Effect of admixtures on the setting times of high-strength concrete

The effect of silica fume (SF), metakaolin (MK), fly ash (FA) and ground granulated blast-furnace slag (GGBS) on the setting times of high-strength concrete has been investigated using the penetration resistance method (ASTM C 403). In addition, the effect of a shrinkage-reducing admixture (SRA) on the setting times of normal and high-strength concrete was also studied. The setting times of the high-strength concrete were generally retarded when the mineral admixtures replaced part of the cement. While the SRA was found to have negligible effect on the setting times of normal strength concrete, it exhibited a rather significant retarding effect when used in combination with superplasticiser in high-strength concrete. The inclusion of GGBS at replacement levels of 40% and greater resulted in significant retardation in setting times. In general, as replacement levels of the mineral admixtures were increased, there was greater retardation in setting times. However, for the concrete containing MK, this was only observed up to a replacement level of 10%.     

The effect of mineral admixtures on the properties of high-performance concrete

The paper presents a laboratory study on the influence of two mineral admixtures, silica fume (SF) and fly ash (FA), on the properties of superplasticised high-performance concrete. Assessment of the concrete mixes was based on short- and long-term testing techniques used for the purpose of designing and controlling the quality of high-performance concrete. These include compressive strength, porosity, oxygen permeability, oxygen diffusion and chloride migration. Measurements were carried out after curing at 20% and 65% relative humidity up to the age of 1 yr. The results, in general, showed that mineral admixtures improved the properties of high-performance concretes, but at different rates depending on the binder type. While SF contributed to both short- and long-term properties of concrete, FA required a relatively longer time to get its beneficial effect. In the long term, both mineral admixtures slightly increased compressive strength by about 10%, but contributed more to the improvement of transport properties of concretes.     

Compressive strength of HPC containing CNI and fly ash after long-term exposure to a marine environment

This study addressed the effect of calcium nitrite based corrosion inhibitor (CNI) and fly ash (FA) on the long-term compressive strength of high performance concrete (HPC). A 3³ full factorial design was developed to evaluate the influence of CNI at addition rates of 0, 12.5 and 25 L/m³ on the compressive strength of HPC manufactured with 8% silica fume blended cement in combination with 0%, 20% and 40% FA replacements and mixed at 0.29, 0.37 and 0.45 water to cementing materials ratios (w/cm). Standard 100 × 200 mm cylinders were prepared and tested for compressive strength at 28 days and 1 year. The 9-year old concrete specimens were obtained from small-scale reinforced concrete slabs that were exposed to a marine environment. Results indicate that the interaction of CNI and FA does not adversely affect the short and long term compressive strength of concrete. In fact, an enhancement on the compressive strength was observed in concretes containing such combination even after long-term exposure to a marine environment.     

Durability performance potential and strength of blended Portland limestone cement concrete

This paper describes a study on the durability potential and strength of composite Portland-limestone cement (PLC) concrete mixtures blended with ground granulated blast furnace slag (GGBS) and/or fly ash (FA). Their performance was compared against ordinary Portland cement, plain PLC and Portland-slag cement concrete mixtures. Using the South African Durability Index approach, results indicate reductions in the penetrability of the composite PLC blends compared to the other mixtures. The durability indicators are chloride conductivity, gas (oxygen) permeability and water sorptivity. Compressive strength of the composite PLC mixtures containing both GGBS and FA showed competitive performance with the comparative mixtures, but FA blended PLC mixtures had diminished compressive strength values. The paper also presents considerations on the practical implications of using blended PLC concrete mixtures.     

An experimental study on optimum usage of GGBS for the compressive strength of concrete

This paper presents a laboratory investigation on optimum level of ground granulated blast-furnace slag (GGBS) on the compressive strength of concrete. GGBS was added according to the partial replacement method in all mixtures. A total of 32 mixtures were prepared in four groups according to their binder content. Eight mixes were prepared as control mixtures with 175, 210, 245 and 280 kg/m³ cement content in order to calculate the Bolomey and Féret coefficients (K_B, K_F). For each group 175, 210, 245 and 280 kg/m³ dosages were determined as initial dosages, which were obtained by removing 30 percent of the cement content of control concretes with 250, 300, 350, and 400 kg/m³ dosages. Test concretes were obtained by adding GGBS to concretes in an amount equivalent to approximately 0%, 15%, 30%, 50%, 70%, 90% and 110% of cement contents of control concretes with 250, 300, 350 and 400 kg/m³ dosages. All specimens were moist cured for 7, 14, 28, 63, 119, 180 and 365 days before compressive strength testing.The test results proved that the compressive strength of concrete mixtures containing GGBS increases as the amount of GGBS increase. After an optimum point, at around 55% of the total binder content, the addition of GGBS does not improve the compressive strength. This can be explained by the presence of unreacted GGBS, acting as a filler material in the paste.     

