Expert: Bruce E. Johnson - 8/29/2010

Question
QUESTION: My father and I have been building a garage and have poured three slabs 15'x26' and every one has come out wavy.  It seems to look level/flat but as soon as you start to trowel it, boom these low spots show up and you can't work them out because the concrete is setting up to much.  So how do you get away from these waves?  What are we doing wrong?  I forgot to mention we used a 2x6 with a vibrater attached to the top as a screed board.   This last batch did not turn out very good as we requested 10% rock and the rocks were huge and so there is low spots and also a golf ball tembling in it in some spots.  My father is very upset at how it turned out.  Thanks for any info you can provide.

ANSWER: Hi landon, finishing concrete is a learned experience and requires a certain amount of training and ability to do it right.  First of all you don't need a vibrating screed.  You are better off vibrating the concrete directly with the vibrator and then dragging the screed after that.  While dragging the screed you need to fill in any foot holes and other low spots as you go.  The above process should be done fairly quickly as you only have two hours or so to get the slab laid and flattened out.  Once you have the slab screeded you need to immediately follow with a bull float.  A bull float comes with six foot handles that can be added to each other to make it long enough to reach across the entire slab.

In your case the width of the slab is 15' so your bull float will need at least two handles, preferably three, to easily reach across the slab.  Bull floating isn't too difficult but it needs to be done properly to flatten out the surface of the slab.  The bull float is set at a slight angle when pushing it across the wet concrete so that the front edge of the bull float is about a half inch to an inch higher than the concrete and the back edge of the float does the flattening of the concrete as you push it forward.  Once your bull float is pushed all the way across the slab you lift up on it and pull it back to you so that now the rear edge is slightly higher than the concrete and the front edge is now doing the flattening (some of the newer bull floats have a special connection on them so that when you twist the handle at the end of your stroke it automatically adjusts the angle of the float for the return stroke). Make one pass forward and back and then move over 3/4 of the width of the bull float  and do it again.  Repeat until the entire slab has been bull floated.

This process helps knock the aggregate down and bring the cream to the surface.  After the concrete sets up to a point where it will hold an edge when you run your edger across the front, then it is time to fresno.  A fresno is attached to the bull float handles and is like the bull float but is thin and sharp like a trowel.  The fresno usually has round corners on it to keep from digging into the concrete surface.  The fresno is set at a little steeper angle than a bull float when pushing because the concrete is harder and needs more edge applied to it.

Usually after a fresno you can call it good if done right.  If you need more work after that you can get out on the slab with knee boards and hand trowel the slab or put a concrete finishing machine on it and let the machine do the work.

All of the tools I mentioned above can be rented at your local tool rental store..

As far as the concrete goes..a 3000 PSI mix with 3/4 minus rock is the best mix for this project.  That means that the aggregate is less than 3/4" in diameter...


I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..

---------- FOLLOW-UP ----------

QUESTION: Could the wavy pattern been caused by the vibrating screed?  We might not of went straight from the screed to the float soon enough.  We did not use a fresno.  I have never even heard of that.  Will have to check into that.  Thanks for the info.

Answer
Hi again landon, a fresno is not really necessary if you do the bull float properly and are able to do a decent job with a trowel..It sounds to me like you didn't do the bull float soon enough or it took too long to lay the concrete down that it was setting up too quickly to bull float properly..
sincerely bruce e johnson