42907 Chap.10 Concrete cracking and shrinkage
Concrete cracking and shrinkage
Weeks 8 and 9 separate the two families of crack and the three movements a concrete member makes. Plastic settlement and bleeding cracking arrive within hours; restrained shrinkage cracking arrives after hardening. Bleeding is a durability problem before it is a cracking problem, because its consequences include increased permeability and continuous bleed channels.
Shrinkage itself is three things of very different size: autogenous at about 50 to 100 microstrain, carbonation at about 50, and drying as the major component.
What this chapter covers
- 01
Two crack families separated by timing: plastic settlement and bleeding against restrained shrinkage
- 02
Bleeding as a form of segregation, and its six consequences
- 03
Channelled bleeding as a permeability path through the cover
- 04
Why concrete bleeds less than the paste in it at the same ratio
- 05
Three components of total deformation: elastic, creep and shrinkage
- 06
Creep under sustained stress, its basic and drying components, and the creep coefficient
- 07
Shrinkage as deformation without any applied load, in an unsaturated environment
- 08
Three shrinkage components and their relative magnitudes
- 09
Nine factors affecting shrinkage, grouped into the mix, the environment and the treatment
- 10
Aggregate as the restraining phase and paste as the shrinking phase
- 11
Curling where a slab can only dry from one face
- 12
What reinforcement does to a crack pattern, and why width matters more than count
- 13
Minimum shrinkage and temperature reinforcement, and the thickness that decides one layer or two
Diagnosing a line of cracks over the top bars
- +1Read the timing. A few hours after placing means the concrete was still plastic, which places this in the first family rather than in restrained shrinkage, which arrives after hardening.
- +1Read the geometry. The cracks trace the reinforcement, which is the signature of plastic settlement: the solids settled as bleed water rose, the bar obstructed the settlement locally, and the concrete above it hung up and tore.
- +1Change the bleed side. Reduce bleeding through the mix, revibrate the surface once settlement is complete, and protect the surface from rapid evaporation. Increasing the cover to that bar also helps, which is convenient given everything else in this guide.
Key terms
- Bleeding
- Segregation of a particular kind, in which part of the mixing water works its way up to the top of freshly placed concrete because the settling solids cannot retain all of it.
- Plastic settlement cracking
- Cracking that occurs while the concrete is still plastic, where settling solids are obstructed locally by a bar or an insert and the concrete above the obstruction tears.
- Channelled bleeding
- Bleed water moving fast enough through the paste to create continuous channels. Those channels are a permeability defect that no diffusion coefficient in this subject accounts for.
- Elastic strain
- The instantaneous strain that appears the moment a load is applied and, for practical purposes, does not change with time. It is the only one of the three movements that is not time dependent.
- Creep
- Strain that keeps climbing for years under a stress held on the member, with a basic part and a drying part. It is quantified as specific creep per unit stress, or as a coefficient relative to the elastic strain.
- Drying shrinkage
- Contraction caused by water evaporating from the concrete to an unsaturated environment, and the major of the three shrinkage components by a wide margin.
- Autogenous shrinkage
- Contraction caused by water being consumed internally as hydration continues, at about 50 to 100 microstrain. Small compared with drying shrinkage but present even in a sealed member.
- Curling
- The tendency of a slab that can only dry from one face to lift at its edges, because the drying face shrinks and the other does not. Self weight resists the movement and puts internal stresses through the section.
Concrete cracking and shrinkage FAQ
Why is bleeding treated as a durability topic rather than a finishing one?
Because four of its six listed consequences bear directly on durability: a weak top layer, reduced bond to the aggregate, reduced bond to the reinforcement and increased permeability. The last of those is the connection to every ingress model in the guide, and channelled bleeding is worse still, because fast moving bleed water can leave continuous channels through the paste.
A cover designed as a pore network has been compromised before the concrete has even set.
How large is shrinkage, in numbers I can hold onto?
Two of the three components are small and one is not. Autogenous shrinkage runs at about 50 to 100 microstrain and carbonation shrinkage at about 50, while drying shrinkage is the major component. For scale, a shrinkage limited cement is one whose drying shrinkage is held below 750 microstrain, so the two small components together are well under a fifth of a controlled drying shrinkage.
Does reinforcement prevent shrinkage cracking?
No, it redistributes it. The same movement has to be accommodated either way, because the shrinkage is a property of the concrete rather than of the steel. An unreinforced slab without joints cracks frequently and widely; a reinforced one develops cracks that are smaller and closer together.
For durability that is a real win, since ingress through a crack depends strongly on its width, which is exactly why the Standard asks for distributed minimum reinforcement rather than for no cracks.
Why does the guide not give me the shrinkage prediction model?
Because the Standard extract available in the published subject materials covers the durability section only, and the shrinkage model sits in a different section that is not among them. Reconstructing a decomposition or its coefficients from memory would be inventing a citation. The components and their magnitudes given here are sourced; for the prediction model itself go to the Standard and to the week 9 material.
Assessment move
Keep the two crack families apart by their timing, and practise diagnosing from a description rather than from a photograph, because that is how the question will be put. Learn the three movements as a set and be able to say which need a load and which do not.
Memorise the relative magnitudes of the three shrinkage components rather than trying to remember an absolute figure for drying shrinkage, since the source gives a direction rather than a number. Group the nine shrinkage factors into the mix, the environment and the treatment, and notice that most of the mix factors act through the single mechanism of paste fraction.
Finally, be ready to explain why crack width rather than crack count is the durability quantity.
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