矿物掺合料对水泥基材料耐海水侵蚀性能的影响研究

加入适当矿物掺合料是使水泥基材料达到高性能的重要手段之一。本课题从工程应用的角度出发,以磨细矿渣和磨细矿渣-粉煤灰复合掺合料为研究对象,着重研究了矿物掺合料对水泥基材料耐海水侵蚀性能的影响。     

Effect of chemical composition of slag on chloride penetration resistance of concrete

Three ground granulated slags (FeMn arc-furnace (GGAS), Corex (GGCS) and blastfurnace (GGBS) slags) of varying chemical composition, and from different sources were used to make concretes using two w/b ratios (0.40 and 0.60) and three slag replacement levels (20%, 35% and 50%). The effect of chemical composition and replacement level of slags on the chloride penetration resistance of the concretes was assessed using the chloride conductivity test. The results showed that the chloride penetration resistance of concrete increases with decreasing w/b ratio and increasing slag replacement level. In the GGAS concretes, despite having relatively low SiO₂ and high MgO content, its significantly high Mn₂O₃ and low Al₂O₃ content was found to have a negative effect on the chloride penetration resistance of the concrete. The significantly high chloride penetration resistance of GGCS concretes was partly attributed to both its high CaO content and particle fineness. Only GGCS concretes showed a trend of increasing chloride penetration resistance with increased particle fineness; GGBS and GGAS concretes did not show any trend between particle fineness and chloride penetration resistance. The slag activity index was found to be a better indicator of chloride penetration resistance in concrete than the slag hydraulic index.     

The influence of supplementary cementing materials on water retaining characteristics of hydrated lime and cement mortars in masonry construction

The purpose of this paper is an investigation of the possible role of supplementary cementing materials (SCMs) on the water retaining ability of hydrated lime (CL90) and Portland cement (PC) mortars. Desorptivity (R) defines the water retaining ability of mortars in the freshly-mixed wet state. Transfer sorptivity (A) defines the ability of the substrate to withdraw water from the wet mix. The time to dewater (t _dw), is an expression derived from the sharp front theory, and enables calculation of the time taken for a wet mortar joint to be dewatered by an absorbent substrate. The results show that the very water retaining CL90 mortars become progressively more water releasing with increased volume fraction replacement levels of both ground granulated blast-furnace slag (GGBS) and fly ash (FA). On the other hand, the very water releasing PC mortars become more water retaining with the addition of silica fume (SF). Results also show that transfer sorptivity increases as the volume fraction replacement levels of GGBS and FA increases in CL90 mortars and decreases with increased volume fraction replacement levels of SF in PC mortars. Since the time taken to dewater a mortar joint (t _dw) is inversely proportional to the squared transfer sorptivity, t _dw can be dramatically altered by the addition of SCMs in both CL90 and PC mortars. These parameters have important practical consequences, not only in the initial adhesion of the mortar to the substrate but also in the strength of the set material. The ability to manipulate the water retaining properties can also allow construction time to be reduced.     

Compressive strength,modulus of elasticity,and water permeability of inorganic polymer concrete

Inorganic polymer concretes (IPCs) were produced from rice husk–bark ash (RHBA) combined with fly ash (FA) as a cementitious raw material. Six different mixtures were used to study the properties of IPC. Since RHBA is rich in silica material, varying the ratio of FA to RHBA results in differing SiO₂/Al₂O₃ ratios. To keep the SiO₂/Al₂O₃ ratio constant, the ratio of FA to RHBA was fixed at 80:20 by weight. High concentration sodium hydroxide solution and sodium silicate solution were used as a liquid component of the concrete mixture. The mixing and curing of these inorganic polymer concretes were performed under ambient conditions. Compressive strength, modulus of elasticity, and water permeability of the IPCs were investigated at specified intervals up to 90 days. The results showed that the compressive strength, modulus of elasticity, and water permeability of IPCs depend on the mix proportions, especially the solution to ash (S/A) ratio and the paste to aggregate (P/Agg) ratio. Moreover, the results showed that the water permeability and the elastic modulus of IPCs were significantly related to their compressive strength.     

