Chapter 4 preliminary design Properties of concrete 8

[rowingengineer]

As many would already know i am writing some notes on an undergrad manual that will go in the FAQ, the next chapter has some simple properties of concrete. These are from notes i have taken over time, two or more are from posts in this forum, other are from the net.

If you have the time please look through them and comment on anything you do not agree with.

Or if you have any properties that you think should be apart of this list, please post.

Properties
1. Temperature and shrinkage causes tensile forces in concrete, due to the interaction of reinforcement and concrete; Cracking levels depend on,
a) Tensile strength of concrete.
b) The cover thickness.
c) The diameter of rebar
d) Rate of corrosion.

2. Poisson’s ratio: A value of about 0.2 is usually considered for design.

3. Shear strength: The strength of concrete in PURE SHEAR has been reported to be in the range of 10 to 20% of its compressive strength.

4. Factors influencing creep:
Creep increases when,
a) Cement content is high,
b) w/c ratio is high,
c) Aggregate content is low,
d) Air entertainment is high,
e) Relative humidity is low,
f) Temperature (causing moisture loss) is high,
g) Size / thickness of the member is small,
h) Loading occurs at an early age &
i) loading is sustained over a long period.

5. Effect of creep:
Detrimental results in RC structures due to creep:
a) Increased deflection of beams and slabs.
b) Increased deflection of slender columns (possibly leading to buckling)
c) Gradual transfer of load from concrete to reinforcing steel in compression members.
d) Loss of prestress in prestressed concrete.

6. In order to reduce the effect of creep-deflection it is advisable to use 0.2% of cross sectional area at the compression face.

7. Symmetrical arrangements of reinforcement will aid to avoid the differential restraint.

8. Reduction of moments on account of moment redistribution is generally NOT APPLIED TO COLUMNS.

9. Reinforcement availability:
Standard diameter sizes (mm): 6, 8, 10, 12, 16, 20, 24, 32, 40
Standard lengths: > 12mm diameter: 12 metres
< 12mm diameter: from a coil

10. These values are approximate and should be used only as a check on the total estimated quantity:
Slabs - 80 - 110 kg/m3 (flat slab120-220kg/m3)
Columns - 200 - 450 kg/m3
Walls - 40 - 100 kg/m3
R/C footings 70-90 kg/m3
Pile caps - 110 - 150 kg/m3
Rafts - 60 - 70 kg/m3
Beams - 150 - 220 kg/m3
Transfer slabs 150kg/m3
Retaining walls-110kg/m3
Stairs – 135kg/m3
Note: The actual reinforcement quantity in the element will vary according to detailing practice and efficiency of the concrete element.

11. In normal circumstances and where N grad concrete is used, forms may generally be removed after the expiry of the following periods:
Type of Form Work (Location) Min period before striking Form Work
a) Vertical formwork to columns, Walls, beam 16 - 24 hrs
b) Soffit formwork to slabs (props to be re-fixed immediately after removal of formwork) 3 days
c) Soffit formwork to beams (props to be re-fixed immediately after removal of formwork) 7 days
d) Props to slabs: (1) Spanning up to 4.5m 7 days (2) Spanning Over 4.5m 14 days
e) Props to beams & arches1) Spanning up to 6m 14 days (2) Spanning Over 6m 21 days

12. CONCRETE MIX RULES OF THUMB
• ADDING 3L OF WATER TO ONE CUBIC METER OF FRESHLY MIXED CONCRETE WILL:
a. Increase slump about 25mm
b. Decrease compressive strength about 1 to 2 mPai
c. Increase shrinkage potential about 10%
d. Waste as much as 1/4 bag of cement
• IF FRESHLY MIXED CONCRETE TEMPERATURE INCREASES 10 DEGREES:
a. About 3L OF WATER TO ONE CUBIC METER maintains equal slump
b. Air content decreases about 1%
c. Compressive strength decreases about .5 to 1.2 mPai
• IF THE AIR CONTENT OF FRESHLY MIXED CONCRETE:
a. Increases 1%, then compressive strength decreases about 5%
b. Decreases 1%, then slump decreases about 10mm
c. Decreases 1%, then durability decreases about 10%

13. The main components of cast-in-place concrete floor systems are concrete, reinforcement (normal and/or post-tensioned), and formwork. The cost of the concrete, including placing and finishing, usually accounts for about 30% to 35% of the overall cost of the floor system.

