SUBGRADES AND SUBBASES FOR CONCRETE SLABS
A well-compacted subgrade keeps construction out of the mud and provides uniform slab support. Lippincott & Jacobs
What lies below your concrete slab is critical to a successful job. This is no different than the foundation for a
building. A slab on ground (or slab on grade) by definition is not intended to be self-supporting. The "soil support
system" beneath it is there to support the slab.
WHAT IS A SUBBASE/SUBGRADE?
The terminology used for soil support systems, unfortunately, is not completely consistent, so let's follow the
American Concrete Institute's definitions, starting from the bottom:
•
Subgrade—this is the native soil (or improved soil), usually compacted
•
Subbase—this is a layer of gravel on top of the subgrade
•
Base (or base course)—this is the layer of material on top of the subbase and directly under the slab
A compacted subbase keeps workers out of the mud. Energy Efficient Building Network
The only layer that is absolutely required is the subgrade—you have to have ground to place a slab on ground on
top of. If the natural soil is relatively clean and compactable, then you can put a slab right on top of it without
any extra layers. The problems with that are that the soil may not drain well and it can be muddy during
construction if it gets wet, it may not compact well, and it can be difficult to get it flat and to the proper grade.
Typically, the top of the subgrade should be graded to within plus or minus 1.5 inches of the specified elevation.
A subbase and base course, or both, provide several good things. The thicker the subbase, the more load the
slab can support, so if there are going to be heavy loads on the slab—like trucks or fork lifts—the designer will
probably specify a thick subbase. A subbase can also act as a capillary break, preventing water from wicking up
from the groundwater table and into the slab. The subbase material is usually a reasonably low cost gravel
without a lot of fines.
Recycled crushed concrete is an excellent source for subbase material. The Concrete Producer
A base course on top of the subbase makes it easier to get to the proper grade and to get it flat. If you use some
sort of a choker course of finer material on the top of the subbase, it will support your people and equipment
during concrete placement. It will also keep your slab thickness uniform, which will save money on concrete—
the most expensive part of the system. A flat base course will also allow the slab to slide easily as it shrinks,
reducing restraint and the risk of cracks as the concrete contracts after placement (drying shrinkage).
The entire subbase and base system should be at least 4 inches thick—thicker if the engineer feels it is needed
for proper support. The base course material, according to ACI 302, "Concrete Floor and Slab Construction,"
should be "compactible, easy to trim, granular fill that will remain stable and support construction traffic." ACI
302 recommends material with 10 to 30% fines (passing the No. 100 sieve) with no clay, silt, or organic
materials. Manufactured aggregate works well—crushed recycled concrete aggregate can also work well.
Tolerances on the base course are +0 inches and minus 1 inch for Class 1 through 3 floors (typical low tolerance
floors) or +0 inches and minus ¾ inches for higher tolerance floors.
WHAT ABOUT THE SOIL?
A sand base course is easy to compress, but can rut easily during construction.
Free Reformed Church of Southern River
The weight of the slab and anything on top of it is going to eventually be supported by the soil. When a building
site is excavated, usually the soil gets moved around—high spots are cut and low spots are filled. Everything
then should get compacted before you place the concrete, subbase and base.
The type of soil determines what needs to happen before placing a slab. There are three basic types of soil and
here's what you should know about each:
•
Organic soils, what you might call top soils, are great in your garden, but awful beneath a slab. Organic
soils can't be compacted and must be removed and replaced with a compressible fill.
•
Granular soils are sand or gravel. You can easily see the individual particles and water drains quite easily
from them. Just like at the beach when you make a sand castle, if you take a wet handful of granular soil
and make a ball, as soon as it dries it will crumble. Granular soils have the highest bearing strength and
compact easily.
•
Cohesive soils are clays. If you take a wet handful, you can roll it into a string just like with modeling
clay. It has a greasy, smooth feeling between your fingers and individual particles are too small to see.
