Did you know concrete is the 2nd most consumed material in the United States? Concrete consumption is second, only to water. Nearly two tons of concrete are used per year for every person in the US. That’s over 500 million tons of concrete produced per year in the US alone.
Whether you’re a seasoned concrete subcontractor or a construction professional looking to learn more about concrete construction, this is the article for you.
Table of Contents
What Is Concrete?
Concrete is produced by mixing coarse aggregates, fine aggregates, portland cement, admixtures, and water to create a rock-like material. Concrete is used in nearly every construction project. Using concrete, you can create building foundations, roads, sidewalks, structures, and much more.
Where Do You Get Concrete?
Ready-mix concrete manufacturers sell concrete. When buying concrete, you need to find ready-mix plants close to your construction project and get quotes from different suppliers.
When searching for a supplier, here are some tips to keep in mind:
- Request estimates based on your project’s specifications.
- Review past jobs and see if they’ve supplied jobs of similar sizes.
- Confirm delivery times.
- Check the quality of the product.
- Confirm that your job site is accessible to ready-mix trucks.
- Have a representative from the suppliers you’re considering visiting the site (if possible) to confirm the viability of the order, location, and overall accessibility.
- If you need multiple trucks, make sure to space their arrival times apart for optimal workflow.
Need help finding a concrete supplier? Simply enter your project zip code in Billd’s easy-to-use supplier searcher.
How Is Concrete Made?
Concrete is made using a mixture of coarse aggregates, fine aggregates, portland cement, admixtures, and water, but it is far more complex. To fully understand how concrete is created, you first have to understand what components go into creating concrete.
Components of concrete
Water – ASTM C1602 outlines the water requirements used in concrete production. Surprisingly enough, your run-of-the-mill water may not be suitable for use in concrete. Drinkable water is ordinarily ideal for making concrete, but it can’t have any harmful substances like clay, salts, or sulfites.
Fine aggregates – Refers to the sand that is used in concrete.
Coarse aggregates- Refers to any “rock” used in concrete. Different types of concrete have different specifications for coarse aggregate sizes based on the reinforcing steel being used in the concrete. Coarse aggregates are generally no larger than three-fourths of the spacing between bars or one-third of the concrete slab depth. This is done so that the coarse aggregates don’t create voids in the concrete.
Admixtures- Ingredients other than fine aggregates, coarse aggregates, cement, and water that are used to alter the properties of concrete:
- Air-entraining admixtures increase the workability of wet concrete. When air-entraining admixtures are used in large quantities, the concrete becomes less structurally stable but extremely light and an excellent insulator, making it perfect for nonstructural building components.
- Water-reducing admixtures allow you to use less water when mixing your concrete, which leads to higher-strength concrete.
- High-range water-reducing admixtures (super-plasticizers) are like standard water-reducing admixtures but cause concrete to be even more liquidy, allowing it to flow easily into forms. Super-plasticizers are used when concrete needs to be placed in areas where concrete would be challenging to place if it were drier, like in a retaining wall.
- Accelerating admixtures make concrete cure faster.
- Retarding admixtures make concrete cure slower.
- Workability agents increase the malleability of concrete, making it easier to finish and place in forms.
- Shrinkage-reducing admixtures help to reduce cracking by reducing drying shrinkage.
- Corrosion inhibitors ensure that reinforcing steel within concrete doesn’t corrode within the dried concrete. This can happen on roadways where concrete is slowly penetrated by corrosion, causing chemicals like oils, de-icing salts, and more.
- Freeze protection admixtures make it possible for concrete to cure at temperatures as low as 20 degrees Fahrenheit.
- Extend set-control admixtures cause the concrete curing process to be delayed for any period of time, up to several days.
- Coloring agents allow concretes color to be altered.
Portland cement – Portland cement is used as a binding agent in concrete. ASTM C150 establishes several types of cement:
- Type I, also called normal cement, is the most common cement used in construction.
- Type IA, also called normal, air-entraining cement, is used when the concrete has direct contact with water containing sulfates.
- Type IIA, also called moderate cement, is moderately resistant to sulfate attack. It is used when the concrete has direct contact with water containing sulfates.
- Type III, also called moderate, air-entraining cement, is moderately resistant to sulfate attack. Type III cement causes concrete to harden more quickly, making it suitable in cold weather conditions.
