System-Built Low Rise Concrete Housing – 1940s - 1970s

Non Traditional Housing

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System-Built Low Rise Concrete Housing – 1940s - 1970s

Introduction

In the period 1945 – 1955 about 20% of the houses built by public authorities in England and Wales were non-traditional; in Scotland this rose to nearly 50%. This total represented some half million properties divided among at least fifty different building systems. These systems included steel, in-situ concrete and pre-cast concrete. It was not until the mid-1950s that financial support was withdrawn as the availability of traditional materials and skilled labour was improved. By the end of the 1950s the post-War programme was almost at an end although new housing initiatives once again explored system building in the 1960s and 1970s. The vast majority of non traditional houses were built in concrete and this section describes some of the most common types.

The 1940s and 1950s

Many of the post-War pre-cast concrete houses have suffered from one common defect –carbonation. This is sometimes referred to (incorrectly) as concrete cancer. However, concrete cancer is a very different type of problem (alkali – silica reaction) and is more common in motorway bridges etc.

Most system-built houses used pre-cast columns or pre-cast wall panels reinforced with steel bars or tubing. Normally, reinforcing steel embedded in concrete does not rust or corrode because, in an alkaline environment, a passive film forms on the steel which protects it against corrosion. Concrete provides a suitably alkaline environment because of the high level of calcium hydroxide which is produced as cement hydrates and hardens. Unfortunately, this protection can be broken down in two ways. First, carbon dioxide from the air can react with the calcium hydroxide. As a result of this action the surface of the concrete becomes carbonated and this is insufficiently alkaline to protect the steel. With time, the carbonated layer extends very slowly into the concrete so the alkaline protection is reduced and moisture eventually reaches the steel reinforcement, which begins to rust. In the design of reinforced concrete structures a sufficient depth of covering should be specified so that the carbonation will not reach the steel within the design life of the building. The rate of carbonation will depend on the quality and density of the concrete as well as on the exposure of the building. Both the photos below, a pre-cast lintel and a pre-cast fence post, show typical examples of carbonation.

Another way in which the passivity of steel in concrete may be broken down is through the presence of chlorides. Chloride ions can permeate and break down the protective film which forms on steel in an alkaline environment. Hence, when chlorides are present in concrete the reinforcement can corrode even if carbonation has not occurred. Calcium chloride was often introduced into the concrete mix to accelerate hardening during cold weather, to enable continuous production of the pre-cast units.

If the protection of the steel is broken down, either through carbonation or by chlorides, rusting can occur. Rust occupies a volume many times larger than that of steel, and as corrosion continues the reinforcement expands and breaks the concrete.


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Specific house types

It would be possible to devote a whole book to the problems of system building due to the large number of designs used. In this section four specific pre-cast house systems are described:

Woolaway

Airey

Unity

Cornish

Woolaway Houses

Some 5,000 of these were built after the second world war, the majority on large estates. These units generally provided three-bedroom properties, usually in pairs with a variety of internal layouts. There are some terraces of four units and a few two-storey one-bedroom flats.

The external and party walls are made from storey-height pre-cast lightweight concrete columns spaced at centres of 750mm (2ft6in). The columns are located on a ring beam at their base and there is an additional ring beam at the roof. There is no mechanical fixing between the columns and the ring beam. The gap between the columns is filled with pre-cast concrete panels bolted in position, as shown in the diagram below. On the outside, the concrete framework is hidden by a textured render which gives the property the appearance of a traditionally built structure. On the inside, the frame is hidden by a lining of plasterboard or fibreboard..

The illustration above-right shows the basic layout of a gable end. The frame is hidden by render.

The main defect In severe exposure conditions these properties have suffered from damp penetration for several years, and serious corrosion has been discovered in both the beams and the columns. The aerated lightweight concrete components are full of tiny air pockets which have absorbed some of the expansion of the rusting steel. Therefore, quite advanced structural failure can go unnoticed. Deterioration of the three ring beams is particularly serious because this member ensures lateral stability of the columns.

Other problems 1. The original timber windows were often badly fitting; these suffered from water penetration, rotting

sills and draughts.

A drawing taken from 1947 publicity (Building Magazine).


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2. The combination of post and panel construction has given rise to uneven ‘U’ values in external walls, with associated problems of cold bridging and condensation.

3. The sound insulation of the party wall is poor.

4. Differential movement of the panels and columns can cause the render to crack, increasing the problem of rain penetration.

Airey Houses

Some 26,000 Airey houses were built after the war, and of all the non-traditional house types these are probably the easiest to recognise by their concrete shiplap cladding. They are constructed from closely spaced, storey-height pre-cast columns reinforced with steel tubing. The cladding is in the form of pre-cast overlapping panels with either a smooth or pebbledash finish. The inside of the framework is usually finished in blockwork downstairs and plasterboard upstairs. The triangular spandrel panels over the gable ends are finished in timber weatherboarding or plain tile hanging.

