Early age concrete

Question 1

What is early age concrete

Answer 1

The timeframe from the measuring of ingredients to the formwork removal

Question 2

How is concrete ingredients often measured by and why

Answer 2

By mass as volume leads to ratio inaccuracies

Question 3

What method of transporting wet concrete is used for deep pours and why

Answer 3

Pipes because it reduces segregation

Question 4

When should formwork be removed (approximate strength in MPa)

Answer 4

6-7MPa enough to carry its own weight

Question 5

Why should concrete be placed as close as possible to its final destination

Answer 5

Reduces segregation and mix remains cohesive

Question 6

How much volume by percentage does air voids make up in freshly placed concrete

Answer 6

5-20

Question 7

How can air voids be removed from freshly placed concrete and what was and is done with

Answer 7

compaction - done manually by tamping before but done today with vibrators (by fluidising the concrete)

Question 8

What is the aim of finishing

Answer 8

produce flat surface and increase density in order to reduce the ingress of degrading substances

Question 9

What are the four types of finishing

Answer 9

screeding, darby, brooming, power floating

Question 10

What is the measurement made in the slump test

Answer 10

Height difference in mm between slump cone and the slumped concrete

Question 11

What is the flow table test

Answer 11

The concrete is compacted into the slump cone on a table that is hinged on one end. After the concrete slumps, the table is lifted on one side opposite to the hinged side and then dropped; this is done 25 times so the concrete spreads out and then the approximate diameter is measured

Question 12

What is the advantage of the flow table test

Answer 12

measures workability differences in very fluid concrete mixes as they will slump a lot in the slump test. Also useful for identifying cohesiveness issues and segregation

Question 13

what are the ranges of high and very high workability mixes’ diameter

Answer 13

400-500mm and 500-650mm

Question 14

what factors affect workability

Answer 14

ingredients, w/c ratio, admixtures, temperature, humidity, time, aggregates’ properties

Question 15

What is curing

Answer 15

keep the environment suitable for the concrete after casting (enough humidity and correct temperatures) which maintains strength gain

Question 16

methods of curing give 4

Answer 16

impermeable sheets, steam curing, curing compounds, autoclaving

Question 17

What is the strength loss percentage per percentage of air voids

Answer 17

5-6% loss in strength per % of air

Question 18

What is bleeding

Answer 18

The rising of water to the top surface of the freshly compacted concrete

Question 19

What is a side effect of bleeding

Answer 19

Trapped water beneath aggregates and rebar that reduces bond strength between cement and aggregates/steel

Question 20

Why isn’t bleeding always visible despite always occuring

Answer 20

It only is visible if rate of bleeding is greater than rate of evaporation

Question 21

What is laitance and what is it an effect of

Answer 21

An effect of bleeding; the water carries unreacted cement to the surface that crystallises into dusty layer at the top

Question 22

What is a benefit of bleeding

Answer 22

The water keeps the surface moist which reduces plastic shrinkage cracking

Question 23

what exacerbates bleeding

Answer 23

Retarders as the hydration reaction is slowed down leaving more time for the water to escape the surface, low temperature, and deeper pours

Question 24

What reduces bleeding

Answer 24

Reducing water ratio, using accelerators and air entraining agents, increase aggregate proportion

Question 25

What is a plastic settlement crack

Answer 25

When concrete settles over an obstruction and this causes a crack to form

Question 26

What is plastic shrinkage cracking

Answer 26

When concrete shrinks due to drying and this applies tensile stress due to being constrained by inner concrete. Tensile stress causes the cracking

Question 27

How to reduce plastic shrinkage cracking

Answer 27

Reduce surface evaporation by curing, maintain temperatures and erect windbreaks/sunshades

Question 28

How is compressive strength of concrete measured

Answer 28

Standard uniaxial strength test

Question 29

What is stronger cube or cylinder

Answer 29

Cube by about 20%

Question 30

What is the direct axial tension test and why is it bad for measuring concrete tensile strength

Answer 30

clamp the ends of the object then pull it apart. Very difficult to avoid secondary stresses

Question 31

What is the splitting tension test

Answer 31

An indirect tensile strength test. Performed by compressing a cylinder on opposite sides which causes the cylinder to split down the middle

Question 32

What is the flexural test

Answer 32

An indirect tensile strength test. Support a beam on two sides and then press down in the middle to cause it to bend which applies tension on the bottom

Question 33

Why is concrete’s tensile strength so much less than its compressive strength

Answer 33

It has cracks in its microstructure so this propagates in tension

Question 34

Concrete is brittle: at what strain does it fail

Answer 34

0.35% which is 0.0035 strain