Material Properties of concrete masonry units (CMU) 3

[lowcosthomes]

I'm doing some Finite element Analysis on custom cinder blocks and need to input material properties. Ideally i would like to obtain the properties of concrete mixes typically used in concrete masonry units (CMU). Thank You!!

 

[Ron]

Call a local masonry supplier and ask. You'll probably find that the mix is a lightweight coarse aggregate, less sand than a typical concrete mix, admixtures for casting ease, and a compressive strength of somewhere between 2500 and 3000 psi.

 

[concretemasonry]

lowcosthomes -

Are you talking about the old cinder blocks only made from waste materials in eastern U.S. many years ago or a concrete masonry unit (CMU) made to exceed the ASTM C-90 requirement?

You will not get a an accurate mix design from a CMU manufacturer and it would be fruitless for your purposes.

The real world requirements are far more critical than the typical minimal ASTM material standards that alway allow exceptions. The gradation and particle shape is critical and aggregates used are only from one supplier and may be determined by actual test samples of products made one the manufacturing equipment and cured using the same curing cycles. Lighweight aggregates represent a very small percentage of the aggregate in a typical block and in many areas, lightweight is not used.

Forget about the compressive strength because almost anything can be acheived. I have seen CMU with net compressive strengths over 8500 psi, but there is no market for that type of unit. Most engineers are in the gutter or rut (3000 psi net compressive strength) relying on the very, very minimal ASTM requirements (I have been a voting member of the ASTM C12 committee that writes the ASTM C90 specifications for over 15 years).

The cheapest way to make a strong unit is to increase the amount of water and adjust the manufacturing cyles, but it may not be suitable for a masonry contractor. A prt of this is that good units must have suitable visual appearances and be economical to lay since labor is the major cost item. The exception may be an architectural unit that can be very costly.

Concerning a finite analysis, there are other shapes that are more efficient, but are not practical and engineers have never been familiar enough with the potentials of masonry design and specifications to really get specific about it. None of the codes can easily applied to anything based on a finite analysis.

Is this for creating a low cost home or for a research project?

Dick

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Engineer and international traveler interested in construction techniques, problems and proper design.

 

[lowcosthomes]

concretemasonry,

Im talking about the CMU made to meet ASTM C-90 standards. What exactly do you mean by "None of the codes can easily applied to anything based on a finite analysis" can you elaborate more on that point of view?
Yes this is a research project and I would like to have some type of material properties to enter into my FEA modeling.

these are typical properties of normal strength Portland cement, Is it possible to tweak some of the numbers around to get a closer CMU material profile? once again any advice helps!

Density : 2240 - 2400 kg/m3 (140 - 150 lb/ft3)
Compressive strength : 20 - 40 MPa (3000 - 6000 psi)
Flexural strength : 3 - 5 MPa (400 - 700 psi)
Tensile strength : 2 - 5 MPa (300 - 700 psi)
Modulus of elasticity : 14000 - 41000 MPa (2 - 6 x 106 psi)
Permeability : 1 x 10-10 cm/sec
Coefficient of thermal expansion : 10-5 oC-1 (5.5 x 10-6 oF-1)
Drying shrinkage : 4 - 8 x 10-4
Drying shrinkage of reinforced concrete : 2 - 3 x 10-4
Poisson's ratio : 0.20 - 0.21
Shear stress : 6000 - 17000 MPa (1 - 3 x 106 psi)
Specific heat capacity : 0.75 kJ/kg K (0.18 Btu/lbm oF (kcal/kg oC))

Thank you!

Dan.

 

[concretemasonry]

lowcosthomes -

First, there is no such definition of a "cinder blok" and very few have been made in the last 60 years and many area have never seen a "cinder block" - It is a slang term used by the uniformed. The only possible exception may be volcanic cinders that are used for specific purposes (color usually)and possibly natural pumic that is also volcanic, but can be more closely specified.

Most of the factors you listed are not a part of the ASTM C90 specifications for a CMU unit and never were. When you are talking about a wall, you must consider the mortar and mortar properties used to determine the wall actions and properties.

The following points are addressed:

1.The are 3 different densities, but other can be obtained. The classicications are normalweight (over 125 pcg), medium weight (105-125 pcf) and lightweight (under 105 pcf). At various times the was a classification a lighterweigh category (under 85 pcf), but that is much costlier unit in most areas.

2. Compressive strength is measured based on the net area of the masonry unit. The minimum is 1900 psi, but it is very difficult to make one that low. Strengths can range up to over 8000 psi or may specified bu the engineer.

3 & 4. The are no requirements. I have no idea of any tensile strength tests conducted on a CMU unit. The tensile strength is a rough function of the compressive strength, but is not used in design. The flexural strength of a CMU unit is also similarly meaningless, especially since there is not even a test ptocedure.

5. The modulous of elasticity is usually assumed to be related to the density and conpressive strength and many textx and design standard have a range of factor to used for design.

6. Permeability is not usually tested and varies widely because of the different types of aggregate, cement content and gradations and admixtures if used.

7. Thermal expansion depends on the weight of the block and percentages of the various aggregates used.

8. Drying shrinkage is addressed by ATMS C426 and is a test for properties to use in design, if needed.

9. Drying shinkage of reinforced conrete is applicable for masonry units. Reinforced concrete masonry has various core filled with grout and reinforced with spacings between 8" and 4' o.c.

10. The remaining items all related to specific units of different densities and aggregates used for different purposes.

Since CMUs are not used without mortar, any testing or analysis for practical use must also include a composite of the block and mortar without any reinforcement. There have been some theoretical FEA, including the National Concrete Masonry Association, but I don't think they were ever published because there were too vague factors to be used in any design codes that the engineers participated in writing.

Good luck on your research project or paper.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[henri2]

Lowcosthomes,

Piggybacking on what Dick wrote above, instead of items 3 and 4 (flexural and tensile strength), have you considered flexural bond strength?

I recall the flexural strength of 6 x 6 x 21 in. (approx 150 x 150 x 525 mm) 1-1/2 inch MSA concrete beams typically tested between 600 - 700 psi. So, I'd imagine the values you have for tensile and flexural strength seem fine.

 

[concretemasonry]

henri -

Thank you for reading and taking the effort to respond to the previous post. Items 3&4 were for the actual CMU units and not for a wall or assembly structure (mortar and CMU), since the O.P. seemed to be concerned with the individual units.

The mortar and grout (including the loose selection of types used in the analysis are separate materials) can have an effect on a wall, but, the codes have adopted allowable stresses.

Rant - Since a real finite analysis for a wall must include te effects of all materials and the interaction of them in composite wall investigated.

There is little question that a CMU unit can be made acheive many different superior specifications, but engineers and standards seem to only deal with the "market" units generally produced and not what could be made economically. FWIW, the cheapest way to increase CMU strength is to just add water to the mix, but users and the common blocklayers do not like appaearance.

I think the original poster may have a much more encompassing FEA subject if a wall performance is included in the and there have been decades of testing of many different assemblie to use in establishing the interaction of the different materials.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[henri2]

Good point Dick.