RCC Concrete Construction, Analysis, and Step-by-step Guide| Concrete – part-2

RCC Concrete construction for Reinforcement Concrete(RCC), concrete slab foundation, concrete residential construction, patio pavers, cement binder, driveway are complex task, there is a series of steps and rules followed for proper High strength RCC structure and for there Durability.

RCC concrete Construction

(Placing of Concrete)

A. Concrete is placed only in the presence of the Employer’s Representative or his authorised representative. After the surface has been prepared satisfactorily, surfaces or construction joints upon which new concrete is to be placed, are flushed with a coat or neat cement grout or covered with a layer of mortar approximately 3 cm thick.  The mortar is the same proportion of cement and sand as the regular concrete mixture, unless otherwise directed.  The water-cement ratio of the mortar is suitable for placing and working in the manner herein specified.  The mortar is spread uniformly and  worked thoroughly into all irregularities of the surfaces.  Concrete then placed immediately upon the fresh mortar.  In placing concrete against formed construction joints, special contact with the surface of the joints are achieved by careful puddling and spading with the aid of suitable tools.

B. Retempering of concrete are not be permitted. Any concrete which has become so stiff that proper placing cannot be assured  discarded and no payment will be made to the Contractor for such wasted concrete.  Concrete should deposited in all cases as nearly as practicable directly in its final position and not flow in a manner to permit or cause segregation.  Excessive separation of coarse aggregates in concrete, caused by allowing the concrete to fall freely from too great a height or at too great an angle from the vertical, or to strike the forms or reinforcement steel, are not  permitted and where such separation would otherwise occur, the Contractor should provide suitable drop chutes and baffles to confine and control the falling concrete.

C. Except as intercepted by joints, all formed concrete placed in continuous horizontal layers, the depths of which generally not exceed 50 cm. The Employer’s Representative reserves the right to require lesser depths of layers where concrete in 50 cm layers cannot be placed in accordance with the requirements of this specification.  All intersections of construction joints with concrete surfaces which will be exposed to view is made straight and level or plumb, unless otherwise shown on the Drawings as ordered by the Employer’s Representative.

D. Large Blocks: In placing Concrete in large thick exposed areas the Contractor should maintain the exposed area of fresh concrete at the practicable minimum by first building up the concrete to full width of structures and to full height of the lift over a restricted area at one end of the structure and then continuing in similar progressive stages across the structure and then continuing in similar progressive stages to the full area of the structure. The slope formed by the unconfined edges of concrete is kept as steep as practicable in order to keep its area to a minimum. Concrete along these edges are not  vibrated until adjacent concrete in the layer is placed except that it  vibrated immediately when conditions are such that the concrete will harden to the extent that later vibrations will not fully consolidate and integrate it with more recently placed adjacent concrete.  Clusters of large aggregate are scattered before new concrete is placed over it.  Once placement of concrete has commenced in a structure, placement is not  interrupted, unless adequate construction joints can be made.

E. Rain: Concrete is not placed during rains sufficiently heavy or prolonged to wash mortar from coarse aggregate on the slopes of the placement.  During such rains mortar should not  spread on construction joints and diluted mortar already spread are removed and replaced before continuing with the work.

F. Buckets: Concrete buckets where used are capable of promptly discharging the low-slump concrete mixes specified and the dumping mechanism is designed to permit the discharge of as little as 0.35 m³ portions of the load in one place. Buckets are suitable for attachment to and use of drop chutes where required in confined locations.

G. Concrete around Openings: If concrete is placed monolithically around openings having vertical dimensions greater than 60 mm, and if concrete in decks, floor slabs, beams, girders or other similar parts of structures if placed monolithically with the concrete of structures, the following instructions are strictly observed;

1. Placing of concrete is delayed from one to three hours at the top of openings and at the bottom of bevels under decks, floor slabs, girders or other similar parts of structure members when bevels are not specified, but in no case is placing be delayed so long that the vibrating unit will not readily penetrate of its own weight into concrete placed before the delay. When consolidating the concrete placed after the delay, the vibrating unit penetrate and vibrate the concrete placed before the delay.
2. The last 60 cm or more of concrete placed immediately before the delay are placed with as low slump as practicable and special care shall be exercised to effect thorough consolidation of the
3. The surface of concrete where delays are made is clean and free from loose and foreign material when concrete is started after the delay.
4. Concrete placed over openings and in decks, floors, beams, girders and other similar parts of structures are placed with as low a slump as practicable and special care is exercised to effect thorough consolidation of concrete.

