Concrete

Question 1

Name the SCMs, the pozzolanics and the differences between them

Answer 1

Scm: supplementary cementitious, react only with water. Blast furnace slag is most common
Pozzolans: need to react with CH as well. Good because produce CSH. Fly ash, silica fume and calcined clay

Question 2

Name all the sources of the mineral admixtures?

Answer 2

Blast furnace slag: steel furnaces
Fly ash: from coal burning
Silica fume: from silicon factory
Calcined clay: from crushing clay
Can be not beneficial when we need to transport this material

Question 3

What are the types of cements?

Answer 3

GU general usage
HS high sulphate resistance - less C3A
MS moderate sulphate resistance - less C3A
HE high early strength - more c3s and finer grinding
MH moderate heat of hydration - less c3a and less c3s
LH low heat of hydration (obsolete) - use pozzolanics

Question 4

What are the reasons to use mineral admixtures?

Answer 4

Replace cement
Better durability
Better workability
Cheaper
Better for environment (reuse waste)
Lower heat of hydration

Question 5

Types of chemical admixtures?

Answer 5

Water reducing
Set accelerating
Set retarding
Air entraining

Question 6

Causes of air entraining?

Answer 6

Freeze thaw prevention
Less segregation
More sulphate resistance
But lower compressive strength

Question 7

When do we use set retarding chemical admixtures?

Answer 7

High temp
Long delay between mixing and casting

Question 8

When do we use set accelerating chemical admixtures?

Answer 8

Lower temp
Repairs (we need high early strength)
When we want to remove the forms faster

Question 9

What affects optimum mixing time?

Answer 9

Type of mixer
Condition of mixer
Ambient temp
How dry or how wet our mixture is
Speed of mixing
Size of ingredients

Question 10

Types of ready mix mixing?

Answer 10

Truck - completely in tuck
Transit
Shrink - partially mixed at plant and finished in truck
Central - mixing at plant

Question 11

What does vibrating mixture do?

Answer 11

Minimizes friction between particles
Over vibrating causes more segregation (bubbles collapse or go up and aggregates go down and break down)

Question 12

What are five causes for segregation?

Answer 12

High specific gravity
Large ratio of large to small particle sizes
Not enough time aggregates
Too wet or too dry
Bad handling! Dropping jt

Question 13

What is latence?

Answer 13

Weak milky or powdery layer of cement dust lime and sand fines on surface of concrete. Not used particles are brought up to surface

Question 14

What is false set?

Answer 14

There is gelling action but the hydration hasn’t yet started no hardening at all

Question 15

What are the ways to measure air content?

Answer 15

Gravimetric, volumetric and pressure methods
Bad because don’t tell you nature of air
Entrained or entrapped? Porosity or permeability?

Question 16

Describe gravimetric method?

Answer 16

Weigh a know volume of particles. Bad because of different packing. Think buckets. See how far you are from calculated weight

Question 17

Describe volumetric method

Answer 17

Measure difference in volume with and without air. Mechanically difficult to remove air

Question 18

Why is time of set important?

Answer 18

To regulate mixing and casting times
To monitor the effectiveness of set changing admixtures
To plan time between casting and finish

Question 19

What is the pseudo mortar test?

Answer 19

Checking the rigidity of concrete. Stick a large needle in (low stress) and decrease needle size to see how much stress it can resist. Time of set test without aggregates

Question 20

What is interrupted curing? Why is it bad?

Answer 20

When you stop the curing process. When interval relative humidity drops below 80% the hydration reaction stops. It’s very hard to resaturate dried our concrete and you’ll never get the strength you would’ve gotten if you hadn’t stopped
Capillaries are blocked of my air bubbles and water lays on top of air bubbke

Question 21

What are the methods of curing?

Answer 21

Sealed: spray or cover
Continuous water supply: fogging ponding or sprinkling