1791

RECONSTRUCTION OF PRC HOUSING USING BRICK & BLOCK MASONRY

J. MICHAEL DYSON, C.Eng., B.Sc ., FICE Principal

Michael Dyson Associates Midway House, 51/53 Huddersfield Road, Meltham, Huddersfield HD7 3AF

Introduction

Prefabricated reinforced concrete (PRC) systems of house construction were developed in the United Kingdom in the years following the Second World War. At this time there was a shortage both of tradesmen and of trad­itional building materials . The use of concrete as a building material, attempted on a smaller scale in the 1920s and 1930s, was encouraged by Government to ease the housing problem.

PRC System Building Some systems utilised slender components, light enough to oe manhandled and erected without cranage. Others used larger panels and a further range of designs was available based on a true concrete frame with further concrete in smaller cladding panels. Concrete components in all of these systems shared a common factor in that the steel reinforcement was located relatively close to the surface of the concrete. In normal reinforced concrete it might be expected that the depth of "cover" to the steel reinforcement would be of the order of 40 mm. PRC elements' cover, however, is rarely more than 20mm and often as little as 5 mm.

Concrete Chemistry At that time knowledge of the chemistry of concrete, as regards its capability to protect embedded steel reinforcement due to its alkaline nature, was probably unknown. We now understand what is referred to as carbonation of concrete and its implications. The fact is that fresh concrete has a high pH value - around 13 - and mild steel reinforcement, even in damp conditions, is prevented from rusting by this degree of alkalinity. The effect of atmospheric oxygen is, however, to neutralise the concrete. Ultimately the pH value will fall to below 9. This effect begins at the surface and gradually penetrates. Open textured, badly consolidated, concrete admits air more easily and therefore the rate of deterioration is faster . Concrete made with a lower cement content also carbonates more quickly . In good concrete it may be expected that the depth of carbonation rarely exceeds 25 mm but in the majority of prc elements steel reinforcement lost its initial protection after as little as 10 years.

Symptoms of trouble are of course the rusting of steel reinforce­ment, consequent expansion of the rust products, and cracking of the concrete due to this expansion.

1792

plasterboard inner lining

timber fillet, secured to prc post, for p'board fi xing

, If'<J;;::;."",:.,.,,', •. : J- copper wires holding

120mm cavity

AIREY

concrete cladding panels

~ ~ ~100mm x 50mm prc post 0 30mm dia, mild steel

tube reinforcement

plastered inner surface

115mm thick breeze block inner lining

150mm x 100mm prc pos t

x A 6. A A A t><= 6. :J.--co pper or gal vani sed I I ::;> S tee 1 s traps seCUrl ng

UNITY plastered inner surface

100mm thick breeze block lining

WATES 30mm thick

concrete cladding pane 1 s to pos ts

12mm dia mild steel reinforcing bars

structural prc panel, storey height, widths from 1m to 2.5mm, ends profiled to contain an in situ concrete column approx 100mm x 100mm to lock panels in correct alignment

Figure 1. Horizontal cross-sections through walls of 3 different prc house types, as constructed

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1793

Concrete Deterioration Where reinforcement is contained in concrete members which are expcsed to the elements, cracking and then spalling of concrete can be seen. The extent of structural decay is easily established and remedial measures can be taken. In many PRC designs, however, the reinforced members lie below the surface and are protected by external cladding paneis which are often concrete themselves, but unreinforced. There is no easy means of inspecting within wall cavities of such types of construction. It was as a result of a fire in an Airey type PRC house in 1980 that a large amount of wall cavity was exposed when the inner lining was destroyed, and the extent of damage to the reinforcement was noted .

Surveys of PRC Houses This incident led to widescale inspection of Airey houses and then of other types of PRC construction. PRC housing had been most commonly utilised by local authorities for what is known as "council housing" for rent to those who were not home owners. These local authorities were therefore instructed by Government to carry out inspections of represent­ative samples of their PRC housing stock and report their findings to the Building Research Establishment, which is the technical wing of the Department of the Environment. The total number of PRC houses was established at over 200,000. This investigation was supplemented by specific surveys commissioned by the BRE and when the statistics became available it was evident that in some house types quite large scale decay was already present. No house of such construction has yet been known to collapse, largely because of innate over-design, but such concern was shown that building societies in general decided almost overnight that such houses should not be regarded as adequate security for normal mortgages.