Microstructure and mechanical properties of accelerated sprayed concrete

Considering the different hydration processes of concrete without accelerator, sprayed concrete with low-alkali accelerator not only presents short setting times and high early-age mechanical properties but also yields different hydration products. This study presents an analysis of the mechanical properties of concrete with and without accelerator and sprayed concrete with three water–binder (w/b) ratios and four dosages of fly ash (FA) after different curing ages. It also examines the setting time, mineral composition, thermogravimetric–differential scanning calorimetry curves and microscopic images of cement pastes with different accelerator amounts. Furthermore, the setting time and microstructure of accelerated sprayed concrete with different w/b ratios and FA contents are examined. Results show that the retarded action of gypsum disappears in the accelerated cement–accelerator–water system. C₃A is quickly hydrated to form calcium aluminate hydrate (CAH) crystals, and a mesh structure is formed by ettringite, albite and CAH. A large amount of hydration heat improves the hydration rate of the cement clinker mineral and the resulting density, thereby improving mechanical properties at early curing ages. The setting times of the pastes increase with increasing w/b ratio and FA dosage. Thus, the hydration level, microstructure and morphology of the hydration products also change. Models of mechanical properties as functions of w/b, FA and curing age, as well as the relationship between compressive strength and splitting tensile strength, are established.     

A site study of durability indexes for concrete in marine conditions

The paper describes an investigation into the validity of durability index tests when used in a site situation and to evaluate the effectiveness of site curing methods. Concretes manufactured and cured under site conditions in a warm, humid coastal environment were investigated using recently developed durability index tests.Three blended binders (GGBS, FA and CSF) were used to cast a series of wall and slab elements. The elements were cured using practical site methods currently employed in the industry. Cores were extracted at early (28-day) and later (120-day) ages and used to determine the durability index properties.The results indicated that it is possible to manufacture, place and cure site concrete to achieve acceptable durability properties. Full wet curing proved to be the most effective method, as expected. While variation in potential durability properties existed at early age (28 days) between site and wet cured samples, at a later age (120 days) the variations had reduced such that, in practical and general terms, the different site curing methods were virtually indistinguishable. It was clear that environmental curing continues after 28 days provided climatic conditions are conducive for this to occur. Thus, on-going environmental curing largely governs the potential durability. To develop a concrete durability performance specification, it is imperative that a system is developed to quantify the effects of the environment on potential durability.     

Transport and mechanical properties of self consolidating concrete with high volume fly ash

This paper presents the transport and mechanical properties of self consolidating concrete that contain high percentages of low-lime and high-lime fly ash (FA). Self consolidating concretes (SCC) containing five different contents of high-lime FA and low-lime FA as a replacement of cement (30, 40, 50, 60 and 70 by weight of total cementitious material) are examined. For comparison, a control SCC mixture without any FA was also produced. The fresh properties of the SCCs were observed through, slump flow time and diameter, V-funnel flow time, L-box height ratio, and segregation ratio. The hardened properties included the compressive strength, split tensile strength, drying shrinkage and transport properties (absorption, sorptivity and rapid chloride permeability tests) up to 365 days. Test results confirm that it is possible to produce SCC with a 70% of cement replacement by both types of FA. The use of high volumes of FA in SCC not only improved the workability and transport properties but also made it possible to produce concretes between 33 and 40 MPa compressive strength at 28 days, which exceeds the nominal compressive strength for normal concrete (30 MPa).     

Characterisation of high-performance cold bitumen emulsion mixtures for surface courses

Cold bitumen emulsion mixture (CBEM) is not yet widely used as a surface course around the world. In this study, 0/14-mm-size dense-graded surface course CBEMs have been investigated. The mechanical performance was evaluated in terms of stiffness modulus over 3 months and resistance to permanent deformation under three different stress levels (100, 200, 300 kPa), whilst durability evaluation was carried out in terms of resistance to moisture and frost damage. The study has also investigated the incorporation of low cement content (1%) with relatively sustainable by-product fillers, namely ground-granulated blast furnace slag (GGBS) and fly ash (FA) type 450-S on both mechanical and durability performance. A comparison has been carried out between the low and high cement content CBEM, as well as with respect to corresponding hot mix asphalt (HMA). The results revealed that the incorporation of GGBS and FA in CBEMs leads to superior performance, similar to CBEMs treated with high cement content and comparable to an equivalent HMA. Furthermore, GGBS replacement exhibited better performance than that of FA replacement. The findings suggest that the new sustainable types of CBEM can be developed for using as a surface layer for medium- to heavy-trafficked roads.     