The main defect The most common form of deterioration is found in the columns in the form of longitudinal cracking. This is caused by expansion of the rusting reinforcement, which is in the form of a steel tube. The corrosion of the steel tube is most common at the column ends where the water has collected after penetrating the cladding, or as a result of condensation.

Other problems 1. The existing steel windows are of poor quality and are often warped, causing severe draught

problems.

The load bearing pre-cast columns support the external shiplap cladding. The cladding is held in place by loops of copper wire twisted around the columns. A strip of timber running on the inside of the column face provides a fixing for the internal plasterboard lining and is held in position by the wire ties.


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2. The timber cladding to the gable end is subject to rot.

3. The thermal insulation is very poor and this can result in high heating costs and problems of condensation.

4. In some properties cracks have been found running across the columns; the probable cause being damage on site during erection.

5. Some properties in private ownership have been double-glazed, and this has sometimes involved removal of the loadbearing mullion in between the windows.

The Housing Defects Act was introduced by the Conservative Government in 1984. Its aim was to restore the mortgageability of PRC (pre-cast reinforced concrete) houses. A number of house types were identified as at risk from carbonation. Tenants who had bought their houses unaware of the defect were eligible for grant aid. Anyone since then who has bought a PRC is deemed to be aware of the potential defects

Unity Houses

There were approximately 15,000 Unity houses built, most of them between 1953 and 1955. The structure comprises a series of pre-cast columns, usually at 3ft centres with pre-cast (but not reinforced) panels forming internal and external skins. In early forms the inner and outer panels were tied together with metal ties but later models had the external panels tied independently to the columns. The inner panels were lightweight concrete to provide some measure of thermal insulation. The outer panels were profiled along their top and bottom edges to provide weather protection. In addition, they were separated from the columns by a vertical dpc. The panels helped stiffen the frame although additional bracing was provided by steel straps (running diagonally between the columns). Lateral bracing was also provided with steel straps. On completion the outer panel joints were pointed. At first floor and roof level steel beams spanned across the building and also joined the ground and first floor columns.

Private owners, who bought their houses in the 1970s and early 1980s, have had financial support to repair their houses through the Housing Defects Act 1984. This support was available even if the house itself was not defective. As long as it was recognised as a defective type support was forthcoming. Grants usually paid for renewal of all external loadbearing elements. No such support was available for tenants.

A special tool (right) tightened the wire ties and held the inner and outer panels against the load bearing columns.


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Most Unity houses were built as two storey houses. However, there were also a number of bungalows and some medium-rise flats. The party walls were formed in cavity construction and comprised two 75mm leaves with a 50mm cavity. Ground floors were usually of solid construction with splayed timber bearers bedded into the screed or slab and supporting softwood boards. Internal partitions were formed from storey-height 50mm reinforced plaster panels or from 50mm breeze blocks. Windows were mostly metal. During the late 1940s and early 1950s there were a number of changes aimed mostly at speeding construction times.

The BRE investigation in the early 1980s identified a number of problems with Unity houses. However, although there were some specific problems, the investigation suggested that,

"Unsafe conditions are unlikely to be present as a result of deterioration of columns due to reinforcement corrosion." (The Structural Condition of Unity Houses, BRE, 1983,page 3).

Nowadays, many of these houses have been repaired or have had their external walling elements renewed. However, some are 'as-built' and surveyors should be aware of the need for detailed inspections.

The left hand photo shows the diagonal wind bracing, designed to prevent deformation of the frame. The right hand chart shows the number of Unity Houses built during the late 1940s and early 1950s.

Specific Defects 1. Corrosion through carbonation and impact damage can result in steel expanding and columns

spalling.

2. Where claddings contain high levels of chlorides it is possible that the ties which connect these panels to the columns or internal leaf can corrode.

3. Tenants have complained about poor sound insulation between units.

4. In some areas there has been damage to the external cladding panels, probably through frost action.


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The plan above is taken form an article in ‘The Builder’, 1949. These Unity houses were very popular because they were large in terms of space, and built to low density.

Cornish Units

By the mid 1950s almost 30,000 Cornish Units had been built in the UK. The walls comprise a series of reinforced columns set, at their base, into a pre-cast concrete base-stone, itself supported on a few courses of brickwork or blockwork on a strip foundation. The columns are at approximately 900mm centres. Early Cornish units had separate columns supporting the internal and external wall panels -later ones had, 'double' columns. Cladding panels are made from concrete (not reinforced) and are positioned by sliding them in between the columns. A series of tie rods in the cavity help keep the columns at the correct spacing and help keep the structure rigid.

