Collapsed carriageway ironwork installations are a far-too common sight. With a little understanding of the factors at play, the number of failures can be significantly reduced.
WRc research, reported in the industry-defining Streetworks CP373 document, points to a solution. This research paper revealed that up to 89% of ironwork installation failures suffered from foundation issues and that, in 44% of cases, foundation collapse was the sole cause of failure.
Care in matching the performance characteristics of the elements which have to work together – the manhole cover or gully grate, the mortar and the supporting manhole chamber structure – and in taking account of several factors during installation, are vital to long-lasting ironwork installations.
If we consider traffic passing over a manhole cover or gully grate, we might expect the compression load from each vehicle to transfer from the cover to the frame, which is largely correct. It therefore stands to reason that this load is then transferred from the frame into compression forces on the bedding materials.
Due to the relatively complex geometry of the cover seating and flanges of the frame, this, however, is not entirely the case.
The geometry of ironwork frames differs by manufacturer and product line and so some apply more damaging tensile and flexural stresses than others.
Mortar materials such as concrete are rated by their compression properties in N/mm2, grade C35 for example refers to 35 N/mm2. There is normally no indication of tensile or flexural capability because the strength properties are compression-based. Concrete will not flex under pressure or withstand tensile forces. Instead, this sort of bedding mortar will simply crack in service causing premature reinstatement failure.
Ironwork has to meet industry standards (e.g. 40 tonne test loads for D400 products) and the main supporting chamber is carefully constructed, but when it comes to what is in between these two, it is rarely adequately considered.
Indeed, bedding material is often treated simply as the ‘adjustment’ material used to align ironwork with the surrounding road surface. However, the bedding material is the most vulnerable structural component in a conventional reinstatement’s construction. But it is the most crucial element and therefore it is vital to limit the bedding’s exposure to forces within its capability, especially in the susceptible [uncured] early stages of its application.
Highways England recognises this in advice note HA104/09; stating that bedding failure is “one of the main factors contributing to the current poor performance of chamber tops”. It goes on to explain that to conform with advice note, bedding mortars should have tensile capability in addition to the conventional compression properties.
Pressure to re-open roads as quickly as possible following reinstatement presents a significant risk to the life expectancy of bedding materials. Some conventional bedding materials can take several hours, days or weeks to reach full strength, particularly if air cured and laid as a thick bed or prevented from contacting air.
To prevent failure at the outset, early development of a mortar’s trafficable properties is crucial; premature exposure to traffic forces will cause damage.
Unsympathetic installation techniques can also induce bedding failure at the point of installation. Vibration compaction equipment being allowed to overrun the ironwork so that it applies those vibration/compaction forces to the bedding via the ironwork frame is an example.
Getting the bedding right
Clearly, getting the bedding mortar right is vital to long-lasting carriageway ironwork reinstatement.
Polyester resin-based bedding mortars are much stronger than their concrete-based counterparts in all the necessary aspects. They cure quickly to form a structure which compresses and flexes with the surrounding ironwork so that the system reacts uniformly.
Engagement between the frame and bedding materials is also important. This can be ensured through use of ironwork with mortar holes and some more sophisticated designs have a patented engagement surface (tread) on the underside.
Having taken all of this into consideration, Wrekin Products Ltd developed the UniPak polyester resin bedding system which combines fast-setting (one-hour), low-notch sensitivity, tensile and flexural-capable bedding mortar with factory-cured HA104/09-conforming packing materials.
It is the only bedding system on the market to do this.
Third party accreditation verifies that products are fit for purpose and Wrekin Products has been awarded HAPAS (Highways Authorities Product Approval Scheme) approval and certification from the British Board of Agrément (BBA) for its UniPak ironwork bedding mortar.
In addition to getting the bedding mortar right, the importance of also choosing ironwork that works in sympathy with the bedding material should not be underestimated, as good selection will extend the life of an installation, thereby, reducing the need for repairs, saving on cost and avoiding disruptive reinstatements.
Wrekin Products is happy to advise on mortar selection and undertake on-site training to help installers understand how they can get it right first time.
Barry Turner, Technical Manager, Wrekin Products