Method Statement for Post tension works .
Method Statement For Implementation of POST TENSIONED SLABS - BUILDINGS Project
:
ORION CITY
Client
:
M/s Sanken Construction Pvt. Ltd.
Consultant
:
Design Consortium Ltd. 85, Kynsey Road Colombo -08, Srilanka.
Tech9 Engineering Solutions Pvt. Ltd H. No 7-1-28/A/2, Park Avenue Colony, Ameerpet, Hyderabad - 500016. Ph. +91 40 23743291, Fax : + 91 40 23743292 mobile: +91 94901 89735, +94 7571 51069 e mail :
[email protected],
[email protected]
-1-
Method Statement for Post tension works .
1.0 STANDARD WORK METHOD OF POST TENSIONING (Multi Strand Flat & Round Duct, Bonded System) The general standard work method for post tensioning would be sequenced as follows: tendon Installation, Concrete placement, tendon stressing and tendon grouting. GENERAL: The construction of a post-tensioned slab is broadly similar to that for an ordinarily reinforced slab. The construction sequence of Post tensioned slabs as follows. 1. Shuttering of Slabs: 2. Placing of Bottom reinforcement 3. Tendon installation. 4. Placing of additional reinforcement (Top reinforcement). 5. Concreting 6. Stressing 7. Grouting 8. De shuttering of slabs 1.
SHUTTERING OF SLABS:
Shuttering/form work for PT slab is must designed to with stand the all external working loads coming to the slab. The shuttering of PT slab should design and approved by Main consultant. The Main contractor should follow the check list formats. 2.
PLACING OF BOTTOM REINFORCEMENT:
The placing of bottom reinforcement should follow the PT rebar drawings, which is approved by main consultant. The reinforcement should have all required development lengths and lapping length. Additional to PT rebar drawings The structural consultant may add additional reinforcement, So, it should get approval from Main consultant. The contractor also should follow the reinforcement check list. 3.
TENDON INSTALLATION 3.1 Set up of pre-stressing tendon components
Lay duct into position and connect ducts with duct couplings The strand is cut to tendon length and insert or push the strand into the duct with one strand at a time Set up bar chairs and profile according to the location given on drawing. Tape all duct joints -2-
Method Statement for Post tension works .
All tendons should be placed in the location and profile as shown by Post Tensioning Shop Drawing.
3.2 Set up of stressing anchorage components
Set up shuttering formwork at end of Slab and stressing recess on shuttering formwork at end of Slab. Install anchor casting to stressing recess formwork. Provide and fix anchorage bursting reinforcement (spiral/ square links for multiple) Support and fix the anchor casting into position using chair Seal all connections of components with tape
3.3 Set up of DE anchorage components 4.
The onion (bulb) dead end type will be used unless noted otherwise in the specification and PT shop drawing. The strand for each tendon will be formed with onion dead end from the work shop or at site by means of a special hydraulic jack. Anchorage bursting reinforcement at DE will be fixed into position as shown on the PT shop drawing.
ADDITIONAL /TOP REINFORMCENT
The placing of top reinforcement should follow the PT rebar drawings, which is approved by main consultant. The reinforcement should have all required development lengths and lapping length. Additional to PT rebar drawings The structural consultant may add additional reinforcement, So, it should get approval from Main consultant. The contractor also should follow the reinforcement check list. 5.
CONCRETING
The following general procedures are recommended for placement of concrete
Pouring concrete shall be done when the steel reinforcement and posttensioning components are completely installed Should construction joints be needed during the pour joints shall be at the location shown on the drawing. Tendons shall be observed during concreting. Any misalignment of tendon shall be rectified. After concreting is complete the concrete should be cured as appropriate until the Strength has attained as specified prior to stressing operation.
-3-
Method Statement for Post tension works .
6.
Sufficient number of cubes shall be taking as samples for testing compressive strength.
STRESSING
General The tensioning operation shall not begin until tests of concrete cylinders/cubes cured under job site Conditions indicate that the concrete in the members has attained the minimum compressive strength specified for stressing in the project contract documents.
