Sustainable concrete is becoming more popular than green concrete because it includes useful service over a long time. In the case of floors and pavement, the concrete must stand up to the service needs of an application for a long period of time, with minimal repair expenses.
But sustainable concrete is only half the story, the subgrade it rests on has much to do with its service life. When the subgrade under a slab fails inside a building or outside in the parking lot, so does the concrete.
The word “subgrade” refers to the soil below the subbase. The subbase is the finely graded compactable material between the subgrade and the concrete. Geotechnical engineers typically are contracted by owners to evaluate soil conditions on a construction site and sometimes they specify the subbase fill materials to be used and how they should be placed. But structural engineers hired by owners to design floors and pavements also get involved in the specifications for subbase materials as part of the concrete system. Pat Harrison, a senior consultant for Structural Systems, Richardson, Texas, says it's standard for his firm to review all base materials for a project and make recommendations based on their experience. But he's careful to point out that they work together with the geotechnical company to solve the problems related to a particular site. It may be discovered, for instance, that soil conditions are inadequate to support a floor slab or parking area and that designing a compactable-fill subbase will solve a problem by spreading loads over a wider area. Sometimes water tables may be high, causing problems and affecting the choice of subbase material.
Types of subbase material
Some believe that if you can cut soil to the proper grade level, concrete can be placed directly on the ground, saving the expense of hauling in stone. But it's difficult to fine grade to the tolerances now required. Len Swederski, president of Swederski Construction, Richmond, Ill., says that parking lot construction is dependent on the weather. When it rains on dirt, mud and vehicle rutting result, making construction difficult. “Compactable fill helps us manage a site by limiting the amount of water that saturates the ground.” He adds that they are having some success stabilizing soil with portland cement or fly ash to tighten up subgrade prior to concreting.
The two most frequently used types of fine grading materials are open-graded or closed-graded crushed rock. Open-graded stone materials lack small particles so water can easily pass through. Kevin MacDonald, the vice president of engineering services for Cemstone, Mendota Heights, Minn., says you can't compact a layer of open-graded stone but you can adjust it—shaking stone into place with a compactor. He adds that a layer of open-graded stone will stop the capillary rise of water, but not the rise of water vapor. Water moves by capillary action through material with small voids between particles. A way to control capillary movement of water is to specify a layer of open-graded stone with a layer of closed-grade stone on top of it. You also can manage both the movement of water by capillary action and the rise of water vapor by placing a vapor retarding membrane on top of the compacted fine grade and under the concrete. There are other good reasons to use vapor retarders too.
Closed-graded fine-grade material is the most frequently used. It's referred to as “Grade 8,” CA-6, “Classified,” and other names common to a region. The least expensive way to buy it is to find local DOT specifications for the material. Readily available around the country, the material is generally composed of ¾-inch crushed rock down to fines. It's the small aggregates in the blend that make it very compactable, especially when it has a 10% to 12% moisture content. Some regions of the country sell “processed” material, meaning that water has been mixed into the aggregate before delivery. But in most regions water must be added onsite.
The greatest advantage of closed-graded material is good compaction. You can achieve nearly 100% compaction, enough that tire prints from a ready-mix truck are hardly noticeable. MacDonald says that closed-graded material should be graded and compacted to a “slip-sheet” surface with no abrupt elevation changes or ruts so that concrete can easily move on the subgrade as it shrinks. The result is minimal cracking.
How thick should fine grade be?
Harrison says that in regions of the country with expansive soils, as much as 5 feet of soil can be removed and replaced with compactable fill. The usual application is 4 to 8 inches. For parking lot applications, Swederski recommends 4 inches—enough material to accurately grade and compact the pitched slopes to drain common parking lot construction. MacDonald says the thickness of granular fill should be determined by the strength of the soils below but the specification should be 8 inches for interior applications. Harrison says his company's specification is 4 to 6 inches placed with a laser-guided grader and compacted.
The responsibility for determining the thickness and type of stone fill for a project rests between the geotechnical engineer and engineering services responsible for the concrete construction. The best decisions are collaborative ones.
“Proof-rolling” is a method for estimating the stability of a subgrade. It's usually done with a loaded 6-wheel truck or with a ready-mix truck loaded with concrete. As these trucks move over the subgrade, the staff look for indications of “pumping” or rutting subgrade, locating areas that will either have to be recompacted, or removed and replaced with suitable material. Another method involves a testing company performing a nuclear density test, which takes about five minutes to perform for each location. It measures soil density and moisture content, providing an accurate record. But the problem with each system is that little is known about areas that aren't checked—most of the construction area.
Measuring compaction as you go
Arthur Taylor, manager of grading solutions for Trimble, Dayton, Ohio, says they developed a unit that attaches to rolling vibratory compactors to estimate the relative level of compaction of a subgrade as the machine is in the process of compacting it. Operators watch a screen mounted in their cockpit with a color-coded display that shows the operator when an area is compacted enough, whether it needs further compaction, or if a local area will not meet the specification for the job and must be replaced. Taylor says that either GPS instruments or Robotic Total Stations track the compactors, mapping all compactor locations along with the condition of the subgrade. This makes it easy to locate areas where unsuitable material must be removed and replaced.
Many specifications require a number of passes over an area with compactors with known compaction ability. When the right numbers of passes are completed, an area is regarded as satisfactory and the machine moves on. But it's unknown whether compaction really is or isn't satisfactory. Taylor says their technology enables one to know when compaction levels reach the specified levels, which could occur in a couple passes or in many. Either way, compaction will be uniform throughout a project area. This new technology can save contractors labor hours, fuel, and machine time.
When a project begins, Taylor says the first step usually involves doing a test strip with the materials that will be used on the job. This strip is checked by either proof rolling or nuclear density for calibration purposes. Volume change also is noted, making it possible for motor graders using the same software and guidance systems to add the volume difference as they place the subbase material, leaving the compacted, finished grade at the precise finished grade.
The importance of subgrade
The settlement and differential settlement of concrete slabs is a serious problem, and although they reflect on concrete, they aren't caused by the concrete. They stem from failures of the subgrade or the subbase. Installing grade work properly is just as important as placing and finishing concrete and using the right mix design.