Method Statement for Concreting of Raft Foundation
5:18 AM 0 Comments

1. TITLE

Method Statement for the Concreting Works of Raft Foundation.

2. SCOPE OF WORKS

2.1 General Description

· The scope and purpose of this method statement is to outline the general requirements and precautionary measures which should be taken into consideration during concreting works of raft foundation and to ensure that works are according to the drawings, standards and project specification.

2.2 Plant and Equipment

§ Concrete Mixer

§ Concrete Pump and standby

§ Tower Lights

§ Vibrators with standby (Diesel & Electric)

§ Masonry trowels and other hand tools

§ Concrete Power Trowel

§ Air Compressor

§ Concrete Hopper/ Bucket

§ Mobile/ Tower Cranes

§ Surveyor equipment (level, Total station)

2.3 Materials

S.N	MIX GRADE	CONCRETE LOCATION	APPROVED MIXED DESIGN
1	C50	Raft foundation	50 Mpa with GGBS + MS
	SRPC20	Blinding	Sulphate Resisting Portland Cement

S.N	Material	Reference	STATUS
1	Steel reinforcement
2	Spacer & chamfer
3	Formworks
4	Tie wire
5	Polyethylene sheet
6	Release agent
7	PVC Waterstop
8	Swellbar
9	Curing compound
10	Hessian cloth

2.4 Site Planning and Preparation

2.4.1 Site Management Plan and Access Route

· Upon receiving construction drawings, all concreting routes / locations will be set out and marked routes plan and access shall be divided by geographical zones. The information will be provided to the construction team for implementation.

2.4.2 Permits

· Concreting Works permits should be secured ahead of time from HSE Department.

2.4.3 Communication

· Communication on site will be by GSM mobile phones.

2.5 Work Methodology

2.5.1 Pre-activity Meetings

Coordination between different disciplines will be done prior to commencing the concreting of particular structure based on the submitted work for inspection, Toolbox meeting should be conducted based on the approved method statement & risk assessment by the execution team. A pre-activity meeting will be held in order to provide the following information:

· The location of the structure to be concreted, particular grid, type, area, volume etc.

· All necessary inspection to be performed for concrete works,

· Related Interfaces, coincidences and integration with all partners involved.

· Related non-conformance, Technical query, Technical deviation or concession request.

· Ensure shop drawings are in place and approved Released for Construction Drawings.

The pre-placement activity meeting Safety task analysis risk reduction talk, has to be set to assign all tools required, rate of pouring, vibration and finishing procedures, staff requirements, testing specialties, traffic flow, number of pumps, emergency plans if any, etc.

2.5.2 Scheduling

The concreting shall be scheduled well in advance to allow the control of geometry, block-outs, insert, embeds, reinforcement and other activities.

A Concrete Requisition shall be prepared by the Site Engineer. The Site Engineer receives from Project Engineer 3 days look ahead concrete requirements with all data related to the pour to prepare a complete and accurate concrete in advance schedule as required by the concrete supplier.

The concrete schedule will be verified for every next day by the Construction Manager and coordinate with QA/QC requirements and approvals, prior sending to the Concrete supplier.

On the day of the pours, concrete orders shall be confirmed; the engineer in charge of the pour confirms each pour requirement and readiness with the concrete coordinator and the concrete coordinator confirms the same (make the order) with the Concrete supplier.

Any changes / amendments / deletions to the previously prepared concrete requisition schedule shall be forwarded to the concrete coordinator without delay. The concrete coordinator shall then amend the requisition to reflect the actual concrete activities and, when confirming the order with the Concrete Supplier, he shall make sure that the confirmation includes the latest information as amended.

2.5.3 Work Sequence

2.5.3.1 Ground Preparation

· As per approved drawings, ground will be excavated aligned with the markings placed by a licensed certified surveyor level and clean the area as per given rock surface level.

· Shutter will be place aligned with the marking points placed by licensed certified surveyor.

2.5.3.2 Blinding

· Subsequent completion of the excavation, once the third party Geotechnical had a site report stating that the soil could eliminate the compaction due to the rocky property, will be cleaned, remove any loose materials to be ready in applying the anti-termite.

· Formwork for the blinding will be laid; as per the approved shop drawings with the help of surveyors and sides of the shuttering will be supported with the steel nails with end cap/ rebar cap. Surveyor will mark finishing levels of the concrete on the formwork with nails.

