4. Cracking, Bulging and Crumbling in Walls

 

4.1 Cracking in Brickwork

4.1.1 Ground Movement

4.1.2 Thermal Movement

4.1.3 Moisture Movement

4.1.4 Movements in Structural Members Built into Walls

4.1.5 Movements in Building Frames into which Walls are Built

4.1.6 Corrosion of Embedded Steel

4.1.7 Sulphate Attack on Mortar

4.2 Deterioration of Walling Materials

4.2.1 Disintegration of Brick

4.2.2 Deterioration of Natrual Stone

4.2.3 The Birds and the Bees

4.3 Basic Repairs

4.3.1 Repairs to Cracking in Brickwork

4.3.2 Replacement of Decayed or Cracked Bricks

4. CRACKING, BULGING AND CRUMBLING IN WALLS

In this chapter we look at those defects that are most likely to be encountered in the run of repointing work and at remedial work that may be carried out. It should be emphasised at the outset that the diagnosis of defects is often a long, complex process requiring observation and measurement of a wall perhaps over several seasons in order to establish a pattern of behaviour. Such diagnosis is beyond the scope of ourselves and requires considerable technical knowledge and experience. What we shall be doing is reviewing the whole range of defects that you might come across and give some indications of where further professional advice should be sought by your client from an architect, surveyor or engineer. It is very important to be clear in your mind at the outset that repointing and/or surface repairs are not going to cure the cause of a defect. They will prevent some of the worse effects (for example rain penetration or continued deterioration of materials) provided the work is done properly (here particularly in the choice of mix proportions). On the other hand, poorly executed repointing could be responsible for further cracking in the wall or deterioration of the wall materials.

4.1 Cracking in Brickwork

Brickwork cracks when the pattern of stress in the wall is changed from that which existed when the wall was originally built. In particular when the brickwork is subjected to tensile (stretching) or shear (sliding) stress which it is far less able to cope with than direct compressive (crushing) stress. The changes in stress can arise from a variety of causes and often, of course, a combination of causes.

4.1.1 Ground Movement

As the building is constructed so the load on the ground is progressively increased and some settlement occurs. Mortars used should of course accommodate these movements.

Subsequent changes in the ground can create large settlements to the point that the superstructure of the building is unable to absorb the movements and the stress in the material increases to the point at which the material cracks. There isn’t the space here (or the need) to go into the reasons why soil behaviour can vary, figure 4.1 illustrates typical cracking patterns arising from ground movements.

4.1.2 Thermal Movements

When materials are heated they expand, contracting again as they cool. Whilst this is most obvious in the metals, ceramics also possess the same property though to a lesser degree. Thus any wall, and in particular one facing in a generally southerly direction, is going to be continually expanding and contracting as the temperature varies between day and night time, again there will be a seasonal variation as between summer and winter, reflecting the difference in air temperature. Even in brickwork these movements can be sufficient in long walls in modern cement-containing mortar to cause a measurable change in say wall length. Where there is, restraint is relieved by a cracking in the wall often along a plane of weakness such as a dpc. The displacement and cracking of the wall often then prevents further movement. Modern construction of long walls tends to include vertical movement (or expansion) joints to take up the changes in dimension so as to avoid the unpredictable cracking that would otherwise result. Traditional construction with lime mortars provided the opposite approach (a flexible mortar absorbing the changes in brick size) we are all familiar with walls like that to Blenheim Park which go on mile after mile without any expansion joints.

4.1.3 Moisture Movement

Shrinkage cracking on drying out is most likely to occur with calcium silicate brickwork (see section 2.2.2 above), figure 4.3 shows typical cracking patterns in calcium silicate brickwork. Again modern practice emphasises the need to provide movement joints. The cracking is unlikely to be progressive and may need repointing possibly to improve appearance or to prevent rain penetration.

Clay bricks when freshly drawn from the kiln start to expand (see 2.2.1 above). In the past in times of building boom, brick shortages got to the point when bricks were loaded from the kiln on to a lorry and taken straight to site where they were built in sometimes whilst the bricks were still warm. Inevitably with these fresh bricks, expansion took place, leading to cracking, often where there were short returns or setbacks in an elevation. As times of boom recede, this problem (which again is not progressive) is less likely to occur.

