Concrete

 

 

 

 

 

 

 

 

 

Concrete is a sturdy, rigid building material which combines graded aggregates, Portland cement, and water to form a consistent and continuous surface. The component parts are mixed to form a liquid slurry, which is then poured into place and allowed to cure, hardening into a strong, rock-like solid. Its common applications include driveways, sidewalks, and all manner of surface conditions; because of its strength, it is also commonly used below grade to support or anchor other site elements.

Concrete technology dates back thousands of years, with the best documented examples being attributed to the engineers of the Roman Empire for use in their various construction projects. In a sense, remarkably little has changed about the general principles of concrete use, although the mixtures have been improved and refined considerably.

 

Pros & Cons

Advantages:

• Because concrete forms a continuous surface as a poured pavement, the installer has control over where surface jointing will occur, as opposed to having it be dictated by the pavement module size
• Concrete can be installed to fit just about any shape, can be very effective in irregular or curved layouts
• Versatile, strong pavement offering economy of scale the larger the area to be paved
• Can be customized based on site conditions; mix additions can add integral color, adjust curing time and compressive strength, increase viscosity/workability of the mixture, etc.
• As a construction material, concrete performs exceptionally well under compression; can tolerate significant loading without degradation of the material

 

Disadvantages:

• Poured concrete will take the shape of its container, so formwork must first be constructed to contain the mixture until it has cured sufficiently to hold its shape. This adds to both the labor and cost required to install
• Regarded as an economy material, can look “cheap” next to premium building materials such as stone or brick
• Although there is some degree of control over where the jointing occurs, concrete pavement requires joints to control cracking and movement over time. Through settlement in the subgrade, weathering, or heavy use, concrete pavement will inevitably crack over time; tooled control joints and expansion joints must be used to control where cracks develop and make them less visually obvious.
• In large areas, it can be difficult to add detail to articulate form, directionality, or scale to a space. Since concrete pavement is so uniform in its consistency, large areas of concrete can feel barren or void of visual interest.

 

Construction

The sequence of construction is usually as follows:

EXCAVATION/GRADING

As a first step, the area to be paved will be excavated to allow for a proper building surface to be constructed beneath the pavement. Depending on the pavement’s intended use, the depth and bearing capacity of the base material will vary. Concrete pavement designed for vehicles (“heavy duty”) will usually require at least a 12” base below the pavement, whereas pedestrian-only surfaces can be designed to 8” below pavement. If you are unsure, a landscape architect or civil engineer can make a recommendation for your project taking into account specific attributes of your site.

Once the excavation is completed to attain the necessary depth, the exposed earth surface below is compacted, usually to a minimum of 95% Proctor Density, or an engineer’s recommendations.

AGGREGATE BASE

Once grading and subgrade preparation is complete, a base course will be laid to provide a stable platform for the pavement surface to rest on. In most cases, a dense-graded aggregate will be used, since the range of particle sizes can lock together to form a very supportive aggregate base. For applications of porous concrete technology, an open-graded aggregate or crushed stone will be specified instead, to allow for free drainage below the pavement (this is less common, discussed in more depth below). The base is distributed over the compacted subgrade in the area to be paved, and is then mechanically compacted itself, either by a steamroller or manual vibrating plate compactor (for smaller areas).

Once the base course is in place, the concrete itself is poured into place.

POUR CONCRETE

Concrete typically arrives at a project site in a truck with a rotating drum to keep the mix uniform and workable. The truck is equipped with an apparatus that will open the drum and direct the slurry into a particular area, as directed by the operator. For the average pedestrian walkway, the pavement will be a single pour into the established formwork, which is then distributed evenly about the form by a team of laborers and smoothed out to the desired surface elevation. If tooled control joints and/or windowpaning are called for, this will be applied while the concrete is still soft and workable; sawcut joints must wait until the pavement has cured sufficiently to allow for clean cuts to be made.

 

 

 

 

 

 

 

For some applications, concrete may be installed as several separate pours. This commonly occurs when different colors of integral-color concrete occur next to each other in a layout, or if there is an expansion joint occurring in a large expanse of pavement.

The most common surface finish applied to concrete pavement is a broom finish, which involves dragging a wire broom across the surface to create fine lines. Finished concrete is usually very smooth, and the broom finish gives it a bit of texture for traction when wet. Broom finish is typically specified as either light or medium, with the medium being slightly more pronounced and offering greater surface grip. If a rectangular or square jointing pattern is used, many designers opt for a tooled pattern called a windowpane, where a narrow strip adjacent to the joints is left smooth to emphasize the rectilinear pattern.

Another style of finish which is also fairly commonly is called exposed aggregate, where the fine particles at the surface of the pavement are washed away to reveal the rougher, coarse aggregates. This gives the surface a greater degree of texture, and is considered by many designers to be a more decorative finish than a standard broom finish. Depending on the aggregate used in the concrete mix, it may also provide a bit of color variability, as opposed to the uniform look of a standard broom finish.

In some cases, designers may call for aggregates to be broadcast into poured concrete, pressing them into the surface to achieve the look of exposed aggregate concrete without as much effort. The same visual aesthetic can be gained, although it may be a less durable construction method.

 

 

 

 

 

 

 

 

Concrete may also have a stamped finish, which is applied when the concrete has begun to cure but is still somewhat workable. A plate with a pattern is positioned on top of the poured mixture, and pressure is applied with a tamper to drive the pattern into the surface. A workman will often follow behind with a tool to ensure that the pattern has been evenly imprinted and clean up any uneven impressions.

 

 

 

 

 

 

 

 

 

 

 

Porous Concrete Technology

Concrete can also be engineered to drain water freely through the profile, typically done by eliminating much of the “fine” particle sizes from the aggregate mix. A pavement of this type will perform similarly to standard concrete, but must be built very differently due to the free action of water movement within the pavement profile. It is common to require around 36” of open-graded aggregate or crushed stone as a base for porous asphalt, to serve as a storage reservoir for surface water draining into the subgrade.

Maintenance of this type of pavement should NOT include sanding for added traction in colder climates – the sand particles will clog the voids in the porous pavement, negating the effect of the porosity and/or clogging the reservoir voids in the aggregate below. It should also be noted that this pavement can look “unfinished” to some viewers, since the missing fine particles in the aggregate mix can make the surface look rougher, so the designer should be cautious about how and where porous concrete is used.

 

Further Reading:

There are numerous books and trade institutes dedicated to the art and science of concrete construction. This article should serve as a primer, but should not be regarded as a complete set of information on this topic. Further reading can be found at the links below (to be updated on an ongoing basis):

American Concrete Institute Website

https://www.concrete.org/

Portland Cement Association Website

http://www.cement.org/

Popular Mechanics, on the history of concrete

https://www.popularmechanics.com/technology/infrastructure/a28502/rock-solid-history-of-concrete/

 

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