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COMPARISON OF ISI AND ACI METHODS FOR ABSOLUTEVOLUME CONCRETE MIX DESIGN

Amarj it Singh,University of Hawaii at Manoa, USAKamal Gautam, University of Hawaii at Manoa, USA

30thConference on OUR WORLD IN CONCRETE & STRUCTURES: 23 - 24 August 2005,Singapore

Article Online Id: 100030048

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1. IntroductionMix design is a process of specifying the mixture of ingredients required to meet anticipated

properties of fresh and hardened concrete. Concrete mix design is a well established practice around theworld. All developed countries, as well as many developing countries, have standardized their concretemix design methods. These methods are mostly based on empirical relations, charts, graphs, and tablesdeveloped as outcomes of extensive experiments and investigations of locally available materials. All ofthose standards and methods follow the same basic trial and error principles.

Some of the prevalent concrete mix design methods are: a) ACI Mix Design Method, b) USBR Mixdesign practice, c) British Mix design Method, and d) ISI Recommended guidelines. The scope of thisstudy is to compare ACI and ISI recommended mix design guidelines. A major part of concrete used inrural and semi-urban areas in India falls in the range of 15 - 20 MPa [1]. Similarly, concrete of strengthsup to 40 MPa are widely used in USA. Therefore, similar ranges of concrete strengths are widelyapplicable in both India and USA. The scope of this paper is limited to absolute volume and concrete mixdesign for compressive strengths less than 6000 psi (~ 40 MPa). In order to compare the two methods,calculation processes are briefly summarized, flow charts are made to illustrate the design steps, andsample tests are performed with the two techniques to produce 20, 30, and 40 MPa concrete. Basic dataused in both methods is illustrated in Table 1.

Table 1: Basic data used in the ISI and ACI Mix Design Methods

Parameter ISI Method ACI MethodCharacteristic compressive strength at 28 days yes yes

Standard deviation of compressive strength yes yes

Degree of workability Compacting factor Slump

Type and maximum size of aggregates yes yes

Nominal maximum size of coarse aggregates (NMSA) yes yes

Dry rodded unit weight of coarse aggregates (DRUW) no yes

Fine aggregates (sand) Grading zone Fineness modulus (FM)

Specific gravity of cement, coarse and fine aggregates yes yes

Water absorption and moisture content adjustment yes yes

Type of construction yes yes

Exposure condition no yes

Air/Non-air entrainment no yes

2. The ISI method:The Indian Standards Institution (ISI) has recommended guidelines for concrete mix design based on

cement and other materials locally available in India [2, 3].These guidelines are applicable for normalconcrete (less than 60 MPa) mix design. Use of gap graded aggregates, various admixtures, andpozzolana is beyond the scope of IS 10262-1982 [2]. The design steps for mix proportioning asrecommended in IS 10262-1982, is presented in the form of a schematic flow chart, Figure 1. The stepsare outlined below:

1. The target average compressive strength (ck

f ) at 28 days is determined by using equation 1:

ck

f stfck (1)where,

ckf = characteristic compressive strength at 28 days,s = standard deviation of compressive strength,t = a statistic, depending upon the accepted proportion of low results and the number of tests.

2. The water cement (w/c) ratio is chosen from an empirical relationship (a graph) for the given 28-daytarget mean strength. The w/c ratio is checked against the limiting w/c ratio to satisfy the durabilityrequirements.

3. Air content, amount of entrapped air in fresh concrete, as percentage of volume of concrete, isestimated based on the nominal maximum size of aggregate (NMSA).

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Figure1: Design flow chart of ISI method

Sanadju

Sp.gravityof agg. &cement

C. agg.content

B C

Watercontent

Check w/c issatisfactory

B

Targetstrength

Cementcontent

Volume of allitems Moisture

contentadjustm

W/Cratio

Aircontent

StandardDeviation

Designstrength

Check minimumcement content

A

Sanconte

Wateradjust

Satisf-actoryNMSA

Comp-

actingfactor

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4. Initially, water content, as mass (kg) per unit volume (m3) of concrete, is selected based on the NMSA

and the target strength. Then, the initially determined water content is adjusted for workabilityconditions depending upon the compacting factor and types of aggregates.

