CE 60 Quiz 4
Hydraulic pressure
In our case our concrete is porous, so the water is squeezed through highly restricted channels of capillary pores.
What is capillary porosity dictated by?
(think the harsh angles in the gaps) the % of capillary porosity is dictated by the w/c ratio
the chord modulus
(we like this one)you take the 40% stress subtract the stress at 50 microstrain, and then divide by the coordinating strain to 40% stress and subtract 50 microstrain Use the points from 40% stress and 50 microstrain to find the slope
What is the standard time of curing for testing the compressive strength of concrete?
28 days
the ability of concrete to resist frost damage depends on:
-location of escape boundaries (the distance by which water has to travel for pressure relief) -pore structure (size, number, and continuity of pores) -degree of saturation (amount of freezable water present) -rate of cooling -tensile strength (of material that must be exceeded to cause rupture) (you can only cause cracks if you exceed to tensile strength)
air entrainment
-provides escape boundaries but decreases the strength :( -each 1% of air addition decreases strength by 5% -aids in workability -air entrainment agents: polymeric materials (long chain organic molecules; soap like foaming agents) surfactants which act at air/water interface causing water to foam during mixing -added (1-5% by volume of cement) 1. during mixing of concrete 2. during grinding of the clinker
physical process of deterioration in frost action
-type of H2O -Damage mechanisms -sign of damage -prevention of damage
What are the two processes of durability?
1. Chemical process: (sulfate attack) (ie corrosion of steel and alkali-silica rxn) 2. physical process: (ie freezing/ thawing/fire) all produce expansive processes
Why do we add aggregates
1. economical, aggregates are cheaper than cement cement 10x more expensive 2. stiffness enhancement, the stiffness, aggregates are stiff because they have ionic or covalent bonds 3. dimensional stability, reduce the shrinkage and creep
Sulfate attack characteristics of ettringite and gypsum
1. ettringite: -expansive products which form in the presence of sulfates in soil/water. Sulfates attack hydration products (monosulfate and csh ch) 2. gypsum: -cause cracks/deterioration
At which strength level (in compression) is concrete considered to be a high strength concrete (in psi)?
6,000 psi (Section 1.3)8,000 psi per ACI 2002 (Sec. 12.2.2)
Concrete by definition contains coarse aggregates. Why is there a reduction in compressive strength (f'c) when coarse aggregates are added to the hydrated cement paste (hcp)?
Because of the interfacial transition zone between the coarse aggregates and the hcp
Would you expect more frost damage in high strength concrete (i.e. f'c > 10,000 psi) compared to normal strength concrete (f'c <6,000psi)? Please explain your answer.
Because the high strength concrete typically has a low w/c ratio (i.e. w/c <0.3), there will be no capillary porosity in the HSC. If there is no capillary porosity, there will be no free water available to freeze at 0°C. Thus, I would expect more frost damage in normal strength concrete.
Two concrete specimens with w/c ratios of 0.3 and 0.4, respectively, are being moist cured. After 100% hydration will the two concrete specimens differ in their permeability? Please explain.
By lowering the w/c ratio, the capillary porosity will be even lower than the 30.6% calculated in the previous problem. Therefore, no, they will not differ in permeability because they both have an isolated pore structure (CP < 30%).
Determine the w/c ratio that would lead to zero capillary porosity after 100% hydration
Capillary porosity is the ratio of the volume of water to the entire volume. An equal volume of water to volume of cement will result in zero capillary porosity. Since w/c ratio is a mass ratio, we find that the ratio is equal to the ratio of the density:
Stress strain curve of an aggregate, hcp and concrete
Ceramic type material, (low ductility, very brittle but a very high stiffness) It is ionic or covalently bonded . Very high strength very high stiffness The curve will end up being different
What causes the sulfate attack?
Chemical rxn between sulfate ions and hydration products leading to ettringite and gypsum formation (very expansion prone compounds)
Spalling
Chipping or pitting of concrete or masonry surfaces.
Is concrete stronger in compression or tension? By how much?
