I have a slab outside my back door which I am interested in adding to, in thickness and overall size. How do I add to the thickness of the existing pad without removing that slab and starting from scratch? There has to be a way to bond the two together. I would like to build a covered patio at somepoint down the road as well. Do I need a thickened edge? My experience with concrete is limited but with the proper giudance I am smart enough to get it done.

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Originally posted by frednaylor:
My experience with concrete is limited but with the proper giudance I am smart enough to get it done.

No, you're not. The biggest cost with pouring concrete is the actual cost of the concrete itself. By comparison, the price of an experienced guy to work the concrete and make a level and smooth slab is very minor - probably only about 20% of the total project cost. If you are not experienced in pouring concrete there is absolutely no reason to try this yourself - just a little inexperience in mis-ordering the amount of the pour will cost far more than the price of skilled labor to work the slab. I don't mean to be harsh with this but it's just a fact - why spend hundreds or thousands of dollars on concrete to get an unsatisfactory result when by spending just a little more it can look perfect.

That said, there are concrete bonding agents that can help two slabs stay together. The old slab can also be scored or pinned to help the two stay joined without cracking apart. The thicker the new slap on top is the better chance you have of making the two slabs work together. There are also different blends of concrete mixes, different gravel sizes and fiber additives that can make this work better. Again, a pro will know what to order here.

If any of the slab is to be used as a footer for a porch structure it will need to be deep enough to meed frost heave requirements in your area and thick enough to withstand the load.

I would make a few phone calls to concrete guys first, get some quotes and then proceed.

This message has been edited. Last edited by: Jaybee,

Fred:

I, too, have limited experience with concrete, but I'll tell you what little I know so that you can at least have something to build on.

Normally, new concrete will not stick to old concrete. In order to make repairs to buried concrete sewer pipe, for example, contractors will use something called a "concrete bonding agent" which you can buy at any construction materials wholesaler (who usually will sell to you if you pay cash) and at many home centers.

Basically, a concrete bonding agent is just like white wood glue...with one very important difference.
And that is that while white wood glue will re-emulsify if it gets wet and stays wet for a while (even if it's previously been dry for years), a concrete bonding agent won't. Instead, a concrete bonding agent has a "time window" during which the moisture from the new wet concrete will re-activate the adhesive so that the new concrete sticks to it. And that time window can be anywhere from a few hours to a few days. After that time window passes, a chemical reaction occurs in the concrete bonding agent making it unaffected by moisture.

So, in practice, if you wanted to bond new concrete to old concrete, you would paint the concrete bonding agent onto the surfaces of the old concrete you wanted the new concrete to stick to, and pour your new concrete within the time window of that concrete bonding agent. The moisture from the new concrete would re-activate the adhesive that had already dried on the old concrete and the new concrete would stick to the old concrete as it cured. Once that time window expires, the concrete bonding agent will be unaffected by moisture, and so you don't have to worry that if your concrete slab gets wet it's going to fall into pieces.

Also, if you're simply extending the area of a slab, it's a good idea to also "pin" the new concrete to the old. Basically, the idea here is that you use a powerful kind of hammer drill (called a "rotary hammer" made specifically for drilling into rock and concrete) to drill holes into the edge(s) of your existing concrete slab. You then clean any dust out of hose holes, fill them partially with an epoxy adhesive, and push a piece of re-bar into the hole so that a tiny amount of epoxy comes oozing out of the hole just as the rebar is fully inserted. So, if you drill 6 inch deep 3/4 inch diameter holes you might use 12 inch long pieces of 5/8 inch diamter rebar in them. After the epoxy cures, then there will be rebar "pins" sticking out of the edge of the old slab. When you pour your new slab, the concrete will cure around those ribbed pins and pin the two slabs together. In this way the two slabs can't separate and their edges will always be level with each other.

For a curb, you can buy rebar bent into "U" shaped pieces which you epoxy into holes drilled into the top of the slab around it's perimeter. Epoxying the pins or "U" shaped hoops in is very much stronger than using a concrete bonding agent, so I don't expect it would ever be necessary to use BOTH a concrete bonding agent and pins. You'd typically use one or the other, depending on the application.

I don't know much on this topic, but at least you know just as much now, and you can start talking to more knowledgeable people about what you want to do.

I totally agree with Jaybee. Hire this one out.