Early age and mechanical properties

Question 1

What’s workability?

Answer 1

Workability: easily mixed, transported, placed in position, compacted and finished

Question 2

what must tests for workability be?

Answer 2

For site use any test for workability must be:
relatively simple to use
capable of providing a rapid result
cheap
able to detect variations in the quality of a concrete mix

Question 3

What’s the slump test for workability?

Answer 3

SLUMP

Distance between the original height of conical mould to highest point of slumped concrete in mm

Question 4

What’s the flow test? (vertical and horizontal diameters considered)

Answer 4

Mean diameter of concrete after

test in mm = (A + B)/2

Question 5

What must workability be for roads, mass/reinforced/pumped concrete, floors, reinforced concrete with congested reinforcement?

Answer 5

Roads - v low
Floors - low
Mass/reinforced/pumped concrete - medium
Reinforced concrete with congested reinforcement - high
self compacting concrete - v high

Question 6

What factors affect workability for cement paste?

Answer 6

Cement paste:

- water/cement
- admixtures (especially superplasticisers)

Question 7

What factors affect workability for aggregate?

Answer 7

Aggregate:

- Content (aggregate reduces workability) 
- Shape and surface texture (smooth/round increases workability)
- Grading (larger proportion of larger particles increases workability as smaller surface area to lubricate with paste, but may lead to SEGREGATION)

Question 8

What’s the aim of compaction and what are requirements for compacted concrete?

Answer 8

The aim of compaction is to achieve the maximum possible density.
Concrete should be sufficiently workable to be thoroughly compacted.
Well compacted concrete has an entrapped air void content of <1.5%.
Low entrapped air void content is necessary to produce strong and durable concrete.

Question 9

What’s effect of entrapped air on strength of concrete?

Answer 9

Entrapped air reduces strength of concrete by 5-6% for every 1% of air by volume.
W/C ratio causes decline in strength of concrete, hand compacted concrete strength increases initially though

Question 10

What’s bleeding?

Answer 10

Aggregate settles while water rises

Question 11

What’s plastic settlement?

Answer 11

Caused by excessive bleeding
Plastic settlement cracking occurs before concrete has set (typically between 10 minutes and 3 hours after casting)
void under reinforcement occurs

Question 12

What’s plastic shrinkage?

Answer 12

Reduction in volume of cement paste caused by hydration, evaporation or absorption (e.g by soil)
If water lost by evaporation exceeds amount replenished by bleeding then shrinkage and cracking (plastic shrinkage cracking) may occur

Question 13

How can we reduce plastic shrinkage cracking?

Answer 13

To minimise plastic shrinkage cracking, reduce rate of evaporation to < 1.0 kg/m2/h

Question 14

What’s effect of temperature on compressive strength?

Answer 14

As temperature rises, the compressive strength is greater relative to lower temperatures and increases with time of curing

Question 15

What are effects of curing in cold weather and recommended precautions?

Answer 15

Effects on concrete
Delayed setting
Reduced strength at early ages
So delay in striking formwork

Precautions
Use rapid hardening cement
Insulate formwork and retain in position longer
Ensure concrete placement at 15-20°C
Protect concrete immediately after placing
Do not place concrete on frozen or snow-covered areas

Question 16

What are effects of curing in hot weather and recommended precautions?

Answer 16

Effects on fresh concrete
Rapid loss of workability
Accelerated set
Increased likelihood of plastic cracking					
Effects on hardened concrete
Increased early-age strength
Often reduced long-term strength

Precautions
Control temperature and minimise evaporation of water
In hot dry weather with high winds, provide wind shields and shade

Question 17

In adiabatic temperature rise which type of concrete gets hottest?

Answer 17

Rapid hardening PC gets to highest temperature in same amount of time as normal PC concrete and low heat PC

Question 18

How does temp increase with time in curing concrete?

Answer 18

Temperature rises until formwork removed then anti-exponential decline

Question 19

What factors influence temperature rise in concrete?

