Why precast foundations and footings might be the solution on your next project.
By Evan Gurley
Like most construction projects, the wide variety of available building materials requires some research and consideration before settling on the best value. With foundation systems, the selection process is unique for each project, but the quality and proper procedures should remain constant.
Precast concrete foundations/footings have many strong attributes that make precast a front runner for use in concrete foundation/footing design. Precast concrete foundations address the numerous problems that can arise during the design and installation processes with typical cast-in-place foundations and foundations that are simply placed into an excavated hole.
Precast Concrete Addresses the Foundation/Footing Design & Accuracy Requirements
Since precast concrete foundations and footings are manufactured in a controlled environment, they can exhibit high quality and uniformity. Adverse factors found on a typical job site such as inaccurate dimensions/measurements, extreme temperatures and improper curing are all but eliminated in a quality controlled environment.
The controlled manufacturing environment also means that dimensional accuracy is seldom an issue. Precast foundations and footings can be manufactured to exacting specifications. By contrast, concrete foundations and footings poured on site may experience lines that are out of level or out of plumb due to poor craftsmanship and adverse conditions. This can create multiple issues ranging from:
• Cracks in the structure/building
• The structure/building becomes unlevel
• In the worst cases, the structure/building eventually becomes uninhabitable or collapses.
Precast concrete foundations/footings manufacturers also can produce mixes with higher compressive strengths and superior durability. Precast concrete foundation systems are constructed with a minimum of 4,000 psi concrete.
Designing a concrete foundation or footing with a lower water-to-cementitious ratio and a controlled curing process will create a denser matrix within the concrete to provide a foundation/footing that is less susceptible to water infiltration.
Precast Concrete Addresses the Subgrade/Soils Requirements
Precast concrete foundation/footing manufacturers specify that the concrete foundations are to be installed on a level, compacted gravel, sand or stone sub-base, which eliminates many of the problems that arise when a concrete foundation/footing is simply poured into an excavated aperture without preparing a proper subgrade or checking dimensional accuracy. Since foundation panels are typically specified to be installed on compacted gravel or stone sub-base, water can drain away from the wall, minimizing the possibility
of settlement. Water standing or running alongside a foundation after rains may cause differential settlement of a foundation. If soil grading is such that water runs alongside a foundation during rains, the water will run under the edge of the foundation and carry away soil supporting the foundation.
Having a level, sound sub-base specified for the foundation/footing to rest on is crucial since the weight of the structure distributes the load from the building to the concrete foundation/footing and then directly to the soil beneath. The soil pressure from this direct loading is greatest directly under the foundation/footing, and the load distribution spreads out as the depth of the affected soil increases. Figure 1 on page 18 illustrates the distribution of the loads beneath the foundation/footing and points out the location of the critical compaction zone.
Using a precast concrete foundation/footing will help eliminate poor practices by requiring that a properly compacted, level subgrade is prepared.
In order for precast concrete foundations/footings to be as effective as possible, contractors and building officials should be educated on installation best practices.
Precast concrete foundations/footings manufacturers typically provide proper installation literature and/or assistance to the end user of the precast product. In addition to providing the appropriate literature and assistance, precast concrete foundations manufacturers typically check the install sites prior to the foundation wall installation. This is another added benefit of using precast concrete foundations.
Follow the regulations & codes
Designing a foundation/footing with proper accuracy (forms) and proper subgrade is essential. Unsound practices can jeopardize the integrity of the structure.
Each component of these codes is essential in construction and design practices, and each has consequences that coincide with using poor methods.
Excavation is one of the first factors that should be outlined in the design of a concrete foundation subgrade.
During excavation the contractor should be required to remove any materials that do not comply with the IBC code (debris, organic material, etc.) or other specified codes and standards. Removing materials and soils listed in the IBC codes helps create a level, firm subgrade and helps eliminate settlement or shifting (causing cracking) that can occur using poor materials.
It also must be realized that while excavating the soil, excavation loosens up the undisturbed soil and mixes air into the soil, which decreases the density. A decrease
in density of soil results in a loss in the load-bearing capacity, or the loss of the soil to support loads. Proper compaction of the disturbed native soil or imported fill material is needed to regain its intended design density properties.
All components, considerations and variables must be realized and outlined in the design stage.
Whether it is a foundation, driveway, patio or sidewalk, native soil or granular fill cannot be simply placed in the excavated fill area and covered with concrete. It is vital that the engineer requires the compaction of all fill materials in proper lifts, regardless if it is a native soil or a granular material to provide a dense, flat subgrade. If the fill material is not compacted in a uniform fashion, uneven settling of the foundation/footing can occur, resulting in cracking of the foundation/footing. The engineer should call out the appropriate backfill/compaction lifts, which coincide with the material/soil being used for backfilling the area. If the engineer calls for excessive backfill lifts, the foundation is subject to leaning, buckling and possible collapse.
Compacted Fill Material
There are six reasons why proper fill materials and compaction methods are essential for designing a proper subgrade for a concrete foundation.
1. Provides stability
2. Reduces water seepage and swelling
3. Reduces settling of soil
4. Prevents soil settlement and frost damage
5. Increases the load-bearing capacity
6. Provides good drainage properties and can be used to avoid moisture concentrations beneath the foundation walls, which lead to a weakening of the foundation
If improper fill materials are used, adverse effects such as settlement, differential settlement, leaning or tipping of the concrete foundation/footing may occur. Differential settlement will damage the concrete foundation/footing by producing (usually vertical, possibly diagonal or stair-stepped) cracks and other symptoms of wall movement.
Also, improper use of compaction machinery may inflict damage to the concrete foundation in the form of breaks, buckling or leaning.
Dimensional accuracy is a key component to any structure, but especially true for concrete foundations/footings, since foundations/footings require precise level lines to provide a flat, level base for construction. If the foundation wall is not designed accurately in accordance with the soil characteristics and with a proper sub-grade preparation, the foundation could be subject to several failures (foundation cracking, differential settlement, etc.).
Building standards and codes require that foundations and footings are designed to a certain dimensional criteria based on the soil loads, backfill height and the estimated wall height. Table 1805.5(5) in IBC Section 1805 states the minimum wall thickness based on these criteria.
When planning for building foundations and footings, precast concrete is the ideal material. Precast makes sense where quality, value and permanence are important.
Evan Gurley is a Technical Services Engineer with NPCA.
International Building Code. 2006 Edition
ACI Manual of Concrete Practice, Part 3. 2007