Cohesive soils are often difficult to compact and take on a rock-hard consistency when dry, but they
have a lower bearing strength than granular soils. Some clays expand when wet and shrink when dry
making them particularly difficult as subgrade materials. The best way to counter this problem is first
with good compaction, then to not let them get wet (by providing drainage). But as the ground beneath
the slab dries over time, it will shrink and the slab will sink. That's not a big problem as long as the slab is
isolated from the footings and columns and from any pipes that penetrate the slab so that it can settle a
little and settle evenly. Often, with expansive clays, the best approach is a structural slab that doesn't
bear on the soil at all or a post-tensioned slab that floats atop the soil but doesn't rely on it for structural
support.
Post tensioning is often the best solution for a slab on poor soil. J.C. Escamilla's Concrete
Most natural soil, of course, is a mixture and so is characterized by the type of material that is predominant. The
amount of weight the soil can support before it fails is its bearing capacity, typically given in pounds per square
foot. The design, however, is based on the allowable soil pressure, which adds a safety factor to the ultimate
bearing capacity.
Let's look at the weight the subgrade soil will typically need to support. A 6-inch-thick slab weighs about 75
pounds per square foot. According to the International Residential Code, the live load (anything that is not part
of the building itself), varies from about 20 to about 60 pounds per square foot—50 pounds per square foot in a
garage. That gives us 125 pounds per square foot for the soil to support. A clean sandy soil might have an
allowable soil pressure as high as 2000 pounds per square foot. Even a poor soil—silt or soft clay—might have an
allowable soil pressure of 400 pounds per square foot.
We can see then that the allowable soil pressure for a slab is seldom a problem. However, there is a need for
uniform support because if one part of the slab settles more than another, that's when we get bending in the
slab—and potentially cracks and differential settlement. Knowing which areas have been cut and which filled is
important—make sure the fill areas have been well compacted. In fact, any soil that's been disturbed during
excavation must be compacted.
Compacting
Subgrade Compaction
Subgrade compaction is the act of grading, shaping, and compacting the natural subgrade materials prior to
placing an aggregate base or pavement. It mechanically increases the unit volume (density) of the soil or base.
Different soil types have different optimum moisture contents and densities. Sandy soils require lower moisture
contents and can typically achieve higher densities than silts or clays do. Also silts and high-fine clays are more
prone to moisture retention and frost heave making them generally less optimal for performing well as a
pavement subgrade.
Compaction achieves the following:
•
increases load bearing capacity
•
helps reduce future rutting/settling
•
reduces voids which increase susceptibility to moisture changes/freeze thaw
•
help ensure that freeze/thaw movement is uniform.
Subgrade compaction is an act that is sometimes overlooked in private, non-inspected, construction projects but
(depending on the soil type and condition) can be critical in future performance of pavements.
It’s important to note that for subgrade areas that are found to be man-made fill, overly moist, or otherwise
questionable, subgrade compaction may actually make the situation worse by disturbing, consolidating, or
working moisture up to the surface. Soils with higher than normal moisture content will ‘pump’ underneath
tires and tracks of grading equipment, compaction equipment, trucks, and paving equipment. In these cases
staying off of the subgrade and leaving it virgin may be the better option ( if undercutting and replacement with
premium material is not in the budget).
Unsuitable, high-moisture, silty-clay subgrade – pumping and rutting
A common test used in conjunction with subgrade compaction is the Proctor Density Test. This test classifies the
existing material and defines the desired maximum density of a particular soil type and it’s optimum moisture
content. A nuclear density gauge is a piece of testing equipment typically used on-site to check the density of a
sub-base or subgrade. It will also indicate the moisture content.
Most engineers recommend an optimum proctor density range of 95-100% for pavement bases and trench
backfill under pavement. Testing the subgrade for passing compaction is less common than for premium
aggregate bases, but it is still sometimes done. The primary reason testing isn’t done on a subgrade is that the
material typically isn’t uniform either in gradation or composition, so to come up with an accurate proctor
sample is difficult and requires time consuming lab testing from various locations of the subgrade.
