Residential Concrete Installation Manuals

Many people are unaware of the fact that concrete is a sophisticated and technical building product. I feel this is partly because three of the four basic ingredients of concrete are so common: sand, gravel and water. The fourth ingredient, cement, is where the science and technology come into play. This key ingredient allows concrete to go from a liquid or plastic state to a solid compound in just hours time. It is a wonderful transformation that, when done properly, can yield a product which is extremely durable.

However, homeowners and trades people often fail to properly install concrete. I think that this is due, in large part, to the vast amount of misinformation which has been passed along concerning concrete. Fortunately, there is an enormous amount of clearly written information dealing with concrete. Much of it is available at public libraries. However, probably the most concentrated source of high quality concrete information is available from The Portland Cement Association (PCA - http://www.cement.org).

The PCA is "an organization of cement manufacturers whose mission is to improve and extend the uses of Portland cement and concrete through market development, engineering, research, education, and public affairs work". They have been around for years and are considered by many to be one of the most authoritative sources of concrete information in the USA.

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The Portland Cement Association has an online catalog listing all of their publications. Many of them are written so that a layman can understand them. You will be amazed at the number of publications available. Some are small five to ten page pamphlets, and are available for free download in PDF format. Others are full blown books. The photos and illustrations are excellent. Following is a partial listing of publications which would be of great use to homeowners. I've listed the item number and its title.

• LT167 THE CONTRACTOR'S GUIDE TO QUALITY CONCRETE CONSTRUCTION

• IS174 CONCRETE FOR SMALL JOBS
• LT209 GUIDE FOR CONCRETE FLOOR & SLAB CONSTRUCTION
• LT202 RESIDENTIAL CONCRETE
• EB122 CONCRETE FINISHER'S GUIDE
• SP038 HOMEOWNER'S GUIDE TO BUILDING WITH CONCRETE, BRICK & STONE
• EB001 DESIGN AND CONTROL OF CONCRETE MIXTURES
• IS177 CONCRETE SLAB SURFACE DEFECTS: CAUSES, PREVENTION, REPAIR

08/2008

Concrete Strength

Not all concrete is the same. The design strength of concrete is a function of the quantity of cement used when it is mixed. The more cement that is used, the higher the strength of the finished product, as long as you place, finish, reinforce and cure the concrete properly. Thus, the final strength of the concrete really depends upon what is done after the concrete is unloaded from the truck! Many people are unaware of this responsibility. However, certain applications require different minimum strengths. For example,exterior slabs (driveways, patios, sidewalks, steps, etc.) should be a minimum 4,000 PSI strength. The PSI refers to pounds per square inch, or its compression strength. As the number gets larger the concrete becomes stronger.

Thickness

The performance of a slab is directly related to its thickness. The thicker you pour a slab the stronger it will be. In fact, increasing the thickness of a slab from four to five inches increases the strength dramatically.Thicker slabs can handle more weight and are better able to bridge depressions which may form due to soil settlement. A thicker driveway slab will possibly be able to handle that once a year heavy delivery truck. When you consider the long term benefits, the slight extra cost to increase the thickness is well worth it.

Cracking

Most homeowners are shocked when I tell them that concrete is supposed to crack. In fact, I guarantee that it will crack in a majority of instances. The reasons are really very simple. Concrete cracks can usually be blamed on one or more of the following: cracks caused by shrinkage related to the curing or drying process, cracks related to expansion and contraction induced by temperature changes, cracks related to concentrated heavy loads, cracks related to poor, uncompacted sub-grade conditions, cracks due to rapid water loss prior to the final finishing stage. These all might sound complicated, but in reality they are not.

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Concrete shrinks when it drys, just like many things. Clothes shrink, wood shrinks, and concrete shrinks. Concrete is similar to wood with regards to shrinkage. As a piece of wood dries, it shrinks. The wood fibers pull closer to one another in response to the loss of water. Eventually a weakness develops between a 'row' of fibers, and a crack or a split develops. This is exactly what happens to concrete. The crystals of concrete are actually pulling away from one another. This pulling or tensile force is enormous and will crack the slab. The rate of shrinkage is very close to 1/16" per ten feet. So, if you poured a 100 foot long slab it will shrink nearly 5/8"! That's a lot! The trick is to anticipate this shrinkage and attempt to force the concrete to crack where it will look the least noticeable.

