权利要求

This invention relates to a manhole cover. The invention also concerns and to a method of forming the manhole cover.

It is known to make manhole covers from concrete. The concrete used is formed from Ordinary Portland Cement (OPC) 4.25, gravel and sand. Water is added to the mixture in a controlled manner to achieve a water to cement ratio of 0.495. The addition of water in this way results in the constituent materials homogenising over a period of about 3 minutes.

In order to form a manhole cover, the concrete mix is added to a pressed pan that is welded at the corners. The pan is galvanised after the welding or pressing operation and is typically formed from 3mm thick steel. The pan defines the shape of the manhole cover.

Various national and international standards specify strength requirements for manhole covers. For example BS EN 124, 1994 specifies that the covers must satisfy a 125kN load test.

In order to form a manhole cover having sufficient strength particularly to meet standards of strength set by various countries, it is necessary to reinforce the concrete within the pan. It is therefore known to provide a steel mesh that is welded to the pan at spaced apart locations. The mesh itself is formed from cut lengths of steel wire having a diameter of approximately 6mm. The lengths are spot welded together at their crossing points, to form a grid structure.

Known concrete formed as described is a thick, viscous non-flowing mixture. In order to fill the pan with the concrete mix, the mix is added to the pan by means of a trowel. The use of a trowel ensures that the corners of the pan are properly filled with the mix and that the grids are properly enveloped by the mix.

Once the mix has been added to the pan, the pan is mechanically vibrated to further ensure appropriate distribution of the concrete mix within the pan and around the mesh. Finally, once sufficient concrete mix has been added to the pan, the top surface of the mix is smoothed with a trowel.

Once the pan has been filled in this way it is left to cure for at least 18 hours. In order to minimise costs, forced curing usually is not employed. In any event the concrete is contained within the pan, thereby facilitating handing of the cover before complete curing has occurred.

A disadvantage of the known concrete mix and the manhole cover formed therefrom is that the method of producing a finished manhole cover is time consuming, labour intensive and therefore expensive. The manufacture and installation of the mesh within the pan is a particularly labour intensive activity.

According to a first aspect of the present invention there is provided a manhole cover comprising a pan including a base having a generally continuous wall upstanding therefrom defining a boundary;

• a plurality of reinforcing wires secured in the pan in a non-reticulated manner; and
• a layer of self-compacting concrete received in the pan within the boundary so as to surround the reinforcing wires.

The use of self-compacting concrete requires fewer reinforcing wires, that do not require assembly into a mesh. Consequently the manhole cover of the invention is simpler and cheaper than the prior art version.

Self-compacting concrete is a concrete that is highly flowable and can spread into place under its own weight. In addition it can achieve excellent consolidation often without internal or external vibration and without exhibiting defects due to segregation and bleeding.

The use of SCC to form a manhole cover therefore enables the process of forming the manhole cover to be simplified. The procedure is no longer labour intensive, and the end product is cheaper than when made from conventional concrete. In particular, the use of SCC ensures that any reinforcing structures such as the steel wires are correctly enveloped within the concrete.

Preferably the reinforcing wires each include a generally straight portion, the straight portions being parallel to one another within the boundary. The benefit of having a parallel wire configuration rather than a grid structure is that fewer spot welds are required.

Conveniently the boundary defined by the upstanding wall is rectangular; and the straight portions of the reinforcing wires lie parallel to a side of the thus defined rectangle. This arrangement is simple to manufacture and provides good strength characteristics.

Conveniently the wires are secured to the pan by welding. This ensures that the manhole has sufficient strength, as tested for example by means of a compressive test after completion of the pan manufacturing steps.

Preferably the self-compacting concrete is cured from a mix of, by weight %, approximately 21.10% cement, 29%-36%, and preferably 35.6% sand, 29%-36%, and preferably 35.60% gravel, 0.19%-0.8%, and preferably 0.19%, superplasticiser and 0.025% fibres in water. This concrete mix provides a good combination of pourability, strength and curing properties; but other SCC concrete mixes are possible within the scope of the invention.

More specifically the nominal diameter of the gravel is 10mm; the superplasticiser is a polycarboxylate superplasticiser; or a superplasticiser formed as a blend of polymeric sulphonate materials; and the fibres are polypropylene fibres.

One range of polycarboxylate superplasticisers that are generally suitable for use in the manufacture of the manhole cover of the invention is the ADVA range manufactured by Grace Construction Products.

A suitable polymeric sulphonate blend superplasticiser is Duracem SP6, that is also manufactured by Grace Construction Products. Other superplasticisers are possible within the scope of the invention. The superplasticisers perform several functions in the concrete mix, including those of increasing the flowability of the mix and hence rendering it self-compacting. The superplasticisers typically also influence the curing of the concrete.

Preferably the polypropylene fibres are nominally 12mm long, although other fibre types and sizes are within the scope of the invention.

The fibres are added to the SCC in order to ensure that the composite has appropriate strength as exemplified by local strength requirements such as the aforementioned BSEN124:1994. The preferred 12mm long fibres also support the gravel particles in suspension during curing of the concrete; and tend not to protrude from the cured, concrete part of the cover.

