GB2028405A - Improvements Relating to Methods of Protecting Structural Members - Google Patents

Abstract

Metal piles partially immersed in water require protection against corrosion, particularly over the zone subjected to alternate wetting and drying. To provide this, a jacket (3) is placed around the pile, leaving a space (5) to be filled with polyurethane foam. At the bottom of this space a barrier (16) of aggregate is provided through which water and air can permeate. Compressed air can be used to expel water from the space through the barrier, and then pre-frothed polyurethane foam is injected to fill the entire space, the barrier preventing its escape at the bottom. The foam can itself be used to expel the water. Similar protection can be applied to concrete or wooden piles. <IMAGE>

Description

SPECIFICATION Improvements Relating to Methods of Protecting Structural Members This invention relates to methods of protecting structural members, such as metal, concrete and wooden piles exposed to the action of water, against corrosion, erosion and degradation through chemical attack. It is a development of the method described in Patent No. 1494072.

In that earlier Patent there is proposed a method of protecting an in situ metal, concrete or wooden pile, partly immersed in water, against corrosion, particularly over a region which is subjected to alternate wetting and drying, the method comprising placing a jacket around the pile, removing water from the space between the jacket and the pile, injecting polyurethane into said space, the foam subsequently setting to complete a protective coating.

The jacket is preferably of black pigmented polypropylene, which is impermeable to ultraviolet light. It is normally held in place by spacers which remain embedded in the foamed polyurethane.

Conveniently, the jacket is assembled in stages at the upper end of the pile and progressively moved down the pile with further sections being added at the upper end. When fuily in place water is expelled from the space to be filled by the polyurethane. This is done by injecting compressed air, preferably dried and heated.

The foam filling previously proposed was a complex operation which necessitated the premixing of liquid chemical constituents which were pumped into the void and allowed to expand as a froth before setting as a rigid polyurethane foam. However, the recent development of prefrothed liquid foams permits the direct injection of these foams into the void, thus avoiding the majority of the frothing expansion phase taking place in the void. Such prefrothed foams enable improvements to be made to the original method.

According to the present invention there is provided a method of protecting an in situ metal, concrete or wooden pile, partly immersed in water, against corrosion, erosion or degradation, particularly over a region which is subjected to alternate wetting and drying, wherein a jacket is placed around the pile and the air and water in the space between the jacket and pile are replaced by polyurethane foam injected into said space, characterised in that the lower end of said space is provided with a water-and air-permeable barrier, such as aggregate or coarse particulate material, of greater density than water, prior to the injection of the foam, the latter being a prefrothed foam which is impeded by said barrier.

The aggregate or other barrier can be introduced before the water is expelled, so that there is no problem of introducing it into a pressurised zone. As well as acting as a barrier between foam and water, it will assist in overcoming any tendency of the jacket to float or rise up the pile before the polyurethane has been injected and set.

This type of foam can be injected at a single point, and this may either be at an intermediate position shortly above the water level, possibly with a downwardly directed continuation pipe within the space, or at or near the top of the pile.

In the latter case, it is preferred that below said point the injected foam passes through guide means to form it into a substantially uniform plug.

Water in said space may be expelled through the barrier either by compressed air or by the foam itself. Preferably, the foam includes an agent which chemically reacts with a potentially harmful substance known to exist at the surface of the pile substantially to remove the latter, and one such agent may be capable of chemically taking up water.

One important advantage of this method is that the pile can be encased without stringent prior cleaning.

For a better understanding of the invention, some embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammtic sectional elevation of a pile using a first method for filling a hollow jacket, Figure 2 is a diagrammatic sectional elevation of a pile using a second method for filling a hollow jacket.

Figure 3 is a detail of the lower part of the pile of Figures 1 and 2, Figure 4 is a section on the line IV .IV of Figure 3, and Figure 5 is a detail of the upper part of the pile of Figure 2.