Plastic shrinkage cracking of concrete incorporating mineral admixtures and its mitigation

In recent years, there has been a rapid increase in the use of mineral admixtures for high performance and durable concrete. Plastic shrinkage cracking in such concretes is a serious concern in large surface area/volume applications. The present study has two objectives: firstly, to investigate the influence of incorporating fly ash and granulated blast furnace slag (GGBS) on the susceptibility to such cracking; and secondly, to assess the techniques, such as fibre and shrinkage reducing admixture (SRA) addition, and spraying of curing compounds, to mitigate the cracking. The results indicate that replacement of ordinary Portland cement (OPC) with fly ash and GGBS increases the possibility of plastic shrinkage cracking significantly, with higher severity as the replacement level increases; 30% replacement of OPC with fly ash and GGBS doubled and quadrupled the crack area, respectively, mainly due to higher binder finesses, and the delay of setting and strength gain. Among the fibres tested, polypropylene and polyester fibres, at the recommended dosages of about 0.9 kg/m³, completely eliminated cracking in the most affected concrete (i.e., with 30% GGBS) while the dosages of the polyacrylonitrile and glass fibres had to be increased to provide a higher volume fraction. Two glycol-based SRAs, and two curing compounds based on acrylic resin and methacrylate mitigated cracking by significantly reducing evaporation from the surface of concrete.     

Fresh properties of self-compacting cold bonded fly ash lightweight aggregate concrete with different mineral admixtures

This paper presents the results of an experimental study on the fresh properties of the self-compacting lightweight concretes made with cold bond fly ash (FA) lightweight aggregates. Binary and ternary use of FA and silica fume (SF) blends have been investigated in the production of self-compacting cold bonded FA lightweight aggregate concretes (SCLWCs). A total of 9 SCLWC mixtures were proportioned having constant water-binder ratio of 0.35 and the total binder content of 550?kg/m³. The control mixture contained only Portland cement (PC) as the binder while the remaining mixtures incorporated binary and ternary blends of PC, FA, and SF. After mixing, the fresh properties of the SCLWC were tested for T ₅₀₀ slump flow time, slump-flow diameter, V-funnel flow time and L-box height ratio. The fresh properties of SCLWCs with and without mineral admixtures were also evaluated by statistical technique, namely GLM-ANOVA. The results indicated that the combination use of FA and SF together decreased the slump flow time and V-funnel flow time. L-box height ratio, on the other hand, improved significantly.     

Residue strength,water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures

In this paper, the effects of high temperature exposure of recycled aggregate concretes in terms of residual strengths, capillary water absorption capacity and pore size distribution are discussed. Two mineral admixtures, fly ash (FA) and ground granulated blast furnace (GGBS) were used in the experiment to partially replace ordinary Portland cement for concrete production. The water to cementitious materials ratio was maintained at 0.50 for all the concrete mixes. The replacement levels of natural aggregates by recycled aggregates were at 0%, 50% and 100%. The concretes were exposed separately to 300 °C, 500 °C and 800 °C, and the compressive and splitting tensile strength, capillary water coefficient, porosity and pore size distribution were determined before and after the exposure to the high temperatures. The results show that the concretes made with recycled aggregates suffered less deteriorations in mechanical and durability properties than the concrete made with natural aggregates after the high temperature exposures.     

Mechanical and cementitious characteristics of ground granulated blast furnace slag and basic oxygen furnace slag blended mortar

