Understanding Compressive Strength in Concrete Testing

What is the compressive strength of concrete tested in a standard test cylinder of 6 inch diameter, which fails under a load of 84,700 lbs?

• A. 479 psi
• B. 749 psi
• C. 2,247 psi
• D. 2,997 psi

Answer: (D)

To determine the compressive strength of concrete tested in a standard test cylinder, you can use the formula that relates load, area, and stress: Compressive Strength (psi) = Load (lbs) / Area (in²) For a standard concrete cylinder with a diameter of 6 inches, the area can be calculated using the formula for the area of a circle: Area = π × (radius)² Since the diameter is 6 inches, the radius is 3 inches. Thus, Area = π × (3 in)² Area = π × 9 in² Area ≈ 28.27 in² (using π ≈ 3.14) Now, plugging the numbers into the compressive strength formula gives: Compressive Strength = 84,700 lbs / 28.27 in² Compressive Strength ≈ 2,997 psi This calculation shows that the compressive strength of the concrete cylinder that failed under a load of 84,700 lbs is approximately 2,997 psi. This value corresponds to the correct answer and illustrates how the compressive strength reflects the material's ability to withstand axial loads without failing.

When you're gearing up for the Massachusetts Unrestricted Construction Supervisor License, there’s a good chance you’ll encounter a question about concrete—specifically, its compressive strength. You see, understanding how this vital building material behaves under load isn’t just crucial for passing exams; it’s essential for ensuring the safety and integrity of structures. So, let’s get into it!

Imagine testing a concrete cylinder that’s the standard 6 inches in diameter. This is a common practice in the field, where engineers and builders evaluate how much weight that concrete can handle before it fails. The question often arises: what is the compressive strength when this cylinder succumbs to a load of 84,700 lbs?

Here’s where it gets interesting. To figure that out, we can use a pretty straightforward formula. You remember your basic physics, right? No? Don’t sweat it; I’ve got you covered! The compressive strength in pounds per square inch (psi) is calculated by dividing the load by the area of the cylinder.

Now, here’s the math. First, find the area of a circle—that’s the cross-section of the cylinder. The formula we know is Area = π × (radius)². Since the diameter is 6 inches, the radius becomes 3 inches. Plugging that into the formula gives you:

Area = π × (3 in)² = π × 9 in² ≈ 28.27 in² (using π ≈ 3.14)

Great so far? Now, you take the load of 84,700 lbs and divide that by the area we just calculated. It looks like this:

Compressive Strength (psi) = Load (lbs) / Area (in²)

So, filling in the numbers:

Compressive Strength = 84,700 lbs / 28.27 in² ≈ 2,997 psi

Voila! That result tells us the concrete cylinder can handle roughly 2,997 psi before it fails. Isn’t that a remarkable way to gauge the durability of materials? It’s like having a window into the strength of your structure, ensuring you’re not cutting corners where it matters most.

Now, while you’re studying, think about how this principle translates into real-world scenarios. A solid understanding of compressive strength aids you as a future supervisor, especially when overseeing projects that require stringent building codes. After all, the higher you can ensure your structures stand against compressive forces, the safer everyone will be.

As you study for your construction supervisor license, keep these calculations in mind—they’re not just numbers. They represent how we can blend science with practicality to create buildings that last a lifetime. It’s the heart of construction, really; understanding these fundamentals equips you for a successful career. So, jot this formula down and practice applying it to different scenarios—you’ll definitely thank yourself later!