MPa at 365 d.

Effects of granulated blast furnace slag and superplasticizer type on the fresh properties and compressive strength of self-compacting concrete

Abstract: This paper presents the results of an experimental investigation carried out to study the effect of granulated blast furnace slag and two types of superplasticizers on the properties of self-compacting concrete (SCC). In control SCC, cement was replaced with 10%, 15%, 20%, and 25% of blast furnace slag. Two types of superplasticizers: polycarboxylate based superplasticizer and naphthalene sulphonate based superplasticizers were used. Tests were conducted for slump flow, the modified slump test, V-Funnel, J-Ring, U-Box, and compressive strength. The results showed that polycarboxylate based superplasticizer concrete mixes give more workability and higher compressive strength, at all ages, than those with naphthalene sulphonate based superplasticizer. Inclusion of blast furnace slag by substitution to cement was found to be very beneficial to fresh self-compacting concrete. An improvement of workability was observed up to 20% of slag content with an optimum content of 15%. Workability retention of about 45 min with 15% and 20% of slag content was obtained using a polycarboxylate based superplasticizer; compressive strength decreased with the increase in slag content, as occurs for vibrated concrete, although at later ages the differences were small.

Reuse of sanitary ceramic wastes as coarse aggregate in eco-efficient concretes

Abstract: The sanitary ceramics industry inevitably generates wastes, irrespective of the improvements introduced in manufacturing processes. The present study investigated the reuse of these wastes as recycled coarse aggregate in partial substitution (15%, 20% and 25%) of natural coarse aggregates in the manufacture of structural concretes. The results demonstrate that recycled, eco-efficient concretes present superior mechanical behaviour compared to conventional concrete and it was moreover appreciated that the recycled ceramic aggregate does not interfere in a negative way during the hydration process. It was also observed that the microstructure in the interfacial transition zone (ITZ) between recycled ceramic aggregate and paste was more compact than in the case of natural aggregate and paste.

The effect of superplasticizers on the mechanical performance of concrete made with fine recycled concrete aggregates

Abstract: It is considered that using crushed recycled concrete as aggregate for concrete production is a viable alternative to dumping and would help to conserve abiotic resources. This use has fundamentally been based on the coarse fraction because the fine fraction is likely to degrade the performance of the resulting concrete. This paper presents results from a research work undertaken at Instituto Superior Tecnico (IST), Lisbon, Portugal, in which the effects of incorporating two types of superplasticizer on the mechanical performance of concrete containing fine recycled aggregate were evaluated. The purpose was to see if the addition of superplasticizer would offset the detrimental effects associated with the use of fine recycled concrete aggregate. The experimental programme is described and the results of tests for splitting tensile strength, modulus of elasticity and abrasion resistance are presented. The relative performance of concrete made with recycled aggregate was found to decrease. However, the same concrete with admixtures in general exhibited a better mechanical performance than the reference mixes without admixtures or with a less active superplasticizer. Therefore, it is argued that the mechanical performance of concrete made with fine recycled concrete aggregates can be as good as that of conventional concrete, if superplasticizers are used to reduce the water–cement ratio of the former concrete.

Effects of colloidal nanoSiO2 on fly ash hydration

Abstract: The influences of colloidal nanoSiO2 (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO2 had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO2 on cement/FA hydration at later ages were proposed.

Fine limestone additions to regulate setting in high volume fly ash mixtures

Abstract: High volume fly ash (HVFA) concrete mixtures are being considered more frequently due to their cost and sustainability advantages. While the long term performance of these HVFA concretes typically meets or exceeds that of conventional concretes, their early age performance is often characterized by excessive retardation of the hydration reactions, delayed setting times, and low strengths. Extending an HVFA mixture to a ternary blend that incorporates a fine limestone powder may provide a viable solution to these deficiencies, particularly the hydration retardation and setting issues. In this paper, a nano-limestone powder and two other limestone fillers of increasing median particle size (4.4 μm and 16.4 μm) are investigated for their propensity to accelerate early age reactions and reduce setting times in a Class C fly ash/cement blend. The fineness of the limestone has measurable effects on its efficacy in accelerating hydration and decreasing setting times. Companion specimens prepared with a fine silica powder suggest that the fine limestone may act favorably through both a physical and a chemical mechanism. Isothermal calorimetry and Vicat needle penetration measurements on pastes are accompanied by strength measurements on mortars, to verify that the limestone powder substitutions are not negatively impacting strength development. A linear relationship with a reasonable correlation is found to exist between 1 d and 7 d compressive strengths of mortars and their accompanying cumulative heat release values as determined using isothermal calorimetry.

