The Science of Pouring Concrete

At 2 AM this past Monday, June 17, several Reeves Young employees and interns visited The Phoenix at Braselton site as concrete was poured for one of the parking decks. Many of you may already be familiar with the process, but it’s also okay if you aren’t because we’re about to break it down for you right here. Let’s get started:

After mixing the concrete, the team works quickly to place it into the formwork. (Photo/Lindsey Adams, LAdams@reevesyoung.com)
After mixing the concrete, the team works quickly to place it into the formwork. (Photo/Lindsey Adams, LAdams@reevesyoung.com)

The Process

#1 Design

Determining the ingredients that will make up the concrete and its proportions is the most important step in the process. However, there are several factors to consider before concrete can be poured: cement type, aggregate size and type, amount of water, and mineral and chemical admixtures.

“Concrete is composed of four main ingredients: cement, water, fine aggregate (sand), and coarse aggregate (rock). Sometimes recycled cementitious materials known as slag and fly ash are substituted for cement, causing varying effects on the concrete’s plastic and hardened properties. Admixtures can also be supplemented to the mix in order to reduce curing time, increase workability, increase strength, or to change the material properties. Varying proportions of these main ingredients are what create the many types of concrete.” – Concrete 101

In order to generate an appropriate mix design, the properties of the required concrete must be determined while also finding the most economical design. These preparations paired with the following considerations will produce a good mix design and give way to positive results:

Loads Supported:  concrete can be made with a wide variety of strengths, so this is often the starting place for the mix design.  Since the cost of concrete scales rather closely with its strength, one does not want to make the concrete stronger than it needs to be.  However, if the application will only be supporting relatively small loads, it is usually not a good idea to specify weak concrete, because weak concrete almost always lacks durability.  For low load applications the quality of the concrete is determined by other factors such as resistance to freezing or wear resistance.

Workability: the workability that is required depends primarily on how the concrete is to be placed.  Concrete can be poured, pumped, and even sprayed into place, and this will affect the workability that is needed.  Other factors such as the shape of the molds, the rebar spacing, and the equipment available at the site for consolidating the fresh concrete after it is placed must also be considered.  Workability is usually defined by the slump, which is the tendency for the fresh concrete tends to spread out under its own weight when placed onto a flat surface. 

Surface Wear: for some applications the physical loads tend to wear away the concrete instead of breaking it.  For roads, parking garages, driveways, and industrial floors the hardness and wear resistance of the top layer of concrete will determine how long the structure lasts. 

#2 Batching

Most concrete is batched and mixed in one location, commonly referred to as the ready-mix plant, before being transported to the project site. This is the best solution for most jobs because the plants have controlled conditions for storage, as well as good equipment for weighing and mixing. As a result, the concrete will usually be high and consistent.

#3 Placing

Once the concrete has been mixed, it is placed in the formwork, defining its final position and shape. Steel rebar may be used to reinforce the concrete, but it must be placed beforehand so the concrete can flow around it.

Mixers keep concrete consistent before it’s placed into the formwork. (Photo/Shutterstock)

#4 Consolidation

After the concrete has been placed, it is consolidated or compacted. In other words, large air voids developed during placement are removed and workers ensure that concrete has flowed into all of the corners and nooks of the formwork. The two most common methods of consolidation are vibration and roller compacting. Vibration is a mechanical process that transfers pulses of shear energy to the concrete, usually by a probe that is inserted several inches into the concrete.  Each pulse of shear energy momentarily liquefies the concrete, allowing it to flow very freely.  This is the standard consolidating method for general construction projects with the exception of roads. Roller compaction is a simpler and more cost-effective technique that is suitable for roads and very large mass concrete structures such as dams. 

#5 Finishing

Finishing refers to any final treatment of the concrete surface after it has been consolidated to achieve the desired properties. This can be as simple as pushing a wide blade over the fresh concrete surface to make it flat, also known as screeding, or utilizing the process of troweling.

#6 Curing

At this point, the concrete hardens and become less vulnerable over time. Therefore, action should be taken to cure it properly: (1) keep it moist and (2) keep it supported. To keep concrete young and fresh, it can be covered to prevent evaporation or sprayed periodically with water. However, pools of water should not be allowed to form on top of the cover’s surface or the concrete with degrade underneath.

It is important to keep in mind that patience and timing are key when it comes to pouring concrete. Remember to keep the formwork for as long as possible before continuing with construction. It’s also imperative to keep loads off the fresh slabs. Otherwise, the concrete will deform, which will eventually lead to cracking, among other things.

The last thing to consider while pouring concrete is the weather. Hot windy weather leads to rapid evaporation. Conversely, cold weather causes the concrete to harden much more slowly than hot weather. However, if fresh concrete freezes, it will most likely be destroyed beyond repair.

Basic Definitions

Slump: measures the consistency of fresh concrete before it sets. It is performed to check the workability of freshly made concrete, and therefore the ease with which concrete flows. It can also be used as an indicator of an improperly mixed batch.

The higher the slump, the wetter the mix. A four-inch slump is very common with normal weight concrete and is a good for pumping. (Photo/Shutterstock)
There are three types of slumps: (1) collapse, shear, and true slump. In a collapse slump, the concrete collapses completely. In a shear slump the top portion of the concrete shears off and slips sideways. In a true slump, the concrete simply subsides, keeping more or less to shape. (Photo/Wikipedia)

Come-Along Rake: using a rake is the quickest way to begin getting the freshly-poured concrete spread more uniformly into place. Garden rakes work but concrete rakes, also called come-along rakes, have a more scooped blade for more easily pre-leveling new concrete. Concrete rakes also have a tine on the back of the blade to help lift rebar or mesh into position before the concrete begins to harden.

Moisture Retarder: used to prevent water vapor from intruding on a finished concrete slab. They are generally placed directly under on grade or below grade slabs.

Tamper: used with low slump concrete to push the aggregate below the slab surface. There are types that are used standing on the wet concrete or roller types that can be used from the slab edge.

Vibrator: helps release trapped air pockets and excess water from the concrete mix to prevent possibly compromising problems in medium to high slump concrete.

Screed: come in a variety of sizes and can be a specific tool (also called straight edges or bump cutters), or can be simple flat pieces of dimensional lumber. The purpose of a screed is to smooth concrete after it has been moved into place by scraping away any excess from the slab surface.

Trowel: helps smooth concrete surfaces for their finish coats before being left to dry. Hand-troweling is common for smaller slabs, or power trowels are often preferred for large slabs. There are varying types of trowels for specific concrete work.

Power trowels are preferred for large slabs. (Photo/Shutterstock)
After just a few hours, the concrete is consolidated and the team can finish and cure the floor to produce a solid foundation. (Photo/Lindsey Adams, LAdams@reevesyoung.com)

Sources:

http://iti.northwestern.edu/cement/monograph/Monograph2_3.html

https://www.wagnermeters.com/concrete-moisture-test/concrete-info/30-top-tools-concrete-construction-site/

https://en.wikipedia.org/wiki/Concrete_slump_test

https://centreconcrete.com/concrete101.html

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