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How to Avoid Concrete Subgrade Uniformity ProblemsHow to Avoid Concrete Subgrade Uniformity Problems
Here are two effective ways to assess the subgrade's condition before placement to assure an acceptable subbase.
5 Min Read
Building & Earth Science
Concrete contractors rely on the general contractor to provide a stable and uniform subbase. The American Concrete Institute's Guide to Concrete Floor and Slab Construction (302) supports this expectation. "Because the performance of a slab-on-ground depends on the integrity of the soil-support system, specific attention should be given to site preparation requirements," states the document.
Unfortunately, placement contractors often have a short window of opportunity to inspect the site's subbase condition. Even so, Robert Adams, senior materials engineer at Building and Earth, an engineering firm that provides geotechnical engineering, construction materials testing, special inspections and environmental consulting, says contractors can be proactive in their preparation.
Adams suggests that concrete contractors adopt a two-step approach to assure an acceptable subbase.
Reviewing the Geotechnical Report
Upon receiving the job order, Adams suggests that the concrete contractors ask for a review copy of the site's geotechnical report. The owner or their design team have often arranged for a thorough geotechnical evaluation of the site months before the start of the project. The bulk of the report's information provides the criteria that structural engineers use to design footings and foundations.
However, there are at least two essential items that can forewarn the placement contractor of a potential challenge:
1. Adams suggests starting with a review of the report's section describing the site's soils. If the geotechnical engineers identify moisture-sensitive clay soils and silts, there could be site problems. These materials will degrade if they are saturated. "Wet weather will soften these soil types, causing the subgrade to become unstable, which the contractor must address before starting placement," says Adams.
2. Another key warning sign identified in the geotechnical report is the presence of groundwater. Most reports will identify the boreholes where drilling encountered groundwater. "A simple check of the borehole's placement's location in relation to the future floor will alert the contractor of a water potential problem," says Adams.
Proofing the Condition
While the geotechnical report provides some general information, site conditions often change. Installations of footings and utilities and weather conditions may alter subbase conditions. Forewarned of potential concerns, contractors can perform a simple check to verify that the soil beneath the potential slab's subgrade is properly conditioned for concrete placement.
Proof rolling is a field technique that requires no special testing equipment. In most cases, "proof-rolling results are much more indicative of the soil support system's ability to withstand loading than from the results of in-place tests of moisture content or density," states the ACI 302 document.
The primary purpose of proof rolling is to identify any weak or unstable areas in the soil or subgrade, not to assess the subgrade's strength. "Uniformity is the key, as the strength of the slab is provided by the combination of concrete strength and reinforcement,” says Adams.
The Army Corps of Engineers first recognized proof rolling as an acceptance field check in the 1940s while constructing several airfields. Noting the procedure's success, the corps later introduced proof rolling into its guide specifications for graded crushed aggregate base courses in 1957.
There is no ASTM standard for this procedure, but there is guidance. In addition to the Army Corps guidelines, several state DOTs have written procedures for using the method on their projects. However, for commercial projects, where these rules aren't specified, there are some general practical guidelines by which a contractor can judge a subgrade's uniformity.
In most situations, the proof roll test is a double check after the geotechnical engineering firm has tested the in-situ material and reported that it has reached the required compaction percentage. Thus, the acceptable deflection of the surface when the vehicle drives on it often ranges between no deflection and 1-inch deflection; however, on stabilized subgrades, the maximum allowable deflection is usually specified as ½ inch.
Contractors are also on the lookout for any signs of instability, such as cracking and rutting during the test. If unsure of the exact requirements, check with the site's structural engineer, as the contract documents might call out a certain amount of deflection and rutting.
Creating a Good Proof Roll Test
Check the subgrade before conducting the proof rolling. Ensure that the moisture content is uniform. Proof rolling wet spots can damage the subgrade.
Select the correct roller weight that provides a practical subgrade assessment. The equipment should be heavy enough to simulate the stress that the material will endure during preplacement activities. On most placements, contractors often use a fully loaded tandem truck or a ready-mix concrete truck with gross weights ranging from 20 to 25 tons. On large areas, contractors often use pneumatic-tired rolling equipment that can exert 30 to 50 tons of load.
Establishing a proof roll test pattern path on the slab's subbase ensures a thorough assessment of the material's stability. On most projects, let the equipment perform a minimum of three passes. It's important not to miss any spots to ensure uniform stability across the entire subgrade.
Operate the test equipment at an optimal speed that demonstrates effective compaction. Too fast, and the tires may not adequately compress the material; too slow, and the tires might overwork the area, causing unnecessary wear. Another error is failing to overlap the vehicle tires passes adequately. If spread too far apart, the tested surface will have weak spots that compromise the subgrade's uniformity.
During the proof roll test, walk alongside the truck or roller as it moves over the prepared subgrade surface. Look for visible deformations or excessive deflection and mark them with paint to identify unstable locations.
Correcting the Problems
When the contractor observes problems during the proof roll, it's best to contact the general contractor.
If some deflections are too deep, delaying placement and allowing the soils to dry out may be best. Sometimes, additional compaction with a plate vibrator can solve concerns on small areas. If conditions are severe, summon geotechnical testing to evaluate the placement site. Unstable material should be removed and replaced with suitable, compacted on-site clays or other approved materials.
To learn more on this subject, listen to Robert Adams’ interview in the free webinar "Prepping a Site for a Defect-Free Slab," which is available on demand. For more technical information, download this report, “Proof Rolling of Foundation Soil and Prepared Subgrade During Construction,” from Purdue University and Indiana’s DOT.
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