avoid cracks in concrete slabs-on-grade
TRANSCRIPT
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AVOID CRACKS IN CONCRETE SLABS-ON-GRADE
By Harvey Haynes-Consulting Concrete Engineer
Part 1 of 2 Types of Cracks and Their Causes
SOIL MOVEMENT
HeaveExpansive clay soils swell or heave with an increase in moisture content. Cracks in concrete
slabs caused by heave can be identified by vertical offsets, cracks running parallel to an exteriorwall, or cracks exhibiting an X-shaped pattern in a small room. It is difficult to distinguish cracks
caused by heave from those caused by settlement.
SettlementSlabs that have been overloaded or have weak soil support will settle due to soil consolidation.
The strength of the slab greatly depends on the strength of the soil support. A slab four inchesthick can be quite strong on firm soil, but may crack easily on weak soil. Cracks due tosettlement can appear as half-circle cracks at the edge of slabs, cracks raveling along the edges,
slabs breaking into small pieces (sections of one or two feet per side), or diagonal cracks across
corners of slabs.
THERMAL BEHAVIOR OF CONCRETE
Seasonal Temperature ChangeIf concrete is cast in the summer, it can experience a temperature decrease of 100 F by the
middle of winter. This decrease in temperature can cause a slab 100 feet long to contract about
3/4 of an inch. This contraction movement may crack the slab. Contraction joints are placed inslabs to encourage the cracks to occur at the joint locations. If the slab is cast in winter, then the
concrete could experience a 100 F increase in temperature by the summer. In this case, the slabwould expand about 3/4 of an inch. Expansion joints are required to allow the slab to expand, or
the slab may buckle.
Daily Temperature ChangeDaily temperature change causes a varying temperature through the thickness of the slab. The
sun heats the top surface, which causes the concrete near the top to expand, and the slab developsa hump shape where the middle is higher than the edges. If the top surface is colder than the
bottom, the slab will have a curled shape where the edges are higher than the middle.
Heat of HydrationHeat of hydration is heat internally generated by the concrete during the chemical process of
cement hydration. Hydration is the mechanism by which cement sets and then gains strength.
During the first night after concrete is cast, a common crack that occurs is caused by the
combination of heat of hydration and warm ambient temperatures on the day the concrete is cast.Typically concrete is cast in the morning, and by the afternoon a hot sun raises the temperature
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of the concrete, especially near the top surface. Internally, concrete generates a considerable
amount of heat due to heat of hydration. By sunset of the first day, the concrete can be quite
warm, easily 120 F. Cool evening temperatures initially reduce the temperature at the topsurface, and this cooler concrete contracts. At this young age the concrete is quite weak, and the
contraction movement can crack the concrete. By the next morning, the slab is found cracked.
For this reason, contraction joints (also called control joints) must be installed the same day thatthe concrete is cast.
SHRINKAGE BEHAVIOR OF CONCRETE
Crazing CracksThese cracks appear at the very top surface layer of the slab where a thin layer of cement pastehas lost water too rapidly and cracked. These cracks are very fine and shallow.
Plastic Shrinkage CracksPlastic shrinkage cracks develop when too much water evaporates while the concrete is fresh, or
plastic in consistency. These cracks have a distinct form. They are quite wide at the surface, theirdepth into the slab is usually limited to about one to two inches, they range in length from about
six inches to five feet, they usually develop parallel to one another, and they don't run to theedges of the slab.
Drying Shrinkage CracksThese are the typical shrinkage cracks, which develop after the concrete is hard. They can appear
randomly across slabs or have a uniform pattern. These cracks also extend from re-entry corners.
If a cube four inches on a side were to dry over several months, each side would decrease in
width by about 0.002 inch. The entire cube will decrease in volume. When cubes are laid end to
end for a distance of 20 feet, the change in length would be about 1/8 inch. In a slab this gapwould be the crack width.
Why does the cube get smaller? Technically, there are two contributing reasons why moisture
loss causes shrinkage, and the explanations relate to two different void sizes within the concrete.