Confirm the cube strength of concrete. Average cube strength for a minimum of 3 cubes shall be 75 % of specified strength Take off side shuttering formwork at the end of Slab and stressing recess formwork prior to stressing operation. Clean up strand prior to stressing operation Place anchor head / Barrels and wedge into each strand. The stressing equipment consists of mono strand stressing hydraulic unit for tendon size as specified and ancillary components (including hydraulic pump unit with pressure gauge). Stressing shall be carried out according to the following procedure
6.1 Installation of the Jacking Unit: The fixing of the jacking unit to the stressable tendon end shall be carried out in the following manner:
The jacking unit is installed over the strand so that the jack nose is in complete contact with the surface of the anchor plate front. The jacking unit is aligned with the axis of the tendon to be stressed After assuring that the pressure hoses are intact and the jacking unit is properly aligned with the tendon axis, the actual stressing operation can commence.
6.2 Stressing of the tendon: The stressing of each tendon is performed along the following steps:
The stressing operation is undertaken by the pre-stressing technicians in the presence of the Engineer’s representative.
-4-
Method Statement for Post tension works .
A stressing record form is prepared for each tendon to be stressed on which all steps and occurrences of the procedure are recorded. The stressing record sheet is filled out by the pre-stressing technician in charge of the operation. A copy is submitted to the Contractor and the Engineer. The stressing records shall also contain the following information: Project name’ number, part of the project and tendon identification number. Type of jacking unit and identification number. Type of pump and pressure gauge with their ID number Tendon size and design jacking load with corresponding design pressure according to the calibration chart. Design cable extension and tolerance variation of ±7% Date of stressing Signature of the responsible pre-stressing technician and the Consultant’s representative. The stressing sequence shall be observed as indicated on the post-tensioning shop drawings. If stressing at both tendon ends is specified. The stressing should be done from one end of tendon at a time and topple up at the other end of tendon, stressing records shall be taken at both ends. For mono strand stressing, initially 25% of the jacking loads are applied and a marking done on the strand near the permanent compact wedge. After marking is done, the remaining from 75% of loads applied and elongations noted. Final elongations calculated on extrapolation basis. When conformity of the design elongation with the measured one is ascertained, tendon force can be anchored. The lever of the valve at the pump is pushed from forward to return, and the safety return valve at the jack is gently opened. The tendon load is uniformly transferred from the jack to anchor head with even draw-in of the anchor wedges. If one piston stroke is not sufficient to stress the cable to its full design jacking load, re-gripping is required, whereby the following procedure has to be observed. The piston stroke is fully retracted by activating the return pressure of the pump. Subsequently the standard stressing procedure is followed again for further strands. Elongations achieved on each strand on average shall be considered for total elongation of each tendon. Upon successfully completed stressing of the tendon, the jacking unit can be removed and installed on the next tendon. Upon approval by the consultants, the strands protruding from the anchor head can be cut off at a distance of not less than 50 mm from the anchor-head surface by means of an abrasive disc cutter. -5-
Method Statement for Post tension works .
7.
The tendon can now be made prepared for grouting. Between stressing and grouting of a tendon not more than two week’s time shall elapse.( a maximum of 21 days)
GROUTING
Grouting should be performed as soon as the stressing operation is complete Grout vents are provided at anchorages for each cable. The anchorage should be capped with concrete or suitable capping material prior to grouting operation After the anchorage has been capped the tendon shall be water flushed through the grout hole which projects from the top of the duct As soon as flushing is completed the grout operation shall start. Grout mix shall have the mix proportion of water/cement ratio as specified by the weight. The additive admixture is mixed with cement in proportion as recommended by additive manufacture. Typical proportions of grout mix are as follow. Water cement ratio = 0.40 by weight. - Sika Intraplast N 200 (200 gm per bag of cement by weight) (OR) Fosroc Cebex-100 (225 gm per bag of cement) additive is mixed to Portland Cement Type I of weight 50 kilograms, Tap water (w) 20 liters is added to the grouting cement (with additive). 3 samples of 100 cubes shall be collected and tested at 7 days and 28days. The compressive stressing of cubes expected shall be 17 and 28 N/sqmm respectively. After completion of grouting the projected portion of grout vent shall be cut flat to the slab level by means of abrasive disc.
Grout preparation process:
The selected water content at the rate of 20 liters for 50 kg of cement, should be accurately measured into the mixture. Slowly add the cement and additive mix continuously for 5 min. Making sure that a smooth consistency is obtained.
-6-
Method Statement for Post tension works .