5. Sand content, as percentage of total aggregate volume, is selected based on the NMSA and thetarget strength. Then, the initially determined sand content is adjusted for workability conditionsdepending on the sand grading zone, w/c ratio, and type of aggregates.

6. The cement content is calculated from the w/c ratio and the water content. The cement content, thuscalculated, is then checked against the minimum cement content to satisfy the durability requirement.

7. With the quantities of water and cement per unit volume of concrete and the percentage of sand inthe total aggregate already determined, the coarse and fine aggregate contents per unit volume ofconcrete are calculated from the following equations, respectively:

)1(1000 pSS

CWVC ca

c

a

....(2)

fa

c

a pSS

CWVf

1000 ...(3)

where,

aC = total mass of coarse aggregate, [kg per m3of concrete],

af = total mass of fine aggregate, [kg per m3of concrete],

V = absolute volume of fresh concrete, equal to gross volume minus the volume of entrapped air,

W = mass of water (kg) per m3of concrete,

C = mass of cement (kg) per m3of concrete,

cS = specific gravity of cement,

p = ratio of fine aggregate to total aggregate by absolute volume,

faS = specific gravities of saturated surface dry fine aggregate,

caS = specific gravities of saturated surface dry coarse aggregate.

8. Finally, water content is adjusted based on the absorption and the current moisture content togenerate equivalent of saturated surface dry condition of the aggregates.

3. ACI Method:In 1991, the American Concrete Institute (ACI) published its guidelines for normal, heavyweight and

mass concrete mix design [4]. The Absolute Volume Method of mix design as described by the ACIMethod [5] is revisited, and the design steps for mix proportioning as recommended by ACI Committee211, is presented in the form of a schematic flow chart, Figure 2. The steps are discussed below:

1. The required (target) average compressive strength ( crf ) at 28 days for mix design is determined byadding up an empirical factor ( k) to the design compressive strength ( f ) as per equation 4:

kff ccr ....(4)2. The W/C ratio is selected based on the target strength and the type of concrete (air-entrained or non

air-entrained).

3. Air content, as percentage of the concrete volume, is estimated depending upon the air-entrained ornon-air-entrained type of concrete, exposure conditions, and NMSA.4. Slump, as measure of workability, is selected depending upon the type of structure and complexity of

the pouring conditions.5. Water content is determined based on the NMSA, type of concrete (air-entrained or non-air-

entrained), and specified slump. Then it is adjusted for the types of aggregates.6. Cement content, is calculated based on the w/c ratio and the water content.7. Coarse aggregates content, as dry rodded bulk (percentage) of concrete unit volume, is determined

based on the NMSA, and the fineness modulus of sand.

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8. Once the water content, cement content, air content, and the coarse aggregate content per unitvolume of the concrete is determined, the fine aggregate (Fagg) is calculated by subtracting theabsolute volume of the known ingredients from unit volume of the fresh concrete (in this case 1 m

3)

as following:

YFagg 1 ... ..(5)where,

Y = sum of all other ingredients (air, water, cement and coarse aggregates) in cubic meter calculatedfor 1 m

3of concrete.

9. Finally, water content is adjusted based on the absorption and the current moisture content of thecoarse and fine aggregates, in account of saturated surface dry condition of the aggregates.

4. Similarities of ISI and ACI Mix Design ProcessBoth of the methods are based on the empirical relations. These relations are derived from extensive

experiments done in each of the countries with the locally available materials. Thus, both methodsextensively use tables and graphs during the design process and follow logical determination of theingredients by establishing the targeted strength for trial batch. Such trial batch strength is derived fromthe required design strength of the structural concrete and the statistical analysis to ensure that the mixdesign meets or exceeds the design strength. This is related to statistics of the quality control. Once thetarget mix design strength is established, both methods advance the process with the determination of

w/c ratio.It is common in both cases that the cement content is calculated based on the relationships of two

parameters: the w/c ratio and the cement content both derived separately and independently. Both ofthese parameters are checked against the limiting values in order to ensure the durability conditions.

