Dusting concrete surfaces, or surfaces that powder or chalk easily,
are caused by a weak wearing surface.
The weakness of the surface can be caused by a number of things,
- Finishing the surface while bleedwater is still present. When you
trowel or work this water back into the top surface, you are weakening
the top ¼" by greatly increasing the water / cement ratio, resulting
in a surface much weaker than the remainder of the slab.
- Placing concrete over a non-absorbing subgrade or polyfilm, which
reduces normal absorption by the subgrade, thereby increasing
bleedwater moving to the surface.
- Improper or no curing of the concrete.
- Inadequate ventilation in closed in areas. Carbon dioxide from
heaters, gasoline engines, or mixer engines can cause a chemical
reaction known as carbonation, which greatly reduces the strength and
hardness of the concrete surface.
- Inadequate protection from rain, snow or winds that can dry out the
To help minimize dusting, use concrete with a moderate slump of 5 inches
or less. If you need a higher slump, use a superplasticizer. Avoid
sprinkling dry cement on the surface to dry up the bleedwater. If you need
to remove bleedwater, drag a hose or something similar across the slab.
NEVER perform finishing operations with water present on the surface.
Provide proper curing by using a liquid membrane curing compound or by
covering the slab with WET CLEAN burlap. When placing concrete in cold
weather, use heated concrete and an accelerator to speed up the setting of
To minimize dusting, apply a floor hardener, or treat the surface with
boiled linseed oil. Be aware that these methods can change the appearance
of the concrete. If the damage is severe, the surface can be wet-grinded
off, and a concrete topping course can be applied. If this proves
impractical, a floor covering or carpet may be a less expensive option.
Scaling of the concrete surface is usually caused by freezing and thawing
(freeze/thaw cycles). The amount of scaling and the degree of the scaling
damage can vary. Light scaling does not expose the coarse aggregate
(gravel or stone). Moderate scaling exposes the coarse aggregate, and may
involve the loss of up to 3/8" of the concrete surface. In severe cases of
scaling, more surface has been lost and the aggregate is clearly exposed.
Some causes of scaling are as follows :
- Use of non-air entrained concrete for applications exposed to
freeze / thaw cycles.
- Applying calcium, de-icing salt, ammonium nitrate or other harsh
de-icers can cause scaling as well as causing severe chemical attack
on the surface of the concrete.
- Finishing concrete while bleedwater is on the surface is a common
cause of scaling. When you finish or work that bleedwater back into
the top ¼" of the concrete, you greatly increase the water / cement
ratio, which causes a very weak surface.
- Inadequate or no curing.
To help prevent scaling, always order air-entrained concrete, and place
that concrete at a moderate slump of 5" or less. If a wetter mix is
desired, use superplasticizer. DO NOT use De-icers such as salt or calcium
chloride. Properly cure the concrete with a liquid membrane curing
compound. This maintains moisture in the concrete, which is needed to
hydrate the cement, which enables the concrete to reach its full strength
potential. If the concrete is allowed to dry out in a few days, the
strength gain from hydration is stopped, resulting in low strength
concrete. DO NOT perform any finishing operations with water on the
Protect the concrete from the harsh winter environment by applying a
sealer specifically made for use on concrete. Boiled Linseed Oil can be
used as well, but tends to darken the surface of the concrete. These
sealing treatments should be applied in late summer.
REPAIRING SCALED SURFACES
Scaled surfaces can be repaired by resurfacing with either a Portland
cement mixture, or a latex modified concrete. Before applying these
products, you must remove all loose and scaling surfaces to assure the new
topping will adhere properly. Also remove any oil or paint on the surface.
Concrete surface cracks are usually caused by improper design and
construction practices, such as :
- Omission of isolation joints and control joints, and / or improper
- The use of high slump concrete or addition of water on the jobsite.
- Improper subgrade preparation.
- Improper finishing.
- Inadequate or no curing.
Minimizing Surface Cracking
All concrete has a tendency to crack and it's not possible to consistently
produce completely crack-free concrete. It is possible to reduce the
occurrences of surface cracking by following these safeguards :
- Properly prepare the subgrade, removing all topsoil and soft spots.
The material under the concrete should be well compacted by rolling,
tamping or vibrating. Smooth, level subgrades help prevent concrete
- Polyethylene vapor barriers increase bleeding of the concrete,
greatly increasing the chance of cracking. The vapor barrier should be
covered with one to two inches of damp sand to minimize the
bleedwater, which will help reduce the chance of cracking.
