New builder - use lintels for hearth base

[RobDodds]

I'm just starting project for modular pizza oven. Have laid 8" thick reinforced foundation onto which I'll build concrete block base. My questions are to do with materials for hearth. I have concept of using concrete lintels 100mm X 65mm (4" X 2.5") laid between the concrete blocks then topped with 12.5mm Hardibacker board, then either making insulating layer of vericrete & CALSIL board or just CALSIL board without Vermicrete. I figure maybe I'll need 4 lintels to bridge the 1.2M square gap adequately. Does anyone have any experience of doing this rather than pouring concrete hearth? Are there any risks or potential poor performance with oven? Cooking surface will be firebricks.

Also thinking if I can use 140mm wide hollow dense concrete blocks rather than normal 215mm wide as they should be lighter to handle, with every other hollow filled with concrete - is this likely to be stable enough?

Thanks in advance for advice - don't want to screw up through trying something new.....

[Lburou]

Re: New builder - use lintels for hearth base

The smaller blocks should be just fine. About the concrete lintels. I'm not exactly sure what you mean there. If the lintel has a lot of steel in it, might be OK, if not, no!. An optimum hearth is going to have steel reinforcement across the space between the blocks, from side to side and front to back.

[RobDodds]

Re: New builder - use lintels for hearth base

The lintels have pre-stressed steel reinforcement and I calculate gaps between lintels will be less than 6 inches. I would expect the Hardibacker board would be strong enough. However my head is telling me not to experiment and just mix up the concrete like everyone else does. I think I'll build up the base then decide. Thanks for the advice. It's a great forum where first timers can learn from those that have done it before.

[deejayoh]

Re: New builder - use lintels for hearth base

I wouldn't have installed my oven on anything but solid concrete under the insulation. Finished ovens probably weigh somewhere close to a ton. Putting that much weight on non-structural materials (e.g. hardibacker, insulation board) that have unsupported spans - even if they are only a few inches long - sounds like asking for trouble. I think chances are very high will sag over time.

I'm not sure what you gain in time or cost savings by using lintels instead of concrete. Pouring the concrete hearth is probably the easiest step of an oven install. If you're in any way thinking in that step is too much effort (and I not saying you are, but if you are) then you probably aren't going to enjoy the rest of the process much.

[RobDodds]

Re: New builder - use lintels for hearth base

You're right that I'm not looking forward to mixing and pouring the concrete and that's why I'm looking for alternatives. I took advantage of having some builders at the house to get them to pour the foundations - it won't be available to me when I do the hearth base. I've never mixed concrete and have no concept of how many bags of materials I'll need and in what proportions. I'm realising now that I need to do some more research and just bite the bullet and do it properly. As you say I don't need a sagging base over time - I'll one get one chance to do the oven.
Once again, thanks for the advice everyone.

[Mingy]

Re: New builder - use lintels for hearth base

The first thing to find out is can you get concrete delivered. Lots of places won't for a small amount. Failing that do you have access to 'reddi mix', or whatever its called where they basically fill up a trailer with concrete and you tow it to your site. You obviously need a heavy enough vehicle to tow it. The last and worst choice it to mix yourself. I'd advise making your own (vs. bags of mix) because it is way cheaper. Whatever you do, make sure you have more than enough to do the work, because 'two pours' is a problem: concrete doesn't like to stick to concrete.

I am lucky in that I live on a farm and own a number of tractors, so I used one to mix the concrete and another to move the material around. Most people have friends. If you have friends, maybe they can help. Lifting buckets of concrete is hard work.

[deejayoh]

Re: New builder - use lintels for hearth base

Rob -
My hearth was my first concrete pour as well. Don't sweat it. watch some online videos or better yet put the call out on facebook for a friend that can guide you. Calling around for someone who can deliver small quantities is another great idea. But the pour is really a pretty easy task, and done in a day. Special bonus is that it has a lot of "wow" factor in terms of feeling like you accomplished something.

.

[Gulf]

Re: New builder - use lintels for hearth base

All good information. You can get all the info you need about pouring concrete. If you decide to borrow or rent a mixer, the placement of the mixer can save a lot of work. For the foundation it can be placed right at the pour to where it can be dumped right into the form. I used a home made trough to help get it to the back. On my foundation I scavenged sand and gravel from the county barn and made it from scratch. On the hearth pour I set the mixer up on saw horses floored with scaffold boards. That gave me the height needed to pour it the same way. I used bagged premixed for the hearth pour. I just backed the pickup with the bags of quikrete right up to the scaffold for easy handling.

Making it from scratch is cheaper if you can acquire the sand and gravel for free. By the time I got to my hearth pour my sources had dried up. Buying the sand and gravel (in my area) made the premixed comparable in price.

It is a two man job though. A half day for each pour if every thing is set up and ready to go before you start.

[UtahBeehiver]

Re: New builder - use lintels for hearth base

RobDobbs,

I poured both the base and the hearth with bag mix since having a "short" load delivery cost was out of sight. The redimix people really don't like to deliver small loads.

Like Gulf said, placement of the mixer is key to saving time and it is a two person job. Plus a sidewalk superintendent (me) in the pic below

Russell

[RobDodds]

Re: New builder - use lintels for hearth base

Thanks guys - definitely some good advice. I'll start to scour the forum and internet for info on concrete mixes etc. You've convinced me to pour the hearth. I'll have to mix it myself as my local companies will only deliver a minimum of 1 cubic metre - I need 0.2 cubic metres max. Access is awkward so I think wheelbarrows will be required after mixing and my 14year old son will be getting extra pocket money as a labourer.

[david s]

Re: New builder - use lintels for hearth base

Buying pre-mixed bags is way more expensive (at least it is here) than buying the sand and aggregate at a landscaping supplies place. If you fill 20 litre drums you can transport them in a sedan in a few trips.

[Lburou]

Re: New builder - use lintels for hearth base

One advantage to using a well chosen prepackaged concrete product is that the proportions of ingredients will be consistent and correct. That could be important to your doing this for the first time.

Premixed bags are also less mess in the back seat of the car It won't cost that much more....Its a Small percentage of the total cost to go ahead and use the premixed bags. I say go with it and use the bags for your first concrete experience. Buy the components separately next project.

[dvm]

Re: New builder - use lintels for hearth base

We poured our hearth this morning - I got a one bag cement mixer from Harbor Freight for around $150. Portable Electric Cement Mixer - 1-1/4 Cubic Ft. I will use the heck out of it on this project then probably sell it. A few small points my sons (20 and 13 years old) and I worked out.
1) Using bagged pre-mix: mix a few bags slowly to calculate the right amount of water needed per bag - mark this volume on a bucket.
2) start with 3/4 the water needed in the mixer
3) cut across the middle of the (90 pound) bag the short direction and lift below the cut - you will end up with two open paper sacs - each holding about 45 pounds of premix
4) dump in one "half bag", then the second (easier than pouring from a 90 pound bag into a small mixer) and add the final bit of water.
5) we then dumped into several 2 gallon buckets for transport (my 13 year old would not have lifted a 5 gal bucket)
We also purchased and uses a concrete vibrator. 3/4 HP Concrete VibratorI don't know how necessary this is, but it absolutely settles the wet cement into the form quickly.
I know I am making memories with my sons they will remember (my dad was also in town and lifted a few buckets) - it was a good day. Good Luck.

[Olly]

Re: New builder - use lintels for hearth base

Hi Robb

We used reinforced concrete lintels for our hearth base. There are two steel reinforcing bars present in each.

xcm length to cover the distance of the block work by 210mm x 75mm.

They do the job and it has saved a load of hassle