Portland cement concrete (or commonly just “concrete”) can be a long lasting, reliable material for foundations, sidewalks, roadways, and driveways. Properly reinforced, proportioned, mixed, placed, and cured, it will provide many years of solid service. However, all is not created equal.
Despite the fact that it contains no moving parts (we hope), ignorance of its properties can cause the final product to fail long before it should. Correctly proportioned, mixed, and placed concrete will be uniform throughout in every direction and from the top to the bottom, and a slice through the finished product would reveal a cross section which looks much like fruitcake. (We’ve all had fruitcake which could easily be confused with concrete, but that’s another story.) This month, we’ll look at some of these aspects briefly so that we can better understand how to use this versatile, durable product.
What’s in it?
Finished, cured concrete can have widely varied properties, and like making a cake, the relative amount of ingredients, the time at which they are added, how the ingredients are mixed, and how it is allowed to cure will all have a measurable, noticeable effect on the final product. Portland cement concrete has three main components: small rocks (“coarse aggregate”), sand (“fine aggregate”), and a fine white powder called Portland cement. Simply, it is processed limestone from which the magnesium has been removed. The final properties of the cured concrete are dependent on many different factors, but the two most important factors are the ratio of Portland cement to water and the relative dry amounts of each of the constituent components. If concrete is to be mixed on site in very small batches (rather than delivered in ready-mix trucks), there are tables available that will help proportion the mix to the desired strength.
Home centers and hardware stores carry pre-combined dry ingredients, and there are mixing instructions on the bag that should be closely followed. In general, if your project will require mixing more than five bags of Sakrete®, it would probably be more worth your while to prepare the area and take delivery from a truck. If you can combine projects to make a larger delivery, or are willing to wait until the batch plant is sending out another “short load” to another customer, you’re likely to get a better product at a lower price. As a point of reference, it would require ten, 90 lb. bags of Sakrete® to make a slab 4" thick about 3' wide and 6' long. Save the bagged product for replacing posts and steps in ones and twos.
Although foundation slabs, driveways, sidewalks, and retaining walls all look like big hunks of rock, there are forces acting in different directions that the concrete structure must resist. The arrangement (or matrix) of Portland cement, sand, and aggregate particles provides compressive (or crushing) strength to the structure. That matrix has very low tensile (or pulling) strength. To provide for this, steel bars and/or wire mesh is arranged in advance of the concrete delivery. The physical geometry of the finished product including the dimensions and diameter of the bars, as well as the amount of concrete above, below and around the steel all have a critical effect on the performance of the slab. And while there are standard methods for placement and design, there is no “cookie cutter” or standard design. Each application should be designed individually with the designer considering the strength of the soil, seasonal groundwater conditions, frost depth, and the loads that the concrete will be supporting.
How strong is it?
The ultimate strength of concrete is a mathematical description of how much pressure the cured mix can support under controlled laboratory conditions. At the time of placement, samples of the concrete are collected and placed into special cylindrical forms, and placed in a warm water bath. After a day, the forms are removed and the “green” concrete cylinders are returned to the bath. This immersed condition represents the best curing conditions for concrete, because it limits the rate at which the concrete releases heat and water. Seven, fourteen, and twenty-eight days after the forming, the concrete is put into a machine which gradually adds pressure to the round ends of the cylinders until it falls apart (“fails”). The data collected during the test is put through some calculations and the resulting figure describes the best expected performance of the concrete structure curing in the field. Typically, we specify material which will withstand 4,000 pounds per square inch (psi) of load, though there are applications where 5,000 or 6,000 psi is required. Very seldom do we specify lower strengths, though it is available.
In general, the less water added to the mix, the higher the ultimate strength of the cured concrete. When calculating the mix proportions, it is important to take into account any moisture included with the aggregate particles, since this also contributes to the water content of the mix. At modern concrete batch plants, the moisture level in stockpiles is monitored continuously, and the water amount to be added is adjusted accordingly.
It should be clear, then, that reliable and consistent concrete mixes are best produced by the truckload at plants that are equipped and accustomed to producing Portland cement concrete for large projects that rely on consistent batches of concrete. Every where you can, take advantage of this product and avoid formulating your own.
