FREQUENTLY ASKED QUESTIONS ABOUT CONCRETE With the following knowledge and understanding of concrete, along with our experience, our materials and workmanship our Customers and Drpatch will create a wonderful experience.
The base should be evaluated before placement of concrete to minimize cracking and any excessive settlement or sliding. Instability may be the result of excessive moisture within the soil. Certain types of soils are very fine and are unstable with high moisture content. Other soils are very heavy and called fat clay, which holds excessive amounts of water and can become unstable over time. This same fat clay can become very dry and shrink in dry periods or expand in heavy moisture times. The different types of soils react differently in temperature and water changes over the weather seasons. Instability within the existing subgrade may be from excessively cracked old surfaces allowing water to be trapped below, water from pour surrounding drainage conditions over time, or even excessive in ground sprinkler water. The soil types react to adverse conditions and changes. The soil base should be firm and stable under construction equipment, i.e. no marking (rutting) from the equipment while work is in progress. Once a firm base is recognized, a layer of rock should be placed, leveled and compacted to establish a uniform and level load distribution base for anticipated uses of the concrete. A base that moves and is excessively wet and moves without much pressure on it will most certainly do the same in the short or long term under new concrete. Excessive cracking, with cracking from a control joint to an edge or another joint may be the result of a pour subgrade. Cracking and lifting of individual sections of any slab between joints also may be the result of a poor subgrade. Uncontrolled excessive water is the main enemy and likely the cause of excessive concrete cracking and failure. Once severe cracking occurs water permeates through the cracks into the subgrade, making the condition worse and replacement more costly and difficult. Most unstable subgrade conditions are improved by removing some minimal amount of additional soil in depth, then replacing the removed zone with additional rock as a new stable base. A reasonable removal would be less than 6 inches. Any probed unstable depth beyond 6 inches maybe cost excessive with no stability improvement in deeper removal. In situations such as this, adding reinforcement within in the concrete may be warranted. (See Reinforcement Reference)
DOES CONCRETE CRACK?Yes: But DrPatch places Control Joints for anticipated cracking.
Control joints allow for movements caused by ambient and ground temperature changes and the expanding and contracting of the concrete in extreme climate changes. Since concrete will crack you want to mostly control where it does and how it will be seen (not seen) There are two types of control joints that everyone sees in all larger masses of concrete. A trowel joint (line) which is a tool of various width and design edge that is used while the concrete is plastic (wet) The other is by saw cutting a joint (line) after the concrete is set, generally less than 24 hrs after new concrete placement. These joints are usually no more than 8-12 feet apart, relative to no more than 2-3 times the thickness of the concrete slab. The joint depth is generally 25% of concrete thickness. The joints in effect create a planned weaker spot in the overall mass of concrete. With any combination of movement, freezing, thawing, subgrade changes, the cracking is more controlled and unseen. Look in an expansion joint of older concrete and you will likely see a crack of some size in width and length. Check the same control joint from an extreme cold or hot time of the year and you will likely see a noticeable size change in the crack within the joint. Cracking from one joint across the slab to another joint does occur. This is not generally due to inferior concrete old or new. Many conditions play a part in random or excessive cracking. (See Subgrade Considerations) Bridges and overpasses have large metal control joints within the concrete decking and the concrete for the same application. Without these large metal joints in bridges, the concrete would snap and fall apart.
WILL THE INSTALLATION OF REINFORCEMENT (WIRE MESH OR REBAR) WITHIN THE CONCRETE PREVENT CRACKING?NO: Reinforcement will not prevent or even minimize cracking, but it may help hold the cracks together if they do occur.
Wire Mesh or Rebar is used as a structural support where heavy loads or excessive traffic is expected. Typically concrete used around homes does not require the application and associated additional costs of any reinforcement
There are various types of Reinforcement such as prefabricated wire mesh (sheets or rolls) or ½-inch (#4) steel rebar, placed in a grid pattern. In these applications, blocks should be used under the reinforcement, placed near centered of the design thickness of the concrete poured. It is impossible to maintain a consistent center placement of the reinforcement as the concrete is poured, without the use of support blocks. Without the supports, the reinforcement is stepped on by the workers during concrete placement. The result then is the reinforcement ends up at random depths within the concrete or likely on the ground below the concrete. Reinforcement not located within the near center of the design thickness of the concrete placed, has no structural benefit. The use of rebar is applicable in a rehab project to hold 2 masses of concrete together. Rebar maybe used to attach a new patio to a house foundation. In this application the intent is not structural, but is intended to hold the patio to the house, where separation is not desired.
