Become Concrete Master - Concrete Mix Designing

[Dnyan Deshmukh]

Storage of Aggregates
-Aggregates are transported to site in lorry / haul truck
-Ensure the actual quantity is received (Better to take aggregates on weight basis if you have weight bridge at your site, else you can take actual measurements of material received in cum or cft)
-For fine aggregates do check for bulkage
-Aggregates should be stacked on a well drain platform in bins with wall
-It should be directly unloaded in bins, avoid double handling.
-Do not allow intermixing of aggregates.

Storage of Water
Do not store water for long time in open area.

Storage of Chemical Admixtures
-Chemical Admixtures generally received in barrel or bulker.
-Stack them separately under sunshade.
-Avoid dilution of it.

Storage of Mineral Admixtures
-Received in powder form.
-Received in bags or bulkers.
-To be handled with care.
-Stored in silo or same like cement storage.

 

[Dnyan Deshmukh]

Properties of Concrete

Plastic State / Fresh Concrete

Workability - Slump
Cohesiveness
Slump retention
Setting time

Hardened State

Compressive strength
Cracking

 

[Dnyan Deshmukh]

Workability Slump Cone Test

Collect sample in wheel barrow. Mix it well using scoop.
Place concrete in four layers, tamp each layer 25 times.
Lift cone steadily upwards.
Measure using stainless steel scale or steel tape.
Report to nearest 5mm.

 

[Dnyan Deshmukh]

Cohesiveness of Concrete
When we do the slump test on concrete, if sample is falling away or shearing off from one side or from portion of the mass.
Repeat test should be made on concrete.

If two consecutive tests on a concrete sample shows a falling away or sharing off a portion of the concrete from the mass of specimen, the concrete probably lack of plasticity and cohesiveness.

 

[Dnyan Deshmukh]

Slump Retention
Retention time is nothing but the workability of the concrete which is retained by use of admixture over period of time.
Retention depends on type of admixture use and workability required at what time, pouring time of concrete etc.

Admixture cause the dispersion in cement particle which stop its interaction with water and further avoid its chemical process.
The effect depends on the properties of admixture.

Admixture use should be checked for its effect on properties of concrete like setting, stiffening, bleeding etc.

We will discuss more about hardened concrete properties later in this thread.

 

[Dnyan Deshmukh]

Concrete Mix Proportioning
Grade of Concrete
The concrete should be of grade as given below in table.
Characteristic Strength of concrete is defined as the strength of concrete below which not more than 5% of results expected to fail.

 

[Dnyan Deshmukh]

Durability of concrete:
A concrete which performs satisfactorily in the environment during its anticipated exposure condition in its service is said to be durable one. The materials and mix proportions specified and used should be such as to maintain its integrity and if applicable to protect embedded steel from corrosion.
The main characteristics influencing the durability of concrete is its permeability to the ingress of water, oxygen, carbon dioxide, chloride, sulphates and other potentially deleterious substances. Impermeable concrete is governed by constituents of concrete and workmanship in making concrete. In normal conditions with normal weight aggregates a suitable low permeability can be achieved by having an adequate cement content, low w/c ratio, ensuring complete compaction and by adequate curing.

The factors influencing durability are
• The environment
• The cover to embedded steel
• The type and quality of constituent materials
• The cement content and w/c ratio used
• Workmanship to obtain full compaction
• Adequate curing and shape of the member.

The various exposure condition are tabulated below

 

[Dnyan Deshmukh]

Minimum Cement content, Maximum water cement ratio and Minimum grade of concrete for different exposure condition with normal weight aggregates of 20mm nominal size.


Requirement of concrete exposed to Sulphate attack

 

[Dnyan Deshmukh]

Chlorides in concrete:
If chloride is present in concrete, there is increased risk of corrosion of embedded steel. The higher the chlorides content or if the structure is exposed to warm moist conditions, the greater is the risk of corrosion. All the constituents may contain chloride and concrete may have contaminated by chloride from environment. To minimize the change of deterioration of concrete from harmful chemical salts, the levels of such harmful salts in concrete coming from concrete making material like cement. aggregate, water and admixture, as well as by diffusion from the environment should be limited. The total acid soluble chloride content should be calculated from the mix proportion and the measured chloride content of each of the constituents. The total chloride content (as Cl ) in concrete at the time of placing shall be as given below;

 

[Dnyan Deshmukh]

Sulphates in concrete
Sulphates are present in most cement and in some aggregates, excessive amounts of water-soluble sulphate from these or other mix constituents can cause expansion and disruption of concrete. To prevent this the total water soluble sulphate content of the concrete mix expressed as SO3 , should not exceed 4 % by ,mass of cement in the mix. The sulphate content to be calculated as the total from the various constituents of the mix.

 

[Dnyan Deshmukh]

Alkali – aggregate reaction
Some aggregates contain particular variety of silica which react with the alkali (Na2O and K2O) of cement, producing expansive reaction which cause cracking and disruption of concrete. Such disruption occurs when

a) A high moisture level is present with in the concrete,
b) A cement with alkali content or another source of alkali,
c) Aggregates containing an alkali reactive mineral

When past data is available on the cement aggregate combination and which does not indicate any cracking due to alkali aggregate reaction, no further precaution is required. If the data is not known take the one of the following precautions

1) Use of non-reactive aggregate from alternate source
2) Use of Low alkali ordinary Portland cement having total alkali content not more than 0.6 percent (as Na2O equivalent) or using 20 % fly ash or 50 % GGBFS as replacement to OPC.
3) Limiting the total cement content in the concrete mix , there by limiting total alkali content in concrete mix.

