- Manufacturer: William Airey & Sons Ltd
- Construction Type: Cast In-Situ-Concrete/Post & Beam
- Designer: William Airey
- Number Built: 4600-4650
- Years Built: 1922-25
- Distribution: Mainly Leeds/Edinburgh
Duo-Slab Brief History
The Duo-Slab system of house building incorporates clinker aggregate concrete blocks and cast-in-situ concrete columns. It was originally developed in early 1920 by William Airey and Sons Ltd, in Leeds. Approximately, 4650 Duo-Slab dwellings were built between 1922 and 1925.
Cast-in-situ construction Concrete was used extensively as an alternative to brickwork during the inter-war period. A number of different systems of cast-in-situ concrete walling were developed with the intention of lowering costs by reducing construction time. Unfortunately, history has shown us that many cast-in-situ concrete houses were in fact considerably more expensive to build, than traditional forms of construction.
The Duo-Slab Cast in-Situ concrete houses were generally built as two-storey houses, but there are also some examples of larger and more impressive two and three-storey blocks of flats.
The term ‘Duo-Slab’ should not be confused with the later 1945-55 Airey house, that was also produced by William Airey, despite the common use of references such as Airey- ‘Duo-Slab 2’ or ‘New Improved Duo-Slab’. The main reason for not comparing the two builds, is due to the fact that 1945 Airey house incorporates the use of several different construction methods.
Cavity walls consisting of precast concrete slabs with cast-in-situ concrete columns
The general construction of the Duo-Slab system is very similar to that of a traditional cavity-walled house. The floor and roof loading is taken directly to the foundations by the inner leaf of load bearing external and internal walls (consisting of precast concrete slabs with cast-in-situ concrete
This type of system is generally considered to be a simple and robust structure, that can tolerate considerable damage, without affecting the overall stability. In addition to this, concrete cross walls provide substantial lateral bracing, which improves overall strength and stability. In the
unlikely event of even a partial external wall failure, it is likely that significant damage to the rest of the structure would be minimal or at least reduced.
Traditional/Conventional concrete strip foundations were laid, and the top surface was set level, to provide a flat horizontal surface for the addition of the wall construction above. A damp-proof course was installed beneath the external and internal walls to prevent moisture rising from the ground and up into the structure.
The external walls are built of cavity construction, which consist of two leaves of 3-in thick clinker concrete blocks that were supported and tied together at regular intervals by cruciform shaped cast-in-situ clinker aggregate concrete columns (Figure 3.4).
The width of the cavity is typically 2-in in 2-storey construction, but this varies in width from 10-ins at ground floor level, up to 4-ins at second-floor level in 3-storey variants.
The edges of the wall blocks and the corresponding edges of the cruciform shaped columns are chamfered in order to provide a keyed or dovetailed joint between them.
The blocks are 8-in deep, 3-in thick, and 3-ft 8-ins long on their outer faces and 3-ft 6-ins long on their inner face. The dimensions of the column section are very much depends on whether they are being used in two or three-storey constructions.
For the two-storey construction, the specification called for the outer face of the trapezoidal sections of each column, to be about 6 ins long, and the inner face to be about 4-ins long, with the parallel side lugs extending some 3-ins into the cavity.
The walls were constructed by laying the panels dry and leaving the appropriate width gap between each of the panels edges.
Shuttering was then placed on the outer edges of the two leaves, forming the wall to close these gaps, and wood was placed directly into the cavity to act as spacers and forming stop-ends, leaving the specified distance from the edges of the panels free. Concrete was then poured directly into the space to form the columns.
The columns, were generally not reinforced, although sometimes, a single plain1/2-inch diameter steel bar was used.
The columns were cast using a 1:6 mixture of clinker aggregate.
The clinker concrete slabs were cast on site in specially constructed moulds using a specified mix of, 5 parts hard clinker, 1 part sand and 1 part Portland cement.
Dense reinforced concrete sills and lintels were installed below and above openings in walls, and possibly a reinforced, or sometimes unreinforced, concrete ring beam at the first floor and eaves level.
Other Du-Slab Construction Observations
The party walls are of a similar construction to the external walls. Information, published at the time of the original Duo-Slab construction, suggests that the brickwork chimneys were actually built before the main walling was erected.
The Internal ground-floor walls generally consist of a single leaf of 3-in thick, clinker concrete blocks, with unreinforced cast-in-situ clinker aggregate concrete columns spaced at approximately 4-ft centres. At the first-floor level, the internal walls are either constructed similar to those at ground-floor level, or they are constructed from lightweight timber studding.
Ground floors can either be formed out of solid clinker aggregate concrete, or of a more traditional timber construction.
Kitchen floors are of solid concrete construction, which may also be deployed in other areas where it was finished with floor boards, fixed to timber battens. Clinker aggregate concrete blocks are used to build the sleeper walls supporting the ground-floor joists.
The ends of the timber joists forming the first floor of houses are located in slots that were cut into the concrete blocks, to form the inner leaf to the external walls.
Intermediate floors of flats are usually constructed with reinforced concrete.
The roofs are pitched, usually hipped or gable, and of conventional timber construction.
The External walls are rendered, and internally, they are either hard-plastered, or dry lined.
