Precast early cracking: Why does this keep happening?

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25 February 2020

Early cracking in concrete can be defined as developing with in the first 7 days after casting or placement. Itís a common problem, so much so that it may not be given much attention in the field. It is certainly the focus of much industry discussion, however, and if youíre actively involved in mix design or project coordination, weíre not telling you much you didnít already know.

In the context of precast concrete, it might be logical to believe that the superior control of the conditions under which the concrete is poured, result in consistently superior results and reduced cracking. Unfortunately, in the real world, this isnít necessarily the case.

CAUSES

There are multiple possible causes, resulting from a combination of the mix composition, the exposure environment, the hydration rate, and other curing conditions. However, the two most common generic causes are (1) insufficient cover (settlement cracking) or (2) inefficient hydration (shrinkage cracking).

INSUFFICIENT COVER

Insufficient cover refers to the reinforcing steel being too close to the surface of the concrete. As the concrete cures and settles, cracks open over the steel. This is called settlement cracking.

Itís also important to understand that even if settlement cracking doesnít develop, insufficient cover means that the steel has less protecting concrete between it and the elements, and the risk of corrosion over time is greatly increased.

INEFFICIENT HYDRATION

The most common cause of inefficient hydration is when moisture from the outer surface of the concrete evaporates too quickly, causing the outer layer of concrete to dry prematurely, resulting in shrinkage cracking.

In the context of precast elements, another potential cause of inefficient hydration arises when steam curing is used. Elements which are over-heated during the curing process may not cure thoroughly within the matrix of the concrete, leading to the risk of delayed ettringite formation (DEF) as the concrete ages.

LONG-TERM RESULTS

Early cracking contributes to an increased likelihood of reduced service life due to premature deterioration.

The reason for this it that reactive contaminants are carried by water, including moisture vapour. Their reactivity also depends on moisture. Chloride, potassium, and other reactive ions move into and around the porosity of the concrete Ė and cracks are an open gateway for their entry.

What happens then? Either the concrete itself may react (e.g. ASR, DEF or carbonation) or the reinforcing steel may corrode. Very likely more than one reaction will occur simultaneously. These reactions are not immediate but occur over time, and have expansive results within the concrete, causing further and more severe cracking.

You can see where this is going. Little cracks cause big cracks, and big cracks end up making the concrete fall apart. Yes, of course thatís putting it simplistically Ė but the deterioration cycle is very real.

WHAT HAPPENS IN THE REAL WORLD?

As noted, there is already much industry discussion on the underlying causes of cracking. Mix designs and casting techniques are clearly defined.

What happens in the real world? Local materials, ambient weather, or substandard casting practices can conspire to nullify the best intentions of the design engineers.

It would be excellent if each precast yard identified its risk factors with a view to eliminating potential defects. However contributing factors are not always clearly identifiable; and furthermore, microcracking may often be overlooked or ignored.

SOLUTIONS PLEASE?

There is a solution available which can be used pro-actively to arrest the deterioration cycle from day one: concrete hydrogel treatment.

Available both as admixture, or spray-applied variants, hydrogel treatments are effective for a very wide spectrum of cracking issues. By immobilising moisture Ė preventing its entrance or movement within the concrete Ė contaminants are prevented from entering, moving, or continuing to react. This breaks the deterioration cycle. By forming a C-S-H non-expansive hydrogel, existing cracks and porosities can be closed.

 

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Source:  Markham Global - www.markhamglobal.com

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