Concrete of residential home quality contains about 20 percent void volume. Part of this volume
is microscopic pores called capillary voids, which were created by the original mixing water.Smaller voids, called gel voids, exist in the concrete within the hydrated cement particles. When
water evaporates from the capillary voids, capillary forces develop which place the water in
tension. Therefore the solids are placed in compression, and shrinkage occurs. When water
within the gel voids evaporates, the hydrated cement particles become smaller and additionalshrinkage occurs.
If ambient conditions are at 100 percent relative humidity a concrete slab will not shrink. At 40percent relative humidity, and many months of drying time, a slab 100 feet long can shrink up to
3/4 inch. Let rain soak the slab for a couple of days and the concrete will swell such that a
permanent shortening of 1/8 to 1/4 inch exists.
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Uniform drying of concrete does not occur in slabs-on-grade because only one face is exposed to
evaporation. A moisture gradient exists through the slab thickness where the top is drier than the
bottom, so shrinkage will be greater at the top. This condition results in the slab having a cuppedor curled shape. This shape can be explained by picturing the 4-inch cube, where the top of the
cube is dry and shrinks by 0.002 inch, while the bottom is moist and does not shrink. The cube
would have a wedged shape. Place these cubes tightly together, end-to-end, for a distance of 20feet and the slab will have a curled shape.
It is difficult to observe the slight curvature of curling, but measurements have been made onhighway pavement. For a slab 15 feet in length, measurements in the morning showed curling
where the edges of the slab picked up about 1/8 inch. In the afternoon, the slab essentially was
flat. The effect of the sun caused the top surface to expand, which countered the shrinkage, and
the slab leveled out. When night returned, the slab cooled and shrinkage again created the curledshape of the slab.
There are situations where curling is restrained, such as a slab joined to a perimeter footing.
When curling is restrained, the stresses on the top surface of the slab are greater than if the slabwas free to curl, so cracks occur sooner. Because curling cannot be prevented, drying shrinkage
cracks in concrete slabs can only be minimized, not prevented.
Part 2 of 2
Recommendations to Avoid Cracks
SOIL MOVEMENT
HeaveThe critical factor in preventing expansive clay soils from causing problems with heave is to be
certain that the clay soil is damp (above its optimum moisture content) before concrete is cast.Watering the clay may take days to get it damp. It is advisable to have a soils engineer provide a
letter stating that the clay is at proper moisture content.
Slabs, such as garage floors or walkways next to a house, should be free to float on top of the soil
as opposed to being tied into the house footing. Place isolation joint material between the slab
and building.
Use reinforcing bars to distribute and limit the width of cracks and prevent vertical offsets. As a
guide, use #3 bars at 18-inch spacing each way in 4-inch thick slabs, and #4 bars at 20-inchspacing in 5-inch slabs. Do not use welded wire mesh of size 6x6-W1.4xW1.4 (also known by its
old designation of 6x6-10x10). Mesh of size 6x6-W2.9xW2.9 is marginal, but acceptable.
SettlementCompacted base rock four inches deep will provide a uniform, firm support for the slab. If base
rock is not used, be sure that the subgrade (native soil) can provide good support even when thesubgrade is wet. Use a 5-inch thick slab, which is 50% stronger than a 4-inch thick slab in
flexural strength.
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THERMAL BEHAVIOR OF CONCRETE
Seasonal Temperature ChangeTo minimize cracks, different types of joints are installed in slabs.
Contraction or Control Joints: These joints are installed by placing plastic inserts or grooves intothe fresh concrete to a depth of one-fourth the thickness of the slab. Saw cuts are also used, but
should be made the same day that the concrete is cast. Spacing and layout are discussed later in
the drying shrinkage section.
Expansion or Isolation Joints: These joints are of a compressible material, such as fiberboard,installed the full depth of the slab. For expansion joints, spacing is between 50 to 100 feet.
Construction Joints: These joints mark where new concrete abuts existing concrete. The joint can
function as a contraction joint when detailed using bond breaker on one concrete face, or whenusing keyways or smooth dowels greased on one side. Constructions joints will not function as
contraction joints when design details call for roughening one concrete face or havingreinforcing bars tie together the new and existing concrete.
Daily Temperature Change and Heat of HydrationTo minimize cracks that appear the morning after concrete is cast, install saw cut contractionjoints on the same day that concrete is cast. Grooves and inserts also are good joints because they
are installed while the concrete is fresh.