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On a small scale subgrade compaction can be done with a simple plate compactor. On a medium scale, subgrade
compaction can be done with a plate tamp attachment for a backhoe or excavator. On a large scale, drum rollers
are utilized. The different types of rollers include: smooth drum, vibratory, pneumatic, and sheepsfoot. A static
application is a non-vibratory pass that reduces disruption of adjacent structures and components vs. vibratory
actions.
On a large scale subgrade compaction is typically done with a small 1-2 man crew equipped with a dozer and a
roller. For sandy or gravely subgrades, a basic roller will often do. For silty and clayey soils, a sheepsfoot roller
or a low amplitude vibratory roller is typically more productive.
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What is the Freeze-Thaw Cycle in Concrete?
Winter, especially farther away from the equator, equals white landscapes and that time
of the year to cozy up to a fireplace. To us humans, winter is just another season. But to
buildings, roads, bridges, and many other types of concrete structures, winter is a time
to suffer the worst type of distress: the freeze-thaw cycle effect.
What is the freeze-thaw cycle and how does it happen?
There are 2 ways this phenomenon occurs in nature. The first is through the repeated
cycle of melting and freezing of water on the natural cracks and grooves of rocks, such
as in mountains, tundra and the like.
The second phenomenon occurs when the surface layer of rock is baked by direct
sunlight, such as in desert regions. This top layer will expand and contract repeatedly,
causing distress and eventually, cracks.
Can’t visualize the end result of years of distress and erosion? Example: think The
Grand Canyon – mighty beautiful, but we want our human-made structures to stand
proud and strong for years.
The Grand Canyon in Arizona, USA, is a great example of the power of water and heat
over time. These forces eroded and created the massive landscapes we know today. In
nature its beautiful, but we want our man-made structures to withstand it all for years.
What are the types of distress concrete structures face due
to weather conditions?
Dry concrete can suffer from both types of distress, either by freezing and thawing of
water or its expansion and contraction under extreme daily temperature variations. Due
to the geographical locations of certain cities like Dubai and Toronto, these types of
distress cannot be avoided. However, concrete can better withstand the effects of both
phenomena with a little help from additives.
What is the freeze-thaw cycle in concrete?
The freeze-thaw phenomenon occurs when concrete is saturated with water and the
temperature drops, freezing the H2O molecules. Since frozen water expands 9% of its
original volume, it causes distress to the concrete structure. Once the warmer months
come by, the H2O molecules melt away and reveal tiny cracks in the surface layer of the
structure. The following winter when those tiny cracks are filled with water once again
and the temperature drops, the H2O molecules expand, making more room for
themselves and causing more distress in the concrete structure.
The continuous freezing and thawing of water causes those tiny cracks to become
larger over time. If not repaired – or prevented by additives in the concrete mixing phase
– will permanently damage the structure.
Above is an infographic summarizing the role of water in the freeze-thaw cycle and how
it affects the concrete surface. The takeaway concept here is understanding that water
at room temperature and frozen water have different volumes. When water freezes, it
occupies 9% more volume and creates microcracks. It melts away after the temperature
rises. The cycle repeats itself, with water being able to penetrate the microcracks.
Why does the freeze-thaw effect damage concrete?
Distress and damage to concrete from this phenomenon happens when concrete is
heavily saturated with water, which happens when more than 90% of its pores are filled
with H2O. Added to the fact that frozen water occupies 9% more volume than water at
room temperature and there is limited space for the volume increase within dry
concrete, the freezing of water causes microcracks. This damage begins from the first
cycle of the freezing and thawing of water and with continuous exposure to winter
seasons, will result in repeated loss of the concrete surface.
In a nutshell, this phenomenon is a process of distress that concrete experiences
through the cyclical freezing and melting of water that has seeped into it. Over time and
without proper care, this cycle can cause total cracking and permanent damage to the
concrete structure.