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Class 1 Flashcards

1
Q

What is the role of C3S (Tri-Calcium Silicate) in concrete?

A

It is hydrated to form calcium silicate hydrates. It provides the majority of concrete’s strength.

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2
Q

What does C3A (Tri-Calcium Aluminate) do in concrete? Why is it desirable?

A

It flash sets. It may also expand if the concrete deteriorates, leading to cracking / spalling.

It is desirable because it reduces kiln temperatures.

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3
Q

What is clinker?

A

Clinker is raw meal that has been heated in a kiln. Clinker is ground with gypsum to create cement.

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4
Q

What is gypsum for?

A

Gypsum is added to clinker when griding to prevent concrete from flash setting due to C3A.

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5
Q

Limestone, clay and chalk are ground up. What is the result called?

A

Raw Meal.

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6
Q

What is ground up to make Raw Meal?

A

Limestone, clay and chalk.

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7
Q

Name 3 chemicals that are commony mixed with portland cement to reduce the amount of OPC required.

A

GGBS (Ground, Granulated Blast furnace Slag)
PFA (Pulverised Fly Ash)
Silica Fume

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8
Q

How is Magnesium Oxide relevant?

A

It is a type of cement, made at lower kiln temperatures. It is expensive to source and not alakaline enough to protect the steel.

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9
Q

What is Cemfree?

A

A OPC-free cement using GGBS and PFA

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10
Q

What is OPC?

A

Ordinary Portland Cement.

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11
Q

What is specified for concrete?

A 
SESME:
Strength
Exposure Class
Slump
Maximum aggregate size
Extras
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12
Q

What do the two Es stand for in SESME?

A

Exposure Class and Extras

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13
Q

What does M stand for in SESME?

A

Maximum Aggregate Size

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14
Q

One S in SESME stands for strength. What does the other one stand for?

A

Slump.

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15
Q

What does A1-A3 measure?

A

Cradle to Gate

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16
Q

What does A4 measure?

A

Transport

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17
Q

What does A5 measure? What are the two subscripts?

A

Construction: A5waste and A5activities

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18
Q

Why is carbonation bad?

A

Lowers pH>Rusting

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19
Q

Why might a lack of OPC lead to carbonation?

Give a material where this is a problem.

A

OPC raises the pH. A lack of OPC makes rusting more likely. For example, Magnesium Oxide cements suffer from carbonation.

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20
Q

What is the typical stress profile block for concrete?

A

0.8x deep, f_cd wide

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21
Q

How wide are T-beams taken to be?

A

0.2*length between M=0 + b_w

The total width at the top of the flange is

0.2 L_0 + b_w

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22
Q

How are columns designed?

A

Assume compression steel is yielding & 0.0035 in concrete
Set N-A somewhere
Calculate what axial load and moment are
Repeat for all N-A

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23
Q

What are the four inequalities on a Magnel diagram?

A

Top and bottom of beam ; concrete tension and compression

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24
Q

How is strength design of pre-stressed concrete performed?

A

Assume yield of top concrete in compression, vary N-A to get equilibrium, (and therefore find steel stress), find M

Note that steel stress is affected by pre-stress and concrete relaxation pre-stress

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25
Q

Why might steel strains be higher than expected in prestressed concrete?

A

Small cracks in the concrete concentrate strain in the steel. Pre-stressing steel isn’t that ductile.

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26
Q

What admixtures are there? Name 3.

A

Accelerators, Plasticizers, Set-retarders

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27
Q

What is segregation?

A

Concrete is over-vibrated, so aggregate falls to the bottom.

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28
Q

What is phenolphthaelin for?

A

See where carbonation front has reached. Colourless if OK, pink if not.

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29
Q

Why is cracking along reinforcement worse?

A

Both the anode and cathode are exposed, so corrosion occurs more readily.

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30
Q

A high strength concrete mix needs what ad mixture?

A

A plasticizer because it will have a low water:cement ratio

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31
Q

Why is a fabric mould good?

A

Lower porosity to carbonation and chlorides. (Potentially) cheaper and lower carbon emissions. More flexible shapes.

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32
Q

How to design a doubly reinforced beam?

