Making sure you have the right materials for a slab-on-grade
- Feb 16
- 4 min read
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Structural engineering plans can be a bit of a nightmare to get through. A whole page of notes in very small text, plans with millions of callouts, and details with a lot of arrows. How do you make sure you have the right materials for your slab on grade?
The quality of materials and installation really matters in making sure your foundation has the right strength and stiffness to support your building, especially if your site has expansive soils. How do you make sure your builder is supplying you with materials that match the engineering drawings? And how do you make sure that those materials are properly installed on site?
There is an entire process for making sure that you are getting what you pay for. This process is called the Quality Assurance/Quality Control Process (the QA/QC process). The process has two parts happening at the same time: quality assurance – having your builder provide documentation that the quality of the materials meets the standards in the engineering specifications, and quality control – making sure that those standards are actually implemented through testing and inspection.
So on one hand, the builder has to prove that they understand the standards and plan to meet them beforehand, and then on the other hand, the builder has to prove that they have met those standards during and after construction.
This process works in two ways. First, the builder sends in a document called a submittal. The submittal is a summary of specifications of the exact product they are proposing to use, from the company they are acquiring it from. This is the quality control portion.
Then during construction, your engineer does site visits to make sure the design is implemented correctly. You can even have a construction materials testing company (CMT) run spot tests on the materials proving that the materials meet the standard that was agreed upon. This is the quality assurance portion.
When you are building a foundation (especially a slab on grade,) there are some some very important elements that come with the whole endeavor, and each element needs to be checked and verified.
Select Fill
The first thing you want to check is what exactly is being used as a select fill. Typically, your engineering drawings or geotechnical report will include a recommendation for a type of fill to be used for the building pad. This select fill will probably be identified on the drawings or report by its USCS classification.
There will also be guidelines in the geotechnical report on how to install the select fill. For instance, the report will include information like how thick to put down each layer before you compact and lay the next layer (lift thickness), the density level that the fill should be compacted to (usually a percentage of the maximum density), and the optimum moisture content of the select fill during installation (usually a range).
Before the fill hits your site, your builder can provide a density and moisture report (or a pit report) from the dirt pit that they are ordering the fill from. This report will have the USCS classification as well as other engineering properties like the moisture level in the pit and the density of the materials. By checking this document, you can make sure that the select fill that your engineer specified is what you are getting.
When the fill is installed, a construction materials testing company (CMT) can provide a moisture content test to check the moisture level. A CMT company can also provide a Proctor test. The proctor test can tell you whether they have compacted the fill to the right density as specified by the engineer.
Hiring a CMT company is an extra cost and one that you will have to discuss with your builder. But if you are somebody who needs that peace of mind, a moisture test and a Proctor test can help prove that your pad is constructed properly.
The Proofroll Test
Before the select fill is installed, your builder will need to excavate the pad to remove the existing soil down to the level specified in the engineering drawings. After the excavation, it is recommended to perform a proofroll test by driving a 15 - 20 ton dump truck all over the subgrade before you install the select fill. This is to check the integrity of the soil that will support your select fill. If your subgrade can't support a dump truck, it can't support a house. Any areas with rutting should be filled with select fill and recompacted.
The Vapor Barrier
Your engineer will usually specify a vapor barrier thickness for your slab-on-grade. The minimum thickness is 10 mil for residential foundations and 15 mil for commercial foundations. These thicknesses provide levels of permeability that have been standardized in ASTM E1745. Your engineer might also specify a thicker vapor barrier if you have sulfates or other contaminants in your soil. You can check the brochure of your vapor barrier product to make sure it matches the thickness specified in your engineering drawings, and that it conforms to ASTM E1745.
The Reinforced Concrete Slab
Your engineering drawings will typically include a list of specifications about the concrete for your slab. These specifications will include the concrete strength, the water-to-cement ratio, the maximum aggregate size, and the type of cement required, as well as other information. You can verify you have the right concrete by by checking the concrete mix report from the concrete supplier. This mix report should also include historical testing data on the strengths of that mix. The historical data does not indicate anything about future performance but it is better than nothing.
You can also request that concrete cylinders be taken and compression tested by a CMT company 7 and 28 days after the pour. Your concrete strength can be verified with these tests.
The rebar specified by your engineer will also be a particular grade of steel. You can request mill certificates from your rebar supplier to make sure the grade matches the engineering drawings. It is also recommended to have your engineer perform a pre-pour site visit to spot-check your rebar sizes, spacing, and cover requirements.
The checklist below includes all the recommended items to be verified during the construction of a slab-on-grade:
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Need help on a project? Call 713-909-0458 or email info@o3structural.com
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