- Use concrete with a moderate slump of 2" to 4". Try to place
concrete quickly enough to reduce the chance of adding water to the
mix (retempering). Be sure to specify air entrained concrete for all
concrete placed outdoors, subject to freeze-thaw cycles.
- DO NOT perform finishing operations with water present on the
surface. If the surface is drying too quickly due to evaporation, try
to erect wind breaks to keep the wind from drying out the surface too
fast. In extreme conditions, consider the use of an Evaporation
- Start curing as soon as possible. Spray the surface with a curing
compound when finishing operations are complete. A second application
of curing compound the next day is a good idea.
- Install control joints by sawing, forming or tooling a groove about
¼ the thickness of the slab, no further apart than about 25 to 30
times the thickness. (ie: 4" thick slab = control joints about 9 to 10
feet apart.) Isolation joints should be provided when slab is against
other structures. These joints should be full depth of the slab.
Expansion Joint is usually used for isolation joints.
Concrete yield is the volume of ready mixed concrete produced from a known
quantity of ingredients. One cubic yard equals 27 cubic feet. Many factors
can cause the Actual "yield" of the concrete to change. The moisture
content of the raw materials, for instance, can cause a considerable
change in the actual yield of the concrete. It takes more "pounds" of wet
sand and gravel to create the same volume as the drier sand and gravel.
That's why the concrete producer checks the moisture content on a regular
basis, usually more than once a day.
Another factor can be the "air content" of the concrete. A typical air
entrained mix will contain about 6 percent air, meaning 6 percent of the
volume in that cubic yard of concrete is actually just that, AIR. As the
concrete sits in the truck during delivery and unloading, the air content
tends to drop. To adjust for this, the concrete producer will batch the
concrete with more than 6 percent air at the batch plant, trying to
predict the actual air loss for that load. If the truck sits on the job
for an extended time, the actual air content may be only 4 percent,
meaning 2 percent of the concrete has been lost.
How to help prevent yield discrepancies
- Measure the formwork accurately. A small difference in measurement
can make a big difference in actual yield.
- Check for over excavation. The subgrade must be very level to
accurately measure its depth, and if the depth is not uniform, a yield
problem can occur. These yield problems can go either way. There are
many yield problems that involve the customer having too much
concrete, wasting money.
- Striking off the surface, or "straight-edging", can greatly
influence the amount of concrete needed for a slab. A bowed straight
edge, depending on which way it is bowed, can either leave a hump of
concrete in the middle of the slab, or a valley. It may not be visable
to the eye, but just ¼" variation can add up on a large slab.
- Always allow a little for waste. Even though the truck seems empty,
a small amount will usually stick to the mixer drum. When added to the
amount spilled, and stuck to boots, tools, etc., It can add up fast.
Most authorities on concrete will recommend 4% to 7% extra. Concrete
producers realize that they ask for a great deal of trust, when
dealing in cubic yards of concrete. Just keep in mind, that the vast
majority of concrete suppliers would not jeopardize their reputation,
and their relationships with their customers, to "save a buck" and
short the customer.
The two major reasons for low compressive strength tests are improper
handling and testing, and reduced concrete quality to due an error in
production, or the addition of too much water at the jobsite. High air
content can lead to low concrete strengths, stressing the need for
accurate test results from the jobsite, when tests are being performed.
Collect all test reports and carefully analyze the results before taking
action. Look at the slump, air content, air and concrete temperature.
Check how many days the test cylinders were left at the jobsite, and any
noted cylinder defects. Some of the most common causes of mis-handling are
as follows :
Under American Concrete Institute standards (ACI), concrete is acceptable
if no one test is lower than the specified strength by more than 500 psi
and the average of three consecutive tests equals at least the specified
strength. If a test falls below by more than 500 psi, an investigation
should be made to determine the problem. Always distribute copies of the
test results to the concrete producer, as he may see a problem before it
- Cylinders left in the field too long without curing. The cylinders
must be taken to be cured in 24 hours.
- Frozen cylinders. Test cylinders on the jobsite must be maintained
between 60 degrees and 80 degrees. Falling out of that temperature
range will severely affect the cylinder strength.
- Impact during transport. Always take great care to handle the
cylinders properly. Any bouncing around could damage the cylinder,
even if it looks okay. Be Careful!
- Improper cylinder capping. The test cylinder must be capped on both
ends prior to being placed in the compression machine. If these caps
are not properly installed, the strength data from the specimen could
be completely useless.
