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A comparative evaluation of the single and combined effect of nanoparticles (NPs) on the static and impact properties of concrete is presented. Different NPs (silicon dioxide, aluminium oxide and ferric oxide) were used separately and in combination as partial replacements of cement at 0.5% and 1% by weight of cement. The effects of the NP additions on the unit weight, compressive strength, split tensile strength, flexural strength, toughness, fracture energy and impact performance of concrete were experimentally examined. The effects of the NPs on the microstructure of concrete was investigated using scanning electron microscopy (SEM). The cost effectiveness of the use of NPs in concrete was also assessed in order to evaluate the commercial production of NP-doped concretes. Desirability function analysis was also conducted to compare the overall performance of the concretes. The addition of NPs had a positive effect on improving the mechanical performance of the concrete, with the addition of NPs increasing the compressive strength, split tensile strength, flexural strength and energy absorption capacity. The NPs did not have any significant influence on the unit weight of the concrete. SEM analysis showed that the use of NPs improved the concrete microstructure, but the NPs did not have a good influence on the impact performance of the concrete. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) is one of the most widely used building materials. However, RC, especially in coastal seaside or humid environments, can suffer reinforcement corrosion with the result that the reinforcement fails to perform its function over time. The effect of a retarder additive used to eliminate loss of consistency in hot, windy and humid weather on the corrosion of RC elements was investigated in this work. Four different concrete types were assessed, with 0%, 1%, 1.5% and 2% additions of the chemical additive. Along with accelerated corrosion and pressure tests, physical properties such as electrical resistivity, water absorption, porosity, unit weight and capillary water absorption were also determined. The results of this work indicate that the use of up to 2% of the setting retarder did not pose a problem in terms of the physical and mechanical properties of the concrete. However, in terms of the frequently encountered corrosion problem, the use of 1.5% is more appropriate. Many chemical additives are used for different purposes in concrete and it is suggested that, alongside physical and mechanical properties, durability characteristics should also be examined when determining the suitable ratios of these additives. [ABSTRACT FROM AUTHOR]

Construction and demolition waste (CDW) management and recycling practices are crucial for transitioning to a circular economy. The focus of this study was on the detailed characterisation of different CDWs (hollow brick (HB), red clay brick (RCB), roof tile (RT), concrete and glass) collected from seven different demolition sites in Turkey. The CDWs were characterised based on particle size distribution, chemical composition and crystalline nature. Pozzolanic activity was evaluated through compressive strength measurements of cement mortars made with 20% cement replacement by CDWs at 7, 28 and 90 days. The results showed that the clayey CDWs exhibited similar physical/chemical properties and crystalline structures. The compositions of the waste concretes varied significantly based on their original materials. All the CDWs satisfied the minimum strength activity index (SAI) for supplementary cementitious materials, with pozzolanic activity influenced by the fineness and silica + alumina contents. The average SAIs for the HB, RCB, RT, concrete and glass were, respectively, 84.5%, 86.3%, 83.4%, 80.7% and 75.8%. Clayey CDWs contributed to mechanical strength development, while the contribution of concretes was related to the hydration of unreacted cementitious particles. Glass exhibited the weakest pozzolanic activity due to its coarser particle size. Overall, the CDWs demonstrated suitable properties for use as supplementary cementitious materials in Portland cement based systems. [ABSTRACT FROM AUTHOR]

The current study reports on the long-term structural performance of novel reinforced marine geopolymer concrete beams under accelerated weathering conditions. The study covers the flexural performance of 40 geopolymer concrete beams reinforced with basalt fibre-reinforced polymer (BFRP) bars, including 12 beams under sustained loading when exposed to 3, 6 and 12 months of accelerated marine environment consisting of tidal cycles of seawater at a temperature of 50°C. The experimental results revealed that the novel marine geopolymer concrete reinforced with BFRP bars reported minimal micro- and macro-mechanical degradation compared to geopolymer concrete or ordinary concrete beams under the same exposure environment, with and without sustained loading. The BFRP-reinforced self-compacting geopolymer concrete (SCGC) beams reported 87% residual ultimate load after 12 months of exposure to marine environments, while the sustained loaded BFRP-SCGC beams reported a residual strength of 79%. In addition, microstructural assessment using scanning electron microscopy and energy dispersive X-ray spectroscopy analysis revealed that after 12 months of exposure there was a trace of chloride salts, indicating the chemical ingress over time; however, the impact on structural properties is not distinct. [ABSTRACT FROM AUTHOR]

