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Mean of the Group of 4 Non-Overlapping Consecutive Test Results In N/mm^2
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es_jayakumar
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PostPosted: Tue Mar 24, 2020 3:10 pm    Post subject: Reply with quote

An article contributed by me to the "Structural Engineering Digest" of "Indian Association of Structural Engineers" (IASE) is enclosed. I hope it will elucidate the present discussion on the acceptance criteria of concrete.

E S Jayakumar



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rajeevan_b
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PostPosted: Wed Mar 25, 2020 4:52 am    Post subject: Cube Tests Reply with quote

In many of the works, only 3 cubes are presented for testing for a particular work. As per this, it means we need to consider 3 cubes are considered as one SAMPLE only and that a cube is considered as a SPECIMEN. That means, for a 30 m^3 concreting we need to take 3 SAMPLES minimum , which is 3 x 3 = 9 cube SPECIMENS to be tested to arrive at the acceptance criteria?
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es_jayakumar
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PostPosted: Wed Mar 25, 2020 6:14 am    Post subject: Reply with quote

Yes, Er.Rajeevan. That much cubes will have to be cast and tested. The Code aims at stringent quality control. Many engineers are seen to test 7 day's compressive test too. This is not essential, unless the situation necessitates a quick estimation of the compressive strength.

E S Jayakumar
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ssmann
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PostPosted: Wed Mar 25, 2020 11:57 am    Post subject: Acceptance criterian of concrete Reply with quote

1. Why we forget the flexural strength criterian besides compressive strength as per 16.1 and 16.2 of Is:456 which may be critical for individual test as per column 3 of table 11(is-456).
2. We should should differentiate between nominal mix criteria and design mix criteria. After going through through CPWD specification(5.9.3.3) vis-a-vis IS:456-2000(15.2.2) regarding test frequency, it seems in IS code(15.2.2) is meant for nominal mix. In CPWD specs it is clearly mentioned frequency of 50 cum for design mix.
3. eg. in design mix(M-30) pour card is of 30cum, then only one sample(28 days-3cubes) will be tested and will be considered for individual test- means not less than 29.7 N/mm2.
4. Standard deviation in table 11 is the established standard deviation.

S.S.Mann
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PostPosted: Thu Mar 26, 2020 2:14 am    Post subject: Reply with quote

We are designating the grade of concrete based on its “Characteristic Compressive Strength” (fck). Hence, this outranks the flexural strength. Moreover, determination of flexural tensile strength (modulous of rupture) of concrete is more involved, as compared to determining the direct compressive strength, which can be easily performed by a compression testing machine. For testing flexural tensile strength, you have to cast a PCC beam and carryout a bending test, by third point loading. I am not aware of, if there is any method to test the flexural compressive strength of concrete directly.

Clause 5.9.3.3 of CPWD Specifications 2019 (Vol-I) is adopted from IS 4926:2003, “Ready-mix Concrete – Code of Practice” , which is reproduced below :
6.3 Specified Strength
6.3.1 Compliance shall be assessed against the requirements of IS 456 or other agreed Indian Standard. The purchaser may perform his own sampling and testing or may enter in to an arrangement with the producer to provide his testing requirements.
6.3.2 Unless otherwise agreed between the parties involved, the minimum testing frequency to be applied by the producer in the absence of a recognized ready-mixed concrete industry method of production control, should be one sample for every 50 m3of production or every 50 batches, whichever is the greater frequency. Three test specimens shall be made up for each sample for testing at 28 days (see also IS 456).

If we go by the first sentence of 6.3.1 above :
The IS 456 : 2000 at 15.2.2 stipulates :
NOTE-At least one sample shall be taken from each Shift. Where concrete is produced at continuous production unit, such as ready-mixed concrete plant, frequency of sampling may be agreed upon mutually by suppliers and purchasers.
The heading of this clause 15 is “SAMPLING AND STRENGTH OF DESIGNED CONCRETE MIX”. That means, it is Design Mix concrete, and not Nominal Mix.
We follow CPWD Specifications in our Department. Under my office, we go by the provision of Clause 15.2.2 table for the minimum frequency of sampling of concrete, irrespective of if the concrete is either RMC or site mixed Design Mix Concrete, for the sake of better quality assurance. It will be prudent for the user to increase the frequency of their own testing at site, even if the supplier tests his RMC at a frequency of 50 m3 of concrete produced.

