Good quality concrete starts with the quality of materials, cost effective designs is actually a by-product of selecting the best quality material and good construction practices. Following are 10 Things to remember during Concrete Mix Design and Concrete Trials.

1. ACI and other standards only serves as a guide, initial designs must be confirmed by laboratory trial and plant trial, adjustments on the design shall be done during trial mixes. Initial design “on paper” is never the final design.

2. Always carry out trial mixes using the materials for actual use.

3. Carry out 2 or 3 design variations for every design target.

4. Consider always the factor of safety, (1.125, 1.2, 1.25, 1.3 X target strength)

5. Before proceeding to plant trials, always confirm the source of materials to be the same as the one used in the laboratory trials.

6. Check calibration of batching plant.

7. Carry out full tests of fresh concrete at the batching plant, specially the air content and yield which is very important in commercial batching plants.

8. Correct quality control procedures at the plant will prevent future concrete problems.

9. Follow admixture recommendations from your supplier

10. Check and verify strength development, most critical stage is the 3 and 7 days strength.

Important note:
Technical knowledge is an advantage for batching plant staff, even if you have good concrete design but uncommon or wrong procedures are practiced it will eventually result to failures.

We at engineeringcivil.com are thankful to Tumi J. Mbaiwa for submitting these 10 points which are helpful to each and every civil engineer.

The temperature resisting cement (Refractory Castable) consists of fine and coarse agregates pre-mixed. When we pour the dry material into gunning machine directly, we normaly land up with 35% rebound waste of material. To reduce this rebound we should first mixed this dry material with 5% water by weight in a paddle mixer machine for 4mins and then pour this pre-dampened mix in the gunning machine. This will give us only 5-10% rebound.

We are thankful to ATIK MIRZA for submitting this very useful information to us.

By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

In India 0.93 kg of CO₂ is emitted in the production of one kg of cement. In the financial year 2009-10 India produces 200 million tonnes of cement. In the production of this cement 186 million tonnes of CO₂ was emitted in the atmosphere during financial year of 2009-10.

The availability of water in India per person per year in 1950 was 5177 cu.m. In the year 2009 it is reduces to 1700 cu.m.

If 50 million tonnes cement in making concrete uses water reducers 7500000 tonnes of cement can be saved. 3750000 kl of potable water will be saved and the saving of Rs. 3300 crores per year to construction industry. This amount is worked out after adjusting the cost of water reducers. Less cement used means less cement required to be produce by the cement factories resulting 6975000 tonnes of CO₂ will be prevented to be emitted to the atmosphere. These are worked out with an average saving of 15% cement and 15% water.

CO₂ emission is word problem, but for India in addition to CO2 it has problems of Air, Water, Soil, Food and Noise pollutions. Less densily populated countries may cope with these problems but for India it is of the top concern. The population figures of 2009 is, India 350 person per sq.km, China 132 person per sq.km and USA only 34 person per sq.km. The figures of 2006 CO¬2 emissions are USA 658.60 tonnes per sq.km, China 611.76 tonnes per sq.km and India 459.35 tonnes per sq.km. Every one should contribute his or her efforts to save the environment from pollution. Those involve in the construction activities can contribute their share by proper design of concrete Mixes. This is best illustrated by the following examples.
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Concrete mix design – M70 grade of concrete provided here is for reference purpose only. Actual site conditions vary and thus this should be adjusted as per the location and other factors.

A. Design Stipulation:
Characteristic comprehensive Strength @ 28 days = 70 N/mm²
Maximum size of aggregate = 20 mm
Degree of workability = Collapsible
Degree of quality control = Good
Type of exposure = Severe
Minimum cement content as per is 456-2000

B. Test data for concrete ingredients
Specific gravity of cement = 3.15
Specific gravity of fly ash = 2.24
Specific gravity of microsilica = 2.21
Setting time of cement initial = 165 min, final = 270min
Cement compressive strength =
39.0 N/mm² @ 3 days
51.0 N/mm² @ 7 days
64.2 N/mm² @ 28 days
Specific gravity of coarse aggregates (ca) and fine aggregates (fa)
20 mm 2.729
10 mm 2.747
R/sand 2.751
C/sand 2.697

Water absorption
20 mm 1.540, 10mm 1.780, R/sand 3.780, C/sand 4.490

Characteristic strength @ 28 days 70 N/mm²
Target mean strength : Depend upon degree of quality control “good” and considering (std. Dev.As 5 N/mm²)

