Recognize, manage and prevent shrinkage cracking in concrete.
By Mitch Rector
Editor’s Note: This article is intended to be a reference guide for entry-level production employees.
One of the first things you learn about the properties of concrete is it has much greater compressive strength than tensile strength. Reinforcing with rebar is a great way to add the missing tensile strength to a concrete structure to prevent any structural cracking from occurring, but sometimes shallow cracks can occur near the surface during production. For most products, these cracks do not threaten the immediate structural integrity of the concrete, but they still create a negative appearance on the concrete surface. This article will focus on how plastic shrinkage cracks develop, as well as precautions you can take to minimize their development.
Overview of shrinkage
When concrete is deposited in a form, gravity causes heavier particles – like aggregate or cement – to segregate and sink, which displaces water. That water has nowhere to go but to the surface. Referred to as bleed water, it will typically evaporate. As the bleed water evaporates, a negative pressure will develop in the paste. Thankfully, the evaporated water is replaced by the additional rising bleed water within the mix. However, when the rising bleed water rate does not match the rate of evaporation, the concrete will dry out and shrink.
The Portland Cement Association identifies six major contributors to rapid surface moisture evaporation as:
- High cementitious materials content. The more cement there is in a mix, the more paste will be created. The paste in concrete is most prone to shrinkage.
- Low water-cement ratio. An excessively low w/c ratio means there may not be enough water to replace what has evaporated.
- High concrete temperature. As the cement hydrates, it creates a large amount of heat. If that heat is allowed to build up, it will cook the water off, similar to letting a pan of water sit on the stove for too long.
- High air temperature. High air temperature is an obvious contributor to evaporation. Just like a damp towel in a hot dryer, concrete will lose moisture rapidly when the air surrounding it is hot.
- Low humidity. Chapped lips and knuckles are one of the worst parts of a long winter. This occurs as dry air wicks away moisture from your hands and the same can happen to concrete.
- Wind. Inside and outside air currents blowing over the surface of concrete will both take any surface moisture. This is one of the reasons special precautions need to be taken when casting outside.
All of these factors can cause some major headaches. Accurately predicting the rate of evaporation is difficult, but the National Ready Mix Concrete Association has created a nomograph that can give an approximation of the evaporation rate. This chart is used by starting with the air temperature and working your way around the graph in a clockwise manner.
For example, let’s say we are casting concrete on a hot summer day outside in the production yard and it is 90 degrees Fahrenheit. On the graph, we will start by finding the 90 F mark by air temperature. We then move upward until we intersect with the line corresponding with the relative humidity. Air temperature and humidity must be measured at approximately 5 feet above the evaporating surface and on the windward side shielded from the sun’s rays. Perhaps it is a dry day. If so, we will then move up to the 20% mark. We must next consider the concrete temperature and follow the graph horizontally to the line corresponding with our concrete temperature. In our case, the concrete is around 80 F. We then move downward to wind velocity by measuring the average horizontal wind speed in miles per hour at approximately 2 inches from the face of the concrete. For our scenario, we will assume the wind is averaging 5 miles per hour. Lastly, we move left to read the approximate rate of evaporation – about 0.12 pounds of water per square foot per hour.
Volume changes are a natural part of concrete. However, when concrete is held in place, shrinkage can transform into internal stresses. This is why it is not a good idea to leave an unopened soda can in the freezer. As the soda freezes, it tries to expand but has nowhere to go. Eventually, the pressure causes the can to fracture to allow the soda to escape.
As the top layer of concrete shrinks, the concrete underneath may not be able to keep up since it is still wet. The concrete underneath will try to hold the top layer in place while the top layer will pull itself together due to the negative pressure from moisture evaporation. These two forces trying to make the concrete behave differently will cause tensile forces to develop in the concrete’s top layer. The tensile stress will eventually overcome the early tensile strength of the concrete and create cracks. Following proper curing procedures is a valuable way to reduce cracking that could occur.
Effects of shrinkage
The most obvious effect of shrinkage cracking is an unsightly appearance on the surface of the concrete. Cracks will begin to form immediately as the cement reacts with the water. They can range in length from a few inches to several feet. When wind is blowing over the surface of the concrete, the cracks will follow a regular pattern, running perpendicular to the wind. If the wind is swirling, cracks can form a random pattern along the surface, running in several different directions. Cracks will typically appear on horizontal surfaces of the product where the water is evaporating.
Even though shrinkage cracking does not immediately threaten the structure of a product, it can potentially reduce the product’s lifespan. Rebar in concrete plays a similar role to the bones in our bodies. And just as our skin protects our bodies from germs or bacteria, the outer layer of concrete plays an important role in protecting the rebar from any deleterious substances. Cracks along the surface essentially offer an open doorway for dirt and corrosive materials to enter a product. This is especially dangerous because it may lead to corrosion of reinforcing steel. The expanding oxidizing steel could potentially lead to additional cracking and more exposed steel and corrosion, which may eventually reduce the strength or service life of the product. Precast concrete wall panels or slabs that are placed in an environment high in chlorides or other corrosive materials are especially at risk of damage.
As the saying goes, “The best defense is a good offense.” In this case, you want to take proactive measures to minimize the possibility of plastic shrinkage cracking.
Start by cooling aggregates and mix water. ACI 305R-10, “Guide to Hot Weather Concreting,” states that water cooled to 32 degrees Fahrenheit may be used as long as the quantity of cooled water does not exceed the batch water requirement. ACI 305 even permits the use of ice as a replacement for part of the batch water. Ice should be crushed or shaved into small pieces to melt completely and uniformly before the concrete mixing is complete. Ice usage will typically not exceed 75% of the batch water requirement.
Adding fibers to the mix is a way to increase the tensile strength of your concrete. This will help keep the tensile stresses from exceeding the strength of your mix. It is important to understand the fiber attributes being used and how to properly batch the fibers into the concrete as they will affect the water demand of the mix.
Windbreaks and sunshades can be erected around and over the concrete in order to reduce the velocity of wind and protect against sunlight. Damp burlap is often used to protect against sun and ensure a moist curing environment.
Fog sprays will increase the relative humidity of the air above the concrete. Because high levels of relative humidity correspond with a lower rate of evaporation, keeping the concrete in a humid environment is preferable. Curing as soon as possible and being as consistent as possible is the best way to ensure high-strength and reliable concrete. ACI 308R-16, “Guide to External Curing Concrete,” contains detailed information on how to ensure a healthy curing environment.
A common mistake to watch out for is bleed water that is worked back into the top layer of concrete. This will increase the w/c ratio of the top layer of concrete, which causes a reduction in strength among other critical surface issues. This also creates a problem by eliminating water that would normally evaporate, increasing the temperature of the concrete and subsequently increasing the occurrence of cracking. Do not begin finishing the concrete before most of the bleed water has evaporated.
If shrinkage cracks have already occurred, sealants and fillers can be used to protect against some intrusive damage. Epoxies, polyurethane and silicones are all common examples of sealants and fillers. Application of a filler should not be considered a common practice because it adds more steps and time to the production of a product. When sealants are used, read the manufacturer’s directions thoroughly.
Develop a preventive approach to cracking
We all have heard the phrase, “Never judge a book by its cover.” However, in the case of concrete, the cover is a good indicator of quality. Keeping your product free of any cracking or crazing is key to delivering a quality product. Managing the rates of evaporation and being consistent with curing procedures should be a part of any plant’s quality control process. By taking preventive measures early, you can save yourself lot of time, effort and money in repairs or touching up after the fact.
Mitch Rector is a technical services engineer with NPCA.
Portland Concrete Association, Design and Control of Concrete Mixtures, 16th Edition