In some situations the use of reinforcement maybe applicable where a stable subgrade is not feasible (see Subgrade Considerations)
WILL SYNTHETIC FIBERS MIXED WITHIN THE CONCRETE PREVENT CRACKING?NO: Synthetic fibers (fiber mesh) will not prevent all cracking but it will minimize cracking during the early stages of curing.
Synthetic fibers (fiber mesh) are added during the mixing process of each load of concrete at the point of origin. Concrete with all the basic components of cement, sand, rock and water make a paste and go through a chemical reaction going from a plastic (fluid) to solid. The percentage of each of these components especially water is critical to the early curing and long term performance. The correct percentage of water increases or decreases the heat and hydration reaction during the initial critical curing time of 3-7 days. Plastic Shrinkage cracks may occur within 24-96 hours or longer as the result of the chemical reaction and hydration concrete goes through. All cross sections of a large mass of concrete is not equal in strength especially during the early curing period. The use of fiber mesh equaling thousands of particles throughout the concrete in all directions block the natural micro cracking taking place as water evaporates from the heated chemical reaction. This intersecting blocking effect of the micro cracking reduces the unsightly visual cracking that also may reduce the concretes long term performance. All concrete goes through the plastic shrinkage effect in the early stages of placement. With the use of fiber mesh the large cracking that tries to form in the early stages of curing without mesh can virtually be eliminated. Some visual hair line plastic shrinkage cracking may occur soon after concrete installation, which is not uncommon. The impact of these hair line plastic shrinkage cracks related to the long term performance is minimal since the cracking generally does not extend the full thickness of the concrete placed. Just as in consideration of reinforcement, a stable subgrade is necessary under any area of concrete.[ ASK DR.PATCH)
WHAT OTHER BASIC GUIDELINES AND CONSIDERATIONS BEYOND THOSE ALREADY NOTED IMPACT THE PERFORMANCE OF NEW AND OLD CONCRETE.
Ground temperature, frozen or extremely hot subgrade; When pouring new concrete, frozen or extremely hot subgrade will likely create thermal shock which will slow drying and extends the plastic state. May prevent the designed curing strength being achieved, does not necessarily cause any short or long term cracking. The chemical hydration process needs to occur within the designed normal range. Single or combined extreme variables in water ratios or ambient and ground temperatures may impact maximum design strength being achieved.
Concrete placed outside; When pouring new concrete, air should be added in the mixing process (air entrainment). Design strength will not be impacted, but without air entrainment cracking likely will occur. Air entrainment allows concrete to go through many freeze-thaw cycles over years. Tiny microscopic air bubbles create frost protection of the water that can penetrate the concrete at any given time. Without the air bubbles to absorb the varying pressures within the concrete, cracking likely will occur as the concrete naturally expands and contracts.
Another cause of outdoor concrete cracking; Water under the slab in extended low ambient temperatures becomes frozen in depth, causing frost heave. The heave can be several inches in rise as the soil below freezes. Daily high temperatures well below 32 degrees for many days within the Missouri region can result in the frost zone within the ground extending as deep as 32 inches. As the zone thickens the heave becomes more severe. Driveways, patios, sidewalks, any outdoor concrete can be raised several inches beyond the normal elevation. Some subsidence (resettling) may happen upon complete thaw out, unfortunately cracking usually occurs over the course of the freeze to thaw cycle. The Midwest does experience yearly temperature ranges from below 0 to over 100 degrees, with some extremes being over a short period of only weeks.
SHOULD CONCRETE BE SEALED?Yes: All concrete should be sealed, especially Colored or Exposed Aggregate.
