CONCRETE MIX DESIGN USING CRUSHED SAND The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance. The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labour. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking. In this study, we have used grit and crushed sand for the design mix and found the characteristic compressive strength at 3,7 and 28 days respectively. The mix design has been done as per IS:10262. Advantages of mix design Mix design aims to achieve good quality concrete at site economically. I. Quality concrete means
Transcript
CONCRETE MIX DESIGN USING CRUSHED SAND
The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance. The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labour. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking.
In this study, we have used grit and crushed sand for the design mix and found the characteristic compressive strength at 3,7 and 28 days respectively. The mix design has been done as per IS:10262.Advantages of mix design
Mix design aims to achieve good quality concrete at site economically.
I. Quality concrete means
• Better strength
• Better imperviousness and durability
• Dense and homogeneous concrete
II. Economy
a) Economy in cement consumption
It is possible to save up to 15% of cement for M20 grade of concrete with the help of concrete mix design. In fact higher the grade of concrete more are the savings. Lower cement content also results in lower heat of hydration and hence reduces shrinkage cracks.
b) Best use of available materials:
Site conditions often restrict the quality and quantity of ingredient materials. Concrete mix design offers a lot of flexibility on type of aggregates to be used in mix design. Mix design can give an economical solution based on the available materials if they meet the basic IS requirements. This can lead to saving in transportation costs from longer distances
c) Other properties:
Mix design can help us to achieve form finishes, high early strengths for early deshuttering, concrete with better flexural strengths, concrete with pumpability and concrete with lower densities.Requirements of mix design
Concrete mix design is the method of correct proportioning of ingredients of concrete, in order to optimize the above properties of concrete as per site requirements.
The site engineer should give following information while giving material for mix design to the mix design laboratory: -
a) Grade of concrete (the characteristic strength)
b) Workability requirement in terms of slump
c) Other properties (if required): -
i. Retardation of initial set (to avoid cold joints in case of longer leads or for ready-mix concrete)
ii. Slump retention (in case of ready mix concrete)
iii. Pumpability (In case of ready mix concrete)
iv. Acceleration of strength (for precast members or where early deshuttering is desired)
v. Flexural strength (normally required for concrete pavements)
d) Ascertain whether condition of exposure to concrete is mild, moderate severe or very severe. Proper investigation of soil should be done to ascertain presence of sulphates &chlorides, in case of doubt.
e) What is the degree of control at site? Following factors indicate degree of control at site: -
i. Batching – weigh batching / volume batching.
ii. Type of aggregates – whether mixed graded aggregate will be usedor 20mm, 10mm aggregates will be used separately.
iii. Testing of concrete – whether casting & testing of concrete cubeswill be done regularly at site.
iv. Source of aggregate – whether sources of sand and aggregate will be standardised or likely to change frequently.
v. Supervision – whether qualified staff will be present to supervise concreting workand make necessary corrections e.g. correction for moisture in sand andchanges in material properties.Factors affecting the choice of mix proportions
The various factors affecting the mix design are:
A. Compressive strength
It is one of the most important properties of concrete and influences many other describable properties of the hardened concrete. The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction. According to
Abraham’s law the strength of fully compacted concrete is inversely proportional to the water-cement ratio.
A. Workability
The degree of workability required depends on three factors. These are the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used. 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.
B. Durability
The durability of concrete is its resistance to the aggressive environmental conditions. High strength concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the water-cement ratio to be used.
C. Maximum nominal size of aggregate
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.
D. Grading and type of aggregate
The grading of aggregate influences the mix proportions for a specified workability and water-cement ratio. 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 aggregate-cement ratio for the desired workability and stipulated water cement ratio. An important feature of a satisfactory aggregate is the uniformity of the grading which can be achieved by mixing different size fractions.
E. Quality Control
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.
I. Mix Proportion designations
The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in massFactors to be considered for mix design
o The grade designation giving the characteristic strength requirement of concrete.
o The type of cement influences the rate of development of compressive strength of concrete.
o Maximum nominal size of aggregates to be used in concrete may be as large as possible
within the limits prescribed by IS 456:2000.
o The cement content is to be limited from shrinkage, cracking and creep.
o The workability of concrete for satisfactory placing and compaction is related to the size
and shape of section, quantity and spacing of reinforcement and technique used for
transportation, placing and compaction.
Mix Design Procedure as per IS:10262.
1. Determine the mean target strength ft from the specified characteristic compressive strength at 28-day fck and the level of quality control.ft = fck + 1.65 S , where S is the standard deviation obtained from the Table of approximate contents given after the design mix.
2. Obtain the water cement ratio for the desired mean target using the emperical relationship between compressive strength and water cement ratio so chosen is checked against the limiting water cement ratio. The water cement ratio so chosen is checked against the limiting water cement ratio for the requirements of durability given in table and adopts the lower of the two values.
3. Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.
4. Select the water content, for the required workability and maximum size of aggregates (for aggregates in saturated surface dry condition) from table.
5. Determine the percentage of fine aggregate in total aggregate by absolute volume from table for the concrete using crushed coarse aggregate.
6. Adjust the values of water content and percentage of sand as provided in the table for any difference in workability, water cement ratio, grading of fine aggregate and for rounded aggregate the values are given in table.
7. Calculate the cement content form the water-cement ratio and the final water content as arrived after adjustment. Check the cement against the minimum cement content from the requirements of the durability, and greater of the two values is adopted.
8. From the quantities of water and cement per unit volume of concrete and the percentage of sand already determined in steps 6 and 7 above, calculate the content of coarse and fine aggregates per unit volume of concrete from the following relations:
where V = absolute volume of concrete= gross volume (1m3 ) 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 fine 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
9. Determine the concrete mix proportions for the first trial mix.
10. 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.
11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.I. MIX DESIGN OF CONCRETE AS PER IS:10262.
The results of mix design indicate that crushed sand can also make as good a concrete as that made of natural sand.The compressive strength obtained is same as of normal mixes.In fact use ofcrushed sand will become inevitable in near future because of dwindling sources of naturalsand. Crushed sand particle though shaped, does not have the spherical shape of naturalsand. Hence the crushed sand will have greater water demand than that of natural sand resulting in slightly higher cement consumption. However, if crushed sand is properly gradedwith adequate fines the mix may have lower water demand when compared to poorly
graded natural sand. Besides crushed sand can afford better control on gradation when compared to natural sand. Hence crushed sand may become an economical option if good quality natural sand is not available.
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TYPES OF CONCRETE MIX DESIGN
Types of Concrete Mix Design
1. Nominal Mixes
In the past the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of strength above that specified. However, due to the variability of mix ingredients the nominal concrete for a given workability varies widely in strength.
2. Standard mixes
The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may result in under- or over-rich mixes. For this reason, the minimum compressive strength has been included in many specifications. These mixes are termed standard mixes.
IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm2. The mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively.
3. Designed Mixes
In these mixes the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. The approach results in the production of concrete with the appropriate properties most economically. However, the designed mix does not serve
as a guide since this does not guarantee the correct mix proportions for the prescribed performance.
For the concrete with undemanding performance nominal or standard mixes (prescribed in the codes by quantities of dry ingredients per cubic meter and by slump) may be used only for very small jobs, when the 28-day strength of concrete does not exceed 30 N/mm2. No control testing is necessary reliance being placed on the masses of the ingredients.
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ACCEPTANCE CRITERIA FOR DESIGN MIX CONCRETE
Design Mix Concrete Acceptance Criteria:
I. The concrete shall be deemed to comply with the strength requirements if:
a) every sample has a test strength not less than the characteristic value;or
b) The strength of one or more samples though less than the characteristic value, is in each case not less than the greater of :
1) The characteristic strength minus 1.35 times the standard deviation; and
2) 0.80 times the characteristic strength ; and the average strength of all the samples is not less than the characteristic strength plus
times the standard deviation.
II. The concrete shall be deemed not to comply with the strength requirements if:
a) The strength of any sample is less than the greater of :
1) the characteristic strength mix is 1.35 times the standard deviation; and
2) 0.80 times the characteristic strength ; or
b) The average strength of all samples is less than the characteristic strength plus
times the standard deviation.
III Concrete which does not meet the strength requirements as specified in I, but has a strength greater than that required by II may, at the discretion
of the designer, be accepted as being structurally adequate without further testing.
IV. Concrete of each grade shall be assessed separately.
V Concrete shall be assessed daily for compliance.
VI. Concrete is liable to be rejected if it is porous or honey-combed; its placing has been interrupted without providing a proper construction joint; the reinforcement has been displaced beyond the tolerances specified; or construction tolerances have not been met. However, the hardened concrete may be accepted after carrying out suitable remedial measures to the satisfaction.
VIII. Where the value of the average strength of the tests (preferably 30 tests or 15 tests) is less than
shall be rejected.
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CONCRETE MIX DESIGN PROCEDURE & EXAMPLE
The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in massFactors to be considered for mix design
o The grade designation giving the characteristic strength requirement of concrete.
o The type of cement influences the rate of development of compressive strength of concrete.
o Maximum nominal size of aggregates to be used in concrete may be as large as possible
within the limits prescribed by IS 456:2000.
o The cement content is to be limited from shrinkage, cracking and creep.
o The workability of concrete for satisfactory placing and compaction is related to the size
and shape of section, quantity and spacing of reinforcement and technique used for
transportation, placing and compaction.