- Improper testing of the compressive strength. When breaking test
cylinders, the proper rate of compression must be used. Using too much
pressure, too quickly, can lead to false readings. Once again, useless
Why measure in-place strength?
Testing of in-place concrete strength may be needed when standard cylinder
strengths are low, and are not attributable to faulty test practices.
In-place testing can be done by rebound hammer, also known as "Swiss
hammer", probe penetration resistance testing, and core testing.
Rebound hammer testing
Test the strength of the in-place concrete using a rebound hammer,
checking both the areas in question, and areas where the strength is not
in question. Sometimes these results will show similar readings from both
areas, avoiding the need for further testing. This rebound hammer method
should be performed by someone experienced in the procedure.
Probe penetration resistance testing
This method is not often used, as it can be expensive, and difficult to
perform. It consists of driving special probes into the concrete, checking
its resistance. A strength curve can be developed for the concrete under
Core strength testing
A common type of "final" testing, cores of the in-place concrete are
drilled out of the slab, and used as strength specimens for compressive
strength. These cores can also be examined for approximate cement content,
air content, water/cement ratio, foreign substances or impurities in the
concrete, as well as deficiencies in the placement and finishing of the
concrete. A minimum of 3 cores should be taken. Be aware that drilled
cores test LOWER than properly made test cylinders. ACI Building code
states that core strength is considered adequate if the cores average at
least 85 percent of the specified strength, with none below 75 percent.
Cora testing can also prove expensive, and should be used only as a last
Curing is the maintaining of a satisfactory moisture content and
temperature in concrete. Curing begins after finishing so that the
concrete may develop the desired strength and hardness, leading to greater
durability. Without an adequate supply of moisture, the cement in the
concrete will not "hydrate", to form a quality finished product.
Temperature is an important factor in curing concrete, since the rate of
hydration is temperature dependent. For concrete exposed to weather,
humidity and wind conditions also play an important part, contributing to
the moisture loss from the concrete. A windy day with low humidity is a
Reasons to cure
- Predictable strength gain. Concrete not properly cured can lose as
much as 50 percent of its potential strength. Placing concrete in hot
weather may result in high strengths at early ages, but the ultimate
strength will be lower, perhaps much lower.
- Improved durability. Well cured concrete has a harder surface, and
is more watertight.
- Better appearance and serviceability. If the concrete is allowed to
dry out prematurely, it will have a soft surface, with poor resistance
to wear and abrasion. Proper curing reduces craze cracking, dusting
- Liquid membrane-forming compounds. These should be applied per
manufacturers instructions, after the bleed water has disappeared from
the surface. Usually about one hour after finishing operations have
- Plastic sheeting or waterproof paper. This method can lead to a
marred surface, and should be done carefully. The edges of the sheets
should be sealed so the moisture will be trapped.
- Burlap or cotton matting can be used, but must be clean, and kept
CONSTANTLY WET. If allowed to dry out, the burlap will actually absorb
water FROM the concrete, actually being worse than not using it at
- Sprinkling on a CONTINUOUS BASIS is suitable, as long as air
temperatures are well above freezing. The soaking must be continuous,
as repetitive drying and wetting can damage the concrete.
The importance of proper curing cannot be overstated. You may get lucky.
You may leave your concrete uncured and never have a problem. At the price
of your installed concrete, don't bet on it! Spend a little money, and a
little time. It can save you big money later.
Placing concrete during periods of hot weather requires special attention
to detail. Hot weather conditions can produce a rapid rate of evaporation
of the moisture from the concretes surface. It can also greatly accelerate
the concretes setting time. High humidity can reduce the effects of high
temperature on concrete. The higher the humidity, the slower the
evaporation of water from the surface.
High temperature causes increased water demand in the concrete, resulting
in lower strengths in higher temperatures. The same mix that gave you 4700
psi in April, may only come up to 4200 psi in August. Depending on the
conditions, the difference in strength can be even greater than that. As
the hot temperatures cause faster setting times, you must be prepared to
place the concrete faster than in cooler weather. Slow placement in hot
weather means loss of slump, which leads to added water, which results in
lower strength. Shrinkage cracking can result in hot weather, especially
if there are windy conditions. Low humidity will add to the problem.
If concrete placed on a hot day is subjected to a cool night, thermal
cracking can occur.
