Guru Vittal, U K

Kumar, Sanjay

English

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Fly Ash Utilisation for Value Added ProductsEds. B. Cliatrerjee, K. K. Singh & N. G. Goswamia 1999, NMi„ Jamshedpur; pp. 172-175Use of fly ash admixed concrete for pavementconstruction*U.K. GURU VITTAL and SATANDER KUMARScientists, Central Road Research Institute, New Delhi-110 0201.0 INTRODUCTIONFly ash is the inorganic portion of the source-coal obtained as waste material fromthermal power stations . It is usually found in the form of spherical, often hollow spheresof silicon, aluminium and iron oxides and unoxidised carbon. Coal fired power plantsproduce millions of tonnes of fly ash annually , but only a fraction is productivelyemployed. The balance must be disposed in ash ponds at great expense. On the otherhand R&D work carried out in India and abroad has established that fly ash can beconverted to meaningful wealth as a construction material, especially as an additive tocement concrete.Fly ash improves the properties of both fresh and hardened concrete. In green state, flyash added to cement concrete results in reduction of heat of hydration and increases theworkability. It improve the strength of hardened concrete at later stages. Fly ash makesefficient use of products of hydration of portland cement. Besides, the economy achievedby addition of fly ash, most important gain, would be increased in durability of con-crete.2.0 DURABILITY OF FLY ASH ADMIXED CONCRETEConcrete in service is exposed to a range of external and internal stresses, environmen-tal and material related, which can lead to a reduction in service life or premature failureof the concrete. Some of the environmental stresses include aggressive chemicals in,groundwater or soils, cycles of freezing and thawing and exposure to de-icing chemi-cals. Material related stresses can be a result of alkali-silica or alkali-carbonate reac-tions, excessive shrinkage and related micro-cracking, thermal stresses from excessiveheat of hydration, high permeability, etc..The traditional method of improving concrete durability by raising the portland cementcontent is not sufficient to protect concrete in service from many of these destructiveforces. Present day cements are typically more finely ground and contain higher C5Scontent to achieve higher early strength. Because of these characteristics, more heat isgenerated rapidly during cement hydration, and more water soluble calcium hydroxideis created. Increased cement content contributes to increased drying shrinkage and ther-mal stress, and provides additional alkalies to support alkali aggregate reactions. Properuse of mineral admixtures such as fly ash will improve concrete properties related todurability.'Full text of paper172U.K. GURU VITTAL and SATANDER KUMAR2.1 Permeability ReductionTo achieve high durability, concrete requires a high degree of impermeability. Highlyimpermeable concrete will resist the intrusion of external chemical attack ( sulphatesand chlorides) as well as water penetration. Moisture is also required to activate alkali-aggregate reactions, and to initiate corrosion of reinforcing steel. A decrease in concretepermeability improves durability.Use of fly ash creates a discontinuous pore structure and clogs channels with theadditional hydration products. The result of these reactions will be slower transport ratesfor both water and chlorides through the cementitious paste. The consumption of water-soluble calcium hydroxide (produced as a by-product of hydration of portland cement)by the pozzolanic reaction creates additional calcium silicate gel, which is much denserthan the gel without the pozzolana. The modification of pore structure and permeabilityreducing effect is more pronounced in case of Class F fly ash (produced by burningAnthracitic coal) than with Class F fly ash ( produced by burning Lignite coal).2.2 Sulphate ResistanceUse of an effective pozzolana will reduce the potential for expansion due to sulphates.The mechanism is three fold: the pozzolanic reaction reduces the calcium hydroxidecontent of the concrete, the use of Class F fly ash reduces the absolute amount ofcalcium aluminate available to support ettringite formation by reducing the cementcontent, and permeability of concrete is reduced, thus diminishing the sulphatepenetration.Research has shown that Class F fly ash is more effective in reducing sulphate attack.Class F fly ash used at a relatively high percentage of the total cementitious content willprovide a high degree of protection, when combined with the appropriate type of ce-ment. The effect is even more pronounced in lean concretes, where additional 'fly ashproduces dramatic permeability reductions.2.3 Alkali-Silica ReactionFly ashes when compared with low alkali cements, typically contain high levels ofsilica- This silica, present as highly reactive silicates, is responsible for the pozzolanicproperties of fly ash- Alkalies ( sodium and potassium hydroxide) form during thehydration of portland cement, and co-exist with calcium hydroxide in the gel/pore waterstructure. The fly ash pozzolanic reaction also combines some of the alkali metals