Guidelines of IS Concrete Mix Proportioning - lceted

IS Concrete Mix Proportioning – Guidelines

Objective

To design a concrete mix in accordance with Indian Standard mix proportioning – Guidelines.

 

Theory and Scope

The concrete mix design method uses the Indian Standard mix proportioning guidelines to achieve specified characteristics, i.e., workability of fresh concrete, and strength and durability requirements of hardened concrete at specified age. The guidelines are applicable to ordinary and standard concrete grades only. All the requirements of IS 456-2000 are also satisfied in the mix design process.

 

Based on the guidelines, the preliminary or trial mixes are made and desired properties of the trial mixes are checked; suitable adjustments are made to produce concrete possessing specified properties both in fresh and hardened states with the maximum overall economy. The design of plastic concrete mixes of medium strength can be based on the following two criteria:

1.   The compressive strength of concrete is governed by its water-cement ratio.

2.   For the given aggregate characteristics, the workability of concrete is governed by its water content.

 

Apparatus

Sieve sets for finding maximum nominal size, and gradings of coarse and fine aggregates; Weighing balance; Trowels; Tamping bar; Moulds, Universal compression testing machine; Graduated cylinder; Slump cone apparatus and Buckets.

 

Procedure

Step 1: Perform sieve analyses of both the fine and coarse aggregates available to determine:

a. The maximum nominal size of coarse aggregate,

b. The gradings of fine and coarse aggregates and

c. The grading zone of fine aggregate.

If necessary, combine two or more different size coarse aggregate fractions so that the overall grading of coarse aggregate conforms to Table 2 of IS-383 for the particular nominal maximum size of aggregate.

 

Step 2: Determine the unit weight, specific gravities, and absorption capacities of both the coarse and fine aggregates.

 

Step 3: Determine the target mean compressive strength f’_ck in MPa from the specified characteristic compressive strength at 28-day f_ck in MPa and the level of quality control.

 

F’_ck = f_ck +1.65S

where S is the standard deviation in MPa obtained from the Table below.

 

Assumed standard deviation

Group  No.	Grade of  concrete	Assumed standard  deviation, MPa	Quality control
1.	M10 M15	3.5	The values correspond to the site control having proper storage of cement; weigh batching of all materials;  controlled addition of water; regular checking of all materials, aggregate grading and moisture content; and periodical checking of workability and strength. Where there is a deviation from the above, values given in this table shall be increased by 1.0 MPa.
2.	M20 M25	4.0
3.	M30 M35 M40 M45 M50 M55	5.0

 

Step 4: Determine the water-cement ratio using the relationship between strength and free water–cement ratio established for the materials used in the job. In the absence of such data, select the preliminary free water-cement ratio (by mass) corresponding to the target mean strength at 28 days using the empirical relationship between compressive strength and water–cement ratio given in  Fig. below Check the selected water–cement ratio against the limiting water–cement ratio for the requirements of durability given in Table below  the lower of the two values is adopted.

 

Table: Minimum cement content and maximum water–cement ratio of concrete with normal weight  aggregates of 20 mm nominal maximum size subjected to different exposures (Adapted from IS  456-2000)

Si.no	Exposure  condition	Plain Concrete			Reinforced Concrete
		Minimum  cement content, kg/  m3	Maximum  free water-  cement ratio	Minimum  grade of  concrete	Minimum  cement content, kg/  m3	Maximum  free water-  cement ratio	Minimum  grade of  concrete
1	Mild	220	0.60	-	300	0.55	M20
2	Moderate	240	0.60	M15	300	0.60	M 25
3	Severe	250	0.50	M20	320	0.45	M30
4	Very severe	260	0.45	M20	340	0.45	M35
5	Extreme	280	0.40	M25	360	0.40	M40
Adjustments to Minimum cement contents for Aggregates other than 20 mm Nominal Maximum Size
Nominal Maximum Size, mm				Adjustments to Minimum cement contents, kg/m3
10				+40
20				0
40				-30

 

Notes:

1. Cement content prescribed is irrespective of the grades of cement and it is inclusive of all supplementary cementitious materials. The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolana and slag specified in IS 1489  (Part I) and IS 455 respectively.

2. Minimum grade for plain concrete under mild exposure conditions is not specified.

 

Step 5: Determine the water content per unit volume of concrete, for the required workability and maximum size of  aggregates (for aggregates in saturated surface dry condition) from Table 10.7 for computing cementitious  material contents for trial batches.

