Table of Contents
The High-performance concrete (HPC) is a mixture of many materials usually 5-10. The interaction between these materials is the reason of the various workability cases, and also the different ratios and portions added causes this variability. Testing the concrete is not the best solution for examining the workability properties, The testing process of the concrete need huge number of labor and materials, which is not cheap. There is, therefore, a need to predict the workability of concrete through a simpler, cheaper laboratory approach. Concrete workability is de¬ned, according to American Concrete Institute (ACI), as the ease of placement of concrete and is usually quanti¬ed by the result of the slump cone test.
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Rheological tests on cement paste were used to select the type and dosage of mineral admixtures that improved concrete workability. The conclusions reached based on cement paste tests were validated by concrete slump tests. It was also deemed interesting to compare the fundamental parameters (yield stress and viscosity) measured with a ¬‚uid rheometer with the results from two commonly used empirical tests, the minislump and the Marsh cone tests. If a relationship could be established, the empirical tests could be used to design materials for a given yield stress and viscosity or, at the very least, rank different materials base on yield stress or viscosity. The decision to study the in¬‚uence of mineral admixtures was dictated by the recent increase in use of mineral admixtures for improved concrete durability. Economics (lower cement requirement) and environmental considerations have also had a role in the growth of mineral admixture usage. The lower cement requirement also leads to a reduction for carbon dioxide generated by the production of cement, while the use of a mineral admixture utilizes a product that would ordinarily be bound for the land ¬ll. Thus, there is a double environmental bene¬t from using mineral admixtures.
Concrete workability characterization
Concrete workability cannot necessarily be sacri¬ced for improved hardened properties, such as durability or strength. Workability is typically quanti¬ed in the ¬eld by the result of the slump cone tet. Nevertheless, a survey conducted by the National Ready-Mixed Concrete Association (NRMCA) and the National Institute of Standards and Technology (NIST) determined that, for I-IPC, the slump cone value is not representative of the ease of handling HPC in the ¬eld. It was reported that concrete mixtures with the same slump might not behave the same way during placement. This implies that the slump value does not give enough data to fully characterize concrete ¬‚ow.
In the construction ¬eld, terms like workability, ¬‚owability, and cohesion are used, sometimes interchangeably, to describe the behavior of concrete under ¬‚ow. The de¬nitions of these terms are very subjective. Therefore, there is a need for a more ¬rndamental and quantitative description of concrete ¬‚ow. Rheological measurements of concentrated suspensions can be used to describe the ¬‚ow of concrete. Numerous researchers have success¬rlly used the Bingharn equation. Two parameters de¬ne the ¬‚ow: yield stress and plastic viscosity. Yield stress is related to slump, but plastic viscosity is usually ignored because only a few type of instruments exist to measure it. However, the viscosity may be related to properties such as stickiness, p1aceability,pumpability, and ¬nishability. In addition, segregation could be de¬ned as the ability of the aggregate to migrate (or sink) in the cement paste. This phenomenon is linked to the viscosity of the cement paste and the concrete mixture design. Therefore, methods to predict concrete workability need to take into account more than just the yield stress.
What are Admixtures?
Admixture is a material or a combination of different materials that are added to the concrete during mixing to enhance a property or more properties of the concrete mix, and providing it with new features that suits the need either if it is made by central batching plants, pre-stressed concrete factories or in-site mixing. Admixtures may enhance the workability, durability or strength of concrete. They can also be used to overcome extreme circumstances such as cold or hot weather w lower water-cement ratio.
Admixtures are materials other than the main concrete mix components (water, cement, aggregate) which means that the admixture is added to the mixing water before or during the mixing process to give the concrete the needed properties, taking into consideration that there are some admixtures are added after a certain duration after mixing.
Admixtures are divided into 2 types according to origin: chemical admixtures and mineral admixtures.
They are materials that are found as fine particles or liquid and are added to the concrete to enhance it and give precise properties that are unobtainable in plain concrete mixtures. Usually, admixture quantities are not more than 5% of the mass of cement and are put into the concrete at the period of mixing. Public kinds of admixtures are.
