Selecting the right mixture for concrete is critical for the long-term life of your steel building. Concrete should be strong and durable. Mixing concrete, however, is relatively simple. The basic ingredients are water, sand, gravel, and Portland cement. Admixtures such as fly ash or a water retardant can be included to help cure the concrete and to strengthen it.
The strength of concrete can be defined in two different ways. Compression strength is the number of pounds per square inch (PSI) the concrete will achieve in 28 days. 28 days is the normal amount of time it takes for concrete to fully cure. Tensile strength denotes concrete’s resistance to stretching or expansion. Steel reinforcing bars (rebar) are usually used to increase the tensile strength of concrete foundations.
The typical compression strength of concrete is 3,000 pounds per square inch after 28 days. To achieve this strength, the following approximate proportions should be used.
- Portland cement: 94 pounds (the size of a typical bag of cement purchased at a retail store)
- Sand: 185 pounds
- Gravel: 360 pounds
- Water: 5½ gallons
Mix these ingredients in a rotating drum mixer or a have it done in a cement truck to ensure the concrete is thoroughly blended.
Before the concrete is poured, forms must be built. Forms are used to hold the concrete together in the shape you desire until it dries. They are usually made from wood two-bys, although they can also be made of metal. They must be strong and rigid enough to withstand the outward pressure of the wet concrete without deforming, bending, or deflecting. They must also be framed tightly enough to keep the liquid concrete in and the rain water out.
The concrete should be poured evenly, ensuring the aggregates have not settled to the bottom. This is seldom a problem because liquid concrete is rarely allowed to sit still. While waiting to be poured, concrete is kept churning in the rotating drum. Make sure all of the rebar has been fully covered and that there are no voids or air bubbles. On larger projects, an electric or pneumatic vibrator is often used to compact the cement and ensure there is no air left in the poured slab.
When pouring deep foundations, water often seeps up to the top due to the immense pressure of the sand and gravel. This bleed-water is called latinate and should be removed immediately as it comes to the surface.
Screeding is the process in which excess concrete is removed from the slab after pouring. Generally, slightly more concrete is poured than is needed to make sure there is enough to fill all the voids. Whatever is left over must be removed to make a flat, even surface. To do so, a large wooden or metal board called a screed is dragged across the tops of the forms. This is generally done by two people who move the screed back and forth as they push it across. The excess cement is pushed over the edge of at the end of the forms on to the ground where it will dry and be removed during cleanup.
Slab finishing is generally done in two steps. The first step is to run large wooden or metal tools called floats over the surface of the wet cement to push the larger aggregates back down into the mixture. The float is pushed and pulled back and forth until a fairly smooth, even surface has been achieved. The concrete is still quite wet at this point.
After it has set a little while and much of the bleed-water has evaporated, a second finishing operation is done with steel trowels. The surface is smoothed out even more which will create a hard, even finish.
If the slab is going to be outside or otherwise wet, a metal rake is then used to create a textured surface which will prevent slippage.
Concrete dries and cures through a chemical interaction between the water and the Portland cement. Temperature and moisture content also have a direct bearing on strength and durability of the final product. The best conditions for proper curing and maximum strength is in dry weather between 50° and 90° Fahrenheit for 72 hours. After three days, the concrete will be strong enough to allow further construction on the building.
If you are building in a region with very cold or extremely hot weather seasonally, it will be necessary to plan ahead and take precautions to ensure your concrete will cure under the proper conditions. Concrete that does not set properly can lose up to 50% of its cohesiveness and strength over time.
Anatomy of a Foundation
There are two components that make up a foundation.
The foundation footing is a structural unit that requires additional excavation and reinforcement. The footings are used to distribute the weight of the building to load-bearing materials.
The foundation wall is basically a poured concrete wall that extends both above and below grade or ground level. They are load-bearing walls that serve as supports for the building walls and columns.
Whereas steel buildings do not present a great deal of vertical load, they do need to be able to withstand very high horizontal loads which tend to push outwards. If not properly distributed, these loads can eventually cause structural failure of the foundation and framing members. There are two common ways to resist or distribute horizontal loads.
Using Tie Bars
With this option, reinforcing steel tie bars are connected to anchor bolts in order to tie the building columns together and evenly distribute the load. In cases where the horizontal load is not as high, spread or hairpin ties can be used to transfer the load directly to the rebar used in making the cement floor.
Increase Footing Size
This method counteracts the horizontal load force, thereby preventing shifting of the foundation. While very effective, this is by far a more expensive option.
Pouring the Floor
In steel building installations, the floor is most often created by pouring a concrete slab—known as a floating slab or a slab on grade—within the foundation’s walls. This slab can be poured either at the same time that the foundation walls are being poured or after they are in place. Steel reinforcing bars are almost always used in the floor to give added strength and prevent cracking over time. If the building is going to be used for heavy machinery or vehicles, it is imperative that rebar be included, and often crack-resistant concrete is used as well.
Before the floor is poured, a sheet of polyethylene plastic, also known as visqueen, is laid on the surface of the ground where the slab is going to be poured. This will prevent water vapor from seeping up into the floor and will further enable the proper curing of the concrete.
The thickness of the floor will determine how heavy of loads it will be able to bear. Typically, the thickness of the slab is the equivalent of the thickness of a 2×4 or a 2×6, but some purposes require that it be even thicker. Many jurisdictions have specific requirements regarding floor thickness and load-bearing ability. Before building, you should check your local building codes for such requirements.
Control joints are required between construction components or between concrete pours. For floors, an expansion joint is used to control the point where normal expansion or contraction takes place, such as where the floor abuts a wall or has penetrations through it by a column or a pier. During the curing process, the concrete will shrink a bit. Expansion joints prevent the floor from cracking during this initial process as well as down the road when varying temperatures cause normal expansion and contraction.
It is usually advisable during the curing process or just after it, to apply a waterproofing substance to the floor to further prevent moisture intrusion. Types of substance and application techniques vary and often depend on the building’s intended use.