Decoding Concrete Mixes: What are you really getting when you ask for 6 bag concrete?

An interesting thing happened the other day. When my field staff was pouring a new concrete foundation, they were given a competitor’s flatwork concrete delivery ticket by mistake.  The ticket has shed some light on how a contractor can meet specifications, while still short changing the customer.   In the end, the customer technically got what was likely specified, but not the long-term durability I am sure was desired.

The ticket shown does need some deciphering to those unfamiliar with concrete jargon. Under quantity it shows the number “seven”. Therefore, seven yards were delivered. The item code states “6 bag AE” and “Mid-Range 8oz”. Let’s break down the “6 bag AE” first.  This refers to the amount of bags of cement per yard in the mix design.  A straight six bag concrete mix with the correct proportion of aggregates and water should yield about 4,000 pounds per square inch for compressive strength. This is an excellent strength for most concrete slab applications. The issue, however, is that adding more cement causes more concrete shrinkage due to the higher amount paste.  This leads to more concrete cracking.  Also, more cement in the mix will also speed the setting time or as we say in the concrete world, increase the rate of hydration.  Speeding up this process will give off more heat and lead to decreased strength of the end product.  

The other component in the item code is “AE” which stands for air entrainment. The admixture adds tiny air pockets to the paste. This air is desirable in exterior concrete poured in cooler climates because it gives water a place to go if it freezes. When water freezes below the surface of the concrete it expands and can fracture the concrete.  In a basement, you do not need to add air for performance. 

If you continue reading the concrete ticket, you will see it noted that the air was doubled.  The drawback of adding air is that it interferes with the cement paste matrix (bond) and weakens the concrete.  A standard air entrainment proportion range is 4 to 6 percent of the cement paste per the American Concrete Institute (ACI). Higher amounts significantly reduce the strength of the concrete. A double air dosage can reduce the strength by as much as 600 psi.

The next item code or ingredient shown on the ticket is mid-range water reducer.

This is added to the concrete to increase the workability of the concrete without adding water to the mix.  (Adding water will directly decrease the concrete’s strength.)  The admixture will, however, accelerate the speed at which the concrete sets.

You can also see two hand written notes on the delivery ticket. One is “1%CC” and the other is “1% HE”. Both of these are accelerating admixtures to reduce labor time during the finishing process.  These will increase the heat of hydration and may then also increase the chance of shrinking before the concrete is strong enough to resist the tensile and flexural forces this will create. If the air temperature is cold, this combination works. If the air temperature is higher than fifty degrees, the risks of adding these admixtures start to outweigh the benefits.

           

So, now you should have a good understanding of what was in this concrete, which brings me the question of why was the mix designed this way? 

To answer this, I now must bring up the challenges associated with pouring concrete directly on a vapor barriers Vapor barriers are necessary to keep water vapor and other gases, such as radon, below the concrete.  When you pour on the vapor barrier, the bleed water (water that is not needed for the bonding chemical reaction in the cement paste) can only go up and out the surface. This is a problem for many reasons.

The biggest problem is the additional cost of labor. You cannot begin the finishing process until the bleed water is gone.  The vapor barrier causes more bleed water and we must wait longer before we can start finishing the surface.

As slabs harden, they will curl towards the air side of the slab.  More bleed water means an increased chance of curling which ultimately can cause an uneven floor surface.  Both of these issues can be controlled through good concrete practices.

My assessment of the situation is that rather than use good concrete practices to control bleed water, this contractor chose to add extra accelerants to the mix and double the air content to alleviate the problems I just mentioned.  The heightened air content will result in a slab with many voids for the bleed water to hide in as it is making its way to the surface.  Therefore, more times than not the finishing process will begin before the bleed water is all out.  Trapped bleed water will cause numerous durability issues as time goes by. In fact, I think it is probably the number one cause of concrete slab call back issues. 

The accelerants added speed up the setting process and decrease the labor time on the job, but will lower the strength curve over time.  The concrete will gain strength quickly, but if used when air temperatures are above 60 degrees, the ultimate strength of the concrete will really suffer.

Therefore, even though the concrete contractor ordered and installed a six bag mix (4,000 psi), the owner received a product that has a slim chance of performing like a six bag mix should. The corners cut gave the contractor labor savings and gave his customer a potentially defective concrete floor.

This brings me to the point of this entry.  We specify our standard slabs to be a minimum 3000 psi concrete mix and know that when tested, the 28 day strength exceeds this specified number.  I am absolutely sure that we are putting out a much better product using our concrete mix than this competitor who may state they are using a higher strength concrete.  Unfortunately, just specifying the strength of concrete does not give you more as the customer.  You need to choose a contractor with the knowledge to properly install the concrete AND the moral integrity to not take short cuts that reduce the structural integrity of your concrete.