Table 2: Maximum water content for the nominal maximum size of aggregate

 

Step 6: Calculate the cement and supplementary cementitious material content per unit volume of concrete from the free water-cement ratio and the water content per unit volume of concrete. Check the cementitious material content so calculated against the minimum content for the requirements of durability; adopt the greater of the two values.

 

Step 7: Estimate the volume of a coarse aggregate of a given nominal maximum size from Table 10.8 for the reference water–cement ratio of 0.5 and grading zone of fine aggregate used; adjust it suitably for the selected water-cement ratios.

For more workable concrete, e.g., pumpable or concrete mixes to be placed around congested reinforcing steel the estimated coarse aggregate content may be reduced up to 10 per cent subject to slump, water-cement ratio and strength properties of concrete remaining consistent with the provisions of IS 456 and project specifications.

 

Table 3: Proportion of coarse aggregate to total aggregate for different zones  of fine aggregate

 

 

Step 8: Estimate the volume of total aggregate by subtracting the sum of absolute volumes of cementitious material, water and the chemical admixture; and entrained air (if considered) from the unit volume of concrete.

 

Step 9: Divide the volume of total aggregate so obtained into coarse and fine aggregate fractions by volume in accordance with the coarse aggregate proportion already determined in Step 7. Determine the coarse and fine aggregate contents by multiplying with their respective specific gravities and multiplying by 1000. Alternatively, determine the volume of coarse and fine aggregate fractions as follows.

 

 

And,

V = absolute volume of fresh concrete,

= gross volume (1.0 m3) minus the volume of entrapped air,

Sc = specific gravity of cement,

W = Mass of water per cubic metre of concrete, kg

C = mass of cement per cubic metre of concrete, kg

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

fa. Ca = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg  and

Sfa, Sca= specific gravities of saturated surface dry fine and coarse aggregates, respectively.

 

Step 10: Determine the concrete mix proportions for the first trial mix or trial mix no. 1. Measure the workability of the trial mix in terms of a slump; carefully observe the mix for freedom from segregation and bleeding and its finishing properties. If the slump of the first trial mix is different from the stipulated value, adjust the water and/or admixture content suitably to obtain the correct slump.

Step 11:  Recalculate the mix proportions keeping the free water–cement ratio at the pre-selected value; this will comprise trial mix no. 2. In addition formulate two more trial mixes no. 3 and 4 with the water content same as trial mix no. 2 and varying the free water–cement ratio by ±10 per cent of the preselected value.

 

Step 12: Test the fresh concrete for unit weight, yield and air content. Prepare trial mix and cast three 150 mm cubes and test them after 28 days of moist curing.

 

Step 13: Analyse mix nos. 2 to 4 for relevant information, including the relationship between compressive strength and water–cement ratio. Compute water–cement ratio required for the mean target strength using the relationship. Recalculate the mix proportions for the changed water–cement ratio keeping water content at the same level as that determined in trial no. 2.

 

For field trials, produce the concrete by the actual concrete production method used in the field.

 

Observations and Calculations

The compressive strength of concrete mix is………..  The designed mix is suitable/it needs further revision.  The mix proportions are…………………

 

Precautions

The water content and the proportion of coarse aggregate should be adjusted for any difference in  workability, water–cement ratio and the grading zone of fine aggregate from the reference values used  in the Table below

 

The slump test, cube casting, curing and testing should be done according to the specifications.

 

the fresh concrete should be carefully observed for freedom from segregation and bleeding, and finishing properties.

 

Discussion

The mix design is really a process of making an initial guess at the optimum combination of ingredients and  the final mix proportion is obtained only on the basis of further trial mixes. The IS guidelines envisage that  the design of concrete mix is based on the following factors:

 

Grade designation gives the characteristic strength requirement of the concrete. The term characteristic strength means that the value of the strength of material below which not more than five per cent of  test results are expected to fall. Depending upon the level of quality control available at the site, the  concrete mix has to be designed for a target mean strength which is greater than the characteristic  strength by a suitable margin. The target mean strength is expressed as

f_t = f_ck + 1.65 S

where f_t = target mean strength,

f_ck = characteristic strength, and

S = standard deviation.

