CONCRETE
SCOPE
This specification establishes the material, mixing, placing, curing, etc., of all types of cast – in - situ concrete to be used in foundations, underground and above ground structures, floors, etc., Any special requirement as shown or noted on the drawings shall govern over the provisions of this specifications. All work should be carried out with proper scientific knowledge and practice and it is obligatory that the contractor shall employ a qualified and competent Engineer for the said purpose.
MATERIALS
CEMENT (IS : 269)
Cement shall be ordinary Portland cement with a 7 days strength of not less than 220 Kg / cm2 unless otherwise specified, obtained from approved suppliers and shall be stored in a waterproof / weatherproof shed in a manner approved by the Engineer. No cement that has been allowed to deteriorate in quality or that has become caked or has perished by dampness or otherwise shall under any circumstances be used on the works. The concrete mix proportions shall be used on the weight of cement bags as delivered at the mixer and not on their theoretical weight. The contractor should allow in his rates for losses in weight of cement bags in transit and handling as indicated in special terms of contract part I.
STORAGE
The contractor shall follow accepted good practice in handling and storing cement. Cement may be stored on site in moisture proof bulk containers, which shall be equipped with venting arrangements. Cement delivered in bags shall be stored off the ground (at lest 15 cms.) in dry, well ventilated, weather – proof / water proof sheds, arranged in separate consignments as received from the manufacturer so that consumption of cement is ensured in the order of receipt. I.e., ‘First in First Out’ rule. The stacks of cement shall be such that there is easy access for proper inspection and identification. The bags shall be piled not more than 15 bags per pile and placed close together in the pile to reduce circulation of air. Each stack of cement shall be covered with good water proof tarpaulin or thick polyethylene sheets.
USE
Cement shall be used in the order in which it is received. Cement in bags in storage for more than 3 months shall be rest before use.
TESTING
Testing shall be carried out on cement delivered to the site for fineness, initial and final setting time, and compressive strength (IS : 4031) and the results should be approved by the Engineer before use of the cement in permanent works. Samples shall be taken immediately on receipt of cement at site. The methods and procedure of sampling shall be in accordance with IS : 3535. The Engineer may specify other forms of sampling and tests, if in his opinion the cement is of doubtful quality, the costs of such additional tests shall be borne by the contractor, if supplied by him.
FINE AGGREGATE (SAND) (IS : 383)
It shall be of river or pit sand or, if permitted by the Engineer, crushed stone sand (produced from crushing stone suitable for concrete aggregates) and all fine aggregates shall be sharp, free from excess fines, loam earth, vegetable matter, soluble salts and other harmful chemical organic impurities and shall be clean. If considered by the Engineer as necessary, the same shall be washed. Washing shall be done at least one day before using it in concrete. The aggregate should be stored in such a manner as to avoid contamination.
Fine aggregates acceptable for the works shall normally have a grading which falls within the following limits
GRADING LIMITS FOR FINE AGGREGATES (IS : 383)
IS Sieve Designation / Equivalent B.S. Sieve No. / Natural Sand or crushed gravel sand / Crushing stone sand4.75 mm / 3/1600 / 90 – 100 / 90 – 100
2.36 mm / No. 7 / 75 – 100 / 60 – 95
1.1.8 mm / No. 14 / 55 – 90 / 30 – 70
600 Micron / No. 25 / 35 – 60 / 15 – 35
300 Micron / No. 52 / 5 – 30 / 5 – 20
150 micron / No. 100 / 0 – 10 / 0 – 12
If grading of fine aggregates can be improved by mixing two varieties of sand, the Engineer may at his discretion specify such mixing, and may permit the use of crushed sand as one of the two sands forming the mixture. The provisions of two types of sand and their mixing in the specified proportions shall be done at the contractors cost.
a)The fine aggregate shall not contain more than a total 4% by weight of loam, dust silt and any.
b)The amount of fine particles shall not exceed 3% by weight for natural sand.
COARSE AGGREGATES (IS : 383)
Coarse aggregate for the works shall be river gravel or crushed stone obtained from sources approved by the Engineer. It shall be clean and free from clay or loamy admixture or quarry refuse or vegetable matter. The pieces of aggregates shall be cubical or rounded in shape and shall have granular or crystalline or smooth (but not glossy non - powdery surfaces.
Aggregates shall be properly screened and it necessary washed and cleaned before use.
