High-Volume Class F Fly Ash Concrete Will Haynes CEE 8813 HVFAC 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 Results
Definition 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). (Reiner and Rens, 2006) 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). (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.
Table 1: Typical mix proportions for different strength levels Strength level (MPa) 28 Days 90 Days to 1 Year Mix Proportions (kg/m^3) Low 20 40 Moderate 30 50 High 40 60 Water Cement, ASTM TypeI/II Fly ash, ASTM Class F 120-130 120-130 125-150 115-125 115-125 180-200 100-120 100-120 200-225 w/cm Fly Ash Replacement 0.4-0.45 55% 0.33-0.35* 60% 0.30-0.32* 65% * Moderate and high-strength concretes need a superplasticizer to obtain a low water/cement ratio (Mehta, 2004) Application: Mat Foundation A Massive 3.5 ft x 129 ft x178 ft Unreinforced HVFAC Foundation-(65% Fly Ash Replacement) Shree Swaminarayan Mandir and Cultural Complex Lilburn, Georgia (Garas, Kurtis, Lopez, Mehta, 2005)
I. Properties of Fresh HVFAC Workability Air Entraining of HVFAC Bleeding Setting Times Workability 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)
Workability (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)
Bleeding 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)
II. Properties of Hardened HVFAC Autogenous Temperature Rise Drying Shrinkage and Creep Strength Properties Durability Autogenous 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)
Fig. 1: Typical autogenous temperature rise of HVFAC compared to that of conventional concrete with similar 28-day compressive strength (CII and CANMET, 2005) Drying 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. (Mehta, 2004) Studies have shown the drying shrinkage of HVFAC to be equal or less than that of conventional concrete. (Malhotra and Ramezanianpour, 1994)
Creep 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 pozzolanic reactions). (Bilodeau and Malhotra, 2000) The early compressive strength is a function of the coarseness of the fly ash used and the amount of cement replaced with fly ash. (Chindaprasirt, 2004)
Strength 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)
Compressive Strength Comparison Compressive Strength (MPa) 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 Age (days) HVFAC 1, w/cm=0.30 PCC 1, w/cm=0.39 HVFAC 2, w/cm=0.33 PCC 2, w/cm=0.45 HVFAC 3, w/cm=0.35 PCC 3, w/cm=0.46 Fig. 3: Fly ash mixtures replaced 55% of cement with Class F fly ash (Data From: Langley, Carette, and Malhotra, 1989) Durability The long term permeability of HVFAC is very low when the concrete has been adequately cured (at least 7 days). Using the Rapid Chloride Permeability Test, typical ranges for HVFAC are 500 to 2000 coulombs at 28 days, and from 200 to 700 coulombs at 91 days. (Bilodeau and Malhotra, 2000) ASTM C1202-Rapid Chloride Permeability Test (Figure: CII and CANMET, 2005)
Durability (cont d) The freezing and thawing resistance of HVFAC is adequate as long sufficient air voids are incorporated. De-icing salt scaling has been shown to be a problem for HVFAC in the lab. HVFAC is not recommended for applications where there will exposure to de-icing salts. The causes of severe deicing scaling of HVFAC has not been determined, further research is needed. (Bilodeau and Malhotra 2000) (Photo: Kosmatka, Kerkhoff, and Panarese, 2002) Durability (cont d) (ACI, 2005) ACI 318 allows a maximum of 25% fly ash replacement when exposed to deicing chemicals. Therefore, HVFAC (replacement>50%) is not allowed for de-icing chemical exposure.
Durability (cont d) Carbonation of HVFAC can be an issue. There is a reduction in the calcium hydroxide content in the concrete due to the inclusion of fly ash. When low w/cm ratios are used with an extended period of curing, carbonation of HVFAC can be controlled through further reduction in permeability. (Malhotra and Mehta, 1996) Mat Foundation in Lilburn, Georgia 3.5 ft x 129 ft x 178 ft unreinforced mat foundation. The project replaced 65% of the Type I cement with Class F fly ash. Water to cementitious materials ratio was 0.35. Target strength for the foundation was 31MPa at 56 days. Target maximum hydration temperature was 60 o C with a maximum differential of 20 o C between the surface and interior of the foundation.
Laboratory Specimens 60 9000 8000 50 7000 Strength (MPa) 40 30 56 Days 6000 5000 4000 Strength (psi) 20 4x8-in Specimens 3000 10 4x8-in Cores 6x12-in Specimens 2000 1000 0 0 1 10 100 1000 Age (days) Temple Foundation Plan (Numbers Represent Coring Locations)
Foundation Compressive Strengths 50 Strength (MPa) 40 30 20 7000 6000 5000 4000 3000 Strength (psi) Location 1 (Top) Location 2 (Mid) Location 3 (Mid) Location 4 Location 5 Location 6 (Top) Location 7 Location 8 Location 9 Location 10 (Bottom) 10 56 Days 2000 1000 0 1 10 100 Age (days) 0 Laboratory Experiment
Laboratory Results Tem perature ( o C) 50 45 40 35 30 25 20 15 10 5 0 Surface Middle Center Ambient 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Age (days) 122 112 102 92 82 72 62 52 42 32 Tem perature ( o F) Heat of Hydration Graph vs Time Foundation Results Temperature (oc) 50 Surface 45 Middle 114 104 Center 35 94 Ambient 30 84 DT max=5 C 25 74 40 20 64 15 54 10 5 44 0 34-5 0 2 4 6 8 10 12 14 16 18 20 22 24-10 14 Age (days) Temperature (of) Heat of Hydration Graph vs Time
Laboratory Results Charge Passed (coulombs) 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 28 days: 3.75-in diameter 31 days: 3.75-in diameter 90 days: 3.75-in diameter High Moderate Low Very Low 1 2 3 4 Specimen number ASTM C1202-Rapid Chloride Permeability Test Project Results The HVFAC used for the foundation did not exhibit any thermal induced cracking. The foundation compressive strengths were very close to the targeted compressive strengths. The drying shrinkage was found to be comparable to conventional concrete. The Rapid Chloride Permeability Test showed low to very low permeability for the specimens at 90 days.
Conclusion HVFAC has proven to be effective in controlling thermal gradients in mass concrete applications, and sufficient research exists for HVFAC to be applied judiciously in other structural applications as well. A lower w/cm ratio is normally used in HVFAC in order to get comparable early age strengths as conventional concrete and for high durability. Adequate curing is required (minimum 7 days) and use well graded aggregate. The quality control of the fly ash used is essential. Coarse fly ashes can have a high impact on strength gain. Do not use HVFAC when there is the potential for deicing salt exposure.