4.1.4 Movements in Structural Members Built into Walls

Until the development of steel and reinforced concrete in terms of building technology in the first quarter of this century, the main material for spanning across a space was timber. In a brick wall, of course, the brickwork could be formed in an arch, which by virtue of its shape allowed the brick to span across an opening in a compression member.

Timber has a number of disadvantages in structural applications, only some of which need concern us here. Firstly its stiffness is fairly low so that under load it will sag a great deal before actually breaking. Where timber is used as a lintel over an opening or as a beam (‘bressumer’ is the term properly) over a shop front or bay window (see fig 4.4), bit by bit the timber bends more and more, more of course than the brickwork can follow, so that cracking occurs. Often this type of failure occurred shortly after a building was altered and for many years now no further movement has taken place. On larger span, however, one must not assume that movement has stopped. It may be necessary to shore up and replace the beam (probably with a steel) section before proceeding with any repointing and repairs to brickwork. Modern regulations generally do not permit timber in this situation.

The destruction of timber by fungus attack (dry rot, wet rot) and/or by insects (“wood worm”, furniture beetle and other species) is likely to cause problems with brickwork where the timber is built into brickwork. This is particularly likely in external walls where built in timber may only be 4” or so back from the external face and, in winter, certainly is likely to be wet, timber must be fairly wet in order to be attacked by fungus or beetles.

Fig. 4.5 shows some possible situations in which decay of timber leads to cracking of brickwork. It should be emphasised that there are many variations on this basic theme, the essential common feature is that the weakening of the timber either increases the bending in a beam or a lintel or allows the brickwork to crush the timber, in each case the movements leading to cracking. Again in these cases it is essential to correct the basic fault (replacement of defective timber and prevention of further decay) before patching or pointing the wall.

Arches in older buildings frequently show signs of cracking or movement. Almost invariably this occurs when there is some failure in the wall at the side of the arch (technically the ‘abutment’), Fig 4.6 shows the common failure modes in arches. It is probably not worthwhile (except perhaps for the sake of appearance) rebuilding an arch once it has been established that further movement has ceased. Where the arch is still ‘alive’ shoring must be provided to check movement and prevent collapse. Often cracking in arches is associated with cracking in cills at the bottom of the opening, these are essentially non structural members and one need only consider the failure in terms of appearance, deterioration of material and water penetration.
Roof structures can cause cracking in walls by subjecting them to horizontal thrusts which the wall is incapable of resisting. Two examples of this are shown on fig 4.7 and again the structure should be stabilized before any repairs are undertaken on the brickwork.

4.1.5 Movements in Building Frames into which Walls are Built

Generally in this section we are thinking of a more modern building in which the steel or concrete frame (see Chapter 1) is infilled or clad with masonry walling. Typically a wall is infilled into a concrete frame between columns and beam soffit and slab (see fig 4.8). This work is done fairly early in the process of construction and subsequently perhaps floors, partitioning, equipment are all loaded onto the floor causing the beam to bend (as the designer allows for) under load. Where the wall fits tightly under the beam the tendency to bend is going to “pinch” the wall and could eventually lead to wall cracking as shown in fig 4.8. Again as illustrated, foundation movements can lead to a similar pattern of cracking in a framed building.

Fig 4.8 illustrates a similar type of problem which has been particularly acute in multi-storey reinforced concrete structures.

4.1.6 Corrosion of Embedded Steel

Where heavy steel sections are built into brickwork and rusting of the steel occurs expansion will give rise to localised cracking of the brickwork.

Sometimes the horizontal bed joints of brickwork are reinforced with a variety of types of steel wire mesh. These would normally be galvanised and should not corrode in the alkaline environment in the joint. However, severe exposure substandard protection to the steel, with possible soluble salts in the mortar could give rise to generalised corrosion of the reinforcement. At first sight the effects would be similar to sulphate attack, further probing of the joint would be necessary to discover the embedded steel.

In natural stonework (ashlar in particular) wrought iron or steel cramps have been used to join together the pieces of stone. Eventual corrosion of the metal cramps gives rise to a characteristic localised spalling of the stonework.