Reusing recycled materials made from concrete wastes is one way of preserving natural resources and promoting sustainable development. The effect of pozzolan additives (silica fume (SF) and nanosilica (NS)) on the strength, durability and microstructural properties of concrete made with 100% recycled aggregates (RAs) was investigated in this study. The concrete mixtures were made with complete substitution of natural aggregates with the same grading of RAs. As proportions of the cementitious materials, the contents of SF were 0%, 5%, 10%, 15% and 20% and the contents of NS were 0%, 1%, 2%, 3% and 4%. Compressive strength, tensile strength, flexural strength, water permeability and chloride permeability were measured at 7, 28 and 90 days. Microstructural investigations were performed using scanning electron microscopy on selected concrete mixtures. The test results showed that the addition of the pozzolan additives improved microstructure and strength and durability properties of the concrete made with 100% RA. According to the results, a concrete mixture with 15% SF content is a suitable and environmentally friendly replacement for ordinary concrete. [ABSTRACT FROM AUTHOR]

The production process of ferronickel generates an enormous amount of slag (FNS) as a by-product, which has potential for use as a fine aggregate in concrete. However, information regarding the durability of FNS aggregate concrete is very limited in the literature. This study evaluates the durability characteristics of concrete using FNS as up to 100% replacement of natural sand and fly ash as 30% replacement of cement. The volume of permeable voids (VPV) of concrete was found to increase with the increase of FNS aggregate. As a result, sorptivity and chloride permeability showed increasing trends with the increase of FNS aggregate. However, the pozzolanic reaction of fly ash reduced the porosity of the concrete, as evidenced by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The specimens with FNS aggregate and fly ash were classified as ‘excellent’ in terms of VPV, ‘low’ in terms of chloride permeability and ‘good’ in terms of sorptivity. The use of fly ash also reduced the strength losses of FNS aggregate concrete subjected to alternate wet–dry cycles. Overall, a durability equivalent to that of conventional concrete could be achieved when FNS was used as a partial replacement of sand together with fly ash as a partial replacement of cement. [ABSTRACT FROM AUTHOR]

In this paper, the bond behaviour of reinforced concrete exposed to high temperature at early age has been investigated. The influence of temperature, curing age before exposing to high temperature and repair agent on the ultimate bond stress and bond displacement of reinforced concrete are analysed. The ultimate bond stress of specimens decreases with the increase in temperature, while the ultimate bond displacement increases with the increase in temperature. After being subjected to the same temperatures, the ultimate bond stress of the specimen heated at a curing age of 28 d is found to be lower than that of specimens that were heated at curing ages of 3, 7 and 14 d. However, the ultimate bond displacement of the specimen heated at the age of 28 d is larger than that of specimens heated at the ages of 3, 7 and 14 d. The ultimate bond stress of the specimen using a repair agent is greater than the ultimate bond stress of the specimen without using the repair agent. On the contrary, the ultimate bond displacement of the specimen using a repair agent is found to be less than that of the specimen without using a repair agent. [ABSTRACT FROM AUTHOR]

An experimental investigation is conducted on the influence of the degradation of fibre-reinforced-polymer (FRP) bars on their bond performance with concrete. A basalt fibre-reinforced vinyl ester (BV) bar, basalt fibre-reinforced epoxy (BE) bar, carbon fibre-reinforced epoxy (CE) bar and E-glass-fibre-reinforced vinyl ester (GV) bar are adopted. The bars are pre-exposed to 40°C alkaline solution for 0, 15, 30 and 45 d and then tested under a direct pull-out condition. The test parameters include the resin type, fibre type and conditioned age. The test results indicate that, for the unconditioned bars, the bond strength of CE bars is the largest, next are the GV bars and BE bars, whose strengths are very close to each other, and the BV bars have the least bond strength. For the conditioned FRP bars, the bond strength retention of BE bars with an epoxy matrix are generally greater than those of BV bars with a vinyl ester matrix. The bond strengths of the conditioned CE bars and BE bars with the epoxy matrix both increase, whereas the BV bars and GV bars show a decreasing trend. It is concluded that the order of bond durability is CE > BE > GV > BV for the bars used in this paper combining the test results with a mechanism analysis. [ABSTRACT FROM AUTHOR]

