Here’s a brief description of the different kinds of cracking that can occur in concrete:
Plastic shrinkage commences almost immediately, and although not necessarily seen by the naked eye small cracks are forming within the body of the concrete as it transitions from its plastic state to a solid state. In its basic form concrete is delivered to site as a mixture of water, cement and aggregate, even at this stage a chemical reaction is taking place and the concrete is already hardening, even if you place the entire mixture under water – it will harden due to the chemical reaction. Once poured and set in place the chemical reaction continues and the mixture hardens or in layman’s terms, it dries and as it does it shrinks. Basically as the water in the mixture evaporates and the chemical hardening process continues shrinkage is inevitable, resulting in cracks in its hardened state. Cracks from plastic shrinkage can be unsightly but rarely impair the durability of what is (unless it is polished or honed as a final finish) just a wearing surface, when used as a slab.
There are several methods used to soften the impact of concrete cracking. The most common is to use steel reinforcement, lets say that a concrete path poured on the ground and is ten metres long, this has the potential to shrink 1 mm for every metre. Not much you may say, but this equates to the possibility of a 10 mm wide crack appearing randomly somewhere in its length. We now add steel mesh which restrains the concrete to a degree as it cures, resulting in more cracks but smaller in width say 10 cracks at 1 mm, which is much more acceptable and pleasing to the eye. We take it a step further and it is not uncommon we see articulation joints in concrete paths and slabs, these act much the same as perforations in toilet paper roll, the individual sheets are easily torn apart at the perforations, with the intention that the concrete will crack at the purposely placed articulations joints. However, there are no guarantees that it will crack at the articulation joint, the same as the toilet paper can rip anywhere. This is accepted by the majority of people but if a crack appears on a house slab, new homeowners get a little nervous.
Concrete is affected by weather, temperature, changes in environmental conditions and contact with adjacent materials and or structures and of course time. Like any other material, concrete will subtly change in shape and dimensions. Concrete will not only continue to shrink for decades but is also subject to expansion and contraction in line with its environment, this can change from one minute to another or from morning to night or from wet to dry or cold and hot. To add to the issues facing builders, clay bricks can and do expand dramatically as time goes on, unlike concrete they come from a hot dry furnace and slowly over time absorb moisture, so they grow while concrete shrinks, any interface between the two – like between brick walls and a concrete slab compounds the possibility of cracking issues. Hot weather causes concrete to expand, cold weather causes concrete to shrink, pushing against adjacent objects like brick walls and timber roof members this can and does cause cracking. These are not structural and are call differential movement cracks or settling cracks by builders. They are for the most part out of the control of builders, it is the laws of physics at work. Although these cracks are often called settling cracks by builders and are often best attended to after a normalization period. Builders will usually attend to these at the six monthly maintenance period on new builds but will advise the client that these are not bad workmanship issues and will fall under the umbrella of regular building maintenance, done by the client for the life of the building.
Heaving occurs from forced upward movement such as the growth of tree roots. Hydration cycles can also push the ground upwards on the concrete and cause it to crack. Settling occurs from shifting soil due to excessive water or from decomposing tree roots nearby. Both are likely to occur over time and are difficult to prevent. But home owners are advised not to plant trees or any plant with aggressive root systems or root systems which can take large volumes of moisture from the ground. Paving and hard stands are to have falls away from the building and stormwater should be directed away from footings and foundations. The trick is to maintain as much as possible a stable environment year round.
Crazing cracks are surface cracks that look almost like shattered glass and occur when the concrete loses moisture too quickly. Crazing or spider web surface cracks are entirely cosmetic, and do not need repair on a standard slab (not so with polished or honed slabs).
Concrete is a strong and durable material but it also has its limits. Overloading can also crack or even break the slab underneath. Cracked driveways are perhaps the most common example. The next step is to evaluate any cracks you find in your property.
Some types of cracks in concrete are primarily aesthetic but others can pose a legitimate safety concern.
Our next blog will deal with cracks which are of concern, they are cracks which appear due to insufficient companion, sand jacking, deleterious materials or clay wash outs and loss of compaction due to neighbouring conditions.
Alex Morrison has worked with a range of businesses giving him an in depth understanding of many different industries including house cleaning, financial support and personal injury lawyer.