Check if you are not receiving (using) sub grade or expired cement.

you mention looking at the main reasons that some cement:sand:gravel mixing cannot reach ... desired strength! However you did not mention the water. Generally strength is largely affected by the amount of water in the mix. to a lesser extent the water quality.

Have you taken measured samples of your mix components oven dried them and determined by the change in weight what the water content is in your mix sand and gravel? You must REDUCE the water that you add to the mix by the amount of water contained in your sand and gravel to prevent ending up with too much good old H2O in the mix. Also things like salts in the water or Chlorine can adversely impact concrete strength.

Check the moisture content first, that is often not taken into account and has ruined more concrete than you can imagine.

Good luck

and how you let your concrete dry will impact the concrete. the concrete should be drying in a moist condition, for it to react it's designed strenght. what are you exposing it to when drying?. are you testing the concrete, ie cone test, density? what type of sand/gravel. how big?

Good catch by Ice Man (good of you to chime in IM). 35 years ago when I first started breaking cylinders you cured test concrete fully submerged under water. the concrete mix does not 'dry' in the traditional sense. Not like a wet mud pack where the water slowly rises to the surface and evaporates. Concrete drys by going thru a chemical reaction where the water reacts with the cement and become a part of the final product. More like the egg baked into a cake, it becomes part of the cake and is nolonger egg. If you are curing your test samples in open air, some if the water will evaporate, drawing the water in the mix middle out and causing an improper product. Just like too much water weakens cement concrete, so does too little.

as for his question about what kind of stone, crushed stone, is very angular and helps the concrete lock up and builds strength into the mix thru the angular surfaces bearing on each other and creating very non-uniform shear planes within it. rounded river stone, smooth and of uniform size is much like many marbles that uniformly pushe each other on more uniform planes and therefore tend to assist shear planes to form.

However, PC reacting with water uses a very small proportion of the water used in a concrete mix. Most of the water used in a concrete mix is to allow some fluidity and workability to the mix. Very dry mixes can and are processed by machines forming and fininshing. The majority of water in a concrete mixes rises to the surface of the leveled concrete and runs/evaporates off (You see this on a concrete slab as it sets after initial finishing). As a rule, the less water the stronger the concrete. You can place concrete at less than a 1" slump and get extremely strong mix if the heat is controlled (you just can not work it properly by hand).

Good call. So many variables to making good 'mud pies'. Spray systems and plastic sheeting can be used to keep the concrete wet until maximum (not final) cure, which takes approximately 28 days. Concrete with too little water will take longer to cure, but if it vibrated or impacted enough to create a wet surface, it will be the strongest of all. Figure a few months for this type of mix. 1:2:4 is the usual for high strength concrete with washed sand and crushed aggregate.

Bill, It was that idiot right wing Republican that put you where you are, and cost you a GA from me.

Happy mud pies.

Thank you tippy . tha 1:2:4 mixing which ive tried do fail. i dont really know the reason behind this. for the 28 days curing, the weather is more likely to be not consistent and ive done curing at my back door. My adviser said that one my classmate got the right strength by 1:2:2.8 mixing. i dont really know whats the big QUESTION MARL behind this failure

Tippy - That's OK, I'm not here for the GA's from others, rather am here to learn and share what I know (here on the CR4)

As for getting me where I am at, I worked with that 'right wing Republican', and his predecessors in the White House - all the way back to the Carter administration. He was ANYTHING but an idiot. but politics need to be argued on another page, don't you think? Say the Sunday Cartoons Page?

Hello cyma atz,

The water mixes with the cement to make a slurry which in turn, holds the gravel and sand in place to make a solid, (similar to a 'glue').

The gravel, and sand do not hold anything in terms of strength until it is totally dry. You then effectively have an lump of rock.

The water percentage is as important as the gravel and sand.

If possible, keep the sand and gravel under cover with, any water run-off directed away from the silo which contains the sand etc.

http://www.pavingexpert.com/drain06.htm ...

This site has the linear dranage similar to the way I drained my silo. The silo was small but covered enough to stop direct rain, and over time the water drained from the gravel and sand. Any aggregates can be recovered from the mini culvert.

http://www.4specs.com/s/03.html. ..........

You may find this useful?

Good luck and keep in touch.

bb

thank you baby bear. but im testing 6 x 6 x 21 inches beam samples.

Hello cyma atz,

Can you tell me if you are using steel in the beam, and are you using a vibrator for compaction?

BTW, I like your avatar.

bb

Concrete does not "dry" it "Cures". What MPA are you trying to reach? Aside from proper curing environments you need good quality cement and cement that is formulated to reach certain MPA's. It has been many years since I tested Concrete but for design several solid suggestions were already mentioned. Consider these key requirements;

Cement (good quality)

Water (clean, free of chemical)

Aggregate (pit run vs. quarry stone) (dirty rounded stone vs clean sharp stone)

How it is mixed

Air entrainment

How the cylinders are cured (mist room at 100% humidity and a constant temp minimum 21 deg C or water tank)

Properly capped (Sulphur / rubber caps)

Have a look at the cylinder during and after the compression test. Did it crumble or shear? Have a look at the larger aggregate, did it break around the stone or through the stone? Analysis of the tested cylinders can tell you allot!

