|
Frequently
Asked Questions
I
had a new concrete driveway put in at my house last November.
Is it ok to use calcium chloride to control ice?
No. Deicing chemicals
should never be used on any concrete, especially new or recently placed
concrete. For this first
winter, we would suggest using clean sand for traction.
Concrete placed late in the fall doesn’t have adequate time to
gain strength and durability before exposure to freezing and thawing.
Curing
and sealing are essential to the long-term durability of your concrete.
Curing is a process performed on freshly placed concrete to keep
moisture in. Sealer is
applied to older concrete to keep moisture out.
Insist that your concrete contractor does both!
top
Can
you tell me more about “curing” concrete?
In my opinion, curing is the most misunderstood part of the art known as
concrete finishing. Curing
is the process of maintaining moisture in concrete following placement.
As long as moisture is present, concrete will continue to develop
strength and durability.
Wet curing requires you to keep the concrete constantly wet for a
minimum of seven days. This
is most commonly done with wet burlap that is kept saturated throughout.
Applying a membrane-forming curing compound is another method and
a little less demanding than covering with wet burlap.
The
quality of concrete can be severely damaged by improper or inadequate
curing even though the concrete was properly proportioned, mixed and
placed. Concrete allowed to
dry without curing will never reach its designed strength or provide the
necessary freeze-thaw resistance needed to withstand our winter climate.
top
Can
I pour concrete in the winter?
Concrete can be placed year round and most ready-mix producers are open
through the winter to service industrial and pre-cast needs as well as
the occasional footer. In
our case, Smith Concrete also sells a good quantity of limestone and
gravel through the winter months.
If
you want to put in a driveway and you have a choice between getting it
done the second week of hunting season or waiting until spring, I would
certainly recommend postponing until April to place any exterior
concrete. Concrete placed
late in the season (after October 15) may take up to six months to
properly cure and develop the durability it needs to resist freeze-thaw
cycles. It takes 28
“curing days” (a day averaging 70 degrees Fahrenheit) for concrete
to reach its designed strength.
Be
prepared to spend a lot more money for concrete construction in the
winter months. Protecting
the ground from freezing prior to placement and then covering the
concrete after placement are critical to the success of the product.
Also, expect to pay as much as $10 more per yard during the
winter months. This
additional charge covers the cost for hot water, concrete accelerator
and even an extra half bag of cement to speed the setting of the
concrete.
top
I was planning
spring construction of a new driveway and had heard about putting fibers
in the concrete. Do fibers
work?
Fiber
reinforcement is very popular because synthetic fibers are affordable,
easy to use and effective in reducing and controlling cracks.
Fibers are an alternate system of nonstructural, secondary
reinforcement. That’s a
mouthful, but many homeowners mistakenly want to replace structural
reinforcement (rebar) with fibers.
On the contrary, fibers are a replacement for wire mesh.
The benefit of using fiber versus wire is the three-dimensional
reinforcement fibers provide as compared to the wire, which most often
ends up on the bottom of the slab.
To be effective in crack control, wire must rest in the top third
of a slab. Fibers are added
into the concrete at the batch plant in dosages of 1.5 pounds per cubic
yard, and are dispersed throughout the entire load of concrete.
Fibers reduce plastic shrinkage cracks are form during the critical
hours of curing and reduce microscopic cracks that in time turn into
bigger cracks.
Economically, fibers cost a little more per square foot than wire
reinforcement. However,
there is no labor cost associated with fibers since they are added at
the concrete batch plant.
top
When
the snow melted, I noticed several spots on my driveway where I can see
the rocks underneath. What
is this?
It sounds to me like you are talking about “scaling” – the loss of
surface mortar exposed to freezing and thawing.
Scaling is most prevalent in new concrete (less than a year old) and we
see more of it following a severe winter.
But, concrete doesn’t have to scale.
The right concrete mix (4000 PSI minimum with good, sound
aggregates and purposely entrained air voids) can withstand even the
harshest winters. But, the
concrete must be placed properly.
