3 Powerful Reasons Why Precast Concrete is the Preferred Choice for Commercial Construction
For many years, North American commercial construction has preferred building above ground structures using precast concrete vs. pouring concrete on site. This trend is being fueled by the need to:
- Avoid costly project delays directly associated with inclement weather.
- Reduce the need for large pools of qualified labor on site to erect and take down concrete forms.
- Take more control of product quality.
Precast concrete is produced indoors where temperature and relative humidity is controlled. The proper curing of cement to concrete is essential to ensure a quality product.
Concrete deliveries are scheduled into the plant where each batch can be easily tested for quality consistency. Something rarely done when pouring concrete at a job site.
At our precast concrete wall panel plant located in Beausejour, Manitoba we routinely evaluate each batch of concrete delivered. Our testing procedure focusses on three distinct tests:
- Concrete Air Content Test
- Concrete Cylinder Test
- Concrete Slump Test
Concrete Air Content Testing
What is a Concrete Air Content Test?
The concrete air content test simply determines the air content within the concrete. In Manitoba, water in the concrete will expand and contract during seasonal freeze & thaw cycles. Tiny air chambers within the concrete need to exist to enable water to expand without creating fissures and cracks in the concrete that leads to degradation, something you don’t wish for when building wall systems for a below grade foundation or an above grade multi-storey building.
How we test for air in the concrete:
- Fill your air pot in three equal layers of concrete from the same batch.
- Tap the outside of the pot to close the voids left by the tamping rod.
- Continue two more times until air pot is full. Level off the top.
- Thoroughly clean the rim of the pot before placing the air pot lid on top and fasten the clamps.
- Make sure all the air is expelled.
- Pump air into the air chamber until the gauge is on the initial pressure line (calibrated). Tap the gauge to stabilize it.
- Open the main air valve and hit the side of the pot with a mallet. Tap the gauge again to stabilize the reading.
- Record the results to the nearest 0.1%.
The typical air content for concrete should be 5% to 7%.
Concrete Cylinder Test
What is Concrete Cylinder Testing?
Superior Walls promises each precast concrete foundation wall meets or exceeds 5000psi in strength. We form samples into cylinders from each batch of concrete. Samples are tested at different time intervals by a certified technician to ensure concrete strength is up to our standards.
How to properly make test cylinders:
- Fill your cylinder in three equal layers.
- Use the tamping rod to compact the concrete starting at the perimeter working in a spiral motion towards the middle. This will ensure no air bubbles remain in the cylinder.
- Tap the outside of the cylinder to close the voids left by the tamping rod.
- Continue two more times until cylinder is overflowing. Level off the top ensuring the cylinder is finished flat with an even surface.
- Clean the top edge and place the lid on the mold.
- After the cylinder is cast, it is labelled and moved to a curing location.
- Typically, cylinders are then tested at 14 hrs. and 28-day intervals.
How to properly test cylinders:
- Place the cured 14 hr.- 28 day cylinder on the bearing block aligning it with the center.
- Before the test, make sure the load is set to zero.
- Apply increasing load continuously until the concrete cylinder breaks or until you are certain the ultimate capacity has been cleared.
- Record the maximum load during the test as well as the type of fracture pattern.
What to look for:
A 14 hour break must exceed 2600psi and a 28-day break must equal a minimum of 5000psi.
See our 28-day high pressure test in the video below.
Concrete Slump Test
What is a Concrete Slump Test?
The concrete slump test is a technique used to measure the constancy of fresh concrete. A metallic cone is filled and then lifted, leaving a heap of concrete that slumps. The results of the tests allow us to measure the consistency of the concrete and guarantee the correct water to cement ratio. The test should be conducted from batch-to-batch at the commencement of the concrete operations. Many businesses agree that the average slump for concrete applications is around 4-5 inches, although there is no real rule.
How to the slump test for concrete is performed:
- Dampen the metallic cone as well as the base with water to ensure the concrete does not stick.
- Place the cone on the base. Hold in position, making sure no concrete can leak from under the cone.
- Using the scoop, fill the cone in three equal layers.
- After filling the first third, use a tamping rod to compact the concrete starting at the perimeter working in a spiral motion towards the middle. This will ensure no air bubbles remain in the cone during the test.
- Fill the second third. Use the rod to probe this layer while also penetrating the previous layer approximately 1 inch.
- Fill the remaining third with concrete, just slightly over the top of the cone.
- Rod the layer penetrating the second layer approximately 1 inch. If at any time the concrete falls below the top of the mold, add additional concrete to keep an excess above the cone.
- Level off the top using the rod and remove any excess concrete around the base of the cone. Be sure to continue applying pressure so no concrete can leak from under.
- Raise the cone in a steady, vertical direction.
What to look for:
After removing the cone, you are left with a pile of slumped concrete. You can now measure the slump. After flipping the cone upside down with the rod laying vertical across the top, a measurement is made between the original height and the displaced center of the settled concrete. If the test falls outside of this range, a check test is typically performed to confirm the results. Record the result to the nearest quarter inch.
Testing concrete on site holds the manufacturer accountable and gives everyone especially the builder the peace of mind that they are using the very best quality concrete available for use in their new construction projects.