A

Assume steel yields everywhere (and then check at the end).

fyAs’+0.8f_{cd}xb=fyAs

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33
Q

What is the P-\Delta effect?

A

M=(e+\Delta)P
As P increases, \Delta > 0, and so line in M-P space isn’t straight. For very slender beams, a buckling instability occurs before reaching the failure envelope.

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34
Q

What shape is the P-M failure envelope for a concrete beam?

A

P on y axis; M on x axis
M is greatest for P a little bit more than zero.
Note that M=eP unless \Delta effects are important

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35
Q

What is a Kern Point in terms of the section properties?

What is the Kern point’s sign for the top surface?

A
  • Zi / A

For the top surface, Z1 is -ve, so Kern point is +ve, so Kern point is below the N-A.

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36
Q

What is the significance of a kern point on a mangel diagram?

A

For P->inf (1/P>0) so need to be applied at kern points for top / bottom stress to be OK.

One kern point represents top stress being zero (and therefore between limits) , other for bottom stress.

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37
Q

What is the y-intercept on a e , 1/P graph called?

A

The Kern point. There are 2, each with two straight lines.

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38
Q

A beam has depth h and cover c. At what depth is the longitudinal reinforcement?

A

h-c-D/2

Don’t forget half of bar diameter!

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39
Q

What is pitting caused by?

A

Chloride ions cause pitting. It is difficult to spot.

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40
Q

What is the outer most point on a P-M diagram representative of?

A

The point of balanced failure.
For higher P, the concrete fails in compression.
For lower P, the steel yields first.
At this point, the tension steel and compression concrete fail at the same time.

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41
Q

How would you find the balanced failure point of a column?

A

The compression concrete fails at the same time as the tension steel yields. Therefore, a strain diagram can be drawn.

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42
Q

How would you find the maximum axial load of a column?

A

P = fy (As + As’) + f_cd * b * h

Note no factors on concrete strength or stress block size

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43
Q

The maximum axial column load is

P = fy (As + As’) + f_cd * b * h

What is the corresponding moment? eccentricity?

A

M = 0 ; e = 0

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44
Q

Where is the neutral axis for balanced failure of a column?

A

This is the x-intercept of line from ec=0.0035 to es=fy/E

Concrete extreme fibre fails in compression
Steel just yields in tension.

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45
Q

How do you design a T-beam?

A

Assume fyAs in the steel. Then 0.8xp* b * fcd = Asfy to find N-A. Then take moments.

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46
Q

Does the steel in a singly reinforced T-beam yield?

A

Yes. Design is performed with 0.8xpb*fcd=Asfy

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47
Q

Why is grout applied to a post-tensioned beam?

A

Grout fills the duct, providing better shear transfer to the beam, and preventing corosion.

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48
Q

Where is the shear transfer in a post-tensioned beam?

A

Mostly at the anchors, although some is via grout in the duct.

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49
Q

Where is the shear transfer in a pre-tensioned beam?

A

Along the strand due to bond with the concrete. Ribbing helps this.

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50
Q

What is the concrete strain in an uncracked unbonded beam?

A

It is approximately parabolic. The total contraction is equal to the total steel contraction. The peak strain is about twice the steel strain (which is a constant).

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51
Q

What shape is the STEEL strain along an uncracked unbonded beam?

A

It is constant.

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52
Q

What is the concrete strain in a CRACKED, unbonded beam?

A

It increases towards the centre of the beam. The peak is about 4x higher than the steel strain (which is a constant)

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53
Q

In the absence of shear reinforcement, how is the strut-and-tie model used?

A

Draw straight lines from load application to reactions.

The tension steel probably forms a tie between the reactions.

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54
Q

Is the strut-and-tie model valid in concrete without shear reinforcement?

A

Yes. Compression struts need to go from load to reactions, and the tension struts are in the longitudinal reinforcement.

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55
Q

A strut-and-tie model in concrete without shear reinforcement has compression struts from the load application to the supports. What other struts are there?

A

A tension strut in the longitudinal reinforcing steel.

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56
Q

A beam has shear reinforcement and is loaded by a point load in the centre. Flexural checks show that the beam is just sufficient to carry the moment. Why is this a problem?