- Avoid
too much water in the mix (4-inch maximum slump) because scaling
problems are most often related to high water/cement ratios.
- Wait
until all the bleed water comes to the concrete surface before
finishing.
- Don’t
add water to the surface as a finishing aid.
- Cure
promptly.
- Allow
a month for air-drying before the first exposure to freezing and
thawing. Also, proper
drainage is critical because saturated concrete is more susceptible
to deterioration than dry concrete.
Scaling is normally not a problem with “bad” concrete.
If the concrete were bad, the deterioration would be throughout
the entire slab.
top
What
is the difference between cement and concrete?
That is a great question because our culture often incorrectly uses
those words interchangeably. We’ve
all heard someone say “cement floor” or “cement truck” when they
should have said “concrete floor” or “concrete truck.”
Cement is a dry powder that reacts chemically when mixed with water.
In this process, called hydration, the cement and water form a
paste or glue that, when added to sand and gravel, will bind or adhere
the aggregates together to make concrete.
By itself, cement has no strength.
The best analogy I’ve heard is that cement is to concrete like flour
is to cake.
The old rule of
thumb for making concrete is
3-2-1
(three parts gravel, two parts
sand and one part cement). Then
add enough water to make the mix workable.
The amount of water used will impact the strength and durability
of the concrete. This
3-2-1
system is still practical
today and with the advent and improvement of chemical admixtures, it is
now possible to economically produce concrete which is stronger and more
durable than that produced 20 years ago.
top
Why
does concrete crack?
Concrete, like other construction materials, contracts and expands with
changes in moisture and temperature, and deflects depending on load and
support conditions. Cracks
can occur when these movements are not compensated for in design and
construction.
Concrete is at its greatest volume when it is being placed.
Although one of the main ingredients in concrete is water, only a
small amount of water is needed to hydrate the cement particles.
The rest of the water is called “water of convenience.”
This water of convenience makes the concrete placement possible.
However, this extra water must either bleed from the concrete or
evaporate out. As the
concrete dries, it shrinks, causing cracks.
Cracks occur for many reasons: Lack
of isolation and contraction joints; improper sub-grade preparation; use
of high slump concrete; poor finishing practices and inadequate or no
curing.
All concrete has a tendency to crack and it is not possible to produce
crack-free concrete. Most
random cracks, although unsightly, rarely affect the integrity or the
life of the concrete.
top
Explain
“plastic shrinkage” cracking.
Plastic shrinkage cracking occurs while concrete is still in a plastic,
or workable, condition rather than in a hardened state.
Thus, plastic shrinkage cracks develop when the concrete is very
new.
Plastic shrinkage cracks are very disturbing to those not familiar with
concrete because they appear so quickly and are so unsightly, but in
reality, they are the least harmful of all cracks.
I must also emphasize that plastic shrinkage cracks are not the
result of bad concrete. These
cracks are most often weather related and can be minimized by proper
finishing.
Concrete cracks because it undergoes a volume change as it dries.
Plastic shrinkage cracks occur when the surface of the concrete
dries out before the rest of the slab.
This happens on dry, windy days when there is little or no
humidity and the rate of evaporation of surface moisture exceeds the
rate at which it is being replaced by bleed water.
This causes shrinkage of the surface.
Most concrete producers can predict the days where concrete finishers
may experience plastic shrinkage crack problems.
We like to say that if the weather is beautiful outside (warm and
windy with low humidity), then any exterior concrete being placed is at
risk for plastic shrinkage cracks and precautions must be taken.
Those precautions include building windbreaks, using evaporation
retarders, dampening the sub-grade and curing promptly and properly.
top
Explain
“drying shrinkage” cracking.
Newly placed concrete has the largest volume that it will ever have.
As it dries, it shrinks. Shrinkage
tendency is increased by excessive drying and/or significant drop in
temperature that can lead to random cracking if steps are not taken to
control the location of the cracks.
I have heard many concrete finishers “guarantee” that the concrete
they have placed will crack. In
essence, that is the truth. However,
the art of finishing is to control where those cracks occur by properly
placing control joints.
