Your Complete Concrete Guide
Whether you have experience working with a concrete contractor, or it’s your first time, Winn Construction wants you to have all the resources you need to understand the process. From the paperwork to the end product, we’ve provided a range of information to give the peace of mind that comes from working with a knowledgeable, experienced and dependable contractor. We value our customers expectations, so we go the extra mile to add value to our services.
When evaluating a concrete proposal, it’s important to make sure you know exactly what you’re paying for. The proposal should account for any special instructions you’ve given the contractor and provide a detailed breakdown of the costs for materials and services for the project. A good proposal should include the following:
- Type, thickness and strength of the concrete material being provided.
- Type, size and spacing of reinforcement (if any) included.
- Type and thickness of any sub base aggregate included to support the concrete.
- Depths of excavations or removals included (if applicable).
- Conditional items or unforeseeable circumstances: Though it is impossible to anticipate all scenarios that may be encountered, negotiating pricing for suspected complications on the front side will help you avoid costly extra charges and will help keep the contractor honest.
- Common units of measurement for quantifying materials include: SF (square foot) SY (square yards) LF (lineal feet) CY (cubic yards) or EA (each)
- What isn’t included: Any items your contractor tells you are not included in the proposal should be noted.
Good Proposal Example:
John Smith Residential Driveway January 1st 2018
Demolish, Haul and Dispose of Existing 5” Concrete Driveway 5,000 SF
Excavate, Haul and Dispose of 6” of Unsatisfactory Subgrade Material 5,000 SF
Supply and Install 6” of ND DOT Class 5 Aggregate Base Material 175 TON
Supply and Install #4 Rebar Spaced at 18” OC EW Chaired Mid Depth 5,000 SF
Pour a 5” Thick 4,000 PSI Concrete Driveway W/ No Fly Ash 5,000 SF
Exclusions: Material Testing, Permit Fees, Moving or Repairing Irrigation Lines
Don’t be afraid to ask questions, no matter how simple or complex they seem. Only move forward if you trust the contractor is experienced and honest. Last, but certainly not least, make sure the price aligns with the value the contractor is providing. If the price seems surprisingly low, chances are the contractor lacks industry experience or a reputation for quality work. You’re spending your hard-earned money. Make sure you entrust your project to a contractor who will get the job done right.
Pavement spalling occurs when the surface of the concrete separates or lets loose exposing the aggregates encapsulated within the concrete to view. Spalling can occur for multiple reasons but the two most common causes are when deicing agents are applied during winter months or when the surface is made weak during the finishing process due to the use of excess water.
As concrete cures it shrinks slightly introducing tension and stress into the slab. Best practices dictate making relief cuts with either a diamond soff-cut saw or a grooving tool during the finishing and curing processes. Cracks in concrete are unavoidable, however, a skilled contractor that understands how and when to make relief cuts can help ensure cracks occur in a controlled manner.
Crazing is when tiny cracks appear in the surface of a concrete slab. It is usually more noticeable on a finished floor or a slab with minimal to no texture. Crazing is caused by hot, dry or windy conditions, and it looks like thousands of corn flakes have been etched into the surface of the concrete. Though the appearance of crazing may cause concerns, it is unlikely it will have a detrimental effect on the concrete.
Pressure Cracking and Heaving
Pressure cracking occurs when two adjacent pieces of concrete are pushed together with such force the edges begin to shatter or crumble. Under extremely hot conditions, concrete will expand and put pressure on adjacent panels. Without well-maintained expansion joints to absorb the pressure, the concrete can fracture or lift panels into the air. The best way to avoid this type of damage is to work with a contractor that understands the mechanics of concrete and the importance of not only installing expansion joints at the proper intervals but also educating you on how to maintain them after the pour.
There are two common types of rust staining that occur with concrete: surface staining and staining caused by the aggregates used to make the concrete. Surface staining is typically the most common type of stain and is caused by runoff from irrigation systems. These types of stains can be minimized by using commercially available cleaners, but results vary. The second most common type of rust staining is caused by the aggregates used in the production of concrete. This stain can make it look like your concrete is bleeding. These stains typically occur after a few years, but there are also products available to help clean up these types of stains as well.
What is Concrete?
