Mixing concrete

[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.