Compression Tensile strength is ~10% of compression strength of low-strength concrete, 9% of moderate strength, and 7% of high strength
Capillary pores (10-1000 nanometer) and freezing
Consider Free Water (not attached by any physical chemical bonds for pore sizes >50nm Freezing points ranges from -1C (30F) to -8C (18F) for capillary pores smaller than 50nm Freezing temp depends on pore solution chemistry (contains alkalis so it isn't really pure water which will alter temp)
Where do cracks initiate in normal strength concrete?
Cracks form at the weakest point of the concrete. In normal strength concrete, this happens in the ITZ, the interfacial transition zone.
gypsum formation
Depends on the sulfate ion sodium sulfate or magnesium sulfate
entrapped air voids (1-5mm) and freezing
Do not contain water so we don't have to worry about expansion and damage
Signs of a sulfate attack:
Extensive cracking and expansion; deposition of ettringite crystals in internal cracks or internal voids characteristic whitish appearance on surface
Pore sizes in hcp and frost action (which one freezes first)
Free water vs water that is chemically bound In these three pores there are different freezing temperature. The free water freezes the fastest so the largest pores will freeze first (so entrapped air voids). In gel pores the water is not free, the water is held in the CHS sheets and you must first break the bonds between the water and the sheets and then you freeze it so it must be at a cooler temperature
Interconnected pore structure in permeability
Hydrated cement goes from interconnected to isolated pore structure. Permeability changes drastically when you go between these two pore structures. The graph shows the coefficient of permeability vs capillary porosity. K is constant until 30% permeability and remains there when you get into the is the isolated pore structure (K=10^-11)
Sulfate attack like many other chemical and physical deterioration processes lead to expansion and cracking in concrete. How could you identify at high magnification (i.e. based on microstructural analysis) that your concrete indeed suffered from sulfate attack?
If your concrete suffers from sulfate attack, you will be able to see the late formation of ettringite throughout the cracks in the concrete at high magnification. The formation of ettringite is expansive and thus creates tensile forces that results in cracking. Their microstructure will be needle-like (as opposed to platey CH and amorphous C-S-H).
air entraining agents
Introduces minute air bubbles in the concrete mix to increase its resistance to freezing. It is an escape from hydraulic pressure. A greater number of air voids decreases the average distance water must travel for pressure relieve, but too many air voids decrease the overall strength of the concrete. Air voids (100 to 200 micrometers
The permeability of concrete depends not only on the overall porosity of concrete but also on the pore structure. What type of pore structure is required in order to prevent reactive agents to infiltrate the concrete?
Isolated pore structure (as opposed to interconnected pore structure) because the pores are individual pores. Thus, the reactive agents will not travel within the concrete, and will be limited to the external pores.
failure in low strength concrete
Lesser negative slope see above. but essentially there is less energy being absorbed so less energy is released after fracture.
What does ettringite turn into after a long time
Monosulfate
To produce high strength concrete with a compressive strength >10,000 psi a w/c ratio of 0.2 is commonly used. Do you expect all cement grains to be fully (i.e. 100%) hydrated with a w/c ratio of 0.2 or will there be unhydrated cement grains left?
Not all cement grains will be 100% hydrated
Pozzolanic Admixture
SiO2 added, you have less CH because this is the reacting compound and producing more CSH
Ex from lecture: If we have .4 w/c ration with 100% hydration and we want to add aggregates specifically igneous rock like granite because granite has that same type of porosity. What will be the permeability of the concrete?
So we know that the strength in the combination will decrease, and that is because of the ITZ. The permeability would increase because of the ITZ and the permeability in the ITZ zone. The increase in aggregate leads to larger microcracks and the larger ITZ.
Some concrete specimens that were tested in splitting tension in your lab session revealed rough fracture surfaces with most of the aggregates pulled out instead of fractured. This type of fracture surface reveals (several answers may be correct):
That the concrete had a low compressive strength, That the concrete was produced with a high w/c ratio
Why is the chord modulus of elasticity a better representation of the elastic behavior of concrete compared to the secant modulus?
The chord modulus of elasticity is a better representation because it corrects for a slight concavity that is often observed at the beginning of the stress-strain curve that does not accurately represent elastic concrete behavior.
How does deicing salt work
The deicing salt lowers the freezing temperature, the salts migrate into the capillary pore, and so then the freezing temp in capillary pores is pushed lower and lower. But due to osmotic pressure, the water migrates out because the gradient is so large, so the new issure is
What is the effect on capillary porosity on strength?