When we cut joints in concrete we are creating areas of weakness where
we want the concrete to crack. Spacing
of these joints and depth of the cut are both important factors.
A good rule of thumb for joint spacing is 30 times the thickness.
Using this rule, in a five-inch thick slab, joints should be 12
to 13 feet apart. Joints
should be cut ¼ the depth of the slab.
In the case of the five-inch thick slab, joints should be cut
1-1/4 inches deep.
If you want to eliminate random drying cracking, there are no exceptions
to these rules. Garage and
basement floors are just as susceptible to random cracking as driveways
and sidewalks. If your
contractor does not cut joints in your garage floor, it will crack.
Structurally, that crack won’t hurt anything, but it will be
unsightly.
top
What
are isolation joints?
Control joints are constructed (either sawed or tooled) to create planes
of weakness in the concrete so that cracks will occur at the desired
location. Remember the rule
of thumb for control joint spacing is 30 times the thickness of the
slab. Another rule is that
joints should be cut one quarter of the depth of the slab.
Keep in mind that we’re trying to make the concrete crack in
the joint and proper depth of the cut helps insure this.
Isolation joints are used to separate or isolate slabs from other parts
of the structure such as walls, footings or columns and driveways and
patios from sidewalks, garage floors, light poles or any other
obstruction. Isolation
joints permit movement of the slab and help minimize cracking caused
when movement is restrained.
Random cracks will often occur off of floor drains or posts.
Ideally, isolation joints would be employed in these situations.
If you feel it is not practical to use isolation joints, be aware
that these are areas prone to cracking.
Another area where random cracking is likely to occur is when a
sidewalk takes off at an unusual angle.
You may have to revise your joint spacing plan to accommodate
areas susceptible to cracking.
Remember that cracks rarely affect the structural integrity of your
concrete, but they are unsightly.
top
I
was thinking about using colored concrete on a patio.
Do you have any recommendations?
Please call Smith Concrete and we can recommend several concrete
finishers with colored concrete experience.
To my knowledge, there are two ways to color concrete.
1. Integral coloring
is where the tint is added to the concrete at the batch plant.
2. Dry-shake
coloring is where tint is sprinkled onto the concrete surface and
troweled in as part of the finishing process.
Integral coloring is very expensive.
Certain colors such as black, red or brown will double the price
of a yard of concrete. Other
colors, like blue or green, can be four or five times as expensive.
Dry-shake coloring is a little less expensive but more labor
intensive for the finisher. The
only drawback to surface coloring is that if a blemish occurs or a rock
pops out, the concrete underneath is the traditional gray.
My best advice is to hire a finisher and a concrete producer experienced
with color concrete. Colored
concrete is much more sensitive than “normal” concrete.
Every load must be batched consistently or you risk having a
patio of different shades.
top
I
saw a brochure on stamped concrete and thought I might like to build a
driveway using that technique. Do
you have any suggestions?
Please call Smith Concrete and we can suggest several local concrete
finishers whose expertise is stamped concrete.
I hope you realize that you will pay considerably more for this
process, but in my mind it is certainly worth it.
From a concrete producer’s viewpoint, there is not much difference
between delivering a load of normal gray concrete or a load that will be
stamped and end up looking like beautiful red brick.
Some of the more experienced finishers my alter the amount of
gravel or sand to best suit their needs.
Stamped
concrete does a wonderful job of hiding flaws often found in everyday
concrete. Cracks are
sometimes not as noticeable (even though they are still there) and
surface blemishes or aggregate popouts aren’t as visible.
Concrete is a building material that has a primary function of
getting you out of the mud. Stamped
concrete is an architectural product designed to be aesthetically
pleasing as well as functional.
I love stamped concrete and believe it is worth the cost.
top
My
recently placed driveway has light and dark areas.
Why?
The cause might be in the mix, the finishing, or the curing.
Regardless of the reason, time, wear and weather normally lessen
color differences.
