9/10/2016
What Grade of Concrete (M20/M25) used in RCC Roof Casting?
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What is the grade of
concrete(M20/M25 etc.) normally used in roof
casting? How much PSC cement is required for
making 1 cubic meter of concrete?
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Home (http://theconstructor.org) / Concrete (http://theconstructor.org/question-category/concrete/) / What is the
grade of concrete(M20/M25 etc.) normally used in roof casting? How much PSC cement is required for making 1 cubic
meter of concrete?
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9/10/2016
What Grade of Concrete (M20/M25) used in RCC Roof Casting?
What is the grade of concrete(M20/M25 etc.)
normally used in roof casting? How much PSC
cement is required for making 1 cubic meter of
concrete?
 Question
I am building a small construction of about 700 square foot(picture attached). In
roof casting my mason has used 60 bags of PSC cement though he estimated 74
bags before. I am in serious doubt what is going on. Could you please help me?
 Attachment (http://theconstructor.org/wpcontent/uploads/2016/06/13321797_1196003027096577_2269556106714869837_n.jpg)
 solved
5
 Concrete (http://theconstructor.org/question-category/concrete/)
 Pal Chakraborty (http://theconstructor.org/pro䕕ሥle/pal 3 months
chakraborty/)
 6 Answer (http://theconstructor.org/questions/grade-concrete-m20-m25-roof-casting-cement-required1m3-concrete/#comments)
 5603 views
Answers ( 6 )
Gopal Mishra (http://theconstructor.org/author/gopal­mishra/)
http://theconstructor.org/questions/grade­concrete­m20­m25­roof­casting­cement­required­1m3­concrete/
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9/10/2016
What Grade of Concrete (M20/M25) used in RCC Roof Casting?
Gopal Mishra (http://theconstructor.org/author/gopal­mishra/)
 Jun 2, 2016 at 12:16 (http://theconstructor.org/questions/grade­concrete­m20­m25­
roof­casting­cement­required­1m3­concrete/#comment­26019)
Hi Pal Chakraborty,
As seen from image you have attached, I have estimated that the roof
area of 66.2 Square Meter and deduction of 3.62 square meter for
staircase area as shown in plan you have provided. So, net area of RCC
Slab is taken as 62.58 square meter. Though image is not clear, I have
taken the dimensions as it appeared to me.
You have also not mentioned the thickness of reinforced concrete slab. I
have assumed that the slab has a thickness of 150mm. Thus the total
volume of concrete required by you 62.58*0.15 = 9.387 cubic meter of
concrete.
If you use the nominal concrete mix, the generally used concrete in
residential building is M20 with cement : sand : aggregate proportion as 1
: 1.5 : 3.
Thus, for the given mix proportion, the quantity of cement required is 7.9
bags per cubic meter of concrete. So, the total quantity of cement
required for your M20 roof construction of 150mm thickness should be
74.157 bags of cement.
If your mason has used only 60 bags of cement, then he has not used
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3/20
9/10/2016
What Grade of Concrete (M20/M25) used in RCC Roof Casting?
If your mason has used only 60 bags of cement, then he has not used
M20 concrete for your roof construction. If we consider M15 concrete, the
mix proportion is 1 : 2 : 4. One cubic meter of M15 concrete requires 6.2
bags of cement. Thus for 9.387 cubic meter, cement bags required will be
58.2 bags.
Thus, your mason has used the ratio of cement sand and aggregates close
to M15 concrete. As per Indian Standard, the minimum grade of concrete
for residential reinforced work shall be M20.
Refer this table for quantities of material required per meter cube of
concrete (http://theconstructor.org/practical­guide/estimated­quantities­
of­materials­required­per­cubic­metre­of­compacted­mortar­or­
concrete/1304/).
 Best answer
Pal Chakraborty (http://theconstructor.org/profile/pal­chakraborty/)
 Jun 3, 2016 at 08:58 (http://theconstructor.org/questions/grade­concrete­m20­m25­
roof­casting­cement­required­1m3­concrete/#comment­26026)
Dear Gopal,
http://theconstructor.org/questions/grade­concrete­m20­m25­roof­casting­cement­required­1m3­concrete/
4/20
9/10/2016
What Grade of Concrete (M20/M25) used in RCC Roof Casting?
As per as the following document(p­20, Table: 5) cement required for
M20 grade concrete is 320 kg/cum.
http://www.nitsri.net/Civil/course­material/IS%20456­2000.pdf
(http://www.nitsri.net/Civil/course­material/IS%20456­2000.pdf)
Please let me know your view
Gopal Mishra (http://theconstructor.org/author/gopal­mishra/)
 Jun 3, 2016 at 13:42 (http://theconstructor.org/questions/grade­concrete­m20­m25­
roof­casting­cement­required­1m3­concrete/#comment­26028)
Dear Pal,
The table 5 of IS456:2000 specifies the minimum quantity of cement
required for different grades of concrete. This does not mean that
minimum quantity of cement has to be used for the construction.
