Vito ALUNNO ROSSETTI, Antonella FERRARO

Do thermal cracks of concrete columns close on cooling ?

Due to the heat of hydration generated by cement in concrete and the consequent warming and cooling of

structural elements, various types of cracks can develop1.

In wall type elements, vertical cracks form on cooling, generally in the first week after casting and not

wider than 0.8 mm, sometimes passing, not always reaching the wall top. Such cracks tend to widen in

time, due to the drying shrinkage.

Figure 1 a - Cracks on a concrete wall at two months since the casting. Water

passage and salt crystallization is visible since 2 days from the casting

Figure 1 b – A special wall: 1.7 m in heigth, 8 meters wide, 30 m in length,

same type of cracks has been observed

In column type elements, the cracks form on warming, due to the temperature distribution (warmer in the

core and colder in the cortical layers). The inner column core in the presence of the outer concrete layers,

1 Vito ALUNNO ROSSETTI “Il Calcestruzzo – Materiali e tecnologia”, Ed. McGraw & Hill, Terza Edizione, Milano 2007, pag. 226

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finds an obstacle to dilatation, both in the horizontal and in the vertical direction) so it is compressed; the

contrary happens for the cortical zone where a tensile stress develops .

Column section

Figure 2 - Thermal expansion crack on a massive concrete

column at six days

Figura 3 – At the crossing of the

green and blue lines a no-stress

zone is present

A FEM calculation for a 35 MPa concrete gives an idea of the situation at short term, generating horizontal

and vertical cracks.

Figura 4- FEM Thermal and

tensional analysis. Tension

distribution at 45 h (when the

maximum tensile stress of

the used concrete is reached).

Compression is negative.

The stresses have a vertical

direction; circular horizontal

and linear vertical cracks are

formed.

A question seems to be reasonable: when the inner core of the column loses heat and starts to cool,

producing a thermal contraction, is it possible that also the cortical outer cylinder of the column contracts

and “shrinks” and a closing (at least partial) of the cracks can be noticed?

First of all, an answer can be found by intuition. It is evident that when the core (still compressed) cools,

the outer concrete cannot contract (excluding the elastic contraction in compression) because its form is an

arc; it will lose the residual initial tensile stress and will go in a compression state. Having also lost on

hardening most of the initial creep ability, such a compression will not cause the closing of the vertical

cracks. In time, the drying shrinkage will cause a widening of the cracks.

Being this the case, some tensile stress can be imagined at the internal boundary between the outer and

the inner cylinder, due to the impossibility of the former to follow the contraction of the latter.

Compression

zone

Tensile zone

Temperature

Distribution

No-stress

line

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A fem analysis at 200 hours of the same column confirms the idea. Between the compressed annular

outer zones and the still compressed core zone, a circular tensile zone is evident.

Figure 5 - A inner annular

tensile zone is evident

On the basis of the preceding considerations, the interpretation of an initially rather obscure experimental

result has become possible.

Figure 6 - Sonic tomography after 5 months. Visible

cracks on the surface in correspondence of

blue spots.

The yellow internal annular zone, is

presumably due to some damage of the

concrete originated by the local existence of

tensile stresses on cooling of the column.

The yellow annular inner zone confirms a damage of the concrete, due to the presence of a tensile zone.

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