Early age and mechanical properties Flashcards
What’s workability?
Workability: easily mixed, transported, placed in position, compacted and finished
what must tests for workability be?
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
What’s the slump test for workability?
SLUMP
Distance between the original height of conical mould to highest point of slumped concrete in mm
What’s the flow test? (vertical and horizontal diameters considered)
Mean diameter of concrete after
test in mm = (A + B)/2
What must workability be for roads, mass/reinforced/pumped concrete, floors, reinforced concrete with congested reinforcement?
Roads - v low Floors - low Mass/reinforced/pumped concrete - medium Reinforced concrete with congested reinforcement - high self compacting concrete - v high
What factors affect workability for cement paste?
Cement paste:
- water/cement - admixtures (especially superplasticisers)
What factors affect workability for aggregate?
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)
What’s the aim of compaction and what are requirements for compacted concrete?
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.
What’s effect of entrapped air on strength of concrete?
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
What’s bleeding?
Aggregate settles while water rises
What’s plastic settlement?
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
What’s plastic shrinkage?
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
How can we reduce plastic shrinkage cracking?
To minimise plastic shrinkage cracking, reduce rate of evaporation to < 1.0 kg/m2/h
What’s effect of temperature on compressive strength?
As temperature rises, the compressive strength is greater relative to lower temperatures and increases with time of curing
What are effects of curing in cold weather and recommended precautions?
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
What are effects of curing in hot weather and recommended precautions?
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
In adiabatic temperature rise which type of concrete gets hottest?
Rapid hardening PC gets to highest temperature in same amount of time as normal PC concrete and low heat PC
How does temp increase with time in curing concrete?
Temperature rises until formwork removed then anti-exponential decline
What factors influence temperature rise in concrete?
Cement Composition Fineness Quantity Concrete Volume/dimensions Rate of placing Temperature
Formwork type
Atmospheric temperature
What’s adiabatic temperature rise per 100kg/m3 of cement?
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
How is compressive strength in concrete tested?
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.
How can compressive strength be increased?
Greater rate of application of compressive strength increases compressive strength exponentially
How does aggregate content and water content affect compressive strength?
Greater aggregate content reduces max compressive strength, and as w/c ratio rises compressive strength falls. Fully compacted concrete has a greater compressive strength
How is compressive strength affected by temperature?
Between 0 and 20 degrees C the strength increases with temp, but after 20 degrees strength declines
What is formula for maturity and how does it relate to strength of concrete?
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.
What is cube strength proportional to?
Cube strength proportional to log of maturity.
What factors affect compressive strength of concrete?
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
What is formula for side strain? (perp to loading), where I1 is original length and I’1 is new length
E1 = [(l1 - l’1)/l1] x 106 microstrain
What’s formula for vertical strain?
E3 = [(l3 - l’3)/l3] x 106 microstrain
What’s formula for young’s modulus?
Young’s Modulus, E = Stress1/strain1
what’s formula for poisson’s ratio?
Poisson ratio, v = E3/E1
What is relationship for microstrain against axial stress?
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
What’s formula for lateral tensile stress in Brazilian test?
Lateral tensile stress
= 2P/pi*DL
P is load, D is stretched diameter and L is length
How does tensile stress show on a graph?
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.
Why is concrete’s tensile strength «_space;its compressive strength?
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)
Why is failure in tension rapid?
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.
