OBVIOUSLY ADD COLD WATER. BUT THE EXACT EFFECT OF EXTERNAL TEMP. VS. TEMP. OF WATER IS YET TO BE ESTABLISHED.

by using GGBFS and replace it with cement

Many Thanks, P. Markandeya Raju. I'm seeking for any update in the techniques with economic value.

Dear Arash Aghaeipour,

Yes, reusing reduces the internal temperatures but what about the external temperatures when the casting is in hot weather

apart from modification of concrete ingredients , fog cooler systems can economically give satisfoctorly results to control external temperatures after enough setting period.

for example:

https://www.youtube.com/watch?v=n9nVYRQg9WA

Problems caused by casting in hot weather

1. Increase the speed of reaction of cement.

2. Decreasing the freezing time of the concrete mix.

3. Low compressive strength resistance.

4. If accompanied by high temperature, low humidity and wind may result in cracking of plastic concrete.

5. Difficulty forming the intended air bubbles.

6. The high temperature of the concrete section after casting may increase the probability of cracking later.

Procedures that can be taken when casting concrete to avoid the effect of hot weather on its properties

1. Reduce the temperature of the components of the concrete by cooling.

2. Use slow-freezing materials.

3. Use of low heat cement type

4. Prevent the evaporation of water from the mixture and should avoid the rapid evaporation in particular.

5. Protect concrete from sunlight.

6. Cooling the concrete after casting with moisturizing with water.

Note: The concrete mixture temperature should not be equal to 16 ° C and above 32 ° C.

I think the best way to eliminate the effect of external temperature due to son is work at night instead of working during day.

The following suggestions and precautions should be taken when casting concrete in high temperatures during summer months:

1. Cement choice. Slightly better conditions prevail if low-heat cements are used and rapid hardening cements are avoided. The rate of hydration, however, increases with temperature for all cement types, and the problem of surface cooling due to low night temperature will remain.

2. Admixtures. Retarders may reduce premature stiffening and delay heat evolution, but the maximum temperature level is not affected significantly. Water reducing admixtures are favourable for maintaining workability of concrete with warm ingredients without adding more water. however, they cannot prevent evaporation of water from concrete in drying conditions.

3. Storage of materials. Limiting the temperature of stored aggregates has the advantage of minimizing the temperature of fresh concrete. Obstructing the sun's rays by use of screens prevents the aggregates from attaining higher temperatures due to the direct exposure to the sun. Continuous fine sprays of water on aggregate stockpiles can help to keep them cool.

4. Water. Water taken from the mains, if the pipes are buried underground, should be able to effect useful cooling.

5. Cement storage: If  the cement is stored in silos, the silo must be shaded or coated with reflective paint to avoid heat absorption. If cement is stored in a store, the store must be well ventilated, dry and maintained at low temperatures. High atmospheric temperatures increase the rate of diffusion of vapour through cement bags, thus starting hydration.

6. Batching, mixing and transporting. There should be no delay between mixing and placing; mixing times should be as small as possible to prevent heat creation through friction.

7. Placing, finishing and curing: Rapid stiffening as a result of excessive drying conditions requires the provision of extra vibrators in order to complete the compaction as soon as possible. Evaporation of water from the concrete surface can take place to such an extent, that water deep in the concrete cannot make good the loss - especially when air movement (wind) promotes undue evaporation. Plastic or wind cracks may form shortly after placing, and re-compacting the concrete might not prevent deep penetration of the cracks, thus allowing premature corrosion of the reinforcement steel. Appropriate precautions to prevent such wind effects include erection of wind breaks windward to the concrete, application of mist sprays to raise the relative humidity in contact with the concrete, provision of covers which may be rolled back as finishing proceeds. Proper curing is essential for concrete under drying (drought) conditions. Polythene sheets, preferably pigmented to reflect radiation, or sprayed curing compounds, can prevent evaporation. To minimize temperature gradients from the surface to the interior of hardening concrete it is easier to limit heat losses from concrete surface than to cool the inner concrete.

8. Testing: Special care of test samples must be ensured. Under drying conditions test samples will suffer more than a large mass of concrete, due to the large surface areas in relation to their volumes. Misleading test results can be avoided if the temperature history of the specimens is known.

I really appreciate your answers:

(H. Süleyman Gökçe, Salim Hrez Jassam, Arash Aghaeipour and Alex Mrema)

Best Regards,

KHALID

when you cast a slab the heavier materials tend to settle and water being the lightest material in the mix will rise to the top, this water that rises to the top is known as bleed water. bleed water will eventually evaporate. the rate of evaporation is as a result of the ambient temprature, relative humidity and wind. if the evaporation rate is faster than the bleed rate there is a good chance of getting shrinkage cracking. with the bleed water evaporating the plastic concrete is loosing volume so tends to shrink but any reinforcing will stop it from shrinking and due to the concrete still being very weak in tension the stresses of the sgrinking rip the concrete itself apart causing shrinkage crackss. once the concrete sets and has enough strength to resist the cracking its no longer a problem.

1. There are plenty of apps available or tables where you can calculate the evaporation rate based on weather readings, the bleed rate you need to establish in a lab but there are also rules of thumb.

We want to therfore limit the amount of evaporation, to do this we need to cast concrete when it is not hot, protect the new slab from wind etc.

we also do not want to delay the concrete setting in any way, the longer the concrete takes to set the longer it is weak in tension and susceptible to cracking so the use of retarders or SCM's that will slow down hydration will promote shrinkage cracking.

Structural synthetic fibres are a very good way of stoppping shrinkage cracking as they give the weak concrete a tensile strength and this helps it overcome the shrinkage forces acting on it. micro fibers also work but thats all they do while macro synthetic fibers are usefull in the slab for long term durability etc. look at this web site. www.elastoplastic.com they are the suppliers of Barchip fibre which has a really good reputation world wide. 

Dear Khalid, the previous answers were indeed very descriptive. There are several techniques to tackle this problem, from materials perspective but also from design perspective. You could consider cooling techniques (cooling pipes, liquid nitrogen, cooling individual constituents etc.), adding ggbs in the mix will indeed decrease the peak temperature but not necessarily decrease cracking susceptibility due to its lower the CEM I tensile strain capacity; yet it can be effective, reevaluate pouring sequence, or in the design stage, increase the reinforcement to control cracking or conduct advanced thermo-mechanical analysis of concrete with FEM to get a clearer representation of the expected early age behaviour of the structure.

Dear Dr. Fragkoulis Kanavaris,

Many thanks. Your answer is very valuable .

Perhaps you should test some concrete mixtures with fibres like I did in my project - steel fibres and PP-fibres are pretty common. PP-fibres will decline the plasticity because of their huge surface, but this may improve the shrinkage, too. In my eyes the usage of fibres can improve the quality of the concrete.