Effect of repeated dosages of superplasticizers on slump,strength and freeze-thaw resistance of concrete

Superplasticizers, when added to fresh concrete, cause large increases in its slump. However, this increase in slump is not sustained over long periods and within 60 minutes or so the concrete reverts to its original slump. In actual field applications of superplasticizers it may be necessary to add additional dosages to maintain the increased slump. This paper gives results of a laboratory investigation to determine the effect of repeated dosages of superplasticizers on workability, strength and durability of concrete. A series of air-entrained concrete mixes was made at a water/ cement ratio of 0.42 with a slump of 50 mm. Four commonly available superplasticizers were repeatedly added to the concrete, at the manufacture's recommended dosage rates, after completion of initial mixing. This was followed by additional mixing for 2 minutes. The properties of the fresh concrete were determined and test cylinders were cast after the addition of each dosage. Test prisms were also cast for strength and durability studies after the addition of the last dosage. The test results indicate that large increases in slumps of superplasticized concretes can be maintained for several hours by the addition of a second dosage. Apart from one instance, the addition of the third dosage is not considered desirable. The repeated additions of sulphonated melamine- and naphthalene-based superplasticizers caused substantial loss in entrained air content of the concrete; however, for concrete incorporating the lignosulphonate based superplasticizer, the reverse was true. The loss of entrained air adversely affected the performance of the concrete in freeze-thaw tests.     

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

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.     

Effects of the molecular architecture of comb-shaped superplasticizers on their performance in cementitious systems

The objective of this study is to link the molecular structure of polycarboxylate-ether-type superplasticizers with the performance of cementitious systems in order to develop new products with enhanced properties, e.g. improved water reduction with a wide range of cements or a reduced retardation of cement hydration. Different experimental superplasticizers have been synthesized varying length and density of the polyether chains as well as the molecular weight of the polymer. The influence of these polymers on the properties of cement pastes and mortars was determined using various characterization methods like mortar flow, rheological and calorimetric measurements, adsorption measurements and mortar compressive strength.Characteristic connections between molecular structure of the polycarboxylate-type water reducers, adsorption behaviour, workability and retarding effect have been determined allowing the synthesis of new superplasticizers with improved performance.     

Defined-performance design of ecological concrete

Energy consumption and CO₂-emission of concrete can be reduced when cement is replaced by secondary materials such as residual products from other industries. However, for the design of such environmentally friendly concretes, predicting its performance is very important. In this article a cyclic design method is presented, which can predict the strength of a concrete mixture based on particle packing technology. In the procedure, the amount of water is estimated from the required workability and calculated packing density. After that, the strength of that mixture is predicted from packing density calculations and the amount of water in the mixture via the cement spacing factor. This cycle is repeated until the mixture composition does not have to be adjusted anymore to comply with the desired performance or strength class. With the presented cyclic design procedure cement contents can be decreased without changing concrete properties in a negative way, thereby saving up to 57 % of Portland cement and reducing CO₂-emission with 25 %. This is shown by experimental results of ecological concrete mixtures tested on compressive strength, tensile strength, modulus of elasticity, shrinkage, creep and electrical resistance. The results confirmed that relationships between cube compressive strength, tensile splitting strength and modulus of elasticity correspond to those for normal concrete. The experimental program showed the possibility to use cube compressive strength as the governing design parameter in the cyclic design procedure for ecological concrete. Furthermore, it is shown how the cyclic design method can be used for defined-performance concrete design.     

Workability and strength behaviour of concrete with cold-bonded fly ash aggregate

This paper discusses the development of empirical models for workability and compressive strength of cold-bonded fly ash aggregate concrete in terms of mixture proportioning variables such as cement content, water content and volume fraction of cold-bonded aggregate through statistically designed experiments based on Response Surface Methodology. Factor level of cement is taken from 250 to 450 kg/m³ to introduce weak as well as strong matrix phase in the concrete. Apart from water content, workability of concrete is highly influenced by main and interaction effect of volume fraction of cold-bonded aggregate in the composition. Response surface indicate that increase in cement content causes to change the predominant failure mode from mortar failure to aggregate fracture and concrete strength decreases with increase in volume fraction of aggregate at higher cement contents. The models developed have been found useful in arriving typical relationship to establish a mixture proportioning methodology for cold-bonded fly ash aggregate concrete.     

