Concrete is an extremely versatile material that can vary its appearance based on finishing, forming, and other things such as additives for tinting.

One thing to always keep in mind is a timeless saying that there are two types of concrete:

• Concrete with cracks
• Concrete that is going to crack

It’s because of  this that many different finishing techniques and control joints have been developed to help shape the aesthetics and control the finish over time.

As-Cast Finishes

Rough Form Finish

The simple method of leaving the concrete as is when the forms are removed. This shows the formwork and joints between forms. Defects and tie holes may be left unfinished or finished. This is essentially forming the concrete with just ‘normal’ forms, while the next is more about aesthetic special finishes.

Smooth Form Finish (See Form Liner below)

Similar in that the profile of the interior of the form will be embossed in the face of the concrete. Smooth forms of wood, metal, hardboard are used and the joints and tie holes are planned so that they are symmetrical or have a nice design logic to them.

• Any excess concrete is removed from the concrete as minimal remedial work is done

Architectural Finish

Architectural finishes are used when the concrete will be exposed and finish is a consideration.

Form Liner (Same as Smooth Form Finish)

Concrete form is constructed with liners of plastic, wood, or metal that are attached to the form. Parallel rib liners are a common type. Joints and form tie holes are either integrated or left exposed. Custom form liners are available to be produced that can give concrete almost any form imaginable. Sometimes these formliners are also designed to allow brick and other elements to be integrated into the formwork. The concrete in many times becomes the “mortar” for these elements. This is common for ‘precast concrete’ facade systems.

Scrubbed

The surface of the concrete is wetted and scrubbed with a wire or fiber brush to remove some of the surface mortar and expose the coarse aggregate.

Acid Wash

The surface of the concrete is wetted with muriatic acid to expose and bring out the full color of the aggregate.

Water Jet

A high-pressure water jet mixed with air is used to remove some of the mortar and expose the aggregate.

Tooled and Sandblasted Finishes

Are produced by mechanically modifying the concrete surface. Less likely for in-situ pours, typically these finishes are applied in factory settings to precast concrete facade pieces.

Bush Hammering

Gives the concrete a rugged, heavy texture by removing a portion of the surface with a manual or electric hammer with a head that has rows of small pyramids (similar to a meat tenderizer).

Grinding

Finishing technique uses a grinder to smooth out the surface of the concrete, giving it a similar appearance to terrazzo as it exposes the aggregate across the surface. Different aggregates provide a different aesthetic when exposed.

Applied

Applied finishes include the application of other materials, such as stucco, to the concrete. Will typically require surface prep or additional layers such as strapping or lathe.

Sandblasted

Produce by removing surface material from the concrete. This exposes the fine and coarse aggregate to varying degrees, depending on whether the sandblasted finish is specified as light, medium, or heavy.

Rubbed Finish

Rubbing gives concrete a smooth, uniform, durable finish. The downside being that rubbed finishes are labor intensive.

Smooth

The surface of the concrete is wetted and rubbed with a carborundum abrasive brick to produce a smooth, uniform color and texture.

Grout Cleaned

Grout is applied over the concrete and smoothed out. This results in a uniform surface with defects concealed.

Concrete Slab Finishes

After a concrete slab is poured, there are several steps and techniques used to begin finishing the slab.

1. Strike Off: Drawing a straight edge of some kind across the forms to yield a roughly even surface. The straight edge is drawn against the borders and will help establish level or pitch to the slab.
2. Floating: If a smooth finish is required, then the slab is floated. This brings cement paste to the surface where it is consolidated and smoothed over the coarse aggregate. Floating can be done with a magnesium float or a bull float.
   1. Float: Handheld trowel.
   2. Bull Float: Wider than a standard float and is attached to a long handle that allows finishers to smooth large concrete surfaces while standing away from the fresh concrete. This helps keep the fresh concrete clean.
      1. If the spec or design calls for Float Finish, the bull float could be the last step which may be desired. This would leave the concrete rough like sandpaper which is appropriate for exterior surfaces where smooth surfaces are not required.
   3. Light Steel-Troweled Finish: Achieved by using a steel trowel several hours after floating. This further consolidates the concrete. Either hand towels or more typical is large, mechanically driven rotary trowels may be used. The mechanical trowels can be in push or riding, and look like a large fan pointed at the concrete.
   4. Hard Steel-Troweled Finish: Continues the consolidation of the concrete and greatly densifies the top ⅛” of the concrete, making a very smooth surface.
   5. Broom Surfaces: Created by running an industrial broom with medium bristles over the surface of the concrete. This process dislodges fine aggregates and produces a rough-textured surface useful for slip-resistance on outdoor slabs.
   6. Superflat Floor Finish: Typically has a hard steel troweled finish, but the term refers to the smoothness and levelness of a concrete slab. A superflat floor is commonly used in industrial warehouses where automated or special forklift vehicles are used to rapidly locate and retrieve materials from high-rack storage. This is because there is an immense need for precision and flat surfaces with machinery.
   7. Stamped Finish: Gives the slab an embossed surface and may be used in locations where the concrete is to become a more decorative element such as a patio. Color can be incorporated into the mix and will look better, but the concrete can also be post-colored. Stamped finishes include brick and stone patterns to make the slab resemble those materials.

Precast Joints 

There are four main types of concrete joints: Control, Construction, Expansion, and Isolation.

• Control Joint: A weak section intentionally created in the slab so that normal temperature and stress cracking will occur along the joint instead of at random locations. Control joints are formed either by sawcutting, tooling while the concrete is still wet, or using a pre-molded strip to create the void. Joints are typically cut to be ¼ the slab thickness (4” slab thickness = 1” cut depth)
• Construction Joint: Occurs when there are two successive pours. The joint is created and a new pour is made up against the already poured concrete. A construction joint creates a plane of weakness, so it should be located at a point of minimum structural shear. Normally reinforcing extends from one pour to the other to tie the two pieces together such as rebar or another embed. A construction joint is also a point where water leakage can occur. To help prevent this, a waterstop that extends from the first pour into the second pour can be inserted in the first pour.
• Expansion Joint: Allows entire sections of a concrete structure to move independently of the other. The movement can be necessary and created by shrinkage or temperature changes. Because the movement can be cyclical, the expansion joint needs to be able to move in both directions.Expansion joints are complex fabrications, portions of which are embedded in the concrete and other portions of which are exposed. In most instances, the expansion joint extends through the entire structure, walls, floors, and roofs, etc. So there is no rigid connection between any two components of adjacent sections.

Isolation Joint: Allows two independent sections to move independently of each other, but this kind of joint is not as complex as an expansion joint. The joint typically is two separate pours and inbetween is a pre-molded material. They are often used to separate columns from slabs and slabs from foundations or other types of walls.