Figures 1 and 2 show a pile 1 supporting a platform 2, the pile being partially immersed in water with a variable water line WL. The pile is encased from the underside of the platform 2 to a zone well below the water line by a black pigmented polypropylene jacket 3 which is held radially in position by spacers 4 to create an annular space or void 5, generally of 2.5 to 5 cm radial extent, which is filled with polyurethane foam. The construction of the jacket is done in sections at the upper end of the pile and as each section is completed the progressively longer jacket is moved down the pile for assembly of the next section. The main departures from Patent No. 1494072 reside in there being a different method of forming the jacket, a single entry of the foam, and a different way of blocking the lower end of the annular space.

Each section 6 of jacket 3 is formed from polypropylene sheet with a glass fibre fabric homogeneously and molecularly fused during manufacture to the internal face. The exposed face of the fabric is itself impregnated with a suitable resin to seal against water and to increase the structural stiffness and strength of the sheet. The polypropylene is specially formulated to resist degradation from ultra-violet (U.V.) light by the addition of carbon black pigment and additional U.V. stabilizers. A watertight vertical joint 7 is formed by butting the edges of the sheet with an internal cover strip 8 bridging the two edge portions. Conveniently, it is secured to one edge portion prior to assembly.

The strip 8 is a compatible resin/glass laminate and is bonded by urethane or GRP resin to the sheet edge portions. Additional security can be provided by rivets 9, and to avoid them pulling through the relatively soft material an apertured metal strip 9A may extend longitudinally along the inside of the cover strip, 8 to act as a communal washer. The horizontal joint between sections of the jacket is made simiiarly by means of a bridging or cover strip 10 secured to the inside of the lower end of the new section to provide a spigot. This is inserted in the upper end of the existing section, with a bonding agent to form a watertight joint. This joint can be similarly squeezed using blind rivets with a metal blocking strip if necessary.

In Figure 1 a joint 11 between the top of the last jacket section and the supported platform 2 is made using a sealant and the jacket restrained from moving vertically. Alternatively, as shown in Figure 2, a joint 12 can be provided by injecting a small volume of urethane foam 13 in the space above an annular soffit 14 formed in a similar manner to soffit 1 5 supporting aggregate 1 6 at the bottom, as described below.

Referring to Figures 3 and 4, the lower end of the jacket has, supported on the lowermost set of spacers 4, an annular soffit 1 5 which provides a shelf with clearances between its inner periphery and the pile 1, and between its outer periphery and the jacket 3. In use, it supports a short depth, for example about twenty cm of aggregate 16 graded in size and providing a barrier, permeable to air and water but highly resistant to the passage of polyurethane foam in its frothed presetting state, at the bottom of the annular space 5. Before the space is filled, in one method, compressed air drives out water through the voids in the aggregate 16 and through the annular spaces between the soffit 1 5 and the pile 1 and jacket 3, and the air pressure is maintained while the foam is injected.A certain amount of foam will penetrate the aggregate and its subsequent passage either up or downwards in the aggregate 1 6 due to variations in pressure will be resisted, thus allowing the foam to set with little or no movement and to form a permanent waterproof barrier or plug highly resistant to marine bores. In another method, the foam itself drives out the water. A pre-froth isotropic foam now exists with a pre-determined excess of isocyanate molecules which react with water to form CO2. This helps the foaming. Any residual water droplets adhering to the structure or to the casing or residual water vapour present in the void 5 will be taken up in a chemical reaction with the excess isocyanate.

The setting time of the polyurethane foam can be adjusted to suit the conditions of application.

During this process of setting, the foam tends to increase up to 20% in volume but this volumetric change can be contained as long as the pressures exerted by the foam can be resisted. In doing so the foam's density is increased and its uniformity improved. The formation of the barrier at the bottom and the seal at the top of the pile is to allow for the foam to set under pressure in the void 5.

