Early end date

The market for raised patios continues to grow where the backyard ground elevation is well below the rear entrance. A construction method using flowable fill can shave off days of construction time for raised patios, depending on their size and design. With a pancake batter consistency, flowable fill is a low-strength (150-200 psi) concrete available in most markets from ready-mix concrete producers.

While the material is about four to five times more expensive than dense-graded aggregate base typically used to raise patios, the labor savings can be more than worth the extra material cost.

Contractor Tom Arthur of West Grove, Pennsylvania was one of the first contractors to use this construction method in 2005 and provided the photos for this article. Arthur has done many raised patio projects with this construction method. For this Pennsylvania project built in 2004, a raised patio with flowable fill “reduced construction time by a week compared to endless compacting of dense-graded base.” He said all of his raised patios use flowable fill as long as the ready-mix truck can access the backyard for delivery.

Construction begins like a typical raised patio, i.e., a footer trench is dug, lined with geotextile and filled with compacted, dense-graded aggregate and a segmental retaining wall course raised to begin forming the rest of the wall. As it rises, the wall interior is lined with geotextile to prevent loss of flowable fill through joints. Any drain pipes and conduit for electrical lines are placed prior to each pour.

The flowable fill is poured in layers. This project required four pours, each 18-inch deep (3 courses) with geogrids at each interval. This minimizes lateral forces against the wall units and the house during curing. Lateral forces are minimized once the flowable fill cures.

After each pour cures, the geotextile is trimmed away and geogrid is installed between the next courses of wall units and positioned on the cured fill. Additional courses are placed, glued and geotextile placed against them in preparation for the next pour. Each pour was done in the afternoon and allowed to cure overnight so it could receive a new pour the next day.

Conduit is positioned for wiring step lights and the same treatment can be done for water, sewer and storm drains, plus natural gas supply lines for barbecue grills, fireplaces and fire pits.

Because cured flowable fill can be dug with a shovel, it can be excavated to install lines after the pour, making it a forgiving material if a conduit is inadvertently left out.

Once the final fill layer has cured, a 4-inch thick aggregate base is placed and compacted. While the outer wall elevations help create a slope for the paver surface, the base layer surface provides an opportunity to fine tune or sculpt the final slope for drainage. After the base is compacted, a 1-inch thick layer of bedding sand is screeded.

Pavers are installed as in any patio project, i.e., placed, cut, compacted, joints sanded and the entire paver area compacted again. A small strip of geotextile is placed against the wall units to prevent migration of bedding sand.

Once the flowable fill is poured in lifts and each allowed to harden, a 4-inch thick compacted aggregate base is installed. An inch of bedding sand and concrete pavers are then installed and cut to fit the patio area.

Time is money on hardscape projects. When labor is saved, money is saved. That’s the idea behind using flowable fill. Assuming site access by the delivery truck and a house foundation that can withstand the additional lateral load, a raised patio project should be estimated comparing costs of an aggregate base to flowable fill. This might render lower labor costs, faster project delivery and potential for doing more projects per year.

The author is technical director for the Interlocking Concrete Paving Institute.