As a final example we must look at wall tie failure in cavity walls. There are signs that this is becoming fairly common. Characteristically cracking will occur (in brickwork) along every fourth course (with sulphate attack every course would be likely to be attacked). Corrosion of the ties has occurred quite widely in area where black ash has been used in mortar, in areas where exposure is severe and sea borne salts can enter brickwork and again as a contributing factor where substandard will ties have been used.

Metal detectors are useful sometimes in locating embedded steel, in particular a “cover meter” will give a measurement of the depth below the surface.

4.1.7 Sulphate Attack on Mortar

The basic mechanism of sulphate attack on portland cements has already been described (see 2.1.2c). Recall that failure occurs because the cement in the mortar swells and what you will see is a generalised cracking of the mortar joints. The mortar itself will eventually disintegrate. This form of attack is most likely to occur where brickwork is wet for long periods of time and is most commonly seen in boundary walls, parapets and chimneys. In chimneys particularly tall ones the greater wetness of the side facing the prevailing wind leads to sulphate attack progressing more quickly on that face than on the drier more sheltered face with the result that the chimney eventually takes a distinct curved lean (see fig 4.9).

Repointing by itself is unlikely to cure the problem even when sulphate resisting cement is used, in the mortar. The sulphates come from the bricks (and in a chimney from the flue gases) and these probably should be replaced. Many chimneys nowadays no longer serve the fireplace or appliance that they were originally built for and it is better to take the stack down to roof level and seal off the roof.

4.2.1 Disintegration of Wall Materials (Brick)

This can of course often occur in walls that are otherwise perfectly sound although many older and neglected buildings will show signs of structural failure and failure of materials as well.

The single most common cause of disintegration of bricks or mortar is frost action which has already been described. Resistance to frost attack although generally related to strength in bricks is not invariably so. There are some very hard bricks of high strength whose pore structure is such as to make them vulnerable to freezing. Correspondingly many weak and porous bricks are frost resistant partly because there is plenty void space inside them to give room for absorbed water to expand into on freezing.

A further cause of disintegration sometimes in combination with frost action is disintegration caused by crystallisation of salts immediately behind the face of the brick. The formation of the crystal “blows off” the face of the brick and with progressive action the face becomes more and more eroded. Sometimes this is called cryptoflorescence (“hidden flowering”) to distinguish it from the normally harmless efflorescence. Cryptoflorescence can sometimes be caused by the application of a water repellent treatment to the surface (particularly with some natural stones). (See also next chapter section on ‘Water Repellent Treatments for Walls’).

4.2.2 Deterioration of Natural Stone

In general natural stone is subject to far more ills than brick. Since the stone has been made by natural forces it is often subject to variation in its “raw materials” and the way they are put together. Thus in a wall, nominally built with the same stone, weathering over the years will show different effects on individual stones within the wall, some crumbling away and perhaps split by frost whilst some still retain the original face. Even weathering in differing parts of the stone.

The subject of deterioration of stone is an extensive and complex one. In cases where generalised decay of stonework has occurred specialist advice should be sought from and architect/surveyor or masonry contractor. For the repointing specialist an outline of possible causes of defects will enable you to appreciate any specialist advice or recommendations that you may be given.

Firstly, there are natural defects in the stone itself, particularly, with certain sedimentary rocks, the occurrence of soft layers within the stone which weather more rapidly than the remainder.

Secondly there are defects in a sense attributable to incorrect workmanship when the wall was originally built. Face bedding of the stone often leads to early deterioration of stone by frost attack. With sedimentary stones there is a ‘natural bed’ for the stone (the layers in which it was originally built up millions of years ago), often these can be seen when looking at a freshly broken face of the stone. Stone should be laid so that the direction of loading is at right angles to the natural bed of the stone, that is in a wall, with the natural bed horizontal (in an arch the natural bed should be radial to the arch). This enables the stone to resist the stresses due to loading most effectively. When a stone is “face bedded” the natural bed is set parallel to the face of the wall (that is vertically), allowing in some stones, rain water to enter between the laminations of the stone. When frost occurs laminations are pushed outward and the stone starts to break up. In patching stonework it is sometimes a temptation to face bed the new stone to minimise the amount of cutting back to fit the stone in (or trimming of the stone to fit the recess (or, as is usually the case, both)).

