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.

 

[Egon]

Check out " The Portland Cement association " all the information required will be found there.

Sand or gravel does not matter as long as the mix is designed to fill the voids. Angular faces are irrelevant as cement is a type of glue that has an infinite hardening life span.

As previously stated: well graded agregate mix is cheapest as it requires less portland cement in the mix to acheive the desired strength. The ultimate strength of concrete will depend on the ultimate strength of the aggragate.

There are also different types of Portland cement for different uses.

Aggragate maximum size must also be considered for rebar spacing. Rebar should also be place a minimum distance from exterior surfaces.

There are also addidtives that may be added to the mix to give more flexibility to the concrete.

Concrete must also be handled and placed following certain directives to avoid segregation and a host of other problems. Most of these directives make the finishing of the product more difficult.

Aggragate is added to portland cement to reduce the cost of the concrete.

Mortar for bricks or concrete blocks may be made using cement, aggragate and lime mixed in the proper portions.

It may also be made useing a mortar cement and aggragate mixed to proper proportions. In this case Portland cement may be added for additional strength but you lose some of the sticky qualities.

 

[drm]

Mortars can be weak especially with a lot of lime. 5000 psi mortar is very strong. Standard concrete design is 3000 psi. .

Standard NON Structural concrete mixes may be as low as 3,000 psi depending on the area but normal STRUCTURAL concrete is normally 4,000 psi to 5,000 psi depending on the geographical area and the application. Concrete compressive strength can be as high as 13,000 psi.

... 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.

Normal mortar compressive strength is much less with the standard mortars only 1,500 psi. The average structural mortar is only 2,500 psi.

If you use a concrete mix without aggregate be sure you use enough reinforcement or you will have lots of cracking even for floor slabs on grade. If pea gravel is used with no large aggregate the potential for cracking over using sand only is reduced somewhat be still exists.

Regardless of the application, reinforcing is required to keep the cracking to a minimum and to keep the widths of the cracks that develop to a minimum and keep the joints tight. In slabs on grade welded wire fabric (mesh) is often used but a small (#4 or ½” diameter) rebar spaced at 12 to 18 inches is a much better option. The size and spacing of the reinforcing is dependent on a number of factors including the slab thickness, joint spacing, slab size, concrete strength, and aggregate size.

 

[MikeD74T]

Egon posted the industry standard for concrete. Coarse aggregate, ie crushed stone, is a filler material. It adds compressive strength only to the point that it has compressive strength and has sufficient bond to the plaster. Light weight concrete mixes use fillers such as vermiculite / perlite that do not have much compressive strength because it's light weight is a more important than it's adequate compressive strength.

To get a better understanding of reinforcing concrete for tensile strength do a search for ferro cement. Ferro cement goes back a couple of hundred years and has a high ratio of steel to concrete. Huge WWII troop ships were built of cement plaster and rebar & chicken wire.

Fiber reinforced concrete goes back to adobe roots. While suitable for some jobs it's neither revolutionary or a substitute for better solutions. I't's certainly not a substitute for steel unless the steel is left in the dirt.

The biggest single factor that TBN members can control, besides not over wetting the concrete mix, is soil preparation. Remove soil that will swell when wet & replace with good draining soil that will compact. Expanding soil places the slab in an upward tensile condition which concrete doesn't handle well unless it's specifically designed to. Similarly uncompacted or slippery (like flowable sand) soils will not support the slab well enough. Poor concrete on good soil will last longer & perform better than good concrete on bad soil.