This article is about Method Statement for Fabrication and construction of Hollow Core slabs.
Hollow Core slabs Method Statement for Fabrication
1. The Mould Preparation:
• The QC shall ensure that the basic bed is level, robust and free face deterioration which could impair the quality of the finished concrete bottom surface.
• The Casting bed will be thoroughly cleaned and an oil base releasing agent will be applied manually.
2. Placement Of strand and Pre-stressing:
• Wooden strips will be placed at certain intervals to prevent the strand from touching the form oil and lose the bond between strands and concrete.
• The Strands will be placed along the bed as per the approved shop drawing that indicates location, numbers and diameters of strands.
• The strands will be fixed by the strand anchor chucks at both ends of the bed.
• Initially, all the strands will be pulled by 10% of the final pulling force.
• The strand will be marked clearly to allow the check of the elongation as specified in the shop drawings after applying the pulling force.
• The Paul stressing jack who is equipped with a hydraulic pump that has digital read-out or gage and pressure relief valves will allow setting the initial and final tension pulling force.
• The required pressure stress (bar) will be set and the jack will stop at that point during pulling.
• The strands will be accommodated individually inside the pulling jack with the force applied as shown in the shop drawing.
• As soon as the force reaches the required strand tension set earlier in the gage the Paul stressing jack will stop automatically.
• As counter measure the elongation will be checked to ensure that the strand is already stressed properly up to the required value.
• In Case it exceeds the allowable tolerance of 5% (PCI MNL-116 Division 5), the QC Manager will take the proper corrective action, such as: calibration of the pulling machine, testing the strand, checks the design calculation…etc.
3. Placing Of Concrete:
• Freshly batched semi-dry (zero slump) concrete shall be transferred to the HC machine; concrete shall be placed in a continuous operation to prevent the formation of seams or planes of weakness.
• Concrete shall be placed in the beds in compact mass with the mix and strength throughout casting in order to avoid segregation or variation in the concrete appearance when finished.
• Concrete will be uniformly consolidated by means of vibrating and compaction in order to consolidate the concrete into dense uniformed mass without perfection free surface.
• The measurements of the slab length and the cut-outs take place immediately after casting, at the same time the identification data are usually marked at the top of the slab (Element mark, date of casting, etc…)
4. The Curing and transferring the force:
• Steam curing is used for the prestressed elements.
• Immediately after casting, hollow core slabs will protected against moisture evaporation using tarpaulin (plastic cover).
• A steam curing will be applied (20-30 min. after casting) to hollow core slabs to expedite gaining the strength.
• The steam vapour will be released under the steel beds and through side holes to cover the whole concrete slab under the tarpaulin.
• The temperature of the steam gradually rises to reach 50-60 c considering that the maximum curing temperature is 82c as per PCI MNL-116 clause C.4.18.2
• The rise in temperature will be gradually with rate not exceed 5.6c to avoid reduction in the concrete strength and long-term durability (PCI MNL-116, clause 4.19.1)
• Applying the steam curing will continue for 10-12 hours.
• A concrete sample (cube) subjected to the same curing of the hollow core slab will be tested to ensure the required strength for releasing.
• Once the strength cube is reaching the releasing strength (refer to PCI MNL 116, clause C5.3.17), the strands will be cut to allow the transmission of the prestressing force to the concrete depending on the bond between the concrete and the strand.
• The References: PCI MNL-116, C4.19 (Accelerated Curing) ACI 308R-18 ACI 318/318R-61
5-Handling:
• Cutting machine provided with a circular diamond saw is used to cut the hollow core slabs as per the shop drawings.
• Cooling system is accompanied to the cutting machine to avoid any hazards. • Handling of the newly cast units is done with the tower crane, special lifting device (spreader beam with two clamps) will be used to prevent any damage to the slabs.
• All elements are to be handled carefully and slowly to avoid damage during transit.
6-Storage:
• Storage area should be relatively level and firm.
• Only slabs with the same lengths and with no larger recess should be stacked together and directly on top of each other
• Wooden spacers should be used, all spacers should be stacked exactly on each other, and spacers should be between 200-400 mm away from the slab end.
• Must be stored in separate piles.
7-Transportation & Delivery:
• Hollow Core Slabs are loaded on flat bed trailers by stacking one above the other, up to a certain (workable) height. Proper spacing is provided between the elements using wooden battens.
• All loads have to be secured with appropriate ropes, straps or chain.