Grout Admixture: Material DATA SHEET BONDIT or CEBEX-100 or DR FIXIT
BONDIT GROUT MIX EXPANSIVE ADMIXTURE AND GROUTING AID Description
BONDIT GROUT MIX is a unique formulation to impart non shrink properties to cement when used for grouting purposes.
Characteristics Compressive Strength
It allows water reduction and hence gives better strength.
Advantages
BONDIT GROUT MIX helps in reducing water and gives flowable cement grout. BONDIT GROUT MIX gives high strength at early stages. BONDIT GROUT MIX is chloride free and iron free, hence there is no fear of corrosion. BONDIT GROUT MIX helps in imparting non-shrink property to properly made cementitious grout. BONDIT GROUT MIX reduces permeability and increases durability of the cementitious grout system as it aids in water reduction.
Areas Of Application
The mixed grout or concrete with BONDIT GROUT MIX should be used within about 20-30 minutes. Care should be taken to ensure cleanliness of the substrates and they should be moist enough. Grout should be used as early as possible maintaining a continuous flow-otherwise it may lead to placing and pumping problems. All loose particles laitance, grease and should be removed from the substrate.
Standards
BONDIT GROUT MIX complies with BS110 part, 1985 as stressing grout admixture.
Dosage
Pressure grouting with only cement: 1 Pack /50 Kg cement; Gravity Grouting Cement grout: 1 Pack/ 50 Kg (cement sand 1:1) of cement; For concrete: 2 Pack/50 Kg of cement. With 50 Kg cement -1 pack of BONDIT GROUT MIX will yield 36 litres of neat cement grout at a w/c ratio of 0.41-0.45 with C: S-1: 1-1 pack of BONDIT GROUT MIX will yield 56-58 litres of grout at a W/C ratio of 0.45
Precautions
It is not compatible with high alumina cement. BONDIT GROUT MIX is nontoxic, non-flammable. It is better to handle with gloves. Splashes if any should be washed with plenty of water. When it comes in contact with eyes take medical advice.
-7-
Method Statement for Post tension works .
Shelf Life
one year (if stored in cool and dry place)
Supply
BONDIT GROUT MIX can be supplied in 225 grams packet.
2.0 Safety Procedures This section is not intended as health and safety guidelines which would address all considerations typically required to maintain safe construction. This section is only intended to advice on common safety issues during post-tensioning construction. 2.1 General Job site safety is one of the most important factors. It is important to be acquainted with, and follow all applicable government safety regulations including all applicable standards, state and local regulations and all job site regulations. This is for the safety and well being of all field personnel. It is important to follow stressing procedures, including stressing sequence as defined on the approved installation drawings. This is important to prevent damage to shoring, to the structure, to you, and to you co-workers. The following safety requirements would be enforced during the stressing operations: 2.1.1 A warning sign, “Stressing Operations in Progress” shall be put up at locations where stressing operations would be carried out. 2.1.2 Strand bundle protruding from the bearing plate must be encased in a plastic or sheet metal pipe to prevent injury by the sharp ends of the strands. 2.1.3 During the process of strand cutting, only abrasive disc cutters may be used, and goggles must be worn during the process. 2.1.4 Only intact and tight pressure hoses shall be used. 2.1.5 Never allow anyone to be in the immediate vicinity of the stressing equipment during stressing to avoid injury in event the tendon or concrete should fail. 2.1.6 The pump and jack should be securely tied to a solid object when working at an elevated level. 2.1.7 Never use stressing equipment or accessories that are not complete as supplied from the manufacturer or post-tensioning supplier. -8-
Method Statement for Post tension works .
2.1.8 Do not stand on top of the slab directly above or near the stressing or fixed-end anchorage location. Also, take precaution that no one is standing behind the jack or along the line of the tendon while stressing is taking place. 2.1.9 Remember that stressing a tendon involves a high force transfer at both ends of the tendon and the areas at each end of the tendon require the same precautions. 2.1.10 Stressing should take place on a proper working platform, as required by the regulation agency. Do not reach over the slab to stress tendons. This area should be clean and free of debris. 2.1.11 Keep hands out of jack while stressing operation is taking place and while retracting. 2.1.12 Stand clear of equipment, hoses or electric cords while stressing is taking place; always use the remote switch and not the toggle switch located on the pump unit. If a tendon should fail or a blowout occur, or the jack grippers fail; the jack, pump, hoses and electrical cords can be thrown off the structure. If this occurs anyone standing close the hoses or cords may be dragged off the structure with the equipment. 2.1.13 If de-tensioning is necessary; it should only be done by qualified personnel using the proper equipment such as:
On a jack with removable nose-pieces, jack feet attached to the jack cylinders can be used if they sit firmly against solid concrete that is perpendicular to the jack.
