Methods used for preparing the ground for concrete placement depend on the job. The types of projects include foundations, industrial and commercial floors, roads and highways, and other exterior slab-on-grade hardscapes. The construction sequence starts with the removal of surface vegetation and topsoil, excavation of high areas of grade, surface preparation and fill placement, followed by concreting. Each successfully completed step makes it possible to continue to the next phase. The reverse is also true. For example, when soils are excavated from one location and are used for fill in other areas without proper compaction, the resulting settlement over time destroys even the best concrete work. This is also the case when foundations and utility trenches are backfilled without compaction and concrete elements placed on top.
Proper soil preparation is very important to the performance of a project. This includes the removal of weak materials from the surface, sub-grade evaluation, the selection of suitable materials for use as fill, proper compaction, control of moisture, and providing flat uniform surfaces for concrete placement. Normally, grading contractors bring the site to rough grade through excavation, fill placement, and compaction while concrete contractors cover fine grading and compaction of the upper few inches in their contracts. Both parties should be included in preconstruction meetings to agree on specifications and details.
The ground under concrete must have the proper engineering characteristics to support anticipated loads without experiencing bearing capacity failure or excessive settlement. For instance, a concrete foundation must support the loads associated with columns or bearing walls without shifting or settling more than can be tolerated by the building's frame. The ground under an industrial floor slab must support the weight of forklifts and materials stored on the slab. Structural engineers specify the load that the ground must support for every application. Then geotechnical engineers sample the soil on the site to determine if it can meet the load support requirements. If it can't, there are several options available to improve support capability. Soil can be removed and replaced with suitable materials, aggregates can be mixed with the soil to improve strength and compressibility, or layers of more suitable materials can be placed above weaker soils to better distribute applied loads. In general the denser a soil is, the more load it can carry. Achieving the proper level of density is accomplished through the control of moisture within the soil and proper compaction.
Kevin MacDonald, vice president of engineering services for Cemstone, Mendota Heights, Minn., says that there are two primary types of soil: coarse grained and fine grained. There are organic soils too, but they aren't appropriate to use as a bedding material for concrete and should be removed (see Table 1). Coarse-grained soils include sand and larger aggregate particles, with sand sizes typically ranging from 0.003 to 0.08 inches in diameter and aggregates as large as 1½ inches. Fine-graded soils include clay and silt. Clay and silt particle sizes can be less than 0.00004 inches in diameter, are cohesive in nature, and are much more affected by water. MacDonald adds that clays and silts have much more surface area than coarse-grained materials and can be chemically active. When you mix coarse-grained aggregates into clay and silt, the ability to support loads can be greatly increased.
The ideal material for compaction and consolidation purposes is well graded—aggregates mixed with sands and silts. With the right moisture content, maximum compaction can be achieved.
Moisture content plays a vital role in the effort to achieve maximum density. Very dry soil doesn't compact well; capillary tension causes individual soil grains to bond together in clumps which cannot be readily broken down or compacted to high density. Very wet soil doesn't compact well either because water pushes particles apart making it impossible to achieve maximum density. Maximum consolidation is possible when just the right amount of water for a given weight of material is present. The trick is to know what the right amount is. So before a project begins, project soil conditions are tested to determine the optimal moisture content needed to attain maximum density. Each soil type on a site has its own characteristic optimum moisture content and field control of these moisture contents are essential to allow for proper compaction of fill.
When the amount of support for a slab is determined, the types of soil and densities that meet the project requirements are specified. Using compaction equipment is the next step to achieving the required densities.
Many types of compaction equipment are available. For fine-grained soils, such as clays and silts, kneading rollers or pneumatic tire rollers are most commonly used. For more coarsely grained soils, vibratory drum rollers are more effective. Vibratory compactors are rated by their frequency and amplitude. They use either rotating eccentric shafts or pistons to create compaction forces. Frequency is measured by vibrations per minute and amplitude refers to the distance that a compactor moves up and down—the force applied to the material.
There are several types of compactors available, from small walk-behind units to large ride-on self-propelled machines. They consolidate by impact, vibration, or kneading movements. Different soils require different equipment (see Table 2). Following are the basic types of equipment available.