5. Differences of ISI and ACI Mix Design ProcessDifferences are manifest throughout the mix design process. The following highlight the major

differences:Target strength: The ISI method uses equation (1) and the ACI method uses equation (2) to

determine the target average compressive strength. Although both methods utilize the standard deviationto calculate the target strength, there is a difference in the technique of calculation. When sufficient dataare not available to establish standard deviation, the ACI method has recommended empirical values todetermine the target strength, whereas the ISI method has suggested the value of standard deviation.

Measure of workability:The ISI Method uses the compacting factor as a measure of workability,whereas ACI uses the slump.W/C Ratio:In the ACI method, w/c ratio is determined in combination with the target strength and the

type of concrete (air/non-air entrainment). Although, the ISI discusses the air entrainment, the selectionof w/c ratio in the ISI Method is a sole function of target strength.

Water content: The ISI method determines the water content based on target strength, type ofaggregates, NMSA and compacting factor. In the case of the ACI method, water content is dependent onair-entrainment, types of aggregates, slump, and NMSA. In the case of the ACI method, water contentcan be determined independent of target strength, whereas in the ISI method, target strength influencesthe water content.

Coarse and fine aggregate content: In the ACI method, coarse aggregate content is determinedwithout knowing the absolute volume of fine aggregates. Contrary to the ACI method, the ISI method firstdetermines the fine aggregate content, as percentage of total aggregate by absolute volume; the coarseaggregate content is determined next once the proportion of all other ingredients are known. In the ISI

method, specific sand grading zones are used as a governing parameter for sand content determination,whereas the fineness modulus is used in the ACI method for selecting the bulk volume of dry roddedcoarse aggregate. The ISI method does not utilize the fineness modulus and dry rodded unit weight ofaggregates.

6. Numerical Example of the Mix designIn order to compare the two design techniques, calculations were performed using the procedures

prescribed by both methods for design strengths of 20 MPa, 30 MPa, and 40 MPa. The mix proportionsare presented in Table 2.

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Table 2: Design mix calculated for 1 m3of fresh concrete

Ingredients, 20 MPa 30 MPa 40 MPa

ISI ACI ISI ACI ISI ACI

Water, kg 162 173 164 175 160 176

Cement, kg 403 361 518 451 618 488

Fine aggregate, kg 555 905 524 823 358 790Coarse aggregate, kg 1302 958 1230 959 1322 959

W/c ratio 0.401 0.479 0.317 0.388 0.259 0.361

The calculations indicate that although the design strength (characteristic strength) is the same, theproportion of the ingredients for ISI and ACI mixes are different. Although, the water content results arequite close, the w/c ratios are different, and therefore, the cement content is different between the ISI and

ACI results. The calculations show that ISI uses higher cement content than ACI, although both methodstend to increase the cement quantity as the desired strength increases. The high cement content of theISI method may be owing to the relatively low quality of Indian cement in earlier decades when the codeswere produced. Another reason for the high content may be the fineness of Indian cement, 225 m

2/kg,

compared to American cement, 300-500 m2/kg [6, 7].

Both methods indicate that fine aggregate content tends to decrease as the desired strength

increases. The coarse aggregate content does not seem to change in the case of the ACI method,whereas this tendency is not stable in the case of the ISI method. The w/c ratio is higher in the ACI mixthan ISI.

7. Experimental Results

Concrete cylinders of 4 and 8 height size were prepared and tested according to ASTM standardsfor 7 days and 14 days. Three samples were cast in each case. The tests were done using localmaterials in Hawaii, such as Hawaiian cement (Type I-II), Halawa aggregates (3/4), and # 4 Halawa sand(fineness modulus 2.93). During the sample preparation it was observed that the workability wasconsistently higher in the ACI method. The strength results are presented in Table 3.

Table 3: Tabulation of experimental results

Target StrengthISI, MPa ACI, MPa

7-Day 14-day 28-day* 7-Day 14-day 28-day*

20 MPa31 40 45 24 27 31

31 36 41 24 29 33

31 36 41 24 27 31

30 MPa21 30 34 33 38 43

24 32 36 33 38 43

22 29 33 33 39 44

40 MPa35 34 39 35 41 47

31 33 38 33 40 46

33 35 40 34 41 47* 28-day strength is projected from 14-day strength: 28-day strength = 14-day strength0.887

8. Analysis of the Experimental ResultsThe test results indicate that the ISI 20 MPa design has significantly higher strength than anticipated.

