Wall Crack Repair Services in Gauteng

Why Do Cracks Occur?

There are many reasons why cracks occur and it would take a book to do the subject justice and explain crack repair processes. Following is a list of some of the most common / the ones we come across most in our contracting work:

  1. Foundation not suitable for unstable founding materials e.g. clayey soil and the collapsing soil need special attention / treatment prior to casting a foundation. If this is not done, it can be almost guaranteed that foundation subsidence will occur in time.
  2. Undersize foundations and insufficient curing of the concrete. For concrete to cure properly, it should be allowed to dry out slowly over a period of 28 days. To this end, the concrete should be sprayed with water every day for the first week; every 2/3 days in the second to fourth weeks – then allowed to dry out completely.
  3. Concrete too weak and porous – mostly due to excess mixing water. This will be the cause for rising damp at a later stage with subsequent cracking.
  4. The use of concrete mixes that have no resistance against attack by acids or sulphates.
  5. Brickwork erected without addition of builders lime or the pre-wetting of bricks.
  6. Insufficiently compacted backfill and the absence of soil stabilizers.
  7. Internal walls built on a concrete slab without the support of strip foundations.
  8. No provision made in the building design for the natural expansion and contraction tendencies of building materials.
  9. Applying plaster to brickwork without first wetting bricks.
  10. The ‘tying-in’ of bricks of an extension to the existing structure.

Following is a list of the most common defects and design faults we come across most in our contracting work. We would like to briefly identify unstable founding materials.

Collapsing Soils: Possible Mechanisms of Distress

Collapsing soils are sensitive to water in cress. Assuming the loads placed on the foundations is to be 50Kpa the collapse settlement that could occur on saturation if no special foundation precaution were taken would be fair in excess of accepted deflection ratio. Deflection ratio (differential settlement divided by the wall length) is used to indicate whether the anticipated movement of foundation such as conventional strip footings can be tolerated by the superstructure.

Heaving Clay: Possible Mechanisms of Distress

There are many expansive clay types that swell considerably by the in cress of water and shrink on drying out. This occurs seasonally within the dynamic moisture zone.

Strip footings founded within this dynamic moisture zone and constructed without the necessary precautions will heave unevenly and crack. These cracks will be carried through into the superstructure. Heaving is often so severe that articulation of the superstructure will not be sufficient to avoid structural wall cracks. It is of utmost importance to keep heaving clay moisture stable. This can be achieved by sinking wells that are equally spaced and water is injected into the clayey soil horizon through the perforations in the well skin.

Understrength, Undersized & Insufficiently Cured Concrete Footings Are A Further Cause of Distress

To attain the design or specified concrete strength, curing is absolutely essential. To this end, the fresh concrete is to be covered with hessian sacking and sprayed with water for at least the first 7 days. Thereafter 2/3 day up to four weeks. Alternatively as second choice cover the fresh concrete with plastic sheets to reduce moisture evaporation and moisture movement in the concrete matrix.

Weak Concrete

Weak concrete is most often not due to the under-dosing with cement but due to the overdosing with mixing water. 22ltrs of mixing water is the permissible amount per 50kg of cement. Any surplus of water cannot be chemically bonded and will be expelled leaving behind a very porous structure. The open pores structure will form the passageways for rising damp.

Concrete Mixes

The use of concrete mixes that have no resistance against the attack by acids or sulphates. This in conjunction with overdosing with mixing water will weaken the concrete to a degree that the loadbearing capacity is reduced too far below its design function.

Surface Bed Sagging

Slab supporting hardcore backfill is often insufficiently compacted, causing the surface bed to dish downward. Soil compaction should be carried out in 150mm thick layered increments. Only suitable backfill material should be used and blended together with soil stabilizer.

The slab deflection causes cracks in the concrete and also in the brickwork.

To stop this ongoing slab deflection it is essential to stabilize the slab supporting backfill with soil stabilizers and power grouting the cavity between slab and backfill with self-expanding grout.

Shrinkage Cracks

Any building material has the tendency of expansion and contraction. Cement stock bricks are more prone to expand and contract, especially those that are not fully cured. Also, pure cement mortars are basically in the same category as cement bricks as far as shrinkage is concerned. With the addition of builders lime elasticity is increased and shrinkage is dramatically reduced. However, walls erected without builders lime and the appropriate design to allow for expansion and contraction cracks are bound to appear.