Accelerators that increase the speed of the hardening of the concrete. Normal materials utilized are calcium chloride, calcium nitrate and sodium nitrate. However, using chlorides could cause rust in the steel underpinning and it is banned in a some states, so nitrates are recommended.
Retarders sluggish the hardening of the concrete and are utilized in colossal or tough dispenses whereas partial setting before the dispense is finished is undesirable. Normal polyol retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.
Air entrainments makes very small air bubbles inside the concrete during the batching process. It helps in protecting it from the harm made by frequent freezing and defrosting cycles. It also makes better workability, better consistency, less segregation and less bleeding. More than 80% of concrete pavements in the USA are made of air-entrained admixtures. Entrapped air, are huge air voids while entrained air, are very small voids range from 10 to 1000 micrometers. The admixture may be a liquid added during mixing or a powder mixed with the cement.
Plasticizers raise the workability of the fresh concrete permitting it be easily placed, without joining efforts. Plasticizers can be utilized to decrease the content of water of concrete as keeping the workability, that’s why they are called sometimes water-reducers. This action improves its strength and durability properties.
Super plasticizers (Also known as High Range Water Reducers HRWR) They have the ability to reduce the amount of water needed for the concrete by 12-40% of the needed water. They produce concrete with the same workability but with lower water content ratio, they are also used to make highly flowable concrete. HRWR made out of polyether-polycarboxylates have higher efficiency and are used more widely. HRWR are added often in the site because of its low workability period, it increases the time of setting but increases the flowability for a longer period. Concrete that have HRWR in it has higher strength than that predicted by water content ratio, as the water content is not high so the permeability and shrinkage are lowered but the durability is increased.
Corrosion inhibitors: rusting of steel reinforcement is a huge problem that encounters the durability of the reinforced concrete structures, from the causesof rusting of steel are the chlorides, they come from many sources like the deicing salts used to melt ice in winter or from sea water or they may come from the admixtures. From the ways of resisting the rust resulting from chloride is the use of corrosion inhibitors admixtures, they are added to the concrete during mixing to protect the steel by delaying the debut of rusting and also decrease the rate of rusting after it starts. They last for many years after the setting of concrete.
There are inorganic materials having pozzolanic characteristics. These extremely fine-grained materials are added to the concrete mixture to enhance the properties of concrete or as a substitute for the cement, they decrease cost, permeability, enhance the strength and change other concrete characteristics, from the types of mineral admixtures are:
Fly ash: A by-product of electrical coal-fired producing plants and can vary according to source; it is utilized to partly substitute the cement. The particles of fly ash are made of spherical glassy particles, its use has started in the USA in the 1930’s, tests has proven that concrete can be replaced by one third with fly ash. Experiments made on high strength concrete has shown that it’s strength levels were up to 100 MPa at age of one year. Air-entrainment admixtures maybe required because of the carbon inside the fly ash.
Ground granulated blast furnace slag (GGBFS or GGBS): is a product of made when the molten iron is put rapidly in water to decrease it’s temperature, it is crystalline shaped granular product of high adhesion, and so it be work as a substitute for part of the cement, the most favorable ratio is 50% of the cement if there is no salts and 25% if there is.
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Silica fume: A by-product aims to achieve both high strength and low permeability requirements made out of the creation of silicon and ferrosilicon alloys. Silica fume is compared to fly ash. The size of its particle is smaller by 100 times. This makes a higher ratio between surface and volume and a way faster reaction. Silica fume is utilized to raise the concrete strength and durability, but usually superplastisizers are needed for the workability.
High reactivity Metakaolin (HRM): the concrete made using metakaolin has the same strength and durability of the concrete made using silica fume. As silica fume is normally dark gray or black in color, high-reactivity metakaolin is normally brilliant white in color, creating it the favored choice for architectural concrete whereas emergence is important.
Types of Admixtures
1. Air entraining admixtures
They are liquid chemicals that are added to the concrete during mixing to produce small air bubbles that are called entrained air. The air helps the concrete to resist the damage made by thawing, freezing and deicing salt. In hard concrete, the air enhances workability and decreases the bleeding and the segregation.
Air entrainment is not important for the concrete used inside the structures as it is not exposed to either freezing or thawing, It ought to be evaded for concrete flatwork that have a flat troweled finish. In high cement content concretes, entrained will cut strength by 5% for every single 1% of air added; but in low cement content concretes, adding air has less result and could even cause a humble increased strength due to decreased water demand for needed slump.
