Concrete Mix Design JINSHAD
General
The design of concrete mix is not a simple task on account of
1. varyin varying g prope propertie rtiess of the consti constitue tuent nt mater material ialss 2. condi condition tionss that that prevai prevaill at the the site site of work, work, in part particu icular lar the the exposure condition 3. Proport Proportion ion of of the the mate materia rials ls of conc concret rete e found found out at at the laboratory requires modification and readjustments to suit the field conditions.
General
The design of concrete mix is not a simple task on account of
1. varyin varying g prope propertie rtiess of the consti constitue tuent nt mater material ialss 2. condi condition tionss that that prevai prevaill at the the site site of work, work, in part particu icular lar the the exposure condition 3. Proport Proportion ion of of the the mate materia rials ls of conc concret rete e found found out at at the laboratory requires modification and readjustments to suit the field conditions.
Procedure of Design Structural designer
stipulates certain minimum strength
Concrete technologist designs the concrete mix with the
knowledge of the materials, site exposure conditions and standard standard of supervision available at the site of work to achieve this minimum strength and durability. Site engineer make the concrete at site, closely following the
parameters suggested suggested by the the mix designer to achieve the minimum strength specified by the structural engineer
Types of Concrete Mixes 1. NOMINAL MIX
Used for relatively unimportant and simpler concrete works. In this type of mix, all the ingredients(cement, fine and coarse aggregates) are prescribed. margin of strength above that specified. They off o ffer er simplicity and under normal circumstances, has margin of strength above that specified.
IS:456-2000 permits nominal mixes for concretes of strength M20 or lower
Types of Concrete Mixes 1. NOMINAL MIX
Grade
Proportions C: FA: CA
M5
1: 5:10
M M M M
7.5 10 15
20
1:4:8 1:3:6 1:2:4 1:1.5:3
Types of Concrete Mixes 1. NOMINAL MIX
Objectives of Mix Design 2.
DESIGN MIX The concrete mix produced under quality control keeping in view the strength, durability, and workability is called the design Mix. Others factors like compaction equipment's available, curing method adopted, type of cement, quality of fine and coarse . mix proportion. The design mix or controlled mix is being used more and more in variety of important structures, because of better strength, reduced variability, leaner mixed with consequent economy, as well as greater assurance of the resultant quality.
Objectives of Mix Design
Objectives of Mix Design 1. To achieve the desired minimum strength in the hardened stage 2. To achieve the designed/ desired workability in the plastic stage .
o ac eve
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conditions 4. To produce concrete as economically as possible.
Factors affecting the choice of mix proportions 1. Grade of Concrete 2. Type and Grade of Cement 3. Maximum nominal size of aggregate 4. Grading and type of aggregate 5. Water Cement Ratio 6. Workability 7. Durability 8. Quality Control
01: Grade of Concrete
The grade of concrete gives characteristic compressive strength of concrete. It is one of the important factor influencing the mix design
Other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction.
The grade M 20 denotes characteristic compressive strength fck of 20 N/mm2. Depending upon the degree of control available at site, the concrete mix is to be designed for a target mean compressive strength
02: Type and Grade of Cement
The rate of development of strength of concrete is influenced by the type of cement.
The higher the strength of cement used in concrete, lesser will be the cement content. The use of 43 grade and 53 grade of cement, gives saving in cement consumption as much as 15 % and 25 % respectively, as compared to 33 grade of cement. For concrete of grade M25 it is advisable to use 43 and 53 grade of cement.
03: Maximum nominal size of Agg
The maximum size of C.A is determined by sieve analysis. It is designated by the sieve size higher than larger size on which 15 % or more of the aggregate is retained. The maximum nominal size of C.A. should not be more than one-forth of minimum thickness of the member.
For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to sum less than the minimum clear distance between the main bars or 5 mm less the minimum cover to the reinforcement, whoever is smaller.
03: Maximum nominal size of Agg
In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio, because the workability of concrete increases with increase in maximum size of the aggregate.
However, the compressive strength tends to increase with the decrease in size of aggregate.
IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate should be as large as possible.
04: Grading and type of aggregate
The relative proportions of the fine and coarse aggregate in a concrete mix is one of the important factors affecting the strength of concrete.
For dense concrete, it is essential that the fine and coarse aggregate be well graded.
In the case when the aggregate available from natural sources do not confirm to the specified grading, grading can be achieved by mixing different size fractions.