Mortgageability As long as the houses were in local authority ownership this caused little problem but the Conservat-ive Government had introduced "right to buy" legislation under which Council tenants could elect to purchase their houses at a substantial discount. About 10% of the total PRC housing stock had been so purchased and when these purchasers wished to move on, the new owner usually required a mortgage. As the houses were now regarded as unmortgageable, the effect was that an owner of such a property could not sell and became trapped within his 4 concrete walls. The Government reacted quickly to this and introduced the Housing Defect Acts in 1984. This Act provided for further grants to be paid covering the cost of "structural reinstatement" of the concrete houses. Structural reinstate­ment meant the removal of prc elements or at least measures to isolate and make redundant all structural prc.

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Reinstatement Standards As a means of ensuring that structural reinstatement was carried out to a universally agreed standard, the Government of the day enlisted the help of the National House Building Council (NHBC) . This body oversees construction of new housing and has a 10 year insurance guarantee system . The Company spawned a subsidiary, PRC Homes Ltd, with the same objectives . PRCH set up an assessment committee with representatives drawn from the BRE, from involved government departments, from the building societies themselves, and from local governement. This 15 man committee is authoritative and has to approve all systems of reinstatement for this type of house.

Principles of Reinstatement It was rapidly established that general opinion of this committee favoured the removal of prc and the substitution of "traditional" building materials. Reconstruction of external walls of the houses was therefore adopted and brick and block cavity walls were almost universally selected by designers of reinstatement systems. The system designer faces the problem of "unbuilding" the prc elements of a house whilst retaining floors, roofs, internal partition walls and often the external wall linings. Most of the work is accomplished with the occupants remaining in residence and therefore systems of temporary propping and the protection of openings in walls, against weather and unauthorised entry, had to be faced. Some of the solutions are predictable, others are innovative. For each type of house there has been keen competition to create a design which is practicable, as convenient as possible to the occupants and, of course, economic. Where possible, home owners who wish to have this work carried out by means of a Government grant, are encouraged to seek alternative quotations from companies offering different approved reinstatement schemes. This has the effect of reducing cost whilst the standards set by the PRC Homes assessment committee ensure that the work is done in such a way that the building societies will accept the finished product as being a mortgageable entity .

-P·:·;a',

b. ·: õ·~ ·

window opening - ..

original galvanised steel window set in timber frame tied back to prc posts with gms fixings

plasterboard retained, timber fillets secured to new loadbearing lightweight blockwork

or

Figure 2. Horizontal section through window in wall of an Airey house, before and after reinstatement

position of first fl oor wi ndow

shiplap style----­concrete cladding panels

position of ground fl oor wi ndow

r

I

, . ;" j .. ,

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storey-height prc posts with steel tube reinforcement

fabricated steel/timber floor beams with bolted connection to prc po sts

prc posts stand on dpc at wall base. Bottom of stee l tube is therefore wetted due t o water

/

penetrating wall cavity in driving rain, starting rusting of steel

20mm asphalt topping

150mm concrete floor slab

BRICK FOOTINGS WALL

Figure 3. Vertical cross-section through Airey house wal l -existing

min 40mm cavity

additional concrete footing pinned to existing foundations using stainless steel dowel s set in resin

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existing plasterboa rd lining can be retained

new structural, lightweight blockwork

fl oor beam load tran·sferred to new blockwork

dpc

Figure 4. Vertical section through Airey house wall after reinstatement

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Joist and connections Floor joists in prc houses were often used as a means of bracing the front and back walls . The concrete elements have to be removed and alternative means devised to transfer loads to the new blockwork. Two different examples are illustrated here.

