3510 student questions: Concrete
provide an example of when concrete with low slump and high slump would be used
Low slump concrete can be used in foundations or footers, that require high strength. High slump concrete can be used for precast that has intricate formwork, as a high slump allows the mixture to spread easily and fill all corners of a formwork without excessive vibration.
name three different types of way to finish concrete
Possible answers include bull floating, screeding, jointing, steel troweling, or blooming.
How would replacing OPC with silica fume (by weight) affect a concrete mix?
The V/SA ratio would increase because of the very small size of the silica fume particles. This hinders the mobility of the added water in the mix which greatly diminishes the chances of segregation/bleeding.
why don't contractors want to use fiber-reinforced concrete despite that it reduces the cracking?
because it will have an effect on the workability by reducing it. workability is a vital property of concrete
In the Cement Micro-structure what hydration product causes to expand and crack a. water hydration b. C-H-S hydration c. ettringite d. Carbon Sulfate
c. ettringite
list the ingredients used to make cement paste, mortar and concrete respectively
cement paste- cement powder, water mortar- cement powder, water, sand concrete- cement powder, water, sand, coarse aggregates
how long should concrete be moist cured for?
for as long as possible so it increases compressive strength
how does concrete gain strength?
hydration
what type of reaction occurs while concrete hardens? what is the name of the process that starts when the cement and water are mixed? how does concrete gain strength when setting?
hydration
what was Portland cement originally developed from?
hydraulic lime (limestone) and clay
what does increasing w/c ratio do to concrete?
increases the workability of concrete
how does increasing the water to cement ratio affect workability?
increases workability
what are the effects of superplasticizer on workability of concrete?
increases workability
explain 3 effects of increasing the w/c ratio of your mixture
increasing the w/c ratio of your mixture will caused increased porosity, decreased strength, decreased durability
what is the first stage of the hydration process?
initial dissolution
what is a type of void in concrete
inter-layered, capillary, entrained air, entrapped air
how do air entrainers work?
Air entrainers create microscopic bubbles by having a tail that repels water and a head that is attracted to water. These bubbles stop cracks in the concrete from growing which would ruin the integrity of the concrete.
Why do states like Ohio use air- entrainers but states like Florida don't?
Air entrainers decrease the void size and decrease the spacing between air bubbles which helps to improve frost resistance. This is useful in states like Ohio which have annual freezing and thawing seasons. Air entrainers also decrease strength by about 20% per 1% increase in air so for a state like Florida that rarely experiences freezing temperatures it is more harmful than helpful.
You are working in an area that is exposed to freezing/thawing. What adjustment should you make to your mix to help fight this.
Air-entrainment
list 2 factors that water content impacts in concrete and how does it effect each one?
Any 2 of the following: Strength- adding water decreases the strength of the concrete. Workability- adding water increases workability when pouring the concrete. Durability- adding water decreases the durability of the concrete. Permeability-adding water increases permeability.
how could a super plasticizer be used to reduce cost?
keeping w/c the same but reducing cement and water content
If concrete has a low water-to-cement ratio, is the workability increased or decreased?
low water to cement ratio corresponds to a lower workability
what is flash setting?
rapid, exothermic hardening that is a result of the gypsum content being too low. this type of setting cannot be fixed
what does fiber do to concrete?
reduces workability and increases flexural strength
If you have three concrete mixes containing zero additives where one has 0.6 w/c, one has 0.5 w/c, and one has 0.3 w/c, which mix will have the highest workability?
the 0.5 w/c mix will have the highest workability
What is the cement chemistry notation for (CaO)_3 SiO_2
the notation is C_3 S
what is the typical range of w/c ratios in concrete
0.3-0.4
what is the typical w/c of a sidewalk?
0.7
T/F: gain of strength is obtained by hydration NOT drying
true
in general, does the strength of concrete depend on the strength of aggregate?
true
What admixture would you use if you are trying to decrease the amount of time before starting finishing operations?
you use an accelerator
If you have a 4000 psi concrete, how much strength in psi would you lose by adding 1% more air?
