i am in a concrete class in college and our proffessor has challenged us to a cylinder break which is in about 20 days away. i need to know the best mixture, and curring procedure to win please,only one or two people have beaten him, i would like to also. i will hunt down any material or additive nescessary. thanks

Low water/cement ratio, a decent dose of superplasticizer, and the best aggregate you can find (3/8" crushed granite or crushed basalt if possible).

Cure at elevated temperature (140 F) in lime-water for as long as you can. BUT make sure that it is as dry as possible when you test it. Not just surface dry, I mean dry internally as well, but no oven dried.

What size cylender do you have to make? If it is allowed in the project you can test a non-standard cylinder. If you make the height about the same as the diameter (or even less) you can boost your strength a bit due to confinement effects.

That is a good advice Injinear but sorry you are wrong about the curing part.

Curing the concrete cylinder at 140 F (60 C) will raise the initial strength (early age up to 6-7 days) but will actually lower the required strength of 28 days with respect to the same concrete cured at a lower temperature.

Studies and experience show that the best temperature to cure concrete in the lab to achieve the maximum compressive strength at 28 days is 13 C (86 F).

Here are some additional hints:

1. Use coarse aggregates with a high maximum size (up to 1 1/2") the higher the aggregate size the lower the voids between them, the lower paste you need and because paste is weaker than aggregate (if the aggregates were picked correctly) it will yield a higher strength.

2. Don't use rounded or smooth gravel, use angular crushed aggregates. The rough surface and the increased surface area will yield in a better bond there for a higher strength.

3. Use a well graded aggregate proportioning. The better the aggregates pack together the less voids between the aggregate with less paste needed to fill these voids. As the aggregate is taking the place of paste and the aggregates is far stronger than the paste the strength increases.

4. Use sand with a high fineness modulus. a low fineness modulus sand means that the sand is finer (smaller in size not smoother) with a larger surface area than coarser sand. This larger surface area needs more paste to coat it and lubricate it. This decreases the workability and increases the paste requirement, which will decrease the strength.

4. Use washed aggregates. Materials finer than #200 will decrease the strength.

5. Use aggregates with a higher specific gravity. The more porous the aggregates are, the lower the strength. as the increased porosity will decrease the surface bond with the paste and increase the abrasion.

6. Give big attention to the percentage of fine aggregates in the total aggregate volume. As the percent of sand increase the voids in the aggretes decreases which increases the workability and it is possible to reduce the w/c ratio, but as the fines increase they will start to take the place of the larger aggretes which means more voids and the paste requirements are increases then you will have to increase the cement content which will increase the paste volume.

7. Manage the air content to its lowest possible level. Increasing the air content will decrease the strength as it will increase the porosity of the paste making it less dense. This can be managed by using large sized aggregates and lowering the fine aggregate percent and making sure that the superplasticizer is not air entraining.

8. Remember, DO NOT over compact the cylinders.


NOW HERE ARE SOME CHEATING HINTS:

1. Use a low fineness cement, As the fineness of cement increase there will be more surface area for the hydration to take place therefore the hydration rate increased. On the other side, The decreased rate of hydration allows the slow formation of cement gel-like structure with a larger surface area which give more space for the hydration to take place and thus more cement grain get hydrated on the long run. ((((Use ASTM Type IV cement)))

2. Add some Pozzolans to the mix. Something like pumicite, opaline shales and cherts. Also calcined diatomaceous earth, burnt clay. but the recommended one to use is fly ash (also known as PFA: pulverized fuel ash).

3. Better than PFA why not use silica fume (also known as micro silica)

4. Don't use both fly ash and silica fume .... Dangerous mixture!!!!!!

5. This one is a major cheating hint---- using a cylinder of 150*150*150 mm will give a higher compressive strength test reading for the same concrete tested in cylinders.

for example: for the same concrete mix the compressive strength of a cube equals 1.25*the strength of the 130*150 mm cylinder. The highest strength possible will yield from testing a cube of 100*100*100 mm.

I hope you will find these hints useful!!! KICK HIS BUTT!!!!!!!