Good luck!

Josh

thank you josh. what MPA? i make certain samples of rectangular beams and all of them failed to reach the standard MPA and that is more likely to be 23 MPA.

in my next study, i am going to make cylinder samples and i hope i can make it a successful one. can you give me good advices to catch up a successful experiment of testings for my next cylinder samples.

Hello cyma atz,

I think "MPA" refers to megapascal MPa 1x106Pa .

Where a 'Pascal' is a measure of pressure This site explains exactly what a 'Pascal', KPa, Kilo Pascal, and MPa, Megapascal, Where kilo is 100, and Mega is a million Pascals.

http://canteach.candu.org/library/20053317.pdf.

Find a list of some other SI Units for comparison.

http://www.efunda.com/units/convert_units.cfm?From=339

megapascal Symbol: MPa

Category: Pressure SI Equivalent: 1×106 Pa Dimension: ML-1T-2 System: SI

Convert MPa 52 conversions, showing those commonly used | Show all 1 MPa = Pressure

Symbol Unit Name

9.86923 atm atmosphere (standard) 10 bar bar

750.062 cmHg (0 °C) centimeter of mercury (0°C)

1.01974×104 cmH2O centimeter of water (4°C)

1×107 dyn/cm2 dyne per square centimeter 334.562 ft H2O foot of water (4°C)

1×10-3 GPa gigapascal 295.3 inHg (0 °C) inch of mercury (0°C)

4018.56 inH2O (15.56 °C) inch of water (15.56°C)

10.1972 kgf/cm2 kilogram force per square centimeter

1.01972×105 kgf/m2 kilogram force per square meter

0.145038 kip/in2, ksi, KSI kilopound force per square inch

1×104 mbar millibar

1×106 Pa, N/m2 pascal

2.08854×104 lbf/ft2 pound force per square foot

145.038 psi, PSI, lbf/in2 pound force per square inch

7500.62 torr torr

I hope this helps.......

bb

I know from experience the aggregate can have a great affect on the strength. It should be clean less then 1% on the 200 screen. Should be crushed not round if you're using gravel it should be cracked on at least two surfaces. you should also also check on the softness of the stone a soft stone like shale will break easier cause more dust to form.

This is probably not relevent to the thread. I am a stonemason and we use a dry mix of coarse sand and cement. (Not enough water the make the mixture sump). If you mix this type of mix in a mixer too long you can "overmix". (I guess when a cake is done, you cook it longer and it gets burnt) The cement makes little hard balls with the sand which are really difficult to use and do not stick well to each other. Sometimes customers and labourers do not like it that I use short mix times but an overmixed batch is very difficult to use and is a bad deal all round. Is it possible that overmixing can occur with your mixes too? Brian

Good example. This makes the point of workability of a PCC concrete. Admittedly tou will not have exactly this same problem with a more typical concrete mix where there is larger aggregate entrained. But Workability is one of the major conserations of PCC, You can develop a good concrete product with less than a 1" slump, and it will develop extremely high strength. However, if you can not work the material, you will just have a huge problem and might not be able to develop the strength in the final product because of voids, proper contact, etc.. Typically a 2" to 4" slump is considered a good workable material, 2" is way harder to work but wetter is weaker always in concrete and leads to more issues like shrinkage.

Hi cyma atz,

First, Welcome to CR4.

As I understand from your OP, you are making beam samples (6" x 6" x 21") from 3 design mixes.

I would also assume that the quality of the Portland Cement, fine & coarse aggregates, water (and admixtures, if any) meet the requirements of ASTM, as well as the procedure of mixing, preparing and curing of your concrete samples.

I will think that you are testing your beam samples for flexural strength as shown in this photo.

The beam samples should break within the middle third so as for the result to be acceptable. If the sample breaks outside, the fracture should be located at least 6" from the end of the 21" long beam for the result to be valid.

Now, what is the flexural strength you require @ 28 days? If it failed (not reach the required strength), then, check what is the nature of the fracture/failure. Is it cement bonding? or is it fracture of the coarse aggregate? Were the beam samples contain some defects (i.e. cracks, voids, etc.) in the first place? How long did you dry your sample before testing?

Hope this helps.

thank you willy for the series of your query advice! ill be back soon to school maybe this monday to check the damage and fracture of my beams. ill inform you later what is the reason behind tose failure. that is my main objective now.to check and know the reason why those beams failed to reach the desired strength. im into now studying of cylindrical beams-- which is my next study assigned by my adviser and will be my thesis. Thank you so much.Glad i found this CR4.

If everything else checks out, I would question the quality of the cement.

Going back to the energy crisis of the 70s here:

When the cost of fuel to produce portland cement went through the roof "back in the day" some unscruplous folks substituted cheaper natural cements (talc, etc.) instead. (I only have this second hand from folks in construction back then.)

Needless to say, this made weaker concrete. Per concrete.org, "Strength and uniformity of natural cements are lower than for portland cements. . ." http://www.cement.org/tech/faq_natural_cement.asp

On the plus side, they invented Controlled Low Strength Material (CLSM) http://www.cement.org/basics/concreteproducts_clsm.asp

I hate to say this, and nothing political or offensive is intended, but- does the cement say "made in China?"