Answer 19

Cement
Composition
Fineness
Quantity			
Concrete
Volume/dimensions
Rate of placing
Temperature

Formwork type

Atmospheric temperature

Question 20

What’s adiabatic temperature rise per 100kg/m3 of cement?

Answer 20

Adiabatic temperature rise in concrete
is approximately 12°C per 100 kg/m3
of cement, for cement contents
between 300 and 600 kg/m3

e.g. If cement content = 500 kg/m3 and initial concrete temperature = 20°C the temperature achieved =
20+(5 x 12) = 80°C

Question 21

How is compressive strength in concrete tested?

Answer 21

Measured by a standard uniaxial test and is the basis for structural design and quality control
100mm or 150 mm cube
In failure, cracks spread everywhere except at the triangles where load is applied. Causes the lateral length to decrease or a crack across the concrete.

Question 22

How can compressive strength be increased?

Answer 22

Greater rate of application of compressive strength increases compressive strength exponentially

Question 23

How does aggregate content and water content affect compressive strength?

Answer 23

Greater aggregate content reduces max compressive strength, and as w/c ratio rises compressive strength falls. Fully compacted concrete has a greater compressive strength

Question 24

How is compressive strength affected by temperature?

Answer 24

Between 0 and 20 degrees C the strength increases with temp, but after 20 degrees strength declines

Question 25

What is formula for maturity and how does it relate to strength of concrete?

Answer 25

Maturity can be expressed as : M = sigma T. *dt where: T = Temperature above a certain datum (usually -11°C) dt = time interval during which temperature is T
Maturity ‘rules’ can be used to estimate the strength of a given concrete when subjected to various curing regimes.
Rules are approximate only since relationship between strength and maturity depends on mix characteristics, loss of water during curing and adverse effect of high early temperature.
For wet curing at temperatures 5-40°C strength is related directly to maturity, i.e. equal strengths are found for concrete with same maturity.

Question 26

What is cube strength proportional to?

Answer 26

Cube strength proportional to log of maturity.

Question 27

What factors affect compressive strength of concrete?

Answer 27

MIX CHARACTERISTICS
Free W/C ratio
Cement type (chemical composition & fineness)
Aggregate shape, surface texture, strength and content
Degree of compaction

HISTORY OF CONCRETE
Age
Curing and maturity

TYPE OF APPLIED OR INDUCED STRESS OR STRAIN
Type of specimen
Rate of loading

Question 28

What is formula for side strain? (perp to loading), where I1 is original length and I’1 is new length

Answer 28

E1 = [(l1 - l’1)/l1] x 106 microstrain

Question 29

What’s formula for vertical strain?

Answer 29

E3 = [(l3 - l’3)/l3] x 106 microstrain

Question 30

What’s formula for young’s modulus?

Answer 30

Young’s Modulus, E = Stress1/strain1

Question 31

what’s formula for poisson’s ratio?

Answer 31

Poisson ratio, v = E3/E1

Question 32

What is relationship for microstrain against axial stress?

Answer 32

Lateral stress has a negative microstrain as axial stress rises, and axial stress positive. Volumetric stress between the two. Minimum volume occurs at end of linear relationship before the straight linear lines curve off to a straight horizontal. Axial stress increases to max with strain and then falls

Question 33

What’s formula for lateral tensile stress in Brazilian test?

Answer 33

Lateral tensile stress
= 2P/pi*DL
P is load, D is stretched diameter and L is length

Question 34

How does tensile stress show on a graph?

Answer 34

Lateral stress, volumetric stress and axial stress increase in negative direction for lateral and positive for axial for microstrains and tail off as tensile stress increased.

Question 35

Why is concrete’s tensile strength &laquo_space;its compressive strength?

Answer 35

Concrete is brittle and contains cracks, voids and other flaws
As concrete is stretched, cracks grow. (Compressive loading does not encourage cracks and other flaws to grow and so they have much less influence on compressive strength)

Question 36

Why is failure in tension rapid?

Answer 36

The developing crack pattern reduces the area resisting load and so the stress in the potential failure zone increases rapidly and failure occurs soon after the initiation of cracking.