chapter 4 preliminary design properties of concrete

huangyhg

chapter 4 preliminary design properties of concrete
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.
  
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item 6...i think "use" should be replaced with "reinforce".
item 10...the reinforcement rate for columns seems very high. if a column is kept to 1% reinforcement the reo rate will be ~=78kg/m^3+lapping+ties. 200-450kg/m^3 looks high but i will check over some of the designs we have in our office and comment.
i know a good one you could include...young's modulus of concrete and young's modulus of high performance concrete.
one thing that stuck in my mind (from india) is to choose the rebars you use so that they are easily perceived as far as size from one to another.  india has bar sizes by 2mm (e.g., 16mm, 18mm, 20mm, 22mm, etc.)  this leads to mixups when the workers go "get some 20 mm bars) - they grab an 18 mm instead - the perception isn't there (18 to 20) and if qc isn't on the ball, the wrong size used.
no 1, please add-e) modulus of elasticity of concrete and reinf f) spacing of reinf.
no 4, please add-j) properties of aggregate k) magnitude of loading.
no.5, please add-e)premature yielding of column reinf.
for:
1...add f. cement content (or factor) of concrete
    add g. water-cement ratio of concrete
    add h. curing method and length of curing
    add i. aggregate gradation and type (high absorption coarse aggregate increases shrinkage)
2.  poisson's ratio is generally taken as 0.15 for concrete
4...add j. coarse aggregate type and gradation
10. is unclear.  state that these are gross section densities that vary with the amount of reinforcing steel.  the slab values relative to, say, retaining walls seem inconsistent.
14. seems to an overly broad generalization
17. should clarify that the aggregate shall have low abrasion loss under standardized testing or high abrasion resistance.  the term low abrasion is misleading.
18. the last sentence is only true under good conditions.  i've seen 6mm rebar corrode completely in 5 years or less under adverse conditions (coastal applications, poor consolidation of concrete, insufficient cover, use of chloride admixtures, etc.)
i'm not sure who your intended audience will be, but you might consider universalizing your terms and units.
nice job.
ron

1 please add, reinf. spacing & coefficient of expansion alpha for aggregate, least for lime stone highest for granite
quote:
i'm not sure who your intended audience will be, but you might consider universalizing your terms and units.
i agree with ron - it would be helpful to include inches, pounds, etc. for us old fashioned dudes.
thanks,

i started out on this course with the intended audience as just being a few fresh faced engineer's that need a starting point for their designs, this is only the starting chapter of rules of thumb. however  evolutions occurs and the intended audience is now anyone whom reads it once it is finished in the faq, thus i have converted as much as i can to inch ect. however i don't know the standards for reo in any other country really, could have a stab but would really just be guessing.
sorry to post the whole thing again, but this is easier fo me
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.
e)     modulus of elasticity of concrete and reinforcement
f)     spacing of reinforcement
f)     cement content (or factor) of concrete
g)     water-cement ratio of concrete
h)     curing method and length of curing
i)    aggregate gradation and type (high absorption coarse aggregate increases shrinkage)
j)     coefficient of expansion for aggregate, least for lime stone highest for granite
2.    poisson's ratio: a value of about 0.15-0.2 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.
j) coarse aggregate type and gradation
k) magnitude of loading
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 reinforce with 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
if you're not going to inspect everything keep the difference in bars sizes greater than 3mm (1/8 inch).
10.    these values are approximate and should be used only as a check on the total estimated quantity:
slabs - 80 - 110 kg/m3 (50-70lb/ft3) (flat slab120-220kg/m3 (75-140lb/ft3))
columns - 200 - 450 kg/m3 (125-280lb/ft3)
walls - 40 - 100 kg/m3 (25-65lb/ft3)
r/c footings 70-90 kg/m3 (45-60lb/ft3)
pile caps - 110 - 150 kg/m3 (70-95lb/ft3)
rafts - 60 - 70 kg/m3 (40-45lb/ft3)
beams - 150 - 220 kg/m3 (95-140lb/ft3)
transfer slabs 150kg/m3 (95lb/ft3)
retaining walls-110kg/m3 (70lb/ft3)
stairs – 135kg/m3 (85lb/ft3)
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-grade (normal) 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:          spanning up to 4.5m (16 ft)                     7 days                  spanning over 4.5m (16ft)                    14 days  
e)    props to beams & arches:    spanning up to 6m     (55ft)                    14 days                 spanning over 6m     (55ft)                    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 (1 inch)
b.    decrease compressive strength about 1 to 2 mpa (200 to 300 psi)
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 (1 gallon) of water to one cubic meter (per cubic yard maintains) maintains equal slump
b.    air content decreases about 1%
c.    compressive strength decreases about 0.5 to 1.2 mpa (150 to 200 psi)
?    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 1/2 inch
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 post-tensioned reinforcement is used, a concrete compressive strength of at least 40mpa (5,000) psi is usually specified to attain, among other things, more cost-effective anchorages and higher resistance intension and shear.
15.    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.
asixth,
i think young's e and high early strength concrete in steam heated curing would be a topic which would require a few pages and i would cover this as a in house training, not a manual, to complex.
ron,
the reo rate per volume are from price 2001 i think from memory, i know the slabs and beams  values are good "estimates", have never used the rw value, so i will check against some designs.
  
when in doubt, just take the next small step.

i think you should include also concrete admixture.  
if by admixtures do you mean water proofing i plan to have a short discription on how to handle.
when in doubt, just take the next small step.