The grading of aggregate influences mix proportions for a specified workability and water-cement ratio.
05: Grading and type of aggregate
Coarser the grading, leaner will be mix which can be used. Very lean mix is not desirable since it does not contain enough finer material to make the concrete cohesive.
The type of aggregate influences strongly the aggregatecement ratio for the desired workability and stipulated water cement ratio.
05: Water Cement Ratio
Water to cement ratio (W/C ratio) is the single most important factor governing the strength and durability of concrete.
Abram’s water/Cement ratio states that for any given condition of test, the strength of a workability concrete mix is dependent only on water/cement ratio.
Strength of concrete depends upon W/C ratio rather than the cement content. The lower the water/Cement ratio, the greater is the compressive strength
06: Workability
Workability of fresh concrete determines the case with which a concrete mixture can be mixed, transported, placed, compacted and finished without harmful segregation and bleeding.
The degree of workability required depends on three factors. a) Size of the section to be concreted b) Amount of reinforcement c) Method of compaction to be used
06: Workability
For the narrow and complicated section with numerous corners or inaccessible parts, the concrete must have a high workability so that full compaction can be achieved with a reasonable amount of effort. This also applies to the embedded steel sections. The desired workability depends on the compacting equipment available at the site.
For concrete mixes required high consistency at the time of placing,
the
use
of
water-reducing
and
set-retarding
admixtures should be used rather than the addition of more water
07: Durability
Durability of concrete is its resistance to the aggressive environmental conditions.
High strength concrete is generally more durable than low strength concrete. n
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g s reng
s no necessary u
the conditions of exposure are such that high durability is vital, the durability requirement will determine the watercement ratio to be used.
07: Durability
Durability require low water/Cement ratio. It is usually achieved not by increasing the cement content, but by lowering the water demand at a given cement content.
Water demand can be lowered by through control of the aggregate grading and by using water reducing admixtures
06: Quality Control at site
The degree of control can be estimated statistically by the variations in test results.
The variation in strength results from the variations in the properties of the mix ingredients and lack of control of accuracy in batching, mixing, placing, curing and testing.
The lower the difference between the mean and minimum strengths of the mix lower will be the cement-content required. The factor controlling this difference is termed as quality control.
Variables in Mix Design Four variables to be considered in connection with specifying a concrete mix are
1. Water-Cement ratio 2. Cement content .
-
4. Gradation of the aggregates 5. Consistency
Two or Three of the factors is specified and the others are adjusted to give minimum workability and economy.
Various Methods of Mix Design 1. Arbitrary proportion 2. Fineness modulus method 3. Maximum density method 4. Surface area method 5. Indian Road Congress, IRC 44 method . H g strengt concrete m x es gn 7. Mix design based on flexural strength 8. Road note No. 4 (Grading Curve method) 9. ACI Committee 211 method 10. DOE method 11. Mix design for pumpable concrete 12. Indian standard Recommended method IS 10262-82
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Indian standard method : IS 10262
The
Bureau of Indian Standards, recommended a set of procedure for design of concrete mix mainly based on the work done in national laboratories.
The methods given can be
applied for both medium strength and high strength concrete. The
procedures of concrete mix design IS 10262 1982 was revised to IS 10262 2009
Mix Design Steps 1. Data Collection 2. Testing of materials 3. Target Mean strength for mix proportioning 4. Selecting water/cement ratio 5. Calculating water content 6. Calculating cement content 7. Estimation of proportions for Coarse aggregate 8. Estimation of proportions for fine aggregate 9. Mix calculations 10.Trial mixing 11.Workability measurement (using slump cone method) 12.Test Results
Step 01: Data Collection
All the required information for designing a concrete mix from the client. Grade designation (whether M10, M15, M20 etc) Type of cement to be used Maximum nominal size of aggregates Minimum & maximum cement content Max mum water-cement rat o Workability Exposure conditions (As per IS-456-Table-4) Maximum temperature of concrete at the time of placing Method of transporting & placing Early age strength requirement (if any) Type of aggregate (angular, sub angular, rounded etc) Type of admixture to be used (if any)
Step 02: Testing of Materials The list of most necessary tests to be done on cement, coarse aggregate, fine aggregate and admixture is given below
Step 03: Target Mean Strength
The target mean compressive ( f ck ’) strength at 28 days is given by
Step 03: Target Mean Strength
According to IS: 456–2000 and IS: 1343–’80, the characteristic strength is defined as that value below which not more than 5 per cent results are expected to fall, in which case the Target mean strength for mix design
The value of the standard deviation has been worked out from the trials conducted in the laboratory or field.