Sealing of new concrete should take place in less than 30 days after installation. Sealing within a few days before any staining from contaminants takes place provides improved resistance to weather, water, stains, and abrasives. Sealers also offer resistance to rain, sun, freezing temperatures, petroleum products, deicing salts, and debris such as leaves laying on the surface. Concrete sealers make clean up easier and also keeps the coloring looking fresh, over time. For older concrete, cleaning by power washing with sometimes a percent mixture of acid is necessary to remove any existing stains. Sealant applied over stains or existing old sealant will result in the new stain not penetrating as it should. What is noticeable before new sealant is applied will become more visible and unsightly. There are many types and qualities of sealers available. Poor preparation and the incorrect sealer can actually result in yellowing, peeling or bubbling of the sealant used. Removal of incorrectly applied sealants is very difficult and time consuming, if not mostly impossible without possible damage to the actual concrete. Often it is best to contact a knowledgeable individual to evaluate the condition of the concrete surface before sealing takes place.
BENEFITS OF CONCRETE TO ASPHALT IN CONSIDERATION OF A DRIVEWAY OR EVEN A PARKING LOT
In thickness asphalt should be nearly double that of the desired concrete thickness for comparisons. At a typical concrete thickness of 4 inches, asphalt would need to be 8 inches. Asphalt for a residential driveway is typically placed less than 4 inches. At a 4 inch application of asphalt the longevity factor has already been greatly reduced.
Asphalt is a very flexible and brittle material, especially in extreme hot or cold temperatures. Asphalt subjected to repeated freeze and thaw cycles expands and contracts readily within the actual mass, allowing any permeating water molecules to become trapped inside. As small hair line cracks develop early on, water enters and freezes in the cold and then dries in hot conditions. In the summer the black surface of asphalt becomes extremely hot and remains hotter longer than concrete. Excessive heating of the surface causes flexing to occur under repeated loaded condition, such as car traffic. Rutting and wrinkling from car tracks in the heat may begin to appear within a few short years after installation. As the cold and hot cycles continue over time, the now possibly rutted/wrinkled asphalt becomes excessively cracked with small pieces beginning to break and pop free. The result is complete failure, making removal necessary. Everyone has heard the saying it's so hot you can fry an egg on the street. Asphalt is heat absorbing while concrete is more reflective. Consider the radiating heat effect from a driveway relative to your adjacent house.
Potholes are more prevalent in asphalt than concrete. In temperatures below 32 degrees, frozen water within or under the asphalt readily expands breaking and popping the cold brittle asphalt apart. Potholes forming in concrete are highly unlikely since concrete has a low permeability to infiltrating water along with a very high tensile strength even in cold temperatures.
In the Midwest temperatures range from below 0 to over 100 degrees over the course of a year. Extreme fluctuations in temperatures over just a few days to weeks or temperatures below 32 degrees for an extended period is much harsher on asphalt then concrete.
Sealing asphalt every 1-3 years is relatively cheap and maybe applied by the homeowner. This is similar to a band aid on a bleeding wound. The sealing covers the cracks from season to season, keeping a small percentage of water from infiltrating the asphalt and below. However the mass of asphalt still reacts as it will to freezing and thawing. The sealing really only lasts and looks nice on the surface for a few season cycles. Microscopic cracks continue to develop and water is actually penetrating the seal and cracks unnoticed. The many days of odor from sealing and the potential tracking of any remaining residue into your home is not a pleasant situation. Recent research has begun to question the contents of certain sealers as to whether the residue that becomes tracked out of place and that the odors may contain harmful carcinogens. Coal-tar based sealants have actually been banned in some cities where water run off contamination is in question. Asphalt based sealants at this time still seem acceptable along with other more expensive types being applied to driveways and parking lots.
Asphalt is a residue left over from refining crude oil in the production of gasoline Asphalt up until the past few years was nearly ½ the price of concrete. Recent oil barrel price increases and the improvement in refining crude oil with less impurities remaining, has driven the cost of asphalt up over 200%. The residue asphaltic cement becomes the binder as cement is to concrete. Then a percent of rock and sand is added as in concrete. After these slight similarities between concrete and asphalt, costs, benefits and longevity vary drastically. The appearance factor and longevity of asphalt is 7-10 years, while concrete is a minimum of 20 years+ if installed properly. In the end asphalt costs can actually be twice the cost of a concrete over 20 years.