The higher the capillary porosity the lower the strength will be
What is the different in energy state between capillary pores and gel pores
The ice is more likely to be in the capillary pores (because they are bigger which mean less of a freezing temp) the water goes from the gel pores to the capillary pores and pushes out the gel pores and creates more damage in the capillary pores
What is the effect of the coarse aggregate size on your compressive strength
The increase in aggregate size will decrease the compressive strength (this is a result of the IT zone!!! The itz zone is a function of coarse aggregate size
Effect of w/c and curing age on f'c
The longer the time period cured the larger the compressive strength. Compressive strength decreases as the water cement ratio increases
Concrete specimens are being tested in compression. All the specimens have the same w/c ratio but some of the specimens are tested dry and some are tested wet. Which one will be stronger and why.
The specimens that are tested dry will be stronger (typ. approx. 20 to 25% higher from air dry to saturated). The extra adsorbed water that remains in the concrete creates a disjoining pressure within the cement paste which reduces Van der Waals forces and therefore, the compressive strength.
Strength as a function of porosity
The strength decrease with increase in porosity ( all materials)
The real stress strain curve of the concrete
The stress strain curves of aggregates and cement past are linear elastic up to failure.... weird properties..
Why are materials weaker than their theoretical strength ( what dictates that strength?)
The thing that dictates the strength is the porosity of the material.
Which type of porosity dictates the strength in concrete? Is it the volume fraction of the pores? Is it the size of the pores? Is it the shape of the pore?
There are three different types of pores. (gel pores (nm), capillary pores (mu-m), and entrapped air voids (mm) ITS THE SHAPE!!! its because of the stress concentrations. So think of the paper, round hole more load applied. long skinny less load applied. SO capillary pores would be the worst for strength (decreases strength)
Correlation to strength and permeability
They are both a function of porosity. So if w/c goes down the strength goes up, and the permeability goes down. So if hydration goes up, strength goes up permeability goes down. If we reduce the ITZ zone, the strength goes up, and the permeability goes down
Freezing of water (and the assosiated 9% volume expansion) damages concrete; but what are the underlying mechanisms that cause this damage?
Think about profs beer bottle in the freezer, the whole bottle will expand and shatter. You create a lot of pressure inside of the container, and when water freezes it expnds by 9%, so the pressure is increasing and eventually will cause fracture. In the case of concrete we do not have a very dense specimen, so we are creating hydraulic pressure
What causes the reduction in f'c when coarse aggregates are added to the matrix
This weakness is a result of the IT zone. when you get close to the aggregate you have a higher porosity, like think about the packing density against a wall. 'wall effect' so in this zone there is a high porosity, which enables the ettringide and calcium hydroxide crystal to grow!! Its our weak link
What precautions do you need to take to prevent moderate sulfate attack?
Use ASTM Type II cement with no more than 0.5 w/c ratio
Tangent modulus
Very inaccurate it is the slope of the line drawn at any point on the curve
capillary porosity as a function f(w/c) ratio
We know that volume of cement = water in cement divided by density of cement, then we do the same for the water find the volume of the water Then you find the total volume and hydrate in 100% Then we know that the volume of the hydrated cement past is 2* volume of cement. Then the left over water is what gives the you the capillary porosity. So you take this volume and divide it by the total volume to get the capillary porosity in %
How to we get to 30% capillary porosity? (K=10^-11)
We must look at the w/c vs capillary porosity graph. We add water to the unhydrated cement grains and now it hydrates, if we need to get to that low capillary porosity we need to have high hydration. We must restrict the w/c ratio to 0.4. The maximum w/c ratio with 100% hydration is .6. (it would take 180 days) Anything higher than .6 you wont ever reach 30% capillary porosity. <-(we need this for the isolated pore structure)
The secant modulus
You take your 40% strain and divide it by the coordinating strain. (not too accurate)
How to we compensate for the reduction in f'c strength due to the added air entrainment?
