Flyash Concrete Mix Design
Study of Mechanical, Durability and Thermal
Dr. S.C. Maiti, Ex-Joint Director, National Council for Cement and Building
Materials, New Delhi & Raj K. Agarwal, Managing Director, Marketing,
Transit (India) Pvt. Ltd., New Delhi.
Flyash, a by-product from thermal power stations, is a pozzolana and is being used in concrete construction. The chemical admixture superplasticizer is also being used in concrete along with ordinary Portland cement (OPC) and flyash. But the quality of flyash should conform to IS 3812 (Part 1)1. The following typical properties of flyash must be satisfied:
Fineness (Blaine’s): Minimum 320 m2/kg.,
Lime Reactivity: Maximum 4.5 N/ mm2,
Loss on ignition: Maximum 5.0%
Fortunately, these days, the quality of cement and the quality of superplasticizer is good, and therefore, concrete construction has also improved. But a proper mix proportioning method had to be followed, so that the site engineers can adapt it easily, in order to obtain the desired workability and compressive strength of concrete. In this paper, an attempt has been made to develop a concrete mix design procedure for normal grade of concrete, using flyash and superplasticizer.
(OPC+flyash) or Portland pozzolana cement (PPC) satisfying the requirements of Indian Standards can be used. The superplasticizer commonly being used for normal grades of concrete say up to M60 grade, is napthalene-based. Coarse aggregates are generally crushed rock aggregates, and the fine aggregate is generally riverbed sand of grading zones I, II, III or IV as per Table 4 of IS 3832. IS 383 gives a cautionary note that “the fine aggregate conforming to grading zone IV should not be used in reinforced concrete unless tests have been made to ascertain the suitability of proposed mix proportions”. Sometimes, riverbed sand is not available, and in that case, crushed stone fine aggregate can be used in concrete, and IS 383 specifies maximum fines content passing 150-micron IS sieve is 20%. For smooth gravel or partly crushed gravel coarse aggregate, the mixing water requirement of concrete will be less than that for concrete using the crushed rock aggregate. IS 102623 stipulates reduction of mixing water by 25kg/ m3 of concrete for rounded gravel aggregate, and by 10kg/ m3 of concrete for sub-angular coarse aggregates.
Available Flyash Concrete Mix Design Procedures
1. Stipulation in the Indian Standard
There are no flyash concrete mix design procedures in the Indian Standard IS 10262. The guidelines however, refer to the standard IS 3812 (Part 1) for flyash, and an illustrative example of mix proportioning for M40 grade concrete using 30% flyash and 2% superplasticizer has been outlined. The illustrative example suggests the following:
a) “Decide the percentage of flyash to be used based on project requirement and quality of materials,
b) In certain situations, increase in cementitious material content may be warranted. The decision to increase in cementitious material content and its percentage may be based on experience and trial”.
The illustrative example considers a 10% increase in cementitious material content than the cement content of corresponding OPC concrete.
2. The British Flyash Concrete Mix Design Method
The British flyash concrete mix design method considers improved workability because of flyash and therefore, a reduction in the water content of the concrete mix. Typical water-reduction values for a workability range of 30-60mm slump suggested are: 10kg/ m3 of concrete for the use of 20% flyash, 20kg/ m3 of concrete for the use of 30% flyash etc. To offset the reduction in early strength, the mass of the cementitious material (compared to the mass of cement in the OPC concrete mix) has been increased. The Department of Environment4 of England States that “flyash is used to replace some of the cement in the mix, but in order to obtain concrete having the same strength at 28 days, the combined weight of the cement plus flyash needs to be greater than that of OPC in a OPC concrete mix. BS 5328 Part15 specifies the increase in the combined mass of OPC plus flyash by about 10% by mass. The British method stipulates the quality of flyash with its loss on ignition restricted to 7% and the residue on the 45 micron sieve restricted to 12.5%. The British method further considers Smith’s6 ‘Cementing efficiency factor (k),’ where KF is the mass of OPC equivalent to a mass F of flyash, the value of K considered as 0.30. To design a flyash concrete mix with a specified 28-day compressive strength, the w (c+0.3F) ratio shall be used as free w/c ratio in the relationship between w/c ratio Vs compressive strength of concrete. The British method provides relationships between water-cement ratio and compressive strength of concrete, charts for finding the wet density of concrete for crushed and uncrushed aggregates, proportion of fine aggregate depending on maximum size of aggregate (MSA), grading of fine aggregate and the required workability of concrete in terms of slump or V-B time.
