Density
A crucial aspect of concrete quality is its density. Density refers to the measure of the mass of a given volume of concrete. The test to determine the density of concrete involves filling a container with a known volume with concrete. The container must have a minimum volume of 0.20 cubic feet, and its weight must be recorded to the nearest tenth of a pound.
Density Test
To perform the density calculation, the first step is to weigh the empty container and record that value to the nearest tenth of a pound. After filling the container in three lifts, performing mallet blows, rodding, and striking off with a strike-off plate, the container's weight with the concrete is then recorded. The weight of the concrete is then calculated by subtracting the empty container's weight from the full container's weight. Finally, the weight of concrete is divided by the known volume to calculate the density, also known as the fresh unit weight. To obtain accurate results, it is essential to adhere to procedures such as ASTM C138 or EN 12390-7.
Additional apparatus may be required based on varying concrete slump. For instance, if the slump value of the concrete is less than one inch, a vibrator capable of at least 9,000 vibrations per minute must be used. If the slump value ranges from one to three inches, either a vibrator or tamping rod may be used. However, if the slump value is more than three inches, only a tamping rod should be used. In case a vibrator is used, the measure should be filled and vibrated in two equal layers, with the vibrator inserted at three different points for each layer, approximately three inches into the underlying layer.
Concrete Yield
Yield is another crucial aspect that is used to determine if the concrete supplier has delivered the amount of concrete ordered. To calculate yield, the total mass of all materials batched, including water, cement, and aggregate, is taken. After calculating the density, the yield per batch in cubic feet or yards is calculated by dividing the total weight of materials by the fresh unit weight. The yield in cubic meters is obtained by multiplying the yield per batch in cubic yards by 0.764. The relative yield is then calculated by dividing the yield per batch by the cubic yards delivered. A relative yield of 1.00 or greater indicates that the yards delivered are equal to or greater than what was ordered.
Air Content
Air content can be calculated in the event that an air meter does not work properly. To calculate air content, the theoretical density is required, which can be obtained from the concrete supplier's mix design. The theoretical density is then divided by the actual density or fresh unit weight, and the result is multiplied by 100 to calculate the calculated air content.
Concrete density, yield, and air content are crucial aspects that must be accurately measured to ensure quality concrete. Adhering to the procedures listed in ASTM C138 or EN 12390-7 ensures that the tests are conducted accurately and consistently, leading to high-quality concrete.