High-Volume Class F Fly Ash ConcreteHigh-Volume Class F Fly Ash ConcreteWill Haynes CEE 8813HVFAC Lecture Overview• Definition of HVFAC• Why Class F Fly Ash?• The Properties of Fresh HVFAC• The Properties of Hardened HVFAC• HVFAC Project Application• Target Goals• ResultsHigh-Volume Class F Fly Ash ConcreteDefinition of High-Volume Fly Ash Concrete• The most commonly accepted definition is: Concrete mixtures containing more than 50% fly ash by mass of cementitious material with a low water content (w/cm < 0.4).• May find other definitions depending on the source. • The term “High-Volume Fly Ash Concrete (HVFAC)” originated from Dr. Malhotra in the 1980’s when working with CANMET (Canada Centre for Energy and Mineral Technology). (Reiner and Rens, 2006)(Burden, 2006)Why Class F Fly Ash?• Most of the research on high-volume fly ash concrete has been done using Class F fly ash.• There was already widespread usage of Class F fly ash in concrete prior to the research, and Class F fly ash was abundant in the area where the CANMET research was being conducted.High-Volume Class F Fly Ash Concrete65%60%55%Fly Ash Replacement0.30-0.32*0.33-0.35*0.4-0.45w/cm200-225180-200125-150Fly ash, ASTM Class F100-120115-125120-130Cement, ASTM TypeI/II100-120115-125120-130WaterMix Proportions (kg/m^3)60504090 Days to 1 Year40302028 DaysHighModerateLowStrength level (MPa)Table 1: Typical mix proportions for different strength levels(Mehta, 2004)* Moderate and high-strength concretes need a superplasticizer to obtain a low water/cement ratioA Massive 3.5 ft x 129 ft x178 ft UnreinforcedHVFAC Foundation-(65% Fly Ash Replacement)Shree Swaminarayan Mandir and Cultural Complex Lilburn, GeorgiaApplication: Mat Foundation(Garas, Kurtis, Lopez, Mehta, 2005)High-Volume Class F Fly Ash ConcreteI. Properties of Fresh HVFAC• Workability• Air Entraining of HVFAC• Bleeding • Setting TimesWorkability• Fly ash increases workability when compared with conventional concrete with the same water content.• However, HVFAC normally incorporates a very low water to cementitious material ratio (~0.30) to achieve comparable early strengths as conventional portland cement mixtures. Therefore, the use of superplasticizers is common.(Jiang and V.M. Malhotra, 2000)High-Volume Class F Fly Ash ConcreteWorkability (cont’d)• Slump values less than 5” can be achieved high volume fly ash mixtures without the use of a superplasticizer. However, the water to cementitious materials ratio of these mixtures will be around 0.40.(Mehta, 2004)Air Entraining of HVFAC• HVFAC often requires higher doses of air entraining admixtures due to adsorption of the AEA by carbon in the fly ash.(Malhotra, 1994)(Photo: Kosmatka, Kerkhoff, and Panarese, 2002)High-Volume Class F Fly Ash ConcreteBleeding• HVFAC is typically made with a very low water to cementitious materials ratio therefore bleeding is not usually a problem.• Precautions when placing HVFAC in hot weather should be considered to avoid plastic shrinkage cracking.(Mehta, 2004)(Photo: Kosmatka, Kerkhoff, and Panarese, 2002)Setting of HVFAC• The low cement content of HVFAC and the slow reacting property of fly ash increases setting times.• An additional 1 to 2 hours to final set has been shown for HVFAC.• Special measures may be required when using HVFAC in cold weather to avoid significant strength retardation.(Ramachandran, 1996)High-Volume Class F Fly Ash ConcreteII. Properties of Hardened HVFAC• Autogenous Temperature Rise• Drying Shrinkage and Creep• Strength Properties• DurabilityAutogenous Temperature Rise• HVFAC has been proven to be beneficial in reducing the potential for cracking in mass foundations due to temperature differentials.• Replacement of cement with Class F fly ash lowers the peak temperature of concrete during hydration.• Some experiments done using 50% fly ash replacement have been shown to reduce the peak temperature by 23%. 70% replacement has been shown to reduce the peak temperature by 45%.(Atis, 2000)High-Volume Class F Fly Ash Concrete(CII and CANMET, 2005)Fig. 1: Typical autogenous temperature rise of HVFAC compared to that of conventional concrete with similar 28-daycompressive strengthDrying Shrinkage• The water reducing effect of fly ash is beneficial in reducing the amount of drying shrinkage.• There is less portland cement paste volume in HVFAC which also helps to reduce shrinkage.• Studies have shown the drying shrinkage of HVFAC to be equal or less than that of conventional concrete.(Mehta, 2004)(Malhotra and Ramezanianpour, 1994)High-Volume Class F Fly Ash ConcreteCreep• The creep strains of HVFAC can be higher or lower than conventional concrete depending on the age of the concrete when loaded.• The strength gain of HVFAC is slower than conventional concrete, therefore higher strains may be noticed early.• The quality of fly ash can also influence the strength gain and therefore the creep strains.(Bilodeau and Malhotra, 2000)Strength Properties• HVFAC requires lower w/cm ratios to obtain comparable early age compressive strengths as conventional concrete.• Adequate curing of HVFAC is critical to strength development. A minimum of 7 days of moist curing of HVFAC required for optimum strength and durability (for continued pozzolanicreactions).• The early compressive strength is a function of the coarseness of the fly ash used and the amount of cement replaced with fly ash.(Bilodeau and Malhotra, 2000)(Chindaprasirt, 2004)High-Volume Class F Fly Ash ConcreteStrength Properties (cont’d)• Higher values of cement replacement with fly ash will require lower water contents to achieve the same compressive strength.• The long term compressive strength of HVFAC normally exceeds that of conventional concrete. Longer term (56 day) compressive strength requirements are often specified.• The ratios of the flexural and tensile strength to the compressive strength are comparable to conventional concrete.(Langley, Carette and Malhotra)(Malhotra, 1994)High-Volume Class F Fly Ash ConcreteFig. 3: Fly ash mixtures replaced 55% of cement with Class F fly ash(Data From: Langley, Carette, and Malhotra, 1989)Compressive Strength Comparison0102030405060700 102030405060708090100Age (days)Compressive Strength (MPa)HVFAC 1, w/cm=0.30PCC 1, w/cm=0.39HVFAC 2, w/cm=0.33PCC 2, w/cm=0.45HVFAC 3, w/cm=0.35PCC 3, w/cm=0.46Durability• The long term permeability of HVFAC is very low when