It was Thursday afternoon about two o'clock. John Newman was on the phone. John and I were to have a pre-placement meeting for a shake on floor hardener project he was working on but he canceled the meeting at the last minute. As I answered the phone, the first words out of John's mouth were, "Phil, the floor is coming up!!"

Philip Smith, P.E., FACI, ASTM

Phil-  
 
Just how bad is it?
 
John-  
 
Well, most of the delamination took place in the last part of the pour.
 
Phil-  
 
What is the size of the delaminated areas?
 
John-  
 
They are anywhere from the size of a quarter to three or four feet across. You can take a pocket-knife and lift the hardener right off the concrete. The hardener just did not stick to the concrete!
 
Phil-  
 
John before we go off the deep end let me get some more information. To start with I will need you to fax me a copy of the mix design. I will look it over this afternoon and meet you at the job site Friday at noon. By the way, how many pours have you made?
 
John-  
 
This was the first. We have six more to do.
 
Phil-  
 
John don't make any more before I get there.
 
John-  
 
You got it! We are down until to you get here.
 

I arrived at the job site just after lunch. I made my way to the guard shack and picked up my badge and brain bucket (hard hat). I found my way to building 37A. I could see that the walls were up and the roof was on. I thought to myself, this is good. At least the slab is protected from the wind and the weather. This should reduce the plastic shrinkage cracks.

As I opened the door, as Yogi Berra would say, "It was dejavu all over again." John was not the first to cancel a pre-placement meeting and he will not be the last. They were all there, the hard hats and the white shirts, all looking down as if viewing a concrete corpse. Yes, they are all here: the owner, the job superintendent, ready mix, the finishing crew, and John. He is the one picking at the slab with his pocket knife. I thought to myself, if they all had been here on Monday two weeks ago we might not be here today.

With the introductions and exchange of business cards out of the way the next act in our drama was to walk the floor. With all eyes down, small groups formed that roamed the floor's surface, not unlike well trained hounds in search of game. But the search this time was for solutions.

As the smaller groups formed into a larger more somber group, someone in the group spoke out," Phil, what do you think?"

Phil-  
 
Well, it appears that the damage area is all at the north end of the building by the overhead door. The rest of the floor seems to be sound.
 
Owner-  
 
Phil, do you know what caused this and can you get me a good floor?
 
Phil-  
 
Yes! we can get you a good floor. And yes, I do know what caused it. I have a copy of the concrete mix design and I have reviewed it. I find that the mix has 33 gallons of water ssd for a 4-1/2 inch slump and that no air entraining agent was used. All of this is required for a mix to receive a shake on hardener. There is enough water in the mix for the hardener to hydrate. I like the slump; it is in the right range for good workability and consolidation. I see that there was a lab at the job site and they checked the air content. Someone should get an 'atta-boy' for that. The air content is right in line with a good non-air entrained mix at 1 %. I also see the design strength is above 4000 psi....right around 4500 psi. I have checked the admixtures in the mix and find no problem there. I see from the lab reports the slump was 4 to 5 inches throughout the pour. I also see that the 7 day strengths are very much in line and you should make 4500 psi in 28 days very easily. I would say that both the concrete mix design and the concrete at the job site gets a clean bill of health.

John, what was the condition of the subbase?
 

John-  
 
Well compacted earthen fill and 6 inches of well compacted granular base.
 
Phil-  
 
Did you pour over poly?
 
John-  
 
No poly.
 
Phil-  
 
Did you wet the subgrade down the night before the pour?
 
John-  
 
We did not have anyone on the job site until the day of the pour. We wet it down just before the pour.
 
Phil-  
 
Was the base dry the day of the pour?
 
John-  
 
It was dry but we wet it down petty good.
 
Phil-  
 
John, as you know concrete work is a lot like driving, there are green lights, there are yellow lights and there are red lights. You just ran a yellow light about to turn red. It is always a good idea to soak the subgrade to a depth of as least 2 inches. Placing concrete over a dry subgrade can increase plastic shrinkage cracks. Not only that but a lot of the top surface bleed water from the concrete will be lost to the subgrade. If the mix has a low water content there may not be enough moisture at the surface of the concrete to hydrate the cement in the floor hardener.

John, when did you apply the first of the hardener?
 

John-  
 
As soon as the screed passed we started to bull float.
 
Phil-  
 
Sounds like you got on it before the bleed water started to come up.
 
John-  
 
We did.
 
Phil-  
 
I hope you did not over do the bull floating. You know you are not trying to work up a finishing mortar.
 
John-  
 
All we did was one or two passes just to cut and fill.
 
Phil-  
 
How long did you wait before applying the first of the hardener?
 
John-  
 
The temperature was about 65°F. I guess it was about an hour.
 
Phil-  
 
Did you see any bleed water?
 
John-  
 
There was not that much bleed water.
 
Phil-  
 
I have found, that when the bleed water looks like beads of sweat, then you are just about right. Sounds like the dry subgrade almost got you!
 
John-  
 
Yes, you're right. We had a hard time getting the juice up! I am glad we only had a pound per square foot to put down. We could have been is big trouble.
 
Phil-  
 
I hope you waited until the hardener darkened in color before you started the floating operation.
 
John-  
 
I did not let my men on the slab until all the dry places were gone. We floated the outside edges of the slab first. We used a power float with detachable float shoes. Just like is says in your literature. I bet you did not think I read it.
 
Phil-  
 
I wondered to myself " before or after the problem"? I bet that he can quote it verbatim now!"

John, you have avoided two pitfalls most finishers fall into. One, you did not use combination float and trowel blades and two, you did not use a bull float to float the hardener into the surface of the slab.
 