The use of iron cramps or dowels particularly in ashlar work has already been mentioned above.

Dense pointing with cement rich mortar is again a frequent source of deterioration of stone. We have already seen the wide variety of mortar mixes available and the way in which a mix should be chosen to suit the particular stone (or brick) (see chapter 3 above).

Probably the biggest single cause of deterioration has been atmospheric pollution particularly from inefficient burning of coal. This puts sulphur dioxide into the air which then combines with rain water to form a mildly acidic rain which then reacts with limestone in particular to form a soft encrustation of gypsum.

In general with repairs to stonework, ashlar work is probably best carried out by a masonry contractor other than operations like the rebedding of a coping. With repairs to rubble stone work small patches should not cause you any difficulties provided a suitable stone can be obtained.

4.2.3 The Birds and the Bees

From time to time in older buildings you will come across walls which have been taken over by animals or plants. The greatest menace under this heading is ivy, which if left alone, can literally pull a wall to pieces. It should always be removed, ideally by severing the main trunk or trunks a season before any work is to be carried out so that the plant dies back and its branches become brittle and easy to remove. Ivy causes the damage it does by sending shooting roots into the joints of the wall where, particularly in older mud mortared walls, they become established and thicken as the plant grows. The thickening roots pushes stones apart eventually loosening them. A number of wall climbing plants secure themselves against the wall by putting out various forms of sucker which stick to the face of the stone or brick, in this case little damage is done to the wall and for part of the year the leaf cover, besides perhaps looking picturesque, will act as an outer rain shield to the wall. Virginia Creeper and various vines are of this type and in general it is not essential to cut them back (apart from the fact that a creeper covered wall is a little difficult to repoint).

Certain species of bees known generally as ‘masonry bees’ or ‘mortar bees’ will sometimes be found in the (soft) joints of the walls (normally they live in earth banks and soft exposed rocks). These bees are solitary and do not form colonies like the familiar honey bee although a group of solitary bees may take over a particular wall. The damage is caused by the female bee who bores into the soft mortar excavating a system of galleries and chambers in which she lays eggs from which the next generation emerge eventually. The eggs are laid in the early spring and hatch by early summer so that any eradication treatment is best carried out in August/September (before the winter frosts come). The treatment recommended is to take out and repoint. BRE suggest mortar designation (iv) (see table 3.1) or (iii) of the work is carried out later in the year when frosts are likely, it should be noted that mortar designation (iii) is really too strong for some natural stones and soft bricks and ideally this work should be programmed for a frost free period. If it is necessary to do the remedial work while the bees are still using the wall an insecticide can be injected into the galleries and on to the surface of the wall when the repointing has been completed to discourage the bees from burrowing into the new, soft mortar. The Pest Control Officer at the local council will usually be able to advise on suitable insecticides.

Birds again will sometimes nest in the thickness of a wall (starlings can be troublesome), repointing in a harder mortar will usually deter the birds. Whilst on repointing it is probably always advisable to check the eaves soffit for any holes where birds may get into the roof space and start to nest.

Birds roosting on a building can also cause nuisance and damage (from their droppings) – pigeons in cities are probably the most common. Two methods seem reasonably effective in deterring the birds – jelly fixed to the top of projecting horizontal ledges and a stainless steel wire tensioned and fixed perhaps 5mm above the ledge.

4.3 Basic Repairs

The need to put right the basic defect has been emphasised in the last section, this should be done before repointing and surface repair is attempted. The biggest problem is that of deciding whether cracking is caused by some kind of structural failure that has ceased or whether the problem continues and the failure becomes progressively worse. Assuming that any major repairs have been done, we have to decide what is to be done about the repairs.

Where progressive movement in a wall is suspected, “tell tales” may be specified in order to monitor the wall and check whether movement is taking place. A common, but in someways not very helpful tell tale, is a strip of glass cemented across a joint, if there is continuing movement then eventually the glass strip breaks. However, little information is available about the amount or nature of the movements. As an alternative tell tale, stainless steel studs with a groove cut into them can be embedded into or mortared onto the surface of a wall. Accurate measurements with a vernier gauge can then be made between the grooves at fixed time intervals thus enabling the rate of movement of the crack to be assessed. Other more specialised techniques such a electrical strain gauges may also be used.