General rules for hot weather concreting
The bottom line is "Don't bite off more than you can chew". Many people
who thought they could handle the big pours in high temperatures, were
very sorry. Don't be afraid to cut down on the amount you pour. Get
smaller loads. And don't hesitate to specify set-retarders in your
concrete. It could be the difference between a good job and doing it over
- Adequate manpower. The concrete must be placed quickly.
- When ordering the concrete, ask for a set-retarder to be added to
the mix. This is a low cost additive that extends the set time of the
concrete. In hot conditions, even the 30 year professional uses
- Limit the addition of water at the jobsite. When the truck arrives,
have him adjust the slump of the concrete to your requirements, and
then try to avoid adding water again. The utilization of more
manpower, and ordering smaller, more manageable loads can be a big
- Don't place slabs directly on polyethylene sheeting. Always cover
the sheeting with one or two inches of damp sand.
- Finish the concrete as soon as the sheen of the bleed water has
left. Start curing after the finishing is completed.
- Before placing concrete, moisten the subgrade. Avoid standing water,
but get it good and damp. This will prevent the subgrade from soaking
up moisture from the concrete, allowing the concrete to use that
moisture to hydrate and gain strength.
- Keep all test cylinders in shaded areas, and cap or cover with
plastic to prevent them from drying out. Keep them between 60 and 80
Concrete blisters are hollow, low-profile bumps on the concrete surface,
typically from the size of a dime, to a couple inches in diameter. A dense
troweled skin of mortar about 1/8" thick covers a void, which moves under
the surface while troweling. There are basically two theories as to the
cause of these "blisters". Some think incidental air voids rise and are
trapped under the dense surface skin produced by troweling. Others believe
that bleed water rises and collects to form a void under the surface.
Eventually this water is absorbed into the concrete, leaving a void.
Blisters are more likely to form if :
To help avoid blisters, do not seal the surface before the bleed water has
escaped and evaporated. Avoid dry shakes on air entrained concrete. In
cold weather, use heated and accelerated concrete to promote even setting
throughout the slab. DO NOT place concrete directly onto polyethylene
sheeting. If blisters are forming, try to either flatten the trowel
blades, or tear the surface with a wooden float, and delay the finishing
process as long as possible.
- The subgrade is cool, causing the concrete at the bottom to set
- Entrained air is used, or is higher than normal.
- A dry shake is used, particularly over air entrained concrete.
- The slab is thick.
- The concrete was placed directly on polyethylene.
- Excessive use of a jitterbug or vibrating screed.
Finishing makes concrete attractive and serviceable. The final texture,
hardness, and joint pattern on slabs, floors, sidewalks, patios, and
driveways depends on the concrete's end use. Industrial floors usually
need to be level and smooth, while an office buildings floors may be
covered with carpet, and don't need to be as exact. Exterior slabs must be
sloped to carry away water, and must provide a texture which will not be
slippery when wet. Having the proper manpower and equipment on hand, as
well as properly timing the operations is critical.
Guidelines to placing concrete
- Select the proper concrete mix for your application. Consult with
your concrete supplier if you're not sure.
- Whenever possible, place concrete directly from the chute, or use
wheelbarrows or buggies. If pumping the concrete, alert your concrete
supplier, as this may require a different mix. If unsure, have your
pump operator contact the concrete supplier to work out the details.
- Spread the concrete with a square end shovel or concrete rake. Using
a typical garden rake will cause the stone in the mix to segregate.
- Use a wooden or metal straightedge, or screed, to strike off and
level the concrete. Be sure the straightedge is indeed "straight", or
an uneven surface will result.
Rules to Finish Concrete
- Float the concrete as soon as it has been straightedged. A wood
float, or a metal bull float can be used to further level the surface,
and embed any stone or gravel near the surface. Floating the surface
must be completed before bleed water appears on the surface. Never
float bleed water back into the surface. This water would severely
weaken the surface, and seriously effect the durability of the slab.
- WAIT. You must wait for the bleed water to evaporate from the
surface before starting any other finishing operations.
- Edge the concrete all the way around the perimeter. Proper edging
will result in a more pleasing appearance.
- Apply joints to the slab. Control joints can be installed by using a
jointer, or groover tool. The depth of the joint should be about ¼ the
depth of your slab.
- Trowel the surface if a smooth surface is desired. Troweling would
not be a good idea for driveways or sidewalks, as the surface would be
slippery when wet. Excessive troweling of the surface can result in a
- Texture the surface. If you do not want a smooth troweled surface,
add texture to the slab. You can "broom" the surface, leaving fine
broom marks in the surface. You can expose aggregate on the surface,
add dry-shake color to the surface, or even stamp patterns into your
concrete. The options are almost endless.