Coarse aggregates containing flat or flaky pieces or mica shall be cleaned of such impurities before use. The grading shall conform to the following as far as possible :
GRADING LIMITS FOR SINGLE SIZED COARSE AGGREGATE
(IS : 383)
12 mm / 10 mm / 4.75 / 2.36
63 mm / 100 / 85 – 100 / 0 – 30 / 0 – 50 / - / - / - / -
40 mm / - / 100 / 85 – 100 / 0 – 20 / - / 0 – 5 / - / -
20 mm / - / - / 100 / 85 – 100 / - / 0 – 20 / 0 – 5 / -
12 mm / - / - / - / 100 / 85 – 100 / 0 – 45 / 0 – 10 / -
10 mm / - / - / - / - / 100 / 85 – 100 / 0 – 20 / -
Limits for use of single sized coarse aggregate in various types of structures shall be as follows :
USE / SIZESlab and walls 200 mm or more in thickness / 40 mm to 10 mm
Columns and Girders with least dimension of 300 mm or more / 40 mm to 10 mm
Slab and walls under 200 mm in thickness / 20 mm to 10 mm
Columns and Girders with at least dimension under 300 mm / 20 mm to 10 mm
Note:The sieve sizes of 80mm, 40mm, 20mm, 12mm, and 10mm will correspond roughly to B.S. Sieve size of 3”, 21/2”. ¾” and 3/8” respectively.
Note:Limit for impurities in Aggregates (IS : 383)
TEST / FINE AGGREGATE / COARSE AGGREGATEClay, Silt & Fine dust in natural sand or crushed Gravel (by weight) / 4% / 1%
Organic impurities / The solution obtained by immersing the samples in 3 percent solution of sodium Hydroxide for 24 hrs. should be light colour.
Coarse and fine aggregates shall be batched separately. All in aggregate may be used only where specifically permitted by the Engineer.
WATER
Before commencing the trial mix design, and in course of the work whenever the source of water changes, or whenever there is in the opinion of the Engineer reason to suspect a change in water quality, the contractor shall get the water to be used for mixing, tested in an approved laboratory. The water in general shall comply with the following requirements.
Water to be used for mixing and curing concrete shall be clean, fresh and free from organic or inorganic matter in solution or suspension in such amounts as may impair the strength or durability of concrete or steel. Water which is potable is normally considered satisfactory for concrete mixing. Sea water or water from excavations shall not be used. In case of doubt suitable checks shall be made using distilled water as the basis of comparison.
Where water can be shown to contain an excess of acid, alkali, sugar or salts, the Engineer may refuse to permit its use. As a guide the following concentrations represent the maximum permissible values.
a)To neutralize 200 ml sample of water, using phenolphthalein as an indicator, it should not require more than 2 ml of 0.1 Normal NaOH. The details of test shall be as given in section 13 of IS : 3025.
b)To neutralize 200 ml sample of water, using methyl - orange as an indicator; it should not require more than 10ml of 0.1 HCL, the details of test shall be as given in section 19 of IS : 3025.
c)Percentage of solid when tested in accordance with IS : 3025 shall not exceed the following :
TABLE
Permissible Limit Max.Organic / 200 mg/l
Inorganic / 3000 mg/l
Sulphate (as SO4) / 500 mg/l
Chlorides (as CL) / 1000 mg/l
Suspended matter / 2000 mg/l
d)The pH value of water shall be 6 - 10.
TRIAL MIXES
The contractor shall be entirely responsible for the design of the concrete mixes. The design is however to be approved by the Engineer before commencing any concreting in the works. The contractor shall make trail mixes using samples of coarse aggregates, sand, water and cement, typical of those to be used in the works, and which have been tested in an approved laboratory. A clean dry mixer shall be used and the first batch discarded.
The cement contents of different grades of concrete and the required average strength at 28 days for which the mixes shall be designed are specified below
TABLE : 2
Grades Designation / Characteristic Strength thk. At 28 days / Estimated minimum cement content / Target average cube strength at 7 days / Target average cube strength at 28 days(Kg / cm2) / (Kg / cm3) / (Kg / cm2) / (Kg / cm2)
M 10 / 100 / 225 / 100 / 140
M 15 / 150 / 300 / 150 / 210
M 20 / 200 / 350 / 195 / 275
M 25 / 250 / 400 / 235 / 335
The estimated cement contents indicated above are for the contractors guidance only. They are for 20 mm maximum nominal size aggregates, and for other sizes different cement contents may be necessary to get the specified strength. For 20 mm aggregates, the above are cement contents with which it should be possible to obtain the required target mean strengths provided the aggregate and other ingredients are of satisfactory quality and grading.