SHRINKAGE BEHAVIOR OF CONCRETE
Crazing Cracks
These cracks are prevented by proper finishing methods; use light tamping, do not overwork thesurface, and do not add dry cement to the surface to absorb bleed water. If necessary, bleed water
can be vacuumed or dragged off.
Plastic Shrinkage CracksWindy or hot weather is a sign of danger. Keep the temperature of the concrete as low aspossible by having transit mix trucks stand-by in the shade, or cool the mixing drum by spraying
water on the outside surface. Have a sufficient crew size available for rapid placement and
finishing. In extreme hot weather, avoid exposing young concrete to the hot part of the day bystarting to cast concrete in the late afternoon or early evening.
Delay evaporation of bleed water by spraying fog mist across the work area; or better, use anevaporation control coating sprayed on the fresh concrete after bullfloating. Store this material
on the job so it is available on windy or hot days.
Drying Shrinkage CracksMix Design: The more water used in making concrete, the greater the amount of shrinkage.
Hence, do not add water beyond the amount necessary for proper slump.
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Use as large an aggregate size as possible. Concrete with 1-1/2 inch maximum size aggregate
will shrink less than concrete with 3/4-inch maximum size, and both of these will shrink
considerably less than pea gravel concrete, which uses a 3/8-inch maximum size aggregate.
Vapor Retarders: Vapor retarders are used primarily to stop ground moisture from moving up
through the concrete slab. Acceptable vapor retarders are:
a. Clean 3/4-inch drain rock, 4 to 6 inches deep
b. Plastic sheeting covered by 1 to 2 inches of clean sandc. Plastic sheeting of minimum 10-mil thickness
The preferred system is (a) and (b) together. System (c) can result in excessive cracks in the slab
if a high slump concrete is used. Therefore, use system (c) only with a low slump concrete (3-
inch maximum).
Contraction Joints: Use saw cuts, inserts, or grooves to install contraction joints. Make certain
that saw cuts are installed on the same day the concrete is cast.
Spacing of contraction joints is given in the following table:
Contraction Joint Spacing
For Slab Thickness Of:
4 inches 5 inches 6 inches
Interior Slab 16 feet 20 feet 24 feet
Exterior Slab 10 feet 13 feet 15 feet
Any slab cast in an open environment is an exterior slab, which means that tilt-up slabs areexterior slabs. Patios and walkways are usually 3.5 inches thick and they should have a
contraction joint spacing of around 6 feet. If pea gravel concrete is used, then reduce therecommended spacing in the table by 3 feet. The simplest and surest method to minimize
most cracks in slabs is to follow this guidance on contraction joint spacing.
The layout of joints is best when square sections are made. When rectangular sections are made,the length should not exceed 1.5 times the width. Re-entry corners must have contraction joints.
Reinforcement: Welded wire mesh of size 6x6-W1.4xW1.4 (or old designation of 6x6-10x10) is
the most common type of reinforcement in slabs-on-grade, and it is essentially useless. If welded
wire mesh is used, then use 6x6-W2.9xW2.9; however, this is still a small amount of steel andwill do little in crack control.
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Slabs-on-grade do not have a code requirement for being reinforced; hence, they can be of
unreinforced concrete with cracks controlled by a close spacing of contraction joints.
Cracks can occur even if reinforcing steel is used. The reinforcement is to control crack width
and spacing. A recommended amount of steel (which only provides partial effectiveness in
controlling crack widths) is the amount given for shrinkage and temperature reinforcement forstructural concrete in the American Concrete Institute Building Code Requirements for
Structural Concrete, ACI 318-95, Section 7.12. This amount is equivalent to #3 bars at 18-inch
spacing in 4-inch thick slabs and #4 bars at 20-inch spacing in 5-inch thick slabs. Contractionjoints are still required, and the spacing can be larger than that for unreinforced slabs. To allow
these joints to properly function, it is advisable to cut every other bar crossing the joints.
To obtain maximum effect in controlling cracks by steel reinforcement, an amount of steel three
times that given above is required. These slabs are considered as continuously reinforced
concrete, and contraction joints can be omitted.
