Expansion Update: Rebar

Almost all modern concrete is reinforced with steel rebar.

Steel and concrete are an excellent combination. Concrete is strong in compression and relatively weak in tension. Steel is strong in tension. They have similar rates of expansion and contraction when the temperature changes. When a reinforced concrete column, a slab, or a beam is subject to a force which tends to bend it, the inside of the bend is in compression and the outside of the bend is in tension. The concrete and steel work together.

The usefulness of reinforced concrete was most dramatically shown in the aftermath of the 1906 San Francisco earthquake when it was seen that the few buildings with reinforced concrete were substantially more successful in surviving the quake.

Rebar has a deformed surface. This surface shape prevents the rebar from pulling through the concrete. Ordinary rebar is made with unfinished, tempered steel.

All rebar is sized to meet the structural needs of the building. The engineer calculates the strength required and the placement needed. The bar is used not only to strengthen columns and floors, for instance, but also to connect them. The engineer specifies every bar, its size, location and bends. The bent rebar is generally delivered by the rebar supplier.

Rebar numbers reflect the number of ¹⁄₈ inches.  #4 is a ½ inch rebar.  #8 is a 1 inch rebar. The Museum’s North Wing Expansion is using a lot of #8s.  In the accompanying photo, there are ten or twelve rebars for reinforcement of this one column. These 10 foot tall columns weigh between 270 and 320 lbs each.

All rebar is shipped unassembled. It is joined together using tie wire in the configuration required by the engineer according to the drawings. The tie wire is often on a spool at the hip.

The rebar is carefully placed in the concrete form. The rebar must be secured in the correct location during the pour to prevent oxide jacking or rust bust, a process that can happen if rust on the rebar expands, causing the concrete to spall (crack away) and expose the rebar. If the rebar is placed too close to the surface, the concrete could crack and expose the rebar to moisture in the air. When placed correctly in the form, the high pH value of the concrete prevents corrosion of the steel.

Once the rebar is secured in position, the concrete columns are poured. These columns will support the new 26,000-square-foot contemporary art gallery building.

To learn more about the North Wing expansion project, visit cmog.org/expansion.