In case however the contractor is unable to obtain the required average strengths with these cement contents he will be required to provide increased cement content at his own cost. On the other hand, cement contents less than estimated minimum are not permitted.
Where the mix is not designed following proportions shall be used for the concreting : (by Volume)
M-10-(1:3:6)
M-15-(1:2:4)
M-20-(1:1.5:3)
M25-(1:1:2)
R.C.C WORK IN CONTROLLED CONCRETE
Strength of concrete of different grades and their proportions for controlled concrete shall be as under
Grade of Concrete / Specified Characteristic compressive cube strength at 28 days in Kg / Sq. cm / Total Qty. by weight dry aggregate (course + 50 kg cement) / Proportion of fine to coarse aggregates / Qty. of water per 50 kg. Cement (Ltrs)M – 5 / 50 / 800 / Generally / 60
M – 7.5 / 75 / 625 / 1 : 2 but / 45
M – 10 / 100 / 480 / Subject / 34
M – 15 / 150 / 350 / To upper / 32
M – 20 / 200 / 300 / Limit / 30
M – 25 / 250 / 250 / 1 : 1.5 & Lower limit 1 : 2.5 / 30
Note :
1.M - 5 and M - 7.5 grades need not be designed for mixes.
2.Coarse aggregate above means graded coarse aggregates.
3.If for obtaining workability etc., quantity of water has to be increased, then the cement content shall also proportionately increased so that specified water cement ratio is not exceeded. The cost of such extra cement shall be deemed to be included in the item rates / prices.
1, 2, 3 and 4 samples for quantity of concrete poured daily not exceeding 5 Cu. M, 16 - 39 Cu.M & 31 - 50 Cu. M respectively shall be taken or as directed by the Engineer. From each sample six test cubes shall be made (150 x 150 x 150 mm ) size, three for testing at seven days and three for testing at twenty eight days. The strength at seven days for use of normal Portland cement shall not be less than two thirds of twenty eight days strength. All tests shall be carried out as per IS : 456, IS: 516. The results of test in any recognized laboratory shall be final and binding on the contractor.
All arrangements for testing concrete cubes shall be made by the contractor at his own cost. All the materials and labour for making, curing and transporting the cubes for carrying out necessary tests including the fees of testing whether at site or elsewhere shall be at the cost of the contractor.
Normally cube strength s substantially in excess of those specified are expected.
In case the compressive strength obtained from the test cubes of concrete is less than the minimum specified in acceptance criteria as per IS : 456 – Clause 15 the work is liable to be rejected.
If however, the strengths are higher than the specified due to good control or otherwise, the contractor shall have no claim for extra payment.
CONCRETE MIX DESIGN (IS : 01262)
Concrete mixes for various specified design strengths shall be worked out by the contractor, generally as per the requirements of IS : 456.
a.The mix proportion shall be selected to ensure that the workability of the fresh concrete is suitable for the conditions of handling and placing, so that after compaction it surrounds all reinforcements or tendons and ducts and completely fills the formwork. When the concrete is hardened, its quality shall be such as to comply with the strength, durability and other requirements taking into account the conditions to which it will be exposed.
b.Any standard method of mix design may be used. For determining the required strength the probability of any test result falling short of the specified minimum shall be taken as per IS : 456. Unless the contractor can prove from his past experience that he is capable of achieving a high degree of control, a fair control should be assumed in the initial mix design, before arriving at the average strength values due regard shall be given to the ‘Criteria of Acceptance’ of the site concrete, which shall be as here under. Laboratory cube strength for mix design shall be 25% higher than the targeted characteristic strength e.g. for M - 20 mix, laboratory 28 day strength for 150 mm size cube should be 250 Kg / Sq. cm so as to get 200 Kg/ Sq. cm field strength.
c.The mixes designed by the contractor shall be used on works only after obtaining a written approval of the Engineer. It is to be understood that the mix design shall be entirely the responsibility of the contractor and such approval by the Engineer shall not relieve the contractor of his responsibility in respect thereof.