Reinforcing bars are good because they can be chaired. Place the bars in the top half of the slab.For slabs of 5 inches or thicker, locate the bars 2 inches below the top of the slab. Don't allow the
chairs, or dobbies, to settle in the sand cushion layer. The reinforcing bars need to be near the topof the slab to minimize the width of the cracks. Cracks are widest at the top because the greatest
shrinkage occurs at the top.
Steel fiber reinforcement works well to control cracks. The fibers are added to the transit mix
truck in quantities of at least 30 to 50 pounds per cubic yard of concrete. Normal placing and
finishing procedures are used. Contraction joints are required, but at a larger spacing. Steel fibersare highly recommended.
Polypropylene and nylon fibers do not provide any benefit in controlling drying shrinkagecracks, but they do have value in controlling plastic shrinkage cracks. These fibers are added to
the transit mix truck in quantities of about 1 to 1.5 pounds per cubic yard of concrete.
Curing: The objective of curing is to allow concrete to gain strength. To do this, the temperature
must be above 40 F and water must be present within the concrete for the cement to hydrate.
The best curing method is to flood concrete, but the most common method is to spray curingcompound on concrete. This method attempts to contain the mix water inside concrete, and it is
moderately effective. In windy or hot weather, the coverage rate specified by the manufacturer
should be increased by 1.5 times. A rough surface also requires more compound than a smooth
surface.
As long as water is held inside the concrete, drying shrinkage does not occur. Eventually the
concrete decreases in moisture content, and then shrinkage begins. Curing allows concrete togain strength; stronger concrete will crack less than poorly cured or weaker concrete.
SUMMARY OF RECOMMENDATIONS
Soil Movement
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o Expansive clay soil must be damp before concrete is placed.
o The subgrade must provide fairly uniform and firm support for the slab, even when the
subgrade is wet. If not, use base rock.
Thermal Behavior of Concrete
o To accommodate seasonal temperature changes, install expansion joints to prevent slabfrom buckling, and install contraction joints (also called control joints) to minimizerandom cracks due to contraction.
o To avoid cracks that appear the day after casting slabs, install contraction joints on the
same day the concrete is cast.
Shrinkage Behavior of Concrete
o
To avoid crazing cracks, prevent excessive paste on the top surface of fresh concrete bylight tamping, do not overwork the surface, and do not apply dry cement to the topsurface.
o To avoid plastic shrinkage cracks, use an evaporation-control coating on fresh concrete
during windy or hot weather. Also, use polypropylene or nylon fiber reinforcement tominimize plastic shrinkage cracks.
o Do not add mix water to concrete beyond the amount for proper slump.
o Use concrete with 3/4-inch maximum size aggregate or larger. For pea gravel concrete,
use extra-close spacing of contraction joints.
o During hot weather, keep the temperature of the fresh concrete cool while in the transit
mix truck by having truck stand-by in the shade and spraying the outside of the drum
with water.o Space contraction joints properly (see table). This is the simplest and surest method to
minimize most cracks.
o Layout contraction joints for square sections. For rectangular sections, make the length
less than 1.5 times the width.
o Put contraction joints at re-entry corners.
o Unreinforced concrete slabs are acceptable, but be certain that contraction joints are
properly spaced.
o Don't use welded wire mesh of size 6x6-W1.4xW1.4. In its place, use mesh of size 6x6-
W2.9xW2.9 or larger.
o For reinforcement to provide a fair benefit in crack control, use #3 bars at 18-inch
spacing in slabs 4-inches thick, and #4 bars at 20-inch spacing in slabs 5-inches thick.
o Chair reinforcing bars and mesh so they are located in the top half of slabs.
o Use steel reinforcement to control shrinkage cracks; steel fibers are more effective than
bars.
o Spray curing compound on fresh concrete immediately after the final finishing operation.Check to be sure the coverage rate is equal or greater than the manufacturer's
recommendation and 1.5 times the recommendation in windy or hot weather.
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ACKNOWLEDGEMENTS
Sincere thanks to the following individuals for reviewing the tape series: Professor Kumar Mehta
of the University of California at Berkeley, Professor Paulo Monteiro of the University ofCalifornia at Berkeley, Professor Lawrence Kahn of the Georgia Institute of Technology, Mr.
