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Saturday, May 5, 2012

CONCRETE WORK

4. CONCRETE WORK
4.1 Description
Concrete work shall consist of mixing, conveying, placing of concrete , for work and reinforcement Work, (it is inclusive of all measures and materials required removal of concrete forms and reinforcement).

4.2 Material
All materials used in the work shall be the best of their kind and shall conform in quality and treatment to the conditions herein specified. The contractor shall submit to the Engineer when required and at his own expense, samples of all materials to be used in the works. The quality of the samples so provided being representative of the Bulk of such materials. The construction of all concrete and reinforced concrete work shall commence at points approved by Engineer and shall be continued and completed in accordance with the
programme of work to be submitted to the Engineer for approval before the concrete work is commenced.
Any work considered by the Engineer to be of inferior workman ship and therefore , to present a potential point of weakness in any part of the work shall be demolished and rebuilt at the expense of the contractor.

4.2.1 Cement
Portland cement for all structural concrete shall conform to ASTM C 150, for all concrete construction below ground level and water-retaining structures, sulphate resisting Portland cement Type II of ASTM C 150 or equivalent shall be used and for above ground level Type I shall be used The contractor shall provide appropriate dry, well ventilated weather and water proof sheds of capacity sufficient to store cement so that the cement can be stored in such a manner as to prevent deterioration or intrusion of foreign matter. Floors of the sheds shall be at least 30 cm above ground. The cement while being conveyed to the site in trucks or other vehicles shall be adequately from the weather. The cement shall be used as soon as possible after delivery. Any cement that has deteriorated or has been contaminated shall not be used for concrete.

4. 2. 2 Aggregates a. General
All aggregates shall conform to the requirements of J3.S 882 or equivalent and be locally available. Aggregates failing to meet above mentioned specifications but which have been shown by special test or to actual service to produce concrete of adequate strength and durability may be used when authorized by the engineer. The aggregates shall be dense, hard durable and free from harmful amount of reactive minerals and other chemical compounds and shall conform to the above mentioned standards. Samples of aggregates used in the work shall so provided from the same aggregate sources stockpile at the site, and be submitted to the laboratory authorized by the employer and the written approval of the authorized laboratory shall be given to the engineer for his approval.
b. Grading of aggregate
The grading of- the fine and coarse aggregate shall be such that whe they are mixed in the proportions decided for each class of concrete, the grading of the combined aggregate shall be suitable for making a dense concrete of appropriate workability with the proportions of cement and water with which the aggregate is to be used. The quantity of flaky aggregate shall be limited as much as possible.
When tested by means of laboratory sieve the fine and coarse aggregate shall conform to the requirements of D.S. 002 for graded aggregate. The contractor shall be responsible for mixing the aggregate in the proportions approved by the Engineer for each class of concrete and each section of the work. It shall submit samples of the re-graded material to the Engineer well in advance before commencing any concrete works, 20 mm size shall apply to all concrete having 200mm or less in thickness, for heavier wall, footing and mass concrete 25 mm size may be used, provided that the space between the reinforcing bars therein is one and half times greater than the maximum aggregate size.

4. 2. 3 Water
Water use in mixing concrete shall be clean and free from injurious amounts of oil, acids, alkalis, salts, organic material or other substance which may be deleterious to concrete or reinforcement. The temperature of water use for making concrete in hot weather shall be low enough to attain the proper mixing temperature of concrete, and in any case shall be Lower than 30 degree centigrade. The contractor shall store on the site an adequate supply of fresh water to meet all needs . The contractor must indicate the origin . of the water he intends to use.
4.2.4 Steel reinforcing bars
Reinforcing bar should be deformed bars for all reinforced concrete work, and should have minimum yield strength of 414 N/mm2 ( 60000 psi) in accordance to ( ASTM A 184 and A 185) . Representative samples of all steel reinforcement that the contractor proposes to use in the Works together with manufacture's certificate stating clearly for each sample, the place of manufacture expected date and size of deliveries to site, and all relevant details as regards composition manufacture, strength and other qualities of the steel shall be submitted to the Engineer for written approval.

