Concrete Best Practices
By John Comfort Jr.
All concrete starts as a mixture (in various proportions) of cement, sand, aggregate (gravel) and water. Changing the proportions of the ingredients will change the properties of the wet mixture as well as the final dried slab. In addition, a number of other admixtures are available to improve the workability of the mixture in various situations as well as changing the properties of the final product. When wet, concrete can be formed into a virtually unlimited number of shapes and sizes that will be very durable after the concrete mix hardens. Concrete can be used for a wide variety of purposes from office buildings to streets and sidewalks, and from bridges across the water to boats floating on it.
Producing a good quality product is dependent on a number of factors, but the 2 most important are the mix supplier and the installer. Concrete mixes are typically about 10-15% cement, 15 – 20% water, and 60 – 75% sand and gravel, but this will vary depending on the desired final product. When the cement is wetted, it will create a paste that completely surrounds the sand and aggregate particles. As it dries, the paste will harden by a chemical process called hydration. In general, mixtures with more cement and less water will be more difficult to work with but ultimately stronger. It is the responsibility of the concrete supplier to balance all of the factors and provide a mix suitable to the project. In addition to compressive strength, many other factors can be manipulated and adjusted with the mix design. For example, standard concrete will typically require 3-5 days of curing before it can be put to its intended use. Concrete can require 30 days or more for a complete cure and full final strength, but the shorter time period will usually allow concrete to achieve 80- 90% of its final strength. If desired, testing can be performed on samples taken during the initial pour to document strength prior to release for intended use. If required, the mix can be designed for faster curing allowing use even the next day. This can be particularly useful when work is done around functioning retail or commercial buildings where closing sections of concrete pavement or sidewalks could create significant business interruption and loss of revenue.
Concrete strength is measured in psi or Pounds per Square Inch. This is a measure of compressive strength of the concrete, and it is tested by actually compressing cured samples of the concrete until it breaks or crushes. Typically, most standard exterior concrete is either 4000 psi or 4500 psi with the higher psi mix being the more expensive. In some cases, lower psi concrete is used where the thickness is significantly greater than normal. This can be in the 2000-3000 psi range and even lower. Alternatively, higher psi concrete is used when it is important to put the concrete to use as quickly as possible. 6000 psi high early concrete will usually reach 3000 psi strength in 24 hours and this is adequate for (at least temporarily) many exterior uses.
While concrete installation looks like an easy matter of pouring the mix into forms, the skills and experience of the installer are important to the quality of the product. When concrete arrives at a jobsite, there is only a limited amount of time available to get it off of the truck and properly placed in the forms. After that time has passed, the concrete will become too hard to work with. When the concrete is placed, it is the responsibility of the installer to form it and finish it to meet the requirements of the job.
In addition to the standard gray concrete we see everywhere, concrete can also be provided in a wide variety of colors. Color can be applied as a stain to the surface after the concrete has been placed, or it can be mixed as a dye into the initial wet concrete before pouring so the color is integral to the entire thickness of the product. In addition, the surface of sidewalks, patios, and pavements can be stamped with different patterns while the concrete is still wet. These are generally considered to be architectural enhancements and can resemble flagstones, tiles, wood planks, or geometric patterns. Stamping is particularly effective when used with colored concrete.
After concrete is cured, it may be prudent to apply a sealer. Sealers can be used to protect the concrete from deterioration due to freeze thaw cycles in typically damp or wet areas. Sealers can also be used to reduce damages caused by the use of deicing salts on the concrete surface especially during the initial several years of service. Sealers can reduce the possibility of accidental staining due to use in high traffic locations or spills in areas subject to accidents such as picnic areas or bar-b-que locations. Sealers can also be used to enhance the appearance of colored concrete and even add a shine to the otherwise matte finish. In any event, if sealers are applied, they are not permanent, and should be re-applied every 2 or 3 years or as appearance dictates.
Exterior concrete flatwork contractors are the appropriate contractor to contact for sidewalks, curb & gutter, drain pans, and a variety of pavement slabs from patios to parking lots and roadways. Because of its environmental friendliness, relatively low lifetime cost, and long lasting durability, concrete is an excellent choice for these structures.