Very sound advice BluesMan. I agree completely.

However, the cylinder break is 20 days away, and based on Donovan's post he is not quite ready to mix his concrete. That is why I mentioned elevated curing temperatures. He may need the early age strength even though it comes at the cost of lowering long term strength.

Injinear

I agree, there is no simple or clear explanation to the fact that increasing the curing temperature results in improved early strength and lower ultimate strength. But one can say that the early strength gain is explained by the increase of the hydration process. The lower ultimate strength is more difficult to explain, but seems to be related to non-uniform development of the microstructure.

But lets not confuse the poor guy!

Donovan, you know better what is the time limit you are left with. So you have two options:

1. If your are testing the concrete at an age of 6 days or less -----> then elevate the curing temperature to 140 F (60 C).

2. If you are testing the concrete at a later age (more than 7 days) -----> lower the temperature to around 13 C (86 F).

PS: you said you would hunt down any materials or ADDITIVES!!!! Additives won't help you in increasing the strength.... additives are something you add to the concrete after it has already hardened or materials added to the cement at manufacturing. This is a common mistake. the suitable word is admixture. Which means materials added to the FRESH concrete in the mixing stage to change its properties (for better or for worse)

We agree on that Injinear, don't we? By the way I always enjoy your replies, sounds like a guy how knows what he is doing! Keep up the good work!

thanks injinear, and bluesman for your input. i will know tomorrow the exact size of the cylinder, it is about 4"x8".if i could get a little more information from you

1. when do i put the clinder in water, should i take it out of the cylinder after 24 hours and then set it in water or hang it?

2. it was mentioned that i shouldnt oven dry it but it needs to be completly dry, how long before the break do i need to take it out of the water?

3. what are the ratio's for the mixture, water content, fly ash, aggregate, superplasticizer, sand, cement?

4. what should i wash the aggregiats and sand in i heard somthing about moranic acid is that correct, if so what are the procedures for that?


again i appreciate the attention you have given to my question with all this help i should succeed

I guess Injinear was right, you are not ready yet to mix-design your concrete.

here is the MIX FROM HELL.. ..THE MONSTER OF ALL CONCRETE MIXES.. .. it should yield a compressive strength of 107 MPa @ 28 days with a good slump Other wise use a superplasticizer with a dose 1-1.5% of the weight of cement + fly ash + silica fume but I doubt you will need it…..

Maximum size aggregate----13 mm
Cement--------------------475 kg/m3
Fly Ash-------------------104 kg/m3 Class C
Silica fume----------------74 kg/m3
w/c-----------------------0.231
Total Water---------------151 kg/m3


By knowing the specific gravity of each of the above ingredient, the volume of the water, cement, fly ash, and silica fume can be computed :
Absolute volume = weight/absolute unit weight
Absolute unit weight = Specific gravity*1000

example: if the specific gravity of cement = 3.15
then the unit weight = 3.15*1000
and the volume of the cement in 1 m3 = 475*(3.15*1000)

now all the volumes are known, knowing that:
volume of a unit weight of concrete = sum of all volumes
1 m3= V cement +V water + V fly ash+ V silica fume + V aggregates (assuming the volume of air is 0)

the volume of aggregates is known...

Knowing the grading of your aggregates, find the best proportioning between coarse and fine to satisfy the combined grading specifications.(( i think you already know how to do that))

simple as that...You are ready to go!!!!!!!

AS FOR YOUR QUESTIONS::::

1. You un-mold the cylinders after 24 hours unless the concrete tends to stick to the mold (if you use superplasticizer)then wait for a few hours.then cure in water/steam immediately.

2. Let it stand dry before testing for at least 6 hours.

3. Wash the aggregates with plain water… don't get into stuff you can't handle.

FINALLY>>>>>>>> make as much cylinders as you can ,if you are allowed too.... the more the cylinders,the more the mean compressive strength will be closer to the targeted strength...... The great lows of STANDARED DEVIATION!!!!

HAVE FUN !!!!!!!!!!!!!