 

The maximum nominal size of aggregates to be used in concrete may be as large as possible within the  limits prescribed by IS: 456-2000 and IS: 1343-1980. In general, increasing the maximum nominal size of aggregate helps increase the workability and reduce the cement requirement for a particular water-cement ratio. However, the size of aggregates also influences the compressive strength  of concrete in that, for a particular volume of aggregate, the compressive strength tends to increase  with a decrease in the size of aggregate. This is due to the fact that the smaller size of aggregates provides a  larger surface area for binding with the mortar matrix. Moreover, increasing the maximum size of aggregate increases the stress concentration in the mortar-aggregate interface. For high-strength concrete, 10 or 20 mm size of aggregate is preferable.

 

The cement content is to be limited from shrinkage cracking and creep considerations. In thick concrete sections restrained against movements, high cement content may give rise to excessive cracking  by differential thermal stresses due to hydration of cement in young concretes. However, the cement  content should not be less than the minimum content prescribed for the requirements of durability.

 

For high strength, concrete increasing cement content beyond a certain value, of the order of 550 kg/m³  or so may not help in increasing the compressive strength. From overall economic considerations, the  maximum cement content in concrete mixes is limited to 530 kg/m³ for prestressed concrete.

The workability of concrete for satisfactory placing and compaction is related to the size and shape  of the section to be concreted, the quantity and spacing of reinforcement and the technique used for  transportation, placing and compaction of concrete.

 

In the case of fly ash cement concrete of comparable workability, the water-cement ratio can be reduced  by about 3 to 5 per cent and the proportion of fine aggregate is reduced by 2 to 4 per cent points.

 

The compressive strength of concrete for the same free water-cement ratio varies with the type of cement and supplementary cementitious materials, maximum size, grading, surface texture, and shape of aggregate. Therefore, the relationship between strength and free water-cement ratio should preferably  be established for the materials actually to be used. In the absence of such data, the preliminary free water-cement ratio (by mass) corresponding to the target strength at 28 days may be selected from the  established relationship, if available, e.g., from Fig. below This relationship is applicable to both ordinary Portland and Portland pozzolana cement. If the 7-day compressive strength of concrete is considered as  an additional parameter influencing the relationship between the water-cement ratio and 28-day compressive  strength Fig. below can be used to make a more precise estimate of the water-cement ratio. Alternatively, the water-cement ratio given in Table 5 of IS 456 for respective environment exposure conditions  may be used as starting point. The supplementary cementitious materials, i.e., mineral admixtures are  included in water–cement ratio calculations in accordance with Table 5 of IS 456.

 

Relationship between the free water-cement ratio and 28-day compressive strength of concrete

 

As in the ACI method, the volume of coarse aggregate in the concrete mix is first determined depending upon the  maximum nominal size of coarse aggregate and grading of fine aggregate. Both the methods use the absolute  volumes of the ingredients in mix proportioning.

 

The method can use the aggregate available at the work site of any grading so long as they are within the  grading limits specified by IS 383, i.e., conform to standard grading. For example, consider the case where the  fine and coarse aggregates available at the work site (given in the table) are to be combined so as to approximate the standard grading also listed in the table. This can be done conveniently by analytical calculations.

 

 

If fine and coarse aggregates are combined in proportion 1: k, then using IS: 4.75 mm sieve size as criteria,

the value of k is given by: k = (f-s/s-c)

 

The grading of the resulting combined aggregate is determined by multiplying the grading of fine and

coarse aggregates by 1.0 and k, respectively, and dividing the sum of corresponding products of percentages

passing the particular sieve size by (1 + k), the values being rounded off to nearest percentage. For this case

k = (f-s/s-c) = (100-30)/(30-7) = 3.043

Therefore, fine and coarse aggregates are to be combined in a mass proportion of 1: 3.043

 

REFERENCES-NATIONAL STANDARDS

IS 383-1970 (2nd revision, reaffirmed 2011): Specification for Coarse and Fine Aggregates from  Natural Sources for Concrete.

IS 456-2000 (4th revision, reaffirmed 2011): Code of Practice for Plain and Reinforced Concrete.

IS 2386 (Part 3) -1963 (reaffirmed 2011): Methods of Test for Aggregates for Concrete: Part 3: Specific  Gravity, Density, Voids, Absorption and Bulking

IS 3812 (Part 1)-2003 (2nd revision, reaffirmed 2007): Specification for Pulverized Fuel Ash: Part1:

For Use as Pozzolana in Cement, Cement Mortar and Concrete.

IS 8112-1989 (1st revision, reaffirmed 2009): Specification for 43-grade Ordinary Portland Cement.

IS 9103-1999 (1st revision, reaffirmed 2008): Specification for Admixtures for Concrete.

IS 10262-2009 (1st revision): Concrete Mix Proportioning-Guidelines.