Published on 8/9/2017
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Beware the cold joint
Cold joints in concrete are your worst nightmare. But how do they occur and how do we prevent them from happening?
Cover source photo by : Alex Klopcic on Unsplash
Cold joints in concrete are your worst nightmare. But how do they occur and how do we prevent them?
Picture yourself trying to reach out and connect with someone, but being so cold you are only able to connect and hug yourself.....
Photo source in order top left to bottom :
Eddie Kopp, Rémi Walle and Dmitry Ratushny All on Unsplash
In the concrete jungle every day around the world if you close your eyes you can hear a concreter screaming down the phone at a concrete dispatcher, "I'm going to get a cold joint in my concrete buddy!!!!" Only not so politely, usually there's a few swear words thrown in.
The problem is that when we place portland cement with water it reacts, in a two stage process known as hydration. When water is added the cement particles dissolve and then enter a process known as precipitation wherein the crosslinking of calcium oxides and silicates within the cement mix occur. Portland cement is a hydraulic cement, hence it derives its strength from chemical reactions between the cement and water. Concrete is completely fluid before the cement sets, then progressively hardens.
But what if your first batch of cement commences the hydration process, and begins to harden before your second batch arrives from the concrete plant?
Beware the dreaded cold joint in concrete.
What are cold joints?
Cold joints are formed primarily between two batches of concrete where the delivery and placement of the second batch has been delayed and the initial placed and compacted concrete has started to set. The full knitting together of the two batches of concrete under vibration to form a homogeneous mass is therefore not possible, unlike the compaction of two fresh workable batches of concrete. This could be a potential plane of weakness, according to the Concrete Society, a member based body in the UK.
The Concrete Society informs us that cold joints, unlike cracks that form in hardened concrete through tensile restraint, are not gaps in the concrete but merely seams containing no appreciable void structure. They are usually linear, closely joined and bonded. However, there is a danger of small voids in areas where the concrete is not fully compacted, as with any concrete pour.
The Concrete Society in the UK concludes that generally cold joints are not a problem structurally if the joint is in compression. However, the location of the joint within the structure, the structural function of the element and aesthetics need to be considered when assessing a cold joint.
How can we prevent cold joints occurring?
I guess cold joints are often beyond our control. A truck from the plant where the concrete is being delivered breaks down, is lost, or delayed due to an unforseen event. Or a concreter doesn't calculate the distance to site and the estimated delivery time properly. It happens. He or she then spends an hour trying to keep the first batch alive by vibrating and working the concrete with trowels, concrete tools and water, before alas! It's too late. The concrete has already set and hardened before the next truck arrives. Hot weather, wind, a lack of retarder in the cement mix and perhaps a lack of personeel on site, may have all contributed to the concreters woes.
In terms of preventing cold joints it is really all in the planning of your pour. Know the distance between you and the plant. Consider the weather conditions. Have enough personeel on site to handle the pour. Use some retarder in the mix if you need to delay the setting time of your concrete. And have a back up plan. Make sure there is another concrete plant nearby who your friendly dispatcher can call on if they have a breakdown or malfunction at the plant.
What can we do in situations where a cold joint is imminent?
If all else fails and you know you are going to be unable to knit your last load of concrete together with your next, and a cold joint is imminent, you need to place a wooden board against your last load of concrete. "Put a board in it", is the cry of the frustrated concreter. For then a joint can be inserted into that section of the slab at a later stage and allow the next section of concrete to be poured free of cracks and weakness.
Related Articles :
TEN (THATS 10) MINUTE CRACK REPAIR. YOUR'E KIDDING AREN'T YOU? : https://www.concretebroker.com.au/Blog/View/51
What makes a good concreter? : https://www.concretebroker.com.au/Blog/View/157
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Tags: cold joint concrete batcher concrete cracking concrete plants concreter hot weather hydration vibration
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