Cracks in Concrete

When initially placed, on-site concrete generally contains more water than is required for hydration. When the concrete hardens, and there is a loss of the excess water. This is when shrinkage begins if this shrinkage is retained cracks will form. This cracking is dependent upon the rate of drying the amount of shrinkage the strength and strain, creep elasticity of the concrete if there is any restraint and other more complex factors.

Cutting control joint -and concrete

Cutting control joint and concrete

Typically cracking does not affect the serviceability or longevity of the structure though all cracks are unsightly. In more severe cases, cracking can reduce the serviceability of the structure and should be kept to a minimum.

Shrinkage of concrete, especially from drying, is caused by many things the ratio of water to cement, the actual amount of the concrete, how fine and the mineral composition of the cement, how stiff is the cement, the shape of the poor, the texture, and the type of aggregate used is it fine or course, all affect how and why concrete cracks

When concrete cracks, surveys had revealed that the four leading causes had been problems with construction or supervision of the installation, defects in the design inferior or substandard materials used and lastly the conditions when the concrete was poured, i.e. temperature or humidity.

Cracking can be classified into two categories-1. Cracking that occurs while hardening or before the hardening process has occurred. 2. Cracking that occurs post hardening. How do we recognise various types of cracking? Insufficient expansion joints generally cause shrinkage cracks that may be caused by stress concentration, cracking in feathered sections of the concrete, cracks at doors or window corners these type of cracks in the concrete or insufficient reinforcing.

Other types of cracking occur because of poor drainage of the subgrade this can either be too much or too little moisture all by the quick loss of water from the concrete. Plastics shrinkage is caused by the evaporation of water on the surface of the concrete at a rate higher than can be replaced by the normal transference of moisture to the surface usually caused by high wind, low humidity or higher temperatures or sometimes all 3. This type of cracking can be remedied by surface treatments covering or shading the surface or installing windbreaks et cetera.

Sometimes you can see a pattern that follows the reinforcing in the concrete this is a vertical crack and is caused by the settling of concrete around reinforcing bars especially when the concrete near the surface sets faster than the rest of the concrete and there is settling.

Sometimes cracking can be caused by the overuse of vibrators or small movements by formwork (twisting or swelling of formwork or popping of nails in the form work) in the 1st process of hardening may cause cracks. If the subgrade is unlevel or poorly compacted excessively muddy or not stable for any other reason cracks may extend through the slab before the concrete has acquired relative strength.

Cracks that form after hardening

Generally concrete cannot change it shape without some damage this change in shape may be caused by shrinking because of dryness or because of movement caused by temperature or sometimes may take place because of exposure. In these circumstances, the structure will need to permit movement or cracking may occur because of the extra stresses.

Crazing

Crazing

Crazing in concrete sometimes referred to as shallow map or pattern cracking, do not generally affect the integrity or durability of concrete. They are not pleasant to look at and spoils the finish, and the shrinkage of the top layer causes them they are rarely more than 3 mm deep and are noticeable on trowelled surfaces.

The cause of crazing is normally because of high evaporation rates because of low humidity, high temperature, sunlight or the drying effect of wins on the surface. To whet a mixture where there is excessive concrete and finds at the surface. The use of concrete powder to remove excess water is a common cause. Or a combination of all the above.

Shrinkage cracks occur and can be managed by cutting control joints that manage the direction of the cracking when movement occurs. If cracking results usually it will occur along the control joint. These control joints are usually at logical points of stress; sometimes, these joints I have a pre-moulded material inserted.

In short too much water at the installation stage of the concrete is as dangerous as too little water during the curing stage.

With concrete floors, regardless of how well they are designed or how well they are installed, it would be unrealistic to have the expectation of no cracking. There will always be some amount of cracking and curling on every concrete job and this does not necessarily reflect on the quality of the floor design or the quality of its construction.

6 Common Varieties of Cracks in Concrete

1. When the concrete has not hardened and is full of water.

As this water dissipates, it may leave significant gaps between solid particles in these areas the cement is not as strong and may crack. These are commonly known as plastic shrinkage. This type of crack generally occurs around pipes or corners.

2.Expansion crack

Expansion Joint in Concrete

Expansion Joint in Concrete

Heat will cause concrete to expand, and when it hits another slab or a solid wall, the expansion may cause concrete to crack. This is a logical point to use an expansion joint or isolation joint.

3.Heaving

The ground movement caused by excesses in moisture or dryness causes the soil to move many centimetres, and if the slab is not available to move with the ground, it will crack. Sometimes in the colder areas where the ground freezes under slabs and then thaws, this will create excessive movement.

Tree roots will have a similar effect on the slab. This is very visible when a large tree grows under concrete.

4.Settling cracks.

This occurs where a tree is removed nearby, and the route started to compose, leaving a void underneath the slab. Or more commonly where the ground underneath slabs has been contacted properly, for example with plumbing electricity.

5. Overloading the slab

This is simply placing loads on concrete slabs in excess of their strength. You can see this occurring where large motor vehicles or trucks are parked in driveways.

6. Cracks caused by premature drying

The cause of crazing is normally because of high evaporation rates because of low humidity, high temperature, sunlight or the drying effect of wins on the surface. To whet a mixture where there is excessive concrete and finds at the surface. The use of concrete powder to remove excess water is a common cause. Or a combination of all the above.

When a building inspector reports that the piers are cracked, and you should get an engineer to have a look. What the building inspector is normally talking about is the rust or corrosion in the reinforcing in the piers. This expansion because of the rust in the reinforcing, causes the concrete to crack or explode and will only accelerate with time.

With the older style concrete piers, it is relatively easy to replace the peer with another concrete pier or even a galvanised steel peer. But before that is done, we would strongly recommend that you investigate the cause of the cracking, i.e. where did the moisture come from was from a leaky tap overflow from drain wastewater or a condensate drain from an air conditioner.

Some common concrete terms:

  • Cast in place – this is formed, poured and cured in a permanent position.
  • Cured concrete – concrete that has reached its maximum strength
  • Green concrete – this is concrete that has not hardened or cured it remains hydrated and in the early stages of curing
  • Lightweight concrete – a lightweight concrete mix that is normally used on roof or upper floors.
  • Monolithic concrete –  this is a concrete structure that is of one poor including the footings and the slab concrete.
  • Precast concrete – this is concrete that is poured and cured in another place. These are normally beams or columns exterior walls sometimes lintels.
  • Prestressed concrete –  this is where concrete is poured over tightly drawn steel cables or rods.
  • Reinforced concrete –  this is concrete where wire cables fibre mesh wire mesh or steel rods have been added to allow concrete to resist cracking or movement
  • Admixtures –  accelerators shorten the curing time fly ash a substitute for cement which reduces shrinkage and slows setting. Retarder slows the rate of hardening
  • Aggregate – this may be sand gravel it increases the bulk and can also increase the compression strength of the concrete.
  • Cold joint –  a stop in a poor where the concrete is cured basically a non-continuous poor.
  • Compaction – this refers to the density of the surface on which a concrete slab is poured this process ensures a suitable surface for pouring concrete and prevents slab movement and/or cracking.
  • Expansion joint – the separation between two slabs to allow for movement in the slab by either expansion or contraction due to variations in temperature these joints are normally filled with a synthetic product.