External structural repairs to brick-clad high-rise buildings

The Scottish Special Housing Association (hereinafter referred to as SSHA or the Association) is a government-sponsored body which designs, builds and manages houses for the people of Scotland on a nonprofit making basis. Set up in 1937 to provide houses in the special (or ‘distressed’) areas, the Association’s function was extended after the war to provide houses for any local authority requiring assistance with their housing problems.

This year is the fiftieth anniversary year of SSHA and for the preceding half-century the Association has played a major part in providing homes in Scotland, from the Shetlands in the north to the Borders in the south.

The Association has developed into an organization employing some 2000 people and has built over 110 000 houses – one tenth of the public sector houses in Scotland.


Mk III - Wyndford, Maryhill, Glasgow - 15 storeys high, reinforced concrete frame with brick infill cladding panels

SSHA stock of multi-storey blocks

The Association owns 73 blocks, which is about 1.5 per cent of the total in Britain of blocks over 5 storeys high. These blocks vary from 8 to 26 storeys. It is interesting to note that in Strathclyde Region there are nearly 50 per cent of all blocks in the UK over 20 storeys high.

Of the 69 SSHA blocks in the west of Scotland the distribution of types is as follows:

Inspection programme

Following the revelations in 1983 about a potential serious fault in the Bison system, and being aware through other inspections with binoculars by our building inspectors, that there was evidence of deterioration to varying degrees in all our blocks, it was decided that full inspections of all the Association’s high-rise blocks were required.

Thus in early 1984, a 5 million inspection and repair programme, to be carried out over four years, commenced on all our multi-storey blocks.

Recognizing that funds were not limitless and that all the Association’s multi-storey blocks had a high occupancy rate, decisions were taken about the extent of the investigations.

The blocks were divided into the various elements: foundations; main frame (where applicable); floors; internal walls; external walls; stairs; and roofs.

There was no evidence of failure of foundations so this was omitted from the investigations at this time.

Top left: Access system, single mast tower with moving cantilevered platform
Base left: Powered suspended cradle
Top right: Typical defects, nib at floor slab - rusting reinforcement due to inadequate cover
Centre right: Slip bricks at nib - cracking due to frame shortening
Base right: Window cills - spalling concrete

A full investigation of the main frame, floors and internal walls would cause considerable disruption to tenants, hence, it was decided to limit this to common areas and a few empty flats (where possible). It was agreed that a full investigation of the external envelope was possible and as this was the area where the main danger to the public lay from falling panels or debris, it was decided to proceed on this basis.

Since the evidence was that all the faults within the buildings were due to bad workmanship and material deterioration, no design check was carried out – this was a deterioration/ condition survey.

Brick-clad blocks - type of construction

Typically the structural frame of this type of multi-storey block is cast in-situ reinforced concrete inner walls, columns, beams and floors with the brickwork cladding constructed on the edge of the floor slab and supported by the frame. The brickwork may be facing brick or common brick roughcast, while the internal leaf is generally a soft thermal block on elevations and an in-situ concrete shear wall on gables.

Result of inspections

Potential problems were identified from maintenance feedback, from other inspection results and from BDA Technical Note No 9 (*). An examination was then made targeting the following areas:

The external inspections discovered the following faults in greater or lesser degrees in all blocks (see also Figure 1):

Elevation – The numbers required by the original drawings were not always present particularly to the side of the windows. The metallurgical results indicated that the galvanized steel ties were nearing the end of their useful life

Gables – In general the dovetailed bronze ties were present in irregular quantities, but the tail was sometimes deformed or the tie mismatched with the slot. In addition L-shaped galvanized mild steel ties from previous repairs were found to be sub-standard.

Overall wall ties were found to be skewed, deformed, off level, rusting, mismatched between dovetail and slot or inadequately anchored. Many of the panels were consequently deemed to be sub-standard and ineffective with respect to wall ties, even to the earlier lower requirements and standards both for quality and quantity.

Faults included: vertical cracks at corners; gable brickwork seating inadequate; slip bricks debonded from nib (see Figure 2); bricks spalled above and below nib level; DPC squeezed out; pointing eroded; and dirty cavities, etc.