Step 03: Target Mean Strength
Step 04: Water Cement Ratio
Different cements, supplementary cementitious materials, aggregates of different sizes and grading, surface texture, shape and other characteristics may produce
concretes of
different compressive strengths for the same water cement ratio.
Therefore, relationships between strength and free water cement ratio should be established based on the materials actually used.
Step 04: Water Cement Ratio
Advantages of low water/cement ratio:
Increased strength
Lower permeability
Increased resistance to weathering
Better bond between concrete and reinforcement.
Reduced drying shrinkage and cracking
Less volume change from wetting and drying
Step 04: Water Cement Ratio
In the absence of such data, preliminary calculation for water cement ratio as given in IS-456-Table 5 for different environmental exposure condition(IS 456 - Table 3) may be used. These conditions have been set based on durability conditions
Step 04: Water Cement Ratio
Step 04: Water Cement Ratio
Water cement ratio ratio as given in Fig 1 of IS-10262:1982 IS-10262:1982 may also be used.
Step 05: Water Content
The water content content i.e. the quantity of maximum mixing water per unit volume of concrete concrete
The water water content content of concrete concrete is influenced by a number of factors, such as 1. 2. 3. 4.
Aggregate s ze s ze, s ape an texture Workability Water-c er-cem emen entt ra ratio tio Cement Cement and other other supple supplemen menta tary ry cement cementiti itious ous mater material ial type and content 5. Chem hemical ical admix dmixtu turre 6. Envi Enviro ronm nmen enta tall cond condit itio ions ns..
Step 05: Water Content
The maximum mixing water per unit volume of concrete concrete based on aggregate size may be determined from IS 10262 Table 2
The water content in Table 2 is for angular coarse aggregate and for 25 to 50 mm slump range.
Step 05: Water Content
1. 2. 3. 4. 5.
1. 2. 3. 4. 5. 6.
Factors that can reduce water demand are as follows Usin Using g inc incrrease eased d agg aggre reg gat ate e siz size e Redu Reducin cing g wate waterr ceme cement nt rati ratio o Redu Reducin cing g the the slu slump mp requ requir irem emen entt Usin Using g rou round nded ed aggr aggreg ega ate Using Using wat water er redu reducing cing admixtu admixture re and plas plastic ticiz izer erss Factors that can increase water demand are as follows Incr Increa ease sed d temp temp.. at site site Incr Increa ease sed d cem cemen entt con conttent ent Inc Increased slump Incr Increa ease sed d wat water er ceme cement nt rat atio io Incr Increa ease sed d aggr aggreg egat ate e ang angul ular arit ity y Decreas Decrease e in propo proporti rtion on of the the coar coarse se aggr aggreg egat ate e to fine fine aggregate
Step 05: Water Content
The above adjustments (if the material used differs from the specified condition) is tabulated below Type of material/condition
Adjustment required
For sub angular aggregate
Reduce the selected value by 10kg
For gravel with crushed stone
Reduce the selected value by 20kg
For rounded gravel
Reduce the selected value by 25kg
For every addition of 25mm slump
Increase the selected value by 3%
If using plasticizer
Decrease the selected value by 5-10%
If using super plasticizer
Decrease the selected value by 20-30%
Increase or decrease in values of compacting factor by 0.1
Increase or decrease the selected value by 3%
Step 06: Cement Content
Different cements, supplementary cementitious materials and aggregates of different maximum size, grading, surface texture, shape and other characteristics may produce concretes of different compressive strength for the same free water-cement ratio.
The cement content per unit volume of concrete may be calculated from free water-cement ratio and the quantity of water per unit volume of concrete
Step 06: Cement Content
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.
Otherwise, the water-cement ratio given in Table 5 of IS 456 for respective environment exposure conditions may be used as starting point.
The purpose of specifying a minimum cement content by IS 456 is to ensure reasonable durability.
Step 06: Cement Content
Step 06: Cement Content
The supplementary cementitious materials, that is, mineral admixtures shall also be considered in water-cement ratio calculations in accordance with Table 5 of IS 456.
The percentage of mineral admixture to be used should be based on project requirement and quality of material.
