4.7.4 Basements
Basements are remarkably versatile and an attractive addition to above ground living. They have a wide range of uses from playrooms and studies to bedrooms, garages, gyms and saunas.
Aircrete block walling should be used in accordance with the Approved Document "Basements for Dwellings". Commonly this takes the form of a cavity wall with a reinforced concrete filled cavity.
Aircrete blocks not less than 7N/mm compressive strength range should be used.
The block thickness may be determined from the rules in DD ENV 1996-3:1999
Eurocode 6 Part 3: Simplified calculation methods and simple rules for masonry.
4.8 Movement
Provision should be made where necessary for the normal movements which occur in masonry from shrinkage of concrete products and expansion of clay products.
Thermal expansion and contraction movements may also occur.
Attempts to calculate the expected movements are not recommended, as the calculations are complex, unrealistic and therefore unreliable.
When movement joints are to be incorporated, the following well-established rules of thumb should be followed to minimise cracking. Aircrete blockwork should be divided into panels approximately 6m long separated by vertical movement joints placed in unobtrusive locations consistent with them being able to fulfil their function.
The spacing of the first movement joint from an external or internal angle should be not more than 3m and preferably less where the masonry is continuous at the angle, due to the effect of end restraint of the wall panel.
Alternatively, walls may be reinforced with bed joint reinforcement to avoid using movement joints e.g. in party walls or long straight lounge walls in dwellings.
Adverse effects of movement will be minimised by protection from extremes of moisture in storage and before, during and after construction of the Aircrete block walls.
In addition, the weakest recommended mortar mix, consistent with the structural and durability requirements, should also be used.
4.8.1 Movement joints
It is not normally necessary to include movement joints in the internal walls and inner leaves of traditional dwellings unless the walls are longer than about three times their height.
The Aircrete manufacturer should be given the opportunity to indicate their recommendations. They should be included in external Aircrete walls of dwellings and the walls of industrial and commercial buildings. Vertical movement joints should always be used to form the joint between walls of dissimilar materials e.g. when an Aircrete wall abuts a brick wall.
Movement joints and any joint filler should be built in as work proceeds. The filler should be set back from the face sufficient distance to allow accommodation of any sealants and should not project into the cavity. |
Where required, both for walls built using general purpose mortar and walls built using thin-layer mortar, movement joints should be spaced not more than 6m apart and not more than 3m from a corner. To maximise their effectiveness and minimise the visual impact they should be located in re-entrant corners, at openings such as window openings in the least obtrusive position, behind downpipes etc. Movement joints should be built in as work proceeds and in external situations should be sealed with waterproof, flexible sealer to prevent the ingress of moisture. Rendering or plastering should never be continuous over a movement joint. Any ties across movement joints should be slip ties and must not restrain movement of the joint in the direction of the length of the wall. Flat strip or rod slip ties should be placed across the movement joint at 450mm centres vertically. In order to assist free movement at the movement joint one half of the movement joint tie should be debonded or proprietary ties (which already have this feature) should be used.
Where movement joints in Aircrete walls are also required to provide fire resistance, an intumescent filler providing the same fire resistance as required by the Aircrete wall should be used. The filler should be installed into the movement joint in accordance with the manufacturer's instructions.
Where ceramic tiling is to be fixed to Aircrete the movement joints should be 3m to 4.5m apart to match the movement joints in the tiling.
4.8.2 Bed joint reinforcement
Bed joint reinforcement designed for use in the horizontal mortar joints of blockwork may also be used to restrain movement within a panel.
Bed joint reinforcement should have at least 13mm of mortar cover on inside wall faces and 25mm on external faces, consideration should be given to stainless steel according to the exposure conditions.
Bed joint reinforcement should be suitable for use in general purpose mortar joints or thin-layer mortar joints as appropriate.
Where external walls are perforated by door or window openings, bed joint reinforcement should be considered under and over the openings and should continue at least 600mm both sides of the opening.
Where Aircrete blockwork partitions are built on floors subject to significant deflection, the bottom two bed joints should be reinforced with bed joint reinforcement (Fig 15).
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