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LEARN MORE ABOUT THE PHOENIX
WHY IS THE WATER CONTENT IMPORTANT?
The water content is arguably the most important parameter in a concrete mixture to determine the strength, consistency, workability or slump, and durability. As water increases in a concrete mixture, the spacing of the cement grains will also increase. This increase in cement grain spacing can improve the workability of the concrete but excessive water will decrease the performance of the concrete. If the water content is too high, the hydration products will have greater difficulty filling the space between the cement grains. This will increase the number of pores in the concrete, decrease the strength, increase the shrinkage from drying, and will reduce the ability of the concrete to resist outside chemical penetration.
WHY IS THE WATER CONTENT SO VARIABLE?
The water content of the concrete is variable for many different reasons. The aggregates used in a concrete mixture contain moisture that varies with the weather. The mixture must be adjusted for this moisture content. Water is used to clean the concrete trucks during batching. This additional water will enter the mix and this will change the water content. After the concrete truck is emptied it also must be cleaned. It is challenging to remove all of the washout water and so this changes the water content of the next mixture added to the truck. Water is also commonly added at the job site. All of these are examples of water being added to the concrete that is not measured and is added inconsistently. This inconsistency between concrete mixtures decreases productivity, increases labor costs, threatens concrete quality, and increases the risk for rejected concrete.
HOW DOES THE PHOENIX MEASURE THE WATER CONTENT OF CONCRETE?
The Phoenix is an oven that uses a specially designed high capacity heating coil system to rapidly evaporate the water from a 4x8 cylinder of fresh concrete. Concrete is used to fill a metal mold of a known volume. The mold is measured before and after filling, and then after emptying. The concrete is emptied into a metal pan and it is weighed. Next, the pan is placed in an oven. The heat from the oven removes the water from the concrete and also the water that is within the aggregates. After 15 minutes in the oven, the concrete is weighed. The sample is then placed back within the oven and one more weight is taken to be sure the water has been removed.
A simple calculation is done based on the batch weights, specific gravities, and the absorption of the aggregates. This information is used to determine the amount of water that will be contained within the aggregate before the concrete sets. The water within the aggregates is subtracted from the total water content measured and this gives you the water content available to react with the cement. This is the water that is responsible for the workability, consistency, and ultimately the strength of the concrete.
The Phoenix has been proven with both lab and field testing. This testing is outlined in a peer reviewed published journal paper that is available here. This initial paper looked at 258 concrete mixtures with 23 different types of aggregates, 9 portland cements, 5 supplementary cementitious materials (SCMs), and 15 different types of admixtures.
HOW DO WE KNOW THAT
THE PHOENIX IS ACCURATE?
LAB TESTING
In the lab testing, all of the mixture ingredients were carefully measured and batched. Since this was done in a lab setting then the exact moisture content of the aggregates and each ingredient could be accurately measured. Concrete samples were made and the theoretical or batched water to cementitious ratio (w/cm) was compared to the w/cm measured by the Phoenix. The results are shown in Figure 1. The line of agreement is shown as a dashed line with a +/- 0.02 w/cm variation. A result from the AASHTO T 318 microwave oven test.
Figure 1 – The average and one standard deviation of all batched and measured w/cm test results
from the Phoenix and tests with the microwave oven test.
Figure 1 – The average and one standard deviation of all batched and measured w/cm test results
from the Phoenix and tests with the microwave oven test.
Figure 1 – The average and one standard deviation of all batched and measured w/cm test results
from the Phoenix and tests with the microwave oven test.
On average, there was no difference between the predicted and measured w/cm. Further, all of the measurements were within 0.02 w/cm. This shows that the test is very accurate. Also, the replicate measurements show that the Phoenix is three times more precise than the microwave oven test (AASHTO T 318). More details are found here.
FIELD TESTING
To investigate the Phoenix in the field, the moisture content of the aggregates at a concrete batch plant was measured and then used to batch 4 cubic yards of concrete with a known mixture design into a ready mix truck. The concrete batch tickets were used to find the exact amount of material that was added to the truck. The drums of the trucks were inspected with cameras to be sure that they did not contain water. The concrete was then batched and tested to determine the water content. Next, a known amount of water was weighed and carefully added to the back of the truck to increase the w/cm by roughly 0.04. The concrete was mixed and then tested again with the Phoenix. This process was repeated four times for eight different trucks. This allowed 32 different concrete mixtures to be investigated. The results of the testing are shown in the Figure below. The line of agreement is shown as a dashed line with a +/- 0.02 w/cm variation. All of the measurements were within +/- 0.02 w/cm, and 88% of the mixtures are within +/- 0.01 w/cm. More details can be found here.
Figure 2 – A summary of the measured w/cm with the Phoenix compared to the batched w/cm. One standard deviation is shown on each data point.
This shows that the Phoenix can be successfully used in both the lab and the field to accurately measure the water content of fresh concrete.
When water is added to concrete for mixing, chemical reactions start occurring that cause the concrete to harden and gain strength. Typically, concrete is designed so that the hardening does not occur until after the concrete is transported, emptied, consolidated, and finished. This requires the concrete to be sampled within roughly 3h after initial mixing to give time for the other construction practices. Laboratory testing has shown that the Phoenix can be used to obtain an accurate water content of the concrete up until 5h after the initial mixing. This means the measurement must be taken before the concrete reaches the initial set. This means that as long as the Phoenix is completed within the time frame allowed for other concrete quality tests then the Phoenix results will be accurate. More details can be found here.
If a specialized concrete is used that changes the set time then the Phoenix should be used within the time allowed for other quality control testing or before the initial set is reached.
HOW SHOULD THE PHOENIX BE USED?
The Phoenix can be used at a concrete batch plant to know the water content before the concrete is placed. The Phoenix could also be powered with a generator at a job site and measured prior to the concrete placement.
HOW LONG CAN WE LEAVE THE SAMPLE IN THE PHOENIX?
The sample should be tested for the water content within 40 minutes of adding the sample to the Phoenix. This time limit is used to keep the aggregates from deteriorating in the test. This is based on testing 10 different aggregates with largely different makeup. More details can be found here.
HOW DOES THE PHOENIX COMPARE TO OTHER WATER CONTENT TESTS?
Field testing was done with the Phoenix, slump, unit weight, compressive strength at 7 and 28d, and resistivity at 7 and 28d to compare their ability to measure a 0.04 w/cm change in the concrete mixture. Only the Phoenix and the resistivity tests were able to reliably determine a change in w/cm. The Phoenix was the only test to be able to make this measurement in the fresh concrete.