- NEVER sprinkle water or cement on the concrete while finishing. This
is a leading cause of scaling and dusting.
Admixtures are natural or manufactured chemicals which are added to
concrete before or during mixing. The most commonly used admixtures are
air-entraining agents, water reducers, retarders and accelerators. The
function of admixtures in concrete is to enhance the durability,
workability or strength characteristics of the mix.
Types of admixtures
- AIR ENTRAINING AGENTS are liquid chemicals added during the
batching process to produce microscopic air bubbles in concrete. These
bubbles greatly improve the concrete's durability and increase its
resistance to damage from freezing and thawing, as well as de-icing
salts. Air entraining agent also improves workability, and reduces
bleed water. Concrete that will be exposed to freeze / thaw cycles
should contain from 4.5% - 7.5% air content.
- WATER REDUCERS are used for two purposes. The first, and most
obvious is to lower the amount of water needed in the mix, thereby
increasing the strength of the mix. The second purpose is to increase
the slump (wetness) of the concrete, while maintaining the original
strength. The average amount of water reduction when using a standard
range water reducer is between 8 and 11 percent. This reduction
increases the potential strength of the concrete considerably.
- RETARDERS are chemicals, which delay the initial set of the concrete
by an hour or more. Retarders are particularly useful in hot weather
to counter the faster set times in hot temperatures. Most retarders
are also water reducers, which enhance the strength of the concrete.
- ACCELERATORS reduce the initial set time of concrete. Accelerators
are mostly used in cold weather, to offset the slower set times in
cold temperatures. It must be pointed out that accelerators used in
their normal dosages ARE NOT ANTI FREEZE. They simply cause the
concrete to set faster than normal, aiding the early strength
development of the concrete. There are a few accelerators on the
market, which can act as an anti-freeze, but the high dosages needed
usually make the cost prohibitive. The common types of accelerators
are calcium chloride and accelerators that contain no chlorides,
- HIGH RANGE WATER REDUCER, also known as "superplasticizer", acts in
a similar way to normal water reducer except the amount of water
reduction is much greater. Reduction between 10% and 25% can be
accomplished using superplasticizer, which can greatly increase the
strength of the concrete, or can increase the slump (wetness) of the
concrete by a considerable amount. When added to a mix at a 3 inch
slump, the superplasticizer can increase its slump to 8 to 9 inches,
making the concrete much more pumpable and workable.
Curling is the distortion of a concrete slab into a curved shape by upward
or downward bending of the edges. This distortion can lift the edges of
the slab from the base, leaving an unsupported edge or corner which can
crack under heavy loads.
Causes of Curling
Typically, curling is caused by shrinkage or contraction of the top
surface, relative to the bottom. When one surface changes size more than
the other, the slab tends to warp at the edges. This curling is most
noticeable at the sides and corners. Most curling is the result of
moisture and temperature gradients in the slab.
How to minimize curling
- Use the lowest practical slump and avoid adding extra water at the
- Use the largest practical maximum size aggregate and/or the highest
practical coarse aggregate content to minimize drying shrinkage.
- Take precautions to avoid excessive bleed water.
- Do not place the concrete directly on polyethylene sheeting. Place
one or two inches of damp sand over the polyethylene.
- Avoid using mixes with high cement content. Dense concrete will
produce larger top to bottom moisture differentials, increasing
- Cure the concrete thoroughly, including joints and edges. If you use
a membrane curing compound, apply it twice in two applications at
right angles to each other.
- Use a thicker slab. Thin slabs tend to curl much more.
Surface discoloration is the non-uniformity of color on the surface of a
single concrete placement. It may take the form of dark blotches or
mottled discoloration on the slabs surface. Some of the main factors
influencing discoloration are the use of calcium chloride in the mix,
variation in cement alkali content, admixtures, hard troweled surfaces,
inadequate curing, incorrect finishing procedures, and changes in the
How to prevent discoloration
- Don't use high alkali cement.
- Limit the use of Calcium Chloride when color is important.
- Eliminate trowel burning of the concrete surface.
- Curing with polyethylene can cause discoloration.
- Improper or uneven curing can cause discoloration.
- Moisten subgrade, properly cure the surface, and protect surface
from excessive drying due to wind and sun.
- Color changes from one day to the next, is common. The amount of
sunshine, humidity, temperature, subbase, and many other factors can
affect the color.
- Use the same finishing techniques and timing on all placements.
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