What should it look like when it arrives?
The amount of water added to the dry ingredients obviously has an effect on how easy it flows, is finished, and is worked. But as mentioned previously, mixed concrete with too much water, though easy to pour into place, will be weak and will allow the aggregates to sink to the bottom. The standard method for evaluating the consistency of concrete is the “slump test.” It uses a 12" tall test cone into which wet concrete is placed, and then packed down with a rod. The cone is then turned upside down, like making a sand castle at the beach. When the cone is removed, the height of the pile of wet concrete is measured and subtracted from the original 12" height. The resulting number is referred to as the batch’s “slump.” Under no circumstances should the slump be less than 1", and only for very special construction should the slump approach 5". In general for construction around camp, the slump should range between 2" and 4". Concrete which is either too stiff or too runny should be rejected and returned to the plant. To avoid wasting a stiff batch, the driver may offer to add water from the tank on the truck, however, this procedure should be closely supervised by your engineer. As we discussed earlier, the proportions of the ingredients were closely checked when the batch was put into the mixer barrel. To “eyeball” the procedure now may be inviting trouble in the long run.
Properly proportioned concrete should be light to medium gray (unless you’ve ordered specially colored concrete) and will look lumpy and flow like fresh fudge. It should not vary as the truck makes its delivery in that the first bunch of stuff flowing from the chute should be identical to the last stuff.
As concrete cures, it undergoes a number of changes, chemically and physically. You may be aware that over time, concrete shrinks. But did you know that initially, the concrete expands? This is due to the water molecules shifting around to match into the Portland cement matrix. Water that doesn’t find a “home” right off rises to the surface in puddles. Over time, the remaining water evaporates out of the matrix leaving tiny holes which actually add strength to the finished product. This is the time when the concrete is shrinking.
How is fresh concrete cared for?
Earlier, we saw that testing samples were kept very moist and at a warm, consistent temperature using a water bath. Any steps that can be taken to mimic these optimum conditions will help the concrete cure uniformly and in a controlled fashion. On hot, humid summer days, the surface may be sprayed with a garden hose to keep the concrete from drying too quickly both from the air temperature and the heat generated by the chemical reactions in the concrete itself. Plastic sheeting may be placed on top to also keep the surface wet. Chemicals can be added to the concrete mix to slow the rate at which the concrete sets up, and ice may be used instead of water in the mix itself. On colder days, plastic sheeting, layers of straw, Styrofoam, and even soil may be used to insulate the fresh concrete. Chemicals can be added to speed the set of the concrete. In some cases, it may even be necessary to build a temporary enclosure to keep the space heated. All of these weather dependent issues can be addressed on very short notice by the designer of the concrete system you’re installing, but they have to be considered before the truck leaves the plant.
My brand new floor has cracks!!!
Most importantly, I need to emphasize that VERY FINE CRACKS ARE PERFECTLY NORMAL. These hairline cracks are the result of stress internal to the concrete matrix and appear as the water evaporates. If reinforcing steel was properly sized and placed before the concrete was delivered, these cracks are altogether cosmetic. If, however, in the first year after the concrete is placed, cracks develop which are wider than 1/8" to 3/16", or if there is a measurable difference in height from one side of the crack to the other, there may be significant problems with the slab or with the earth beneath the slab. These should be investigated and documented by an engineer.
There are methods and additives that will help to minimize shrinkage cracking. Often, contractors will score lines with a trowel in the surface of the wet concrete (usually one-third the depth of the slab) to provide an inconspicuous place for these cracks to form. Sometimes poly fibers are added to the mix truck. These fibers are incredibly strong and serve to help bind the matrix together. This material costs about an extra $100 per truck load. Under no circumstances, though, should this material be considered a substitute for traditional wire mesh or reinforcing bars. Remember that shrinkage cracking is cosmetic, and that the additional cost for reducing shrinkage cracking is really only justified where appearances are critical.
The costs associated with a solid design, tailored for your site conditions and structure, and then hiring a competent, experienced contractor will always prove to be far lower than throwing something together. Installing concrete on a wing and a prayer almost always either leads to failure from under-design, or a structure t