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

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 Técnico (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.     

Packing optimization of paste and aggregate phases for sustainability and performance improvement of concrete

Previous research demonstrated that the packing density, water film thickness and paste film thickness have great effects on the performance of a concrete mix. On this basis, it is herein proposed a strategy of adding a powder waste as both paste and aggregate replacements to reduce the cement and aggregate consumptions for sustainable development and to improve the packing densities of both the paste phase and aggregate phase for performance improvement. To evaluate such strategy, 25 concrete mixes incorporating granite polishing waste (GPW) as paste and aggregate replacements were tested. The results revealed that the addition of GPW as paste replacement up to 7.5% and as aggregate replacement up to 10% would most effectively increase the packing densities of the paste phase, aggregate phase and whole concrete mix, and thereby increasing the strength of the concrete, despite reduction in cement content. Such increases in packing density would also increase the excess water and excess paste to avoid excessive reductions in the water and paste film thicknesses, which are needed to maintain workability. Last but not least, separate optimization of the paste phase and aggregate phase is an effective way of optimizing the concrete mix design.     

Effect of temperature on the rheology of flowable mortars

In addition to the characteristics of mixture constituents and mix design, the rheological behavior of concrete is influenced by material temperature and time after water–cement contact. The study presented herein evaluates the combined influence of time and temperature on the workability of micro mortars. The mixtures were proportioned with polymelamine (PMS), polynaphtalene (PNS), polycarboxylate (PCP) polymer, and made of different supplementary cementitious materials. Seven micro mortars proportioned with various binder compositions and water-to-binder ratio of 0.42 and 0.53 were prepared at 10–33 °C. Test results show that the yield stress and plastic viscosity vary linearly with the coupled effect of time and temperature for mixtures made with PNS or PMS superplasticizers. However, for mixtures made with PCP superplasticizer, both the material temperature and type of supplementary cementitious materials are shown to influence the evolution of rheology with time.     

A method for optimal design of steel fiber reinforced concrete composition

Existing design approaches for steel fiber reinforced concrete composition practically do not consider the interaction between the concrete components. It decreases the design efficiency and accuracy. The paper deals with methodology for design of optimal steel fibered fine-grained concrete composition based on stiff mixtures. Such concrete is used for production of thin walled precise elements. The current investigation enables to find the influence of the main factors (water–cement ratio, fiber content, fineness and quantity of sand) on the concrete mixture stiffness, compressive and flexural strength of concrete. The study has also enabled to obtain corresponding mathematical models of concrete properties. Based on the models a methodology for design of steel fibered concrete was developed and appropriate nomograms were prepared. The proposed methodology allows obtaining of optimal steel fibered fine-grained concrete composition, taking into account the required flexural strength of concrete, sand fineness and concrete mixture workability.     

Steel–concrete bond strength of lightweight self-consolidating concrete

The bond behavior of lightweight self-consolidating concrete (LWSCC) must be understood in order to use this type of high performance concrete in structural members. The objective of this research program is to assess the bond behavior of reinforcing steel bars embedded in LWSCC members. Three different classes of LWSCC mixtures were developed with two different types of lightweight aggregates. In addition, one normal weight SCC (NWSCC) was developed and used as a control mixture. A total of twenty four pullout tests were conducted on deformed reinforcing bars with an embedded length of either 100 or 200 mm and the load-slip responses, failure modes and bond strengths of LWSCC and NWSCC were compared. Based on the results of this study, the bond strength of deformed bars for LWSCCs are found to be less (between 16 and 38%) as compared with NWSCC. Under the conditions of equivalent workability properties and compressive strength, bond slip properties were shown to be significantly influenced by the type of lightweight aggregate used. In this study, the use of expanded shale in the production of LWSCC significantly enhanced the pullout strength when compared with lightweight slag aggregate.     