Sidewalks made of concrete are typically 4” thick, but sometimes they can be thicker for site specific reasons. When sidewalks are made, they are formed on soils that are compacted to around 95% density. This is done to provide strong support for the sidewalk panels. While concrete can span small voids without having problems, longer unsupported spans such as relatively thin sidewalks can be subject to cracking, and this is never a desired outcome. Compacting the dirt beneath a sidewalk is a relatively inexpensive way to reduce the chances for cracks. While sidewalks can and have been made of many different types of material, concrete is by far the most suitable especially where ADA concerns come into play. Among other things, The Americans with Disabilities Act requires that sidewalks used by people with disabilities be flat and smooth, provide good traction or grip, have very consistent and small slopes, and drain freely. This can be almost impossible to achieve to the degree required by the ADA using other materials. Concrete can be initially formed and poured to precise dimensional requirements with very tight tolerances. The surface can be finished in a variety of ways to form a flat surface with good traction that will drain. If needed, truncated dome plates can be embedded in the surface, and the concrete can be colored both to provide indications and warnings to blind individuals as required. All of this can be difficult to impossible to achieve with other sidewalk materials ranging from asphalt to stone and brick pavers and even tile or breeze (small, flat gravel mixed with fines).
Concrete is an especially good material to use for curb and gutter work. The purpose of curb and gutters is provide a pathway around the edges of a pavement surface where water can be directed and contained, and will flow to a desired location. Concrete can be formed to a very flat surface that will allow water to flow along slopes as low as 1%. If small sections of a curb and gutter system are damaged, they can be easily and individually replaced. The straight lines and smooth curves in a concrete curb and gutter system are aesthetically pleasing and will enhance “curb appeal” in any location. Surrounding an asphalt pavement parking lot or roadway with a concrete curb and gutter will contain and significantly strengthen the asphalt surface. In general, there are 2 types of curbs. The first is the barrier curb. In this type, the face of the curb is vertical, and typically 6” or more in height. The purpose of this curb is to contain vehicles and, to a degree, pedestrians on the lower or pavement side. The second type is the mountable or rollover type. Here, the face of the curb will be sloped at 45 degrees or less, and it is typically 6” high like the barrier curb. The mountable curb is often used at the start of driveways and will allow vehicles to cross at lower speeds without damaging wheels or tires.
Drain pans have a similar function to the gutter part of curb and gutters. Drain pans are used to contain and transport water from one location to another. Drain pans can work effectively at a slope of 1% or greater. They can be used in asphalt or concrete pavements, and can also be used in landscaped areas to transport water and reduce erosion of the landscaped area. The design of the pan (width and profile) will determine how much water the pan can carry without overflowing.
Concrete pavements are similar to sidewalks in that they are flat surfaces with a consistent thickness designed to carry a certain load and for certain traffic considerations. While sidewalks are typically 4” thick and made with 4000 psi concrete, pavements are usually thicker, and the concrete used is designed for the desired characteristics of the roadway. In addition to concrete mix design, reinforcement such as fiber mesh, welded wire mesh, or rebar are often included to provide additional strength and resistance to cracking as well as vertical and lateral separation. Usually, concrete pavements will have cut or troweled joints to control cracking, but in pavements, the joints will be farther apart and deeper than they would be with sidewalks. Joints can create panels of up to 100 square feet or sometimes more without additional engineering and design work. Two specialized concrete pavements are loading docks and trash pads. Concrete pavement is considered a good alternative to asphalt because of its relatively long life expectancy and low maintenance cost over the lifetime of the pavement.
One thing that can be guaranteed of almost any concrete installation is that the concrete will crack. It is unfortunate, but concrete is very strong in compression, but very brittle where bending is concerned. Always, the ground supporting the concrete should be compacted and tamped down fully. The ground the concrete is placed on should be as flat as possible with no irregularities. The onsite soil can be replaced to a specified depth with road base. This road base is very stable, relatively inexpensive, and is not subject to expansion and contraction due to wetting and drying. More expensive solutions can include adding various sizes of plastic or metal fibers to the wet mix or embedding wire mesh or rebar in the pads before they harden.
While every effort is made to reduce the chances for concrete cracking due to settling, there will always be a possibility that the concrete pad will crack either when is shrinks slightly as a normal result of the initial curing process, or later on as a result of expanding and shrinking during hot summer days or cold winter nights. Concrete has very little flexibility. The solution to this is to place grooves or joints strategically in the finished product. Grooves called joints are either formed into the wet panel using trowels or saw cut into the freshly hardened concrete pad. While these joints will not prevent cracking, proper placement by an experienced installer will cause the concrete pad to crack unobtrusively in the joint where the crack will be almost invisible.
Overall, concrete is an excellent choice of material for outdoor structures. Driveways, sidewalks, curb and gutters, roadways, and drainage structures will all benefit from the use of properly designed and fabricated concrete.