General Specifications of Cement Concrete Work In Building
All concreting, especially cement concrete work, is a specialized task in respect of bringing out quality: finish, strength, and leak-proof performance.
Cement concrete work can be categorized (for separate measurements to be billed separately as per varying rates of payment because of differing specifications) as follows :
(a) Plain concrete work
(b) Reinforced concrete work
(c) Pre-stressed concrete work
(d) Pre-cast concrete work – plain or reinforced
(e) Cast-in-situ concrete work – plain or reinforced.
Unit of measurement is cubic metre if no qualifying statement is made in its detailed specifications.Cement Concrete Work In Building
Reinforcement in an RCC work, if not accounted for in the composite rate analysis, is measured as a separate item. Similarly, formwork (a necessary item in an RCC work) is measured separately.Description of a cement concrete work should include the type of finishing of the surface that is desired. Special finishes shall be paid separately in m2 measure.
Special concrete, such as cooled (for dams) heated, cellular, heat resisting has to be fully and in detail described; and is measured as a separate item.
Damp proof course (DPC), while, stating its thickness, is measured in m2. Its rate shall include formwork and fair finish to edges which includes levelling and full preparation of the brick or stone masonry on which it is to be laid.
Only sufficient water, and no more, shall be used to give the slump test specified for the work. Consistency shall be frequently tested by carrying out slump tests. For this test, concrete is filled into a metallic conical mould open at both ends, and conforming to the frustum of a cone of specified dimensions. The mould shall be placed on a horizontal metal sheet, and the concrete (that is prepared) filled into it in, say, 10 cm layers. Each layer is lightly prodded by a bar. Its top layer is struck off level with the top of the mould, which then shall be withdrawn by means of the handles provided on it, by a gentle, steady upwards pull. Later the pile of concrete, that is left, will settle as much as is determined by it, consistency – this amount of settlement in height is known as slump.
Fine and coarse aggregates shall be measured loose or placed (thrown) in the measuring box and struck off. The measured quantity of cement shall be placed on the top of the measured quantity of fine aggregate, and the whole is mixed dry, three times or more to bring it to a uniform colour. Next the measured quantity of coarse aggregate shall be added to the mixture and the whole mixed dry once. Later, the required quantity of water (measured volumetrically or weighed for each batch) shall be added gently (with a rose) and, the process of turning over goes on until the whole mass of this mixture has been turned wet three times or more till a homogeneous mixture of the required consistency is obtained. Hand mixing shall only be done on a smooth platform.
Concrete shall be handled, from the mixing platform to the final placement point, as quickly as possible in the ways that prevent the segregation (separation) or loss of its ingredients. It is placed in the forms as nearly as possible in its final position to avoid re-handling. It shall in no circumstance be dropped, short or tipped from a height greater than 30 cm. It shall be deposited such that to maintain, until the completion of the particular unit of work, a plastic surface approximately horizontal. After concrete is
deposited, it shall be rodded, tamped or worked to ensure no hollow places are left in its mass – now-a-days electrically working vibrators are much in use which should be used as per specified instructions. Such tamping or working shall be completed within half an hour of adding water to the cement. Any voids, hollows left in the mass contribute, later on, to undesirable seepage through it – a very annoying feature of roof slabs. Water proof compounds that are added to the mix way fail to give results if bigger voids are there to exist.
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Laitance (a milky accumulation of fine particles forming on the surface of freshly laid concrete when the concrete is too wet or is vibrated or tamped too much) shall be prevented on horizontal layers when stopping day’s work. If it occurs, it shall be removed (i.e. removing any “creaming” or excess water) as soon as concrete is laid and compacted. Where laitance has formed and hardened, it shall be removed before recommencing work, by chipping gently so as not to disturb the concrete that has already partially set.
Re-tempering of concrete or mortar that has set partially – i.e. remixing with/without additional cement, aggregate or water – is absolutely unacceptable. Moreover, never shall concrete, that has partially hardened, be deposited in the work.
The top surface, if it stands exposed (but not under any wear), shall be smoothed with a wood float. A steel trowel is not to be allowed. Any excess water (or cream) that has formed on the surface shall first be removed. Dry cement, or a dry mixture of element and sand, shall not be sprinkled on the surface with the intentions of absorbing such excess moisture.
Concrete shall be cured after laying by being covered with gunny bags, sand or saw dust which shall be maintained wet constantly for 15 days. Shuttering shall not be removed before the specified period regarding different structural parts.
Special case needs to be devoted on the storage of cement (as also for lime, and surkhi). All cement shall be stored in a weather proof shed whose floor has to be damp proof in nature, and at least 30 cm above the natural ground surface. Cement shall not be stored in contact with walls. To reduce deterioration by aeration, cement should be stored in bulk wherever feasible. When stored in bags, these shall be placed horizontally in contiguous lines and layers.
No cement that has been stacked throughout a monsoon or for more than six months shall be used for reinforced concrete until samples have been tested against laid down properties/quantities.Cement Concrete Work In Building
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