· A certified technician from the approved Third Party applicator will apply the Anti-termite solution and lay 1000 Gauge polythene sheet prior to blinding.

· Anti-termite treatment will be applied inside the formwork area by approved applier in the presence of client/consultant if requested. (Please refer to Method Statement of Anti-termite approval).

· After application of anti-termite allow a gap of 1 hour prior to a pour blinding.

· Concrete of approved mix will be ordered after getting approval from the client representative, and will be placed by means of pump or chute.

· All the quality control checks for the concrete placement will be done, before the placement with the help of laboratory technicians.

· Final levels of the concrete will be rechecked with the help of surveyor.

· Compaction of blinding concrete will be done using a rigid metal or wood performed in a vertical stroke.

· One set of 150 mm cubes of any particular mix shall be taken by the contractor every 100 m3 of concrete and/or once a day at least. Refer to appendix 6.2 Inspection and Test Plan

· Blinding concrete shall have a minimum cube crushing strength of 20 N/mm2 at 28 days.

· Continuous curing of newly casted blinding shall be properly done for at least 3 days consecutively.

2.5.3.3 Water Proofing

· Ensure that the concrete substrate is well leveled and free of damage prior to receive the waterproofing system, otherwise proper repair shall be done.

· Once blinding is cleaned and prepared for waterproofing, a certified waterproofing mason will apply water proofing primer then lay membrane as per approved separate method of statement.

2.5.3.4 Screeding

· Damp proofing membrane using 1000 gauge polyethylene sheet will be placed above membrane prior to screed.

· Subsequent completion of the waterproofing once approved as per application. Formwork will be placed aligned with the marking points placed by the surveyor.

· Concrete of approved mix will be ordered after getting approval from the client representative, and will be placed by means of pump or chute.

· All the quality control checks for the concrete placement will be done, before the placement with the help of laboratory technicians.

· Final levels of the concrete will be rechecked with the help of surveyor.

Ø One set of 150 mm cubes of any particular mix shall be taken by the contractor every 100 m3 of concrete and/or once a day at least.

· Screeding concrete shall have a minimum cube crushing strength of 20 N/mm2 at 28 days.

· Continuous curing of newly casted blinding shall be properly done for at least 3 days consecutively.

2.5.3.5 Steel Reinforcement Fixing

· Steel reinforcement will be fixed as per the approved shop drawings using the approved pre-cut and bend bars as scheduled on the approved BBS.

· All required concrete covering for raft foundation will be followed as per the specification
· All intersection of steel reinforcement will be tied properly and a concrete cover will be placed same strength with the required concrete.

· After bottom layer of rebar for raft foundation inspection request will be submitted.

· Top reinforcement will be inspected after the approval on bottom layer.

· All lapping shall be in staggered position.

2.5.3.6 Raft Foundation

· After the completion of the curing period of the blinding concrete, waterproofing works will commence as per approved shop drawing and Method Statement for Substructure Waterproofing Works.

· Checks completion of waterproofing membrane, 50 mm protection screed shall be provided in horizontal bottom and protection board in vertical surfaces. Water stop to be installed as per approved shop drawings.

· After completion and approval of waterproofing works, formworks and reinforcement steel for the raft foundation will be laid with approved materials and as per the approved shop drawings. Pouring sequence of raft foundation is attached under attachment 6.5.

· Checks will be done for the formworks, which should be sufficiently tight, rigid and reinforcements in details as per the approved shop drawings.

· The surveyor will check the coordinates, levels and control points for neck columns, wall and all structural members as per approved shop drawings.

· One coat of approved form releasing agent will be applied to the formwork before placing the steel reinforcement.

· Fixing and installation of reinforcement shall conform to BS 4466 and as per approved shop drawing.

· All the plumbing and HVAC box outs, bolts, inserts, wall plates, sleeves, and electrical conduits will be placed as per the approved shop drawings and will be tied to the reinforcement to arrest any possible movements during the concreting. For this proper co-ordination will be done during the steel installation.

· Final levels of the concrete will be marked as per approved shop drawing with the help of surveyor.

· All the precautions will be taken for the hot weather concreting as per the ACI 305-72 specification for hot weather concreting OR project specification.

· (CLIENT/CONSULTANT) will be notified at least 24 hours before each day’s operation of placing the concrete.

· Set of concrete cube samples will be done as mentioned in appendix 6.2 Inspection and Test Plan.

· A designated area for quality control check of fresh concrete delivery will be set and all concrete deliveries will be checked and monitored by the QA/QC Engineer (Concrete specialist) and technician from third party laboratory.