Procedure for Concrete Mix Design – IS456:2000:
1. Determine the mean target strength ft from the specified characteristic compressive strength at 28-day fck and the level of quality control.
ft = fck + 1.65 S
where S is the standard deviation obtained from the Table of approximate contents given after the design mix.
2. Obtain the water cement ratio for the desired mean target using the empirical relationship between compressive strength and water cement ratio so chosen is checked against the limiting water cement ratio. The water cement ratio so chosen is checked against the limiting water cement ratio for the requirements of durability given in table and adopts the lower of the two values.
3. Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.
4. Select the water content, for the required workability and maximum size of aggregates (for aggregates in saturated surface dry condition) from table.
5. Determine the percentage of fine aggregate in total aggregate by absolute volume from table for the concrete using crushed coarse aggregate.
6. Adjust the values of water content and percentage of sand as provided in the table for any difference in workability, water cement ratio, grading of fine aggregate and for rounded aggregate the values are given in table.
7. Calculate the cement content form the water-cement ratio and the final water content as arrived after adjustment. Check the cement against the minimum cement content from the requirements of the durability, and greater of the two values is adopted.
8. From the quantities of water and cement per unit volume of concrete and the percentage of sand already determined in steps 6 and 7 above, calculate the content of coarse and fine aggregates per unit volume of concrete from the following relations:
where V = absolute volume of concrete = gross volume (1m3) 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 fine 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
9. Determine the concrete mix proportions for the first trial mix.
10. 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.
11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.CONCRETE MIX DESIGN EXAMPLE – M50 GRADE CONCRETE
Grade Designation = M-50 Type of cement = O.P.C-43 grade Brand of cement = Vikram ( Grasim ) Admixture = Sika [Sikament 170 ( H ) ] Fine Aggregate = Zone-II
Minimum Cement (As per contract) =400 kg / m3 Maximum water cement ratio (As per contract) = 0.45Mix Calculation: -
1. Target Mean Strength = 50 + ( 5 X 1.65 ) = 58.25 Mpa
2. Selection of water cement ratio:-
Assume water cement ratio = 0.35
3. Calculation of water: -
Approximate water content for 20mm max. Size of aggregate = 180 kg /m3 (As per Table No. 5 , IS : 10262 ). As plasticizer is proposed we can reduce water content by 20%.
Now water content = 180 X 0.8 = 144 kg /m3
4. Calculation of cement content:-
Water cement ratio = 0.35 Water content per cum of concrete = 144 kg Cement content = 144/0.35 = 411.4 kg / m3 Say cement content = 412 kg / m3 (As per contract Minimum cement content 400 kg / m3 ) Hence O.K.
5. Calculation for C.A. & F.A.: -
Volume of concrete = 1 m3 Volume of cement = 412 / ( 3.15 X 1000 ) = 0.1308 m3 Volume of water = 144 / ( 1 X 1000 ) = 0.1440 m3 Volume of Admixture = 4.994 / (1.145 X 1000 ) = 0.0043 m3 Total weight of other materials except coarse aggregate = 0.1308 + 0.1440 +0.0043 = 0.2791 m3
Volume of coarse and fine aggregate = 1 – 0.2791 = 0.7209 m3 Volume of F.A. = 0.7209 X 0.33 = 0.2379 m3 (Assuming 33% by volume of total aggregate )
Volume of C.A. = 0.7209 – 0.2379 = 0.4830 m3
Therefore weight of F.A. = 0.2379 X 2.61 X 1000 = 620.919 kg/ m3
Say weight of F.A. = 621 kg/ m3
Therefore weight of C.A. = 0.4830 X 2.655 X 1000 = 1282.365 kg/ m3
Say weight of C.A. = 1284 kg/ m3
Considering 20 mm: 10mm = 0.55: 0.45 20mm = 706 kg . 10mm = 578 kg .
Hence Mix details per m3 Increasing cement, water, admixture by 2.5% for this trial
Cement = 412 X 1.025 = 422 kg Water = 144 X 1.025 = 147.6 kg Fine aggregate = 621 kg Coarse aggregate 20 mm = 706 kg Coarse aggregate 10 mm = 578 kg Admixture = 1.2 % by weight of cement = 5.064 kg.
Water: cement: F.A.: C.A. = 0.35: 1: 1.472: 3.043
Observations from Concrete Mix Design: -
A. Mix was cohesive and homogeneous. B. Slump = 120 mm C. No. of cube casted = 9 Nos. 7 days average compressive strength = 52.07 MPa. 28 days average compressive strength = 62.52 MPa which is greater than 58.25MPa Hence the mix accepted.Percentage strength of concrete at various ages:
The strength of concrete increases with age. Table shows the strength of concrete different ages in comparison with the strength at 28 days.
Age Strength per cent
1 day 16%3 days 40%7 days 65%14 days 90%28 days 99%
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