As a large part of the construction market, concrete manufacturing makes up more than $60 billion in revenue across the United States, so research is ongoing to provide the best means and methods to produce it. Perlo has been known for decades as the ‘Tilt King’ because of the immense number of concrete tilt-up projects we perform. Concrete work takes place on almost every commercial project, at least as part of the foundation work, if not as part of the entire structure. Today we explore both the art and the science of concrete.
What is concrete?
According to the Merriam-Webster dictionary, concrete is:
“A hard, strong building material made by mixing a cementing material (such as portland cement) and a mineral aggregate (such as sand and gravel) with sufficient water to cause the cement to set and bind the entire mass.”
Importantly, there is a distinction between concrete and cement: cement is an ingredient used to make concrete, along with sand, gravel, and water. Concrete is the finished product.
When completed properly, concrete is a solid, strong surface or structure that forms foundations, walls, sidewalks, mezzanines, columns and a variety of other building elements or walking surfaces. It can be decorative or functional or a combination of both.
Unlike some building materials, once concrete is poured and cures, it’s relatively permanent. Aside from minor patching, if concrete is not installed correctly, the process to replace it includes saw-cutting and removing the damaged area and replacing it. This involves significant time and expense. It’s best to leave concrete work to professionals with extensive experience to avoid costly mistakes.
The Science of Concrete
Similar to baking a loaf of bread, concrete is made up of a specific list and ratio of ingredients and mixed together into a malleable form, which then ‘bakes’ into a more solid form. The baking in this case is actually a chemical reaction, known as ‘hydration’, which causes the cement, water and aggregate ingredients to harden and strengthen over time. The process of hardening is known as curing, and sufficient time must be provided to the concrete for it to turn into the hardened state that is desired.
The ratio of ingredients is adjusted depending on the desired strength and finished look the concrete requires. Let’s look at a typical ratio of ingredients:
Aggregates (sand and gravel)
60 – 75 %
15 – 20%
10 – 15%
5 – 8%
These ingredients may be adjusted to hasten or slow the concrete curing process. Admixtures are chemicals or additives included in the mix to adjust the cure time in response to environmental factors. For instance, on cold days, water might be added to the mix at a warmer temperature to keep the ingredients from freezing as the concrete cures. Adjustments to the raw materials ratios and added chemicals are also made to increase or decrease the strength of the finished product.
The ingredients to make concrete can be hand mixed, such as when an individual wants to use a bagged concrete, mix it with water and pour it into a fence post hole. There are also portable concrete mixers for smaller batches of concrete that needs to be mixed onsite. For larger commercial projects, however, the materials are typically mixed at a local batch plant, loaded onto one, or many, concrete trucks and driven to the site, where it is either dumped directly from the back of the truck, or pumped to its final location.
Concrete Placement Types
Concrete can be mixed, poured and cured on the actual jobsite, or in an offsite manufacturing space. Let’s look at the definitions that create the distinction between these two methods:
Concrete that is placed in liquid form and cures on the actual jobsite.
Concrete that is cast offsite at a manufacturing facility and then transported in hardened form to the site for final installation. Pre-cast items are typically things like walls, columns, decorative pieces, wheel stops, or barriers. This can be particularly helpful in challenging climates where excessive cold, heat or moisture make pouring concrete outdoors challenging.
What are the standards for quality of concrete?
When placing concrete for flat surfaces like a slab, contractors must consider Floor Flatness (FF) and Floor Levelness (FL). These measurements are what helps an architect specify how flat and level the floor slab needs to be and provides a way for all parties to set expectations and then verify that those were met. More information on these definitions and how they were developed can be found here.
In addition, items like texture and strength play a large role in the cost and time involved in placing concrete. Slabs, walls, footings and columns will all vary in terms of size, thickness and strength and the finished surface may appear rocky, grooved or flat and shiny. How the finished product should look must be determined prior to pouring the concrete so that the correct method for placement can be applied.
Structurally, the strength of the concrete mixture once it has cured must be specified. Measured in pounds per square inch, or PSI, a rating such as 3000 PSI indicates that the concrete should be able to support up to 3000 pounds per square inch before cracking or failing. For slabs or walls that need additional strength, the mix design can be adjusted to attain a higher PSI.
The Art of Concrete
Placement of concrete is both a science and an art. Even with so much in the way of research and science going into concrete, there is still an element of creating a great finished concrete product that involves a stroke of luck and a lot of experience. When asked how long it takes to be an expert in concrete, long time Perlo superintendent Gary Lundervold says, ‘A lifetime. You’re always learning about concrete and how to get the best result. You have to know the science but reading a book won’t make you an expert. You need time working with it to really start to know it.’
What makes concrete so difficult to know? The variables that go into placing concrete are extensive, and include but are not limited to:
- Environmental factors such as heat, cold, wind and humidity
- Admixtures, or the additives included to speed up or slow down the curing process
- Condition of the subgrade
- Geographical location
- Available crew size
- Available concrete supply
- Specified thickness and strength
In addition to the concrete itself, there are several entities involved in properly placing and finishing concrete:
Engineering consultant: providing subgrade and reinforcing design
Geotechnical engineer: verifying grades and compaction of subgrade
Special Inspectors: providing testing and reporting to owner and local jurisdiction
Excavation: Proper preparation and grading of the surface where concrete is to be placed
Rebar fabrication and installation: providing materials and installation of the reinforcing steel inside the slab
Concrete supplier: providing the raw materials and transportation to the site
Concrete pumping: providing equipment and manpower to operate the pump that delivers concrete from the truck to the final placement location
Placement, finishing and curing: includes the form work, placement and finishing to desired finished product
Coordination of so many parties involves an extensive amount of planning ahead of time, and supervision by an experienced superintendent to adjust as needed to in-the-moment circumstances. While the pre-planning is extensive, we can’t ultimately control things like the weather, traffic or suppliers, and all those factors can lead to calling off a concrete pour within hours or even in the middle of placing concrete.
Once concrete is poured and finished, there are still several steps that require expertise to achieve a quality finished product. For instance, slab joints are cut into the concrete following the pour. This work is risky in that if it’s done too early or too late, problems can arise. Additionally, the curing process needs to be controlled so that the concrete doesn’t set up too quickly or too slowly. Heating blankets and cooling blankets in addition to the utilization of hot water, cold water, or water baths are used to help control the temperature of the slab as it cures.
With such a wide variety of variables involved in the process, every concrete pour will be different from the last, even if only slightly. This is where the trade becomes less of a science and more of an art.
Concrete over Time
Concrete does require some maintenance to remain solid over time, particularly if it is exposed to abrasive materials, forklifts or vehicular traffic, or freezing weather. It’s critical to keep debris out of the slab joints. When wheeled forklifts or automobiles drive over slab joints with debris inside, it can cause the joints to deteriorate. While these can be repaired when small, if neglected they can cause significant damage that requires cutting out and replacing large sections of slab.
Similarly, tilt walls should be maintained if they are to be expected to last for decades. Re-caulking the panel joints and re-painting buildings at approximately 5 year intervals and repairing any cracks that may occur will increase the lifespan of the concrete.
This blog simply brushes the surface of the intricacies of concrete and the complexity involved in the process of producing a quality product. If you’re contemplating concrete work for your commercial building, we encourage you to call our experienced team members to discuss your options and how we can help you.