· All the quality control checks for the concrete such as slump, temperature will be done prior to casting. The entire QC check sheet will be maintained for the concrete traceability.

· Slump of the concrete should be as per approved concrete mix design and the Concrete temp. Should not exceed 32 C°, whereas the ambient temperature should be less than 40 C°.

· Fog spray to the installed reinforcement will be done to reduce the rebar temperature and allow it to cool prior to receive concrete.

· During placement care will be taken, not to lose any ingredients from the concrete, any type of segregation and the displacement of the reinforcement.

· Consolidation will be done using adequate numbers of concrete vibrators of appropriate size with same trained persons for each pump, and care will be taken during the consolidation for honey combings or over vibrations.

· Pipes will be laid above to be used as a reference for final casting level of concrete.

· Immediately after the completion of the placement and concrete finishing is done, the concrete will be covered with gauge 1000 polythene sheet. And soon as the concrete has stiffened sufficiently wet hessian cloth will be laid and kept wet continuously for minimum 7 days.

2.5.3.7 Formworks

· Formwork for the raft will be placed by skilled carpenters; as per the approved shop drawings aligned with the surveyors marking. Surveyors will mark points for the boundaries and level of concrete prior to pouring.

· A kicker form shall be placed on retaining walls, columns and other vertical structures in compliance with the approved shop drawings.

2.5.3.8 MEP and other embedded works

· Embedded installations will be proposed by contractor and submitted for consultant’s approval.

2.5.3.9 Water Stopper

· All construction joint below the ground will be sealed using PVC water stops and swell bar.

2.5.3.10 Construction Joint

· Care will be taken during joint installation to maintain proper joint function and assure proper formwork support is in place and not to destroy the joint after concrete placement.

· A shear key will be provided for structural joints as per structural details and shall be applied with a bonding agent as per approved materials prior to the next casting sequence.

2.5.3.11 Preparation and Approval (Prior Casting inspections):

· Site Engineer will submit to the quality department the work inspection for concrete works prior target pouring schedule.

· QC Inspector (Civil) will check the preparation, steel placement, overlapping, anchoring as specified and in accord to the shop drawings.

· QC Inspector (Electrical & Mechanical) will check the sleeves, embeds, tubes and conduits and all items are set properly in its place.

· QC Inspector (Civil) will check for joints needed and preparation, existing joints preparation, cleanliness and laittance removal, spacers and concrete cover if it is in accordance with the Specifications and provisions of the project.

· After the inspection and approval of the QA/QC Inspector, works will be offered for consultant Inspection and once approved by the consultant concrete of approved concrete mix design will be ordered.

· HSE TEAM will make visual inspection for assurance of the compliance status of MS&RA in addition to the Temporary Work Designers Calculation, PPE and competency of the Execution Team.

2.5.3.12 Concrete placement

Concrete placement shall be carried out in accordance with Project specification and standards.

· Quality control on concrete mixtures shall be monitored to ensure smooth running, consistent pumping operation and good quality of pumped concrete.

· The QA/QC Department in coordination with the Construction Department will monitor the uniformity of the concrete materials and consistency of mixes.

· Avoid displacing reinforcement, ducts and anchorages, or formwork, and damage to the faces of formwork, especially when the concrete is allowed to fall freely through the depth of lift. Consider the effect of lift height in massive section of the concreted area. Follow the depth of each lift every 450 mm in layer.

· Do not spread the concrete across the surface of open textured formwork by means of internal vibrators as this may lead to localized honey-combing, differential mixes and inadequate bonding between the concrete and the reinforcement.

· All concrete shall be compacted using a mechanical vibration process.

· Inclined the boom of the concrete pump to minimize free fall distance to avoid segregation, smaller size can be used for thin walls.

· Pumped concrete circulation frequency shall be in line with the rate of supply and pouring rate.

· Do not add anything to the pump hopper during casting.

· Pipe aligning should not be vertically installed.

· The concrete pump hopper shall be always full of concrete to avoid additional air entrapment to the mixes and maintain the pumping pressure rates.

· If concreting should stop for a period exceeding 30 minutes, the concrete in the pipes and hopper shall be pumped outside the structure

2.5.3.13 Deshuttering of formwork and Concrete Curing

· Exposed surface of concrete shall be covered with hessian cloth and shall be kept wet by water ponding water on the concrete surface for 7 days.