Excessive bleeding (often caused by extra water) produces a lighter
surface while low water/cement ratios darken a surface.
Two adjacent slabs placed a day apart may look different because
they were placed at different slumps.
A main culprit of spots is the use of calcium chloride.
Calcium-based concrete accelerators tend to darken the surface
while bagged calcium chloride – if not completely dissolved before
being introduced into the concrete – will produce dark spots.
Early troweling on wet surfaces lightens color while extended hard
troweling darkens a surface, especially if the concrete contains calcium
chloride. Finally, curing
darkens color, but, more importantly, improves durability.
top
My
sidewalk is cracking and chipping.
Three pads need replaced. Should
I hire someone to fix it or can I do it myself?
How much can I expect to spend?
You can do it yourself or you can hire it out depending upon you level
of ability and your budget. Personally,
I would recommend that you call a local concrete contractor to
professionally do the work. Placing
concrete is hard work and it can become even more challenging as the
days get hotter. Remember,
concrete is a perishable product and needs to be placed within 90
minutes of batching.
As far as cost is concerned, 4000-PSI concrete (which we recommend for
all exterior concrete) costs in the range of $70 per yard depending upon
delivery location. Concrete
finishers normally charge by the square foot to form, place and finish
concrete. Obviously, you
can do it cheaper if you do the work yourself, but can you do it better?
Why is your concrete cracking and chipping?
Is it old or was there a problem the first time it was done?
These are questions a concrete contractor can answer.
From a concrete producer’s standpoint, we suggest using fiber
reinforcement in all exterior concrete and please don’t forget to cure
and seal the concrete.
top
I’ve heard people refer to chemicals in concrete.
Why would you put chemicals in concrete?
Admixtures are natural or manufactured chemicals which are added to the
concrete before or after mixing. Admixtures
are used to give special properties to plastic or hardened concrete.
Air entrainment is the most common admixture.
Air-entraining agents are liquid chemicals added during mixing to
produce microscopic bubbles in the concrete.
These bubbles improve the concrete’s durability and increase
its resistance to damage from freezing and thawing.
All exterior concrete should be ordered “with air.”
Water reducers are another common admixture found in most everyday
concrete. Water reducers
are used to achieve a higher slump with the same water content.
Excess water is very damaging to concrete.
Water reducers provide workability without the harmful effects of
extra water.
A high-range water reducer, called a superplasticizer, is the concrete
industry’s answer to sliced bread.
Superplasticizers reduce the water content of a given mix up to
25 percent, which increases strength.
A normal dosage of superplasticizer will turn a three-inch slump
into an eight-inch slump, providing more workability and pumpability.
In 60 to 90 minutes, the concrete will return to its original
slump. Superplasticizers
are expensive, but they are worth every penny.
They can prevent a variety of sins caused by an overused water
valve.
Other frequently ordered admixtures are retarders (used in hot water)
and accelerators (used in cold weather).
Please contact Smith Concrete for recommended dosages of these
chemicals.
top
What
are crazing cracks?
Crazing cracks are sometimes referred to as map or pattern cracks
because the cracking resembles a road map.
Crazing is the development of a network of fine random cracks on
the surface of concrete caused by shrinkage of the surface layer.
To prevent crazing, start curing the concrete as soon as possible.
Curing retains the moisture required for the proper reaction of
cement with water.
Use moderate slump concrete. If
you need additional workability, request water-reducing admixtures.
Never sprinkle or trowel dry cement on the surface of plastic concrete
to absorb bleed water. Do
not sprinkle water on the slab to make finishing easier.
Do not perform any finishing operation while bleed water is
present on the concrete surface.
When high evaporation rates are possible, lightly dampen the sub-grade
prior to placement to prevent it from absorbing too much water from the
concrete.
Crazing cracks develop at an early age and are most readily visible when
the surface is wet. They
are particularly conspicuous when concrete contains calcium chloride –
an additive used to accelerate the set of the concrete.
Crazing cracks to not affect the structural integrity of the
concrete nor do they affect long-term durability.
