what is the relation between concrete strength and casting time?

Question

for example, i cast concrete for every 1 hour. does the strength at 2nd hour lower than the first hour?why?

Answer 1

*http://www.highwaysmaintenance.com/concbits.htm
*Concrete is the product of mixing, aggregate, cement and water.
The setting of concrete is a chemical reaction between the cement and the water, not a drying process.
This reaction is called hydration, it evolves heat as does any chemical reaction, and the process is irreversible.
There is an initial set when the concrete will cease to be liquid but have little strength (e.g. 6 to 24hrs. old), thereafter the concrete will gradually gain strength over time until it achieves the strength required.
Differing mix proportions and cement types will achieve required strengths in differing time spans.
*TIME TAKEN TO PLACE CONCRETE ATER BATCHING

From the time of adding water to the cement the chemical reaction has begun and you only have a limited amount of time to place and compact the concrete, this is usually specified as 90 minutes.
The delivery ticket of the load of concrete will be stamped with the time of batching.

ADDITION OF WATER

Given a set amount of cement and aggregate there is an optimum amount of water to be added to produce a chemical reaction to give the maximum obtainable strength, too little or too much water
will produce a weaker concrete.
Unfortunately as in all things, life is not that simple, and the workability of the concrete has to be considered when placing concrete, especially in difficult situations.
These situations can be areas of high density of reinforcing bars, complicated formwork design, or where the concrete needs to be suitable for pumping.
In these situations water content is increased to make the concrete more workable, BUT this increase in water content is calculated at the design stage and the cement content is increased accordingly to retain the strength of the mix.

*MEASURING CONCRETE STRENGTH:
The strength is measured by crushing concrete cubes to failure and recording this strength.

Concrete cubes are made from fresh concrete sampled at the time of pouring by placing correctly sampled concrete into a steel mould and compacting to remove air voids.
The concrete is allowed an initial "set" period of 24 hours, the mould is then stripped and the cube is cured in water at a temperature of 20 deg.c for 28 days prior to crushing.

If you wish to strike shuttering before 28 days, extra cubes will be required to determine that the in-situ concrete has achieved the appropriate strength at the time you wish to strike the shuttering.
This is usually an arrangement agreed by the contractor, the concrete supplier and the engineer.

Answer 2

The mix design should yield a theoretical strength, based on the components in the mix. This should include all the admixtures and plasticizers that are planned to be used. And you should note that if you are adding additional admixtures and plasticizers, then you are not pouring the concrete you had designed. The compressive strength you get from physical testing of concrete cores is used to verify you have met or exceeded the strength you thought you were getting on the concrete. This matters because the load calculations for the particular concrete is based on meeting a minimum strength level, and the structure would need to be re-examined if this minimum strength is not met.

Answer 3

The hydration process that makes the cement harden takes several days to complete. normal mixes do not reach full strength until 28 days. the cement might not need to be at full design strength to remove the casting forms which in some cases can be done earlier. special high-early mixes can get to ultimate strength in 7 days, but it will not be as strong as a comparable standard mix. accellerator chemicals added to speed the hydration process have the side affect of generating the "heat of hydration" over a shorter period of time and the mix can potentially exceed maximum allowable temperatures or require significant external cooling.