H. Consolidation: Each layer of concrete is consolidated to the maximum practicable density, so that it is free from pockets of aggregates and closes snugly against all surfaces of forms and embedded materials. In consolidating each layer of concrete, the vibrating head of the vibrator is allowed to penetrate and revibrate the concrete in the upper portion of the underlying layer. All concrete for major structures are consolidated with electric or pneumatic power driven immersion type vibrators, operating at speeds of at least 7,000 revolutions per minute when immersed in the concrete.  Hand consolidation, using slice bars, are employed for minor structures at the Employer’s Representative’s direction. Additional layers of concrete are not placed until the layers previously placed have been compacted thoroughly as specified.  Equipment used for consolidating the concrete are subject to the approval of the Employer’s Representative.

Temperature of Concrete

The temperature of concrete when it is being placed, in general, not more than 30⁰C.  When the temperature is high, the concrete is mixed at the job site and  placed immediately after mixing.  If the weather conditions are such that the temperature rise above + 32⁰ C, the contractor could take the necessary steps to keep the concrete temperature below that specified above (e.g. placing of concrete during the night).

Removal of Forms 

• The time and method of removal and moving of forms is subject to approval by the Employer’s Representative, and this work is done with care so as to avoid injury to the concrete. No loading on “green” concrete is permitted.  As soon as the forms are removed, the surface of the concrete is carefully examined and any irregularities in the surface are immediately repaired to the satisfaction of the Employer’s Representative.
• In general, the minimum time between placing of the concrete and the removal of forms is two days for unloaded walls and other side forms, seven days for supporting walls and conduits, and 14 days for bridge decks.

 Curing of Concrete and Protection of Surfaces

• All concrete are cured by water or by white pigmented sealing compound, as hereinafter specified. The Employer’s Representative have the right to determine which type of curing is used by the Contractor in the different portions of the work in order to prevent damage.
• Concrete cured with water is kept wet for at least fourteen consecutive days immediately following placement, beginning as soon as the concrete has hardened sufficiently to prevent damage. Curing consist of covering the exposed concrete surfaces and the forms with water saturated material or a system of perforated pipes, mechanical sprinklers or porous hose, or by any other approved method which will keep all surfaces to  cured continuously (not periodically) wet.  Water used for curing should meet the requirements of the Specification for water used for mixing concrete.
• Exposed surfaces of all concrete, except surfaces of concrete coated with white pigmented sealing compound, are protected against direct access of sun rays for at least the first three days after placing. Such protection is made effective as soon as practicable after the removal of the forms. The Contractor have to protect all concrete against injury until final acceptance by the Employer’s Representative.
• Testing of Irregularities: 

1. Finishing and repair of concrete surfaces are performed only by skilled workmen and in the presence of the Employer’s Representative. Concrete surfaces is examined by the Employer’s Representative where necessary to determine whether surface irregularities are within the limits hereinafter specified.
2. Surface irregularities are classified as “abrupt” or “gradual”. Offsets caused by displaced sheeting or lining of form sections or by loose knots or otherwise defective form lumber is considered as abrupt irregularities and tested by direct measurement.

All other irregularities are considered as gradual irregularities and is tested by use of a template, consisting of a straight edge or the equivalent thereof for curved surface. The length of the template will be 1.5 m for testing or formed surfaces and 3 m for testing of unformed surfaces.

• Finishing of Formed Surfaces: Finishing of formed surfaces is consist of filling all recesses larger that 5 mm, or which are caused by removal of form ties, and grinding or filling as necessary to produce surfaces with no abrupt irregularities exceeding 6 mm and no gradual irregularities exceeding 13 mm. Before acceptance of the work, the Contractor should clean all exposed surface of unsightly encrustation and stains.
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1. Sloping of Unformed Surface: Interior unformed surfaces is sloped for drainage where shown on the Drawings or as directed by the Employer’s Representative. Surfaces which are exposed to the weather shall be sloped for drainage. Unless the use of other slopes on normally level surfaces in indicated on the Drawings or directed by the Employer’s Representative, narrow surfaces such as tops of walls and curbs are sloped approximately 20 mm per metre of width; broader surfaces such as platforms and decks are sloped approximately 10 mm per metre.

• Finishing of Unformed Surfaces: Clauses of finish for unformed surfaces are as follows:

1. Unformed surfaces that covered by backfill or by concrete are finished by sufficient levelling and screening to produce an even uniform surface. Surface irregularities measured as described in paragraph D above are not exceed 1.9 cm.
2. A hard steel trowel finish is applied to unformed surfaces that subjected to the action of flowing water.  Bridge deck surfaces which are subject to pedestrian or vehicular traffic is given a standard light broomed finish.  Floating and trowelling is performed by use of hand or power driven equipment.  Floating and trowelling are started as soon as the screened surface has stiffened sufficiently and the minimum necessary to produce a surface that is free from screed marks and is uniform in texture.