Concrete is a mixture of various sized aggregates, cement powder, water and chemical admixtures. When combined in the correct proportions, these materials cause a chemical reaction which hardens them into a durable, long-lasting product.
Pavement design refers to the different layers of materials that compose the structural elements of a concrete surface. Several variables can affect pavement design including subsoil or native subgrade conditions, ground moisture content, type and thickness of available aggregate bases, PSI of concrete used and the intended use of the surface. Correct pavement design is vital to ensuring the longevity of your concrete.
Designing a pavement section with the appropriate thickness for the project is vital to the integrity of the concrete. Walking paths, sidewalks, patios and residential floors are generally 4 to 5 inches thick. Driveways and garage floors are around 5 to 6 inches thick, depending on traffic rates. Parking lots, loading docks and areas that see heavy traffic can range from 5 to 8 inches in thickness.
Aggregate Types and Thickness
Aggregate bases such as compactable sand or DOT specified gravel are commonly used under concrete to add strength to the pavement section. Typically 4 inches of base material is placed beneath walkways, patios, floors and areas that receive foot traffic. Areas with frequent vehicle traffic are bedded with 6 to 10 inches of base material. Increasing the amount of aggregate base strengthens concrete while minimizing its required thickness.
Concrete Strength (PSI)
Concrete strength is measured in pounds per square inch, which is tested by compressing a cylinder of cured concrete in a machine until it breaks. The machine measures and displays the strength achieved right before the concrete cylinder failed. Ready mix suppliers have mix designs that specify material proportions needed to achieve desired strengths. Common concrete mix designs start at 3,000 PSI and increase in increments of 500 PSI. Most concrete poured for footings and foundations ranges from 3,000 to 4,000 PSI. Exterior flatwork and finished floors are typically 4,000 to 5,000 PSI.
Concrete has excellent compressive strength properties, which means it can handle immense amounts of downward pressure. However, unreinforced concrete can lack in flexural strength. This flexural strength is improved by adding reinforcement to the design, most commonly steel bars, micro fibers and macro fibers. These reinforcements act like a bridge that helps the concrete distribute weight over a greater surface area.
Admixtures are chemicals that may be integrated into a concrete mix to alter its properties. There are several common types of admixtures. Viscosity modifying admixtures (VMAs) are used to make the concrete more fluid without compromising its strength. Non-chloride accelerators (NCAs) are commonly used in cooler temperatures to help the concrete set up faster. Hydration stabilizers are used to control the concrete’s set time, which gives the contractor more time to work with and finish it. Air entraining admixtures add air voids to concrete being poured in climates where it will be exposed to moisture and deicers during the freeze/thaw cycles.
When selecting a concrete contractor, it is important to partner with someone that is knowledgeable and truly understands the product. The right contractor will be able to assist you in designing a project that will give you years of trouble-free service.
Cold Weather Care When Pouring
Pouring concrete in cold weather is a difficult task. If concrete freezes, the product will be compromised. If your project requires concrete to be poured during winter months, it is important to hire a contractor that has experience pouring in cold temperatures. Most contractors that pour in cold conditions own their own concrete blankets and heating/curing machines.
Hot Weather Care When Pouring
Pouring concrete in hot weather also presents challenges. On hot days concrete has a tendency to set up faster, which reduces the time your contractor will have to finish the surface. If the concrete cures before your contractor is able to properly finish it, you will likely be left with a poor surface.
Pouring in Windy and Dry Conditions
On hot, windy days the surface of concrete can dry out faster than the concrete beneath it. This causes crazing or surface cracking. Although this cracking typically has no detrimental effect to the concrete’s longevity, it can be unsightly. Special care must be taken when pouring concrete on windy and/or hot days to minimize the appearance of such cracks.
Saw Cutting and Jointing
Proper sawing and jointing of concrete is key to reducing the risk of random cracking in the slab.
Knowing the best time to saw largely depends on how fast the concrete is curing. Saw it too soon and the concrete can spall and chip, but wait too long and the concrete may crack. The frequency and depth of sawed joints are also important. Cuts spaced too far apart may allow random cracking within a panel. Although it is impossible to stop random cracking in concrete, a knowledgeable contractor can take steps to greatly reduce the likelihood of cracks.