Hydration of slag-blended cements

Abstract: In this paper, the hydration of slag in blended cements is investigated through the measurement of hydration reaction indicators such as portlandite content, non-evaporable and free water, and hydration heat Three substitution rates of cement by slag were used (30%, 50% and 70%) The tests were performed at two constant temperatures (20 °C and 40 °C) in order to assess the activation energy of the different components A multiphasic hydratation model is proposed to take account of the difference of kinetics of each main phase (clinker and slag) and the hydration kinetic law proposed considers interactions between the two phases It includes the activation of the dissolution of slag by alkalis released by the clinker phases in the pore solution, the portlandite consumption by slag and the effect of temperature and moisture content on the reaction kinetics The model is able to simulate the evolution of hydration products and adjust the hydration product stoechimetry to the rates of slag and the current temperature automatically and instantaneously Its reliability is shown through its ability to fit the whole experimental plan results with a single parameter set Among these parameters are the hydration heat of slag and its water consumption The model and its parameters should be useful to simulate other types of slag-blended cement

Effect of metakaolin and silica fume on the durability of self-consolidating concrete

Abstract: Metakaolin (MK) is a valuable admixture for concrete/cement applications that can enhance the performance of cementitious composites through high pozzolanic reactivity, much like silica fume (SF). While SF concrete is characterized by superior mechanical and durability performance, concrete containing MK achieves comparable properties at a lower price and with better workability. The objective of this study is to investigate the effect of cement replacement by MK on the durability of self-consolidating concrete (SCC); the effect of SF at similar levels of MK replacement has also been included for comparison. The durability performance of SCC was evaluated based on the results of drying shrinkage, freezing and thawing, salt scaling, and rapid chloride permeability tests. The results of these tests indicate that highly durable SCC mixtures can be produced using a high MK content with an optimum percentage of around 20%. The results also show that the durability of SCC, especially with high MK content, is higher than that of SCC containing SF.

Compressive strength and electrical properties of concrete with white Portland cement and blast-furnace slag

Abstract: Electrical resistivity is an important characteristic of concrete because it allows evaluation of the accessibility of aggressive agents prior to the beginning of the corrosive process and estimation of the corrosion propagation. This study investigated the apparent electrical resistivity of concrete mixes with white Portland cement and with and without blast-furnace slag using Wenner’s four-electrode method. The compressive strength of concrete cylinders and the electrical conductivity of the pore solution were tested. Examined slag contents were 50% and 70% by mass and the results were compared to reference mixtures of 100% white Portland cement and 100% grey Portland cement, as well as to mixtures with equal percentages of slag and grey Portland cement. Larger amounts of slag resulted in increased electrical resistivity and decreases in the electrical conductivity of the pore solution, when compared to the reference concretes. The mixture made of 50% slag and 50% white Portland cement showed, on average, compressive resistance levels between 35 MPa and 60 MPa, electrical resistivity values that were approximately five times greater, costs that were 14.6% less per m3, and whiteness similar to the reference concrete. These results indicate that white Portland cement can be partially substituted by blast-furnace slag.

Alkali activation of a slag at ambient and elevated temperatures

Abstract: Strength development of alkali activated slag (AAS) mortars, activated using alkali hydroxide and sodium silicate, was investigated at room and elevated temperatures. Heat evolution at room temperature was measured using isothermal calorimetry. Important differences were observed between critical activation parameters. Heat cured specimens gain strength rapidly, humid oven conditions being favorable, but given sufficient time room temperature curing yields comparable strengths. Both activators are needed for high strength at room temperature, NaOH solution is more critical and its concentration greatly influences strength. At 80 °C however, sodium silicate is essential and even sufficient. KOH is more effective than NaOH at 80 °C, but not at room temperature. Lower water-to-slag ratios give higher strength at early ages. AAS hydration evolves less heat than Portland cement hydration. Time to significant strength gain of mixtures can be predicted using their time and heat evolution at setting. Twenty eight-day strength of AAS mortars is roughly related to total evolved heat and increases nearly linearly with the amount of NaOH activator for fixed water glass content.