200 psi
describe the differences between false set and flash set including causes and solutions
False set is the premature stiffening of concrete that can be mixed by more mixing. It is caused by too much gypsum in the cement mixture. Flash set is the rapid, exothermic hardening of concrete. Caused by too little gypsum in the cement mixture. This cannot be fixed with more mixing.
true or false: Water content and strength of the concrete have a direct correlation to one another. The relationship is the higher the amount of added water, the stronger the resulting concrete will be.
False! While water content and concrete strength do have a relationship, the effects of the water are opposite to the previous statement. The more water added to a mix, the weaker the resulting concrete will be.
Structures made of Roman concrete have lasted for centuries because of their great strength.
False. They have lasted so long due to Roman concrete's durability. Durability is a concrete's ability to withstand forces over a period of time.
what is the main contributor to concretes strength?
W/C ratio
what is the use of a water reducing agent in concrete?
Water reducers can improve workability and increase strength at a lower w/c ratio, which will result in a lower cost.
what is the most used material in the world (besides water)
concrete
As the water-cement ratio increases in a mixture, what happens to the slump, workability, and chances of segregation of the mixture?
The slump and chance of segregation both increase with a higher water-cement ratio. Slump increasing also means that the workability will also increase.
can flash/quick set be fixed?
no
w/c is directly related to
porosity, strength, durability
for every 1% increase of air content in a mixture, there is a 5% decrease in...
strength
what happens to concrete strength as water to cement ratio increases
strength decreases
Increasing the w/c ratio A. Increases Strength B. Increases Stiffness C. Decreases Strength D. Increases Both Strength & Stiffness E. Decreases Both Strength & Stiffness
E. Decreases Both Strength & Stiffness
what does water reducers do?
- Increases dispersion of cement particles Improves workability and flow Limit time effect
what are the 5 concrete ingredients
-Aggregates -Cement -Water -Supplementary Cementitians Materials (SCM's) -Admixtures
what are the effects of concrete hydration?
-Building of microstructure (Ca silicates to C-S-H) -Release of heat from exothermic reactions -Mix expands then contracts due to: cooling from initial hydration temp increase, consumption of water (capillary forces get stronger as water is removed from pores and crystallized into the structures)
What are at least two benefits to using a superplasticizer in your mix? Give an short explanation for both.
-First benefit is increasing the workability of the concrete mix, without promoting segregation. This is important for allowing the concrete to move around any reinforced areas and really work into all the pockets and voids in your form work. -Second benefit, which ties into the first, is that this allows the concrete to get the previous benefits without having to add more water. This means the concrete will be stronger, while still workable.
What is super-plasticizer and how can it help to improve a concrete mix?
A water reducing admixture that improves the workability of a concrete mixture, allowing for the mix to have a lower w/c ratio, which also improves strength and durability.
what kind of admixture is needed most when the concrete operation starts before winter and why?
Accelerators, the cold weather will have a negative impact on the fresh concrete, so the work should be done as soon as possible before winter.
what is the specific gravity of cement
3.15
If the w/c ratio for a specific type of concrete is 0.4, and a total amount of 37 lbs of water is necessary, how many lbs of cement is necessary?
37lb/c = 0.4 --> c = 37lb / 0.4 = 92.5 lbs Cement
what is the minimum amount of time you should let normal concrete cure for?
7 days
what is the typical weight of a sack of cement
94 pounds
if you were making a sidewalk what would you take into consideration in your mix?
A "high" wc ration because the workability would be increased and strength is not as important for an application like a sidewalk.
what is the purpose of pre or post tensioning concrete?
Concrete is pre or post tensioned in order to keep the concrete in compression. The metal strands inside the concrete constantly pull the concrete together. Concrete is typically about 10 times stronger under compressive loads as compared to tensile loads.
why is reinforced rebar placed within concrete?