However, they may cause concern to someone who has not seen this
situation before.
top
How
do I know if I’m ordering the right amount of concrete?
This
is a great question and one that is mathematically easy to answer.
However, in the real world of construction, someone always comes
up a yard short and the agony begins for the concrete dispatcher who is
short on available trucks as well as the finishing crew that has six
guys standing around.
To
determine how much concrete you need, simply multiply length x width
(square footage) x depth (cubic footage).
A 20 x 20 slab, five inches thick equals 400 square feet or 168
cubic feet (20 x 20 x .42). Since
there are 27 cubic feet in a cubic yard of concrete, divide 168 by 27 to
get 6.2 cubic yards.
This
is where things get interesting. Most
commercial contractors add anywhere between five and 10 percent for
grade variations and then order their concrete.
The difference between five and 10 percent on the above example
is the difference between 6.5 yards and 7 yards.
Many
who have come up the proverbial “yard short” claim their pour was
exactly on grade. That is
often a point of contention after the fact.
But the truth is that a ¼ inch variance in the above example
(5-1/4 inch depth rather than 5-inch) translates into more than a ¼
cubic yard error in ordering.
Please
be as precise as possible when measuring and realize how small grade
variations can add up, especially on larger pours.
Also, do your concrete supplier and your work crew a favor by
ordering enough concrete to complete the job.
top
Can
you tell me what I have to do to get a driveway ready for concrete to be
poured?
The first decision you must make is whether you will do the job yourself
or hire a concrete contractor. Many
contractors can be hired to do nothing but finish concrete.
This leaves all the preparation up to you.
Or, you may ask the contractor to give you a price to form, place
and finish the concrete.
In preparing for concrete, make sure the sub-grade is free of all soft
spots. If you leave a wet
area, you’ll notice it when your concrete is placed and it could cause
you grief down the road. Make
sure your drainage is adequate. You
don’t want to realize you have a problem when puddles form on your new
concrete.
Develop a control joint plan. Remember
that joints should be cut (both vertically and horizontally)
approximately 30 times the thickness of the slab.
Buy curing compound and have a sprayer on hand to apply the curing.
If you choose to use wet burlap, have the burlap ready so that
the concrete can be covered immediately after finishing.
Watch the weather. As an
insurance policy you should have plenty of plastic available to cover
your concrete in case a storm comes up before the concrete is finished.
There is much more to building a driveway than ordering concrete.
Preparation and planning are critical.
Placing concrete is hard work, especially as temperatures rise.
top
Summer
is approaching. What
precautions need to be taken this time of year when planning to place
concrete?
Hot weather must be taken into account when planning concrete projects
because of the potential effects on fresh and recently placed concrete.
High temperatures alone cause increased water demand which, in
turn, results in lower concrete strength.
Higher temperatures can also cause loss of entrained air in
concrete, which could affect long-term durability.
High temperatures, high wind velocity and low relative humidity can
affect fresh concrete in two ways.
One, the high rate of evaporation may cause early plastic
shrinkage shrinkage. Two,
the evaporation rate can remove surface water necessary for hydration
unless proper curing methods are employed.
If you decide to tackle your project in the heat of the summer, make
sure you have adequate manpower to quickly place, finish and cure the
concrete. Limit the
addition of water at the jobsite. Excess
water is always detrimental to concrete.
Talk to your ready-mix supplier about fly ash and/or retarder.
Both products slow the heat of hydration and are beneficial
summertime additives.
Begin final finishing operation as soon as the water sheen has left the
surface. Then, start curing
as soon as finishing is completed.
Continue curing for at least three days.
Wet burlap is the best way but you must keep the burlap
continually wet. You
don’t want periods of dry followed by periods of wet.
Liquid membrane curing compounds are also effective at keeping
moisture in the slab. White
pigmented curing compounds reflect heat away from the concrete surface.
top
What
is flowable fill?