A de-tensioning stool or stressing chair that sits firmly against solid concrete and is perpendicular to the jack.
A special de-tensioning nose-piece that sits firmly against the anchor.
2.1.14 If a jack gets “hung up” or should the anchor wedges fail, NEVER, UNDER ANY CIRCUMSTANCES use a second jack on the back of the one that is “hung up” (piggy backing). In this situation, special equipment or procedures must be used such as:
A troubleshooting anchor can be used with most types of jacks.
A de-tensioning stool or chair can be used if it sits firmly against solid concrete and is perpendicular to the jack.
If accessible, the anchor at the far end may be de-tensioned.
-9-
Method Statement for Post tension works .
Before any of these procedures are used, it is recommended that the posttensioning system installer be contacted and the situation discussed before any action is taken.
2.1.15 If the concrete is visibly honeycombed or cracked, DO NOT STRESS until it has been properly repaired and inspected. 2.2 DON’Ts of Stressing 2.2.1 DON’T stress any tendon which contains concrete slurry inside the anchor cavity. Concrete slurry in the anchor cavity will prevent proper seating of the wedges. 2.2.2 DON’T use the jack when it is not positioned properly on the face of the anchor. 2.2.3 DON’T overstress tendons in order to achieve proper elongation. 2.2.4 DON’T allow obstruction in the path of the jack extension. 2.2.5 DON’T use extension cords longer than 100 ft. (30 m), 2 wire cords or cords less than 12 gauge. 2.2.6 DON’T continue stressing if you suspect that something is not working properly. 2.2.7 DON’T detension with loose plates, spacing shims or by ‘piggy backing’ of jacks. 2.2.8 DON’T stand in the immediate vicinity of the jack, or between the jack and pump during stressing or detensioning. 2.2.9 DON’T permit other workman in the immediate area of the jack, or pump, or along the tendon during stressing application. 2.2.10 DON’T hammer or beat on the jack or jack cylinders. 2.2.11 DON’T activate equipment if you are not sure; ask someone who knows.
3.0 Quality Measures The following measures would be adhered to, on site and specifically to the post tensioning works, in addition to the QA/QC polices of the Organization:
- 10 -
Method Statement for Post tension works .
Inspection Before, During and After Stressing Tendons
Is the stressing anchor cavity free of concrete slurry, dirt and plastic?
Is a consistent reference dimension used for the elongation datum mark on the strand?
The accuracy of the datum mark is critical to achieving accurate elongation measurements and special care should be taken when marking the tendon tails. The same device should be used throughout the entire project.
Are the wedges new, with undeformed teeth; free of rust and steel shavings; and of consistent quality?
Are the wedges of proper size and length for the strand and anchor being used?
Are the wedges seated as evenly as possible? All pairs should be properly aligned and equally spaced.
Did you use a hand seating tool?
Is the stressing equipment well maintained and are current calibration charts available?
Is the stressing operator experienced and careful with the equipment and consistent from tendon to tendon?
Are the measured elongations in agreement with the calculated values shown on the approved installation drawings?
Are measured elongations properly records?
Have the measured elongations been approved?
After the measured elongations have been approved, have the tendon tails been cut of at the proper depth? Are pocket surfaces sufficiently clean to allow good grout bond during and after patching?
- 11 -
Method Statement for Post tension works .
4.0 Site Documentation The following records would be maintained on site and later analyzed and referred to where applicable: 9.1 Inspection Check List The Inspection check list would serve as a guideline and an integral part of the quality control measures as stated in Section 3. 9.2 Stressing Record Form The stressing record form would be used during the process of stressing: 9.3 Grouting Record Form The grouting record form would be used during the process of grouting.
EQUIPMENT DATA SHEET:
- 12 -
Method Statement for Post tension works .
- 13 -
Method Statement for Post tension works .
EQUIPMENT DATA SHEET: GROUTING
- 14 -
Method Statement for Post tension works .
- 15 -