This paper describes the influence of pore structure parameters – connected porosity and tortuosity – on the water permeability coefficient of cement mortar. Plain and fibre-reinforced mortar specimens were prepared to evaluate the permeability and characterise the microstructure. Polypropylene microfibres were incorporated at 0·25% volume fraction in the fibre-reinforced mortar specimens. The permeability of the mortar specimens was determined using an in situ method where hollow cylinders were subjected to uniaxial compressive stress. The specimens were loaded in the range of 0–90% of the ultimate compressive strength in five different stages. X-ray computed micro-tomography was employed to characterise the pore system of the mortar specimens. By analysing the tomographic images, both connected porosity and tortuosity of the pore network were evaluated. The results showed that the coefficient of water permeability has a correlation with connected porosity across the load levels. While no significant correlation was found with tortuosity alone, the water permeability was found to depend on the ratio between connected porosity and tortuosity as expressed by the Kozeny–Carman formulation. [ABSTRACT FROM AUTHOR]

An innovative hybrid fibre system of steel fibres, polyvinyl alcohol (PVA) fibres and inexpensive (approximately US$230/t) calcium carbonate whiskers (CW) was assessed as a way of improving the mechanical behaviour of fibre-reinforced concrete under static and impact loads and reducing production costs. Static mechanical properties and repeated drop-weight impact tests of the designed hybrid-fibre-reinforced cementitious composites (HyFRCCs) are presented in this paper. The reinforcing effect of the new hybrid fibre system was found to be very close to that of conventional HyFRCCs (incorporating only steel and PVA fibres) under both static and impact loads. It is thus suggested that steel and PVA fibres can be partly replaced with CW to reduce production costs in large-scale construction applications. Ultrasonic pulse velocity, dynamic elastic modulus and crack width assessments were carried out to quantify the degree of damage in samples during mechanical testing. The impact energy absorbed by the specimens could be predicted using linear modelling. There is therefore potential for using such tools to evaluate the in situ structural damage under dynamic loads. [ABSTRACT FROM AUTHOR]

This paper presents a laboratory study on the use of cement kiln dust (CKD) as an activator of fly ash when used in high volumes within concrete. Two separate batches of fly ash and CKD were tested to assess the effect of material variability on binder properties and compressive strength gain. Ternary blends of fly ash (55-65%), CEM 1 (30%) and CKD (5-15%) and quaternary blends that included moderate amounts (15-18·5%) of ground granulated blast-furnace slag (GGBS) were prepared. Physico-chemical properties of individual binder materials were compared and concrete compressive strength was measured at 2 d, 7 d and 28 d. Ternary blends of 60% fly ash, 30% cement and 10% CKD resulted in moderate early age and 28 d strength and addition of GGBS enhanced strength significantly due to increased ettringite formation. Particle fineness, water demand and loss on ignition content of fly ash, and calcium oxide and sulfur trioxide content of CKD were found to be the main physico-chemical factors that influence compressive strength gain. [ABSTRACT FROM AUTHOR]

The effect of accelerated curing and steel fibre volume on ultra-high performance concrete (UHPC) is the focus of this paper. The compressive strength and microstructural properties of UHPC were evaluated under water, steam and heat curing. The results show that heat-cured samples have higher mechanical properties than those undergoing the other curing techniques. Experimental studies were conducted on heat-treated specimens with different steel fibre volumes and aspect ratios (2·5% and 2·0% of 13 mm and 6 mm length fibres of diameter 0·16 mm) to examine stress-strain behaviour, tensile behaviour and flexure behaviour. The stress-strain behaviour of UHPC was evaluated by uniaxial compression tests on cylinders to propose a new stress-strain model for UHPC under compression. Size-dependent and size-independent fracture energies were determined as per the Rilem procedure and the - tail correction method. Flexural and residual strengths were evaluated under four-point bending. [ABSTRACT FROM AUTHOR]