· The hessian cloth placed on top of exposed concrete shall be wet at all times.

2.5.3.14 Concrete Quality and Repair

· Before any other activities will commence, upon curing is completed, concrete surfaces shall be inspected for its quality for any defects such as cold joints, honey combing, cracking, or surface imperfections. Any concrete repair will be made on case-by-case and submitted under separate cover for approval.

2.5.4 Summer Weather Concreting

In summer weather special precautions are required to avoid the rapid loss of surface moisture and/or rapid stiffening of the concrete which prevents its proper compaction, further to the increased heat of hydration and setting.

High temperatures, high wind speed, low humidity are all under the precautions of hot weather which will be the directed cause of settlement and plastic cracking and a reduction in strength and durability of the concrete, further to the potential formation of cold joints.

· Concrete should not be placed if the shade temperature exceeds 40°C on a rising thermometer.

· At the time of placing, concrete shall have a temperature lower than 32°C.

· Particular attention will be given in placing, consolidation, floating or any other mean to protect the surface from drying prior to bleeding.

· To prevent moisture loss from the concrete, provide curing system immediately to open surfaces initial floating.

· If the surface is not to be marked, it is best to provide the initial curing with sheets of impervious material (polyethylene), preferably pigmented to reflect radiation, supported away from the surface and fastened at the edges to prevent droughts.

· Rate of evaporation from the concrete surface shall be determined prior scheduling the pour; accordingly the QA/QC team shall decide the pouring and protection procedure or advice for rescheduling the pour if the rate is too high

· Provision of shading from wind if practicable and additional available water to keep the surface wet after bull floating will aid in decreasing the afore-mentioned risks

· Use of curing compounds where allowed, will aid in reducing the evaporation.

· Continuous flow of concrete is essential.

· Increased rate of pouring and double crews for faster application

· Preparatory for cracking recovery in case it developed fast prior concrete hardening

2.5.5 Consolidation

To cover properly the congested steel structures, shove away coarse aggregates particles from the forms, and to facilitate movements of excess water and air pockets and entrapped air to the top surface of concrete.

· Compact thoroughly the concrete using applicable vibrator or other means during placing. Vibrate the concrete until the expulsion of air has practically ceased and in a manner that does not promote segregation.

· Vibration on the location of congested reinforcement, duct, sleeves, anchors, embedded items should be carefully executed to eliminate the voids or honey combs that may affect the appearance or integrity of the concrete.

· Over-vibration should be avoided to minimize the risk of forming a weak surface layer (sand streak, scalling and lower concrete strength)

· Consolidate concrete by layers. The concrete will be placed in a continuous operation in two to three lifts as per the required thickness or 450 mm high by making passes around the perimeter or section of the wall.

· Consolidate the concrete by inserting Vibrator heads vertically and slowly into concrete. QA/QC Engineer/Inspector will monitor any irregularities on the use of internal vibrators.

· Vibrate the flowable concrete between 3 to 5 seconds, and up to 15 seconds for low slump/workability concrete.

· Pull out the vibrator vertically and at a maximum rate of 7 cm/sec. Insertion offsets of 1.5 times the radius of influence of the vibrator.

· Insertion shall penetrate slightly to the preceding layer to knit both layers together.

· In case of external vibrators, use those that conform to the design of the formworks to ensure efficient compaction and to avoid surface blemishes. There must be 2 vibrators in each concrete pump.

· Care should be taken on the type of vibrating procedure to account for the energy transition whenever a permanent formwork is incorporated.

· Extra care is required to ensure full compaction of the concrete since this cannot be checked when the formwork is removed.

· Vibrate thoroughly the concrete containing Slag cement to decrease the effect of settlement due to self-consolidation.

· Concrete containing fibre reinforcement materials affect the workability and should be vibrated thoroughly to have a good compacted mass.

2.5.6 Finishing & Setting

Top surfaces of concrete, buttresses and floors shall be finished to the proper level and, if specified, to the required slopes. No holes or rough patches shall remain. Surfaces requiring subsequent plastering or tiling shall be rough-finished to provide a proper key for the finishing layer.

After initial setting of the concrete, any disturbance thereon (e.g. from walking, wheeling over or vibration of the formwork) shall be prevented until the concrete has sufficiently hardened.

Protect the reinforcement, anchor bolts, embeds, etc. projecting from the concrete from disturbance during hardening of the concrete.