- Type IIIA also called high early strength cement, causes concrete to harden more quickly, making it suitable for cold weather conditions.
- Type IV, also called low heat of hydration cement, is used when concrete is being used in massive structures. When large structures are curing, they have an incredible heat output during the curing process that can and will damage the concrete structure, but if Type IV cement is used, the curing process is slowed so that the heat output won’t damage the structure.
- Type V cement has a high resistance to sulfate attacks. It is used when the concrete is going to have direct contact with water containing sulfates.
Supplementary Cementitious Materials (SCMs) – Are used to replace a portion of the cement used in the concrete mixture, which decreases the needed water, reduces the heat of hydration, decreases the amount of shrinkage, and yields a stronger concrete. Mixing SCMs and cement creates a product called blended hydraulic cement, standardized by ASTM C595.
- Coal-fired power plants produce a fine powder called fly ash. Fly ash is widespread and is now used in most concrete mixes. It causes the concrete to have increased strength and workability and decreases the water needed.
- Silica fumes, also known as micro silica, is a byproduct created during semiconductor manufacturing. It is comprised of particles 100 times smaller than cement, causing the concrete to have increased strength and lower permeability.
- Natural pozzolans are created from shales or clays and reduce the temperature of concrete that is curing.
- Blast furnace slag, also called slag cement, is a byproduct of iron manufacturing similar to portland cement.
The terms “Pay When Paid” or “Pay If Paid” are contractual red flags to watch out for. Also referred to as “contingent payment clauses,” both indicate that a subcontractor’s payment will in some way depend on when the GC gets paid by the owner. These phrases ultimately gauge who will take on the risk of the project: the contractor or the subcontractor.
Proportioning Concrete
Different combinations of admixtures, fine aggregates, coarse aggregates, cement, and supplementary cementitious materials (SCMs) create different types and strengths of concrete. The proportions of each concrete component will change depending on what the final use case is. Concrete compressive strength is measured in psi. Concrete strengths also range significantly depending on what the concrete will be used for.
Concrete Formwork
When pouring concrete, it is a liquid and has no form. That’s why it is essential to have formwork in place. Formwork can fall under three categories, custom forms, reusable forms, and insulated concrete forms. You can read more about the different types of formwork and why we use forms in this article on cast-in-place concrete.
Concrete Reinforcing
We enforce concrete because concrete has incredibly high compressive strength but no tensile strength. By adding reinforcing elements to concrete, we increase the tensile strength of the concrete.
Steel Bars
Steel reinforcing bars, also known as rebar, are round steel bars with deformation on the faces so that they can bond to concrete more effectively. The following chart displays how rebar is sized:
In addition to rebar, concrete will also have other reinforcing elements such as hooks, stirrups, chairs, bolsters, ties, column spirals, column ties, and more.
Pouring Concrete
Once formwork and reinforcing steel are in place concrete can be poured into the forms. There are several methods that can be used to pour concrete into forms. Concrete can be poured directly from ready-mix trucks, using pump trucks, using a concrete conveyor, manually with wheelbarrows, using a crane and bucket, and a plethora of other ways.
Concrete is poured and finished back to back so it is important that you have the manpower available to vibrate and finish the concrete. In warm climates concrete might be poured in the early hours of the morning before the sun rises so that the concrete takes longer to cure. In colder climates, you might wait until the hottest part of the day so that concrete can adequately cure.
Concrete Curing
Concrete cures in a chemical process called hydration when the cement and water bond together. Hydration does not occur quickly, concrete normally takes 28 days to reach its full strength, and normally continues to get stronger even after 28 days. During that time it is important that the concrete remains moist otherwise the concrete can lose strength, and be more prone to cracking. To keep concrete moist and prevent premature drying you can place plastic over the curing concrete to trap moisture in, add curing compound to exposed surfaces, or repeatedly spray water over the exposed concrete.
Concrete Testing
Concrete testing is an essential part of the concrete construction process. It proves that a concrete structure is stable and can potentially save lives. It is also imperative that you test your concrete as specified in your contract to prevent legal ramifications should concrete begin to degrade for any reason. Most construction contracts require you to test concrete using a third party to ensure that all testing results are accurate and undoctored. This helps both parties because the testing is no longer the responsibility of the subcontractor and it means the third-party testing company is providing another level of quality control assurance.
