Method Statement for Precast Concrete Fabrication and Installation

method statement for precast

Purpose of Method Statement for Precast Concrete Fabrication and Installation

The purpose of this method statement for precast concrete fabrication and installation is to establish systematic procedures in performing safely the detailed precast methodology and ensuring that quality control objectives are maintained with accurate established records for all construction activities associated with the project.

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What is precast concrete construction?

Precast concrete construction is the process of producing casted elements by the use of reusable mold or form from setup/off-site yard then cured in the controlled environment, transported to the construction site and erected into place.

Complete template of Method Statement for Precast Concrete Fabrication and Installation along with the related attachments can be downloaded here.

Scope of Method Statement for Precast Concrete Fabrication and Installation

The scope of this procedure covers the steps to be followed for the Fabrication and Installation of Precast Elements such as foundations, columns, beams, staircases, manholes, and hollow core slab. The precast will be produced in xxxx precast factory located in xxxxx. This includes general requirements for materials, tools, equipment and all services and operations necessary for the complete supply and execution not limited with the following scope of works:

a) Mould Fabrication

b) Steel reinforcement assembly

c) MEP inserts, embedded items installation (if any)

d) Prestressing Wire Strands

e) Casting of Precast Concrete Elements

f) Sampling and Testing of Concrete

g) De-moulding and stripping

h) Detensioning

i) Repairing of damaged elements (if any)

j) Curing of precast elements

k) Site installation of precast elements

Definition and Abbreviations

SC           Supervision Consultant

COSHH  Control of Substances Hazardous to Health

H&S       Health and Safety

ISO         International Organization for Standardization

ITP          Inspection and Test Plan

MAS      Material Submittal

MST       Method Statement for Precast Concrete Fabrication and Installation

MSDS    Material Safety Data Sheet

PPE        Personal Protective Equipment

QA/QC  Quality Assurance/ Quality Control

RA          Risk Assessment

NTS        Notice To Start

PTW       Permit To Work

SUB        Sub-Contractor

TPL         Third Party Laboratory

ICF          Interface Control Form

RO          Road Opening

WIR        Work Inspection Request

MIR        Material Inspection Request

PCI         Precast concrete Institute

TBD        To be Determined

Mandatory Requirement

Induction and Training

Safety and Health induction shall be conducted by the Health & Safety Department prior to commencement of works to each person involved in works. All applicators involved in the works execution and the workers undertaken critical tasks shall be competent and certified to undertake the works. All workers will receive a daily briefing about the works they are to undertake that day.

Training in the factory and in site shall be a must requirement harnessing the competencies of the workers directly involved in the precast production and precast installation. In line with xxxx Precast Factory quality control procedure Quality Engineers will ensure adherence to the production process flow and Standards through quality tool box among personnel, operators, supervisors and skilled workers assigned in the factory.

The Contractor will guarantee updated certifications to its personnel and operators, and will ensure that works at the factory and in the project site to follow the Project Specifications and governing Standards.

Instruction and Supervision

The precast Factory Manager thru its Managers of different departments and Engineers will be responsible in production of precast elements compliance to the requirements of Project specifications and Reference Drawings.

Equipment and transportation requirements for the delivery and installation of precast elements will be arrange by logistic coordinator following the delivery route plan as per Appendix G.

The Construction Manager thru its Site Engineer and Erection Personnel is responsible for the job and proper execution of works in accordance to manufacturers’ installation instruction and approved method statement. He will be present at site during working hours together with Safety Officers to monitor all activities and ensure safety of the workers.

The Site Engineer will be in-charge and directly supervising Supervisors and Foremen, and carry the job at site. Any completed activity must be channeled and coordinated to the Quality Department. Acceptance of completed activity in line to the approved ITP will be closed through Work Inspection Request (WIR) approval by the Supervision Engineer.

Plant and Equipment Certifications and Calibrations:

XXXXX Precast Factory will ensure that updated and recent certifications and calibrations of batching plants and equipment are in place though out the production timeframe of precast elements. Equipment that is near to expiration shall be monitored and timely calibrated.

Preliminaries

Permits and Licenses

The Stakeholder and Permit Coordinator will be responsible acquire the Authority approval of the necessary permits required to commence site activities.

The following Permits are required and will be obtained:

Permit Scope     Issued By

Permit To Work (PTW)   xxxxx

Road Opening Clearance               xxxxx

Traffic Diversion (If applicable)   xxxxx (Traffic Department)

Appendix F details all the permits obtained.

Notice to Start (NTS)

The contractor will secure approval from Supervision Engineer prior to start any activity through Notice to Start NTS submittal. The contractor will ensure that all relevant documents approval is in-place as it is the primary requirement of NTS.

Notice to Start or NTS must be submitted 7 days from the start of the activity in order for Supervision Engineer to review and correlate the received relevant submittals with respect to the subsequent activity.

