By Ethan Davis
This blog continues our discussion of Foundation Basics with information about residential concrete construction materials.
A residential designer using concrete and masonry materials must have a basic understanding of such materials, as well as an appreciation of variations in the materials’ composition and structural properties. In addition, soils are considered a foundation material. A brief discussion of the properties of concrete and masonry follows.
The concrete compressive strength used in residential construction is typically either 2,500 or 3,000 psi, although other values may be specified. For example, 3,500 psi concrete may be used for improved weathering resistance in particularly severe climates or unusual applications. The concrete compressive strength may be verified in accordance with ASTM C39. Given that concrete strength increases at a diminishing rate with time, the specified compressive strength is usually associated with the strength attained after 28 days of curing time. At that time, concrete generally attains about 85% of its fully cured compressive strength. Concrete is a mixture of cement, water, sand, gravel, crushed rock, or other aggregates. Sometimes, one or more admixtures are added to change certain characteristics of the concrete, such as workability, durability, and time of hardening. The proportions of the components determine the concrete mix’s compressive strength and durability.
Porrtland cement is classified into several types in accordance with ASTM C150. Residential foundation walls are typically constructed with Type I cement, which is a general-purpose Portland cement used for the vast majority of construction projects. Other types of cement are appropriate in accommodating conditions related to heat of hydration in massive pours and sulfate resistance. In some regions, sulfates in soils have caused durability problems with concrete. The designer should check into local conditions and practices.
The weight of concrete varies depending on the type of aggregates used in the concrete mix. Concrete is typically referred to as lightweight or normal-weight. The density of unreinforced normal weight concrete ranges between 144 and 156 pounds per cubic foot (pcf) and is typically assumed to be 150 pcf. Residential foundations are constructed with normal-weight concrete.
Slump is the measure of concrete consistency; the higher the slump, the wetter the concrete and the easier it flows. Slump is measured in accordance with ASTM C143 by inverting a standard 12-inch-high metal cone, filling it with concrete, and then removing the cone; the amount the concrete settles in units of inches is the slump. Most foundations, slabs, and walls consolidated by hand methods have a slump between 4 and 6 inches. One problem associated with a high-slump concrete is segregation of the aggregate, which leads to cracking and scaling. Therefore, a slump of greater than 6 should be avoided.
Admixtures are materials added to the concrete mix to improve workability and durability and to retard or accelerate curing. Some of the most common admixtures include:
water reducers to improve the workability of concrete without reducing its strength;
retarders used in hot weather to allow more time for placing and finishing concrete. Retarders may also reduce the early strength of concrete;
accelerators to reduce the setting time, allowing less time for placing and finishing concrete. Accelerators may also increase the early strength of concrete; and
air-entrainers used for concrete that will be exposed to freeze-thaw conditions and de-icing salts. Less water is needed, and desegregation of aggregate is reduced when air-entrainers are added.
Concrete has high compressive strength but low tensile strength; therefore, reinforcing steel is often embedded in the concrete to provide additional tensile strength and ductility. In the rare event that the capacity may be exceeded, the reinforcing steel begins to yield, eliminating an abrupt failure that may otherwise occur in plain, unreinforced concrete. For this reason, a larger safety margin is used in the design of plain concrete construction than in reinforced concrete construction.
Steel reinforcement is available in Grade 40 or Grade 60; the grade number refers to the minimum tensile yield strength of the steel (Grade 40 is minimum 40 ksi steel and Grade 60 is minimum 60 ksi steel). Either grade may be used for residential construction; however, most reinforcement in the U.S. market today is Grade 60. It is also important that the concrete mix or slump be adjusted through the addition of an appropriate amount of water to allow the concrete to flow easily around the reinforcement bars, particularly when the bars are closely spaced or crowed at points of overlap. However, close spacing is rarely required in residential construction and should be avoided in design.
The most common steel reinforcement or rebar sizes in residential construction are No. 3, No. 4, and No. 5, which correspond to diameters of 3/8-inch, 1/2-inch, and 5/8-inch, respectively. These three sizes of rebar are easily handled at the jobsite by using manual bending and cutting devices. Table 4.1 provides useful relationships among the rebar number, diameter, and cross-sectional areas for reinforced concrete and masonry design.
In our next blog, we'll cover some information on residential concrete masonry units.
(This information is taken from an article by Nick Gromicko and Ben Gromiko on the International Association of Certified Home Inspections website)