Concrete has always been the most preferred material over asphalt for all traffic uses in terms of durability and longevity. Cost in past years for asphalt made the initial prices more bearable with frequent maintenance costs and earlier replacement to be dealt with later. Concrete usage in new construction or replacement now is nearly beating out asphalt in every instance. The long term benefits are out weighing the near compatible cost of concrete and asphalt.
Asphalt may still be slightly cheaper, but initial and lasting overall appearance, durability, with low maintenance and longevity makes concrete the current winner.
CAN ASPHALT OR CONCRETE BE COVERED (OVER LAYED) WITH THE SAME OR ANOTHER LIKE MATERIAL?Yes and no.
Asphalt is commonly placed over the top of other asphalt if only minor cracking of less than 1/8 inch in width is present. This is acceptable only if no deferential across the crack is present in the existing layer (the crack being at the same height from one side to the other) Any small dislodged loose pieces within the old layer should be removed, as they will likely cause surface distress in the new layer. Typically only 2 inches of new asphalt is placed over the old layer. The concern here is that in a very short period of time a crack or loose moving pieces within the old layer will reflect up through the new layer in the precise location. With that result the new layer will look mostly the same as the old layer very quickly. If the overall condition is not severe then a second layer of asphalt maybe considered as a short term gain. Even in near perfect conditions no more than 2-5 years of improved performance and appearance gains can be expected with an asphalt over lay. If the asphalt looks unsightly with cracks, has high and low areas of heave or settlement with pieces missing, then complete removal is warranted to evaluate the conditions below and apply a new surface. (See the Benefits of Concrete to Asphalt) Concrete is not likely to be placed over old concrete. The same concerns relate here as in asphalt with any diverse existing conditions reflecting from the old to the new. The concert being considered likely looks unsightly with cracks, has high and low spots with pieces missing, thus the reason and desire to do something as risen. Removal and the evaluation of the conditions below and the surrounding area are necessary before any new concrete is placed.
Asphalt is sometimes applied over concrete, but again the same concerns are that the conditions of the old surface will reflect at some time into the new surface.
Even if existing surface conditions are conducive to an over lay of any material, matching the nearby street, garage floor, house and yard height (elevation) may not be feasible without additional costs and effort.
Just call DR.PATCH to setup and appointment for a FREE estimate.
WHAT ARE SOME APPEARANCE TYPES FOR CONCRETE AND HOW ARE THEY CREATED?Ordinary concrete;
Leveling and finishing of the surface and edges is accomplished with trowels, while the concrete is wet. The surface of the wet concrete may be "Broom Finished" in a uniform procedure, which creates a slightly rough look. Or another look created while wet is called a "Mag Swirl". This trowel method creates a surface that appears as though it was touched with swirling fan blades. Both types of finish have a slightly rough surface to resist slipping.
During the finishing, expansion joints are troweled into the wet concrete to aid in controlling short and long term cracking. (See Does Concrete Crack) (See Should New and Older Concrete be Sealed) "Ordinary" is the most basic look of concrete
A base color is selected and added into the wet concrete typically after it arrives in a truck to the work site. This manual addition of the color (liquid or powder) selected is controlled based on the volume of concrete delivered. (per gallon, lighter shade colors are generally cheaper than darker)
Leveling and finishing of the surface and edges is accomplished with trowels, while the concrete is wet. As with ordinary concrete, the surface of the wet concrete may be "Broom Finished" in a uniform procedure, which creates a slightly rough look. Or another look created while wet is called a "Mag Swirl". This trowel method creates a surface that appears as though it was touched with swirling fan blades. Both types of finish have a slightly rough surface to resist slipping.
During the finishing, expansion joints are troweled into the wet concrete to aid in controlling short and long term cracking. (See Does Concrete Crack)
Power washing to remove the color residue dust and then sealing should take place within a short period of time before any surface staining occurs. Sealing will create a beautiful and long lasting look of the vibrant color selected. (See Should New and Older Concrete be Sealed) "Colored" is one step above Ordinary concrete