How to Prevent Concrete Cracks

We have all experienced it, the formwork comes off and there we have it, cracks on the brand-new pour. It no longer has to be that way and there are steps we can take as engineers to prevent concrete cracks from forming. With careful detailing of the concrete mix and in some cases implementing modern technology, we can have that crack free dam, powerhouse or garage slab in our homes.  

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“crack free dam, powerhouse or garage slab in our homes”

First, let’s take a step back and understand how a crack is formed. Cracks occur when the concrete’s tensile stress resistance is exceeded, causing a discontinuity in the mass, i.e. a crack. Along the length of the crack and through the full depth of the crack, the concrete member is no longer monolithic and allows the opportunity for a more rapid water infiltration, which under the right circumstances can cause deterioration of the member. Now don’t worry, most cracks do not impact the structural integrity of the member, but in many cases on elements exposed to the environment they do impact the serviceability and longevity. Each case is unique and is important to have the deficiencies reviewed by a qualified engineer.

“rapid water loss through evaporation, then we get what is called shrinkage cracks”

So, what can cause the concrete to crack? There are number of reasons, from differential settlement of a structure, loading stresses, thermal cycling and the most frequent; shrinkage cracks. Mass concrete pours are notoriously prone to shrinkage cracks. Shrinkage cracks are a result of a reduction in mass during the curing process of concrete. As the concrete hydrates (cures), water reacts with cement to form a silica gel which is credited for giving concrete its strength. The more gel is formed, the stronger the concrete. The reaction of cement with water also produces large amounts of heat. The generated heat is counteractive to the curing process because it rapidly evaporates the water content which is needed to produce more silica gel and make the concrete stronger.

“prevent concrete shrinkage cracks, from the traditional methods to the modern take on concrete cooling”

If there is rapid water loss through evaporation, then we get what is called drying shrinkage cracks, where there is a reduction in volume as the concrete transitions from a wet fluid mix to a hardened product. So, what can we as engineers do to mitigate this cause, particularly on mass concrete pours which generate large amounts of heat? We will go over different approaches our team has used to prevent concrete shrinkage cracks, from the traditional methods to the modern take on concrete cooling that is emerging in the market.

Wet Curing

The most effective and accepted method in the industry is to follow an extended wet curing of 7 days. Any water loss that would evaporate can be compensated by continuously saturating the new concrete element. The water also keeps the concrete cool and prevents the internal core temperature from rising above the limits.

Steel Reinforcement Detailing

We rely on steel reinforcement to withstand the tensile forces after the induced stresses exceed the concrete tensile stress. But did you know that reinforcement increases the frequency of cracks? A non-reinforced concrete element such as a sidewalk would experience fewer cracks then a reinforced slab. The non-reinforced member would however have wider cracks formed in a more regular formation versus narrower cracks formed in a scattered pattern formation for a reinforced slab. With reinforcement, each design case is unique, and you must look for the right balance in content, bar diameter and spacing for the best performance.

Larger Aggregates

Another popular method is to use larger aggregates in the mix, such as 40 mm stone. The larger stone reduces the cement content requirements, which also means less water content if the water-cement ratio is kept the same. Lower cement content would result in less heat generated during curing and a reduction in evaporation rate. Another benefit is a lower cost of concrete since we are reducing the content of cement which is the most expensive ingredient in the mix.

Cement Type

The most readily available cement is Portland Type General Use (GU). There are also other Portland cement types of the same chemical composition but varying ingredient content. For crack control, it is recommended to use a Low Heat (LH) of hydration Portland cement. The Type LH has a much lower percentage of tricalcium aluminate (C3A) and higher percentage of dicalcium silicate (C2S) when compared to Type GU.

A common alternate to Type LH is to use Supplementary Cementing Materials (SCM) up to 50% in content. The SCM can be ground granulated blast furnace slag, fly ash, or silica fume or any combination of two or all of the materials. There are numerous types of blended cements mixes suited for specific application such as reducing water content, mitigating Alkali-Silica Reactions and crack control.  

Joints      

There are three type of joints; control, expansion and construction. Large concrete structures typically have all three joints, strategically placed to facilitate crack control and allow for construction to take place.

Control joints allow cracks to be induced in a controlled manner at the location of the joint. Expansion and isolation joints provide a complete discontinuity of the concrete to accommodate differential movement. Construction joints also known as cold joints do not provide relief for crack control, but rather they contribute to crack formation along the two different pours.

Ice and Cold-Water Chillers

Many batch plants have the capability to add ice or cold-water to control the heat of the fresh concrete, particularly during hot weather applications. This allows for a cooler concrete mix delivered to the site and a lower temperature maximum during hydration. Lower concrete temperatures during hydration result in less evaporation of water which can reduce crack formation.  

Nitrogen Cooling

Lastly, we will explore a new modern way that has emerged to reduce plastic shrinkage by controlling evaporation rates. Companies such as NITROcrete systems provide nitrogen automated solutions that can cool concrete up to 20 degrees Fahrenheit. Their patented system is advantageous over ice as it uses liquid nitrogen to pre-cool the aggregate during the batching process. This allows for consistent results, stable mix and also reduces the environmental impact due to cooling by 80% through the use of nitrogen over ice. 

This technology was used in the construction of the concrete core structure of the CIBC Square in Toronto, Ontario. NITROcrete was used to lower and control the concrete temperature of the 80 MPa high strength concrete to be no higher than 25 degrees at the time of the delivery. This modern approach also allowed to control concrete temperatures for a longer duration as needed to overcome varying truck travel times due to downtown traffic.