3. The ACI High-Strength Flyash Concrete Mix Design Method
The American high-strength flyash concrete mix proportioning method7 as reported by ACI Committee 211 provides guidelines for selecting proportions for high-strength flyash concrete mixes using high-range water-reducing admixtures, the 28-day cylinder compressive strength of such concrete being 42-84 N/ mm2. The method recommends 15-25% flyash replacement for class F flyash, and 20-35% replacement for class C flyash (the flyash produced from lignite coal and containing lime more than 10%). The preferred flyash for use in high-strength concrete should have ‘loss on ignition’ value not greater than 3%, and should have high fineness. The suggested MSA is 20-25mm for concrete compressive strength less than 63 MPa and 10-12mm for concrete compressive strength more than 63 MPa. The fineness modulus of sand suggested is 2.5-3.2. The American method is a little complicated, as it considers the volume of oven-dry rodded aggregates, calculates void content in fine aggregate, and requires trial mixture to be conducted for the basic concrete mixture (without any flyash), for obtaining 25-50mm slump of concrete, before adding any superplasticizer. They state that trial is needed to ensure an adequate amount of watet for mixing, and the superplasticizer to be effective. The American method provides specific tables for obtaining (a) fractional volume of even-dry rodded coarse aggregate for different MSA, and for sand fineness modulus of 2.5 to 3.2, and (b) mixing water content for the desired slump of concrete for different MSA. The table includes entrapped air content to be considered for different MSA in concrete. Also the method provides maximum water / (cement + flyash) i.e W/(C+F) ratios for concrete with and without any high-range water-reducing admixtures, for different MSA and different field strength of concrete to be achieved at 28 and 56 days.
Comments on the Mix Design Methods
The Indian Standard IS 10262 does not provide much detail on the flyash concrete mix design procedure. The illustrative example on M40 grade concrete using 30% flyash and 2% superplasticizer considers increase in cementitious material content of 10% than the cement content of corresponding OPC concrete. The British mix design procedure considers improved workability of concrete because of the inclusion of flyash in concrete and hence provides reduced water content of concrete for inclusion of different percentage of flyash in concrete. This may not be true for all flyashes. Also, in our cement factories, coarse flyash is ground and blended with OPC and in this process, the requirement of water content of concrete mix using PPC becomes more than that of concrete with (OPC + good quality flyash). The British method considers flyash of much finer variety, i.e. particles retained on 45 micron sieve restricted to 12.5%, whereas in IS 3812 (Part 1), it has been restricted to 34%. With 12.5% residue on 45 micron sieve, the Blaine’s fineness of flyash will be very high. The ACI high-strength concrete mix design method also considers very good quality of flyash, where the ‘loss on ignition’ value has been restricted to 3%, and the flyash is of high-fineness. The method considers the cylinder compressive strength of concrete, and calculates void content in aggregates based on oven-dry rodded unit weight of aggregates. As such, the British and American methods of flyash concrete mix design are based on specific tables and charts, which cannot be followed by us, for the quality of Indian concrete-making materials.
The proposed flyash concrete mix proportioning procedure, using a Superplasticizer.