For example, anticipated 7-day test results of ISI 20 MPa should be in the range of 75% of 20 MPa, i.e.,15 MPa; instead, the test results of ISI 20 MPa were 31 MPa, which is more than double the anticipatedstrength. The 7-day test results for ISI 30 MPa and ISI 40 MPa were within the expected range. The 40MPa results show that the ISI technique makes for slower strength gain compared to the ACI, in spite ofincreased cement use, and may not meet the strength requirements, as well.

Basic statistical analysis, such as deviation measurement and correlation analysis were undertakenfor 28-day compressive strength results. A summary of the analysis is presented in Table 4.

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Table 4: Analysis of 28-day compressive strength

Target StrengthMPa(1)

Average Strength, MPa Strength Deviation,MPa

Correlation coefficient, r

ISI(2)

ACI(3)

ISI(4)

ACI(5)

ISI(1) vs. (2)

ACI(1) vs. (3)

20 42.42 31.44 22.42 11.44 -0.48 0.9430 34.47 43.56 4.47 13.56

40 38.64 46.21 -1.36 6.21

9. Conclus ionsBased on the analysis of the pilot tests, the following conclusions can be drawn:

1. The ISI technique performs better for lower strengths than higher strengths.2. Whereas, the ISI technique meets the strength criteria for 20 MPa, the actual strength achieved is

almost double. This appears to indicate a waste of materials, since such a high strength is notrequired. It may be that the high strength obtained is specified to offset hand mix conditions in ruraland remote India.

3. The correlation between desired strength and actual strength is r = 0.94 for ACI and r = - 0.48 for ISI.This indicates a higher correlation between designed and actual strength for ACI, indicating not only

that it is a more consistent and reliable technique, but that the ISI technique may give the inverse ofwhat is desired.

4. The ACI method is more likely to meet 30 MPa and 40 MPa design strengths.5. Higher strength fluctuations for ISI are evidenced by fluctuations ranging from -1.36 MPa to 22.42

MPa compared to ACI ranging from 6.21 MPa to 11.44 MPa. This indicates that the ACI technique ismore reliable.

6. The fines content in ACI is higher, which makes for higher workability. Presumably, It alsocontributes to increased strength as the voids are filled, especially as observed in the 30 and 40 MPatest cases. In the case of ISI, fine aggregate content is reduced as the design strength requirementgoes up. Therefore, voids are likely to be higher for higher strengths, thus leading to decreasedstrength in such cases.

7. The cement content for higher strengths in ISI increases at the expense of fine aggregates, makingfor an overall lower fines to coarse ratio, which possibly affects the strength achievement.

The conclusions are based only on a limited number of trial batches. Results may vary for larger numberof samples and with the use of different quality of cement, sand and coarse aggregates.

10. Acknowledgements:

The authors thank Mr. Timothy S. Folks, Manager of Technical Services, Hawaiian Cement /Hawaiian Laboratories, for use of their concrete lab facility.

11. References:1. Kumar, P. and Kaushik S. K., 2003, Some trends in the use of concrete: Indian scenario, The Indian

Concrete Journal, December 2003, pp.1503-1508.2. Indian Standard, Recommended guidelines for concrete mix design, IS 10262-1982, 1983, Indian

Standard Institution, New Delhi, India.

3. Handbook on concrete mixes (based on Indian Standards), 1983 (SP: 23-1982), Bureau of IndianStandards, New Delhi, India4. ACI Committee 211, 1991 (re-approved in 2002), Standard practice for selecting proportions for

normal, heavyweight and mass concrete.American Concrete Institute, USA.5. Kosmatka S. H., Kerkhoff B. and Panarese W. C., 2002, Design and control of concrete mixtures,

Portland Cement Association, Skokie, Illinois, USA.6. Indian Standard, Ordinary Portland cement, 33 grade- specification, IS 269:1989 , 1990 (third print

2000), Bureau of Indian Standards, New Delhi, India.7. Portland Cement, U. S. Department of Transportation, Federal Highway Administration,

www.fhwa.dot.gov/infrastructure/materialsgrp/cement.html, accessed on July 5, 2005.