St Paul’s Cathedral

Project brief:

A monumental restoration project for the iconic St Paul’s Cathedral spanning 15 years and costing £40 million. It was the first comprehensive restoration project that the Cathedral has gone through and this extensive cleaning, repair and renovation programme was one of the largest of its kind ever in the UK.

The Project:

It doesn’t get much more high profile than this when it comes to historic building restoration. As part of the London skyline for over 300 years, the requirement was for materials that perfectly balanced the strength and performance to do the job with the delicate nature of working with such a fragile and weathered structure. The sheer scale of the project also dictated the need for an extremely consistent product.

Help from Tarmac Building Products – St. Pauls Grout Mix:

Pozament, from Tarmac Building Products Ltd, boasts the heritage range of Limelite Grouts and Mortars specifically designed for such tasks. Yet in a project of such epic proportions and with the stakes so high our technicians instead produced an all new low strength blend of PFA, lime and special additives, that was capable of penetrating the fine voids and fissures in the Cathedral’s structure, whilst remaining sympathetic to the existing materials.

The new grout, now named St. Paul’s Grout Mix, is easily mixed and in addition can be placed using gravity feed techniques, which are often used to ensure low pressures that are needed to avoid damage to delicate structures, such as in the case of St. Paul’s Cathedral.

St. Paul’s Grout was designed to achieve a low strength of approximately 2.0 N/mm2 at 28 days, which was sufficient to strengthen and fill voids in the heartening of the masonry walls at St. Pauls Cathedral. The mix also contains a special plasticiser/wetting aid, so that when used at a water solids ratio of 0.4, a very fluid mix capable of penetrating fine voids and fissures is produced.

Outcomes:

The St. Paul’s Restoration project was finally finished in June 2011. To this day Tarmac continue to produce the St. Paul’s Grout when required and since then, it has been used successfully on many similar restoration projects including York City Wall, Winchester Cathedral and other English Heritage projects.