Estimation of W/(C+F) Ratio The target 28-day compressive strength of concrete (fck/) has to be calculated first for the grade of concrete, from the following formula:
fck/ = fck + 1.65 x S.D., where fck = characteristic compressive strength of concrete at 28 days in N/ mm2, and S.D. = Standard deviation in N/ mm2. It is better to use the actual standard deviation value obtained from the sites of construction. For a ‘good’ control, the standard deviation value of 4 N/ mm2 can be assumed irrespective of the grade of concrete4. Using an established correlation between W/ (C+F) ratio and the 28-day compressive strength of concrete, the W/(C+F) ratio is to be estimated for the target strength. Typical correlation is shown in fig. 1.
Estmation of Water Content of Concrete
The water requirement of the concrete mix is to be fixed, for the desired workability of concrete. Generally, a naphthalenebased superplasticizer is being used, either to reduce the water content or to increase the workability of concrete. About 0.80 to 1.2% by weight of (C+F) is being used. For a low slump concrete, say 25-30mm slump, about 20% mixing water can be reduced using this type of superplasticizer. For increasing the slump from 50-60mm to about 100-120mm, this much percentage of superplasticizer, i.e. 0.8-1.2% is required to be added to a concrete mix having water content of say 160- 180kg/ m3 of concrete. The water requirement of concrete mix is primarily dependent on the MSA used in the concrete mix. The mixing water content of concrete is also dependent on the quality of flyash and its quantity in concrete. Typically, using a good quality flyash (the Blaine’s fineness of about 320 m2/kg and a lime reactivity value of about 4.5 N/ mm2), using 20mm MSA in concrete, the mixing water requirement is about 170kg/ m3 of concrete, for a normal 50-60mm slump of concrete. For 40mm MSA in concrete, the water requirement will be less, say about 150kg/ m3 of concrete. This water content of concrete is for crushed rock saturated surface dry (S.S.D.) aggregates. For gravel aggregates or partly crushed gravel aggregates, the mixing water requirement will be lower, typically 160kg/ m3 and 140kg/ m3 of concrete for 20mm MSA and 40mm MSA is concrete, respectively. The water requirement of concrete for a desired workability of concrete will also depend on the efficiency of the superplasticizer used.
The Quantity of Flyash in Concrete
The percentage of flyash in a concrete mix should be based on project requirements. For example, for hydroelectric projects, IS 4568 stipulates at least 25% flyash in concrete, in order to combat the probable alkali-silica reaction in concrete. For mass concrete construction, even 40% flyash has been used in the raft foundation concrete of M50 grade, for the highest tower of the world, Burj Khalifa9. The quantity of flyash in concrete also will depend on the quality of OPC and the quality of flyash. With OPC 53-grade, more quantity of flyash (say 35 or 40%) can be used in concrete. If the quality of flyash is very good, say its fineness is more than 400 m2/kg and the lime reactivity value is more than 5N/ mm2, with OPC 43-grade, 35% flyash can safely be used.
The Air Content of Concrete
The air content of concrete shall be considered as 2% in concrete with 20mm MSA, 1% in concrete with 40mm MSA and 3% in concrete with 10mm MSA. These air contents of concrete are for non-air-entrained concrete, and is always entrapped in fully compacted concrete.
Estimation of Coarse and Fine Aggregate Content in Concrete.
With the quantities of cement, flyash, water, and superplasticizer estimated, and the air content assumed, the total absolute volume occupied by these materials in a cubic metre of concrete is calculated. The remaining volume in a cubic metre of concrete will be occupied by the coarse and fine aggregates. The volume of coarse aggregate will depend on MSA, grading of fine aggregate and the workability of concrete. For a normal workability of say 50-60mm slump, for 20mm MSA (crushed rock) and for fine aggregate of medium grading (say of zone II or zone III as per Table 4 of IS: 383), the quantity of coarse aggregate will be about 62-64%), of total aggregate by absolute volume. If sand is of coarse variety (say of grading Zone I as per Table 4 of IS: 383), the volume of coarse aggregate will be around 60% of total aggregate by absolute volume. For 40mm maximum size of crushed rock coarse aggregate, the above mentioned volume of coarse aggregate will be higher by 0.09 m3 approximately, for different grading zones of fine aggregate. For a high workability concrete mix of say 100-120 mm slump, the volume of crushed rock coarse aggregate will be 10% less than the values mentioned above for 20mm and 40mm MSA in concrete.
