Thread: Structural strength of builders cement

Originally Posted by Malcolm Eaton

Concrete Mix
Light domestic footings
1 cement
2.5 clean sharp sand
5 aggregate ( 50/50 mix ie size 50 of 20mm/ 50 of 10mm)

Reinforced beams,floor slabs , drive ways and paths
1 cement
2 sand clean sharp
4 aggregate ( 50/50)

High Strength, thin reinforced walls and columns
1 cement
1 sand clean sharp
2 aggregate ( 50/50)

In your situation I would have gone for a mix as follows
1 cement
3 sand
4 aggregate

Cheers.
Mac

Hi Mac

4:2:1 should give you 20MPa

4:3:1 would be something less

and for Mr Potatohead, you should mix all the dry components before adding water.

http://www.concrete.net.au/publicati...retebasics.pdf

Originally Posted by movay_2008

Mixing the cement for a good while by itself in water with a good paint mixer gives a stronger result.

I've never heard that one. Do you have a reference?

Wow, thats a lot of good advice guys!

Theres a few theories about how deep the holes should be. One website says 'half the height of the exposed post' whereas another says about 1m. I dont particularly mind.

The concrete is done using a mixer, and its interesting to mix the dry first, Ive always done it the other way round (start with a heap of water, add the dry then add water til the right consistency) but im no concreter.

Thanks a bunch guys!

Pete

Try to keep it covered with a waterproof material to slow the setting process for as long as possible. This will also increase the final strength.

I cheated and used coppers logs 4 meters long, sunk 1 metre into the ground off set arout 5 degrees from vertical passed the plane (i.e. tension to the left - offset to the right) with a hand mix of 4:2:1

That was about 7 wet seasons ago now and 1 near miss cyclone and they haven't moved.

Movay - that is an awesome pdf! Tells me everything I need to know for various strengths of concrete.

Cheers!

It has to do with cement not dissolving as well when sand granules are present.

I saw some test results but can't find them now. You might find it here somewhere ACI General Information & Introduction





There is this on Wiki.




Thorough mixing is essential for the production of uniform, high quality concrete. Therefore, equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work.
Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete.

Concrete - Wikipedia, the free encyclopedia

Originally Posted by movay_2008

It has to do with cement not dissolving as well when sand granules are present.

I saw some test results but can't find them now. You might find it here somewhere ACI General Information & Introduction





There is this on Wiki.

Thorough mixing is essential for the production of uniform, high quality concrete. Therefore, equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work.
Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete.

Concrete - Wikipedia, the free encyclopedia

Thanks for that, but a caution for those who make concrete in small batches and are thinking of premixing the cement and water ...
The abstract of the article referenced on Wikipedia ends with
"Evaluation of data shows specific parameters are necessary for optimum performance"
which suggests to me that you would want to read and understand the whole article before attempting the pre-mix technique.

Thanks for the info Movay.

What does the 20 mPa mean...is that a compressive load it will take? If so does the 20mPa assume for that load a certain foundation must be used??? ie sand.

MPa

Concrete strengths are customarily denominated in psi (pounds per square inch) in the imperial system and in MPa's (megapascals) in metric. These are units of pressure.

Concrete Strengths
Nominal MPa values of equivalent psi concrete strengths
In metric, concrete strength is denominated in megapascals (MPa)
In imperial, concrete strength is denominated in pounds per square inch (psi)
2500 psi = 18 MPa (17.23 MPa exact)
3000 psi = 20 MPa (20.67 MPa exact)
3500 psi = 25 MPa (24.12 MPa exact)
4000 psi = 30 MPa (27.57 MPa exact)
5000 psi = 35 MPa (34.46 MPa exact)
6000 psi = 40 MPa (41.35 MPa exact)
Use 0.0068915 to convert psi to MPa

Newtons, psi, concrete strength and prestressed slabs

Concrete strengths are customarily denominated in psi (pounds per square inch) in the imperial system and in MPa's (megapascals) in metric. These are units of pressure.
The newton (N) is a measure of force. 1 newton is that force which pushes 1 gram of matter with an acceleration of 1 centimeter per second per second (or per second ²) or, equivalently, the force that accelerates 1 kilogram of matter to 1 meter per second ².
Force = mass x acceleration
Velocity is a measure of constant speed (i.e., meters per second, miles per hour, furlongs per fortnight)
Velocity is speed in a certain direction
Acceleration is the rate of change in velocity over time
Acceleration can be either positive or negative (deceleration)
1 N = 1 kg x (1 meter / second ²) -----> 1 N = 1 kg.meter / second ²
1 N = 1 g x (1 cm / second ²)
When you apply the force of 1 newton to a 1 meter ² area, you have pressure.
Pressure = force per area
Pressure can be measured in pascals (Pa). 1 Pa = 1 N / meter ²
The strength of concrete is commonly stated, in metric, in megapascals (MPa).
1 MPa = 1,000,000 Pa = 1,000,000 Newtons / meter ²

It is to do with the amount of crushing pressure applied to a cylinder of concrete in laboratory testing .
The concrete get crushed and the pressure exerted at the time of collapse is the MPa.

Yepp know all that...i wanted to know how you use to to design a certain load. but thanks anyway. I should just go and read how to do it..
Ok thks for that..

Originally Posted by sinjin

Yepp know all that...i wanted to know how you use to to design a certain load. but thanks anyway. I should just go and read how to do it..
Ok thks for that..

sinjin
concrete has three basic properties
drying shrinkage -- which leads to the gaps you see between slabs. In some applications you want to limit this
flexural strength -- which in practical terms is not much (from memory it's ~15% of the compressive strength)
compressive strength -- the main measure used

most domestic concrete is reinforced, you normally only find prestressed and post stressed concrete in commercial buildings and bridges.
the reinforcing (usuallu steel) is to make up for the low flexural sttrength of the concrete.

Leaving aside all the calcs needed to work out how much reinforcing steel to use and where to put it to hold the bottom of a concrete beam together, the importance of the concrete's compressive strength relates to determining when the top of a loaded beam (which is in compression) will spall because the load on the beam exceeds the concrete's compressive strength.
Support to the underside of a slab -- gravel, compacted sand, etc -- are not directly related to the ultimate strength of the slab

hope this helps

Cement is strong in compression, weak in tension, thats why anything handling tension ie beams etc are usually pre tensioned with reo cable, when the Opera house was being built they had a area set aside for this with two winches either end ot the beam slowly tightening the cables that ran through it this gave it strength in tension.