Step 07: Estimation of Coarse Agg
The coarse aggregate Coarse aggregate of suitable proportions conforming to Table 2 size of aggregate
used shall conform to IS 383 – 1970. different sizes may be combined in so as result in an overall grading of IS 383 – 1970 for nominal maximum
Step 07: Estimation of Coarse Agg
For equal workability, the volume of coarse aggregate in a unit volume of concrete is dependent only on its nominal maximum size and grading zone of fine aggregate.
Volume of coarse aggregate corresponding to unit volume of total (saturated and surface dry) aggregate for different zones of fine aggregate for a water-cement ratio of 0.5 is given in the Table 3 of IS 10262
Step 07: Estimation of Coarse Agg
The above volume has been arrived at on the assumption that aggregates are saturated and surface dry. For any deviation from this condition i.e., when aggregate are moist or air dry or bone dry, correction has to be applied on quantity of mixing water as well to the aggregate.
Step 07: Estimation of Coarse Agg
Change in Condition Each 0.05 increase or decrease in free water cement ratio umpa e concrete or wor ng around congested reinforcing steel
Adjustment Required in Coarse Aggregate ±
0.01
- 10 %
Step 08: Estimation of Fine Agg
Quantity of Fine Aggregate can be determined by difference if the volume of coarse aggregates subtracted from the total volume of Aggregates to obtain the required volume of fine aggregate.
Subtract the volume of coarse aggregate from 1, to find out the volume of fine aggregate.
Step 09: Mix Calculations
The mix calculations per unit volume of concrete shall be done as follows.
Step 10: Trial Mix
Conduct a trial mix as per the amount of material calculated above.
Step 11: Measurement of Workability
The workability of the trial mix no.1 shall be measured.
The mix shall be carefully observed for freedom from segregation and bleeding and its finishing properties.
If the measured workability of Trial Mix No. 1 is different from the stipulated value, the mix proportion shall be recalculated keeping the free water-cement ratio at the pre-selected value, which will comprise Trial Mix No. 2.
Step 11: Measurement of Workability
In addition two more Trial Mixes No. 3 and 4 shall be made with the water content same as Trial Mix No. 2 and varying the free water-cement ratio by ±10 percent of the preselected value.
Mix No. 2 to 4 normally provides sufficient information, including the relationship between compressive strength and water-cement ratio, from which the mix proportions for field trials may be arrived at.
Step 12: Tests
Prepare the concrete using the calculated proportions and cast three cubes of 150 mm size and test them wet after 28-days moist curing and check for the strength.
Modifications in IS 10262:2009
ACI Method : 211.1
Introduction
This method of proportioning was first published in 1944 by ACI committee 613. In 1954 the method was revised to include, among other modifications, the use of entrained air.
Almost all of the major multipurpose concrete dams in India built during 1950 have been designed by using then prevalent ACI Committee method of mix design.
The ACI Standard 211.1 “Recommended Practice for Selecting Proportions for Concrete” gives the method of mix design which was last updated in 1991.
Assumptions in ACI
ACI Committee mix design method assume certain basic facts which have been substantiated by field experiments or large works.
1. Over a considerable range of practical proportions, fresh concrete of given slump and containing a reasonably well graded aggregate of given maximum size will have practically a constant total water content regardless of variations in water/cement ratio and cement content, which are necessarily interrelated.
Assumptions in ACI 2. Optimum dry rodded volume of coarse aggregate per unit volume of concrete depends on its maximum size and the fineness modulus of the fine aggregate regardless of shape of particles. The effect of angularity is reflected in the void content, thus angular coarse aggregates require more mortar than rounded coarse aggregate. 3. Irrespective of the methods of compaction, even after complete compaction is done, a definite percentage of air remains which is inversely proportional to the maximum size of the aggregate.
Mix Design Steps 1. Data Collection 2. Choice of Slump 3. Choice of maximum size of aggregate 4. Estimation of mixing water and air content. . e ec on o wa er cemen ra o. 6. Calculation of cement content. 7. Estimation of coarse aggregate content. 8. Estimation of Fine Aggregate Content. 9. Adjustments for Aggregate Moisture. 10.Trial Batch Adjustments and Testing
Step 01: Data Collection
The following parameters must be determined by suitable tests a) Fineness modulus of selected fine aggregate b) Unit weight of dry rodded coarse aggregate. c) Sp. gravity of coarse and fine aggregates in SSD condition d) Absorption characteristics of both coarse and fine aggregates. e) Specific gravity of cement.
