These cracks may occasionally have some structural significance but are not usually deemed serious. Often these cracks are more visible inside buildings than in external brickwork. Would generally be located at points of structural weakness in a building e. Indicates slight foundation movement, particularly if isolated.
FINE — Internally cracks can be filled or covered by wall covering and redecorated. Externally, cracks may be visible, sometimes repairs required for weather tightness or aesthetics. Note: Plaster cracks may, in time, become visible again if not covered by a wall covering. These cracks are likely to have some structural significance and will almost always occur at points of weakness or hinge points. Generally, cracks will be visible internally and externally and will indicate foundation or other structural movement enough to distort door and window frames and make doors and windows stick.
Weather tightness may be an issue that needs to be investigated as may the structural integrity of the building. The crack will inevitably become visible again in time if these measures are not carried out.
Windows and door frames distorted, floor sloping noticeably. Walls leaning or bulging noticeably, some loss of bearing in beams. Service pipes disrupted. Typical crack widths are 15 to 25 mm, but also depends on number of cracks. Beams lose bearing, walls lean badly and require shoring.
Windows broken with distortion. Danger of instability. Typical crack widths are greater than 25 mm, but depends on number of cracks. BRE Digest , and in particular the table above, is now used widely in the industry as a way of categorising cracks and determining whether any intervention is necessary. It should be stressed that these comments are a simplification of the assessment needed to properly classify damage to housing.
Figure 1 shows a related problem where the short return was restrained by wall ties. The problem can be reduced in new work by not placing ties too near to returns DAS The only satisfactory remedy is to install a movement joint to allow for any subsequent expansion. In some cases this can be achieved by widening the existing crack but often the brickwork must be restitched and the joint placed elsewhere. The stability of the modified structure must be checked before such work is carried out and additional ties or supports may be necessary.
Another problem arises from differential expansion of different units bonded together into one structure. A contemporary example of this problem is the combination of low-expansion decorative string courses of contrasting colour within high-expansion 7 brickwork of another colour.
The difference in colour will also result in different thermal movements. The result is that the expanding mass of brickwork puts the string courses into tension and they crack - Fig BS Part 3 does not identify this case since all clay units are deemed to perform the same in this respect.
But BRE workO has shown that the expansion can vary by a factor of , or even more, between different types of clay bricks. Similar problems occur where clay bricks are used as a make-up string in concrete masonry, and where diaphragm walls with a concrete inner and clay brick outer are bonded at the webs.
If different materials are to be combined in a single structure, their respective movement characteristics must be considered; if they are significantly different, the specification or the structure design must be changed to accommodate the movements, for example by inserting slip planes.
Most clay brick manufacturers can supply movement data for their own products. Vertical moisture expansion of brickwork can also cause trouble in conjunction with drying shrinkage of in-situ concrete walls or columns. In Fig 18, the moisture expansion has put the infilling brickwork under eccentric compression.
The resulting stress was relieved by outward bowing of the brickwork. The magnitude of the forces involved is illustrated by the way in which the intermediate nib was pulled away. Weathertightness and stability were impaired and a satisfactory repair required rebuilding the damaged elements. Advice on this problem and suggested repairs are included in DAS 2 and Digest The effect is to convert the strongly alkaline material pH 12 to 14 to a weakly acid state pH 8.
The process is progressive from the outer skin inwards at a rate largely dependent on the porosity, taking one or two years to penetrate 50 mm in exposed AAC blocks but 50 to years for 25 mm in well-compacted concrete. The process occurs only in the presence of moisture: generally, the wetter the conditions the worse is the effect. Normal winter condensation can provide sufficient moisture. The products of the reaction are usually of greater volume; this results in an expansion and causes random 'map cracking' at the surface and has the effect of weakening the resistance to tensile and shear forces.
They react expansively with moisture to form the respective hydroxide and often spall small pieces from the surface. The typical failure is termed 'lime popping' and is of aesthetic rather than structural concern. Figure 19 shows an extreme example of a lime pop originating from a larger than normal particle that passed a faulty sieve. Occasionally, clinker aggregates cause popping or general expansive failures of concrete blocks due to hydration processes or through moisture sensitivity of excessive unburnt coal - see BRE Report BR and BS Occasionally, however, drying shrinkage in an asymmetrically reinforced slab can contribute markedly to further deflection.