Flowable fill – also referred to as controlled density fill, low
strength mortar or lean concrete backfill – is a self-compacting, low
strength material with a flowable consistency that is used as an
alternative to compacted granular fill.
A good flowable fill mix is made up of cement, large quantities of fly
ash, sand and enough water to make the mix self-leveling.
Mixtures with entrained air in excess of 20 percent are also used
to keep strengths low.
Flowable fill is an economical alternative to compacted granular fill,
considering the savings in labor costs, equipment and time.
Since it does not need manual compaction, trench width or the
size of excavation is significantly reduced.
Placing flowable fill does not require people to enter an
excavation, which can be a major safety concern.
If the material is going to be excavated at a later date by mechanical
means (a backhoe), then the ultimate strength of the flowable fill
should be kept below 200-PSI. For
manual excavation, the ultimate strength should be less than 50-PSI.
Compressive strengths of 50 to 100-PSI provide an
allowable bearing capacity similar to well-compacted soil.
Please call Smith Concrete if you believe you have a situation where
flowable fill might be a practical solution.
top
What
is the most common concrete complain?
Random cracking is by far the most common concrete “problem” as
perceived by the do-it-yourself concrete finisher.
Earlier, we identified various types of cracks and reasons those
cracks occurred. In all
those discussions, I finished by saying that most cracks will not affect
the structural integrity of the concrete but that they are unsightly.
Most veteran concrete finishers and commercial contractors understand
concrete and why and where it will crack.
The inexperienced finisher often doesn’t understand why
concrete cracks (it shrinks) or how to control where it cracks
(contraction joints). The
resulting random cracking then raises unfounded questions about the
quality of the concrete. Ready-mix
producers can batch the best cubic yard of concrete ever produced, but
if the proper precautions are not taken, it will crack and often in the
most undesirable location.
A much more severe “problem” than cracking is scaling.
Scaling involves the actual loss of surface mortar and is most
prevalent with new concrete in conjunction with a bad winter.
Scaling can be caused by lack of curing, exposure to de-icing
chemicals, excessive water in the mix or the use of a non air-entrained
mix in concrete which will experience freezing and thawing.
top
What
is water/cement ratio and what impact does it have on concrete?
Water/cement ratio refers to the weight of water divided by the weight
of cement in any given concrete mix.
For example, in an Ohio Department of Transportation Class C
concrete mix, there is 600 pounds of cement and 300 pounds of water (36
gallons) for a water/cement ratio of .50 (300 divided by 600).
It is important to note that when we talk about weight of water that we
are talking about total water in a mix and not just batch water.
Moisture is present in both gravel and sand and must be
compensated for when discussing W/C ratio and when batching a yard of
concrete.
In my opinion, water/cement ratio is the most critical factor in
controlling strength and durability of concrete.
Most experts state that all concrete exposed to freezing and
thawing should have a water/cement ratio under .50; many commercial
specifications say it should be under .45; and most bridge decks are
specified under .40. But
what do these numbers mean to you and your driveway?
During the summer months, the water valve on a concrete mixer becomes
very enticing. As the
temperatures increase, it becomes more difficult to place four-inch
slump concrete. But, each
addition of water to a properly batched yard of concrete increases the
water/cement ratio and decreases the long-term durability of your
concrete.
The solution, instead of the water valve, is chemical admixtures.
Mid-range water reducers and superplasticizers provide
workability without affecting water/cement ratios.
top
I
was helping a neighbor pour a driveway and it seemed like he added a lot
of water to the load of concrete. What
will this do to the concrete?
It is a ready-mix concrete supplier’s responsibility to deliver
concrete with the appropriate slump to a job.
In most cases, mixes are designed to be placed at a four-inch
slump. The problem lies in
the fact that there is a big difference between four or five-inch slump
concrete and concrete that is self-leveling.
To answer your question, I’m going to assume that the concrete
supplier delivered the proper mix to the job with the right slump.
Then, the addition of water starts a snowball effect of bad
things that can happen to the concrete.
- One
gallon of water per yard will increase your slump approximately one
inch.