Rammers. Also referred to as “jumping jacks,” these tools look like an engine mounted on top of a rectangular steel foot. They work by delivering high-impact forces with smaller frequencies, usually between 500 to 700 blows per minute. Rammers are perfect for confined areas and often used in trenches. They are especially good for densifying fine-grain soils such as clay and silt. They perform all three types of compaction: impact, vibration, and kneading.
Vibratory plates. These are perhaps the most popular compactor. They deliver low amplitude and high frequency by spinning eccentric weights at high speed, transferring the force to a flat plate that travels in a forward motion along the ground. Frequencies can range from 2500 to more than 6000 vibrations per minute, and are best suited for compacting granular soils.
Reversible vibratory plates. By using two opposing eccentric weights on a shaft, these compactors make a smooth transition from a forward to a backward motion. You also can make them stand in one position to increase the compaction of soft spots. Because of the dual weights, the compacting force is greater than standard vibratory plates. Of all compactors, they are the most able to treat all soil types.
Rolling compactors. There are many types on the market: walk-behind, ride-on, smooth drums or drums with cleats or “sheep foots” mounted on them, vibratory, and nonvibratory. Achieving the highest productivity rates of all compactors, they are best for asphalt applications and for compacting sand and clay. Drums with cleats are ideal for compacting trenches and can be operated remotely to reduce operator fatigue due to the vibration of the machine. Units with cleats also provide a kneading action.
Rubber tire. Used for large projects such as road building, these compactors consist of 7 to 11 pneumatic tires on a heavily weighted frame. Weights typically range from 10 to 35 tons. Compaction is accomplished by kneading and exerting pressure on the soil.
One mistake often made in fill placement and compaction has to do with the thickness of each lift for proper compaction to occur. Some contractors think they can compact any thickness by compacting the top of it. But this isn't the case and over time serious problems can result. Jim Niehoff, chief engineer for Professional Service Industries, Thornton, Colo., says there are recommended lifts for each type of material and also for each type of compactor (see Table 3). “Typically clay soil should be compacted in 8-inch lifts or less, and sandy soil shouldn't exceed 12 inches. In addition, the top 6 inches of material must have a higher level of compaction than the soil below it.” He adds that when doing trench backfills, lifts shouldn't exceed 6 inches, compacted to the same level as surrounding soil.
There are time-honored ways that contractors use to estimate how well soil is compacted. One method is to “proof roll” an area with a six-wheel dump truck loaded with aggregate or soil, or by using a loaded ready-mix truck. MacDonald says that if you see cracking in the soil, “bow waves” in front of the tires, or rutting deeper than 1 inch, more grade stabilization work is necessary. He adds that another way to estimate the level of compaction is to drive a steel pin into the ground with a hammer. If the stake is hard to drive, chances are that the compaction is good.
Most testing companies frequently use nuclear density meters to measure compaction. The test is easy to perform, takes approximately 5 minutes to complete, can measure approximately 2 feet below the surface, and reports the percentage of compaction. This makes it easy to compare with job specifications that usually stipulate minimum percentages of compaction.
As stated earlier, compaction has much to do with the moisture content of the soil. MacDonald says that one way to determine if there is too much water is to roll some soil between your hands. If you can create a thread ¼ inch in diameter and about 3 inches long, the soil won't offer good support. If you are placing standard ¾-inch coarse aggregate with fines processed with water, take a handful of material and squeeze it. If it just barely sticks together, the moisture level is about right.
Concrete contractors typically are responsible for the top layer of grade plus maintaining elevation. Scott Tarr, a forensic engineer and partner of Concrete Engineering Specialists, Dover, N.H., says that rutting caused by tires from ready-mix trucks, grading equipment, and laser screeds can be the cause of cracking in floors. “The flatness of the bottom of a slab is important,” he says. “I like to see concrete contractors bring skidsteer loaders with box graders guided by lasers on the jobsite because they can take care of ruts as they occur.” After regrading areas, they should be recompacted. He adds that placing 3 or 4 inches of compactable stone grade provides a stable work platform.
It's easy to blame failures on the concrete itself. But some concrete problems are the result of poor soil preparation and control of the grade that the concrete rests on. Well-prepared flat ground surfaces, uniformly compacted to the specified densities will properly support the loads imposed on the concrete above. In addition, by controlling water content in the ground over time, soils won't shrink or expand.