The void 5 can be filled in various ways. In Figure 1, if compressed air is used to expel the water first, a quantity of pre-frothed foam is injected through an entrance pipe 1 7 located as low as convenient and the foam either falls to the bottom or is dispensed through a length of plastics tube 1 7A to form in the aggregate 1 6 a permanent barrier at the bottom. Air pressure is maintained and slightly increased during the setting time. The remaining part of the void 5 to be filled is then filled with pre-frothed foam, the contained air being allowed to escape through a release valve 18 at the top of the jacket. The tube 1 7A, if used, is expendable and will remain embedded in the set foam. If the water is not first expelled by compressed air, the foam will fill the space above the water-line before progressively displacing the water.With the arrangement shown in Figure 2, foam is injected through an entrance pipe 1 9 located at the top of the jacket, and passes through a restricting annular filter 20, conveniently made of ribbed cardboard, which creates a uniform plug of descending foam. This drives out the air and/or water from the void through the aggregate 1 6 and spaces between the annular soffit 15, the pile 1 and jacket 3. The aggregate plug will act as described before as a fluid permeable barrier and a matrix in which the foam can set.

This system of protection is applicable both to new and existing structures, even those in which severe corrosion or degradation has already set in.

Shot-blast or other stringent cleaning methods capable of exposing the base material are not necessary, although it will be normal to remove foreign bodies or accretions susceptible to cursory cleaning. Any rust, for example, is completely encapsulated and denied moisture and oxygen.

Any residual moisture trapped within the products- of corrosion is removed by chemical reaction with the excess isocyanate component in the foam.

Residual oxygen can only cause infinitesimal further corrosion before being exhausted. Even if there are any volumetric increases in the corrosion products after the foam has set, the linear compression and collapse characteristics of the foam can adequately absorb them, and even benefit from the process by making an even tighter interface structure to resist hydrostatic pressure changes.

The system also affords efficient protection to non-metallic piles, for example concrete, which is subject to degradation by water-borne chemicals.

The casing, the foam and the bonding agents are themselves highly resistant to chemical attack. If there is residual chemical attack potential likely to be encapsulated by the system and if this can be identified, then an appropriate chemical can be added to the foam to precipitate out the injurious ions causing the degradation. For example, injurious sulphate ions can be precipitated out with Barium Carbonate as insoluble Barium Sulphate, with any residual water aiding the ionisation interchange.

Claims (12)

Claims

1. A method of protecting an in situ metal, concrete or wooden pile, partly immersed in water, against corrosion, erosion or degradation, particularly over a region which is subjected to alternate wetting and drying, wherein a jacket is placed around the pile and the air and water in the space between the jacket and pile are replaced by polyurethane foam injected into said space, characterized in that the lower end of said space is provided with a water-and air-permeable barrier, such as aggregate or coarse particulate material, of greater density than water, prior to the injection of the foam, the latter being a prefrothed foam which is impeded by said barrier.

2. A method as claimed in claim 1, wherein the foam is injected at a single point.

3. A method as claimed in claim 2, wherein said point is at an intermediate position shortly above the water level.

4. A method as claimed in claim 2, wherein said point is at or near the top of the pile.

5. A method as claimed in claim 4, wherein below said point the injected foam passes through guide means to form it into a substantially uniform plug.

6. A method as claimed in any preceding claim, wherein water in said space is expelled through said barrier by compressed air.

7. A method as claimed in claim 6 as appendant to claim 3, wherein a pipe within the space guides the injected foam towards the bottom of said space, the pipe being left embedded when the foam has set.

8. A method as claimed in any one of claims 1 to 5, wherein water in said space is expelled through said barrier by the foam.

9. A method as claimed in any preceding claim, wherein the foam includes an agent which chemically reacts with a potentially harmful substance known to exist at the surface of the pile substantially to remove the substance.

10. A method as claimed in claim 9, wherein said agent chemically takes up water.

11. A method as claimed in any preceding claim, wherein the pile is encased without stringent prior cleaning.

12. A method of protecting an in situ metal, concrete or wooden pile substantially as hereinbefore described with reference to the accompanying drawings.