? about fiber in Sakrete

[Jim Mackell]

I have a small patio patching job I need to do this spring. The concrete will be about 4 inches thick and I'm worried about cracking. I estimate it's only about 7 or 8 bags of Sakrete or equivalent. Is there a fiber product that can be added to the Sakrete mix? This job is way too small to order ready mix.

Thanks!!

[Sandy Smith]

I wouldn't worry about adding fiber. Can you get 5000 lb mix from your local supplier? That product is plenty strong for any home owner patio project. 4 inches with this product is way strong. Just remember to use as little water as possible. About 1 gallon for 80lb bag should be plenty.

[Jim Mackell]

Can you get 5000 lb mix from your local supplier? That product is plenty strong for any home owner patio project.

I plan on picking up bags of Sakrete or equivalent at Lowes or HD. Do they make a 5000 lb mix?

[Jim Rimmer]

I don't know if it is available in your area but here there are a couple of places that have concrete in a mixer you pull behind your truck for jobs that are to small for a truckload and too large to hand mix. I've never used them so I don't know about the cost but thought I would at least check them out if I did have a need.

[Stephen Tashiro]

In my area, Lowes Hardware stocks a "crack resistant" version of Quickrete. I suggest you visit the Sakrete website, if there is one. They probably sell a variety of concrete mixes.

To me, the bagged mixes look like they don't contain enough aggregate.

[Scott T Smith]

The most important thing to preventing cracks in concrete is to use as little water as possible, do not use too much sand, and to mix it thoroughly. I like to mix the sand, cement and aggregate well before adding water. If your mix is "soupy", you've got too much water.

To increase the strenth of a pre-mix such as sacrete, simply buy an extra bag of cement and add some of it to the sacrete mix. Usually a shovel full of cement added to a bag of sacrete will increase the cured strength by around 500 lbs. Thus, if you add 4 shovels of cement to a bag of 3000 psi pre-mix, you should end up with 5000 psi cured concrete.

[Stephen Tashiro]

It's an interesting question how much the cracking of your slab is related to the strength of the concrete and how much it is related to its ability to expand and contract horizontally. If you had a big void under the surface and rolled heavy machinery across it, I suppose the stronger concrete is better. But how does strength vs downward pressure relate to cracking due to uneven heating over the surface and side to side stress?

I've been casting some simple wall cap block with concrete and have developed a great curiosity about how fiber additives would change the material (not that I need any reinforcement - I'm just curious because I've been working with concrete for the past few weeks). I don't see any sources where a homeowner can buy the fiber additives. Is this because of lack of demand? Or does it require some industrial process to get them mixed with the other ingredients?

I see that Sakrete makes both a "crack-resistant 5000" and a "crack-resistant" concrete mix. http://www.sakrete.com/products/product.aspx?ID=CrackResistantConcrete

[Brian J McMillan]

The fiber that is used in concrete as well as stucco base coat is chopped fiber glass. Long strand is used in concrete and short strand in stucco. Usually a whole bag to a yard of concrete, bag and all as the bag will dissolve in water. If your hand mixing and want to use the fiber use the short strand, a small handful to a wheel barrow as the long strand is made for a ready-mix type of mixer and will ball up on you. You should be able to get short strand at a plaster-stucco supplier.

An alternative is to locate some hemp rope and chop you own.

[Harlan Coverdale]

The PSI rating of concrete has less affect on cracking than several other factors, particularly the flexural strength - which is helped with fibers - and the percentage of air in the mix. Pretty much what Steven said above. The crack-resistant Sakrete has fibers in the mix. In freezing climates, it's also common to add air entraining agents to the mix to serve the same purpose. I don't know if there are such agents available at the Borgs, though.

Using too rich of mix can actually increase the chance of hairline cracking due to heat during curing. And as mentioned above, too much water can also cause cracking problems due to shrinkage during curing.