During the mix design calculation, if the cement required is below 300
kg/m3 of concrete, then 300 kg/m3 is only taken as the cement content.
Let us take an example to find out the cement quantity calculation for M20
Concrete.
Cement Calculation for M20 (1:1.5:3 )
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9/10/2016
What Grade of Concrete (M20/M25) used in RCC Roof Casting?
Cement Calculation for M20 (1:1.5:3 )
Concrete:
For a normal reinforced concrete slabs, the medium workability of
concrete required. This corresponds to 50 to 10mm slump value (Refer
Page­7, IS456:2000).
Consider, the slump value required in our case as 75mm for a good
workable concrete with use of vibrator. If vibrator is not used, then
100mm slump should be considered.
Refer Table­2, section 4.2 page 2 of IS 10262­2009. This section provides
the details for maximum quantity of water to be used for 20mm
aggregates for slump 25 to 50mm. In our case slump considered is
75mm. So, quantity of water required is:
Water content = 186 + (3/186)x100 = 191.6 kg (liters)
(increase water
content by 3%)
Thus, if we consider a water cement ratio of 0.5, then
quantity of cement required = 191.6/0.5 = 383.2 kg (7.66 bags of
cement).
Thus, 383.2 kg of cement is greater than 300 kg minimum as per IS456.
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What Grade of Concrete (M20/M25) used in RCC Roof Casting?
Remember, we have considered only a slum of 75mm, generally used
slump in residential construction is 100mm. Also, we have assumed a
water­cement ratio of 0.5, for better strength normally 0.45 is used.
So, if you consider 100mm slump and 0.45 water cement ratio, then
water quantity required = 186 + (6/186)x100 = 197.16 kg of water
Cement quantity required = (197.16 / 0.45 ) = 425.8 kg (8.5 bags)/m3 of
concrete.
The maximum quantity of water is considered in this example. There are
many factors which influence the calculation of water and cement
quantities for 1 cubic meter of concrete. This calculation is just to get an
idea.
Above example is the most simplest method of cement quantity
calculation. Hope, I answered your question.
You can also refer calculations section ANNEX­A of IS 10262­2009
(https://law.resource.org/pub/in/bis/S03/is.10262.2009.pdf).
Pal Chakraborty (http://theconstructor.org/profile/pal­chakraborty/)
 Jun 3, 2016 at 15:24 (http://theconstructor.org/questions/grade­concrete­m20­m25­
roof­casting­cement­required­1m3­concrete/#comment­26029)
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What Grade of Concrete (M20/M25) used in RCC Roof Casting?
Dear Gopal,
Thanks a lot for your clarifications. Could you please let me know what are
the problems I shall face due to the construction of roof with M15
standard concrete? Please let me know do I need to break the casting and
re­construct? Please suggest me some remedy to prevent any damage in
future.
Gopal Mishra (http://theconstructor.org/author/gopal­mishra/)
 Jun 4, 2016 at 00:17 (http://theconstructor.org/questions/grade­concrete­m20­m25­
roof­casting­cement­required­1m3­concrete/#comment­26030)
Hi Pal,
There can be problems and there may not be any problem with M15 grade
concrete in RCC roof. It all depends on the structural design of slab. For
structural design of slab following factors are considered:
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Based on all the above data, when we calculate the reinforcement
required for a M15 concrete, and compare it with the reinforcement
provided, then only we can say whether the slab is fit for its intended
purpose or not. Whether the slab is safe for the live load considered or
not.
So, Just by knowing that concrete of M15 has been used, it does not make
the roof slab unsafe. We have to technically prove that the slab is unfit for
the intended purpose only by calculation.
Prior to IS456­2000, the then M20 concrete mix proportion was current
M15 mix proportion (i.e. 1:2:4). But considering the poor workmanship
and quality control at construction site, the M20 concrete mix proportion
was changed to 1:1.5:3 just to make sure the RCC structure has enough
strength even in the worst conditions of poor quality control.

So, hope that if the quality of construction is good for your roof slab, then
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this should not cause any problem. But, I am not able to clearly say
whether it is suitable or unfit as I do not have required data.
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 Aug 20, 2016 at 16:27 (http://theconstructor.org/questions/grade­concrete­m20­m25­
roof­casting­cement­required­1m3­concrete/#comment­26150)
Hi Gopal,
thank you for the information. but the data shown in table shows that
weight in kg is 395kg of cement for M20 and interms of bag its 7.9 bags.
that means the total kgs for per cubic meter is equal to 3120kgs?
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