Reusing waste materials and reducing carbon emissions are crucial environmental concerns. Ground granulated basic oxygen furnace slag (GGBOS) and ground granulated blast furnace slag (GGBS) are the by-products of the steel industry and has positive effects on the environment because it reduces the problems associated waste disposal. This study reused these two products to completely replace cementitious materials, thus contributing to waste recycling, reducing the production demand for cement, and mitigating carbon emissions. Twelve mixture proportions were designed in this study, including an ordinary Portland mortar (OPM) as the control group and 11 steel/iron slag blended mortar (SISBM) experimental groups for the mechanical and cementitious characteristic experiments. The optimal mixing ratio for SISBM compressive strength ranged from GGBOS (steel slag): GGBS (iron slag) = 3:7 to 5:5 (by weight). At the age of 91 days, the compressive strength of SISBM reached 80–90% compared with that of the control group. Based on the pH values, free-CaO, and microanalysis results, the cementitious characteristics were mainly generated because the GGBOS increased the free-CaO or Ca(OH)₂ concentrations in the SISBM curing water and provided alkaline environments for Ca(OH)₂ to engage in the pozzolanic reaction with the SiO₂ and Al₂O₃ in GGBS, forming crystals such as calcium aluminum silicate hydrate, (C–A–S–H), calcium silicate hydrate (C–S–H), and calcium–magnesium–alumina–silicate (C–M–A–S), which generated strength and strengthened microstructure.     

Full-scale test of a pile supported steel fibre concrete slab

The aim of the short-term studies is to investigate the structural behaviour of pile supported slabs made of steel fibre concrete (SFC) only and combined reinforced steel fibre concrete. The studies include tests on an elevated slab where a combination of reinforcement bars and steel fibres have been used in one half of the slab and SFC only in the other half. The tests were performed on a column-supported elevated slab that simulates a half scale model of an industrial pile-supported floor slab. The short-term tests showed considerable structural and crack arresting performance that also increased with a higher dosage of fibres. A small addition of conventional reinforcement bars further increased the ultimate load capacity P _Max. P _Max was in the range of 125–298 kN for the two types of slab. The results indicate that SFC can be used with verifiable results in structural applications for elevated slabs and pile-supported floor slabs despite that the material testing from the ordered SFC showed a larger scatter in properties and that the calculated load capacities were only 40–220 kN. Main causes of deviance are arch and membrane effects.     

Feasible use of large volumes of GGBS in 100% recycled glass architectural mortar

The use of 100% recycled glass as aggregates in architectural mortar is regarded as an environmentally friendly, cost-effective and attractive feature for construction applications due to the natural characteristics of glass (e.g. aesthetic pleasing, impermeability, chemical resistance properties). However, the need to use large quantities of white cement for architectural products may increase the overall cost of production. Therefore, the possibility of using a near-white coloured ground granulated blast furnace slag (GGBS) to replace white cement for architectural mortar production is an attractive option. This paper reports a study which is an extension of our previous work aiming to investigate the feasibility of using large volumes of GGBS (ranging from 15% to 75% white cement replacements) to produce self-compacting-based architectural mortars. To improve the appearance (whiteness) of the mortar, a small quantity of titanium dioxide (TiO₂) was added to the selected mixes for comparison purposes. Fresh and hardened properties of the mortar including mini-slump flow, density, water absorption, flexural strength, equivalent compressive strength, drying shrinkage, alkali silica reaction (ASR) and acid attack resistance were investigated. The overall performance showed that it is feasible to use GGBS for the production of architectural mortar and 60% replacement of white cement by GGBS was determined to be optimal. The replacement significantly increased the flexural strength, and reduced the drying shrinkage and risk of ASR expansion, as well as improved the ability to resist acid attack of the mortar produced.     

Engineering performance of a new siloxane-based corrosion inhibitor

This paper presents an evaluation of a new non-toxic corrosion inhibitor on selected engineering properties of concrete mixes with different cementitious materials following a corrosion and durability study on concrete samples. Corrosion inhibitors consist of powders or solutions which are added to concrete when mixed to prevent or delay corrosion of steel by their reaction with ferrous ions to form a stable and passive ferric oxide film on the steel surface. The new inhibitor functions slightly differently and its corrosion inhibition effect is due to the formation of a siloxane coating on the steel surface. Therefore, the performance of the new inhibitor in concrete mixes manufactured with CEM I, PFA and GGBS cements was compared against a well known and established corrosion inhibitor on the market, namely calcium nitrite in terms of their effect on workability (measured in terms of slump), compressive strength, freeze–thaw durability and macro-cell corrosion. The results from this experimental programme have demonstrated that the new inhibitor is effective in reducing or slowing down corrosion. In addition, it was found that CEM I concrete containing the new inhibitor was less penetrable to chlorides than that without. A similar set of results was obtained for the freeze–thaw resistance, but the compressive strength was found to decrease with the addition of the new inhibitor. In the case of concretes containing PFA and GGBS, the new inhibitor was found to be less effective. Further, long-term investigations are recommended to assess the effectiveness over time.