d.The contractor shall prepare all calculations, tabulations, graphs etc., pertaining to concrete mix designs and / or test results and supply copies of such calculations, tabulations, graphs etc., as required by the Engineer.
e.Not withstanding the mix design prepared by the contractor, the absolute minimum quantity of cement which must be used for various grades of concrete shall be as follows
MIXK.G. PER CU. M
M 15300
M20350
M 25400
f.Either ordinary Portland or Portland pozzolana cement will be supplied. No deviation either in rate or schedule of work will be entertained on this account. The mix design should be based on the use of ordinary Portland cement / Portland pozzalana cement. Due regard should also be taken regarding minimum cement content specified as above.
g.The contractor shall furnish the cement content assumed for various mixes for approval by the Engineer within on (1) week after award of contract. In case it becomes necessary to use Portland pozzolana cement during the course of the work, the contractor shall design fresh mixes and the difference in the consumption of cement due to use of pozzolana cement shall be taken into account for reconciliation purpose also.
In all cases the contractor shall make trial cubes from each consignment of cement and test the same before actually using in the work.
h.The ‘Criteria Acceptance’ for concrete shall be as follows:
The Criteria of acceptance of site concrete shall be as per clause 15 of IS : 456. Concrete which does not fulfill requirements specified in clause 15.1 of IS : 456. but has a strength greater than that required by clause 15.2 is liable for rejection at all consequences of rejection of concrete shall be totally at the contractors expense.
For each grade a total of 18 cubes shall be made. Of these 18 cubes made not more than 6 may be made on any day and further. Of the 6 cubes made in one day not more than 2 cubes may be from any single batch. 9 of these cubes. Each representing a different batch of concrete shall be tested at the age of 7 days and the remaining 9 cubes shall be tested at the age of 28 days. The making of the cubes, their curing, storing, transporting and testing shall be in accordance with Indian Standard IS : 516. The test shall be carried out in a laboratory approved by the Engineer.
ACCEPTANCE CRITERIA (IS : 456)
The contractor shall submit test results in statistical form as described in IS : 456. Standard Deviation calculations shall be on a set consisting of not less than 30 samples.
Concrete will be considered acceptable if
The average strength of set is not less than
Not more than 5% of the samples of the set fall below characteristic strength of the concrete.
Lowest samples strength value in a set does not fall below greater of the following two values.
If the concrete produced at site does not satisfy the above strength requirements, the Engineer will reserve the right to require the contractor to improve the methods of batching, the quality of the ingredients and redesign the mix with increased cement content if necessary. The contractor shall not be entitled to claim any extra cost for the extra cement used for the modifications stipulated by the Engineer for fulfilling the strength requirements specified.
If from the test results it appears that some portion of the works has not attained the required strength, the Engineer may order the testing of the suspected as well as adjacent portions of the structure. Such testing shall be at the contractor’s cost. The Engineer may also reject the work and order its demolition and reconstruction at the contractor’s cost.
If the strength of concrete in any portion of the structure is lower than the required strength, but it is considered never the less adequate by the Engineer so that demolition is not necessary, the contractor shall be paid a lower rate for such lower strength concrete as determined by the Engineer.
If the average strength of the concrete cubes falls below the required strength fresh preliminary mixes for that grade shall be made as before, until the trial mixes yield cubes of compressive strength at 28 days greater than the required average strength at that age.
When there is a significant change in the quality of any of the ingredients of concrete, the Engineer may at his discretion order the carrying out of fresh trial mixes. All costs for trial mixes and tests shall be to the contractor’s account and held to be included in the contract rates.
Before commencing the works the contractors shall submit to the Engineer, for approval full details of all preliminary trial mixes and tests.
When the proportions of a concrete mix have been approved by the Engineer, the contractor shall not vary the quality or source of the materials or the mix without the written approval of the Engineer.
CONCRETE CUBE TESTS
The quantity of hardened concrete will be verified by the following procedure
The engineer shall select random batches of concrete for examination without warning the contractor and sampling will generally be done at the point of discharge from the mixer.
From the batches thus selected 6 concrete cubes shall be made in accordance with Indian Standards. However not more than 2 cubes may be made from any single batch. Of these 6 cubes thus made 3 cubes (each representing concrete of different batches) shall be tested 7 days and the remaining 3 cubes shall be tested at 28 days.