Hi

Cracking at early age is manly due to internal temperature (and humidity sometimes) gradients induced by the processes of setting and curing of concrete.Temperature and water content distributions across the concrete section depends also of the structure boundary conditions and on the environment. Appropriate curing and mixture design (limiting heat generation during setting) may mitigate the risk of cracking.

You will find a lot of papers on that phenomenon on RG website.

Good luck

Many factors affects the cracking in concrete. The first one is SHRINKAGE of concrete which comes due to the lake of water in hydration process. second is temperature at the time of curing.these are main reasons for initial cracking in the concrete.

False mix design also leads to these type of cracking.More cement content creats the problem.

We have many reason for cracking in the concrete or cement paste at the early age

the first thing is the temperature , when the hydration heat is high and the concrete is already form, crack may be appear due to the rapid  evaporation of water the main thing can appears in cement.

the second way may be the hydration products  like CaO or MgO in the concrete you have to measure the quantity of free CaO or MgO.

dears

it is well known the reasons behind cracks at early age,like natural concrete shrinkage combined with outer boundary conditions restraint this volume changes. in addition changing the properties of materials with time

the standards codes and committees work to solve this issue. unfortunately this issue still not solved yet.

The question: is it this standers and committees not understand clearly this phenomenon?

I think the pores of concrete structure become more and more with water disappeared gradually . Inner water is more than surface, which is influneced by tempreture and strucutre.

early age cracking is due to a combination of hydration heat, cooling, shrinkage, creep and variation in time of stiffness and resistence. You can read some publications i've written on the topic, as I'm working on this problem since 7 years and I'm into some international commitees working on this issue.

dear Gabriele Bertagnoli :

The volume changes restrained by adjacent structures work without any external load, while the creep work with load.

i think the creep doesn't work within this stage. 

its depend upon water cement ratio ....early stage crack is know as shrinkage crack

Cracking in cement-based materials occurs because of differential volume change. This volume change can be shrinkage due to drying (related to moisture gradient), temperature change (related to differential expansion/shrinkage capacity of different phases) and hydration. If the concrete structure is subjected to external restraints, the cracking will be generally more severe than that without any external restraints. Note that cracking still occurs even the structure is not subjected to any external restraints. Recently, I published two relevant paper related to the drying-induced microcracking without external restraints. You can find them in my RG.

DEAR Zhigen Wu

all the structural restraint to some degree either internally by variation of temperature and moisture or externally by adjoining  elements.

This phenomena in covered by ACI code and Euro code , even application of these two codes the cracks still occur !!!

the question way not prevent this cracks ? 

means these codes need more improvement.

I react from one of your answers : i think the creep doesn't work within this stage. 

Yes, creep occurs even at very early age, and in fact, is actually a predominant phenomenon during the early life of concrete. Creep of concrete covers a very wide spectrum of relaxation times, and affects both the long-term and the short-term properties of concrete. There are plenty of studies showing this effect for the early age (I am not sure, however, if this is accounted for in your model), and the interaction between drying shrinkage (which might be the cause of your early-age cracking) and creep is well known as the Pickett effect (and yes, it occurs even at early ages). If you do not have creep in your model, you might overestimate the amount of damage in your material.

Please look at following Literature (sorry in German)

Röhling

Zwangsspannungen infolge Hydratationswärme

isbn 3 - 7640 - 0435 - 5

John Newman - Ban Seng Choco

Advanced Concrete Technology / Concrete Properties Part 2

isbn 0 7506 5104 0

This is due to shrinkage effect: and it can be  classified in the following way:

(a) Plastic Shrinkage

(b) Drying Shrinkage

(c) Autogeneous Shrinkage

Cracks may be due to shrinkage of cement and thermal gradient

this due to  the shrinkage and temperature effect .

Shrinkage and temperature effect, (we have the same kind of phenomenon in steel, at the very beginning of loading actions, the cracks initiate probably due to the new environment)

Concrete cracks when it’s not duly watered during setting. In this case the concrete surface gets dry; there is an increase of suction and a reduction of volume that produces the cracks. After setting the strength of concrete avoids this cracking.

Plastic shrinkage cracks occur while the concrete is still plastic, before setting. Drying shrinkage cracks occur after the concrete has taken set. Drying shrinkage cracks but have little effect on cracks that form while the concrete is highly plastic. During the early stages of hydration, the cement paste is very weak. 

If you follow the standards and specifications carefully, would would eliminate the cracks. For example, cover and protect the concrete just after finishing, protect from winds, start curing as early as possible (final setting time is a good start point),.........

Also you have to ensure good mix design,

Example: Samples prepared in the lab and precast concrete elements do not crack.

dear  Hisham Qasrawi

even, If you follow the standards and specifications carefully, cracks still appears at early age becouse these standers not understood fully the phenomen of cracks at early age and much factors inflounse on it.

on the other hand, some time we have dis agreement betwen standers :

for example in castin slab on ground :the attached figure the recommended stile of sequense casting according to ACI and Euro-code standers

it is not agrement with other local standers in some countaries

Dear HishamQasrawi

Most of Desighn codes pay attention to concrete under loading bt these cracs can occur due to open concrete forms early,wrong mixing of materials,form your concrete wrongly,undesirable materials,undesirable rebars,havnig too much air in it ,wrong vibration and finally, human wrongs and do not pay enough attention to early age concrete by most of codes causes these cracks. So i suggest you to read codes and their subscripts carefuly,do concrete experiments,check mixing materilas and your workers  to find the reson.If you find the main result please E-mail it to me if it is possible.I hope you be successful.

concrete can crack at the early stages due to plastic shrinkage which is the first stage, then aggregate settlement and drying shrinkage. Volumetric changes and sudden change in the heterogeneous mixture of concrete can lead to hairline cracks which can further develop into major visible cracks.

Cracks may appear not only due to shrinkage but also due settlement or deflection of forms after the initial setting .The engineer should check forms carefully specially in slabs and good curing.

the cracks observed in concrete at early stage without load may appear from the properties of cement like the hydration heat of cement, it is very important to know about the hydration heat of cement used.

I agree with @Jessly Rajan

you should follow the civil codes for mixing of materials composition to prepare concrete.