Concrete is very brittle and poor in tension, so rebar is encased within the concrete to reduce cracking and breaking.
You are designing the concrete mix for an outdoor structure in Columbus. Name the admixture that you would design into your mix that would account for the weather. Then explain what that admixture would do to your concrete in terms of durability, workability, and strength.
The admixture that I would put into the concrete would be an air entrainer. This admixture would create very tiny pockets of air in the concrete that would increase the concrete's resistance to freeze and thaw conditions, thus increasing durability. It would also increase the workability of the mixture, but decrease the overall strength of the concrete.
While the cement is being mixed on site, it begins to stiffen or harden. You feel the side of the mixer, and it feels warm to the touch. You should: A. Add water or superplasticizer to increase the workability of the concrete B. Add gypsum in order to offset the low gypsum content C. This cannot be fixed D. Remix vigorously without adding anything
The correct answer is C, the warmth indicates flash set has occurred, it cannot be fixed
explain why drying concrete does not lead to strength gain
The reaction between water and cement is what gives the concrete its strength, so if you remove water through drying, you stop the reaction, which stops the gaining of strength.
Would the water/cement ratio be lower for concrete used in a sidewalk or a pedestrian bridge overhanging a highway.
The water/cement ratio would be higher for the sidewalk and lower for the pedestrian bridge.
True or false: Super plasticizers separate the cement particles and fluidize the mixture.
This is true. Super plasticizers do this and are able to increase slump and workability without the addition of more water to the mix.
T/F: At early ages, increasing the temperature of the mix increases the rate of hydration and therefore the rate of strength development.
True
what causes plastic shrinkage cracking
Volume changes at the surface as drying occurs, but is restrained at the bottom and so cannot change
You are building a sidewalk in a flood prone area. What kind of concrete (properties) would you want in the concrete?
Would want space inside of the concrete to allow high porosity so the water can pass through. Anything else that has the same effect.
You are working on a detailed concrete wall with intricate designs. Your concrete is too thick and needs to have more workability in order to produce the design effects. What are some ways you can increase the workability of the mix and what are the effects of using these?
You can add more water to the mix. However, adding water will decrease the early strength and the durability of the concrete. Another alternative is that you can add superplasticizer or other water reducers to the mix. This will increase the workability without affecting the early strength of the concrete.
What are 3 things you would want to modify in a standard concrete mix if you are pouring a bridge deck that requires high strength, has re-bar close together, and will be exposed to freeze-thaw conditions?
You would want to lower your w/c ratio to increase strength, add superplasticizer to bring back some workability to allow the concrete to get into the small spaces, and you would want to entrain air so the concrete doesn't fail when it freezes and thaws.
Which of the following is not an SCM? a. Superplasticizer b. Fly ash c. Ground granulated blast furnace slag d. Silica Fume e. Natural pozzolan
a. Superplasticizer
increasing w/c of your mix does what to your concrete's strength?
decreases
what is the purpose of the slump test?
detemine the workability of the mix
how does drying affect strength gain in concrete mixtures?
drying slows the rate of strength gain in concrete
Which one of these is NOT a mechanical property of concrete? a.Compressive Strength b.Tensile Strength c.Modulus of Elasticity d.Creep Resistance e.Permeability
e.Permeability -Permeability is a durability property of concrete. Permeability is a quality of material that allows liquid or gases to flow through the concrete.
