Question: Over the summer, our concrete producer experienced air content variations in its exterior mixes. It was getting to the point where technicians considered testing every load of concrete before leaving the plant. How can the producer solve the problem?
Answer: Air entraining problems are some of the toughest to figure out. Many factors affect air content. People often focus on one item as the cause of air fluctuations. But many times, the solution is something very simple but difficult to determine.
First, the producer should make sure there actually is an issue. Pressure meters can be incorrect and the volumetric testing method requires experience to perform correctly. Start by checking your unit weight. Unit weight is a very simple procedure, and change in a mixture’s unit weight from truck to truck will indicate changes in the proportions. Another approach is to run a hardened air content test on a sample of concrete.
Also, the producer should review the sampling procedures on the site. Is the concrete being tested taken off the first 10% of the load or is it being sampled after it passes through a pump or paving machine? The air content at the point of placement can be very different than the air content in the truck, depending on where and how the concrete is sampled. Concrete acceptance must take into consideration where the air content is tested in the mixing and placement process.
Next, the producer’s admixture dispenser system should be checked. Make sure the admixture is actually getting into the mix. Check calibrations of the meter and perhaps manually dose for a period of time. Often air entraining admixture is dosed by dispensing the admixture on the sand in the bins. This is a great way to dose, as the mortar fraction generates stable air content.
The admixture moving to the side of the bins and sticking to the side walls of the bins can cause issues. This would cause one load to get too little admixture and another load too much. There is a possibility of the admixture not entering the sand at all. I have seen the admixture dispense from the valve in a ball, hit the sand, and roll off the sand to the conveyer, then to the floor.
To fix this and make sure the admixture is getting into the sand, a method to extend the valve into the sand bin is required. This is usually accomplished by installing a steel pipe extending down into the bin where the sand is during batching. This pipe would stop where the top of the sand would be in the bin for the smallest load size.
The flexible pipe for the admixture and the valve is then inserted down this steel pipe and stopped about 6 inches from the bottom of the pipe. The discharge valve is now in the sand portion of the bin and protected from the sand by the steel pipe. The valve must stay inside the steel pipe so it does not come in contact with the sand or it will plug. Now, every time they dispense admixture, it will have no choice but to enter the sand.
The sand gradation is the next item to check. Fluctuations in gradation will cause swings in air content. The fraction on the No. 30 and No. 100 sieves are the most critical for consistent air entraining. If there are large changes in these sizes, it will be difficult to control your air content. This problem must be fixed at the source or during handling of the sand.
Cementitious products can affect the air entraining, although they do not usually change wildly in concrete production. Sulfate content and fineness of cement will cause changes in the air content, along with loss of ignition and fineness of fly ash. A load of cement or fly ash is typically used over several truckloads of concrete and therefore should not cause large fluctuations between individual loads of concrete. More often, they show up as gradual changes in dosage requirements.
Water has never been the culprit when I have conducted investigations into air content, but I would not rule it out. Changes in water chemistry can affect the air entrainers. These effects are typically seen when the admixture is first dispensed into the water, then into the load of concrete.
Hardness of the water can affect the air entrainers just as it can affect your bath soap. These effects are not usually seen when dosing separate from the water. The solution chemistry is governed more by dissolution of the cement and hydration products.
Finally, the producer should check admixture compatibility. Concrete mixes and admixtures are getting more complicated. Polycarboxylate admixtures are known to cause air-entraining issues. These are very powerful water reducers and they typically come with a defoaming agent already incorporated into them. Polycarboxylate admixtures, in combination with synthetic air entrainers, have caused some producers grief.
These admixtures are a valuable tool to producers, but when the concrete has to hit the ground at a specific air content, the producer must control the mixture to get the correct result. Understanding an admixture’s effects and performance will help control the outcome.
When investigating these issues, consider all of the factors. Each air entraining issue may have a different cause.
Contributed by Alfred Gardiner, principal engineer with Braun Intertec. For more, visit www.braunintertec.com. This article originally appeared in the October 2012 issue of The Concrete Producer, Concrete Construction’s sister publication.