 

[Dnyan Deshmukh]

Mix proportioning (design mix calculation)

The concrete mix should be proportioned in such a way, it should give a uniform colour, easily transported handled and placed in its final position without difficulty and has given the required shape of the member as designed.

According to IS 456-2000, “As the guarantor of quality of concrete used in the construction, the constructor shall carryout the mix design and mix so designed (not the method of design) shall be approved by the employer within the limitations of parameters and other stipulations laid down by the standard IS 456-2000”.

Before starting the mix design, the following details are to be looked into

Basic Data

• Grade of concrete
• Workability of concrete
• Exposure condition
• Minimum cement content
• Maximum size of aggregate to be used.

Technical Data

• Type and brand of cement to be used
• Specific gravity of cement, fly ash (if used), coarse aggregate, fine aggregates & admixture (if any)
• Source of coarse aggregate, fine aggregates, fly ash (if used)
• Water absorption of coarse and fine aggregates
• Proportion of coarse aggregates and fine aggregates in the mix proportion derived from sieve analysis.
• Dosage of admixture to be used (if any)

 

[Dnyan Deshmukh]

Design calculations

Step 1 Find out the target strength of the mix designed


Where,
fck is grade of concrete
k is expected percentage of failure ( in general it is 1.65 i.e. 5 %)
s is the assumed standard deviation

 

[Dnyan Deshmukh]

Step 2 : Estimate the amount of mixing water for the slump and for the selected/given maximum size of aggregate from table

 

[Dnyan Deshmukh]

Step 3 : Select the water cement ratio for the type of exposure and for sulphate attack from table .

Step 4 : Calculate cement content as per water cement ration water quantity
cement quantity = estimated mixing water quantity from step 1 / water cement ration from step 3

Step 5 : Check the cement content calculated for durability of concrete (click here to go to tables)

Step 6 : Find the absolute volume of cement and water per cum.

absolute volume = Weight of material (Kg) / Specific Gravity of material x 1000

Step 7 : Find the volume of coarse and fine aggregate

For this subtract the absolute volume determined in step 6 from 1 cum.

Step 8 : Multiply the value obtained by the weighted average specific gravity of coarse and fine aggregate and by 1000 to get quantity of coarse and fine aggregate

Step 9 : Proportion the quantities of coarse and fine aggregates derived from step 9 in the ratio proportioned in sieve analysis.

Step 10 : Adjust for moisture in aggregate and water absorption by aggregates.

Step 11 : Adjust weight of aggregate to compensate for moisture content in them

Step 12 : Make trial mixes.

Step 13 : After doing trial mix, If adjustments are made to achieve the desired properties of concrete, recalculate the revised proportions based on the adjustments made.

Step 14 : Cast 15 x 15 x 15 cm cube specimen to check compressive strength at 7 days and 28 days. Minimum of 3 cubes to be cast for each age for each trial.

 

[Dnyan Deshmukh]

Adjustment of trial proportions

a) Express the trial proportion in the form of cement : fine aggregate : coarse aggregate by weight and determine the batch quantities for a convenient batch (say 0.025 cum.)

b) Batch the required quantity of material.

c) For hand mixing take a clean G I tray and first mix cement with fine aggregate. Then add coarse aggregate to it and mix. Finally add water and mix well to get a homogeneous mix.

d) For mechanical mixing first butter the mixer drum with either a 1:2 cement mortar of pasty consistency or a mix proportion with coarse aggregate removed from it. Discard the excess buttering mix from the drum. Mixer drum is now coated with a lining of mortar. Add the batch material in to the mixer and mix it for atleast 1 ½ to 2 minutes.

e) Discharge the mix into a clean surface and observe the following.

• Whether slurry (cement and water) separate from the aggregates. If so increase fine aggregate content and correspondingly reduce coarse aggregate content. Weight of coarse aggregate + fine aggregate is kept constant always.

Concrete mix with less of fine aggregate is termed as “Under sanded”

Press a trowel gently down on the heap of concrete and slide it once or twice horizontally. If fine aggregates (sand) particles come out of the concrete surface as excess then reduce the fine aggregate content and correspondingly increase the coarse aggregate. Concrete mix with more of fine aggregate is termed as “over sanded”.
Click here to understand how to optimize aggregates content in mix

Note : use over sanded concrete mix for tremie and pumped concrete.


f) Check the slump of the mix. If the slump is not in the range aimed, verify the following:

• Too much delay in checking slump after discharging from mixer. Slump test should be done within 5 to 10 minutes after mixing.
• Too high an ambient temperature . Lower slumps are obtained at temperature above 30 degrees.
• Quantity of water added should be the quantity calculated after correction for absorption by aggregates and moisture in aggregates.
• If above steps are all checked and still the slump is not achieved then, decrease or increase the quantity of water to get the correct slump. Note the quantity of water removed or added and correct the total quantity of water suitably for further trials.

g) For trial proportion finalized cast six cubes atleast three cubes being available for 28 days.

 

[Dnyan Deshmukh]

Will post a mix design calculation for say M20 grade concrete and explain its done in next post.

Go to the top of this thread and click on watch, this will email you about new posts in this thread, so you wont miss it!

 

[Pankaj Sharma]

Awesome Article Sir

 

[Shubham Gomare]

Physical Requirements of Cement as per IS Codes

View attachment 260

Which cement should be preferred for plaster to avoid cracks. Does fineness affect the strength of cement & how

 

[köustav Mükhêrjèé]

What a content sir?!!!