High-performance concrete appears to be a good choice for strong and durable reinforced concrete structures. Suitable addition of mineral admixture can improve the strength and durability owing to improvement in the microstructure at the transition zone. This paper presents an experimental study on the mechanical properties and durability of high-performance concrete containing porcelain waste powder. Concrete mixes possessing a target mean compressive strength of 70 MPa were prepared with 5%, 10%, 15% and 20% cement replacement by porcelain waste powder (water-to-binder ratio, W/B = 0·33). The mechanical and durability performance of porcelain waste based concrete is assessed by means of mechanical, rapid chloride penetration, accelerated corrosion and sorptivity tests. Results show that concrete mixtures with 15% porcelain waste perform better than the control mixtures in relation to compressive strength, flexural strength, chloride penetration, corrosion and sorptivity. Replacement by porcelain waste powder also has major environmental benefits. [ABSTRACT FROM AUTHOR]

In this paper, municipal solid waste incineration (MSWI) bottom ash was characterised before and after chemical treatment and the effect of ash addition on the performance of concrete as a partial replacement of fine aggregate was evaluated. The chemical treatment aimed to eliminate the side effect of MSWI ash - the creation of a network of bubbles - which can eventually lead to a significant reduction of the overall performance of concrete. Petrographic examinations, energy dispersive X-ray spectroscopy and X-ray diffraction, were carried out to chemically characterise the MSWI bottom ash. The mechanical performance of the ash-combined concrete was evaluated by measuring its compressive strength. Analysis of the measured data demonstrates that the chemical treatment successfully transformed metallic aluminium in the ash into a stable form and hence expansion of the concrete due to hydrogen gas evolution was no longer detected in the concrete containing treated ash. Consequently, compared with specimens with untreated ash, concrete specimens with treated bottom ash showed improved performance. [ABSTRACT FROM AUTHOR]

A self-designed test and the elliptical ring test were applied to test the early autogenous shrinkage and crack resistance of fly ash high-strength lightweight aggregate concrete in which 10-30% of cement was replaced by equivalent amounts of fly ash of different fineness. Test data of reference lightweight aggregate concrete were provided. The results showed that (a) the early autogenous shrinkage of concrete was restrained in fly ash high-strength lightweight aggregate concrete, and further analysis indicated higher fly ash content and higher fly ash fineness resulted in less autogenous shrinkage; (b) in a lower water-binder ratio, early autogenous shrinkage increased significantly as the water-binder ratio decreased, and, moreover, pre-wetting of lightweight aggregate had a great effect on controlling early autogenous shrinkage; (c) the elliptical ring test results indicated that the addition of fly ash significantly improved crack resistance of lightweight aggregate concrete and that this improvement becomes obvious with increasing fineness of fly ash. Therefore, in practice, the addition of fly ash could be a technical alternative to reduce early autogenous shrinkage and to enhance the crack resistance of lightweight aggregate concrete. [ABSTRACT FROM AUTHOR]

To reduce the shrinkage of concrete so as to alleviate the various problems so caused, a common strategy is to decrease the cement paste volume. Herein, it is proposed to add limestone fines to fill into the voids between aggregate particles so that the cement paste volume can be decreased more substantially. Following a previous study on the effectiveness of adding limestone fines as cement paste replacement in reducing the heat generation of curing concrete, the aim of this study is to evaluate the effectiveness in reducing the shrinkage of hardened concrete. A number of concrete samples with limestone fines added to replace an equal volume of cement paste are tested and the results show that the addition of limestone fines as cement paste replacement could substantially decrease the ultimate shrinkage strain by as much as 28%, and significantly increase the compressive strength. [ABSTRACT FROM AUTHOR]