Influence of a new type of additive on the performance of polymer-lightweight mortar composites

This paper shows how a new powder polymer additive (PPA), containing a waterproofing agent, a rheology control agent and air-entrainers, affects the workability, mechanical properties and setting times of polymer-lightweight mortar composites (PLMC). The waterproofing agent was a mixture of redispersible polyethylene vinyl acetate and redispersible silane based polymer powder. The rheology control agent was a redispersible hydroxypropyl carboxymethyl ether of patato starch based polymer powder. Air-entraining agent was a redispersible and an unmodified sodium laurly sulphate based polymer powder. Pumice fine aggregate at 0–3 mm size fraction was used as lightweight aggregate throughout the research work. In order to examine the effects of powder polymer additive on flowability and the performance when the additive is mixed in a mortar, the mixture proportions were set in four trial batches. The volume proportions of cement and pumice lightweight fine aggregate were fixed at 1:9, 1:8, 1:7 and 1:6, respectively, defining the mixture of mortar for measuring the compressive strength and workability of lightweight mortar. In this research study, PLMC mortars with 28 different mixture proportions (M1–M28) by weight of cement contents of 0.2%, 0.4%, 0.6%, 0.8%, 1.0% and 1.2% were adopted for the mortar mixture batches, respectively. Flow value of mortar was measured using a flow table method in accordance with the regulation in ASTM C230, “flow table for use in tests of hydraulic cement”. The target flow was fixed at 130 mm for each mixture proportion, which is regarded as the most suitable fluidity to secure workability at a site. For each mixture, 12 fresh plastic mortar samples were prepared according to the method specified in ASTM C305 and cured in a humidified atmosphere for 24 h, removed from the mould after 24 h, cured in water for 7 days, and then cured in air. The compressive strength test results were evaluated in accordance with ASTM C270.The suitability of using a new powder polymer additive in terms of workability and required compressive strength in PLMC mortar applications is also presented in this paper. It is observed that PLMC mortars have adequate strength and more convenient workability for their use in general masonry construction applications.     

Effect of addition time of a superplasticizer on cement adsorption and on concrete workability

The effect of addition time of a naphthalene-based superplasticizer (SNF) on the adsorption behavior on type I Portland cement slurries and on the concrete workability was studied. Test results indicate that the adsorption behavior of SNF on cement particles follows a Langmuir isothermal adsorption model. As the addition time increases, the saturated adsorption amount of SNF decreases sharply at the beginning and then more slowly. In comparison, the concrete workability decreases slightly in the early phase and then falls off abruptly. Most importantly, the transition points in both cases appear to be the same, at about 10–15 min. This strongly suggests that a close relationship exists between the SNF adsorption behavior on cement particles and the workability of concrete. In addition, the optimum addition time of SNF to concrete should be in this period, which corresponds to the beginning of the dormant period of the cement hydration process.     

自密实道面抢修混凝土试验研究

针对机场水泥混凝土道面破坏时如何能够在短时间内进行快速修复的迫切问题,采用快硬早强型特种水泥、高效外加剂和矿物掺合料,进行配合比优化设计和性能试验研究。试验表明,所配制的混凝土工作性能良好,加水拌合后4h即可达到某新型战机起飞最低强度要求,能够满足机场道面抢修技术要求,这种混凝土也可用于公路水泥混凝土路面抢修。     

Development of engineered cementitious composites with limestone powder and blast furnace slag