What would you exclude from a cement mix to create pervious concrete and why?
exclude fine aggregates to increase voids
T/F an increase in w/c ratio leads to an increase in durability
false
T or F: by decreasing the w/c of a concrete mix, workability is increased
false, workability would decrease
true or false: curing concrete has little to no effect on the strength of concrete
false. curing concrete increases the strength the longer the mix is cured. the optimal curing is for 28 days
T/F: increased permeability = increased durability
false. higher permeability=reduced durability
describe the relationship between concrete's rate of strength gain and fineness of the cement?
fineness controls rate of hydration: finer cement=faster strength gain
Explain the difference between the effects that fly ash versus silica fume has in concrete. also explain where these materials are from.
fly ash- increases durability and workability because all particles are shaped like spheres. this is created from the burning or coal. silica fume- has very high strength, low workability because it has small particle size to fills the voids. this is created from the waste of silicon wafers and chip production
how does air-entrainer work?
has a hydrophillic head and a hydrophobic tail which allows for air bubbles to be created in the concrete
super-plasticizer is best known to be used for
increasing workability while lowering the demand for water
where and why would pervious concrete be used?
large areas such as parking lots, in order to minimize the opportunity of the parking lot flooding form rain waters
what are the three curing approaches and which one is the best?
maintaining the presence of water, sealing the surface, and heat/steam are the three curing approaches. Maintaining the presence of water is the best curing approach.
in what scenario would you want to use low workability
making a retaining wall
how does a super plasticizer affect a concrete mix
plasticizers increase the workability of concrete without increasing the water to cement ratio
What is false set?
premature stiffening of mixture, usually due to too high of a gypsum content
what does the acronym SCM stand for and name 2 types of SCMs
supplementary cementious material: fly ash, silica fume, GGBFS
how do you know if your concrete is setting?
there will be a release of heat since the reaction that occurs is exothermic
T/F: Substituting a portion of portland cement with fly ash in the mix increases the density and the workability of the concrete.
true
does a higher water/cement ratio make the concrete stronger or weaker?
weaker
Portland cement should be measure by __________ and never measured by volume.
weight
is cement always measured by weight or volume
weight
What is the difference between entrained and entrapped air bubbles? Which is desirable within a concrete mixture?
Entrained air bubbles are from 0.1 to 1 mm in diameter, while entrapped air bubbles are typically larger than 1 mm. Entrained air helps concrete resist the negative effects of freezing and thawing cycles, while entrapped air tends to reduce the f'c of the concrete. Entrapped air is typically the result of poor mixing procedures, while entrained air is intrinsic to the process.
I have a total of 30 tests and my standard deviation is 400 psi. What should my f'c be in order for the average concrete compressive strength (f'cr) to be 950 psi?
F-1.00(30 Tests) s=400psi since my deviation is less than 500, The equation f'cr=f'c+(1.34s) should be used f'cr=950 psi 950=f'c(1.00)+(1.34(400)) 950-(1.34(400))=f'c f'c=414 psi
A client is looking to improve the concrete they originally designed for use in a bridge deck in Columbus, OH, claiming it had too much work ability and not enough strength. What suggestions would you make to improve the mixture?
First consider the water ratio. Suggest a w/c around 0.4 and the possibility of using a super plasticizer if the work ability is too low. Decreasing water increases strength and durability. Ensure an air entrainer is used as the bridge will be exposed to freezing/thawing in Columbus. Although air entrainers result in slight decrease in strength (5% for every 1% increase in air), allows for lower w/c to compensate. Additionally, you could consider using fly ash if workability is too low after decreasing w/c, also decreasing cost of cement and increasing strength.
Which SCM could be added to help fill a complex concrete mold and why
Fly ash could be added for increased workability
discuss how fly ash is used to reduce the carbon footprint of concrete
Fly ash is used to supplement roughly 20 percent of the cement used in concrete. The production of cement produces 1 kilogram of CO2 for every 1 kilogram of portland cement. If less cement is used, then the amount of CO2 output will also be reduced with the presence of fly ash as a supplementary material.
What kind of admixture is the best to use in freeze-thaw conditions (much like Ohio is experiencing right now)? Describe why this admixture is effective in reducing the damage done to concrete.
Freeze-thaw conditions would immediately suggest the need for air entrainer to be added to a mix. Air entrainers create stable, evenly-distributed bubbles of air throughout concrete. As water within the concrete freezes, it will expand towards the nearest void and can cause major cracking if one isn't nearby. By using air entrainers to create intentionally tiny and spread out voids, the water will hopefully always have a close place to go, minimizing cracking and damage in the winter.
