Mixing concrete

[czechsonofagun]

Good Morning,

I talked to this guy, who poured concrete floor in his basement - using only sand and cement. He was saying it does not make too much of a difference as far as the concrete strength - and it is easier to level than gravel based concrete.

Sounds interesting to me - anybody tried/heard of that?

 

[zeuspaul]

You will need a well graded concrete (angular) sand but you need that with ordinary concrete. You will also need more cement in the mix as there is more surface area to cover with the cement if you are using just sand so the cost is higher for the same strength concrete.

When mixing by hand I prefer leaving out the rock and using extra cement as it is easier to work with.

Zeuspaul

 

[czechsonofagun]

Thank you.

I need to put down some floor - adding a hay storage to the barn - and I was thinking about skipping the gravel this time. Now what is the mix ratio without the gravel - 5 parts of sand and one part of cement?

 

[EddieWalker]

Prokop,

The guy is giving you very bad advice. Sand and cement created morter. It's good for holding brick and stone together, but has no structural strength.

The rock is there to provid something for the cement to bond to. It's critical for have rock in any sort of pad or footing!!!!! Rebar is added to keep the concrete from cracking, but isn't as important as rock for strength.

You can increase the strength of concrete by increaseing the amount of cement you add to the mix, but there is a limit to the effectiveness to this. Adding cement to Morter doesn't really do very much.

Morter is good for it's bonding ability. Not it's structural strength!!!

Don't do it.

Eddie

 

[hsvhobbit]

Your friend has a floor of mortor, not concrete. Eddies right, you need the rock to provide real strength to the mix.

 

[czechsonofagun]

But AFAIK for mortar - ok, at least overseas - they use mix of lime, sand and cement. Thats for brick laying and stones.

But i understand what you are sayng about the gravel, thank you.

 

[zeuspaul]

Mortars can be weak especially with a lot of lime. 5000 psi mortar is very strong. Standard concrete design is 3000 psi. I don't use any lime in a sand mix.


Quoted

http://www.masonryinstitute.com/guide/part1/gen3-p2.html

2.3. Compressive strength of mortars. Masonry compressive strength depends on both the unit and the mortar. As with concrete, the strength of mortar is determined by the cement content and the water/cement ratio of the mix. Since plastic properties are often adjusted to achieve proper workability and flow, and since bond strength is ultimately of more importance, higher compressive values are sometimes sacrificed to increase or alter other characteristics. For loadbearing construction, the building codes generally provide minimum allowable working stresses, and required compressive strengths may easily be calculated using accepted engineering methods. Strengths of standard mixes may be as high as 5000 psi, but need not exceed either the requirements of the construction or the strength of the units themselves. Although compressive strength is less important than bond, simple and reliable testing procedures make it a widely accepted basis for comparing mortars. Basically, compressive strength in creases with the proportion of cement in the mix and decreases as the lime content is maximized. Increases in air entrainment, sand, or mixing water beyond normal requirements reduce strength values.

 

[TwinWillows]

What Eddie said about the rock (aka coarse aggregate) is spot on. Concrete is extremely strong in compression 3,000 to +5,000 pis is not uncommon and extremely weak in tension. Thus rebar is needed to "carry" the tensile forces & keep the item from cracking. For the rebar to work it needs to be placed in the area of the item that has the tension forces. For a variety of reasons, including ease of installation, in most cases this ends up being the middle of a wall or floor - beams are totally different since the location of tension forces changes from the ends to the middle.

Way back when lime was used in mortar because the cement making technology was not very advanced nor was cement widely available @ a reasonable price. Lime mortar is much weaker than good portland cement based mortar.

 

[zeuspaul]

The question isn't how to make the best or strongest concrete or using mortar as a basement floor or where to place the rebar. Mortar generally has lime. This would better be classified as concrete made with fine aggregate.

The basement floor is not supporting any structural loads at least none were indicated. 3000 psi concrete is more than adequate. 2000 psi concrete would probably suffice. This can easily be achieved without larger aggregate.

You can make concrete without the coarse aggregate that will be sufficiently strong for a basement floor. In some cases strength is not the guiding factor. Fine aggregate (sand) is easier to handle for the home owner. I can unload a yard of sand from my pickup about three times faster than a yard of gravel. Also mixing concrete with larger aggregate is a lot noisier. So out of respect for my neighbors and to minimize my labor sometimes I opt for sand only concrete.

If I were having ready mix concrete delivered or if I were using it for a foundation I would always use rock. However for a sidewalk or a basement floor that I was doing myself I wouldn't have a problem trading in some strength for for some reduced labor.

Zeuspaul

 

[unreconstructed]

Referring to the post about needing less strength in a basement floor, you might consider a smaller rock. On a tip from a builder, I have used pea gravel for a lot of my little projects and it works fine-easier to shovel and easier to finish. I am but an amateur, but I have done a lot of masonry work. I have a hard time visualizing pouring any kind of slab with just sand and cement. I always thought that was called mortar.