Cast-in-Place (Poured) Concrete Walls

 

 

 

 

 

 

 

 

 

 

 

Cast-in-place (poured) concrete walls are commonly used in the landscape, either as freestanding structures or as retaining walls to hold up slopes or areas of elevated grade. As liquid concrete will assume the shape of its container, a temporary support structure called formwork must first be constructed to hold the concrete in the desired shape until it cures enough to maintain stability on its own.

Purpose/Uses

Concrete walls are usually installed in situations where a sturdy solution is needed to retain grade, and visual appearance is not usually a driving factor in their design. The flexibility to create any shape desired is often a key reason to elect concrete as a building material instead of a more cost-effective modular block or gabion wall construction. In some cases, concrete or concrete masonry walls will be built as a reinforcing core, around which veneer or brick will be added for greater visual appeal.

Basic Engineering Considerations*

• Note that for any work performed in the public domain, building codes may have certain fall height restrictions which should be taken into account when designing walls beyond a certain height.
• Support footings should extend further on the upslope side, to take advantage of the soil’s own weight in stabilizing the wall.
• As with any landscape construction, the behavior of water must be taken into account. Concrete retaining walls should be constructed with weep holes and any appropriate perforated pipe/gravel backfill to alleviate hydrostatic pressure from groundwater buildup. Without these, the wall can push out or fail entirely.
• When in doubt, consult a civil and/or structural engineer

*Provided for general informational purposes only – do not apply this information or detail without consulting a licensed engineer or landscape architect practicing in your jurisdiction for project-specific guidance.

Construction

Base Preparation

Concrete is a very heavy building material, and will require significant subsurface preparation in order to stabilize the wall above. As with most site elements, the larger or more substantial the installation, the more important it will be to properly compact your subgrade to be sure the ground will not subside or shift beneath the weight of the site wall.

Most concrete walls will be set on a footing, extending below grade to reach frost depth. In addition, most designers and engineers recommend an additional 12” be excavated and backfilled with compacted aggregate to ensure the feature doesn’t shift with the freeze-thaw cycle. Excavation on this scale will usually require medium or heavy machinery, so it is a good idea to plan out in advance how this equipment will access the site  and protect existing site features accordingly (trees, and anything else which could be damaged by being struck or run over by equipment).

Formwork and Rebar Assembly

With the base prepared to receive the new wall structure, assembly of the formwork and rebar and/or other integral reinforcing steel can begin. Any time reinforced concrete is used in architectural or landscape construction, it is recommended that a civil and/or structural engineer review the reinforcement to be sure it is appropriately sized for the wall application (within reason). Care should also be taken to keep any steel reinforcing at least 3” away from any exposed concrete surfaces, since insufficient coverage can cause the concrete to break off or reveal rusting of the interior rebar (which leaves unattractive reddish-brown streaks on the surface).

Concrete is essentially liquid stone and will be very heavy when poured, so the formwork should be substantial enough not to fail when subjected to the outward pressure of the concrete mix within. Beyond the scope of small home landscape concrete construction, it is a good idea to have an experienced concrete contractor perform this work.

Pouring Concrete and Finish Work

The final step will be to pour the concrete and apply any finish work needed to complete the wall. A few ideas for best results:

-Pouring concrete should generally be avoided during temperature or humidity extremes, as these can adversely affect the chemical hydration reaction which occurs as concrete cures. If these circumstances cannot be avoided, the effects can be mitigated with the addition of either a chemical accelerant or retardant, which will adjust the rate of reaction for optimal results

-If there is a significant amount of concrete work to be done, with large footings below, it may make sense to stage concrete pouring activities in multiple phases. ALWAYS begin with the deepest site elements (usually footings) and work toward the surface. It can be easier to pour a vertical wall, for instance, if the footing it will sit on is already in place and solid (so the formwork can sit on top of it also).

Decorative Finishes and Colors

At this point, any decorative finishes and/or colors are also added to the concrete work. There are a variety of products and techniques which can be used to create artistic designs and finishes on cast concrete.

Boardform

A technique in which boards are mounted in a regular pattern to the inside of the formwork, resulting in the impression of wood grain and detailing in the finished concrete surface. Many experienced professionals will alternately shim the boards away from the formwork using a nail or other small spacer, creating a small offset from board to board and making the final pattern much more pronounced.

 

 

 

 

 

 

 

 

 

 

 

Formliner

A product which is applied to the interior of the formwork prior to pouring the concrete. Formliners can create a wide range of artistic effects and impressions, and some manufacturers may even be able to create custom patterns if a suitable existing product does not already exist.

 

 

 

 

 

 

 

Integral Color

Refers to a colored admixture which is added as the concrete is being mixed and before it is poured, resulting in an even color distribution throughout the whole concrete mix. This is usually a preferable alternative to painting, since it ensures that the color will be consistent even if the surface of the wall chips or is otherwise damaged. Many manufacturers will offer a wide range of available colors, and with different dosages to achieve different color intensities.

Sock Rub Finish

Exactly what the title indicates – a sock or towel is rubbed on the surface of concrete as it is beginning to cure, to roughen up the surface and provide a bit of texture to the finish.

Anti-Graffiti Coating

Many designers (or their clients) will elect to apply an anti-graffiti coating to finished concrete walls. This will seal the wall, enabling any vandalism attempts to be more readily cleaned from the surface.

 

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