Concrete mix design procedure is the process in which the suitable ingredients of concrete and their relative amounts are selected. Such that the concrete produces required strength, workability and durability. The performance of the concrete mix design is depend upon its two characteristics, plastic and hardened state. Plastic concrete mix design helps in compaction and placing of concrete while the hardened concrete mix design procedure gives strength to the concrete.
However this property of concrete depends upon many factors such as quality and quantity of cement, quantity of water and aggregate, batching and mixing, placing the concrete, compacting, workability and lastly curing. But rich mix concrete may lead to the shrinkage and cracks on the structure. Concrete works leads to the high budget economy, therefore its cost depends on the cost of material, plant for production and labour.
Must Read :- CEMENT PRODUCTION METHODS AND TECHNIQUES
Requirements of concrete mix design procedure
The requirements which form the basis of selection and proportioning of mix ingredients are :
a ) The minimum compressive strength required from structural consideration
b) The adequate workability necessary for full compaction with the compacting equipment available.
c) Maximum water-cement ratio to give adequate durability for the particular site conditions
d) Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete.
EFFECTS OF CHEMICAL ADMIXTURES ON CONCRETE PROPORTIONS
ACI 211.1-91, Reapproved 2002, states: “Chemical admixtures, pozzolanic, and other materials can be added to concrete mix to alter some properties or to produce desired characteristics. Additives are used to affect the workability, consistency, density, strength, and durability of the concrete.”
Concrete Mix Design Procedure
- First step involve in the concrete mix design procedure is to determine the mean target strength f_{t} from the specified characteristic compressive strength at 28-day f_{ck} and the level of quality control.
f_{t} = f_{ck} + 1.65 S
where S is the standard deviation obtained from the Table of approximate contents given after the design mix.
- Limitations on the water-cement ratio and the minimum cement content to ensure adequate durability
- In 3rd step of concrete mix design procedure we have to estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.
- Select the water content, for the required workability and maximum size of aggregates (for aggregates in saturated surface dry condition) from table.
- Determine the percentage of fine aggregate in total aggregate by absolute volume from table for the concrete using crushed coarse aggregate.The table given below is for the 5th step of concrete mix design procedure which shows the list of most necessary tests to be done on cement, coarse aggregate, fine aggregate and admixture. After doing the test, store the test data for further calculation.
Concrete Ingredients
Tests to be done
Cement
Specific gravity
—
—
—
Coarse aggregate
Specific gravity
Water absorption
Free surface moisture
Sieve analysis
Fine aggregate
Specific gravity
Water absorption
Free surface moisture
Sieve analysis
Admixture
(if any)
Specific gravity
—
—
—
- 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 for the concrete mix design procedure. Table-A for finding out water-cement ratio of Plain Concrete. Table-B for finding out water-cement ratio of Reinforced Concrete.
Table -A ( W/C For Plain Concrete) Sl.No. Environmental Exposure Condition Plain Concrete Minimum Cement Content (kg/m3) Maximum Free Water-Cement Ratio Minimum Grade of Concrete 1 Mild 220 0.60 — 2 Moderate 240 0.60 M15 3 Severe 250 0.50 M20 4 Very Severe 260 0.45 M20 5 Extreme 280 0.40 M25 Table -B ( W/C For Reinforced Concrete) Sl.No. Environmental Exposure Condition Reinforced Concrete Minimum Cement Content (kg/m3) Maximum Free Water-Cement Ratio Minimum Grade of Concrete 1 Mild 300 0.55 M20 2 Moderate 300 0.50 M25 3 Severe 320 0.45 M30 4 Very Severe 340 0.45 M35 5 Extreme 360 - 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.
- 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 (1m^{3}) minus the volume of entrapped air
S_{c} = 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
f_{a}, C_{a} = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and
S_{fa}, S_{ca} = specific gravities of saturated surface dry fine and coarse aggregates, respectively
Sl.No. |
Nominal Maximum Size of Aggregate (mm) |
Volume of coarse aggregate per unit volume of total aggregate for different zones of fine aggregate |
|||
Zone IV |
Zone III |
Zone II |
Zone I |
||
1 |
10 |
0.50 |
0.48 |
0.46 |
0.44 |
2 |
20 |
0.66 |
0.64 |
0.62 |
0.60 |
3 |
40 |
0.75 |
0.73 |
0.71 |
0.69 |
- Determine the concrete mix design proportions for the first trial mix.
