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Materials > Concrete > Flashcards

Concrete Flashcards

1
Q

What is concrete?

A

Its a mix of water, cement and aggregate

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2
Q

Difference between Concrete, mortar and grout?

A

Different sizes of aggregate

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3
Q

What is Cast insitu concrete?

A

Concrete poured into timber, metal or plastic formwork/ shuttering.

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4
Q

What is pre-cast concrete? some examples

A

Factory made concrete/ prefabricated parts.
bridges, modular buildings, footings for windmills, stairs

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5
Q

Typical ingredients of concrete?

A

7.5%water
15%Cement
77.5% aggregate (33% Sand 66% Gravel)

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6
Q

Typical properties of concrete?

A

Density 2.4 t/m3
Compressive strength 40N/mm

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7
Q

Tests for concrete workability?

A

Vebe Test
Slump Test
Degree of compactability test
Flow Table Test

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8
Q

What is the vebe test?

A

similar to the slum test but for low workability concrete.
slump test on a vibration plate, measuring the time until the concrete flatens out.

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9
Q

what is the slump test?

A

fill cone upside down on a board, 1/3 1/3 1/3 with 25 tamps in between.
remove cone and measure the slump from top of cone to concrete

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10
Q

What is the degree of compactability test?

A

Fill up a rectangular box with concrete without compacting as you go along (200x200x400)
smooth over the top and scrap of excess
compact as much as possible.
measure the distance from the top it has compacted.
the higher the figure the less workable it is.
Results classified S1-S5

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11
Q

What is a flow test?

A

Like a slump but on a tilt
cone is created
table is lifted 40 mm and dropped 15 times
the concrete is then measured in 2 perpendicular dimentions and the average of the two is taken
flo is clasified F1-6
If cement flows but aggregate stays near the centre, there may be problems with seggregation

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12
Q

What slump classification does pumped concrete require?

A

at least S3

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13
Q

What factors affect workability? how can the be mitigated?

A

Amount of small particles - larger SSA
Aggregate shape - angular gives a larger ssa
Texture - a rougher surface area will make it less workable
addition of plasticiser

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14
Q

what is a sign of a poor cohesive mix?

A

Bleed - where water bleeds to the top of the mix and accumulates on the surface

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15
Q

how can we reduce the risks of segregation?

A

increasing the cement content
increasing the sand content or using finer sand
reducing the water content
using rounded aggregat instead of angular

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16
Q

what is plastic shrinkage?

A

where the rate of surface evaporation exceeds the rate of bleed, cracking can occur

17
Q

what is plastic settlement cracking?

A

where settlement of dense aggregate particles fall and cause cracking close to the surface on the rebar

18
Q

what is a danger of casting concrete in cold weather?

A

concrete should not be poured below 5c for the first 48hrs
it may work against the concretes exothermic reaction and early set may struggle to achieve initial strength

19
Q

what is entrained air?

A

millions of separate bubbles between .1 and .5 mm.
5-8% entrained air can help resist freeze thaw

20
Q

What is entrapped air?

A

entrapped air is large connected voids from poor tamping.
5% entrapped air can reduce overall strength by 25%

21
Q

What are the two sizes of cube and cylander for testing compressive strength?

A

cube - 100 and 150
cylander - 100 by 200 and 150 by 300

22
Q

how many times larger than the maximum aggregate should the cube or cylndar be?

A

minimum of 3

23
Q

describe the cube test?

A

make your concrete mix
fill, tamp and smooth over the cube
leave for 24 hours minimum before removing from the mould
write on the top face of the cube
allow to cure in a 20c bath of water for 28 days
measure the density relative to water
place cube in platens
measure the compressive strength using machine
bottom platen moves up whilst the top has play in case faces arnt completely parralell

24
Q

how do we measure relative density?

A

weigh the cube in air and in water,
density of concrete is given as
(mass in air/ (mass in air- mass in water))x density of water

25
Q

What degree of acuracy does the test machine give us prior to breaking?

A

1 Kn

26
Q

what size cube it traditionally used in the uk for a cube test?

A

100mm

27
Q

how do we calculate strength?

A

by measureing the stress.
stress (f) = P (compressive force) / area force is applied to (pie D^2/4) or (l^2)

28
Q

What are an example of low normal and high compressive strength classes? and why is the cubes compressive strength higher the the cylinder?

A

C8/10 low
C40/50 typical
C100/115 Highest
cube has more restraint top and bottom relative to the sides (poissons ratio)

29
Q

What compressive strengths are cubes measured in?

A

N/mm2 = MN/mm2 = MPa and tons

30
Q

What is the standard deviation assumed as for testing cubes if insufficient data is available?

A

deviation of 8N/mm2

31
Q

What is the effect of water on cement?

A

less water = more compressive strength
however some cement may not hydrate and compaction becomes difficult weakening the mix

32
Q

Why does crused rock add strength to mixes?

A

because it causes interlock between particals also partly because it has a higher SSA

33
Q

what percentage of the compressive strength of concret is the tensile strength?

A

10%

34
Q

how do we measure tension in concrete?

A

Direction tension
Splitting Test
Flexural test

35
Q

How can we increase tensile stress in concrete?

A

Rebar
fibre reinforce concrete (steel, glass, polyester, polypropylene, basalt fibre, graphene)

36
Q

In relation to durability, what are the exposure conditions classes?

A

No risk - inside a building
Carbonation - water causes iron in the mix to rust when carbonation occurs over time
Chloride attack - internal (marine) external salty environments. cause rust then spall
Chemical attack - sulphates on brownfeild sites react with the cement increasing volume and causing spall
AAR
Freeze Thaw

Materials (11 decks)

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