Effects of activator characteristics on the reaction product formation in slag binders activated using alkali silicate powder and NaOH

Abstract: The influence of different levels of alkalinity, expressed using the Na2O-to-source material ratio (n) and activator SiO2-to-Na2O ratio (Ms), on the compressive strength development of, and reaction product formation in sodium silicate and NaOH powder activated slag binder systems is discussed. Higher n value mixtures are found to exhibit higher early and later age compressive strengths. An increase in Ms results in reduced early age and slightly increased later age strengths. Compositional coefficients, which are functions of n and Ms are proposed, that relate to the early and later age strengths of the activated slag binders as well as to the shift in the FTIR spectra. The reaction product formation in these systems as a function of the total alkalinity is explained using the shifts of the dominant peak in the FTIR spectra. Fundamental changes in reaction products of powder activated binders as a function of alkalinity is observed. The deductions from the peak shifts are substantiated using the FTIR spectra of the pastes before and after salicylic acid–methanol (SAM) attack.

Improved fiber distribution and mechanical properties of engineered cementitious composites by adjusting the mixing sequence

Abstract: Engineered cementitious composites (ECC) is a class of ultra ductile fiber reinforced cementitious composites, characterized by high ductility and tight crack width control. The polyvinyl alcohol (PVA) fiber with a diameter of 39 μm and a length of 6–12 mm is often used. Unlike plain concrete and normal fiber reinforced concrete, ECC shows a strain-hardening behavior under tensile load. Apart from the mix design, the fiber distribution is another crucial factor for the mechanical properties of ECC, especially the ductility. In order to obtain a good fiber distribution, the plastic viscosity of the ECC mortar before adding fibers needs to be controlled, for example, by adjusting water-to-powder ratio or chemical admixtures. However, such adjustments have some limitations and may result in poor mechanical properties of ECC. This research explores an innovative approach to improve the fiber distribution by adjusting the mixing sequence. With the standard mixing sequence, fibers are added after all solid and liquid materials are mixed. The undesirable plastic viscosity before the fiber addition may cause poor fiber distribution and results in poor hardened properties. With the adjusted mixing sequence, the mix of solid materials with the liquid material is divided into two steps and the addition of fibers is between the two steps. In this paper, the influence of different water mixing sequences is investigated by comparing the experimental results of the uniaxial tensile test and the fiber distribution analysis. Compared with the standard mixing sequence, the adjusted mixing sequence increases the tensile strain capacity and ultimate tensile strength of ECC and improves the fiber distribution. This concept is further applied in the development of ECC with high volume of sand.

Design of green concrete made of plant-derived aggregates and a pumice-lime binder

Abstract: The use of particles from agricultural lignocellulosic resources in concrete gives it desirable environmental and multiphysics qualities. In this study, parallels are drawn between particles derived from hemp and sunflower stems, in terms of their morphological and physical properties. A pumice–lime binder is proposed as an alternative to the traditional cement or lime based solutions for both environmentally friendly and mechanical qualities. Compaction is applied during casting and its effects on mechanical properties are analysed. A principal finding of this study is that the hemp and sunflower materials show large similarities in terms of morphology and mechanical performance of the resulting concrete. The pumice–lime binder provides desirable properties even with raw pumice sand, which represent 90% of the binder mass proportion. Compaction level during casting induces an orthotropy, even with low plant content, and increases the compressive strength. A simple analytical model using Powers’ equation is proposed to predict plant concrete compressive strength with low plant quantities.

Electrical characteristics and pressure-sensitive response measurements of carboxyl MWNT/cement composites

Abstract: In this study, electrical characteristics of pressure-sensitive carboxyl multi-walled carbon nanotube (MWNT)/cement composites with and without compressive loading were investigated. Experimental results indicate that the carboxyl MWNT/cement composites have both resistance and capacitance characteristics. Capacitance is insensitive to compressive loading, but the charging of the capacitor causes a linear increase in the measured resistance during DC measurement. The reversible pressure-sensitive responses of resistance to compressive loading can be extracted by removing the linear increase component. An AC measurement method can also be used to eliminate the effect of capacitor charging and discharging on the pressure-sensitive responses of carboxyl MWNT/cement composites.

Application of internal curing for mixtures containing high volumes of fly ash

Abstract: This paper focuses on testing performed on mixtures that would be consistent with the mortar portion of a concrete bridge deck mixture for many state departments of transportation. In this work a relatively large percentage of cement (40%, 60%, or 80% by volume) is replaced with Class C fly ash. To overcome concerns associated with slow set and early-age strength development that are often expressed with the high volume fly ash mixtures (HVFA), the water-to-cementitious materials ratio (w/cm) by mass has been reduced from a conventional value of 0.42 to 0.30. To overcome potential complications that the low w/cm may cause in terms of self-desiccation, internal curing (IC) with prewetted lightweight aggregate was used to reduce shrinkage and increase hydration. By adopting this approach (lowering the w/c and using IC) IC HVFA mixtures show additional benefits that should permit their broader application.