Cornish units were available as two storey cottages (left) or as three storey flats (right). Many of the two storey houses were erected on infill sites in the existing suburbs (often among 'middle class' areas of 1930s speculative housing). The three storey versions tend to be on 1950s purpose built estates. They are all immediately recognisable because of their mansard roofs. The construction above the ring beam (at the top of the columns) is entirely traditional.


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Party walls are usually built from brick or block and are typically 225mm thick. Partitions can be brick, block or timber studding covered with plasterboard. Windows can vary but the most common type were steel - the pre-cast columns form mullions between groups of windows.

The BRE report, 'The Structural Condition of Cornish Unit Houses', (BRE 1983), concluded that there were a number of cases of longitudinal cracking in columns, these were caused by rusting of the steel reinforcement, itself caused by carbonation. Cracks were also found in ring beams, cladding panels and the pre-cast floors used in flats. The BRE also found evidence of high chloride levels in some components. However, one of their conclusions was, "No structurally unsafe conditions were found......". However, don't forget that this report was written nearly 20 years ago.

There is also other evidence suggesting that many of the columns etc were actually damaged on site. Once the column is cracked, rust can begin very quickly.

Systems other than Pre-cast Concrete

No-fines building systems

The No-fines system used a concrete mix which contained no sand, (the ‘fines’). This mix was poured into reusable shutters to form both internal and external walls. Omitting the sand from the mix provided cost savings, reduced weight and, at the same time, gave a measure of thermal insulation (because of the air voids). It also helped minimise damp penetration because, without the sand, the open textured concrete was less prone to capillary action. The overall strength of the concrete, however, was reduced which placed limits on the size of window openings. The finished houses looked just the same as a traditional house with a rendered finish.

This graphic looks a bit complicated but the principles are fairly easy to follow. Two outer shutters and two cavity plates form the mould into which the concrete is poured. Steel bars act as spacers to keep the cavity plates apart and form the cavity. When the concrete has set the shutters are raised to form the next lift. Some systems produced solid rather than cavity walls.


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Wimpey and Laings both produced thousands of low rise ‘no-fines’ houses. The system could also be adapted for high rise construction. Unlike the PRC houses there were no problems of carbonation and many of these houses have provided durable, cost effective dwellings. The ‘no fines’ system was not confined to the post war years, as late as the 1960s ‘no fines’ houses were still being built (below).

Other system built house types from the 1960s and 1970s

During the 1960s the government had ambitious plans for public housing. Although most houses were built using traditional construction a significant proportion employed a variety of system-built methods. Many of the systems were primarily designed for high rise housing (beyond the scope of this topic) but a large number (well over 100) were aimed at low rise. Three typical systems are shown below:

Insitu concrete

Pre-cast panel systems

Composite systems using concrete and timber

This system (left) is another insitu concrete process but one which depends on an external cladding of brickwork. One might question the use of industrial and traditional methods side by side but the purpose of the brickwork was to ensure that the house looked ‘traditionally’ built. In practice many of these systems, however well designed, were not cost effective. They relied on large scale contracts involving hundreds of houses. As large estates of new social housing became progressively rarer in the 1970s most of the systems disappeared.


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Another system is shown below. This relies almost entirely on pre-cast panels. External decoration in the form of tile hanging or weatherboarding hid the concrete panels. These qgain depended on large scale contracts to be cost effective. In addition the pre-cast panels could only provide limited levels of thermal insulation. The rise in fuel costs in the early 1970s led to new requirements in the Building Regulations for thermal efficiency and concrete systems were no match for traditional construction or timber framing. Concrete also led to problems of cold bridging and condensation.

party wall orgable panels

Spandril panel withweather boarding or tilehanging finish.

Proprietary system using pre-cast concrete panels. Houses or flats upto four storeys.

Groundbeam

Loadbearing concrete panelswith render or other cladding.

spinewall

party wall orgable eavesfiller

Gable walls clad inbrickwork

Trussed rafter roofs prefabricatedand hoisted in position - supportedon pre-cast concrete wall plateswhich also act as parapet gutter.Gutter factory-finished in asphalt.

In situ or pre-castground floors. Pre-castconcrete or timberupper floors.

The system shown on the left is a composite system which comprises pre-cast concrete cross walls and upper floors, with timber framed infill panels, front and back. This form of construction was very popular in the late 1960s and early 1970s, for private as well as social housing. Levels of thermal insulation are not adequate for modern needs and there are problems of cold bridging, differential movement and poor sound insulation between units.

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System-Built Low Rise Concrete Housing – 1940s - 1970s