Horizontal slabs of thick reinforced concrete often form the floors and ceilings of buildings around the world. Sometimes, the section is prefabricated in a manufacturing facility and transported to the site; prefabricated concrete, also known as precast concrete, is especially frequently used as a cladding material.

Creating a reinforced concrete slab, image by Marcus Mitanis

Most construction however sites utilize a different method; in situ concrete — also called cast-in-place concrete — is constructed directly on the building site using formwork, a mould in which the wet concrete is poured. If the concrete needs to be reinforced, and nearly all sections are, rebar is placed within the formwork before the pour is made to strengthen it; we'll look more at rebar shortly.

The wet concrete is then compacted to ensure the formwork is completely filled. The process of consolidation is conducted to remove trapped air from freshly poured concrete. The most common practice uses an internal vibrator, which is driven down into the wet pour and slowly lifted to help air bubbles escape. 

Wood formwork awaits a concrete pour atop a finished floor slab, image by Marcus Mitanis

When poured and fully set, concrete on its own is relatively durable — think sidewalks — yet its weak tensile strength can result in cracks when the slab expands and contracts. To compensate for fluctuations in temperature, ground water, and air quality, reinforcing steel bars are placed within the concrete. Known as rebar for short, these carbon steel rods help distribute structural loads and reinforce hardened concrete.

Several reinforced concrete slab columns, image by Marcus Mitanis

Steel is an ideal reinforcing material, as concrete and steel expand and contract at about the same rate. Separation between the two is kept to a minimum as a result, which prevents cracks from forming. Rebar rods can be tied together to create a grid or cage of steel mesh, which is often installed in larger building projects. Though rebar is mostly found in load-bearing concrete structures, it can be used in brick and mortar projects as well. This is known as reinforced masonry.

Rebar poking out of a concrete slab, image by Marcus Mitani

Rebar installation is an intricate process. Its susceptibility to rust must be considered carefully, so rebar has to be installed at the correct depth to avoid contact with external water and moisture. In locations where water seepage through the concrete may be an issue, polymer coated rebar is a popular rust-resistant solution. 

Rebar cages awaiting placement, image by Marcus Mitanis

Rebar rods are made with a continuous set of ribs to create slip-free bond with the concrete that forms tightly around it. As columns continue from floor to floor, but floors are formed one at a time, rebar is placed in the forms so that it sticks out above the top of one floor's column or wall, so that once the next slab has been formed, the rebar from the column below can be wired together with rebar for the next floor's column or wall, thus transferring the strength and hold from floor to floor to floor, down to the foundations.

Rebar extending past the top of one floor's column, image by Craig White

Have any other construction and development related terms that you would like to see featured on Explainer? Share your thoughts and questions in the comments section below!

From 2015 to 2017, UrbanToronto and its sister publication, SkyriseCities, ran an occasional series of articles under the heading Explainer. Each one took a concept from Urban Planning, Architecture, Construction, or other topics that often wind up in our publications, and presented an in depth look at it. It's time to revisit (and update where necessary) those articles for readers who are unfamiliar with them. While you may already know what some of these terms mean, others may be new to you. We are publishing or updating and republishing Explainer on a weekly basis. This article is an update and combination of two that first appeared in 2016.

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