Accelerated Strength Testing of Concrete
The boiling water method of accelerated curing can be employed to estimate, the 28-day compressive strength of concrete in 28½ hours. The method is quite accurate and explained in IS: 901310. Using this method 9 concrete cubes from the three concrete mixes (with a variation of +10% in W/ (C+F) ratio cast, can be kept under boiling water for 3½ hours, after the concrete cubes (in the mould) are normal-cured for 23 hours. The concrete moulds need to be covered by cover plates and screwed, before placing them in boiling water. After removing from boiling water, the concrete cubes are cooled for 2 hours, demoulded and tested. The derived equation from the latest data of the Central Road Research Institute in IRC:8511 for PPC (with 25-35% flyash) is :
R28 = 1.85 Ra N/ mm2
where, R28 = 28-day compressive strength of normal cured concrete, N/ mm2 and
Ra = Accelerated compressive strength of concrete, N/ mm2 The W/(C+F) ratios Vs estimated average 28-day compressive strength of concrete for the 3 mixes are next plotted, and a correlation can be obtained, from which the W/ (C+F) ratio required for the target 28-day compressive strength of concrete can be estimated. The concrete mix proportions for this W/(C+F) ratio are then calculated, and can be recommended for the field trial.
Typical worked out example on M25 grade of flyash concrete of workability 100-120mm slump, for reinforced concrete construction.
Degree of Quality Control : Good
Exposure condition: Moderate (Ref. IS: 456). Quantity of flyash to be used: 25% by weight of (Cement + Flyash).
Test Data on Materials:
Cement: OPC, 43 grade
Maximum Size of coarse aggregate (crushed rock): 20mm Grading of fine aggregate: Zone II (as per Table 4 of IS:383) Combined grading of coarse aggregate (Two fractions: 10- 20 mm size & 10-4.75mm size combined in the proportions 60:40, satisfying the combined grading requirement of Table2 of IS: 383. sp.gr of coarse aggregate: 2.74, water absorption: 0.5%. sp.gr of fine aggregate: 2.62, water absorption: 1.0%
Condition of coarse aggregate: Dry
Condition of fine aggregate: wet (total moisture : 6%) Quality of flyash: Good (Properties : Blaine’s Fineness = 325 m2/kg, lime reactivity = 4.6 N/ mm2), satisfying the requirement of IS: 3812 (Part 1), sp.gr=2.20.
Superplasticizer (Naphthalene- based), sp.gr.=1.1
Steps of Mix Design
(i) Target 28-day compressive strength of concrete = 25 + 1.64 x 4 = 31.6 N/ mm2
(ii) W/C+F ratio selected from Fig. 1 = 0.44
(iii) With the superplasticizer and flyash in hand, by trial, it was observed that with 1% superplasticizer [by weight of (C+F)], with 25% flyash, the desired workability of concrete of 100mm slump was achieved with 170l of water per cubic metre of concrete.
(iv) The (cement + flyash) content of concrete = 170/0.44 = 386 kg/ m3 of concrete OPC = 290 kg, Flyash (25%) = 96 kg. The (C+F) content and W/(C+F) ratio satisfied the requirements of minimum cementitious material content (300 kg) and maximum W/cementitious materials ratio (of 0.50) as per Table 1, i.e. durability requirement for reinforced concrete under ‘moderate’, exposure condition.
(v) Superplasticizer [1% of (C+F)] = 3.86 kg.
(vi) Volume of coarse aggregate (20mm MSA), using a fine aggregate of grading zone II for high workability concrete reducing by 10% =0.62 m3 x 0.90 = 0.56 m3 / m3 of total aggregate by absolute volume.