Nowadays limestone powder and blast furnace slag (BFS) are widely used in concrete as blended materials in cement. The replacement of Portland cement by limestone powder and BFS can lower the cost and enhance the greenness of concrete, since the production of these two materials needs less energy and causes less CO₂ emission than Portland cement. Moreover, the use of limestone powder and BFS improves the properties of fresh and hardened concrete, such as workability and durability. Engineered cementitious composites (ECC) is a class of ultra ductile fiber reinforced cementitious composites, characterized by high ductility, tight crack width control and relatively low fiber content. The limestone powder and BFS are used to produce ECC in this research. The mix proportion is designed experimentally by adjusting the amount of limestone powder and BFS, accompanied by four-point bending test and uniaxial tensile test. This study results in an ECC mix proportion with the Portland cement content as low as 15% of powder by weight. This mixture, at 28 days, exhibits a high tensile strain capacity of 3.3%, a tight crack width of 57 μm and a moderate compressive strength of 38 MPa. In order to promote a wide use of ECC, it was tried to simplify the mixing of ECC with only two matrix materials, i.e. BFS cement and limestone powder, instead of three matrix materials. By replacing Portland cement and BFS in the aforementioned ECC mixture with BFS cement, the ECC with BFS cement and limestone powder exhibits a tensile strain capacity of 3.1%, a crack width of 76 μm and a compressive strength of 40 MPa after 28 days of curing.     

Investigation on the scattering-filling coarse aggregate self-consolidating concrete

To secure good flowability and workability of SCC, the volume fraction of coarse aggregate keep at an extremely low level. A new kind of SCC pouring method named scattering-filling coarse aggregate process was invented: it was method to scatter 20% (volume fraction to the finished concrete) of extra coarse aggregate into the fresh SCC mixture to replace the fresh concrete mixture while the concrete was pouring. A high strength (82 MPa) SCC just composing 360 kg/m³ cement and 120 kg/m³ class F fly ash was prepared with this process. With an increase of the extra coarse aggregate replacing ratio from 0 to 30%, the compressive strength of SCC increased steadily and reached a peak value when this ratio is 20%, then the strength dropped sharply. The drying shrinkage ratio and the chloride ion permeability decreased with the increase of that ratio. The scattering-filling coarse aggregate process can cast high strength SCC with lower cementitious materials content and produce concrete with better performance than the ordinary process.     

Influences of superplasticizer modification and mixture composition on the performance of self-compacting concrete at varied ambient temperatures

The fresh behaviour of self-compacting concrete (SCC) at varying temperatures differs from that of normal vibrated concrete. This is because the rheology of SCC depends not only on degree of cement hydration, but also on the adsorption of superplasticizers – mostly polycarboxylate based polymers (PCE) -, which is affected by the time and hydration progress. Due to the variety of PCEs and mixture compositions for SCC a prediction of the rheology at varying temperatures is complicated. The charge densities of PCEs as well as the water to solid ratio in the paste are identified to be the main decisive parameters for robust fresh concrete properties.Rheometric concrete investigations with different SCC mixture compositions and varied anionic charge densities of the PCE were conducted. SCC which is rich in powder components showed robust performance at low temperatures while SCC with low powder content was favourable at high temperatures. High charge density PCE pointed out to be very robust at low temperatures but at high temperatures it significantly reduced the flow retention. Low charge density PCE could not generate self-compacting properties at low temperatures but retained the flow performance over sufficiently long time. Based on considerations about particle interactions and adsorption mechanisms of PCEs, the relevant processes are explained and options for the development of robust mixture compositions for individual temperature ranges are itemised.     