You would reduce the water cement ratio! check out the table
WHat is not in high strength concrete
any aggregate larger than 3/8th an inch because of the itz
What type of structure is the property durability in?
atomic structure
Where do these attacks occur
dams, bridge piers, hydraulic structures
Osmotic pressure
due to concentration gradient in pore solution chemistry capillary pores contain dissolved alkalis (Na+, K+) and calcium. Ice crystal formation consumes only water molecules. Dissolved alkalis and calcium are not incorporated into ice formations. Alkali concentrations increase because less water is available to maintain the original pore solution chemistry
capillary pressure
due to different in the energy state
E-modulus and loading rate
fast: e-modulus is higher, higher strength slow:lower e-mod, lower e-mod
Which curve do we not use when using the chord modulus on the 40% graph with three curves
first curve
The effect of size in the whole cast of the concrete on compressive strength
for ductile materials it wouldnt matter, but concrete is a brittle material. So the the strength is proportional to 1/(flaws)^.5 So the larger the specimen to more the flaws so the lower the strength because it has a higher probability to have larger flaws.
Darcy's Law
for steady state flow the coefficient of permeability K indetermined from this law. Dq/dt=K(delH)(A)/Lv
Type of water and how it effects frost action
freezing temperature is a function of pore size and how the water is held in pores
what causes the non linearity of concrete?!
has to do with stiffness... there are microcracks in the itz (1) we can go up to 30% ultimate stress around 50% ultimate stress, everything is still stable not too many cracks, still linear after 50% the microcracks within the matrix begin to form (hcp) at the ultimate strength there is formed the microcrack, because it is at the ultimate stress. Once you go beyond 50% you get microcracks that is why it is not linear
dry specimen f'c
higher f'c because of the CSH layers get closer together (the porosity between these layers)
Fast loading f'c
higher strength suppression of microcrack formation
what is Kq
it is the relative measure of flow through concrete in cubic feet per year per squarefoot of area for a hydraulic gradient.
D-cracking and scaling
localized regions of weak concrete or high salt concentration or high salt concentration form a network of curved cracks. often in expansion joints.
wet specimen f'c
lower strength due to disjointing pressure
slow loading f'c
lower strength, more time for microcracks to form
what structure is associated to strength
macro-structure
What factors influence E-modulus
matrix: the porosity -w/c ratio -curing -air content aggregate: -volum of -aggregate -size of agg. (ITZ) -type of aggregate Testing parameter: -fast/ slow loading rate -wet/dry
normal strength concrete failure characteristics
pull out aggregate, this is because of the ITZ, they are very weak w/c high
Water in gel pores (1-5 nanometers) in freezing
resists freezing due to chemical bonding typically freezes at -78C (-172C)
Sources of sulfate ions
sea water organic rich environments, (sewer stagnant water) industrial waster, soils and groundwater
How does permeability inhibit the aggressive agents to infiltrate the concrete?
so we know that the permeability is a function of porosity, but which one (isolated vs interconnected) spoiler its the interconnected that inhibits the infiltration and isolated causes less infiltration.
A problem in water in capillary pores, mainly free water; build up of hydraulic pressure causes cracks in hcp
solution: air entrainment (provides space for water to escape)
The intrinsic/ theoretical strength:
the derivative of the cordon morse curve
The elastic modulus (the linear slope in the stress strain curve) at about 40% of f'c what happens after
the non linear zone is due to micro cracking. You must determine the e modulus before the 40% f'c
failure in High strength concrete (hsc)
the part after failure is different. The steep negative slope in the hsc, this is because there is a lot of energy absorbed and releases more energy after fracture there are grooves and cracks, the cracks go straight through the aggregates because the stronger ITZ areas (W/C ratio is low)
Why do we cure the concrete?
to protect concrete from drying out, and the hydration products will not be formed and then the strength will be lower.
wet/dry parameters and e-modulus
wet testing: you take it directly out of fog room and then test. wet: higher e-mod, because water can take some load. lower f'c strength due to disjointing pressure dry: lower e-mod due to increase in microcracking in the ITZ higher f'c because of CSH layers get closer together
How else can you identify a sulfate attack
x-ray diffraction analysis of undamaged and damaged concrete, you can see CH, cant see CSH, in the undamaged concrete. But in the damaged concrete, you can see the gypsum and the ettringite, and you can no longer see the CH because its 'eaten up' chemically
What does durability entail
you have a bunch of aggressive agents that infiltrate the concrete, but how easily can they infiltrate? permeability!