14. Where normal 500 mPa reinforcement is utilized, a concrete mix with a compressive strength of 32mPa yields the least expensive system.

15. Where post-tensioned reinforcement is used, a concrete compressive strength of at least 40mPa psi is usually specified to attain, among other things, more cost-effective anchorages and higher resistance intension and shear.

16. Having the greatest influence on the overall cost of the floor system is the formwork, which is about 45% to 55% of the total cost.
Three basic principles govern formwork economy for site-cast concrete structures:
• Specify readily available standard form sizes. This is essential to achieve economical formwork. Most projects do not have the budget to accommodate custom forms, unless they are required in a quantity that allows mass production.
• Repeat sizes and shapes of the concrete members wherever possible. Repetition allows forms to be reused from bay to bay and from floor to floor, resulting in maximum overall savings.
• Strive for simple formwork. There are countless variables that must be evaluated and then integrated into the design of a building. Economy has traditionally meant a time-consuming search for ways to reduce the quantities of materials. For example, it may seem appropriate to vary the depth of beams with the loading and span variations, providing shallower beams where the loads or spans are smaller. This approach would result in moderate savings in materials, but would create additional costs in formwork, resulting in a substantially more expensive structure—quite the opposite effect of that intended. Providing a constant beam depth while varying the amounts of reinforcement along the span length is the simplest and most cost-effective solution.

17. ABRASIVE RESISTANCE of concrete increases with compressive strength and use of aggregate having low abrasion.

18. Do you know that:
For steel bars to lose one mm diameter due to corrosion, it takes about 12.5 years. But due to practical reasons the number of years reduces due to hostile corrosive environment. For 6mm dia To corrode completely it takes about 75 years.


When in doubt, just take the next small step.

 

[rowingengineer]

Good suggestions, I have am working through the “rules of thumb” at the moment and will add fire requirements and also if time permits add some generally fire deign. Only problem is at the end of this week I resign and the grade manual stops its development for a while I would imagine, starting a new job probably won’t have the time again.

Reason for the manual: I currently have an undergraduate student, he did a thesis in association with our company, and did a great job, and unfortunately it was in wind turbines such won’t help him much with the general day to day stuff. So the reason for the manual comes about because I am leaving this fresh face to an office with one senior engineer whom is has no time. And I would hate for this guy to become only half of what he could.

I am going to post a lot of it in the FAQ now and see i it gets the nod.

When in doubt, just take the next small step.

 

[rowingengineer]

I would take the chance to thank everyone for their input thus far, I have start to post the first chapters in the FAQ, Just thought you would like to know.

When in doubt, just take the next small step.

 

[Ron]

RE...good job. All of us with gray hair should be doing the same thing...you've shamed me to get back to writing. My current project is a stucco course.

 

[rapt]

rowingengineer,

Item 6 - reinforcement at the compression face is useless if it is not in compression and not much use until it is significantly in compression (being at the compression face does not put it in compression).

For lightly reinforced slabs and shallow T beams, it is a waste of money. In those cases, extra tension reinforcement is much more useful.

 

[rowingengineer]

Ron,
Good to see that a positive impact has occurred, hope the course runs well for you, I always enjoy attending or giving courses. Mind you some of the younger folk are starting to look at me strangely when I go to these courses for inexperienced engineers. I don’t think it occurs to them that that I’m not there to learn about the subject (however every now and then I get two or three helpful hints at a time) the reason I am there is to learn about presentation, some guys really set the place alight while others don’t.

