Last edited 17 Aug 2016

Cracking and building movement

Cracking.jpg

Contents

[edit] Introduction

Buildings and other built structures are moving all the time, but usually these movements are so small magnitude as to be unnoticeable. Movement can be caused by defects, movement of the ground, foundation failure, decay of the building fabric and so on.

If a structure is unable to accommodate this movement, cracking is likely to occur. The appearance of distortions and cracks can be visually unattractive and disconcerting for occupants, and are more of a cause for concern as if left untreated they can affect the integrity, safety and stability of the structure.

Effective treatment requires that the causes of cracking are first understood. Only then can a strategy for repair be implemented.

[edit] Causes of cracks

Several of the most common causes of cracking are:

[edit] Size of cracks

In order to help building surveyors identify when ground movement has occurred and intervention is necessary, BRE assessed a number of properties as part of a study that was published as BRE Digest 251 Assessment of damage in low-rise buildings. This identified six categories of cracks together with the typical damage caused and the remedy required:

[edit] 0 - Hairline cracks

Less than 0.1 mm in width. No repair action required.

[edit] 1 - Fine cracks

Up to 1 mm in width. Generally restricted to internal wall finishes. Easily treated using normal decoration.

[edit] 2 - Cracks easily filled

Up to 5 mm in width. Not necessarily visible externally, but doors and windows may require adjusting to prevent sticking. Can be masked by suitable linings.

[edit] 3 - Cracks that require opening up

Widths of 5-15 mm. Weather-tightness and service pipes may be affected. External brickwork may need repointing or, in some cases, to be replaced.

[edit] 4 - Extensive damage

Widths of 15-25 mm. Windows and door frames become distorted, walls lean or bulge noticeably. Requires breaking-out and replacement of wall sections.

[edit] 5 – Structural damage

Widths greater than 25 mm. Beams lose their bearing, walls require shoring, and the structure is generally unstable. May require major repair works.

[edit] Shapes of cracks

  • Hairline cracks are common in many buildings, particularly in plaster, which is prone to shrink and is very sensitive to movement.
  • Stepped cracks tend to follow the lines of horizontal and vertical joints in buildings, such as beds of mortar between bricks or blocks and may indicate structural movement.
  • Vertical cracks may indicate that structural components such as bricks or blocks have failed and so can be a sign of significant stresses within the building structure.
  • Cracks that are wider at the top or at the bottom may indicate that there has been foundation movement, with the direction of the widening giving an indication of the likely direction of the movement.
  • Horizontal cracks may indicate that an element such as a wall is failing and this may present a safety concern.

Parapet wall cracking.jpg

[edit] Dealing with cracks

Designers of new buildings may try to ‘design out’ many of the causes that may lead to cracking by careful design of the foundations and superstructure, introducing movement joints, specifying materials that can tolerate movement, such as lime putty and hydraulic lime and so on.

Existing buildings should be examined for the likely cause of movement, and whether this is due to an external factor (such as subsidence) or is caused by the fabric of the building itself (such as a poor grade of cement). It is important to establish is whether the cracking is likely to be progressive and to deteriorate further, and if so, to what extent and over what period of time. Research of archival information, such as previous surveys, can help with this assessment.

Repair strategies can range from major interventions to limited cosmetic works. However, unless the underlying cause has been dealt with, it is likely that further maintenance work will be required in the future. Movement caused for example by vibration or by thermal expansion and contraction is likely to persist. In this case, repair materials must be chosen carefully; those with the ability to deform in a plastic manner will be more successful than those that simply increase strain around the crack and so are likely to cause the repair to fail.

For example, hydraulic or fat limes may provide enough plasticity to accommodate strain. Corrosion-resistant stainless steel reinforcements can be inserted into bed joints which act to reduce the risk of failure by redistributing strain and stress over a wider area.

Instruments such as boroscopes can be used to locate voids that may have opened up as a result of movement. Grouting voids to fill them with cementitious material is often the best way of dealing with this.

Where underpinning of foundations is required, a number of alternative techniques are available. See underpinning for more information.

Rather than simply covering up problems, it is important to understand the reason that cracks have occurred, particularly if they are large, or if they are increasing in size. In this case it is wise to seek professional advice.

[edit] Find out more

Related articles on Designing Buildings Wiki:

[edit] External references