Problems included: reinforcement very close to surface and rusting locally; some minor stress cracking; face of nib spalling off on gables at corner; balustrade posts corroding where inserted into the balcony; and patches on balcony soffit loose or missing. Chloride ion levels were found to be low and carbonation depths were very variable.

Concrete creep in the frame is a phenomenon which was not fully understood at the time of the design. The consequential frame shortening has transferred undue loads to the brickwork which is now showing signs of distress. However, this frame shortening has produced a clamping action on the panel and undoubtedly contributes significantly to overall stability of the brickwork.

The main problem discovered on inspecting the concrete elements was at the projecting 'nib' at the edge of each floor slab, supporting a storey-height panel of outer leaf brickwork. The nib, only 72mm deep, contains complex reinforcement, often placed out of position with inadequate cover. At certain locations, deterioration was severe and the continuing structural support from this nib to the panel of brick above was in doubt.

To rectify the major defect of the deteriorating concrete nib, trial installations involving angle iron support and 'hit and miss' brickwork replacement were conducted. The results were declared unacceptable visually and the proposals impractical to execute.

Other forms of support were examined and after extensive trials and investigations, the installation of a type of grouted dowel anchor was selected as the most appropriate type of repair to provide an alternative means of support for the outer leaf brick panels. This patented corbel anchor fixing, trade named 'CINTEC' anchor, originates from Belgium and comprises a stainless steel rolled hollow square section with circular flange ends, surrounded by a specially formulated cementitious grout retained within a fabric sleeve (see Figure 3). For this particular application, the anchors used were 235mm long, 30mm square in section, 3mm thick, placed at 700mm centres, with approximately 90mm overall embedment length in the inner leaf of concrete, approximately 90mm overall embedment length in the outer leaf brickwork and about 55mm length in the cavity.

Holes, 60mm diameter and at 5 º downwards were cored, using water-cooled tungsten-tipped coring bits to a controlled depth of 250mm. After cleaning dust and debris, the anchor was loosely positioned in the hole by hand and then the pre-mixed homogeneous slurry grout was pumped into the anchor.

The grout, having a 28-day compressive strength in excess of 40N/mm2, was injected via a nonreturn valve, under pressure of about 2.5 bars, filled the sock and formed a bulge and drip check in the cavity.

The grout, composed of sand, cement, water and inorganic bonding agents was contained within the closed sleeve and hardened as a result of a hydrating and silicating process to form an anchor to withstand the design loading.

The 15mm gap at the outer end of the anchor was filled with a high-bond expanding mortar of a colour to match the brickwork.

Top: Preparatory work for concrete repairs at nib
Top left: Coring in progress in outer leaf brickwork
Base right: Trial 'CINTEC' anchors and ties
Top right: Hole in brickwork after coring
Base right: The grout injection apparatus for the CINTEC anchors and ties

The innovative application of this anchor presented a few teething technical problems initially, overcome jointly between manufacturer, supplier, contractor and engineer, including:

In addition, similar 10mm-diameter 'wall tie' fixings, at an approximate rate of three per square metre, were installed, thereby stiffening the panel.

The approximate installed cost at summer 1986 of the anchors and ties excluding access and general items, was:

To safeguard the specifier/client an insurance-backed guarantee covering the design, manufacture and installation of the system could be obtained for an extended period of say, ten years, without prejudice to other contractual rights.

Five contracts, involving the installation of over 40 000 of these anchors and ties plus other brickwork and concrete repairs, including the removal of all slip bricks, were carried out in summer 1986 to a value of almost l million, on 18 blocks of flats.

Apart from the initial teething problems these novel anchors were installed successfully providing a technically and visually satisfactory answer to a difficult problem. Other situations where these anchors might be applicable are in materials such as weak natural masonry, friable 'dense' concrete, no-fines concrete or perforated brick.

Additional repair work included concrete repairs to the floor slab nibs, balcony soffits, balcony edges, and also concrete window cill replacement.

Reference

* - Foster, D. Brick Development Association – Further observations on the design of brickwork cladding to multi-storey reinforced concrete frame structures, 1975.