How to Prevent Concrete Cracking

It’s the one question everyone wants answered when it comes to concrete – how do we prevent cracking?

We often find our customers want the durability and visual appeal of decorative concrete, but they’re concerned about the risk of cracks in their concrete slab or driveway.

Like all construction projects, concrete relies on strong foundations. So site preparation is key to ensuring a smooth and long-lasting finish, avoiding cracks and the need for concrete resurfacing and repair.

We’ve developed this guide to help our customers and installers get the best possible results from Geostone.

Why does concrete crack?

There are several major reasons and sometimes unique combinations of factors that can contribute to cracks appearing in concrete. Our parent company Holcim Australia explains the cracking phenomenon is complex and can depend on the rate and amount of drying, drying shrinkage, tensile strength, tensile strain, creep, elasticity, degree of restraint, and other factors. Holcim has revealed survey results which found the three most common causes of concrete cracking are:

• Construction and supervision problems – approx 36%
• Design defects – approx 27%
• Ambient conditions during pouring (temperature, humidity, wind) – approx 21%

In addition to this, Holcim advises all cracks can be grouped into two categories. Either cracks that occur before and during hardening, or cracks that occur after hardening of concrete.

“While most types of cracking do not affect structural stability or durability, all cracks are unsightly and in extreme cases cracking can reduce the use and serviceability of the structure. For this reason, cracking should be kept to a minimum.” – Holcim.

Taking proactive steps to avoid cracks in your concrete can help prevent the need for repairs and resurfacing later on.

“Cracks in Concrete: The Cracking Phenomenon”, Holcim, 2017, http://www.holcim.com.au/products-and-services/tools-faqs-and-resources/do-it-yourself-diy/cracks-in-concrete.html

Methods to avoid concrete cracking

Sometimes the underlying causes of concrete cracks are unavoidable, such as extreme weather, or the movement and settlement of earth over time. But measures can be taken at the installation stage to better allow for these possibilities, helping to mitigate the risk of concrete cracking.

The Geostone Best Practice Placement Guide was developed to assist both our customers and concrete service providers with the placement of Geostone’s exposed aggregate concrete and decorative concrete range.

Some of the Guide’s key tips for avoiding cracks include:

• Preparation is Key: Prepare the site where you will lay your Geostone and ensure it is graded and compacted. Be aware that areas which have been excavated alongside buildings, retaining walls and pipework are often poorly compacted. This may result in movement and dampness in and around these areas which can cause cracking and other issues associated with water ingress.
• Call in the Professionals: A compaction certificate from an experienced professional will provide peace of mind for the steps that follow. Unless a water vapour barrier is used, it is also recommended the area is well watered prior to concreting.
• Reinforcement: Steel reinforcement gives concrete additional tensile strength. It helps to control movement and cracking as well as transferring loads from expansion and contraction, and direct loads placed on the concrete. Your installer should use commercial bar chairs which are designed to hold steel reinforcement above the ground when placing steel mesh. Lifting or hooking up the reinforcing mesh is not recommended. In honed or exposed aggregate Geostone projects, we recommend SL62 or higher reinforcing mesh. SL82 or heavier slab mesh should be used in all polished or Geostone internal ground slabs.
• Weather: Avoid pouring Geostone in hot and windy conditions. The concrete should be protected from drying out too quickly, and we recommend installers apply an evaporation retardant to the concrete surface after levelling the surface to keep moisture at optimal levels.
• Product: Geostone is designed to be installed at a slump of 80mm. Adding excess water to the mix should be avoided as it can alter the colour, finish and strength of the concrete. Sprinkling water on the surface during or after installation should also be avoided.
• Control Joints: It is inevitable that concrete will lose moisture over time. Cutting control joints into the concrete during the setting stage allows the concrete to move without cracking from drying out, tension and restraint. See the Best Practice Placement Guide for detailed recommendations on the depth and spacing of control joints for Geostone.

When planning your Geostone concrete project, you should discuss these points with your concrete installer or service provider to help ensure your decorative concrete looks great for years to come.