EXISTING

AIREY

WATES

floor beam end bracket straddles prc posts

and i s bo lted through

wall plate bolted to prc ring beam

Figure 5. Floor beam connections

AFTER REINSTATEMENT 'top hat' style load transfer bracket inserted when prc post removed, bolted to end bracket

and load taken by new blockwork

prc beam removed, wall plate re-secured to gms angle supported

by ne~1

blockwork

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Centreline Oetails When a pair of semi-detached houses is reinstated together, or a whole terrace, there are no major aesthetic problems. Often however a privately-owned house is attached to one which is still Council-owned. Grant is available to reinstate the private house but not for the other half. "Half a pair" is therefore reinstated. Aesthetically this makes no sense at all, but economic considerations 'force' many local authorities to leave the reinstatement of their own prc houses in favour of more pressing demands on their money. Reinstatement schemes have to be designed for single houses even in the middle of a terrace, in such a way that the house is structurally independent of its neighbour(s) afterwards. Care in detailing brickwork is required. Contrasting colours and the expression of a centreline 'pier' can help to give an illusion of purpose in this area.

AIREY HOUSE

in feature

,~ ....

cut concrete cladding, support by new masonry and weatherproof

Figure 6. Centreline detail when one house is not reinstated

Occupants in Residence When the Government drafted the Housing Defects Act, rules for the payment of grants to cover the reinstatement work were relatively tight. It was anticipated that it would be practicable for owners to remain in residence, with suitable protection being provided to window and door openings whilst these were being reformed. These rules were drafted before the full extent of the reinstatement schemes was known. Initially it was expected that work would only be required to the external walls but subsequently the discovery of internal prc components in some houses was followed by confirmation that these elements also must be regarded as suspect and removed.

Designers chose the easiest houses to work on first and in such cases, staying put became the order of the day and owners adapted to the inconvenience of the reinstatement scheme remarkably well. A good contractor could finish the work in 4 to 5 weeks although on some occasions owners have been inconvenienced for as long as 24 weeks. As designers turned towards the more complicated house types however, and reinstatement schemes involved the total removal of party walls, sometimes floor beams and other partition walls, then to remain in residence became less attractive. For these more complicated house types, therefore, it is permitted to "decant" the occupants when the level of inconvenience is greatest.

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Variable Structural Oetailing Although each of the different PRC house types appears at the outset to be entirely modular, it has neverthless been discovered that the basic kit of parts provided by the manufacturer has been modified in some cases. There are also dozens of different variants of each house type. Single and 2-storey houses are most common and numbers of bedrooms range from 2 to 4. Floor plans vary in layout and houses in size. Internal partitions can be arranged differently. These are predictable variations. On the other hand it is as though the supplier could not always provide every necessary component and in such a situation a builder has been found to have resorted to the use of traditional materials for certain sections of the building. We therefore find tile hung gables, tile hung first floors, brick gable or ground floors, rendered panels and occasionally timber in the external elevations. Such alternative detailing keeps both designers and builders on their toes and certainly removes the "sameness" in a contract involving perhaps 50 houses which at the outset appear to be identical.

lhe Finished Product The fact that schemes of reinstatement are approved by PRC Homes Ltd means that on completion, as long as the rules laid down by PRCH have been observed, each house is provided with a 10 year insurance guarantee in almost identical terms to that provided for new houses by NHBC. At a cost of up to fl,400 for a lump sum premium, this is not cheap, but it is probably the best form of protection yet available.

Aesthetically the effect of a whole estate of PRC houses reinstated in brickwork, and with sympathetic architectural treatment, is most dramatic. Modern housing estates are usually devoid of space, with houses being crammed together in order to achieve maximum density. PRC properties were built in days when space was not at such a premium and proportions of estates are more generous. In addition, gardens are mature and in situations where occupants are proud of their surroundings, the effect of a new, brick house set wi thin a mature garden is extremely pleasing. Opportunities are there for owners to elaborate on the brickwork detailing and it ís good to see that an increasing number are prepared t o pay a little extra and achieve an even more pleasing resulto

Conclusion

If I am allowed to conclude with a plea to the brick manufacturing industry in Britain, I should add that as a Civil Engineer I knew little of the intricacies of bricks and brick laying. Some of these mysteries have now unfolded and I am now aware of another dimension in designo Quality facing bricks are now more widely used. Architectural effect can be heightened by design details, many of which need splay bricks, bull noses, cant bricks etc. These are not so readily available and seem inordinately expensive. The industry would gain in stature if this position could be eased.