Remedial Measures and Crack Repair

  • Survey of each individual dwelling:
    • The dwelling must be inspected so that crack patterns, crack widths and crack lengths and any other deformations of the external and internal walls could be assessed. In addition, the condition of paving, driveways or other features even swimming pools must be recorded.
    • Subsurface investigations are sometimes in order to determine the founding material load-bearing capacity below the footing levels.
    • DCP tests must also be carried out wherever possible.
    • Auger holes sunk to a maximum depth of 1.5m will reveal the geology below the dwelling.
    • Cross hair gauges must be installed at selected cracks, so that the rate of ongoing movement can be measured.
    • As can be seen from the above, it is of utmost importance to establish the cause of cracking before any attempt is made to rehabilitate the dwelling.
    • To make only cosmetic repairs, disregarding possible future movements is often very costly as the cracks might reappear in a very short period of time
    • Dwellings, where movement has not stopped the most suitable repair mode, is to sink mini-piles down to refusal and repair cracks as per our standard crack repair specifications.
    • Articulation points (Expansion joints) are necessary wherever they were omitted in the original design like house extension junctions or where the superstructure has been significantly deformed.

Crack Repair Specifications

PILING (JACKED PILING) SPECIFICATION:

  1. Work pits (1m x 1m x 1,5m below foundation level) are excavated at the areas indicated on the layout.
  2. In order to arrest further movement, piles are installed at the positions indicated. These piles comprise 127mm diameter steel tubes, which are jacked in against the resistance of the existing foundations.
  3. Jacking continues until sufficient end resistance is mobilized to lift the super-structure.
  4. The tubes are filled with 25Mpa concrete and securely wedged and propped with a 130mm x 75mm x 500mm rectangle on which two concrete-filled tubes are installed on either end of the horizontally placed rectangle, to support the building loads.
  5. This is called the “jack head” and the loads are transferred onto this “head”

UNDERPINNING SPECIFICATION:

  1. Dig openings +/- 700 x 700mm every 1.5 metres, 400mm lower than the foundation bottom edge.
  2. Rake out all the soil beneath the foundation and cast a high-strength Concrete base.
  3. After 4 days curing, position hydraulic jacks between the base and the Existing foundation (2 jacks per base) and lift structure back, as close as possible, to the original level.
  4. Then place a stack of bricks between the jacks and remove the jacks.
  5. Remove soil spallings and cast a continuous concrete skirt beneath the foundation, thus widening the foundation footprint.
  6. The concrete is self-expanding to compensate for natural shrinkage.

UNDERPINNING OF FLOORS LAB (SURFACE BED) SPECIFICATION:

  1. Drill injection holes 24mm vertically through the concrete of the surface bed or alternatively horizontally through the brick plinth.
  2. Inject self-expanding grout into the cavity between concrete and backfill.
  3. Pumping pressure is in the order of 30 Bar. The self-expanding and will exert an upward / downward pressure of approximately 3 metric tons to compact insufficiently compacted backfill.

STRUCTURAL WALL CRACK REPAIR SPECIFICATION:

  1. Mask all repair areas and protect carpets against spillage.
  2. Remove all loose plaster after the joint is opened down to sound base and chamfer the edges on both sides to 45%.
  3. Drill injection holes at 150mm intervals. Soak all repair areas liberally with Water and allow penetrating.
  4. Inject Structure bond into the injection holes and so filling the complete crack void.
  5. Cut back all excess Structure bond, smooth down and allow to set. Brush apply Structure bond over the repair area, then imbed Samforce Re-inforce Prop Gash while still wet. Apply second coat of Structure bond.
  6. Re-plaster the repair area with low shrinkage plaster to match the surrounding plaster.
  7. Apply a permanent elastic vapour seal to all repaired areas.
    PLEASE NOTE:
  8. All aforementioned steps will guarantee secure bonding of the crack repair. However, it will not prevent any further settling or shrinkage in any other area of the structure.

EXPANSION JOINT SPECIFICATIONS:

  1. Open up the joint for a true plumb gap from ceiling to skirting.
  2. Wash down liberally with water and allow penetrating.
  3. Inject permanent elastic joint filler and provide a 35mm deep recess.
  4. Re-plaster the damaged area and cut a vertical V-groove through the plaster.
  5. Fill the V-groove joint with a permanent elastic filler.

BEAM REPAIR SPECIFICATION:

  1. Prop up the sagging beam.
  2. Rout out delaminated mortar.
  3. Pump Structure Bond Cement Bonding Agent into the space between the bricks in lieu of brick mortar.
  4. Insert steel re-enforcing.

PROPS TO REMAIN SUFFICIENT TIME FOR SETTING

  1. Repair internal/external structural wall cracks.
    PLEASE NOTE:
    All aforementioned steps will guarantee secure bonding of the crack repair. However, it will not prevent any further settling or shrinkage in any other area of the structure.

CLAY SOIL STABILIZATION SPECIFICATION:

  1. Drill injection holes 22mm diameter through the brick plinth just below the floor slab.
  2. Inject a watery solution with a PH value of 12.4 – 12.7 to saturation point.
  3. Allow a reaction period of two days before self-expanded grout is injected

AGRICULTURAL DRAIN SPECIFICATION:

  1. Dig a trench adjacent to the brick plinth providing 2% slope.
  2. Lay a 50mm river sand bed and embed agricultural pipe overlaid with a fill of random sized stones and covered with Geo technical cloth overlaid with 19mm stone fill.