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Concrete Flatwork

user-284410 | Posted in Construction Techniques on May 2, 2002 07:21am

This is my first time I have used this forum so I don’t know what to expect or how it works. 

I am looking for some advise from someone with experience in concrete work.  We recently poured a front porch slab only to have the homeowner tell us she wanted the concrete level to be about 2″ higher.  The obvious solution is to pour a new 2″ thick slab on top of the first slab, but of course I need to do it correctly so I don’t end up tearing the whole thing out and starting over again.  I also want to add 2″ to the face of the porch so I don’t end up with an unsightly line between the two slabs.  Any suggestions?

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  1. slash | May 02, 2002 03:53pm | #1

    Adding 2" to the top should be no problem. I'd add about 4" to the face so I could get a vibrator between the existing foundation wall and the formwork. How much does the footing project? Adding some welded wire fabric where the slab meets the wall could be tough unless it was mat type. Fiber mesh might help.

    1. user-284410 | May 02, 2002 06:06pm | #2

      The footing does not extend beyond the face of the porch so I need to somehow tie the new face to the old.  Also 2" is really the most I can add to the face.  I plan on using a six sack pea gravel mix so I was hoping to work it between the face of the concrete and the form by vibrating the outside of the form.  Would you recommend using some sort of adhisive between the old concrete and the new?

      1. slash | May 02, 2002 11:03pm | #3

        A bonding agent could only help. Could you just use thin pavers at the edge?

        1. user-284410 | May 03, 2002 03:58am | #4

          Thin pavers at the edge is a good idea, but I don't think it would work in this application.  Is your concern that a 2" thick concrete face might crack?  I should tell you that my plan is to use wire mesh attached to the face of the old concrete and wraped up over the top edge using either concrete nails or masonry screws to hold it in place.  If your experience tells you that all of this would be to no avail I could always break back the old concrete 2" so as to end up with a 4" thick face.

          1. Gabe | May 03, 2002 04:03am | #5

            Bush hammer the surface, use a latex bonding agent and apply a non-shrinking grout instead of any concrete mix.

            Gabe

          2. user-284410 | May 03, 2002 04:27am | #6

            What is your reasoning for using non-shrink grout as opposed to concrete?  I know that non-shrink grout is far stronger but I will be needing over one cubic yard.  To my knowledge non-shrink grout comes in 60# bags and would end up being very expensive.

          3. Gabe | May 03, 2002 02:24pm | #7

            It's the only way that you can be sure that you won't have a failure on the vertical plane.

            The rate of drying is different behind forms as opposed to exposed flat work, even if you do a controlled wet cure.

            The shrinking during drying could cause separation, even with bonding agents.

            Gabe

          4. user-284410 | May 04, 2002 08:05am | #10

            I should have mentioned that we would be stripping the forms as soon as the concrete has cured enough in order to finish the face.  Would that make any difference?  Also as Slash has suggested I am thinking of pouring the concrete flush with the edge and finishing the face with stucco.  Do either of you have any suggestions as to how I might apply the stucco so the face will match the concrete surface?

          5. Gabe | May 04, 2002 03:56pm | #11

            If you're only going to top it off, use a sack rub on ALL vertical planes to blend it in.

            Gabe

          6. user-284410 | May 04, 2002 05:47pm | #12

            My experience is mainly in carpentry.  Would you explain the proper technique for sacking and the type of cement mix used?

          7. Gabe | May 04, 2002 10:55pm | #13

            A lot has to do with the colour of the concrete you want to cover or match.

            In a nutshell sack rub is simply using a burlap bag as a brush and rubbing on the surface.

            Here, portland cement is used to give a darker look. Other types give lighter tones. A fine sand (50/50)is mixed for texture.

            You can also use the same grout for a sack rub by the way.

            But I'm more curious as to where this porch is located. If you're in a cold climate, subject to freezing, you still have to add air in your mix to protect it during freeze thaw cycles.

            Is this porch too big to simply do over?

            Gabe

          8. DaveRicheson | May 05, 2002 12:19am | #14

            Gabe,

            I thought you had laeft with so many of the others. Good advice, as always, and I'm glad you still check on this site.

            Dave

          9. Gabe | May 05, 2002 01:46am | #16

            I still hang around, but I do spend most of the time at Joe's site. It's pretty laid back and familiar with James and Ron heading the pack.

            Joe keeps encouraging us to try new things with the computers and I need the push every once in a while.

            Gabe

          10. user-284410 | May 05, 2002 12:29am | #15

            I live in the San Francisco Bay Area so the climate is not a problem.  The porch is 20'x6' with  a substatial footing and has #4 bars @ 16" o.c. both ways.  I really don't want to tear it out and re-do it.  With all the information I have received from you and Slash I am now planning to pour the new slab flush with the existing face.  I will just have to experiment with the sacking until I get it right. 

            Thanks so much for all of the input. 

          11. Gabe | May 05, 2002 01:47am | #17

            Any chance that you could post a picture so that I could maybe give you a formula to try and match the existing?

            Gabe

          12. user-284410 | May 05, 2002 04:15am | #18

            I do not have the means to post a picture.  The  new concrete mix will be a six sack pea gravel mix using portland cement.  Does that help?  (Of course I want to match the new pour as it is the one that will be exposed)

          13. Gabe | May 05, 2002 03:28pm | #19

            Portland is dark so use it with an even amount of fine sand or even silica sand. Wet down the surface really well with water and apply the mix.

            Gabe

          14. slash | May 03, 2002 11:19pm | #8

            Why not pour the 2" topping flush, then stucco the walls to hide the new concrete?

          15. user-284410 | May 04, 2002 07:49am | #9

            Kind of what I have been thinking.

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