Testing Agency

The contractor will ensure that approved 3rd Party Laboratory is present throughout the production stage at the plant to carry out the sampling and testing regiments.

Work Execution of Method Statement for Precast Concrete Fabrication and Installation

As per Project Specification Section, and PCI MNL 116 & 4th Edition and PCI MNL 135.

General

The contractor will comply to Precast Concrete Institute (PCI) MNL 116 and 135 Manual for Quality Control for Plants and Production of Structural Precast Concrete Products for manufacturing procedures, testing requirements, quality control recommendations and dimensional tolerances for the types of units required.

For fabrication of precast/prestressed structural products, self-stressing forms, bed abutments, and anchorage shall be designed by qualified engineers.

The contractor will deploy qualified concrete installer certified by PCI to install units for load-bearing members.

Build mock-up will be prepared complete with anchors connections, flashings, joint fillers and all accessories. The first completed structural unit of each type of a system will serve as a Quality Benchmark and will be raised thru Work Inspection Request (WIR) where the location to be agreed by the Supervision Engineer.

The procedures for the Fabrication and Installation of Building Precast Concrete Elements are detailed below:

1.1      Pre-Production Requirements

Factory Manager shall ensure the approval of the below documents from Supervision Engineer:

a.    Pre-qualification of Precast Factory

b.     Shop Drawings

c.     Design Mix

d.     Materials

e.   Method Statement for Rebar Works & ITP

1.2  Production of Precast Elements

Concrete Constituents

Concrete raw material components (Cement, sand, aggregates, water & Admixture) being used in the production of precast elements from the factory batching plant.

1.3      Testing of Concrete Constituents

The testing procedures and frequency of concrete constituents testing shall be as per Project Specifications Section xxxxxxxx

1.4      Sequence of Work for Precast Element

Unless otherwise specified:

Mould fabrication & preparation
Fabrication and assembly of steel reinforcement
Placement of steel reinforcement into the mould
Embedded parts (if any)
Prestressing of wire strands
Concreting works
Curing
De-moulding
Stripping
Detensioning
Defects and Repairs if any
Curing
Storage & Stacking
Handling
Delivery
Site installation/Erection – Proper surveying and setting out of the installation area is required

1.5      Embedded Parts/Items

a)       Place the embedded items as per approved drawings and approved submittals (if required)

b)        Embedded items must be checked by production engineer/supervisor prior to casting of element before concrete. All embedded items, insulation grouts; sealants, backing rods, and other materials to be used shall be as per approved Material Submittal (MAS). All cast in parts shall be fixed in position as per approved Shop drawings.

MEP Inserts

Prior to casting of precast elements, it will ensure that all MEP inserts & points i.e. Electrical boxes, conduits etc. are provided based on approved drawing locations. QC will ensure that its properly fixed based on approved drawings.

2.1    Concreting

a)            Precast casting is planned in such a way that concreting work is done in one complete operation without breaks. Concrete is brought by either the concrete transport shuttle from the batch plant or through the transit mixer discharged via mixer chute or extended chute, to the casting mould. Time interval is observed to avoid the occurrence of cold joint, QC Inspector will ensure the batching with approved mix design & its batching record at the check point of lab at the check point material engineer will conduct all fresh concrete tests according to Inspection & test plan. QC Engineer will ensure the concrete temperature at the time of concrete placing it should not exceed 32°C.Concrete casting in hot weather shall be according to Project Specifications.

b)            Lay the concrete up to the mould in such a manner that the entire mould surface is covered evenly. Air concrete thermometers shall be kept on the Site. Concrete shall not be placed if the shade temperature exceeds 36°C or the concrete temperature exceeds 30°C, and 25°C for large pours.

c)            Vibration: For casting on table base mould with built-in high frequency vibrators: Switch on the vibrator at pre-determined period of time as instructed by the Production in charge or QC inspector.

d)            For casting on table base mould without built-in vibrators: Compact the concrete by using poker vibrators, immersion type. Vibration is applied continuously for predetermined time during the placement of each concrete batch. Special attention is given to difficult areas where heavy reinforcement occurs to prevent the presence of void or honeycomb in the concrete. Level the top surface and remove excess concrete after concreting. After the initial setting of concrete surface, apply the required surface finish as per requirement of the Specification.

2.2    Prestressing of Wire Strands

As per PCI MNL 116 4111 Edition

The Contractor will comply to PCI MNL 116 in prestressing of tendons for precast concrete units.

This is accomplished using hydraulic pumps and jacks/cylinders, with the tendons anchored on abutments at either end of the bed.