Supplementary cementitious materials (SCM) are commonly added to improve the performance of concrete. However, the effects of SCM on the behaviour of cement paste/mortar/concrete depend on many factors and have so far remained elusive, making the mix design of high-performance concrete containing SCM rather difficult. This paper evaluates the effects of SCM, the fresh and hardened properties of mortar made with superfine cement and condensed silica fume added at different water/cementitious materials ratios. First, the packing densities of the solid particles in the mortar samples were measured directly by a new wet packing method. Then, based on the measured packing densities, a mix parameter called water film thickness (WFT) was quantitatively evaluated. The results showed that the WFT plays important roles in the fresh and hardened properties of mortar, as demonstrated by the good correlations established between the WFT and the rheology as well as the strength of the mortar samples tested. [ABSTRACT FROM AUTHOR]

Superplasticiser (SP) is now an indispensable ingredient for the production of concrete. However, its proper use is a great concern because its over-dosage could lead to drastic reduction in the cohesiveness of concrete. Furthermore, experience has indicated that the SP demand, saturation dosage and dosage causing segregation are highly dependent on the powder content and therefore the effects of SP are not simple functions of dosage. This study aims to evaluate the effects of SP dosage on the fresh properties of cement paste containing condensed silica fume (CSF). A number of cement paste samples with different SP dosages and CSF contents were tested. The results showed that the addition of SP would improve the packing density, flowability and rheology but impair the cohesiveness of cement paste. It would also render the cement paste shear thickening. To cater for the effects of powder fineness, it is proposed that the SP dosage should be measured in terms of the quantity of SP per surface area of the powder content. [ABSTRACT FROM AUTHOR]

Concrete of a desired strength for any specific application can be obtained by using any of the accepted methods of design. However, the addition of mineral admixtures, such as fly ash, in concrete, requires a systematic design procedure for obtaining the required strength. It is possible to design a fly ash concrete of the required strength through the 'efficiency concept', and the same is also true for the design of self-compacting concrete (SCC). In this paper, a new mix design methodology is proposed for proportioning of self-compacting fly ash concretes based on the efficiency concept. The methodology has already been successfully verified through a proper experimental investigation and the self-compacting fly ash concretes were evaluated for their self compactability and strength characteristics. The results indicate that the proposed method is capable of producing high-quality SCC. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate the effectiveness of a mechanism responsible for the mitigation of expansion when reactive aggregate powders (RAP) were used in concrete containing the same reactive aggregate. In the first part, it was shown that RAP were efficient in reducing the expansion of concrete by 35 to 95%. The most efficient powders were those coming from aggregates having high active silica contents (glass, opal). Powders of siliceous limestone were less effective. In a second part, experimental evidence of the pozzolanic activity of RAP was obtained. It was shown that RAP were similar to pozzolan, since they consumed lime and produce calcium-silicate-hydrate (C-S-H) having a lower calcium/silica (C/S) ratio than normal C-S-H. The third part showed experimentally that these low C/S C-S-H could increase the redistribution of alkalis of the pore solution, thus reducing the alkalinity of the pore solution to a level that was sufficiently low to limit the attack of coarser reactive aggregates. In the final part, numerical simulations using a modified version of the hydration model CEMHYD3D were carried out to support this assumption. [ABSTRACT FROM AUTHOR]

The current paper presents a model for predicting compressive strength of no-slump concrete with and without fly ash by considering the effect of elevated curing temperature. The model was constructed based on the current authors' test results of compressive strength of no-slump concrete. Additionally, the influences of other parameters, such as mix proportion, chemical composition and physical properties of binders, were also taken into account. The model was established based on a relationship between gel/space ratio of paste and compressive strength development ratio of concrete. In the present study, the gel/space ratio concept for cement paste proposed by Powers and Brownyard in 1946 was adopted and extended for cement–fly-ash system application. The gel/space ratio was found to have a good correlation with compressive strength development ratio of concrete. The proposed model was calibrated and verified by the data obtained from the studies of the current authors and other researchers. The verification of the model showed satisfactory accuracy. [ABSTRACT FROM AUTHOR]