Reliable onset time determination and source location of acoustic emissions in concrete structures

Abstract: Acoustic emission (AE) technique, as an effective method to monitor the crack characterization in concrete materials is investigated in this paper. An improved approach, based on the Akaike Information Criterion (AIC), is proposed to provide more reliable onset time determination of AE signals. The introduced parameters, quantification of the certainty degree and the apparent velocity in the improved method can help to eliminate the false or doubtful picked onset results automatically. The improved AIC method is successfully applied to the AE detection during a three-point bending test of a fiber reinforced concrete beam to analyze the crack pattern. It is shown that the proposed method is a reliable tool for automatic onset time determination and useful for the crack source location in concrete structures.

The role of hydrotalcite in chloride binding and corrosion protection in concretes with ground granulated blast furnace slag

Abstract: This paper presents an investigation into the extent and reasons of the observed improvement in performance that ground granulated blast furnace slag (GGBFS) contributes against chloride initiated corrosion. Tests conducted on concretes with blended cement included Rapid Chloride Permeability Test (RCPT), long term ponding, corrosion current monitoring, pore size distribution and X-ray diffraction analyses. Values of the RCPT and corrosion current were significantly reduced as the proportion of GGBFS increased. The results showed only small refinements in pore size distribution, as well as indications of the formation of Friedel’s salt. The tests however, revealed the formation of hydrotalcite as a significant hydration product in GGBFS blends. The results further demonstrated the efficiency of hydrotalcite in binding chloride ions. The authors attribute the reduction in corrosion current to the efficient binding of chloride ions by the hydrotalcite that forms in GGBFS hydration products.

Highly concentrated carbon nanotube admixture for nano-fiber reinforced cementitious materials

Abstract: The use of effectively dispersed multiwalled carbon nanotube (MWCNT)/aqueous/surfactant suspensions in cement based materials have been shown to substantially improve their mechanical properties. The produced MWCNT suspensions have a high aqueous content, which corresponds to the mixing water. In the present work, a method for preparing highly concentrated MWCNT suspensions is presented, thus reducing the volume of the resulting admixture that is required in cement based materials. A centrifugal process, that uses two different ultracentrifuge rotors, was employed to reduce the quantity of water in the suspensions. Optical absorbance spectroscopy shows that the ultracentrifugation process increases the concentration of the MWCNT suspensions by a factor of 5. Using the highly concentrated MWCNT suspensions following dilution results in nanocomposites with mechanical properties that are comparable to the performance of samples prepared using the non-concentrated suspensions. These results verify that the ultracentrifugation concentration method successfully preserves the solubility of the MWCNT suspensions without affecting the reinforcing properties of the admixture. In this manner, the ultracentrifugation concentration method may constitute an effective preparation step for large-scale implementation of MWCNT admixtures.

Properties of high performance concrete containing fine-ground ceramics as supplementary cementitious material

Abstract: This paper presents experimental work regarding the basic physical characteristics, mechanical and fracture-mechanics properties, durability characteristics, hydric and thermal properties of high performance concrete (HPC) with up to 60% of Portland cement replaced by fine-ground ceramics. Experimental results show that the amount of the ceramics in the mix is limited mainly by the resistance against de-icing salts which is found satisfactory only up to the cement replacement level of 10%. The mechanical and water transport properties are not significantly impaired by ceramic additions of up to 20%, whereas the effective fracture toughness, specific fracture energy, and chemical resistance (to MgCl2 ,N H 4Cl, Na2SO4, HCl) are effectively maintained up to 40%. The frost resistance, water vapor transport and storage parameters and thermal properties are not significantly impaired even up to a 60% replacement level. 2011 Elsevier Ltd. All rights reserved.

Improved geopolymerization of bottom ash by incorporating fly ash and using waste gypsum as additive

Abstract: This research studied the improvement of the geopolymerization of bottom ash (BA) by incorporating fly ash (FA) and using flue gas desulfurization gypsum (FGDG) as additive. The BA:FA ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 were used as the blended source materials. The source materials were then replaced with 0%, 5%, 10%, and 15% of FGDG. NaOH, sodium silicate and temperature curing were used to activate the geopolymer. Test results indicated that the increase in FA content in the BA–FA blends improved the strengths of geopolymer mortars owing to the high glassy phase content and high reactivity of FA compared to those of BA. The use of up to 10% of FGDG as additive also significantly increased the strengths of geopolymer. In this case, the compressive strength enhancement was due to the increase in the Al3+ leached from BA in the presence of SO 4 2 - and the formation of additional calcium silicate hydrate.