Early age concrete Flashcards
(34 cards)
What is early age concrete
The timeframe from the measuring of ingredients to the formwork removal
How is concrete ingredients often measured by and why
By mass as volume leads to ratio inaccuracies
What method of transporting wet concrete is used for deep pours and why
Pipes because it reduces segregation
When should formwork be removed (approximate strength in MPa)
6-7MPa enough to carry its own weight
Why should concrete be placed as close as possible to its final destination
Reduces segregation and mix remains cohesive
How much volume by percentage does air voids make up in freshly placed concrete
5-20
How can air voids be removed from freshly placed concrete and what was and is done with
compaction - done manually by tamping before but done today with vibrators (by fluidising the concrete)
What is the aim of finishing
produce flat surface and increase density in order to reduce the ingress of degrading substances
What are the four types of finishing
screeding, darby, brooming, power floating
What is the measurement made in the slump test
Height difference in mm between slump cone and the slumped concrete
What is the flow table test
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
What is the advantage of the flow table test
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
what are the ranges of high and very high workability mixes’ diameter
400-500mm and 500-650mm
what factors affect workability
ingredients, w/c ratio, admixtures, temperature, humidity, time, aggregates’ properties
What is curing
keep the environment suitable for the concrete after casting (enough humidity and correct temperatures) which maintains strength gain
methods of curing give 4
impermeable sheets, steam curing, curing compounds, autoclaving
What is the strength loss percentage per percentage of air voids
5-6% loss in strength per % of air
What is bleeding
The rising of water to the top surface of the freshly compacted concrete
What is a side effect of bleeding
Trapped water beneath aggregates and rebar that reduces bond strength between cement and aggregates/steel
Why isn’t bleeding always visible despite always occuring
It only is visible if rate of bleeding is greater than rate of evaporation
What is laitance and what is it an effect of
An effect of bleeding; the water carries unreacted cement to the surface that crystallises into dusty layer at the top
What is a benefit of bleeding
The water keeps the surface moist which reduces plastic shrinkage cracking
what exacerbates bleeding
Retarders as the hydration reaction is slowed down leaving more time for the water to escape the surface, low temperature, and deeper pours
What reduces bleeding
Reducing water ratio, using accelerators and air entraining agents, increase aggregate proportion