The installation of control joints every 10 feet in large slabs will generally account for this shrinkage. A control joint is an induced crack point. You may notice them as tooled lines that occur on a regular basis in sidewalks and driveways. These tooled lines should be deep enough (1/4 of the thickness of the slab) so as to create a natural weak spot in the slab. Isolation joints are points where concrete meets up against something that for all intents and purposes will not move. These joints are made of a material that will react to the expansion and contraction of the slab. These materials are sold under different names such as expansion paper, homasote, fiberboard, etc. It should be installed in strips the same thickness as the slab which is being poured.

Reinforcing Steel

Steel is an inexpensive and user friendly material. Concrete and steel work very well together. The addition of steel to concrete increases concrete's strength enormously. Concrete has very little tensile strength - that is, it can be pulled apart easily. Steel, on the other hand, has tremendous tensile strength. So, if you add enough steel to concrete, it is virtually impossible to pull it apart. Cracks commonly occur in concrete due to its low tensile strength. So, adding steel helps to minimize cracks that might develop as a result of tension. Cracks that do develop often will remain as hairline cracks. If the steel is large enough (1/2"reinforcing rods), frequently there will be no vertical displacement between the two cracked sections.

Steel is very inexpensive. I can purchase 1/2" rods locally for $0.15 a linear foot. Using this number, you can purchase enough steel for a 10 x 20 foot slab for less than $40.00!! I calculated that sum by creating a mat of steel rods, two feet on center both directions, one foot in from each edge. That matrix creates a very strong slab. Would you be willing to pay an additional $40 in materials to significantly increase the strength of your slab? I thought so. An experienced mason with a helper can install this steel in just several hours. It is well worth the price.

You can also install welded wire mesh in slabs. This material is usually available in two sizes, 21# and 42#. The mesh is generally five feet wide and is comprised of squares six inches on a side. The 21# mesh is constructed of steel wire approximately 1/16" thick. The 42# mesh wire is close to 1/8" thick. Mesh is very effective at strengthening slabs. Although I personally prefer steel rods, mesh is a good alternative for those who feel they cannot install the heavier steel rods.

Please take some time and do additional reading concerning concrete. I guarantee that you will be amazed at how much science and technology is involved. This added knowledge will enable you to spot the informed professional that you intend to hire for your next job. Ask him or her some direct, pointed questions. Ask them if they know what the PCA is. See how they respond. Hopefully, you will be able to locate an individual who can answer a large majority of your questions correctly. Good reading and good luck!

Remember to check to see if the sealant will discolor your existing concrete. Most of them will not. Some of the acrylic sealants look milky colored when applied, but will dry completely clear. However, be sure to verify this with the manufacturer before applying to your concrete!

Manufacturers of Concrete Sealants

• Saver Systems
• W.R. Meadows, Inc.
• Abatron, Inc.
• Metalcrete Industries, Inc.
• Sika Chemical Corp.
• Chemprobe Corp.
• Thompson's Water Seal
• Conproco Corp.

Related Articles:  Deicing Salts And Concrete, Concrete Sealants, Concrete Life Maximized

08/2008

Reinforcing Steel

Concrete has very little "tensile" strength. Concrete, which has not been reinforced, will crack when subjected to tension. This can be illustrated quite easily. Pour a piece of concrete 8 feet long, 1 foot wide and 4 inches thick. Let it dry for 28 days. Put two concrete blocks on end and lift the piece of concrete onto the blocks so as to make a table or bridge. Begin to stack concrete blocks in the middle of your "bridge." Before long, the concrete will crack and your bridge will collapse. Watch out for your feet and hands, if you really decide to do this! By adding the concrete blocks to the "bridge," you caused it to bend or 'stretch' in the middle. This is tension.

Concrete reinforcing steel is designed specifically for strengthening concrete. It comes in different diameters. Usually, 1/2" and 5/8" is used in residential construction. The use of this type of steel in your concrete drives, sidewalks, patios, etc. will significantly lengthen the life of your concrete. The steel will minimize cracking. It actually holds the concrete together when you subject it to tension. You subject concrete to tension when you do the following things: drive across it, push against it, subject it to freeze/thaw cycles, concentrate loads on it, etc.

In the event a slab develops a crack, the presence of steel reinforcing rods will keep the concrete from spreading apart or offsetting. Offsetting is where one portion of a slab is higher than the other portion after it cracks. This steel is not very expensive in relation to the cost of having to replace the concrete. A good spacing for these bars is 2 foot on center both directions. While this may seem like an excessive amount, it really will go a long way in strengthening your slabs. Be sure that the steel ends up close to the middle of the concrete slab. The concrete must surround the steel to be effective. If it is too close to the bottom or top of the slab it will be a waste of time and money.