Use of ultrafine rice husk ash with high-carbon content as pozzolan in high performance concrete

Rice husk ash (RHA) has been generated in large quantities in rice producing countries. This by-product can contain non-crystalline silica and thus has a high potential to be used as cement replacement in mortar and concrete. However, as the RHA produced by uncontrolled burning conditions usually contains high-carbon content in its composition, the pozzolanic activity of the ash and the rheology of mortar or concrete can be adversely affected. In this paper the influence of different grinding times in a vibratory mill, operating in dry open-circuit, on the particle size distribution, BET specific surface area and pozzolanic activity of the RHA is studied, in order to improve RHA’s performance. In addition, four high-performance concretes were produced with 0%, 10%, 15%, and 20% of the cement (by mass) replaced by ultrafine RHA. For these mixtures, rheological, mechanical and durability tests were performed. For all levels of cement replacement, especially for the 20%, the ultra-fine RHA concretes achieved superior performance in the mechanical and durability tests compared with the reference mixture. The workability of the concrete, however, was reduced with the increase of cement replacement by RHA.     

Eco-efficient ultra-high performance concrete development by means of response surface methodology

The research described in this paper represents a statistically based model with the help of response surface methodology (RSM) aiming to study the applicability of this method to ultra-high performance concrete (UHPC) mixture design and its optimization. Besides, the effects of silica fume, ultra-fine fly ash (UFFA) and sand as three main variable constituents of UHPC on workability and compressive strength as the main performance criteria and responses of this high-tech material were investigated. The models proposed here demonstrate a perfect correlation among variables and responses. Furthermore, through performing a multi-objective optimization, cement and silica fume, as two main constituents of UHPC affecting its eco-efficiency and cost, were substituted by UFFA and sand as much as possible. Finally, an eco-efficient UHPC with cement and silica fume content of 640 kg/m³ and 56.3 kg/m³ respectively and compressive strength and flow diameter of 160.3 MPa and 19 cm was developed.     

The effect of chemical admixtures and mineral additives on the properties of self-compacting mortars

Mortar serves as the basis for the workability properties of self-compacting concrete (SCC) and these properties could be assessed by self-compacting mortars (SCM). In fact, assessing the properties of SCM is an integral part of SCC design. The objective of this study was to evaluate the effectiveness of various mineral additives and chemical admixtures in producing SCMs. For this purpose, four mineral additives (fly ash, brick powder, limestone powder, and kaolinite), three superplasticizers (SP), and two viscosity modifying admixtures (VMA) were used. Within the scope of the experimental program, 43 mixtures of SCM were prepared keeping the amount of mixing water and total powder content (portland cement and mineral additives) constant. Workability of the fresh mortar was determined using mini V-funnel and mini slump flow tests. The setting time of the mortars, were also determined. The hardened properties that were determined included ultrasonic pulse velocity and strength determined at 28 and 56 days. It was concluded that among the mineral additives used, fly ash and limestone powder significantly increased the workability of SCMs. On the other hand, especially fly ash significantly increased the setting time of the mortars, which can, however, be eliminated through the use of ternary mixtures, such as mixing fly ash with limestone powder. The two polycarboxyl based SPs yield approximately the same workability and the melamine formaldehyde based SP was not as effective as the other two.     

Application of fiber reinforced high performance composites in spun-cast elements

Spin casting is an effective method to produce concrete pylons, masts or pipes. Through the centrifugation process the concrete is compacted and the desired shape, mostly round or ellipsoidal, is obtained. The pre-cast elements made using conventional concrete generally have to be reinforced with steel bars which are susceptible to corrosion. Furthermore, the placement of the steel reinforcement is time consuming and hence expensive and leads to rather thick and heavy structural elements. The application of short fiber reinforced cement (FRC) or mortar, as presented in this paper, is a suitable alternative for such weak-loaded bending elements. Special requirements regarding workability and strength have to be considered. Optimization of cement matrix was achieved with a blend of microfine cement and ordinary Portland cement, improving the rheological properties of the fresh mixture and resulting in a very dense cement matrix with excellent mechanical properties. Reinforcement with different kinds of short fibers of carbon and polyvinylalcohol was studied. Flow properties of the FRC were optimized with regard to the centrifugation process applying a new cone-consistency test. The mechanical properties of conventionally cast specimens and of centrifuged prototypes were investigated.