- Prepare the concrete mix design 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.
- Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.
Concrete Mix Design Procedure and Example – M40 Grade Concrete
The concrete mix design procedure for M-40 grade for Pier (Using Admixture – Fosroc) is provided here for reference purpose only. Actual site conditions vary and thus this should be adjusted as per the location and other factors.
Parameters for mix design M40
Grade Designation = M-40
Type of cement = O.P.C-43 grade
Brand of cement = Vikram ( Grasim )
Admixture = Fosroc ( Conplast SP 430 G8M )
Fine Aggregate = Zone-II
Sp. Gravity Cement = 3.15
Fine Aggregate = 2.61
Coarse Aggregate (20mm) = 2.65
Coarse Aggregate (10mm) = 2.66
Minimum Cement (As per contract) = 400 kg / m^{3}
Maximum water cement ratio (As per contract) = 0.45
Mix Calculation for concrete mix design procedure :-
1. Target Mean Strength = 40 + (5 X 1.65) = 48.25 Mpa
2. Selection of water cement ratio:-
Assume water cement ratio = 0.4
3. Calculation of cement content: –
Assume cement content 400 kg / m^{3}
(As per contract Minimum cement content 400 kg / m^{3})
4. Calculation of water: –
400 X 0.4 = 160 kg Which is less than 186 kg (As per Table No. 4, IS: 10262)
Hence o.k.
5. Calculation for C.A. & F.A.: – As per IS : 10262 , Cl. No. 3.5.1
V = [ W + (C/S_{c}) + (1/p) . (f_{a}/S_{fa}) ] x (1/1000)
V = [ W + (C/S_{c}) + {1/(1-p)} . (ca/S_{ca}) ] x (1/1000)
Where
V = absolute volume of fresh concrete, which is equal to gross volume (m^{3}) minus the volume of entrapped air ,
W = mass of water ( kg ) per m^{3} of concrete ,
C = mass of cement ( kg ) per m^{3} of concrete ,
S_{c} = specific gravity of cement,
(p) = Ratio of fine aggregate to total aggregate by absolute volume ,
(fa) , (ca) = total mass of fine aggregate and coarse aggregate (kg) per m^{3} of
Concrete respectively, and
Sfa , Sca = specific gravities of saturated surface dry fine aggregate and Coarse aggregate respectively.
As per Table No. 3 , IS-10262, for 20mm maximum size entrapped air is 2% .
Assume F.A. by % of volume of total aggregate = 36.5 %
0.98 = [ 160 + ( 400 / 3.15 ) + ( 1 / 0.365 ) ( Fa / 2.61 )] ( 1 /1000 )
=> Fa = 660.2 kg
Say Fa = 660 kg.
0.98 = [ 160 + ( 400 / 3.15 ) + ( 1 / 0.635 ) ( Ca / 2.655 )] ( 1 /1000 )
=> Ca = 1168.37 kg.
Say Ca = 1168 kg.
Considering 20 mm : 10mm = 0.6 : 0.4
20mm = 701 kg .
10mm = 467 kg .
Hence Mix details per m^{3}
Cement = 400 kg
Water = 160 kg
Fine aggregate = 660 kg
Coarse aggregate 20 mm = 701 kg
Coarse aggregate 10 mm = 467 kg
Admixture = 0.6 % by weight of cement = 2.4 kg.
Recron 3S = 900 gm
Water: cement: F.A.: C.A. = 0.4: 1: 1.65: 2.92
Also Read :-
Observation: –
A. Mix was cohesive and homogeneous.
B. Slump = 110mm
C. No. of cube casted = 12 Nos.
7 days average compressive strength = 51.26 MPa.
28 days average compressive strength = 62.96 MPa which is greater than 48.25 MPa