2. Water reducers
Are utilized for two purposes: (1) to decrease the water content in plastic concrete and rise its strength; (2) to attain higher slump without the need of adding water. Water-reducers will usually cut the needed water content of a concrete combination for a given slump. These admixtures disperse the cement particles in concrete and create extra ef¬cient use of cement. This rises strength or permits the cement content to be decreased as maintaining the alike strength. Water-reducers are utilized to rise slump of concrete lacking adding water and are functional for impelling concrete and in hot meteorological conditions to offset the increased water demand. A little water-reducers could aggravate the rate of slump defeat alongside time.
Mid-range water reducers are nowadays usually utilized and they have a larger skill to cut the water content. These admixtures are accepted as they enhance the finishability of concrete ¬‚atwork.
Are chemicals that postpone the early hardening of concrete by an hour or more. Retarders are regularly used in hot climatic conditions to counter the quick hardening caused by elevated temperatures. For colossal jobs, or in hot climate, specify concrete alongside retarder to permit extra period for pumping and ¬nishing. Most retarders additionally work as reducers.
Reduce the early hardening period of concrete and give higher strength after small period. They do not work as an antifreeze; they decrease the time of the hardening and rate of strength increase, thus the concrete would be stronger to endure damage from cold in chilly weather. Accelerators are additionally utilized in fast trail assembly that needs early removal of forms.
Accelerating admixtures consists of two types: chloride and non-chloride based. One of the extra competent and frugal is calcium chloride that is obtainable in fluid or ¬‚ake form. For non-reinforced concrete, The CaCl can be utilized to an amount of 2% by the heaviness of the cement due to worries of corrosion of reinforcement instigated by Cl.
Pre-stressed concrete and concrete that have aluminum or invigorated metal inside it ought not to have chloride-based materials because of the increased possibility for the rusting of the metal. Non-chloride established accelerators are utilized whereas there is fear of rusting of embedded metals or concrete reinforcement.
5. HIGH RANGE WATER-REDUCERS (HRWR)
It is a distinct type of water-reducer. Also known as, super plasticizers, HRWRs decrease the water content of a concrete mix by 12% to 25%. HRWRs are consequently utilized to raise the strength and decrease permeability of concrete by decreasing the water content in the mix; or considerably raise the slump to make “¬‚owing” concrete lacking the addition of any extra water. These admixtures are vital for elevated strength and enhanced concrete mixes that have higher contents of adhesive materials and mixes having silica fume.
Beside these kinds of admixtures, there are other materials obtainable for improving concrete characteristics for different types of applications. These products include rust resistors; admixtures that decrease shrinkage, anti-washout admixtures, and admixtures to decrease possibility for alkali aggregate reactivity, admixtures that helps pumping and a collection of colors and products that improves the concrete aesthetics.
Fly ash is a bi-product resulted from the finely divided residues generated from the combustion of coal electrically in thermal power plants; the dust collection system removes the fly ash, as a fine particulate residue, from the combustion gases before they are discharged into the atmosphere. The particles are finer than cement usually and consist off spherical glassy particles.
The fly ash reacts with the calcium hydroxide chemically during the hydration process forming calcium aluminate and calcium silicate hydrate which decreases the risk of infiltration of calcium hydroxide and the permeability of concrete. The fly ash enhances the permeability of concrete also by decreasing the ratio of water to cement, which decreases the volume of pores. Its spherical shape also enhances the consolidation of concrete which reduces permeability.
Fly ash has a spherical shape that allows it to create a ball bearing effect in the mix which improves the workability without increasing the water and also helps in increasing the pump-ability by decreasing the friction between the concrete and the pump line. It also increases the concrete pump-ability by increasing its cohesion and making it less subject to segregation. It also increases the gaining of strength of mixes for periods much longer than that of Portland cement only.
The most important use of fly ash is that it increases the life cycle expectancy and durability. It also increases the resistance of corrosion of concrete reinforcement.
Fly ash is used in concrete by replacing a portion of the Portland cement that may goes up to 30% of the total cement mix.
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