Step 02: Choice of Slump
Generally specified for a particular job. Generally, the lowest slump that permit adequate placement should be selected.
If slump is not specified, a value appropriate for the work can be selected from the below Table Type of Construction
Slump (mm)
(inches)
Reinforced foundation walls and footings
25 - 75
1-3
Plain footings, caissons and substructure walls
25 - 75
1-3
Beams and reinforced walls
25 - 100
1-4
Building columns
25 - 100
1-4
Pavements and slabs
25 - 75
1-3
Mass concrete
25 - 50
1-2
Step 03: Choice of Max Agg Size
Large maximum sizes of aggregates produce less voids than smaller sizes.
ACI method is based on the principle that “ Maximum Size Of Aggregate Should Be The Largest Available So Long It Is Consistent With The Dimensions Of The Structure”
In practice, dimensions of the forms or the spacing of the reinforcement bars controls the maximum Course Aggregate size
Step 03: Choice of Max Agg Size
ACI 211.1 states that the maximum CA size should not exceed: 1. 1/5 the minimum dimension of structural members, 2. 1/3 the thickness of a slab, or 3. 3/4 the clearance between reinforcing rods and forms. These restrictions limit maximum aggregate size to 1.5 inches, except in mass applications
Step 04: Mixing water and air content.
ACI Method uses past experience to give a first estimate for the quantity of water per unit volume of concrete required to produce a given slump.
In general the quantity of water per unit volume of concrete required to produce a given slump is dependent on the maximum CA size, the grading of both CA and FA, as well as the amount of entrained air.
The approximate amount of water required for average aggregates is given in Table.
Step 04: Mixing water and air content Mixing Water Quantity in kg/m3 (lb/yd3) for the listed Nominal Maximum Aggregate Size
9.5 mm (0.375 in.)
12.5 mm (0.5 in.)
19 mm (0.75 in.)
25 mm (1 in.)
37.5 mm (1.5 in.)
50 mm (2 in.)
75 mm (3 in.)
100 mm (4 in.)
25 - 50 (1 - 2)
207 (350)
199 (335)
190 (315)
179 (300)
166 (275)
154 (260)
130 (220)
113 (190)
75 - 100 (3 - 4)
228 (385)
216 (365)
205 (340)
193 (325)
181 (300)
169 (285)
145 (245)
124 (210)
150 - 175 -
243
228
216
202
190
178
160
Typical entrapped air (percentage)
3
2.5
2
1.5
1
0.5
0.3
0.2
25 - 50 (1 - 2)
181 (305)
175 (295)
168 (280)
160 (270)
148 (250)
142 (240)
122 (205)
107 (180)
75 - 100 (3 - 4)
202 (340)
193 (325)
184 (305)
175 (295)
165 (275)
157 (265)
133 (225)
119 (200)
150 - 175 (6 - 7)
216 (365)
205 (345)
197 (325)
184 (310)
174 (290)
166 (280)
154 (260)
-
Slump Non-Air-Entrained
-
Air-Entrained
Recommended Air Content (percent) Mild Exposure
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Moderate Exposure
6.0
5.5
5.0
4.5
4.5
4.0
3.5
3.0
Severe Ex osure
7.5
7.0
6.0
6.0
5.5
5.0
4.5
4.0
Step 05: Selection of w/c ratio
The required water/cement ratio is determined by strength, durability and finish ability.
A conservative estimate can be made for the accepted 28-day compressive strength from prior testing of a given system of cement and aggregate as shown in Table 10.3
Maximum permissible value also depends upon the type of structure as shown in Table 10.4.
Step 05: Selection of w/c ratio Table 10.3: Water-Cement Ratio and Compressive Strength Relationship 28-Day Compressive Strength in MPa (psi)
Water-cement ratio by weight Non-AirEntrained
AirEntrained
41.4 (6000)
0.41
-
34.5 (5000)
0.48
0.40
27.6 (4000)
0.57
0.48
20.7 (3000)
0.68
0.59
13.8 (2000)
0.82
0.74
Table 10.4 Maximum Permissible Water/Cement
Step 06: Calculation of cement content
The amount of cement per unit volume of the concrete is found by dividing the estimated water content by the w/c ratio.
However, a minimum cement content is required to ensure good finishability, workability, and strength.
Step 07: Estimation of CA content
The most economical concrete will have as much as possible space occupied by CA since it will require no cement in the space filled by CA.