Duo-Slab General Stability – BRE (Building Research Establishment) Inspection
The stability of the Duo-Slab form of construction is dependent on the bond between the blocks and the
columns. The extent to which the columns successfully bond the blocks together was found to vary widely between inspection and survey sites, and is heavily dependent on the quality of the original workmanship in forming the columns.
In some properties, bowing of external flank walls at first-floor level had occurred because of a
lack of mechanical tying between the first-floor joists and external wall components. In some cases,
local authorities had installed ‘L’ shaped metal straps between the top of the first-floor joists and the external face of the outer leaf.
The BRE findings support reports from consultants (commissioned by local authorities) that express
concern at the stability of the external walls under lateral loading, particularly for three-storey construction. At some sites, remedial measures were being taken as part of a rehabilitation scheme to
improve stability by tying internal and external leaves together, and providing ties to existing floor and roof construction.
There were reports from building owners’ consultants that some houses had cast-in-situ concrete ring beams at floor and eaves levels, immediately beneath the floor and ceiling joists. At some sites these ring beams were reinforced and at others unreinforced. On the one property, where detailed explorations were carried out, unreinforced clinker concrete beams were found bridging the cavity, but these were not always continuous around the perimeter of the dwelling.
At some positions the ring beam was found to consist simply of additional isolated blocks running parallel with, and bridging, the existing external wall cavity.
External and party walls were tied together by cast-in-situ concrete columns formed in a similar way to the external walls. However, there were no ties between external and internal partition walls and, in some cases, a wide gap was found at the junction between these walls caused by outwards movement of the external wall.
Both horizontal and vertical cracking of the render to external walls were present and were coincident with the joints between the blocks and the columns. The size and extent of the cracking was found to differ widely between sites. At one site inspected by BRE, coarse horizontal cracking was present above second-floor window level on gable walls.
At another site, mastic sealant had been used to fill cracks that had opened up between the blocks and the cast-in-situ concrete columns prior to re-rendering.
The bearing of lintels was found to vary considerably between sites, ranging from as much as 6 inches to less than 1 inch. Reinforcement was generally found to project from the ends of lintels into the cast-in-situ clinker concrete columns forming the reveals.
Duo-Slab Clinker aggregate concrete components
The evidence from wet chemical analysis, a mineralogical analysis, and moisture sensitivity tests,
suggested that at three of the four sites inspected, unsound aggregates (i.e. containing high levels of non-carbonate carbon) had been used.
The presence of ettringite (a hydrous calcium aluminium sulfate mineral) in one of the samples examined petrographically is consistent with chemical attack of concrete by sulphates. However, the ettringite crystals had formed in the voids of the specimen and were not causing any disruption of the concrete. This would suggest that, despite the relatively high levels of sulphate present, the cracking observed on external walls was primarily the result of moisture induced movement. For further information regarding Ettringite, see download and the bottom of this page.
- The Duo-Slab houses and flats inspected were generally found to have been constructed in accordance with design intentions. However, the quality of construction varied significantly between the sites inspected.
- The performance of the columns is dependent on: (a) the quality of workmanship during construction which varied between sites and, (b) the susceptibility of the reinforcement, where present, to corrosion.
- The clinker aggregate concrete used to form the blocks and columns was, in many cases, susceptible to moisture movement. This was a principal cause of the cracking and movement observed at three of the four sites.
- Thermal and moisture movements of the external walls had been exacerbated by the absence of mechanical connections between the walls and floors, particularly where the first-floor joists run
- parallel to the wall and between internal and external walls.
- Where reinforcement was used in the external clinker aggregate concrete columns, corrosion is likely to cause cracking and disruption of the concrete and eventually impair the stability of the structure.
- Cracking of the external render was found. This was caused by temperature and moisture related movements of the clinker aggregate concrete components containing non-carbonate carbon fractions (unburnt coal). The inability of strong renders to accommodate the relative movements of the underlying components exacerbates the cracking. Further deterioration of external walls will occur, particularly if embedded reinforcement corrodes or there are changes in the moisture content of the walls.
Notes for surveyors
- The stability of the Duo-Slab form of construction is dependent on the performance of the columns in tying the two leaves of blocks in the external and party walls together.
- The ends of precast concrete lintels should be examined for sufficient bearing. This will necessitate the removal of internal or external wall finishes.
- The ends of the timber joists, where they are built into external walls, should be examined for signs of rot, particularly in walls exposed to prevailing weather or where rain penetration is known to have occurred.
- The external walls of the structure should be examined for signs of cracking, movement and de-bonding of render, particularly at building corners.
- When making an assessment of the cracking, consideration should be given to how recently the property has been re-rendered or redecorated.
- Sampling and analysis of the clinker aggregate concrete in the blocks and columns of external walls should be carried out to determine its susceptibility to moisture movement. The non-carbonate carbon content of ten concrete samples should be determined, since direct moisture sensitivity tests are not likely to be generally available. The samples should be taken from columns and both leaves of the external walls.
- An assessment should be made of how effectively the columns tie the two leaves of blocks in the external walls together. Ideally, this should involve removing render locally from external surfaces and inspecting the interface between the columns and blocks. If possible, the cavity should be broken into or inspected using a fibre optic device to check that the columns have been formed correctly. If steel is located in columns in the external walls, consideration should be given to the effects of corrosion on the overall stability of the structure.