Strength of concrete Flashcards
(35 cards)
What other properties of concrete can be measure when testing concrete strength?`
Elastic modulus, impermeability, resistance to weathering agents
What are the 3 phases when studying the structural behavior of concrete?
1) Coherent morter phase
2) Aggregate phase
3) ITZ
What are characteristics of ITZ?
- Large crystals of ettringite and CH with prefferect orientation
- Porous structure.
- IT typically 10-15 um thick around agg.
*
Why is there a higher w/c ratio around large agg. in comparison to the w/c ratio in bulk mortar?
Because in freshly compacted concrete, water film forms around large aggregate .
Whats the strength ratio of tensile strength to compressive strength?
Tensile strength is almost 1/10 of compressive strength.
How much lower is the permeability of mortar compared to concrete? (with same w/c)
coefficient of permeability of mortar is about 1/100 lower than the coefficient permeability of typical concrete.
What are ways to improve ITZ?
- Low w/c ratio
- Silica fume (high surface area)
- Different types of agg.
What minerals reduce ITZ?
lime stones. this is because of the calcium composition of lime stone. lime stone is a weak particle however, so it must be crushed to the appropriate size.
What is the definition of concrete strength?
The stress required to cause fracture of material.
What are some factors affecting terminal compressive strength?
- W/C
- Curing conditions (time, temperature,relative humidity).
- Effect of materials and mix proportions
- testing parrameters
How does water content affect compressive strength?
Excess water becomes capillary pores. these pores are weak points in the concrete. A higher water concentration also leads yields weaker ITZ’s.
Why does a w/c ratio < .3 yield stronger ITZ’s?
Because ITZ are highly composed of CH and Ettringite crystals ( in exclusive concentration act as weak points). Low water content decreases the size of calcium hydroxide crystals.
Is it possible to concrete to gains strength through air-entrainment?
Yes, some lucky batches of low strength concrete with a low cement content may sometimes gain strength through air-entrainment.
What is a way to increase concrete durability without increasing the w/c ratio?
- Air-entrainment
* Admixtures
Why is there a mandated maximum aggregate size (MSA) to be included with the concrete mix?
Because larger aggregates tend to form weaker transition zones.
Why does limestone produce higher strengths in concrete?
because calcium (CaCO3.Ca(OH)2.xH20 is formed at ITZ, strengthening the ITZ’s.
At what percent can water be harmful towards concrete strength yet still be used in the mix?
10% biiitch
What is the relationship between humidity and concrete strength?
The longer concrete is kept moist (especially during the curing process) the stronger the concrete. Chemical reactions in continuously moist concrete continue to have chemical reactions, (grow in strength) but slowly reach a peak.
What is the minimum time curing concrete should be kept moist?
7 days. Pozzolanic concretes require longer curing times.
What are some ways to moist concrete while curing?
- Spraying
- Ponding
- Covering concrete with wet sand, sawdust, or cotton mats.
How does temperature affect the cement hydration process when two batches are placed and cured in the same temperature?
Up to 28 days, the hotter the temperature, the more rapid the cement hydration, boosting the strength in concrete (granted the concrete is placed and cured at the same temp).
How does hydration speed affect the curing process?
The faster the hydration, the stronger the concrete (given that the water content in concrete remains at a constant healthy level).
How is strength affected when two identical batches of concrete are placed in the different temperatures but cured at the same temperature?
Up to 2 hours, the concrete that is placed in a cooler environment yields more strength compared to the same batch that is placed at a warmer temperature, then cured at the same temperature. This is because at cooler levels, the microstructure of hydrated cement paste and pore size distribution are more uniform.
How is strength affected when two identical batches of concrete are placed in the same temperature but cured at different temperatures?
The concrete curing in the lower temperature will be weaker. sucks for him.