Surface irregularities, measured as described in paragraph D above is not exceed 6 mm, no trowel marks or abrupt irregularities can permitted.  Joints and edges are tooled.

• Repair of Concrete Surface:

1. If, after stripping of forms, any concrete is found to be not formed as shown on the Drawings or is out of alignment of level or shows a defective surface, it is considered as not conforming with the intent of the specification and removed and replaced by the Contractor within 72 hours of concrete age at his expense unless the Employer’s Representative grants permission to patch the defective area, in which case patching performed as described in the following paragraphs and subject to further detailed instruction as the Employer’s Representative may consider necessary.
2. Defects that require replacement or repair are those that consist of honeycombs, damage due to stripping of forms, loose pieces of concrete, bolt holes, tie-rod holes, ridges at formed joints, and bulges due to the movement of the forms.
3. Ridges and bulges removed by chipping or tooling followed by rubbing with a grinding stone.
4. Honeycombed and other defective concrete chipped out, the chipped openings being sharp-edged and shaped so that the filling keyed in place. All holes thoroughly moistened for 24 hours before the filling is placed.  The surface or the filling finished flush with the surrounding concrete surface and have the same texture.  All patches are cured as specified above for concrete, and to the satisfaction of the Employer’s Representative.
5. Imperfections, bolt and tie-rod holes, and the chipped out honeycomb areas to be repaired, and filled with dry patching mortar composed of one part of Portland cement and two parts of regular concrete sand (volume measurement) together with a non-shrink patching compound approved by the Employer’s Representative, in the amount specified by the manufacturer, and just enough water, so that after the ingredients are thoroughly mixed the mortar stick together on being moulded into a ball by slight pressure of the hands and not extrude free water. Mortar repairs are placed in thin layers and thoroughly compacted by suitable tools.  Care taken in filling rod, bolt and pipe holes so that the entire depths of the holes are completely filled with compacted mortar.
6. Where concrete be exposed to view, the mortar made to match the colour of concrete by substituting white Portland cement in the required amount for a portion of the regular cement.  When, in the opinion of the Employer’s Representative, the extent of the imperfections in structures exposed to view is such that patching alone is produce a surface of satisfactory appearance the Contractor required to give such surfaces, as well as adjacent surfaces a sack rubbed mortar finish in accordance with the Employer’s Representative’s instructions.

Joints in Concrete

Joints only be formed in the positions shown on the Drawings or approved by the Employer’s Representative.  Construction, contraction and expansion joints formed to the details shown on the Drawings and as specified herein, and from the specified materials.

Construction Joints:

• Whenever concrete is placed on or against previously placed concrete which has become so hard that the new concrete cannot be monolithically compacted therewith, the resulting place of separation defined as a construction joint.

Where the locations of construction joints have not been shown on the Drawings then the Contractor can decide the locations of construction joints to suit the concreting programme. The positions of construction joints to the approval of the Employer’s Representative, which are obtained before any concreting takes place.  The Contractor should  submit to the Employer’s Representative drawings showing the positions of constructions joints. Construction joints should  located so that in conjunction with the programme for concreting, the effects of shrinkage and temperature are minimized.  Construction joints formed in straight lines at right angles to the general direction of the member and take account of shear and other stresses, and as far as is practical located at points of least shear. Construction joints is formed with clean sharp arises.

Construction joints are formed using rigid shuttering or other approved method, with joint lines at exposed surfaces of concrete being straight and continuous, plumb, level or inclined.  Before placing new concrete against that which has already set, the surface is cleaned over the full section to leave a sound irregular clean surface free from laitance, loose and foreign matter by green cutting using an air/water jet, by chipping or other approved means.

Contraction Joints : 

Contraction joints  as located and detailed on the Drawings.  The joints made by forming with smooth forms the concrete on the side of the joint and allowing it to set before placing concrete on the other side of the joint.  The concrete surface on one side of each joint is given two coats of approved bitumen before the concrete on the other side of the joints is placed.  Waterstops can placed in the joints in accordance with the Drawings.

Expansion Joints: 

Expansion joints is constructed in the locations and to the details shown on the Drawings.  Preformed joint filler, bond breaker and joint sealing compound and waterstops, dowel bars and bitumen coating can placed in the joints in accordance with the Drawings.