In Table-11, one has to use the established value of Standard Deviation. For computing this, one has to test a minimum of 30 samples. Till 30 samples are reached, the assumed SD values given in Table-8 can be adopted.

E S Jayakumar
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ssmann
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PostPosted: Fri Mar 27, 2020 6:39 am    Post subject: Acceptance criterian of concrete Reply with quote

Er. E.S. JayaKumar, I admire yr knowledge and contribution in this forum. Can you clarify more on:
1. Concrete Grade designated based on its “Characteristic Compressive Strength” (fck)  outranks the flexural strength which is acceptance critera of conc measured in terms of compressive strength. Finding the testing procedure for flexural strength is like finding derivation of empirical relation?
2. What would be accepted compressive strength in resembling query of Er Rajeevan_B in case of individual test (3 cubes from casting 4 cum of M-30 grade conc.).
3. What is sample frequency of ordinary grade conc. of nominal mix as per IS:456-2000?



Above are my queries with limited knowledge.Hope revised IS Code gets simpler and more clarified. Thanks in advance.

S.S. Mann
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es_jayakumar
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PostPosted: Fri Mar 27, 2020 9:54 am    Post subject: Reply with quote

Thank you, Er.Mann, for your complimenting words.  I try to answer your queries, though those are not fully understood by me :
1. IS 456 : 2000 says :
16.2 FIexural Strength
When both the following conditions are met, the concrete complies with the specified flexural strength.

It is not clear to me, as to in which situation does some entity specify strength requirement (or designation) of concrete in terms of Flexural Strength. Does “Flexural Strength” correspond to compressive or tensile ? If it is flexural tensile strength ( or Modulus of Rupture – fcr), then it can be obtained by conducting beam bending test. It is also possible to correlate it to fck using the equation fcr = 0.7 x sqrt(fck) (Cl.6.2.2 of IS 456 : 2000)

2. Question is not clear. If  3 cubes were cast for 4 cum of M-30 concrete, then the compressive strength requirement for the average strength of the 3  cubes (one sample) is fck + 4, ie, 30+4 = 34 N/mm2.

3. The following excerpt from IS 456 : 2000 on Mix Proportion is worth considering here :
9.1.1 The determination of the proportions of cement, aggregates and water to attain the required strengths shall be made as follows:
a) By designing the concrete mix; such concrete shall be called ‘Design mix concrete’, or
b) By adopting nominal concrete mix; such concrete shall be called ‘Nominal mix concrete’.
Design mix concrete is preferred to nominal mix. If design mix concrete cannot be used for any reason on the work for grades of M 20 or lower, nominal mixes may be used with the permission of engineer-in-charge, which, however, is likely to involve a higher cement content.

The above indicates that the use of Nominal Mix Concrete Should be discouraged. Maybe due to that, probably, there is no Acceptance Criteria for Nominal Mix concrete prescribed in IS 456 : 2000.
The Explanatory Handbook SP 24 : 1983 provides :
I4. SAMPLING AND STRENGTH TEST OF CONCRETE
14.0 The sampling scheme given in 14 and the acceptance criteria given in 15 are
applicable to both design mix concrete and nominal mix concrete.
…………………………………………………………………………..
14.2.2……………………………………………………………………

For batches of volume less than 1 m3; it will not be necessary to obtain a sample for
every batch or every day but it is preferable to ensure that at least one sample (consisting
of 3 specimens) is available for every 5 m3 of concrete placed.


Table 9 of IS 456 : 2000 gives the details of the proportion for nominal mix based on weight of the ingredients.
SP 24 : 1983 at Cl.8.3 further suggests :
A rough guide for the nominal mix proportions by volume will be 1:3:6, 1:2:4 and 1:11/2:3 for M 10, M I5 and M 20 concretes respectively, provided that the nominal maximum size of aggregate is 20 mm. [ This may have become obsolete (or discontinued) in the present era !]
For nominal mix of concrete, either weight batching or volume batching, one can also go by CPWD Specifications 2019 (Vol.I) to determine the frequency of sampling, as given blow.

E S Jayakumar



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