In Fig 20, shrinkage has been accentuated by the inclusion of shrinkable aggregates in the concrete. The resulting additional deflection has removed support from the foot of the wall above, and cracking has occurred. In cases like this it is extremely difficult to say how many factors are collectively responsible for cracking poor design and workmanship may also contribute ; still less is it possible to say what each has contributed in producing the defect.
The inclusion of this example is a reminder of the difficulty of clear-cut diagnosis of the causes of cracking in practice. All clay soils shrink when dried and expand when wetted. These volume changes tend to be worse for the stiff plastic clays that are common throughout the South of England.
Their location and behaviour are described in Digest In these soils, seasonal volume changes can produce vertical movements in open ground of 25 mm or more. The effect is normally restricted to the surface layers and is likely to be only a few millimetres at 1 m depth. The process is reversible and cracks which form during a dry spell will often close during the winter. Foundations can move either as a result of the loads applied to the ground by the building settlement or as a result of external factors that act independently of these loads.
Varying amounts of foundation movement differential movement distort the building and can cause damage. Foundation settlement is likely to be tolerated by the building provided the loads do not exceed the 'allowable bearing pressure'. Values given in BS suggest that most natural soils in the UK, except loose sands, very soft clays and organic soils, are capable of supporting typical low-rise building loads up to 75 kPa. Special care has to be taken when building on made ground or fill - Digests and Overloads occur most commonly where loads are concentrated on to a small area via a joist, beam or joist hanger.
Joist hangers result in particularly high stresses on the outer fibre of the wall adjacent to the load and failures have been recorded. Figure 21 shows a failure in flats on a filled quarry where some of the piles in the foundation did not reach to the quarry floor and slipped during a period of heavy rain. Removal of moisture from the ground by trees tends to exacerbate the seasonal volume changes - Digest Typically, ground movements associated with large trees are about four times those measured for clear ground.
Trees also have a long-term effect because the winter rainfall may not fully replace all the moisture lost in the growing season. The region of permanently desiccated soil so formed expands as the tree grows, producing settlement over an increasing radius; it can reach depths of to 5 to 6 m.
Figure 22 shows typical damage due to clay shrinkage exacerbated by nearby trees. If a tree is removed, the moisture returns to the soil slowly causing it to expand heave by as much as mm.
Because the effect of the tree is localised, the movements generated in nearby foundations are differential and can be much more damaging than uniform movements. Digest gives some guidance on acceptable levels and how to measure the intensity. Continuous daily and annually. Walls and roofs, especially well-insulated dark cladding and south-facing roofs.
Initial drying Shrinkage over weeks or years. Mortar, concrete, aerated concrete, sandlime units, timber. Large, low aspect ratio masonry walls; timber frames and floors; large concrete frames. Wetting and drying Timber and concrete directly exposed to Most materials, especially Expansion and contraction.
Loss of volatiles Shrinkage over hours to years. Solvent-based paints, mastics, plasticised plastics. Freezing and thawing of absorbed water Expansion; internal damage; spalling. Intermittent related to weather. Aggregates should be of a grade which ensures adequate durability of the concrete. Certain types of aggregate are shrinkable and require special precautions in mixing.
Certain types of aggregate may be susceptible to alkali attack or excessive moisture movement. Proprietary and recovered aggregates should only be specified where they have been assessed in accordance with Technical Requirement R3. Also see:. Concrete shall be specified correctly to ensure adequate strength and durability. Issues to be taken into account include: concrete in non-hazardous conditions exposure to climatic and atmospheric conditions exposure to aggressive ground conditions exposure to sulfates and acids in groundwater effects of chlorides effects of alkali-silica reaction aggregates.
Notes 1 Consistence class S3 should be used for strip foundation concrete and consistence class S4 should be used for trench fill foundation concrete. Exposure class Environment Exposure conditions XC1 Dry or permanently wet Concrete inside buildings with low air humidity.
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