John -  
 
I understand the reason for not using the combination blades. You do not want to run the risk of closing the slab to soon, which causes blisters. I do not use combination blades on any of my concrete for that reason. But, what have you got against the bull float?
 
Phil-  
 
The bull float is a good tool for what it is designed for...that is, to cut and fill and to work a little butter to the surface for finishing. But John, you must remember you are working with a dry shake at the surface of the concrete. While it is well mixed in the bag it is not mixed with water.
 
John-  
 
But, I have pulled moisture from the concrete into the hardener using a bull float before.
 
Phil-  
 
I'm sure you have, you have to work the cement paste around all the aggregate particles. However, the best tool for mixing and consolidating the hardener into the surface of the concrete is the power float using detachable float shoes.
 
John-  
 
I have used a bull float to float the hardener in but we always come back with the power float to work up a little butter for finishing. I wonder if those sand pockets in the surface of the hardener were caused by how I floated it.
 
Phil-  
 
Sounds like you did not get the cement paste in the hardener to coat all the aggregate particles.
 
John-  
 
Yeah, you're right, I will not make that mistake again! What do you think about the disk floats....or what my guys call pizza pans? I understand they can get you a real flat floor.
 
Phil-  
 
Yes, they can. But they should be used late in the floating operation.
 
John-  
 
Why is that?
 
Phil-  
 
The disk float gives you about the same mixing action as the bull float. A good finish needs the mixing action of the individual float shoes of the power float to work the cement paste around the aggregate particles.
 
John-  
 
It sounds like you have stock in some company that makes float shoes!
 
Phil-  
 
Not a bad idea. But honestly, I just use the tools that work. John, how did you put the hardener on the concrete?
 
John-  
 
We used a material spreader. One pass. We were putting down only one pound per square foot.
 
Phil-  
 
That's good. I would say about a pound and a quarter would be about the max for one pass. Much more than that you would have trouble getting the moisture worked through the hardener.
 
John-  
 
Phil, we got the right amount of hardener on the floor. We floated the hardener the way you said to do it. The hardener had moisture uniformly throughout. I cut the joints the same day. We cured it the way your literature required. In fact, we used your Dress & Seal. When I left the job my crew had just finished the final floating and every thing was looking good!
 
Phil-  
 
John, you lost this race at the finish line. You ran a red light and just got killed.
 
John-  
 
What do you mean?
 
Phil-  
 
You left the job before it was over.
 
John-  
 
The job was over! All that was left to do was to burn it out, put on a little cure and cut the joints.
 
Phil-  
 
Most of the hardener failures occur at the end of the floating phase and the beginning of the hard troweling. This is the time when you are setting yourself up for delamination. We have been doing all the talking, it is time we allow the concrete to speak for itself.
 
John-  
 
What do you mean?
 
Phil-  
 
Everything that has happened to this slab is written in stone or should I say concrete. Let's look at this area where the delamination occurred. John, you can see from this piece of delaminated hardener that it is hard and the cement paste covers all the aggregate particles. If you look at the floor there are no plastic shrinkage cracks. I can see you must have used our E-Con to control evaporation during finishing and you got the Dress & Seal on in time to control drying.
 
John-  
 
If we did such a good job, why did some of the floor come up?
 
Phil-  
 
You ran a red light!
 
John-  
 
Just tell me what we did wrong!
 
Phil-  
 
Again, let's let the concrete speak for itself. John, come over here to the area near the over head door. The concrete is trying to talk to us.
 
John-  
 
I have got to see this or should I say hear this. Ok Phil, what is it saying?
 
Phil-  
 
Do you see those little bumps on the surface of the floor? Those are blisters.
 
John-  
 
Yes, and what is the concrete saying?
 
Phil-  
 
The concrete is saying that it was closed to soon.
 
John-  
 
What do you mean "to soon"? That concrete was plenty hard!
 
Phil-  
 
John, even though the concrete may look hard, it was shrinking and trying to squeeze out some air and water vapor. By troweling the concrete too tight and too soon the air and water vapor could not get out of the concrete and collected at the interface between the hardener and the concrete. Being denser than the concrete, when the air and water vapor got to the hardener, they were stopped cold. The up lift pressure from the air and water vapor lifted the hardener off the concrete.
 
John-  
 
What could we have done?
 
Phil-  
 
When blisters start to appear, that is a red light and it means to stop troweling the concrete! You must rupture the blisters to release the air and water vapor by flattening out your trowel blades and stay off the slab for at least hour. When you do get back on the slab keep a sharp eye out for more blisters. Blistering is the way concrete is telling you that it cannot breath. Concrete needs to breath or it will blister!
 
John-  
 
My specs require a tight finish.
 
Phil-  
 
John, you can finish a floor hardener job just a tight as any other concrete floor. No problem. You just got to get your timing right.
 
John-  
 
So, you are telling me that my finishing caused the hardener to come up!
 
Phil-  
 
No, not me!! It is the concrete telling you that.
 
John-  
 
Ok! Ok! Phil, I guess I had to learn the hard way. Thanks Phil. On the next pour I will be looking for those yellow and red lights.
 
Phil-  
 
Sounds like you have got it. Oh, by the way, I hear that you have a big project coming up in Louisiana this fall.
 
John-  
 
Yes, that is right.
 
Phil-  
 
Let us plan a concrete emergency, for say, the first day of hunting season!
 
John-  
 
You are on! Hey Phil, don't run any red lights on the way to the airport!
 
Phil-  
 
You got it! I will see you in Louisiana on or about the first day of hunting season!
 

Back to ConcreteNews

© 2001 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2001.

Subscribe to ConcreteNews