Forming systems are designed for tight, quick assemblies. Specified strengths of 3000 psi are typical to withstand back-filling pressures more than holding a building up. Slump is very important however-high slump results in more cracking and reduced strengths.
Pumping or conveying concrete into forms is a sign of quality because contractors can easily place lower slump concrete. Jim Baty, technical director for the Concrete Foundation Association (CFA), Mount Vernon, Iowa, says that the majority of their members own their own pumps. He adds that too much water in concrete causes aggregate segregation, resulting in plugged lines. So contractors using pumps add superplasticizers or midrange water reducers for better pumpability.
The CFA recommends including engineered steel reinforcement to help resist backfilling pressures and to restrict cracks to hairline widths.
Baty also recommends that foundation walls be supported at the top and bottom before backfilling operations begin. “It’s best to cast concrete floors and to install floor joists at the top of the wall first,” he says.
Above-grade Walls
Perhaps the highest quality concrete for any residential application is above-grade walls. They contain structural reinforcement designed and stamped by engineers to withstand loads, wind, and seismic forces. And concrete is carefully monitored during placement. If the walls are precast, concrete is placed and cured under ideal conditions making 8000 psi compressive strengths possible. Concrete for ICF applications also is cured under ideal conditions because it’s encapsulated in foam. Specifications typically require compressive strengths of 3500 psi according to the Insulating Concrete Forms Association (ICFA), Glenview, Ill, but within a year strengths average 4300 psi. Specifications for above-grade removable form wall construction typically require 4000 psi compressive strengths. These walls are frequently only 4 inches thick so placing high quality concrete is important.
Floors
Next to foundation walls, the quality of concrete for floor construction is the greatest concern. Usually placed under difficult circumstances, basement floors are cast with poor access. Specified 3000 psi compressive strengths are normal but adding water to create slumps of 9 inches reduces strength to less than 2000 psi. Contractors should add midrange water reducers or superplasticizing admixtures to achieve the needed flowability. To prevent cracking in floors contractors should install control joints in a timely fashion during placement or shortly after initial set.
Exterior Flatwork
In nonfreeze/thaw climates 3000 psi compressive strength concrete is adequate. But in northern climates 4000 psi strengths are standard. This concrete requires the addition of air entraining admixtures. An average concrete mix with ¾ inch top-sized aggregate requires air contents of 6% ±1.5%-entrapped air being 1.5%, entrained bubbles being 4.5%. Air entrainment percentages change with different mix designs however, but the basic requirement remains the same. Bubbles shouldn’t exceed 0.0001 inches in diameter nor be more than 0.0008 apart. Varying amounts of water can change this relationship so care is needed. When air entrainment is too low scaling and spalled surfaces result. And when it’s too high, low compressive strengths will occur.
Installing good exterior concrete also includes the following:
Ensuring Quality Construction
Use the ACI 332-04 Requirements for Residential Concrete Construction and Commentary standards document as a resource and consider the companion ACIU 332 Guide to Residential Concrete. Also obtain the CFA’s Concrete Foundation Standards document (consider membership in this organization too). These documents are either newly published or completing revisions. The CFA is also in the process of developing a contractor certification program.
How to Install Good Exterior Concrete
- Don’t add water beyond the specified amount.
- Observe the proper timing for finishing steps. Don’t trap water below the surface.
- Develop good strength and durability by curing concrete after placement.
- Don’t pitch slabs less than 1/8 inch per foot. (Keep in mind that when the pitch for two legs of a triangle is ¼ inch per foot, the diagonal becomes 1/8 inch per foot.)
- Allow exterior slabs to move up and down freely. Changing moisture conditions in the subgrade or frozen ground causes significant vertical movement.
–Joe Nasvik is a senior editor for Residential Concrete. He can be reached at 773-24-25410 or jnasvik@hanleywood.com