A

The shear carrying scheme is a strut and tie model. This loads the tension steel in tension, so the longitudinal reinforcement may be insufficient.

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57
Q

A strut and tie model has inclined compression struts. What other struts are there?

A

Tension struts in longitudinal steel, and compression struts at the top of the truss.

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58
Q

What is aggregate interlock?

A

Shear forces carried by interlocking aggregate in beam.

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59
Q

What is dowel action?

A

Shear links prevent the flexural reinforcement fom falling of the bottom of the beam. In cracks, the longitudinal reinforcement is no longer flat, and so a component of its tension helps to resist shear.

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60
Q

What is the size effect in shear?

A

Larger beams resist less shear than they might be expected to by plasticity theory. For very large beams, elastic theory becomes a better model.

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61
Q

What model predicts shear failure in very large beams?

A

Elastic theory is a better estimate for very large beams.

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62
Q

What is the Compressive Force Path Method for concrete beams?

A

The top of the beam is in pure compression, and the bottom of the beam is reinforcing steel in tension. Below the compression concrete are vertical ‘teeth’ serve only to build up the tension and compression loads. The compression concrete carries all of the shear force.

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63
Q

What is peridynamics?

A

Numerical simulation of concrete. Can be used to predict cracking behaviour. Nodes interact if they are sufficiently close together.

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64
Q

Concrete shear failure can be predicted using a strut-and-tie model. Name TWO OTHER methods.

A

Compressive Force Path Method and Peridynamics.

CFPM assumes all shear is carried in ‘compression zone’ at the top of the beam.

65
Q

Why might existing codes for steel shear reinforcement not apply to FRP reinforcement?

A

FRP isn’t as ductile as steel.

66
Q

How is a beam subject to torsion and shear assessed?

A

V/Vult + T/Tult < 1

67
Q

Why are very shallow ribs on longitudinal steel dangerous?

A

The shallow angle allows for slip, and wedging action pries the concrete apart.

68
Q

Two longitudinal reinforcement bars are spliced to transfer the load. What do we need to be careful of?

A

The spliced bars need to transfer force from one to the other. This creates a 45 degree compression field in the concrete. To prevent the two bars being pushed apart, reinforcement around the section - a bit like torsional reinforcement - needs to be provided.

69
Q

A long beam is being post-tensioned. What needs to be considered?

A

Frictional losses during jacking will be large. Jack from both ends will make the cable force more uniform.

70
Q

What is wedge pull-in?

A

After jacking in a post-tensioned beam, the wedge moves into the collet slightly. This reduces the steel strain. It is more significant for shorter beams.

71
Q

What is low relaxation steel?

A

It has recieved extra heat treatment and relaxes less than normal steel when under high strain for a long time (eg in a prestressed concrete)

72
Q

Pre-stressing cables in a particular order affects the stress they experience at the end. How can this effect be mitigated?

A

Pre-stress cables on alternating sides of the section. (Don’t do one side, and then the other).

73
Q

The Capstan equation is

T1 = T2 exp(mu * theta)

How is it modified for post-tensioned concrete?

A

There is ‘wobble’ in the duct, which adds a kx term:

T1 = T2 exp(mu * theta + k * x)

74
Q

In ribbed bars, the bond strength is provided by adhesion and …

A

Rib bearing.

75
Q

In unribbed bars, rib bearing is zero. All transfer is from…

A

adhesion.

76
Q

How does peridynamics work?

A

Nodes interact within a certain distance \delta. Beyond this distance, they do not interact.

77
Q

Peridynamics analyses interactions between sufficiently close nodes. How can this be remembered?

A 
Peri = near / surrounding
Dynamics = Force / Power
78
Q

Name a type of concrete used to reduce density. How is it made?

A

Lightweight concrete uses lower density aggregate.

79
Q

Lower density aggregate is used to make a concrete. What is the concrete called?

A

Lightweight concrete.

80
Q

What is air-entrained concrete?

A

Concrete that has things added to introduce air bubbles. These relieve freeze-thaw pressures.

81
Q

Concrete can be lightweight, air-entrained, or normal. Name two other types.

A

High strength concrete (low water/cement ratio, plasticizer needed)
Rapid-setting concrete (accelerants added).

