Concrete Lintels

In older domestic construction, openings were bridged by using reinforced concrete, in the form of either a pre-cast lintel or one that is cast in-situ. For the latter, timber or metal formwork is erected around the top, or head, of the opening and wet concrete is poured into it to form the lintel. In both cases, the lintel will have steel reinforcing bars in the bottom portion to overcome the weakness of concrete under tension. Where concrete lintels are used today they are invariably pre-cast, due to the length of time taken to cast concrete in-situ and the consequent delay to the construction process.

Concrete lintels tend not to be used in the external walls of new domestic buildings, as they have a number of disadvantages:

• compared to steel, they are costly and are heavy to lift into position (often requiring lifting machinery);

• they have poor thermal insulating qualities, resulting in condensation on the inside around the top of the window opening;

• they allow rainwater to track across to the inside;

• they are available only in a limited range of sizes;

• they cannot span very wide openings; and

• they are not particularly attractive to look at.

To prevent moisture inside the cavity from tracking across to the internal leaf, a DPC tray is introduced over the top of the lintel, to deflect any water in the cavity towards the external leaf. The tray is formed using a length of felt, with the front edge positioned in the bed joint directly above the lintel on the outer leaf of the cavity wall. It is turned up the cavity to form a slope and its back edge is bedded in a higher mortar joint in the blockwork of the inner leaf of the cavity wall. The length of the felt should be sufficient so that the cavity tray projects into the cavity, beyond each end of the lintel, by at least 150mm. Accordingly, the felt needs to be as long as the lintel, plus 300mm. Alternatively, the tray can be provided with proprietary stop-ends or cloaks, in which case it will not need to be extended beyond the ends of the lintel.

In areas of severe or very severe exposure to rain, and especially when full-fill cavity insulation has been employed, it is a requirement to provide proprietary cloaks or stop-ends to cavity trays above lintels. The tray must not be allowed to terminate open-ended where it runs past the ends of the lintel into the cavity.

The width of felt used to form a cavity tray will depend somewhat on the width of cavity but, to ensure that the felt is adequately bedded into each leaf of the cavity wall, a width of 450mm is usually sufficient. Weepholes must always be provided in the external leaf of cavity walls, in the course of brickwork beneath which the DPC tray is bedded, in order to provide an escape route for any water in the cavity.

Concrete lintels are difficult to disguise or make visually pleasing within facing brickwork, so they tend to be limited to bridging openings in blockwork internal partition walls where they are not seen. One way of attempting to improve the aesthetics was to use a pre-cast ‘boot’ lintel (so called because of its shape in cross-section), which had a reduced depth at the front so that less concrete was visible. The shape and design of more modern steel lintels reference the concrete boot lintel. Pre-cast concrete lintels require an end-bearing on the masonry either side of the opening of at least 100mm on a bed of mortar.

Concrete Lintels

Fig. 185 Simple pre-cast reinforced concrete lintels.

Steel lintels (see Fig 186) for bridging openings in external cavity walls are commonly made of either stainless steel or powder-coated galvanized steel. They come in numerous sizes, sectional shapes and lengths to suit various thickness of cavity wall and width of opening. Heavy-duty versions are capable of spanning clear openings of over 6m. Steel lintels typically have an end-bearing on the masonry either side of the opening of 150mm on a very thin bed of mortar in order to allow brickwork to be continued across the front of the lintel. The advantages of steel over concrete are that it is cheaper, lighter, and easier to position, allowing for faster construction. A steel lintel is also more visually appealing than a concrete one, as the end-bearing is concealed within a mortar bed joint and only a narrow lip of steel is visible externally over the top of an opening.

Concrete Lintels

Fig. 186 Insulated box lintel.

The sloping profile or cross-sectional shape of the lintel is a deliberate design feature that means the lintel forms its own DPC tray. A proprietary stop-end or cloak is inserted at each end, which forms a vertical up-stand and provides an additional measure to ensure that any water in the cavity is directed toward the external leaf. A separate cavity tray of felt is not normally required but some consider it good practice to include one. Again, the piece of felt should be long enough so that the cavity tray projects into the cavity at least 150mm beyond each end of the lintel. The length of felt required is therefore equal to the length of the lintel plus 300mm. Alternatively, the tray can be provided with proprietary stop-ends or cloaks and it will not then need to be extended beyond the ends of the lintel.

Подпись: When a concrete beam or lintel is under load it will sag. This causes the top half of the lintel to shorten and to be ‘in compression', with the bottom half essentially lengthening as it is subjected to tensile stretching forces. Concrete is incredibly strong under compression but very weak under tension, so concrete lintels require steel reinforcing bars to be installed in the lower section during the casting process. The reinforcement has a ribbed surface and ‘hooked' ends, so as to remain solidly gripped within the concrete that surrounds it. When steel reinforcement is cast inside concrete, it must have a minimum of 50mm of concrete around it. This concrete cover protects the steel from moisture in the atmosphere and penetrating rain. The steel must not be allowed to rust, since steel expands when it rusts and will crack or otherwise damage the concrete, causing the component to fail. Accordingly, a lintel must be installed the right way up in order to avoid structural failure. Manufacturers usually mark the top of a lintel for this purpose.

Again, in areas of severe or very severe exposure to rain, and especially when full-fill cavity insulation has been employed, proprietary cloaks or stop-ends must be provided to cavity trays above lintels, instead of letting the tray terminate open-ended where it runs past the ends of the lintel into the cavity.

To avoid a ‘cold bridge’ through the cavity wall, steel lintels must incorporate continuous insulation along the length of any box section. Without insulation, ‘cold spots’ would form on the inside wall above the top of the opening, and on the underside or soffit of the opening beneath the lintel, which could lead to condensation forming.

An alternative to the box-section steel lintel is the open-backed lintel, which has a number of advantages. Blockwork, as opposed to steel at the back, provides a better key for plaster. In addition, there is no requirement for blockwork to be cut round a box section and such lintels can be utilized where fair-faced blockwork (or brickwork for that matter) is desired internally.

Concrete Lintels

Fig. 187 Insulated open-backed lintel.