Step 07: Estimation of CA content Table 10.5 Volume of dry-rodded coarse aggregate per unit volume of concrete for different coarse aggregates and fineness moduli of fine aggregates Nominal Maximum Aggregate Size
Fine Aggregate Fineness Modulus 2.40
2.60
2.80
3.00
9.5 mm (0.375 inches)
0.50
0.48
0.46
0.44
12.5 mm (0.5 inches)
0.59
0.57
0.55
0.53
19 mm (0.75 inches)
0.66
0.64
0.62
0.60
25 mm (1 inches)
0.71
0.69
0.67
0.65
37.5 mm (1.5 inches)
0.75
0.73
0.71
0.69
50 mm (2 inches)
0.78
0.76
0.74
0.72
75 mm (3 inches)
0.82
0.80
0.78
0.76
150 mm (6 inches)
0.87
0.85
0.83
0.81
Notes: These values can be increased by up to about 10 percentge for pavement applications.
To convert from Oven Dry(OD) to saturated surface dry (SSD) weights, multiply by [1 + absorption capacity (AC)].
Step 08: Estimation of FA content
Quantity of Fine Aggregate can be determined by difference if the “absolute volume” displaced by the known ingredients, (i.e., water, air, cement, and coarse aggregate), is subtracted from the unit volume of concrete to obtain the required volume of fine aggregate.
once the volumes are know the weights of each ingredient can be calculated from the specific gravities.
Step 09: Adjustment for moisture
Aggregate volumes are calculated based on oven dry unit weights
Any moisture in the aggregate will increase its weight and stockpiled aggregates almost always contain some moisture. Without correcting for this, the batched aggregate volumes will be incorrect.
Step 09: Adjustment for moisture
The weight of aggregate from the stock pile is
Step 10: Trial Batch and Testing
Using the proportions developed in the preceding steps, mix a trial batch of concrete using only a much water as is needed to reach the desired slump (but not exceeding the permissible w/c ratio).
The fresh concrete should be tested for slump, unit weight, yield, air content, and its tendencies to segregate, bleed, and finishing characteristics.
Hardened samples should be tested for compressive and flexural strength.
Road Note Method(RM)
Introduction
This method of designing concrete mix proportions is mainly based on the extensive laboratory and field experiments carried out by the Road Research Laboratory, U.K.
They have established relationship between various properties of concrete and variable parameters.
A series of standard grading curves have been established to give grading limits for all-in aggregates graded down from 20 mm and 40 mm.
Introduction
This method of mix design was popular and was widely used up to 1970’s all over the world. Most of our concrete roads and air field pavements where designed by this method
The procedure of mix design by Road Note No 4 is also called
Grading Curve Method.
The Building Research Establishment of Department of Environment (DOE) U.K. has evolved another method called
DOE method to replace the earlier Road Note No 4 method.
Rapid Method (RM)
Why RM?
The approaches outlined above will need at least 28 days for the trial mix of concrete, and 56 days if cement is to be tested to use
With shortage of cement with interrupted supplies, there is a tendency to straightaway use cement arrives at site without waiting for trial mixes.
Difficulties in storing adequate quantities of cement and aggregates at site is another factor
Rapid Method (RM)
With accelerated curing on the rise, the trial of mixes can be related to target ‘accelerated strength’ rather than the “target 28 days strength”, with the help of correlation between the two
Methods of accelerated curing of concrete for strength tests are now standardised and covered in IS: 9013–1978.
A typical correlation is shown in next figure which is based on cements from all plants in the country and different grades of concrete, using boiling water method of accelerated curing.
Rapid Method (RM)
Reference Concrete Mix
The ‘reference’ concrete mix has w/C = 0.35 and workability = 0.80 (compacting factor).
The nominal maximum size of natural crushed aggregate should be 10 mm and fine aggregate conforming to zone II of Table 4 of IS: 383– 1970 are used.
Typical composition of such a reference concrete mix, per m3 of concrete is
Steps
Step 01 : Determine the accelerated strength (boiling water method) of 150 mm cube specimens of a standard concrete mix, using the cement at hand as per IS: 9013– 1978.
Step 02 : Corresponding to the accelerated strength in step (i) determine the water cement ratio for the required target strength of concrete mix from following figure given in IS 40311968 and IS 10262:1982.
Steps
Steps
Step 03 : Work out the remaining mix proportions as per any accepted method of mix design and check the workability of fresh concrete, against the desired value.
Step 04 : Determine the accelerated compressive strength of the trial mix