82
Q

Z1 refers to the top fibre. What is its sign?

A

Z1 is negative.

83
Q

Z2 refers to the bottom fibre. What is its sign?

A

Z2 is positive.

84
Q

The Magnel point for the top fibre is at -Z1/A. What does this tell you about the sign of Z1?

A

The Magnel point for the top fibre is below the centroid, so e is +ve. Therefore, -Z1 is +ve

Z1 is NEGATIVE.

85
Q

The inequality for a Magnel diagram is:

fc > P/A + Pe/Z - M/Z > ft

What is the sign of e?

A

e is positive when it is below the N-A.

86
Q

The inequality for a Magnel diagram is:

fc > P/A + Pe/Z - M/Z > ft

What is the sign of M?

A

M is positive in sagging.

87
Q

Z1 is defined as I / y1.

What is its sign?

A

Z1 refers to the top fibre, so y1 is -ve, Z1 is -ve.

88
Q

The equation:

fc > P/A + Pe/Z - M/Z > ft

Allows P and e to be designed. What is e?

A

e is the effective point of action e_p of the cable.

e_p = e_s - M2 / P

89
Q

How is the equation:

fc > P/A + Pe/Z - M/Z > ft

modified to include secondary moments?

A

Replace M with M + M2 to get:

fc > P/A + P e_s /Z - (M + M2)/Z > ft

Note that this is now for e_s since e_p = e_s - M2

90
Q

What causes secondary moments, M2?

A

A redistribution of the support reactions.

91
Q

If the bottom flange is large, where do we WANT to put ep?

A

At the bottom / below the section.

92
Q

The bottom flange is large, so we would like ep to be below the section. How can this be achieved?

A

Negative secondary moments (hogging).

ep = es - M2/P

M2 -ve makes ep > es

therefore ep can be below the section.

93
Q

In the equation:
fc > P/A + Pe/Z - M/Z > ft
What is the sign convention on M?

A

M +ve for sagging (ie M is generally positive).

94
Q

The top flange is large, so we would like ep to be above the section. How can this be achieved?

A

Positive secondary moments (sagging).

ep = es - M2/P

M2 +ve makes ep < es

Therefore ep can be above the section.

95
Q

One of the Magnel equations is:

-Z2/A + fcZ2/P + M/P > ep

How can this be used to create an inequality for the cable’s true profile?

A

es = ep + M2/P

so:

-Z2/A + fcZ2/P + (M+M2)/P > es

96
Q

Magnesium Oxide cements don’t use OPC. What is ANOTHER advantage?

A

They are made at lower kiln temperatures.

97
Q

MgO cements are expensive. Name another disadvantage.

A

They aren’t alkaline enough to protect the reinforcing steel.

98
Q

What does CFPM stand for?

A

Compressive Force Path Method

99
Q

What is the name for the concrete model that has a single compression zone at the top of the beam that carries all of the shear?

A

Compressive Force Path Method.

100
Q

A concrete has f_cd = 45MPa

In order to calculate unreinforced shear resistance, we need f_ck. What is f_ck?

A

f_ck = 45MPa * 1.5 = 67.5MPa.

Note that in the unreinforced shear resistance calc, we need to use f_ck in MPa, and b_w and d in mm.

101
Q

The Compressive Force Path Method has a block of compression at the top of the beam, and tensile reinforcement at the bottom. What else is there?

A

The concrete below the compression acts as cantilevering teeth to carry tensile/compressive loads between the top and bottom of the beam.

102
Q

How might heat generation be mitigated in a large pour?

A

Use non-OPC materials e.g. GGBS, PFA and MgO.

Chilled water used for hydration.

Refrigeration pipes within pour (expensive but effective)

Set retarders

Extra reinforcement to deal with stresses.

103
Q

A large concrete pour generates heat and therefore thermal stresses upon cooling. What other problems might there be?

A

Cold joints if the concrete pump stops for any reason.

Compaction / Vibration may be difficult.

Early setting - need to use set retarders.

Large loads on formwork

104
Q

What happened at the Ferry Bridge Cooling Towers?

A

Improperly designed for wind (in particular gusts and vortices), led to structural failure under moderate but not extreme winds. Human failure.