Installation of Joint Sealer

The Contractor prepare the surface of the joint and is fill or caulk the recess completely with the joint sealer strictly in accordance with the manufacture’s instructions.  The application of joint sealer is not  commenced without the Contractor having first obtained the approval of the Employer’s Representative.

Where a polysulphide joint sealer is used an approved bind breaker is provided between the joint filler and the sealer.

Water Stops and Elastomeric Bearing Pads

• Rubber or uPVC water stops approved by the Employer’s Representative is placed in joints of structures as shown on the Drawings. The Contractor should furnish all waterstops, including rubber plugs, gum rubber, rubber cement, bolts, nuts, washers and other articles required for construction of the joints.  The Contractor make all splices, joints and bonds, and fabricate any special intersections in accordance with the Drawings or as directed by the Employer’s Representative.
• All joints, splices, bonds and are made in strict accordance with the manufacturer’s printed instructions using material approved by the manufacturer, and formed to produce strong, sound and watertight joints.
• The Contractor take suitable precautions to support and protect the water-stops during the progress of the work, and to ensure their proper incorporation in the Works.
• Elastomeric bearing pads is neoprene, bonded in laminations by vulcanising to metal plates to prevent extrusion under load. Class of material have had at least five years successful usage as bridge bearing pads. Thickness and dimensions are as shown on the Drawings.

 Measurement 

• No separate payment made for handling cement or for formwork (shuttering and centering), contraction joints, fine concrete topping or grout, and all costs in connection therewith are included in the prices quoted for the various types of concrete.
• Expansion joint material and elastomeric bearing pads each be measured for payment by the square metre.  Quantities for payment is determined by direct measurement of the joint material and bearing pads in place.
• Payment for expansion joint material and elastomeric bearing pads is each be made at the unit price per square metre; except in case where the Bill of Quantities do not show expansion joint material in the Schedule, such material is included in the price for concrete.
• Waterstops measured for payment by the linear metre. Quantities for payment is determined by direct measurement of the water stop in place.
• Payment for Waterstops is made at the unit price per linear metre; except in cases where the bills of quantities do not show Waterstops in the Schedule, such material is included in the price for concrete.
• Concrete is measured for payment by the cubic metre. Quantities for payment for formed concrete and slabs are computed from the line, grades and dimensions shown on the Drawings.  Concrete used as backfill of holes and other areas directed by the Employer’s Representative is measured for payment at the mixer.

• Concrete is paid for at the respective unit prices per cubic metre quoted; except that where backfill concrete is required due to fault for negligence of the Contractor such concrete are at the Contractor’s expense.
• No separate payment is made for the Sampling and Testing of concrete.

Measurement and Payment of Reinforcing Steel

Measurement for payment for furnishing and placing reinforcement bars are made only of the weight of the bars placed in the concrete in accordance with the Drawings or as directed by the Employer’s Representative.  Payment for furnishing and placing reinforcement bars is made at the Contract rate per kg in the Bill of Quantities, which contract rate shall include the cost of furnishing reinforcement bars, furnishing and attaching wire tied and metal supports of use, and of delivering unloading, hauling, storing, sorting, cutting bending, cleaning, placing and securing and maintaining in position all reinforcement bars as shown on the Drawings or as directed by Employer’s Representative.

Payment for handling and installing Employer furnished Steel reinforcing Bars is made at the unit price per kg quoted therefore in the Bill of Quantities which price that include full compensation for furnishing labour, tools, equipment, and incidentals, and for doing all the work involved in handling, cutting, bending and placing the bar reinforcing steel, complete in place as shown or directed.

Concrete Sampling and Testing

Sampling and testing is in accordance with standards ACI, IS or BSS  1881.

The Contractor should establish a system for regular sampling and testing of concrete to the approval of the Employer’s Representative.  Samples can taken from randomly selected batches of each class of concrete used in the Works, and at approximately equal intervals throughout the placement.  Where more than one mixer is in use, sampling is from each mixer in turn.  Rates of sampling and testing is generally be as directed by the Employer’s Representative.  Higher rates of sampling and testing may be used at the start of the Works to establish the quality quickly or during periods of production when quality is in doubt.

The Contractor should provide the necessary equipment, labour and transport for carrying out the sampling and testing in the Laboratory.

The concrete test cubes are stored and tested at the Site Laboratory under the supervision of the Employer’s Representative.

Records of concrete sampling and testing are kept by the Contractor and forwarded to the Employer’s Representative within 24 hours of sampling and testing.  Samples for testing are taken at the point of discharge into the Works.

RCC construction

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