Tensioning will follow the below statement:

Well trained personnel will be deployed to perform and record the tensioning process.

a)            Precast QC Inspector will ensure that jacking equipment and gauging system are in good condition with updated calibrations.

b)            Initial tensioning will be done, and care will be taken in applying the initial force ensuring a valid starting point is established for elongation measurement. Initial force ranging 5 and 25% of final force will be applied to each strand.

c)            Reference marks shall be established for which elongation by final tensioning can be measured. A straight-line marking will be done across the strands on each row and along the face of anchorage.

d)            Full strand force shall be applied to either single strands or multiple strands whereby the final force on the strand shall not exceed 80% of the specified tensile strength of the strand after seating.

e)            Verify by measurement of movement on opposite sides of the anchorage and check plumb position before and after application of final force.

f)             Verify by measurement of movement on opposite sides of the anchorage and check plumb position before and after application of final force.

2.3    Hot Weather Concreting

As per Project Specification :

a)            The Contractor shall establish a thermometer at Precast Factory that records the ambient air shade temperature. The thermometer shall be established at a position to provide representative air temperature for the Site conditions.

b)            During hot weather period as defined in clause 15.3 of QCS 2014 Section 5 Part 15, no concreting will take place between the hours of 10:00 hours and 17:00 hours unless authorized by the Supervision Engineer.

c)            The “Concrete in Hot Weather” as defined from Project Specifications states:

Air concrete thermometers shall be kept on the Site. Concrete shall not be placed if the shade temperature exceeds 36°C or the concrete temperature exceeds 30°C, and 25°C for large pours.

d)            As per Project Specification Hot weather concreting shall be carried in accordance with ACI 305 under a combination of high air temperature, low relative humidity and wind velocity that tends to impair the quality of fresh or hardened concrete.

e)            Where trunking or chutes are to be used, they shall be kept clean and used in such a way as to avoid segregation.

f)             Measures shall be taken to avoid premature stiffening of concrete placed in contact with hot, dry surfaces. Surfaces including reinforcement against which concrete is to be placed shall be shielded against the direct rays of the sun and shall be sprayed with water to prevent excessive absorption by the surfaces of water from the fresh concrete.

g)            During hot weather, placing of concrete shall largely be avoided during the midday period, and if necessary, shall be carried out at night with adequate lighting.

h)            Lower the water temperature by mechanical refrigeration or by mixing water with crushed ice, or by pre-cooling the aggregates ensuring that the temperature of the concrete as delivered shall not exceed 26°C for large pour construction and 30°C for other concrete structures.

i)             Submit for the Engineer’s acceptance calculations or experiments showing that the dry bulb temperature of the placed concrete does not exceed 38oC and that the rate of water evaporation does not exceed 980g/ m2 per hour, taking into consideration the heat of hydration of cement.

j)             The admixture shall meet the requirements specified within this Section. During the placing of all concrete exposed or subject to rapid evaporation from drying conditions including hot weather, drying winds and sunlight, the work shall be protected immediately following screeding.

k)            Spray protective coating over the entire surface of fresh, plastic concrete in accordance with the protective coating manufacturer’s recommendations following screeding at a rate of not less than 10 gallons of sprayable solution for each 2000m2 of surface area, 5m2 per liter.

2.4    Curing

Panel/elements are covered with Polythene sheet and hessian cloth to avoid dehydration. All produced panels/elements will be cured by water continuously for 7 days after tilting and stacking. Alternatively, approved curing compound may also be applied after 24 hours of water curing as per Engineer’s approval.

2.5   De-moulding and Stripping

a)            De-mould and strip the precast units off the mould once they have attained the required lifting strength or as per approved design. The lifting strength is determined by crushing one Concrete cube at 8 hours after casting. The crushing results for every batch of cast units are reflected on the Daily Cube Crushing Data. At the time of lifting, visually check the cast.

b)            As per PCI MNL 116 4th Edition-Division 4 Concrete as per Commentary C4.18.3 Curing to Attain Specified Stripping or Transfer Strength: Stripping or prestress transfer strengths are typically specified at a minimum of 2000 psi (13.8 MPa) for non-prestressed units and 3000 psi (20.7 MPa) or greater for prestressed units. Specified release strengths higher than this may require special mix designs, special curing provisions or longer curing cycles.

c)            Once the precast unit is stripped off the mould, mark the precast element taking into account the identification number and casting date as per approved unique numbering style. The marking is placed in such a way that it is visible even when the unit is in stacking yard.

d)            Precast elements are stripped off the mould and loaded to a truck and get pick up eventually by the overhead tower crane and stack in the stockyard. If immediate transfer is deemed not possible, precast elements are stripped off

the mould and stack temporarily in a holding area provided in the factory until their transfer on-site is required for transfer of units from factory to on-site stack yard by the flatbed trailer or A-Frame trailer depending on geometry of precast panel.