2.5.6.1 Screeding and Strike-Off

· Set the level guide using rigid metal or wood true to level and achieving the specified grade in accordance with project requirement and as indicated in the shop drawing. Set the level guide in a manner that it can be easily removed after screeding and strike off process.

· Screed surface of the last layer with a straight edge and strike off (preferably aluminium bar) concrete surface to correct elevation directly after vibrating the last layer.

· One person shall be holding the straight edge from each side, and careful backward and forward motions like sawing and occasional tamping where required then repeating the strike off process is helpful to maintain a smooth surface to facilitate bull floating.

· The area to be screeded shall not be extended very long, where it will be very difficult to control the screeding operators and increased drying of the surface will occur. A good practice to avoid this and speed up the available concrete is to pour a layer up to the reinforcing level preceded by another layer to the required level or double the finishing crews.

2.5.6.2 Bull Floating/Power Floating

· The direct step after screeding and strike off for all kinds of finishes.

· Proceed to initial floating using bull float, Darby, or modified highway straightedges before any excess moisture or bleeding water is present on the surface to prevent dusting or scaling. This process will set the remaining voids and drain aggregates, level all area and cover air buckets left after screeding until we get a smooth surface.

· In the event that multiple floating passes are required, each floating operation should be made perpendicular to the direction of the previous pass.

· Do not disturb the surface after floating for a specified time to allow bleeding to take place.

· Power floating can also be applied by the time the concrete is hard enough to carry the load of the power float. This is to eliminate the minor cracks appearing as the concrete hardens.

· Prevent the surface from drying during this time by fog spraying or spraying softly water incorporated with liquid solvent curing compound.

2.5.6.3 Troweling

· Start finishing the surface with metal trowels directly after bleeding and when the concrete is accessible, covering the whole area. Wet the surface if necessary prior to trawling if the surface is very dry.

· Proceed to additional trowelling if necessary until we get the desire surface finish.

· Final finishing of the concrete surface will depend on the use and the type of sub-sequent layer/screeds. (e.g. sheet Vinyl, self-finished resin, safety flooring, carpet tiles, etc.)

· Re-trowelling may be needed if plastic shrinkage crack appears in some areas after the last trawling operation, additional water may be used to assist troweling to close the cracks.

· There will be at one person to monitor those hairline cracks resulting from plastic shrinkage cracking to cover prior to expanding.

· Mechanical trawling is recommended especially in hot weather and large areas

· Two passes of mechanical trowelling will produce a smooth finish

· When cracking are spreading drastically only mechanical trowels will aid to recover it prior drying of the concrete.

2.5.7 Concrete Stripping

The operations of stripping the formwork and striking will be conducted progressively, without impacts. The following minimum times before stripping the formworks as per the project specifications.

Type of Formworks	Minimum Period Before Stripping (Times are from Concrete Placement
Beam Sides, walls and column	refer to specification or standards
Soffits of Slab (Props left Under)	refer to specification or standards
Soffits of beams, joists and girders (props left under)	refer to specification or standards
Props to slabs	refer to specification or standards
Props to beams	refer to specification or standards

Remove formwork, for beams sides, walls, columns and similar parts of the Work that does not support weight of concrete after cumulatively curing for 24 hours after placing concrete provided that concrete is hard enough and not to be damaged by form-removal operations will be discussed for the permission of consultant as well to save time and having records to the past projects as per standards and approval of consultants.

While performing de-shuttering, all safety measures will be implemented. Sufficient staff will be provided to handle the loads properly with corresponding platforms provided. The platform system will be installed as per the manufacturer’s recommendation that would be checked from time to time by the Scaffolding Inspectors.

Support of tower crane in order to place and remove the formworks will be done accordingly. The Lifting Procedure will be provided in cooperation with the HSE Department.

It is important to mention that boxes will be provided on designated areas for de-nailing during removal of formworks as per the schedule provided.

Clean and repair surfaces of forms to be reused in the work. Split, fray, delaminated, or otherwise damaged form-facing material will not be acceptable for exposed surfaces. Remedy for the formworks such as placing tape to the laminated plywood could be done accordingly to prevent the product damage. Apply new form-release agent. When forms are reused, clean surfaces, remove fins and surplus, and tighten to close joints. Align and secure joints to avoid offsets. Do not use patched forms for exposed concrete surfaces unless approved.

Maintain properly the formworks material to be re-used; the execution team shall take into account the bearing capacity of reused forms, stiffness and safety.

METHOD STATEMENT FOR CONSTRUCTION