Characteristic strength given by the relation 70 +(1.65 *5 ) = 78.25 N/mm²

C. Quantities of ingredients (By Absolute Volume Method )
Actual cement used = 486 kg/cum
Actual fly ash used = 90 kg/cum
Actual microsilica used = 24 kg/cum
W/C fixed = 0.26
Absolute volume of cement = 0.154
Absolute volume of air = 0.02
Absolute vol of water. = 0.156
Absolute vol of fly ash. = 0.040
Absolute vol of microsilica = 0.011

Total volume of CA and FA used = 1.00-(0.155+0.044+0.022+0.02 +0.154)
= 0.619 Cum

D. Aggregate percent used.
20 Mm = 24, 10 mm = 36, r/sand = 20, c/sand = 20

(2.729*0.24) + (2.747*0.36) +(2.751* 0.20 )+(2.697*0.20) *0.619*1000
405+612+340+334=1691

Aggt: cement = 2.82 : 1

Mix proportion = 0.26:1:0.57:0.56:1.02:0.67

E. Abstract:
20 mm = 405 kg/cum
10 mm = 612 kg/cum
r/sand = 340 kg /cum
c/sand = 334 kg/cum
water = 154 kg/cum

Admixture 0.50 % BY WT OF (C+F+MS) ASTP-1 OF BASF

Cube Compressive Strength (N/mm²)
3 days = 49.13
7 Days = 59.57
28 Days = 81.49

Note: Mix design is same for Crane bucket and Pump concrete only admixture dosage will fine tuned by 0.05 to 0.10%
We are thankful to Deshmukh D S for submitting this very useful mix design information to us.

Concrete mix design – M60 grade of concrete provided here is for reference purpose only. Actual site conditions vary and thus this should be adjusted as per the location and other factors.

A. Design Stipulation:
Charastaristic comprehensive Strength @ 28 days = 60 N/mm²
Maximum size of aggregate = 20 mm
Degree of workability = Collapsible
Degree of quality control = Good
Type of exposure = Severe
Minimum cement content as per is 456-2000

B. Test data for concrete ingredients
Specific gravity of cement = 3.15
Specific gravity of fly ash = 2.24
Specific gravity of microsilica = 2.21
Setting time of cement initial = 120 min, final = 185 min
Cement compressive strength =
45.21 N/mm² @ 3 days
54.82 N/mm² @ 7 days
69.32 N/mm² @ 28 days
Specific gravity of coarse aggregates (ca) and fine aggregates (fa)
20 mm 2.729
10 mm 2.747
R/sand 2.751
C/sand 2.697

Water absorption
20 mm 1.540, 10mm 1.780, R/sand 3.780, C/sand 4.490

Characterstic strength @ 28 days 60 N/mm²
Target mean strength : Depend upon degree of quality control “good” and considering (std. Dev.As 5 N/mm²)

Characteristic strength given by the relation 60 +(1.65 *5 ) = 68.25 N/mm²

C. Quantities of ingredients ( by absolute volume Method )
Actual cement used = 450 kg/cum
Actual fly ash used = 80 kg/cum
Actual microsilica used = 40 kg/cum
W/C fixed = 0.24
Absolute volume of cement = 0.143
Absolute volume of air = 0.02
Absolute vol of water. = 0.137
Absolute vol of fly ash. = 0.036
Absolute vol of microsilica = 0.018

Total volume of CA and FA used = 1.00-(0.143+0.036+0.018+0.02 +0.137)
= 0.619 Cum

D. Aggregate percent used.
20 Mm = 31, 10 mm = 25, r/sand = 34, c/sand = 10

(2.729*0.31) + (2.747*0.25) +(2.751* 0.34 )+(2.697*0.10) *0.619*1000
546+444+604+174=1768
Aggt: cement = 3.10 : 1

Mix proportion = 0.24:1:1.06:0.30:0.78:0.96

E. Abstract:
20 mm 546 kg/cum
10 mm 444 kg/cum
r/sand 604 kg /cum
c/sand 174 kg/cum
water 137 kg/cum

Admixture 1.80 % By wt of (C+F+MS) chemsonite SP 450XL-B

Cube Compressive Strength (N/mm²)
3 days = 40.98
7 Days = 57.71
28 Days = 70.96

Note: Mix design is same for crane bucket and pump concrete only admixture dosage will fine tuned by 0.10%.
We are thankful to Deshmukh D S for submitting this very useful mix design information to us.