(vii) Volume of fine aggregate = 1-0.56 = 0.44 m3 / m3 of total aggregate by absolute volume.
(viii) Absolute volume of materials + (air content) except that of aggregates :
a) Absolute volume of OPC = 290/3150 = 0.0920 m3 (assuming sp.gr of OPC = 3.15)
b) Absolute volume of flyash (sp.gr=2.2) = 96/2200 = 0.0436 m3
c) Absolute volume of water = 170/1000 = 0.1700 m3
d) Absolute volume of superplasticizer = 3.86/1100 = 0.0035 m3
e) Air content of concrete with 20mm MSA (2%)=0.0200 m3 Total = 0.329 m3
(ix) Absolute volume of (Coarse + fine aggregate) = 1- 0.329 = 0.671 m3
(x) Quantity of Coarse aggregate (S.S.D.) (with sp.gr = 2.74) = 0.671 x 0.56 x 2740 = 1030 Kg.
(xi) Quantity of Fine aggregate (S.D.D.) (with sp.gr = 2.62) = 0.67 x 0.44 x 2620 = 772 Kg.
(xii)Quantity of Coarse aggregate (dry) (capable of absorbing 0.5% moisture) = 1030 /1.005 = 1025 Kg.
(xiii) Quantity of Fine aggregate (wet) with 6% total moisture = quantity of dry fine aggregate (which can absorb 1% moisture) x 1.06 = 772/1.01 x 1.06 = 810 Kg The coarse aggregate is dry,and the fine aggregate is wet. Therefore, (1030 - 1025) ie 5 kg water is to be added for the absorption of coarse aggregate, and (810-772) i.e. 38 kg. of surface water of fine aggregate is to be reduced from the mixing water.
(xiv) Therefore, finally, mixing water content = 170+5-38=137 kg. Quantities of all ingredients/ m3 of concrete :
OPC (43 grade) = 290 kg. Flyash (25%) = 96 kg. Superplasticizer (1%) = 3.86 kg. The quantity of coarse aggregate (dry) = 1025 Kg.
10-20mm size (60%) = 615 Kg.
10-4.5mm size (40%) = 410 Kg.
Fine aggregate (wet) = 810 Kg.
Mixing water = 137 Kg.
1. The maximum cement content or cementitious material content is to be reduced by 30 kg/ m3 for 40mm MSA, and is to be increased by 40kg/ m3 for 10mm MSA.
2. The different exposure conditions are explained in IS 456. References
1. IS 3812 (Part 1) Indian Standard Specification for pulverized fuel ash, for use as pozzolana in cement, cement mortar and concrete. Bureau of Indian Standards, New Delhi.
2. IS 383 Indian Standard specification for coarse and fine aggregates from natural sources for concrete. Bureau of Indian Standards, New Delhi.
3. IS 10262 Indian Standard guidelines for concrete mix proportioning. Bureau of Indian Standards, New Delhi.
4. Teychenne, D.C., Nicholls, J.C., Franklin, R.E. and Hobbs, D.W. Design of normal concrete mixes, The Department of Environment, London.
5. BS 5328: Part 1. Guide to specifying concrete. British Standards Institution, London.
6. Smith, I.A. The design of flyash concrete. Journal of Institution of civil Engineers, Vol. 36, 1967, London.
7. ACI 211.4 R Guide for selecting proportions for highstrength concrete with Portland cement and flyash. American Concrete Institute.
8. IS: 456, Code of practice for plain and reinforced concrete. Bureau of Indian Standards, New Delhi.
9. Baker, W.F. Burj Khalifa : A new paradigm. The Indian Concrete Journal, Vol. 85, No. 7, July 2011, pp 8-13.
10. IS 9013 Method of making, curing and determining compressive strength of accelerated cured concrete test specimens, Bureau of Indian Standards, New Delhi.
11. IRC: 85 Recommended practice for accelerated strength testing and evaluation of concrete. Indian Roads Congress, New Delhi.
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