Centrifugal molding of prestressed piles

The method for forming prestressed centrifugal piles by: PA0 disposing a spiral within one of the two constituent half-shells of the mould for the pile to be formed, PA0 securing within this spiral a plurality of strands having at least one end emerging from a mould headpiece, PA0 pouring the concrete into said half-shell, PA0 joining the two half-shells together, PA0 inserting axial restraint elements for the strands through suitable apertures provided in the half-shells, PA0 tensioning the strands at those ends which emerge from the headpiece and, PA0 centrifuging the mould prepared in this manner.

BACKGROUND 
This invention relates to a method for forming prestressed centrifugal 
piles, a mould for implementing the method, and a pile obtained by the 
method. 
Methods are known for forming prestressed centrifugal piles of 
frusto-conical shape. One of these known methods uses two metal 
half-shells forming a mould inside which a reinforcement is placed 
consisting of a ring of steel strands of length slightly greater than the 
length of the mould, together with a spiral located on the outside of the 
strands. 
In order to maintain the strand cross-section/pile cross-section ratio 
substantially constant, and thus prevent fracture of the concrete when 
subjected to non-uniform prestressing, it is known to progressively 
deviate the strands towards the mould interior, so that portions of them 
lie within the central cavity which is obtained during the subsequent 
centrifuging stage, or to cover end portions with a sheath. In this 
manner, although the strands extend over a length equal to the pile 
length, they do not adhere to the concrete beyond a certain point, and 
therefore do not transmit their load fraction thereto. 
This method however has the drawback of a certain laboriousness due to the 
need to prepare the end portions of the strands. 
In another known method, the reinforcement consists of strands of different 
lengths, secured to a spiral disposed on their inside. 
This known method has the drawback that the hooping effect is lost, so that 
the pile has limited load resistance. 
An object of the invention is to obviate the drawbacks of known methods, by 
providing a method for forming prestressed centrifugal piles of high load 
resistance. 
A further object of the invention is to provide a method which enables the 
strands to be easily anchored within the mould. 
SUMMARY OF THE INVENTION 
These objects are attained according to the invention by a method for 
forming prestressed centrifugal piles, characterised by: 
disposing a spiral within one of the two constituent half-shells of the 
mould for the pile to be formed, 
securing within this spiral a plurality of strands having at least one end 
emerging from a mould headpiece, 
pouring the concrete into said half-shell, 
joining the two half-shells together, 
inserting axial restraint elements for the strands through suitable 
apertures provided in the half-shells, 
tensioning the strands at those ends which emerge from the headpiece, 
centrifuging the mould prepared in this manner. 
Preferably, the method is characterised by: 
disposing a spiral within one of the two constituent half-shells of the 
mould for the pile to be formed, 
securing within this spiral a plurality of strands having at least one end 
emerging from a mould headpiece, 
radially inserting, through suitable apertures provided in the half-shell, 
axial restraint elements for the strands disposed within said half-shell, 
pouring the concrete into said half-shell, 
joining the two half-shells together, 
radially inserting, through the other half-shell, axial restraint elements 
for the strands disposed within said half-shell, 
tensioning the strands at those ends which emerge from the headpiece, 
centrifuging the mould prepared in this manner.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As can be seen from the figures, the method according to the invention uses 
for its implementation a metal mould of substantially frusto-conical 
shape, formed from two half-shells 1,1'. 
Housings 3 (four in number in the illustrated example) are welded at 
equiangular spacing along the half-shell 1 in predetermined positions 
determined by static calculations, and are diametrically opposite housings 
3' welded to the half-shell 1'. The housings 3,3' define apertures 4 for 
communication with the interior of the half-shells 1,1'. Forks 5 are 
removably inserted through said apertures and can be rigidly locked 
therein by means of plugs or wedges 6. 
Each fork 5 and each plug 6 can also be locked with respect to the 
corresponding housing 3,3' by pins (not shown on the drawings). A gasket 7 
provides a seal between the fork 5 and the corresponding half-shell 1,1'. 
Perforated headpieces 8,8' are fixed to the ends of the half-shell 1. In 
the illustrated example, the headpiece 8 has twice the number of holes as 
the headpiece 8'. 
The method for forming the prestressed centrifugal pile according to the 
invention is as follows: 
A metal lattice 9 of preferably frusto-conical shape about which a metal 
spiral 10 is wound is positioned within the half-shell 1. 
Harmonic steel strands 11,11' of different lengths are inserted into the 
inside of the spiral 10 and are secured to it by means of iron wire. In 
particular, the strands 11,11' are positioned equidistant from each other 
such that each of them passes by a housing 3,3'. In the illustrated 
example, the strands are sixteen in number, eight of which, 11, extend 
over the entire length of the mould and have their ends emerging from the 
headpieces 8,8' respectively. The other eight strands 11' are of gradually 
decreasing length in pairs so that whereas all the strands 11' emerge from 
the headpiece 8 of larger section, they extend for a length which slightly 
exceeds the distance between the headpiece 8 and the corresponding 
aperture 4. 
Conventional axial restraint clamps (not shown on the drawings) are then 
applied to those ends of the strands 11 which emerge from the headpiece 
8'. Cylindrical blocks 12,12' are clamped to those ends of the strands 11' 
lying within the half-shell 1, in such a manner that said blocks 12, 12' 
straddle the aperture 4 in the vicinity of which the corresponding strand 
terminates. 
In particular, the block 12 has that base facing the block 12' curved to 
correspond to a curved portion of the prongs of the forks 5. 
The forks 5 are then inserted through the apertures 4 of the half-shell 1 
so that their prongs engage the end of the corresponding strand 11' within 
the portion bounded by the two blocks 12,12'. The plugs 6 are then 
inserted and are locked by means of pins. 
Concrete in a semi-fluid state is then poured into the half-shell 1 
arranged in this manner, after which the half-shell 1' is placed on and 
fixed to the half-shell 1 in the conventional manner. 
The forks 5 are then inserted through the apertures 4 of the half-shell 1' 
so that they engage the ends of the strands 11', by an operation similar 
to that heretofore described. 
A tensioning device is then fitted to those ends of the strands 11,11' 
which emerge from the headpiece 8 in order to stretch the strands 11,11', 
after which plugs are fitted to said ends to prevent the strands 11,11' 
returning to their original configuration. 
The mould arranged in this manner is positioned on a conventional 
vibrating-centrifuging machine for compacting the concrete. When curing is 
complete,, the plugs 6 and forks 5 are removed, and after opening the two 
half-shells 1,1' the pile is removed and the mould prepared for a new 
cycle. 
It is apparent that because of the compressive force to which the strands 
are subjected, it could be difficult at this stage to disengage the forks 
from them. This is facilitated by the particular curved shape of the base 
of the blocks 12, which allows the forks to be slightly rotated. It should 
also be noted that the purpose of the blocks 12' is to prevent the strands 
from sliding within the concrete after this has hardened. In this respect, 
when the forks 5 have been disengaged, the blocks 12 are no longer 
restrained at their front as they have a hollow space in front of them. 
From the aforegoing it is apparent that the method for forming prestressed 
centrifugal piles according to the invention results in numerous 
advantages, and in particular: 
it enables high-strenth piles to be formed because of the possibility of 
using a spiral disposed on the outside of the ring of strands, 
it enables the strands to be secured simply and comfortably. 
Inasmuch as the invention is subject to many variations and modifications, 
it is intended that the foregoing description and drawings shall be 
interpreted as merely illustrative of the invention whose scope is defined 
by the following claims.