Wayne Ferree of TerraTech Inc., Mr. Ashok Kakade of Concrete Science, Mr. Donald Pearman
of Pearman Construction Inc., and Mr. Andrew Bardakos of R.H. Wehner Construction Co.
REFERENCES FOR FURTHER STUDY
Concrete: Structure, Properties and Materials, by P. Kumar Mehta and Paulo J. M. Monteiro,
Prentice Hall, New Jersey, 2nd Edition, 1993, pp. 548.
Properties of Concrete, by A.M. Neville, John Wiley & Sons Inc., New York, 7th Edition, 1996,
pp. 842.
ACI Committee 224, Control of Cracking in Concrete Structures, Manual of Concrete Practice,
Part 3, American Concrete Institute, Detroit, Michigan, published annually, pp. 42.
ACI Committee 302, Guide for Concrete Floor and Slab Construction, Manual of Concrete
Practice, Part 2, American Concrete Institute, Detroit, Michigan, published annually, pp. 46.
Concrete Floors on Ground, Portland Cement Association, Skokie, Illinois, 1983, pp. 36.
Designing Floor Slabs On Grade, by Boyd C. Ringo and Robert B. Anderson, The Aberdeen
Group, Addision, Illinois, 1992, pp.199.
This is the first of an every-other-month column on slabs, based on the bookDesigning FloorSlabs on Grade by Boyd C. Ringo and Robert B. Anderson. Bob Anderson is in the process of
updating this book and a third edition is due out later this year.
Mary Hurd's introduction to the book starts with seven questions:
How thick should the slab be?
How strong should the concrete be? Is reinforcement needed?
Where should the joints be placed?
Can adding fibers enhance the slab's performance?
When is post-tensioning appropriate?
What can be done to control cracking?
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Although generally these may be considered decisions made by the designer, contractors should
understand the basics of slab design in order to be an involved partner in a project. Design
includes all of the decisions, specifications, and details made and documented beforeconstruction can begin. It is based on both the subgrade support and the concrete material. The
authors regard design as a two-step procedure: thickness selection is done by one of the
recognized design methods, then other features such as joint location and treatment andconstruction tolerances are determined.
This month, we will start at the bottom and look at what information about the supporting soil isneeded.
Introduction. A slab on grade cannot be designed without numerical values that come directly
from knowing what supports the slab. At the very least, a value is needed for the modulus of
subgrade reaction, commonly referred to as k; however, the grade support system is morecomplicated than is indicated by a single value. In addition to k, it is necessary to know the
properties of the underlying soil and available fill material. In other words, to design and
construct a quality slab on grade, one needs to know as much as possible about the grade systemthat supports that slab. The flow chart summarizes an orderly approach to obtaining thisinformation.
Working with a soils specialist. The first consideration of any slab on grade design should bethat of securing adequate geotechnical information. This should put the person responsible for
the floor design into the process at the very beginning of any planning, which must include site
considerations. When alternative sites are being evaluated for a project, soils conditions are often
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a significant economic factor. The floor designer should be able to advise the owner as to what
soils information will be needed. He should do this along with the geotechnical engineer in order
to provide an optimum geotechnical report. Too often the team effort of floor designer andgeotechnical engineer is missing. This can lead either to costly overspending in obtaining soils
information or to unexpected construction overruns due to omissions or errors in initial
information. It must be emphasized that the slab on ground designer should be engaged eitherbefore or at the same time as the geotechnical firm.
Limit risk with insufficient information. The authors have found that in much routine slab ongrade design no soils information is available other than the floor designer's experience. This
experience is occasionally in the jobsite area, but frequently is not within that geographical area.
This situation often leads to relying on what previous experience dictated, such as 6 incheshas always worked or the soil is good. If forced into this situation, the designer mustprotect himself by stating on the construction drawings what assumptions were made inthe design process. The designer should also limit his liability by noting in writing therisks and possible consequences of inadequate soil information. Such steps not only
protect the floor designer and inform the client but often result in the client's favorablereconsideration in providing geotechnical backup.