Towers were rebuilt and those still standing were strengthened.

105
Q

What is the name of the towers-collapsing-under-wind case study?

A

Ferry Bridge Cooling Towers.

106
Q

How does the volumetric strain evolve with stress?

A

Volume decreases to a minimum, then increases towards the failure stress.

107
Q

How does lateral strian evolve with stress?

A

Initially controlled by poisson’s ratio, so negative and linear, then grows more negative (leading to overall volumetric increase)

108
Q

The equation:

-Z1/A + fc Z1 / P + M / P <= e

is in the D.B. What is the sign of M?

A

M is positive for sagging. It is therefore normally +ve.

109
Q

e<= -Z1/A + ft Z1 / P + M / P

What is the sign of ft?

A

ft is negative in tension and therefore normally negative. e.g. if the maximum tensile stress is 1MPa, fct = -1MPa

110
Q

e<= -Z1/A + ft Z1 / P + M / P

What is the sign of Z1?

A

Z1 is always negative. It refers to the top surface.

111
Q

What do chlorides cause?

A

Pitting

112
Q

Concrete has had chemicals added to introduce small air bubbles. These air bubbles help to relieve sweeling pressures during freeze-thawing. What is the name of this concrete?

A

Air-entrained.

113
Q

Some shear force is carried by longitudinal reinforcement. What is this effect called?

A

Dowel action.

114
Q

What are the 4 C’s of concrete?

A

Cement Content - provides alkilinity and strenth.
Cover - prevents ingress of eg chlorides
Compaction - removes air bubbles
Curing - moisture retained long enough for concrete to fully hydrate

115
Q

How might further corrosion be stopped in a corroding beam?

A

Cathodic protection

Silane coating to prevent ingress of water and chlorides

116
Q

How might corrosion be identified?

A

Phenolphthalein identifies carbonation. Hammer tests identify delamination. Spalling concrete is normally visible.

117
Q

Corrosion can be identified by phenolphthalein and spalling. How else?

A

Hammer tests identify delamination.

118
Q

On a stress-strain graph, how does ‘high-strength’ concrete differ from normal strength concrete?

A

It is stiffer originally, reaches a higher peak stress at about the same strain, and fails more brittley.

119
Q

What chemical is used to find the carbonation depth of concrete?

A

Phenolphthalein.

120
Q

The 4 C’s of concrete include cover, cement content and what else?

A

Compaction
Curing
Water / cement ratio often also included

121
Q

The 4 C’s of concrete include compaction, curing, and what else?

A

Cement content and cover.

122
Q

Prestress concrete stresses are governed by “k1” and “k2” variables. To what does “k1” refer?

A

The compressive force is k1 * A * fcd

123
Q

Prestress concrete stresses are governed by “k1” and “k2” variables. To what does “k2” refer?

A

The compressive force acts at k2 * x. If k2 = 0.5, then it has no effect.

124
Q

What value of “k1” has no effect?

A

k1 = 1. It is usually substantially lower.

125
Q

What value of “k2” has no effect?

A

k2 = 0.5. It is usually about 0.5, or slightly less. If it is less, this is generous to the designer!

126
Q

The neutral axis is 100mm below the top of a beam. The prestressing steel is 200mm below the top of the beam. If k1 = 0.4 and k2 = 0.4, what is the lever arm of the tension steel?

A

(200 - 0.4 * 100) = 160.

If k2 were 0.5, the lever arm would be reduced to 150.

127
Q

A prestressed beam has concrete with fcd = 60MPa, k1 = 0.4 and k2 = 0.45. What does the effective stress block look like?

A

60 * k1 * xp * b

acting at a height of xp * k2 below the top of the beam.

128
Q

How would you find the tensile force in a prestress cable in a prestressed beam?

A

The total strain in the cable is the initial strain (about 0.4%) + the strain at failure (using a straight line from 0.0035 to the N-A) + the slight stress in the adjacent concrete.

The final term accounts for the fact that the steel was strained MORE THAN 0.4% to get to the desired pre-stressing force.

This strain is fed into a stress/strain graph, from which stress and hence T can be calculated.

129
Q

How is Z1 calculated?

