CEPCO prestressed spun concrete piles generally comply with the following specification:-
1. Standards
Piles shall be manufactured to comply generally with the requirements of:-
BS 8004: 1986 - Foundation
BS 8110: 1985 - Structural Use Of Concrete
JIS A5335: 1987 - Pretensioned Spun Concrete Piles
MS 1314: Pt 1: 1993 - Specification for Precast Concrete Piles
2. Materials
Portland Cement: shall be ordinary Portland cement meeting the requirements of B.S. 12.
Aggregates: shall be clean granite that is durable and strong. Soundness and the maximum
amount of dust, clay, organic matter or other deleterious substances in the aggregate shall
conform to the requirements of B.S. 882. Grading shall be consistent with the grading used
in the approved mix designs. Aggregates shall be uniformly graded. The maximum size of
coarse aggregate shall not exceed 20 mm, nor 2/5 the thickness of the pile.
Fine aggregates shall be clean river sand or washed river sand and NOT sea or beach sand.
Water: used in mixing concrete shall be clean and free from injurious amounts of oils, acids,
alkalis, salts, organic materials or other substances that be deleterious to concrete or steel
and shall conform to the requirements of B.S. 3148.
Admixtures: Admixtures used shall meet the requirements of B.S. 5075.
Steel Reinforcement: Prestressing steel bar shall conform to JIS G 3137 for pretensioned
construction. Deformed reinforcing bars shall conform to B.S. 4449.
Joints: Joints shall consist of steel plates meeting the requirements of B.S. 4360, Grade
43A as applicable.
Cast-in-place concrete: Cast-in-place concrete plug used in capping the piles shall be
Grade 30 concrete.
3. Manufacturing of Piles
Piles shall be manufactured by the centrifugal casting process. Metal forms shall be used and they shall be well braced and stiffened against deformations caused by the hydrostatic pressure of the wet concrete while spinning. The metal forms shall have smooth joints and inside surfaces. The forms shall be accessible for adequate cleaning.
The portions of the forms which form the end surfaces of the pile sections must be a true plane perpendicular to the axis of the section within the following tolerances. Maximum allowable deviation for abutting end surfaces 1.5mm; for pile head end surfaces 5mm; and for pile tip end surfaces 25mm.
The concrete mix shall provide the most uniform possible distribution of aggregates across the wall of the pile sections. Fine and coarse aggregates shall have as nearly as possible the same specific gravity. The water to cement ratio by weight shall not exceed 0.35. The concrete shall be Grade 60 or 78.5 (in conformance to B.S. 5400: Part 7) and have a minimum compressive strength of 60 or 78.5 N/mm2 at twenty eight (28) days.
The spiral reinforcement shall be outside the longitudinal reinforcement. The spiral cage shall be securely held in position during the casting and spinning operations.
The piles may be initially steam cured. Steam shall not be applied until 60 minutes have elapsed following casting. The section may then steam cured between 55ºC and 70ºC. Steam shall be so introduced that the temperature in the curing chamber is raised uniformly. After the section has been steam cured for a minimum of (3) hours, the forms may, if the concrete has reached sufficient strength to sustain all handling stresses be removed from the section. The piles shall not be taken out of the curing chamber until the temperature in the curing chamber has been gradually reduced to that prevailing outside. The curing shall be carried out by such a method that may not affect adversely the quality of the product.
The method of handling of the pile sections shall be such that no damage to the concrete will occur. However, the minimum prestress in the concrete after all losses shall be adequate to resist stresses induced by handling.
The main body of the prestressed concrete pile shall, when subjected to the bending test, not develop crack of width more than 0.05 mm under the Calculated Cracking Bending Moment.
The bending strength test shall be performed by supporting the pile section at points located 1/5 L from its ends (where L is the length of the pile section). The vertical load shall be applied at the centre of the span. Two piles from each lot shall be so tested. If one of the two does not meet the test criteria, four other pile sections shall be further sampled and tested. If all four satisfy the test, then all piles in the lot will be accepted; otherwise all piles in that lot shall be rejected.
Both ends of each length of pile is to be provided with a steel end plate. The ends of the prestressing steel bars shall be fixed to the steel end plate such that the full prestressing force is transmitted over to the steel end plate. The ends of the prestressing steel bars shall be flushed with the top of the steel end plate. The end surface of the steel end plate shall be in plane which is at right angles to the longitudinal axis of the pile.
The barrel of the steel casing welded to the end plate shall be flushed with the barrel of the concrete shaft of the pile, with a chamfer to accommodate the required depth of welding when two lengths of pile are jointed.