According to a second aspect of the invention there is provided a method of forming a manhole cover comprising the steps of

• (a) placing in a pan, comprising a base having upstanding therefrom a generally continuous wall defining a boundary, a plurality of reinforcing wires, the wires defining a non-reticulated pattern;
• (b) pouring a pourable, self-compacting concrete in liquid form into the pan so as to form a layer therein that surrounds the reinforcing wires; and
• (c) allowing the self-compacting concrete to cure in the pan.

Preferably the self-compacting concrete comprises a mix of, by weight %, approximately 21.10% cement, 29%-36%, and preferably 35.60% sand, 29%-36%, and preferably 35.60% gravel, 0.19%-0.8%, and preferably 0.19% superplasticiser and 0.025% fibres in water. The advantages of such a mix are set out hereinabove.

More specifically the mass of water in the mix is approximately 35% of the mass of the cement.

It is also preferable that the superplasticiser is a polycarboxylate superplasticiser; or a superplasticiser formed as a blend of polymeric sulphonate materials; and/or that the fibres are polypropylene fibres that preferably are as specified hereinabove. The advantages of these aspects of the invention are also set out hereinabove.

Preferably the method includes, before step (a), fabricating the pan by processes of pressing and welding. The method may also optionally include the step of, before step (b), galvanising the pan; and optionally the step of, before step (b), mixing the self-compacting concrete to a pourable form

Thus the method of the invention conveniently is a self-contained method all the steps of which may, if desired, be carried out at a single site.

The reinforcing wires preferably are secured to the pan before step (b) occurs. Such securing may occur through welding of the wires to the pan, although other securing methods are possible.

Preferred embodiments of the method of the invention omit the shaking and trowelling steps of the prior art methods.

Other embodiments of the method of the invention include a limited amount of oscillation of the pan, e.g. oscillating the pan continuously at a preferred frequency of 50 Hz for no more than 10 seconds.

The invention will now be further described by way of non-limiting example only with reference to the accompanying drawings in which:

• Figure 1 is a schematic, cross-sectional representation of a known manhole cover; and
• Figure 2 is a schematic cross-sectional representation of a manhole cover according to the invention.

Referring to Figure 1, a known manhole cover 1 is designated generally by the reference numeral 1. The manhole cover is formed from a known, non-self-compacting concrete reinforced by steel wires 2. The wires are welded together to a form mesh 3 of wires in a grid pattern. Mesh 3 is e.g. welded to a pan 4 (which is rectangular and has an upstanding peripheral wall 4a welded at its comers 4b) containing the concrete. The mesh 3 is welded at spaced apart intervals along and across the length of the base of the pan. The mesh 3 is embedded in the concrete.

The method of manufacturing the prior art cover of Figure 1 is to form the pan 4 by notching a 3mm thick steel plate and forming the four corners therein to define the upstanding sidewalls thereof; cut the wires for forming the mesh to length; assemble the mesh 3 by welding; weld lifting key assemblies to the pan; weld the mesh 3 to the pan 4; galvanise the entire, resulting metal assembly; and pour the concrete into the pan. The concrete is mixed in the locality including gradual addition of water until the water to cement ratio reaches 0.495:1.

The typical time to mix the concrete is about 3 minutes, but the time required to fabricate the pan 4 with the mesh 3 secured therein is considerably longer.

Following pouring of the concrete so that it substantially fills the pan, the pan is shaken for approximately 25 seconds, depending on its size and the upper surface trowelled to a smooth finish. These steps are also time consuming.

Finally the concrete is left to cure for 18 hours.

Referring now to Figure 2, a manhole cover according to the invention is designated generally by the reference numeral 10. The manhole cover is formed from a steel pressed pan 14 (that is similar to pan 4 of Figure 1) into which self-compacting concrete 16 has been poured and cured. The concrete is reinforced with wires 15 that extend generally parallel to the minor axis of the manhole cover. The wires 15 are lain in the pan 14 (that is galvanised before pouring of the concrete occurs) in a non-reticulated pattern.

The pan 14 need not be rectangular shown. Circular and polygonal pans shapes, for example, are within the scope of the invention. The depths and other dimensions of the pans are variable within the scope of the invention, as is the depth of concrete therein.

The method of manufacturing the cover of Figure 2 is simplified compared with the above described prior art method because:

• There is no need for the time consuming step of forming the wires 15 into a mesh;
• The step of securing (eg. welding) the wires 15 to the pan 14 takes less time than the assembling and securing of a mesh;
• There is no need for the shaking step; although it may optionally occur. Under the latter circumstance the shaking or oscillation of the pan occurs typically at a frequency of 50 Hz, for no more than 10 seconds.

The concrete used in the manhole cover of the invention is manufactured by using a high concentration of cement. Water is added in a water to cement ratio of 0.35. Sand and 10mm gravel are added in equal quantities. To ensure that the concrete mixture is fluid, the superplasticiser is added at 0.9% of the cement weight. Finally, fibres are added to the mixture at 0.12% of the cement weight.

The fibres provide added strength and maintain the gravel in suspension while the mixture is mixed.

Set out below in Table 1 are details of a known (traditional) concrete mix used in the prior art covers described above, compared to a self-compacting concrete mix for use in manufacture of covers according to the invention.

By means of the present invention, fewer reinforcing wires are required to achieve appropriate strengths. In addition the method for forming the manhole cover according to the invention is quicker, more efficient and therefore cheaper than known methods.