On another topic, I have no idea what stucco is, did a quick search found a place called NOCSA, and I would suggest they added a photo or two to their website for guys like me. I assume that it is like cement sheeting of some kind, but as they say "assumes make an ass out of u and me".




When in doubt, just take the next small step.

 

[rowingengineer]

Rapt,
Thanks for the insight, will have to give this some thought, because I am talking about deflection creep, and while your comments are correct about the efficiency of the slab with regards to Ieff ect which will intern affect deflection, will just need to work out a good clarification note.
Thanks again.

When in doubt, just take the next small step.

 

[Ron]

RE...stucco is,technically, portland cement plaster. It is used as a wall surfacing. Almost universally improperly applied resulting in severe deterioration of wood framing and substrate.

 

[rowingengineer]

So kind of like compressed FC (fibre cement sheeting), would be interested to know what problems are associated with this type of product with regards to application.

I really only know of a few problems, if you have a relatively dry atmosphere and very large seasonal temperature variations the stresses built up in FC sheeting were at times large enough to crack the corners of the sheeting, hence you need joints lots of joints. The other problem I know of is the thermal grade difference if you put it on steel in an external situation, the steel will expand more and crack it up.


When in doubt, just take the next small step.

 

[Ron]

RE...stucco is a "field fabricated" material. It is essentially 3 coats of mortar (slightly different than typical mortar, but not much) applied in a prescribed manner. For a wood-framed, sheathed structure, here's the process....

1. Apply an air infiltration barrier (Tyvek or similar)
2. Apply a water resistive barrier (Asphalt saturated felt)
3. Apply a layer of self-furring, galvanized steel lath (diamond mesh is most common) attached only at the structural members.
4. Apply a 3/8" (9.5mm) thick layer of stucco mortar onto the lath, pushing the material through the lath to create embedment. Before the layer sets up, scarify the surface in a horizontal direction. Allow to cure for at least 24 hours, preferably more. Moist curing is necessary. This is called the "scratch coat".
5. Apply another layer of stucco mortar on top of the first coat, again to a thickness of 3/8". Do not scarify this layer. This is called the "brown coat" (there's a reason for this name...has to do with differences in the mix proportions for this coat). Allow to moist cure for 24 hours or more.
5. Apply a 1/8" thick finish layer. Texturing may be added on top of the finish layer, but is not included in the total thickness requirement of 7/8".
6. Paint the surface.

This is a simplified summary...it gets a bit more complicated for joints, wall penetrations such as windows and doors, and at junctions with other materials.

The most common problems we see are improper flashing and waterproofing at the windows (you have to treat the wall as if it were a roof), improper installation and fastening of the lath (usually too much fastening, causing the lath to be too flat against the substrate, thus little embedment), improper installation of accessories such as control joints and weep screeds, improper installation of the stucco layers (insufficient curing time between layers), and improper design/layout of necessary joints.

Volumes have been written about stucco. It can be a very durable, attractive surfacing for walls. It can also be a nightmare, particularly when not done properly or bastardized by "synthetic" cure-all materials and procedures (i.e., "one-coat" stucco systems, poorly designed EIFS, etc.).

 

[rowingengineer]

Never heard of the stuff before (did a quick Aust search and yes the product is used in Aust), So thanks for the information. I have a feeling that i will run into in the future and thanks to you Ron, I will be better equipped. Hope the presentation goes great.

I would give you a star but apparently one is the limit per thread.

When in doubt, just take the next small step.

 

[rowingengineer]

Parts of the “rule of thumb” section of the grad manual are posted in the FAQ. Still have footing, caps and fire (and a few others) to go, but alas I am out of time for now, so this now goes on the back burner until such time I get another grad student.

Feel free to comment, suggest amendments ect.

When in doubt, just take the next small step.