A

Z1 has units of m^3 so it is I / y

130
Q

How is the kern point of a section with properties I and distance from centroid to extreme fibre y calculated?

A

Kern point = - Zi / A

Zi = I / y

So Kern point = - I / (y * A)

131
Q

A rectangular section has dimensions b, d. What is Z1?

A

I = bd^3 / 12

and y1 = -d/2

So Z1 = I / y1 = - bd^2/6

132
Q

What dimensions (in terms of m) does Z have?

A

Z has units m^3

133
Q

The embodied carbon of a beam might be reduced by using non-OPC cements, and reducing the size of the beam at non-critical sections. What else might be done?

A

Holes could be placed in the web.

134
Q

What are the 4Cs normally combined with?

A

Water / Cement Ratio

135
Q

What does sEsMe stand for?

A

Exposure Class

Maximum Aggregate Size

136
Q

The 4Cs include

Compaction
Cover
Cement Content

and what TWO other things?

A

Curing

Water Content.

137
Q

What is water content normally included with when describing concrete quality?

A

Water content is normally included with the 4Cs - compaction, curing, cement content and cover.

138
Q

What is the concrete chimney disaster called?

A

Ferry Bridge Cooling Towers

139
Q

What is the name of the bridge that failed in shear?

A

De la Concorde overpass

140
Q

At the De la concorde overpass, there were the following problems:

It was a novel design that made it difficult to inspect the concrete.

They did not anticipate increasing vehicle weight and frequency.

What else?

A

The shear strength of unreinforced slabs wasn’t fully understood, and fell when the concrete deteriorated.

The half-joint in the deck was not safe.

141
Q

What was the detail that failed in the de la concorde bridge?

A

The half-joint (ie 2 L shapes that intertwine)

142
Q

Why was the De la Concorde bridge not suitable when it collapsed?

A

The designers hadn’t anticipated the additional vehicle loads.

143
Q

What code changes came from the De La Concorde bridge collapse?

A

Deep unreinforced slabs are no longer permitted.

The half-joint detail that failed is no longer permitted.

144
Q

Where does GGBS come from?

A

The steel industry.

145
Q

Where does PFA come from?

A

The coal industry.

146
Q

Other than reduced emissions, what other advantage might come from using OPC-replacements?

A

There is less heat of hydration, and so less shrinkage cracks when cooling.

147
Q

Where do emissions from cement production come from?

A

The production of clinker. About 50% is the chemical reaction, and 40% is from heating the kiln.

148
Q

40% of clinker emissions come from heating the kiln. Where do the other 60% come from?

A

About 10% from raw material transport, and 50% from the chemical reaction

149
Q

What is a high-sulfate cement? Where might it be used?

A

Made with lots of GGBS. Good against sea-water.

150
Q

High sulfate cements are made with lots of GGBS and are good against sea water. What is a disadvantage of high-sulfate cements?

A

They need to be stored in very dry conditions.

151
Q

Air-entrained concrete is porous. Is it also permeable?

A

No - not necessarily.

152
Q

Why do fabric moulds make less permeable concrete?

A

The porous mould allows air and water to escape, so there are fewer pours after setting.

153
Q

What are \lambda and \eta?

A

\lambda is the DEPTH of the stress block = 0.8

\eta is the WIDTH of the stress block = 1.0

154
Q

What effect might GGBS and PFA have on the curing time of concrete?

A

It may well be slower. This could be a problem on sites with time constraints, and especially in the manufacture of precast concrete.

155
Q

The total tendon strain for strength design is made of 3 elements. What are they?

A

The initial pre-stress strain, the extra strain due to concrete shrinkage, and the strain due to the applied momen.

The strain due to concrete shrinkage is likely negligible.

156
Q

What is the effect of a cable with drape h and length L?

A

w = 8 * T * h / L^2

157
Q

How might a concordant profile be found?

A

Take a moment diagram that is compatible with the supports. Set e = M/P. e is the concordant profile.

158
Q

OPC-replacement materials reduce embodied carbon. Name TWO disadvantages.

A

The curing time may increase

The pH may be lower

159
Q

OPC-replacement materials in high quantities have some advantages. What are they?

A

High-sulfate cements (eg high GGBS content) can resist salt water well.

C & PC (5 decks)

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