4.3.1 Repairs to Cracking in Brickwork

Cracking may be confirmed to joints in which case if the wall is to be repointed, no more need be said, chapter 3 has already described repointing techniques. Where the bricks themselves have cracked there are three possibilities – (1) leave well alone, (2) cut out the crack and point it, or (3) cut out the brick and replace it.

Fine cracks (less than 1.5mm wide) through absorbent bricks are unlikely to give rise to water penetration problems. If these are not too noticeable, particularly where they are high up in the elevation, then there is little sense in doing anything to the cracks. If the cracking is conspicuous then your client may wish to tidy up the appearance of the building. In this case cutting out of the cracked bricks and the bedding in of new ones will be satisfactory provided a reasonable match in colour, texture and size can be found. If this is difficult then the patching may well be more conspicuous than the original cracking.

Fine cracks in dense, non-absorbent bricks may lead to water penetration into the wall. In an inconspicuous position it will probably be economical to cut out the crack in the brick and point this up. In a conspicuous position it would be advisable to replace the brick.

Cracks wider than 1.5mm up to say 25mm should be cleaned out and repointed in the normal way. When the joint starts getting more than perhaps 15mm wide shrinkage of the mortar away from the brick can be a problem and checks should be made after the joint has been struck so that any cracks can be sealed by retooling the joint. On very wide joints shrinkage problems can be reduced by setting into the joint a slip of suitably coloured roofing tile and pointing around this.

Often the structural movements that caused the cracking will also have displaced the wall horizontally so that across a crack the face of brickwork steps in or out. On vertical joints the pointing is best kept flush with the innermost brick face and no attempt made to form a fillet. However on horizontal joints where the displacement allow water perhaps to lodge on a horizontal surface, an attempt should be made to weather the exposed surface with a cement fillet (see fig 4.10).

4.3.2 Replacement of Decayed or Cracked Bricks

The treatment of stonework is covered separately in the next section (4.3.3).

The first problem to overcome here is that of finding matching bricks and again one should think about the possibility of the patched work looking worse than the previously decayed brickwork. Mortar patching particularly on elevations which are not readily seen may sometimes give an acceptable finish.

In patching, break away all loose material and build up the necessary thickness in layers. It is sometimes difficult to build up to thicknesses greater than say 25mm in a single application, in this case the first layer should be scratched criss-cross in order to help with the bonding to it of subsequent layers. The patch can be made less conspicuous by mixing into the mortar brick dust obtained by crushing some of the bricks used in the wall or by mixing in a pigment. The final surface should be finished with a piece of wood (or wooden float if the area is large enough) to give a slightly “grainy” texture. Hard trowelling with a steel trowel tends to give a shiny surface and may bring the cement to the surface thus giving a hard crust over a softer interior which could break down prematurely.

Where you have agreed to replace decayed bricks there should be little problem in cutting out soft decayed bricks to give a clean, regular cavity into which the new brick will fit. However, where cracked bricks are to be cut out (see also previous section) they will almost certainly be hard and dense and set in a hard mortar. Care is needed in chopping out to avoid damaging surrounding work or internal plastering, ornaments on walls, etc., often an electrical drill with masonry bit is best used before starting with club hammer and bolster.

As an alternative brick slips may be used to “face up” decayed brickwork or to limit the depth to be cut out where dense bricks have cracked. Manufactured slips may be available and are likely to be at least 25mm thick, requiring cutting back say 40mm from the face to ensure that they are properly bedded in. It may be necessary to cut your own slips from whole bricks in which case with care the thickness can be reduced perhaps to 15mm. With slips it is important to make sure than the whole of the cavity behind the slips is filled with mortar. A partly filled cavity could collect water, which in turn could freeze and detach the slips from the wall.

 

(click images below to view full-size in a new tab/window)

Figure 4.1            Figure 4.2                   Figure 4.3                   Figure 4.4                     Figure 4.5

       
Figure 4.6           Figure 4.7           Figure 4.8                     Figure 4.9

 

The above information should not be taken as recommendations for any individual contract/project and are guidelines only. Consult your local licencee for advice on the projects in your area.

 

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