4.2.5 Formwork
Icuan ply wood, 5-ply, 12 mm thick, or wood boards shall be used . Wood boards or ply wood for shuttering shall be such as not to damage the placed concrete owing to its containing imparities and shall be able to withstand loading occurring during placing of concrete.
4.2.6 Preformed expansion joint sealer and filler
Bitumen Impregnated fiber board to be used for joint filler, shall be robust,durable, waterproof, rat-proof , non- staining, made from selected hard and soft wood fibers impregnated with bitumen emulsion, preformed expansion joint seals shall consist of synthetic resin cover , hard PVC cap, steel anchors and elastic filler as shown on the drawings .
4. 3 Concrete Mixes
Concrete shall be proportioned to have the following specified compressive strengths, as determined by the specified testing and test evaluation procedure, specified compressive strength ( f'c) shall be as indicated on the drawings.
4.4 Water-cement ratio
Water-cement ratio shall be determine so as to achieve the required workability and to obtain the specified concrete strength, which shall be subject to the approval of the Engineer.

4. 5 Test of Concrete
Work cylinder test ( cubes in case B.S. is to be applied) shall be made on concrete sampled during the works. Samples shall be taken for each new grade concrete, from each 100 m3 of concrete when the same grade is being used continuously, except for lean concrete and other non load bearing concrete. The number of specimens taken shall not less than 3 for each compressive strength test, All tests shall be performed in accordance with ASTM C. 39, B.S, 1881. and shall be carried out in an authorized laboratory at the contractor's own expense. If the results of the 28 days test are unsatisfactory, all concrete work shall be stopped at contractor's expense and shall not proceed further without the written permission of the Engineer. Should the test prove that the concrete is not satisfactory or the Engineer Judge any section to be defective , the condemned concrete shall be cut out, removed and replaced by the contractor.
4.6 Mixing and placing concrete 4. 6. 1 Mechanical Mixing, batching .
All concrete shall be Machine Mixed. The location for the batching and minding plant shall be agreed by the Engineer, and the contractor must submit to the Engineer for approval before erection of any mixing plant his proposed arrangements for the storage of aggregate and batching and mixing of concrete. The contractor must also submit details on the type or types of mixers and machines to be used and proposals for the moans of convoying nixed concrete from the miser to the points of deposition.
All concrete shall be batched by weight and the weight-batching machines used shall be of a type approved by the Engineer and shall be kept accurate and in good condition while in use at the works. Checks shall be made as required by the engineer to determine whether the weighting devices are registering correctly. Each mixer shall be fitted with a water measuring device. If aggregate batching by volume is allowed, the cement shall be batched by weight and the water by weight or volume.
Any deposit of old concrete in the mixer drum shall be cleaned out by rotating clean aggregate and water in the drum before any fresh concrete is mixed.
4. 6. 2 Placing and compacting
Immediately after Mixing, the concrete shall be transported to the place of final deposit- by method which prevent separation , loss contamination of any of the ingredients.. Any method involving the use of pipes or chutes for transporting concrete shall not be permitted, except with the written approval of the Engineer.
Transport of concrete from the mixers must be as rapid. as possible and the contractor shall always be responsible to place and compact the concrete.
Before any concrete is poured, the formwork must be thoroughly cleaned of all dirt, shavings, loose stones, etc, and the wood form which will come in contact with the concrete shall be soaked well with an approved mould oil. The concrete shall be placed gently in position and shall normally not have a free fall of more than one meter. To convey, the concrete as near as possible to its final position, rubber or metal drop chutes shall be used for small sections, and bottom opening buckets or other suitable vessels for large sections. The concrete shall be placed in such a manner so as to prevent water from collecting at the ends, corner or along the faces of the forms, and it shall not be placed in large quantities at a given point and allowed to run or b be worked over a long distance in the form . All concrete shall be placed and compacted in even layers with each batch adjoining the previous one. The thickness of the layers shall be between 15-30 cm for reinforced concrete and up to 45 cm for un-reinforced concrete in relation to the width of the forms.
The concrete shall be carefully and continuously compacted and worked around the reinforcement and into the corners of the formwork so that it will be in close contact with the reinforcement and free from honeycombing. Over-vibration causing segregation shall be carefully avoided and the redistribution of concrete in the formwork by means of vibrators shall not be permitted.
The concrete shall be compacted by mechanical or electro-mechanical vibrators of a type approved by the Engineer. The plunger type vibrators shall have a diameter compatible with the spacing of the reinforcement., and sufficiently high frequency.
All vibration, compaction and finishing operations shall be completed immediately after placing of the concrete in its final position, workers shall not be permitted to walk over freshly placed concrete until it has hardened sufficiently to carry their weight without distortion, and great care shall be taken to ensure that reinforcement projecting from recently placed concrete is sot shaken ox1 disturbed so as not to destroy or damage the initial set of the concrete in contact with it.
Concreting in any one part or section of the work shall be carried out in one continuous operation and no interruption of concreting work shall be allowed without the approval of the Engineer .
4. 7 Concreting in Hot weather
The recommendations set out in (( Recommended practice for hot weather concreting) ( ACI- 305-72) . The temperature of the mixed concrete shall not exceed 35 before placing in hot weather. In ambient temperatures coarse aggregates shall be sprayed with clean water for cooling. In order to keep the temperature of mixed concrete below the specified temperature, the contractor may use cooling equipment. Fresh concrete shall be protected from premature drying. Concrete in transit shall be shaded from direct sunlight and special care shall be taken to maintain the correct water/ cement ratio. The formwork and reinforcement in and around which the concrete is to be placed shall be sprayed with water to cool the surfaces and prevent excessive absorption from the concrete when placed. Generally all concrete shall be shaded from direct sunlight and drying winds and from excessive ambient temperature during placement.