Joint Sealing and Crack Repair
Joint sealing and crack repair are the first lines of defense in protecting your pavement investment. Pavement fails when water is allowed to penetrate its surface and soften the aggregate base material below. When the base gets wet, it loses its ability to support the pavement, which causes it to fatigue and fail. Sealing the joints and any random cracks with a flexible joint sealant keeps the supporting base material dry and preserves its structural integrity. Pavement is particularly susceptible to damage from water penetration in the winter months of northern climates. Water trapped beneath the pavement can freeze and thaw, which causes it to lift and drop accelerating deterioration.
Although spalling rarely has any effect on the structural stability of concrete, it most certainly can be unsightly. Repairing spalled concrete can be difficult, and it often leaves an undesirable aesthetic look afterward. Skim coating the affected area with a repair or patch product is the most commonly used technique. However, since the color of the produce may vary slightly from the color of the concrete, a seamless match is almost impossible to obtain. This type of repair is also susceptible to failure as it usually only leaves a very thin layer of material covering the spalled area.
Removal and Replacement
When concrete has failed to the point where it no longer has structural properties, it needs to be replaced. The failed area of concrete should be cut back to the nearest sawed joint and removed.
The aggregate base should be inspected to ensure it is still suitable to support the new concrete. If it is not, new aggregates should be installed and compacted. Steel dowels should be drilled into the slabs adjacent to the repair area to ensure the new concrete cannot move independently of the existing slab. If a mat of rebar existed in the removed concrete then new rebar of the same size and spacing should be installed in the repair. After the bar is tied in place, chairs should be set under the rebar to hold it in the top half of the slab during concrete placement. After the new concrete has been poured and finished, it should be blocked off and traffic should not cross it for seven days to allow for a proper curing period.
Sealers and Coatings
Epoxy coatings, densifiers and clear coats are among the hundreds of products and sealers that can enhance concrete’s durability. After initial concrete installation, most contractors typically apply either a dissipating curing compound or a curing and sealing compound. These products generally offer the concrete slab a basic level of protection that is adequate for typical day-to-day usage. In some cases the concrete slab may require additional protection from chemicals or deicers. In those cases we can assist you in selecting a coating that will help you meet the objectives of your slabs end usage.
Deicing agents can be very harmful to concrete. Some products, such as salt, lower water’s freezing temperatures allowing it to penetrate concrete’s porous surface. As temperatures drop, water can refreeze, expand and pop the concrete’s surface. Other products react chemically with the cement in concrete, which causes accelerated surface deterioration. Although there are products that claim to be concrete friendly, they are not. The best way to protect your investment, especially early in its lifecycle, is to avoid using such products and substitute a sand or gravel mixture instead.
Allowing Proper Cure Time
In most cases, no working load should be applied to concrete for seven days. The curing period can be shortened or extended based on factors such as temperature, humidity and admixtures used. It is important to ask your contractor how long you should remain off of the new concrete. If you apply a load to it too soon, it may cause structural damage.
Sealing and Coatings
Depending on how you intend to use your concrete, it may be beneficial to add a special sealer or coating to it. Sealers and coatings can help protect the slab from water penetration or add impact resistance. They can even change the color of the slab if you desire. A reputable concrete contractor will be able to help you determine if your concrete could benefit from one of the many products available.
Even the strongest concrete will give out over time if there is nothing supporting it. It is extremely important for concrete that is placed in areas where water flows to be well sealed. If water is able to get beneath the pavement and washout the aggregate supporting it, in time that concrete will fail and create a dangerous hole. It is important to address these areas to prevent the need for costly repairs.
From time to time, concrete driveways or walkways may require cleaning. Using a mild dish soap and a soft-bristled brush with some warm water will help lessen the appearance of stains caused by petroleum-based products. Caustic cleaners are not good products to use as they can etch and damage the surface of concrete. A pressure washer may be used as well, but care must be taken not to use too much pressure as this may damage the concrete’s surface.
It is important to make sure water runs freely off of concrete slabs. Trapped water can find its way underneath the slab and soften the aggregate base, which compromises its integrity and diminishes the support it gives. Wet conditions under concrete can also cause frost movement that may crack or severely damage it. Keep flow lines free of dirt, snow or ice to minimize the opportunity for water to penetrate to the aggregate below.