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Pouring Conditions

Most people think that the best time to pour and finish concrete is on a sunny, breezy, hot day. Wrong! The best results can be achieved if you pour concrete on an overcast, damp day with an air temperature around 55 F. If the temperature were to stay at 55 F for three to four days, it would be ideal. Temperature extremes (hot or cold) are bad for concrete. Windy days can cause too much water to evaporate from the surface too rapidly. Plastic shrinkage cracks will develop. These can severely diminish the quality of the surface of the slab. If you must pour on a hot sunny day, try somehow to erect a temporary tent or screen to keep the sun from beating down on the slab. It does not need the extra heat from the sun!

If you must pour in cold weather, be sure to cover the slab as soon as it is finished with insulating blankets, straw and plastic, or some other insulating material. Keep this cover on for at least 72 hours. As the concrete hardens, a chemical reaction is taking place (exothermic) which actually generates heat. You must store this heat so that the fresh concrete will not freeze.

Curing Compounds

Immediately after you put the final finish on concrete, you should apply a curing compound. This is EXTREMELY important. The water which was added to mix the concrete must not be allowed to evaporate from the slab!

The process of concrete drying is called hydration. This is a two step process. The first step occurs in the first few hours after you mix the concrete. This is why you are able to put a finish on the concrete. Each minute that passes after you add water to the sand, gravel, and cement mixture, calcium carbonate crystals begin to grow. After so many minutes enough crystals have grown to allow the concrete to become "stiff." The crystals MUST have water in order to grow.

These crystals continue to grow for weeks and weeks after you first mixed the concrete. If you allow water to evaporate from the slab, the crystals will stop growing and you will have severely weakened concrete. It's that simple! Only a fool would go to all the trouble and expense of pouring a slab and then not apply curing compound.

If you cannot afford curing compound you have several other alternatives. You can cover the slab completely with plastic. You must seal the edges with dirt so that air or wind does not get between the plastic and the slab. This could cause water to evaporate. Another method is to keep the slab wet with damp burlap or other rags. This is very labor intensive and a great waste of water. You must continually monitor the burlap to be sure that it is constantly wet. You must also figure out a way to keep the burlap from blowing away in windy conditions.

Sealant Varieties: Solvent or Acrylic Based

There are many varieties of sealants currently on the market. Some are solvent based (petroleum based), while others are acrylic based (water based). Some are clear, while others are colored. You have many to choose from.

Some sealants penetrate into the concrete, while others merely create a film on the surface. The penetrating sealers generally perform much better. Try to use a penetrating sealant, if at all possible.

Non-Breathable Sealants

Some sealants have compounds in them which virtually prohibits water from being absorbed by the concrete. In certain instances, these sealants also inhibit the passage of water vapor from the slab to the air above the slab. These sealants are often referred to as non-breathable. This property can be detrimental depending upon the climate. Sealants containing compounds which do not breathe should be avoided in climates where temperatures drop below freezing (32 F) for extended periods.

The reason for this is quite simple. Concrete slabs can absorb water from two directions. They can absorb water from rainfall or surface runoff on top of the surface, or they can absorb water from the soil beneath the slab. The water from beneath the slab is actually drawn to the surface as water vapor. Without a sealant, this water can evaporate from the surface.

However, if you seal the concrete with a non-breathable sealant, a barrier is created which the water vapor cannot easily penetrate. During cold weather, this water vapor can freeze and cause the surface of the concrete to flake. There are sealants which contain compounds which allow them to breathe. Often these sealants are clearly labeled as to whether or not they do, in fact, breathe. Consider using these if you live in a cold climate.

Before Using a Sealant

When using sealants, it is EXTREMELY IMPORTANT to follow the manufacturer's directions and instructions. I can't emphasize this enough. Read the directions carefully before opening the product. Pay attention to the air temperature. When the manufacturer says the concrete should be clean and dry, they mean it. If in doubt, clean the concrete with soap and water using a stiff push broom. Thoroughly rinse the surface to remove dirt, soap, etc. Let the slab dry, possibly for a day or two, to ensure that it is totally dry. Generally, the best time to do this type of work is during the summer months. This way you can let the hot sun dry the slab quickly and thoroughly.

Also, WEAR PROTECTIVE CLOTHING and/or EYE WEAR when dealing with these sealants. They can harm your skin and eyes. Avoid using petroleum based sealants indoors, due to the possibility of fire or explosion hazards. These products generate flammable and/or explosive fumes as they dry. BE CAREFUL!