intothe calcium silicate matrix, rendering them unavailable to support alkali-silica reaction.Studies have shown that alkali-silica reaction is substantially reduced in concretes con-taining very high percentages of fly ash. Where alkali-silica reaction is a concern,cement contents (the major source of alkalies) should be kept as low as possible. Ingeneral, a higher fly ash content means a lower potential for alkali silica reaction.3.0 PAVEMENT CONSTRUCTION USING FLY ASH ADMIXED CONCRETEUse of cement concrete for road pavement construction is increasing due to its several173U.K. GURU VITTAL and SATANDER KUMARbenefits like improved serviceability, reduced maintenance cost, ability to withstandtraffic loading, etc. Fly ash admixed concrete can be used for constructing rigid pave-ments in many ways. They include Dry lean fly ash concrete, Roller compacted cementconcrete, Fly ash cement concrete pavements, Fly ash admixed concrete paving blocks,etc.3.1 Dry Lean Fly ash Concrete (Lean Cement Fly ash Concrete)The beneficial effects of addition of fly ash like reduced bleeding, segregation andimproved plasticity, cohesiveness in wet concrete are more pronounced in the case oflean cement concrete, permitting its easier placement and finishing. Cement content islow in Dry lean Fly ash concrete, and hence replacement of sand upto 50 percent isusually adopted. R&D work at CRRI has shown that 100-175 percent increase in thecompressive strength of lean cement fly ash concrete (in which 50 percent of sand hasbeen replaced by Fly ash) over plain mixes of equivalent proportions. Typical mixproportions of lean cement fly ash concrete that have been found to be satisfactory arel cement : 3.5 sand : 3.5 Fly ash: 14 Coarse aggregate (by weight).3.2 Cement Fly ash Concrete PavementIn concrete roads and runways, a part of cement and sand can be replaced by goodquality fly ash to the extent of 10-30 percent and 5-15 percent respectively. This wouldresult in lowering cost of resultant concrete and saving in cement without any loss ofstrength. The design and construction techniques of cement-fly ash concrete pavementare identical with that of conventional cement concrete pavement.3.3 Roller Compacted ConcreteIn roller compacted concrete paving technology, 30-50 percent of cement can be re-placed using fly ash. Since this type of paving requires zero slump concrete, betterworkability can be achieved by using fly ash keeping the same water/ cement ratio.Ordinary static road rollers can be used for compaction of roller compacted concrete.4.0 CONSTRUCTION OF DEMONSTRATION ROAD STRETCHESTo demonstrate construction of road stretches using fly ash admixed Dry lean concreteand roller compacted concrete, 200 m stretch was selected in BC soil area near athermal power station. The road stretch was in MDR and traffic density was low. Drylean fly ash concrete was used for base course and roller compacted concrete for wear-ing course. The cross section of the pavement adopted is shown in fig. 1.Dry lean fly ash concrete (1:3.5:3.5:14) was used for base course. During laboratoryinvestigations, the 28 days compressive strength of dry lean concrete was found to be75 kg/sq.cm. The 28 days compressive strength of roller compacted concrete (1:1.5:3)with 40 percent replacement of cement by fly ash was found to be 270 kg/sq. cm.However taking into consideration , quality control limitations during field construction,the percentage of fly ash admixture was limited to 30 percent. The stretch has beenconstructed successfully and its performance has been found to be very good after oneyear. Further monitoring of its performance is proposed.174U.K. GURU VITTAL and SATANDER KUMARRoller CompactedConcreteDry Lean Fly ashConcreteMoorum + SandSubgradeFig. I : Pavement Cross Section5.0 CONCLUSIONS100 mm100 mm100 mmThe technical benefits of Fly ash use and the mechanism of Fly ash cement reaction arewell known. Adequate amount of pozzolana like fly ash impart properties to concretethat can not be achieved by increasing cement content alone. The goal of producing adurable concrete for longer design life and expected level of service invariably needsfly ash. At a time, when many cement concrete road works are being planned, judicioususe of Fly ash as an admixture will go a long way in helping us construct better roads.ACKNOWLEDGEMENTSThe authors express their sincere thanks to Director, Central Road Research Institutefor his kind permission to publish the paper.BIBLIOGRAPHY1. Ellis W.E. For Durable Concrete Fly ash does not "Replace" cement', Concrete International,July 1992.2_ Rehsi, 'studies of Indian Fly ashes and their use in Structural Concrete', Proceedings of thethird International Seminar on Utilisation of Fly ash, Pittsburgh, 1973.3. Gordon G. J., Ellis W. E., Shelton D., 'ECO Cement', Emerging Technologies Symposiumon Cement and Concrete in the Global Environment, March 1993.4. Murty AV SR, Guru Vittal U. K. & Swami R.K. , 'Use of Coal ash in Road Construction',International Conference on Fly ash Disposal & Utilisation, New Delhi, 1998.175

Use of fly ash admixed concrete for pavement

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