Sephaku Cement explains why incorporating concrete control joints in the placement of concrete can assist in preventing cracks, fissures and unsightly flaws in concrete applications.

While durable, steadfast and one of the strongest and most commonly used materials in construction today, concrete and the placement of Portland cement concrete requires intricate attention to detail, precision and planning. From the overall quality to additives and other ingredients blended with Ordinary Portland Cement (OPC), to the water mix ratio, curing and curing conditions as well as concrete compaction, there are various factors that will influence the conclusive strength and lasting durability of cured concrete, with that said however even when prepared and poured to perfection, concrete installations are prone to crack and weather over time and especially under extreme conditions.

A sure way to prevent concrete installations from cracking is by placing concrete in conjunction with crack control joints. These are especially effective in applications including concrete slabs, flooring and large concrete flatwork which helps prevent these large-scale flat surfaces from cracking.

Why Does Concrete Crack?

Concrete cracks happens mainly due to a few conditions: 

  • Dry windy conditions when casting floors or any concrete with a large open surface. The water evaporates faster than bleeding water comes to the surface. This can be prevented by shading your cast, timing your casting not to happen on the warmest time of the day and by proper mix designing limiting the amount of mixing water needed to place and finish your concrete.
  • Secondly the concrete are prone to cracking if the concrete is not consolidated/compacted properly and you have plastic settlement cracks where the concrete settle and is not able to settle where reinforcing steel is installed or where large aggregates are used which prevents settlement of the concrete directly above the aggregate/steel the result is that concrete settle adjacent and break its back over the aggregate or steel
  • When too much water is used for mixing and placing, when the water evaporates and leaves pores where the water was and then the concrete is prone to shrinkage cracks as it wants to fill the “empty spots”.

How Do Concrete Control Joints Work

Concrete control joins allows concrete to shrink during curing, as well as expand and contract when exposed to moisture/heat/cold. Although in the presence of installed control joints this is not to say that the concrete will not crack, However instead of the concrete cracking causing unsightly blemishes at random intervals, the control joins steers the crack to take place at predetermined areas. This results in the crack occurring beneath the surface of the concrete slab or floor and thus maintaining the consistent appearance and an unblemished concrete surface.

With three different types of control joints available, these include:

  • Isolation joints
  • Contraction joints
  • Construction joints
  • Isolation Joints
    Also known as expansion joints, isolation joints offer a solution to tensile stresses that cause cracks in concrete by allowing the concrete to move as it expands and contracts, installing expansion joints allows for movement between adjoining structures either horizontally or vertically by separating or isolating the concrete from other parts of the construct.
  • Contraction Joints
    Installed with the intention to create areas of “weakness” beneath the surface of the concrete (as explained above) concrete contraction joints create predetermined cracks or fissures in the concrete installation by saw cutting a joint in the concrete. In doing so this prevents random cracking by channelling the expansion and contraction to take place in the pre-planned “crack”. 
  • Construction joints
    Effectively used when flooring or when the placement of concrete slabs take place at different intervals. Construction joints are installed at pre-planned locations in-between phases of pouring cement slabs, allowing each slab to shrink while curing at different intervals minimises the likelihood of random cracking taking place.