While doing research for finding ways to increase workability of concrete, a researcher finds out that increasing water-to-cement content ratio is the easiest way to increase workability. If the researcher proceeds to built a structure with very high w/c ratio concrete mix, what problems could arise?
High w/c ratio mix have tendency to segregates, which makes the concrete more prone to cracking. High water content also causes voids that reduce bond between concrete and rebar, which decrease the strength. Voids also increase permeability of water which could reduce durability and also magnify shrinkage of concrete upon drying.
How does permeability effect the durability of concrete?
Increasing permeability causes increased porosity and voids which in return can decrease durability.
why is adding more water to a mix harmful to the concrete?
Increasing the amount of water in a mix increases the amount of voids. Increasing the voids decreases strength and decreases durability.
Explain why it is impractical to use a low water cement ratio (such as 0.4) for a residential sidewalk compared to using a higher ratio (such as 0.7).
Lower water cement ratios are used in applications where there are high loads and low permeability is integral. Additionally, such applications are more expensive. A sidewalk is a relatively inexpensive project and the loads it is subject to are lower. Therefore, a high water-cement ratio is not necessary.
name the three most common approaches for protecting concrete as it cures.
Maintaining the presence of water (spraying, fogging, wet covering), Sealing the surface (plastic sheets or curing compound), Heat and adding moisture (good for early strength at the cost of long term strength).
what is pervious concrete and what is a situation in which it could be used
Pervious concrete is designed to allow water (and other liquids) to pass through it. One application of this type of concrete is large parking lots where water can easily pool but it can not be used for anything that requires high strength.
Name and explain the use of the different ways of keeping water present for concrete:
Ponding-smaller jobs, flat work, and laboratory Spraying-Expensive and requires a high amount of water Wet Covering- wet concrete and cover with a sheet of sorts
Given that the weight of a coarse aggregate in a concrete mix is 1500 lbs/yd^3 and has 3% absorption. Calculate the weight needed for the mix if it has a moisture content of 5%.
1575 lbs/yd^3
Given a weight of coarse aggregate of 1625 lb/cy, a moisture content for coarse aggregate of 8%, and an absorption percentage of 2%, what is the batch weight of coarse aggregate for the cement mixture? What is the change to the water content of this mixture?
1625 lb/cy(1+0.08)=1755 lb/cy, Delta_W=8%-2%=6%, Water Content= 1625(0.06)= 97.5 lb/cy, so 97.5 lb/cy less water needed in the batch.
list two ways to increase the workability of concrete
1. increase the water content 2. add a water reducer such as a superplasticizer
What is the cement chemist notion for : (CaO)6 Al2O3 (SO3)3 (H2O)32
C6 A S H 32 (THE S HAS A LINE ON THE TOP)
What is the Cement chemistry for Ca(OH)2?
C= CaO H=H2O CaO + H2O=CH
A concrete mix design includes 600 lb cement, 1,000 lb coarse aggregate, 850 lb fine aggregate, and 750 lb water. the adsorption of the coarse aggregate is 3% and the adsorption of the fine aggregate is 7%. If the moisture content on site is 6% for both the coarse and the fine aggregate what are the adjusted weights of each component of the concrete mix?
Cement=600 lb Coarse Agg=1060 lb Fine Agg=901 lb Water=728.5 lb
Give one example of a structure in which ultra-high performance concrete would be desired and explain why UHPC would be beneficial in that structure.
UHPC would be beneficial in skyscrapers because UHPC has a high fiber content to increase flexural strength to prevent brittle failure and a super optimized aggregate gradation, which increases the strength to above 10ksi and gives the concrete a high modulus of elasticity. This is desirable in a skyscraper because the concrete needs to support huge loads and also minimize sway for especially tall structures.