- One
gallon of water per yard will reduce the compressive strength by as
much as 200 PSI.
- One
gallon of water per yard will waste the effect of 25 pounds of
cement.
- One
gallon of water per yard will increase the shrinkage potential by
10%. (Shrinkage is the cause of most cracking.)
- One
gallon of water per yard will decrease the freeze-thaw resistance by
20%. (Freeze-thaw resistance keeps the concrete from scaling.)
If more workability is needed, consider using superplasticizer.
Also, remember concrete was not designed to be self-leveling.
Finishing concrete of the proper slump is hard work.
top
What
can I do to insure that the new concrete driveway I’m having poured
provides me with years of trouble-free service?
I would suggest that you check references and inspect the work of the
person you hire to place your driveway.
If you call your local ready-mix concrete supplier, they will be
glad to provide you with names of reputable concrete finishers who
perform quality work.
Other than hiring the right person to do the job, and equally important
part of your insurance policy is curing the concrete.
Concrete allowed to dry before curing will never achieve its
designed strength or durability.
Curing is essential, but it is absolutely critical during the heat of
the summer. We would
suggest either covering with wet burlap for three days minimum (seven
days is better) or spraying a liquid membrane curing compound on the
freshly-placed concrete. If
you choose to cure using wet burlap, we would then suggest air drying
the concrete for a month before applying a good concrete sealer.
Depending upon the curing compound you choose, we would suggest
the same procedure. Smith
Concrete sells a product that doubles as a curing compound/sealer and
keeps you from having to make a second application.
We also recommend resealing your driveway every other year and avoid
using salt or other de-icing chemicals during the winter months.
top
You’ve
mentioned 4000-PSI concrete several times.
What do you mean by PSI?
The compressive strength of concrete is expressed in pounds per square
inch (also referred to as PSI). Strengths
are measured using six-inch diameter and 12-inch high test cylinders.
Strength is normally tested at 28 days, but some jobs might
require a 24-hour or 7-day strength measurement.
4000-PSI means the concrete in discussion can withstand in excess of
4000 pounds of lead per square inch without crushing.
Most general-use concrete has a compressive strength between 3000 PSI
and 5000 PSI. High strength
concrete is defined as concrete with a compressive strength of at least
6000 PSI. Compressive
strengths of 20,000 PSI have been achieved in some high-rise building
applications.
In this area, most footers are poured using 3000 PSI concrete.
We recommend that any concrete exposed to freezing and thawing be
at least 4000 PSI. Smith
Concrete’s “perfect” driveway mix would be 4500 PSI with fiber
reinforcement and a mid-range water reducer to provide slump followed by
the application of a proper curing compound.
Today most concrete is ordered and delivered using a strength
designation rather than specifying a five or six-bag mix.
top
I
had some concrete poured last summer and I now notice several small pits
on the concrete surface. Have
you seen this before?
It sounds to me as if you are describing what we call a “popout.”
Popouts are caused by pieces of porous rock, such as chert,
lignite or shale, that absorb enough water after concrete placement to
be saturated. When the
saturated aggregate particle freezes, it fractures due to internal
pressure causing the mortar surface to rupture.
Popouts normally range in size from about ¼ inch to several inches in
diameter, and usually a portion of the aggregate that caused the popout
remains at the bottom of the hole.
Popouts tend to very random in size and location.
Scaling, on the other hand, is the general loss of surface mortar
due to freezing and thawing. It
is normally more widespread and troublesome.
The ready-mix suppliers in this area use state approved aggregates in
their respective concrete mixes. However,
even aggregates meeting applicable standards may contain enough
deleterious materials to cause some popouts.
From a concrete finishing standpoint, two things can be done to keep
popouts to a minimum. Place
the concrete as dry as possible and cure promptly.
Since these materials are lighter than normal-weight aggregates,
they tend to float to the surface.
This popout potential is made worse when the concrete is placed
excessively wet.
Popouts
are considered a cosmetic detraction and generally do not affect the
service life of the concrete.
top
|