Concrete Composition, Properties, and Reinforcement

Measuring Strength (PSI)

Concrete strength is measured in PSI (pounds per square inch). It is important to note that concrete gains strength gradually over time:

• 3 Days: Approximately $50%$ of ultimate strength.
• 7 Days: Approximately $70%$ of ultimate strength.
• 28 Days: Approximately $99%$ of ultimate strength.

While the vast majority of strength is achieved within a month, the remaining $1%$ continues to develop over many years through a very slow process.

Exam Tip: Remember that strength is a function of time. If a question asks for the approximate strength at 7 days, the answer is $70%$, not the final PSI value.

Reinforcement: Overcoming Weaknesses

Despite its impressive compressive strength (which can range from $3,000$to$22,000$ PSI), concrete is inherently weak in tensile strength. It is also prone to shrinking and curling at the edges. To counteract these physical limitations, reinforcement materials are integrated into the mix.

Steel Rebar (Reinforcing Bars)

Rebar is the most common method of providing tensile strength. Steel and concrete are complementary materials: they adhere well to one another, and the alkaline nature of the concrete protects the steel from oxidation (rusting).

Rebar is identified by a number that represents increments of $1/8"$. For example:

• A #3 bar has a diameter of $3/8"$.
• A #4 bar has a diameter of $4/8"$, or $1/2"$.

To improve the bond between the steel and the concrete, rebar features ribs—a pattern of longitudinal ridges and perpendicular cross-ridges (an "X" pattern). These ribs increase the surface area, ensuring the two materials work together to resist tension, shear, bending, and compression.

Alternative Reinforcements

Other methods to resist bending and cracking include:

• Welded Wire Mesh: Often used to provide distributed tensile strength.
• Fibermesh: Synthetic, glass, or steel fibers (such as polypropylene) that can be added directly to the mix.

Proper Placement: Rebar Chairs

For reinforcement to function correctly, it must be positioned in the correct part of the slab. Rebar chairs are used to maintain the proper elevation of the steel, ensuring it is not sitting on the bottom of the form but is instead positioned where it can effectively resist bending, curling, and cracking.

Environmental Considerations

Weather conditions play a massive role in the success of a concrete pour.

Hot Weather

In high temperatures, heat is released during the chemical reaction, which can cause water to evaporate too quickly. This leads to shrinking and cracking. As mentioned, damping the surface or using covers is the standard remedy.

Cold Weather

In cold weather, the primary danger is the evaporation of water crystallizing on the surface, which weakens the material. To prevent this, several methods are used:

1. Freeze Protection Admixtures: Chemicals added to the mix that allow concrete to cure in temperatures as low as $20^\circ\text{F}$.
2. Insulation and Covers: Using polyethylene covers or insulation blankets to retain heat.
3. Temporary Heating: In extreme cases where temperatures approach $0^\circ\text{F}$, the area may need to be tented and heated.