Concrete setting vs. cure time and cracking

[Alan CA]

Hi all,

Is concrete setting time the same as the cure time? Hence can concrete cracking occurs after setting time?
For example, if you pour fast-curing concrete that sets within an hour, can it cure after a day if temperature is so low?

 

[oldestguy]

You are mixing up some terms. Setting to me is the hardening process to the point you can walk on it, etc. Curing is the long term hardening process, probably starting as soon as the mix is made and never ending.

 

[Alan CA]

Thank you oldestguy for your answer. However, the most important part of the question: when do cracks form in concrete? While or up to setting? Or even during cure time up to 28 days or something like this?

 

[JAE]

Cracking can be caused but many different things. Even 30 years after concrete placement.

That said, if you are only referring to shrinkage cracking then it can occur well after 28 days.

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[Alan CA]

Thank you Jay. So, if fast curing concrete (BASF 1060 for example) is to be poured in freezing cold weather, using hot water in the mix or providing one hour warm environment until it sets will not be enough to prevent cracking, correct?
How long do we need to keep repair patch heated to prevent cracking if mix setting time is 1hr?

 

[oldestguy]

Sorry about not covering cracking. I once had a group of UW Wisconsin grad students in a class dealing with highway engineering. I took them out to an interstate concrete paving job using slip forming. Not sure of the exact timing, but maybe 15 minute after placing the concrete cracks were visible in the slab surface spaced 3 to 5 feet. We could mainly see them at the exposed edge of the slab. Later they were visible on the surface. So, shrinkage starts early on in a stiff mix. From this it-would appear that visible cracks may or may not be noticeable depending on many things, such as stiffness of mix.

 

[hokie66]

One thing to remember is that concrete doesn't crack due to shrinkage after it is hard, but rather to restraint of the shrinkage. Cracking before hardening is called plastic shrinkage cracking. Cracking after hardening is due to restraint.

 

[Alan CA]

Much appreciated gentlemen, but didn't answer my main part of question: during what phase will cracking occur? Time-wise, how long after placement may normal and rapid cure concrete crack?

 

[hokie66]

You might have to accept an answer to your question that reads something like "I don't know". Part of your problem is in your definition of concrete. We thought you were talking about concrete, but BASF 1060 is a specialized repair mortar. For technical questions about that material, you would be better to ask BASF.

 

[Ron]

You have a combined process that exacerbates cracking. Using hot water in a mix placed in cold weather makes for probable thermal shock with thermal shrinkage. Couple that with drying shrinkage and you have high crack potential.

When can cracks occur? Any time after placement, depending on conditions. As hokie66 noted, there are two significant types of shrinkage cracking. The first and earliest occurring is plastic shrinkage cracking. This occurs while the concrete is still predominately in a plastic state and is caused by the surface of the concrete drying before the subsurface sets up. The most common cause of this issue is placing the concrete in windy weather. These cracks are usually short, discontinuous cracks that, because of the reduction in cross section they cause, then become connected as typical full section drying shrinkage cracks.

Further as hokie66 noted, restraint is the cause of drying shrinkage cracks, although there are other influencing factors, particularly when there are significant changes in cross section thickness. Unrestrained concrete doesn't crack...it only shrinks.

There is a difference between concrete setting and concrete curing. Conventional concrete will go through initial set in about 2 to 3 hours, depending on the mix. Curing is the process of preventing fast moisture loss in the concrete and properly done takes at least 7 days and often longer. The key is to keep moisture in the concrete long enough that it gains enough strength to resist the stresses caused by shrinkage restraint.

Most cracks occur very early in the curing process but may not be clearly visible for many days after placement. As the concrete continues to shrink with the hydration process, the cracks will widen and become more visible. The cracks will continue to widen with time. You should see most of the cracks that will occur in the placement within the first month.

 

[birneys]

Alan CA, it might be helpful for you to take a look at ACI 306 which gives criteria and recommendations for cold weather concreting. I don't have it open in front of me, but I'm fairly certain that it will give you an approximate time frame for protecting concrete from cold temperatures. As hokie66 mentioned, since you are using a specialized mortar from BASF, you're best course of action would be to contact BASF directly. There should be a regional representative that can provide guidance for your specific situation. This will also help you maintain any available warranty from the manufacturer for the product you're using.

 

[epoxybot]

While BASF 1060, according to the SDS, has a good deal of Portland Cement, Master Builders does not use the term "Portland Cement" to describe the product. The very short working time of the mix, 15 minutes, would not allow for the use of hot water. Master Builders will have a minimum application temperature for their material and no doubt a fair amount of knowledge about placement conditions of this sort. Being a rapid set material, it will generate a high exothermic heat in a short amount of time. This is cause for concern, AFTER the material has set. The material will contract as it cools. If the substrate is poorly prepared, there could be delamination. If the BASF 1060 bonds well to the substrate, there could be restraint cracking. It all depends on the mass/depth being placed. The BASF 1060 tech data sheet gives instructions for the addition of pea gravel to extend the product and aid in the control of heat generated by the product. The aggregate acts as a heat sink to limit the heat build up. Master Builders recommends the aggregate & bagged material be "conditioned" to 70F/21C. If it is so cold that water almost rapidly freezes when it comes in contact with concrete, the substrate will need to be warmed.

 

[Alan CA]

Ron, Birneys and Epoxybot: I highly appreciate your answers here. Valuable information that covered my questions and what I am looking for. Thank you very much.