Experimental and theoretical research has shown that in predicting its elastic modulus, concrete should be modelled as a three-phase material at a mesoscopic level and the proportions, mechanical properties and interaction of these three-phase constituents should all be considered in the prediction. The current paper attempts to develop an analytical method for prediction of the elastic modulus of concrete. A three-phase composite circle model for the concrete matrix is proposed to represent the heterogeneous nature of concrete. Based on the classic theory of elasticity, a closed-form solution to the plane strain bulk modulus of concrete is derived. After verifying the developed method with both experimental and numerical results, the effects of the aggregate area fraction, the elastic modulus of aggregate, the elastic modulus of interfacial transition zone (ITZ), the maximum aggregate diameter, the maximum cement diameter and the aggregate gradation on the elastic modulus of concrete are examined in a quantitative manner. It is found that the elastic modulus of concrete increases with the increase of the aggregate area fraction, the elastic modulus of aggregate, the elastic modulus of ITZ and the maximum aggregate diameter, but decreases with the increase of the maximum cement diameter. It is also found that the aggregate gradation affects the elastic modulus of concrete to a certain extent. The paper concludes that the analytical solution derived herein can predict the elastic modulus of concrete with reasonable accuracy. [ABSTRACT FROM AUTHOR]

In this paper, the second of the series, examining the role of cement content in specifying concrete durability, the influences of aggregate characteristics are considered. As for the first paper, the reported work was carried out within a framework of mix adjustments, required to produce practical concretes, that is, with the inclusion of filler and superplasticising admixture. A total of five UK normalweight aggregates and a lightweight aggregate were examined. It is shown that, for a given w/c ratio and aggregate type, reduction in cement and water contents (and the associated mix adjustments) did not adversely affect concrete properties (fresh, bulk engineering, permeation and durability). Indeed, many aspects of concrete performance were enhanced, with the improvements with cement reduction generally being greatest for aggregates having low absorption. Furthermore, the influence of cement content at a given w/c ratio on durability was found to be less significant than that of a change in aggregate type for the range of UK materials tested. The implications of the study for concrete construction practice are discussed. [ABSTRACT FROM AUTHOR]

Idealised data have been derived for the compressive strength of a number of concretes for fire safety design. The data are derived from the author's own research including test series not published before and from more than 400 other test series comprising approximately 3000 specimens known from literature and personal contact. The data cover a variety of concretes with aggregates such as siliceous materials, limestone, granite, sea gravel, pumice, and expanded clay to fire-resistant concrete based on chamotte or Danish mo-clay. Processes are briefly described that are responsible for the deterioration of the materials when they are heated and when they are cooled down after a fire. In addition, it is explained how variations of the concrete composition influences the deterioration. Necessary characteristics of design methods are derived for variable fire courses based on analyses of consequences of material deterioration. The paper serves as a basis for a number of papers presenting calculation methods developed by the author for load-bearing capacity of any structure of any concrete at any time of any fire exposure. Some of these methods and related materials data are adopted in the CEN: ENV 1992-1-2 code and national codes such as the Danish DS411. The present paper serves as a part of the documentation for the methods and it is therefore also a supporting document for these standards. [ABSTRACT FROM AUTHOR]

Research in concrete technology has served the interests of durability, competitiveness and sustainability of concrete construction through development of mix design. Work in this field is mainly aimed at making efficient use of aggregate and developing binders with optimum cement content and industrial by-products as binder components, for example, pulverised fuel ash and ground granulated blast furnace slag. Compared with the traditional Portland cement concrete, concrete elements made with such mixes are expected to have enhanced durability that is attributable to improvement in their microstructure, reduction in voids and increase in resistance to ingress of moisture, gases, etc. This paper examines whether such attributes of different concretes could also improve their engineering properties, e.g. increase in the ratio of flexural tensile strength to compressive strength, better bond between steel and concrete and, in the end, enhanced shear capacity. It is also proposed to examine whether flexural tensile strength should be used as a parameter in the design rules to obtain realistic estimates of shear capacity, in place of the compressive strength. This approach could promote the use of modern mix design techniques and it may have some positive influence on economy of concrete construction, particularly flat slabs, where depth of a concrete member is often governed by contribution of concrete to the shear capacity. [ABSTRACT FROM AUTHOR]