Lightweight concrete for fireplaces and decorative stone

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Lightweight concrete for fireplaces and decorative stone

Suncoast stone sells and ships concrete stone fireplaces out of our Palm Beach County Florida location. Part of the difficulty with selling fireplace surrounds online is that they are very heavy items to ship and also difficult to install for the average DIY-er just because of the huge amount of weight. We had to come up with a solution.

Firstly I have to say it was not as simple as you might think to make lightweight concrete. You would think that all you have to do is get a lightweight aggregate and you are done , right?
Well…. yes and no. Using a lightweight aggregate is one answer. But it is only one answer out of a variety of options we tried, and lightweight aggregates have their own problems.

First we identified the problems of shipping fireplaces to DIY-ers. These are not so obvious at first, but the more we did this , the more we hit these roadblocks.

  • a DIY-er with limited tools should be able to install a fireplace surround.
  • Preferably it must be light enough for 2 men to install.
  • It must not be so heavy as to add more cost in shipping.
  • Dimensions of shipped product should be kept low so the shipping costs are low enough to pass on savings.
  • The stone fireplace must be strong, so that it does not get broken when shipped. Lightweight mixes seemed to be weak.
  • The packaging has to be good so that any minor bumps would not hurt the fireplace
  • Easy mounting solutions.
  • Fireplaces must be Priced low enough, that even with shipping it can beat the competition.

We  soon figured out that weight and shipping costs are the most important factors. Here are the reasons.

  • If the fireplace is easy to install, then the customer will be happy. A lighter fireplace is easier to install.
  • A light fireplace has less dead weight, and so is less likely to break. This is because the inertia increases with weight, and higher inertia means higher impact leading to higher chance of breakage. A low weight means less breakage. The shape of each fireplace piece is actually a strong shape and when combined with light weight drastically reduces the chance of a breakage.

Here is a list of some solutions that were available to us to make lightweight concrete fireplace surrounds.

  • Do not make it solid, make it hollow to save on the volume of concrete (obvious solution)
  • Play with the mix design to get a lighter mix
    • lightweight aggregates
    • very thin wall fireplace construction
    • reinforcing with different fibers
    • using modern concrete admixtures to improve strength
    • air entrainment to reduce weight.
  • packaging design is crucial to reduce breakage.
  • packaging design can reduce dimension of shipping packages, and so reduce the shipping cost of concrete products.

Making fireplace surrounds hollow.

This was not too difficult to solve, there are more ways than one to do this.

We could create a double skin fireplace surround mold so that the wall thickness is about an inch. This would mean that the mix design we create has to be flowable concrete, so that we could just pour the concrete into the mold.

Another option is to do thin wall GFRC. This could be self consolidating GFRC , or sprayed up and then a backer coat added, the thickness would be less than an inch.

Yet a third option is to use something like hydrocal plaster layup. But that limited us when it came to surface texture, and the curing time was quite high, and price of materials was also high.

We now do GFRC and regular pourable concrete. It just depends on the mold.

Lightweight Aggregate Concrete Mix Design

This is where we did the most experimentation. We have probably extensively trialled over 20 mix designs, with over 10 aggregates. Our main source of pain here is the cost of getting aggregates to our facility, since most are not local.

There are only 3 lightweight aggregates that are easy to get (for us). Those are 1. Perlite and 2. Vermiculite 3. regular polystyrene beads.

here are a few of the other lightweight aggregates we tried.

  • expanded clay
  • expanded slag
  • expanded glass
  • expanded slate
  • expanded shale
  • scoria
  • pumice
  • fly ash

For our needs we did not really require course aggregate, anything up to 1/4 inch was fine for us, however we predominantly chose only fine aggregate for our mix designs, this was because it was usually hard to find lightweight coarse aggregate that had sufficient strength.

For the 3 lightweight aggregates that were easily available to us (perlite, foam beads, and vermiculite) we found that vermiculate and plain polystyrene foam beads were too difficult to work with, and also produced undesirable results. For example, we were not able to make a mix with plain polystyrene beads that was pourable, and also did not separate (beads floated up through the mix). Vermiculate was ok when soaked, but seemed to be very weak, when used with a good percentage of the aggregate.

Perlite was fairly weak, but when used in combination with water reducers, GFRC polymers and AR glass fiber, produced adequate strength, although the mix was quite expensive with all those additives. By itself perlite is too weak.

The heat expanded aggregates were stronger than the others, and were all quite good, however due to shipping costs, these were not financially viable for us. If we could get the delivered cost down , we would probably choose the expanded glass product.

Air entrainment proved to be an expensive experiment.

Using chemicals to induce air entrainment did work, and there is a product from surecrete that is pre-packaged as a lightweight mix, but is very expensive, and also has color issues (its comes out a blotchy earthy color).

Using the method of air entrainment by adding foam suds into a concrete mix seemed promising at first, but results were difficult to get consistent, and it is very hard to source a foaming product (Goodcell seemed ok, but are hard to get hold of).
When we finally tested the strength it was obvious that air entrainment was too weak. We tried to go back to our strategy of using polymer modifiers and glass fiber, but the polymers include a defoamer, and the defoamer would end up killing the foam. It’s just too bad we could not find a polymer that did not include a defoamer.

The pre-foam method had the added benefit of making the mix flow extremely well, however either the bubbles burst or the mix was weak.

Interesting facts

During all our tests we found that we could get extremely good flexural strength with some of the mixes that we tried. And in one instance using city-mix polystyrene, we got a product that was like wood. However this mix was better as a backing coat.

Our Current lightweight concrete

We currently do not tell anyone what is in our lightweight mix. It’s not really a secret, but we don’t want to make it easy for our competition. There is a delicate balance between these 3 factors. 1. the aggregate, 2. the strength of the aggregate 3. Additives adding strength. Getting the balance right gets us to our goal weight of 85 pounds per cubic foot of concrete. (60% weight), and our goal strength of 5000 psi, with good flexural strength.

We also do still use regular and SCC GFRC. We have not continued with GFRG, however it was promising.

Please view our concrete fireplace surrounds, we are in Delray Beach Florida .

 

By | 2017-02-23T13:58:28+00:00 August 16th, 2016|Categories: fireplaces, lightweight|0 Comments

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