2.6    Detensioning

a)            Force shall not be transferred to the pretensioned members until concrete strength determined from test results is in accordance with the specified transfer strength.

b)            As per PCI MNL 116 4th Edition – Division 5 – The Reinforcement and Prestressing Part 5.3.17 Detensioning- Force shall not be transferred to pretensioned members until concrete strength, as indicated by test cylinders or other properly calibrated nondestructive test techniques, is in accordance with the specified transfer strength., Commentary C5.3.17-Detensioning A minimum concrete transfer strength of 3000 psi (20.7 MPa) is recommended.

c)            Detensioning tendons will be done by gradually releasing tensioning jacks or by heat-cutting to prevent shock or unbalanced loading.

d)            Detensioning heat cured concrete will be done while concrete still warm and moist to avoid dimensional changes and avoid cracking or undesirable stresses.

e)            Apply approved bituminous, zinc-rich or epoxy paint at the strand ends to avoid corrosion and possible rust spots.

2.7    Repair of Damaged Precast Elements (if any)

Inspection and Preparation

a)            QC inspector will conduct the inspection. Standard identification of defects and conducts repairs if applicable, only approved material will be used for repair work by following procedures recommended by manufacturer.

b)            Repairs to be carried-out either in the Factory or at the site.

c)            Inspection will be carried out to check the repairable concrete elements. Subject to repair elements will be kept in separate designated area. Prior to start repair works, it will be verified that elements surfaces are free of dirt, grease, oil, curing compound, paints and all loose material.

d)            It will be confirmed that the concrete has been cured enough to achieve design strength.

e)            The surface will be roughened by chipping, sandblasting or other mechanical means to assure bond of the repair mortar to the existing concrete.

f)             Prior to conduct the concrete repair/patching, the defect/damaged precast elements shall be inspected by the AEC inspector for acceptance to proceed with the repair activity.

Substrate Preparation

a)            Areas to be repaired will be determined by visual inspection and hammer sounding. Highlight all areas where concrete quality is suspected.

b)            Saw cut perimeter maximum 10mm depth but do not cut steel.

c)            Remove poor quality concrete in repair area until sound concrete appears. Ensure surface is left in rough condition. All edges ideally should be cut at right angles.

d)            The concrete steel reinforcement to receive the repair must be cleaned. If half of the diameter of the bar is exposed, chip out behind the rebar minimum of 10mm.

Resurfacing Application

a)            Pre-wet the concrete surfaces before application of repair mortar with clean water. Surface to be repair must not be allowed to dry out before application.

b)            Mix repair material following the manufacturer’s instructions.

c)            Apply repair mortar to the prepared area by hand or trowel.

d)            Apply the repair material as recommended by manufacturer technical data sheet.

e)            Finish off and smooth the repaired area.

Fabrication Tolerance

Precast concrete units must straight and true to size and shape and complies with PCI MNL 116 and the following product tolerances:

a)       Overall height and width of units, measured at the face exposed to view, as follows:

1. 3m or under, ±3mm.

2. 3m to 6m, to +3mm, -5mm.

3. 6m to 12m, ±6mm.

4. Each additional 3m, ±1 .5mm.

b)            Overall height and width of units, measured at the face not exposed to view, as follows:

1. 3m or under, ±6mm.

2. 3m to 6m, +6mm, -10mm.

3. 6m to 12m, ±10mm.

4. Each additional 3m, ±3mm.

Storage and Stacking

a)            Precast units will be stored with adequate dunnage and bracing and protect units to prevent contact with soil to prevent staining and to prevent cracking, distortion, warping or other physical damage.

b)            It must be stacked the right way up with the timber bearers placed just in from the ends, (maximum four times unit depth, “H”) vertically above one another. Similar lengths should be stacked together.

c)            Place stored units so identification marks are clearly visible and units can be inspected.

Delivery and Handling

a)            Precast units will be handled and transported in a position consistent to their shape and design to avoid excessive stress which would cause damage.

b)            Precast concrete shall not be not be moved, struck or propped until the concrete has reached a cube strength of at least 3 times the maximum stress to which it is subjected.

c)            Lift and support precast units only at designated points on the Shop Drawing. Lifting shall be carried out through designed lifting points.

d)            Delivery vehicle operators shall drive with care and will be assisted by a signaler knowledgeable with traffic control procedures in areas wherein maneuvering is difficult and operators view is obstructed.

e)            Cranes supplied will be of adequate size to safely lift the precast units in off-loading from the delivery vehicles.

f)             Delivery vehicle will be position as close to the crane unloading area as possible. It must be situated on terrain that is as level as possible.

g)            Lifting Supervisor will ensure safe working load of the hoisting equipment, rigging and hardware before use.

h)            Before mounting the precast units ensure that all safety precautions are in place.

i)             Make sure all loads are properly rigged, balanced, and stable before hoisting. Tag lines must be in place and be used to control the loads and to prevent rotation.