4.8 Concreting in cold weather
Ho concrete shall be made with frozen aggregates or placed up on frozen surfaces. In cold weather, the contactor shall take special precautions, such as heating aggregates and mixing water to ensure that the temperature of the concrete when placed is not less than 4C .
4.9 Curing
Curing shall start as soon as practical after placing or finishing . concrete shall be cured with water unless membrane curing is employed.
The surface of placed concrete shall be covered with damped mats or other approved materials for a suffocation period talking into consideration weather conditions during the period. Horizontal surfaces shall be covered by a suitable method so as to avoid the effect of sunshine, drying wind and other harmful effects, vertical surfaces such as walls and column sides shall be wetted for a sufficient period by sprinkling water to forms, or other suitable methods.

4.10 Reinforcement
This article shall apply to the fabrication and erection of reinforcing steel bars. The materials to be used shall be specified in section 4.2.4 of this specification.
4.10.1 Bending and anchorage
Bending specifications shall be drawn up as applicable in accordance with the approved codes, and each reinforcement bar shall be bent to the exact dimensions specified in the relevant specification . All bars shall be bent cold. Bars shall not be welded without the approval of the Engineer. No splices shall be made in the reinforcement except where approved by the Engineer, and all splices or overlaps shall comply entirety with the requirements of proved.
4.10.2 Fixing of reinforcement
The steel reinforcement shall be assembled to the exact shapes and dimensions as approved by the Engineer. The rods shall have the approved cross-sectional area and shall be fixed accurately in the moulds. The ends of all tying wires shall be turned into the main body of the concrete and shall not be allowed to project towards the surface. Spacing blocks shall be used to ensure accurate cover to the reinforcement, where necessary, and these blocks shall be of precast concrete of a strength at least equal to that of the concrete being placed. They shall be as small as possible in view of practicality and shall be securely fixed in position by means of wires to be cast into them.
No temporary supports for the reinforcement shall be allowed to be incorporated into the finished concrete. At the time of concreting all reinforcement shall have been thoroughly cleaned and made free
of all loss rust (crude oil or any other coatings that might destroy or reduce the bond).

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