Plastic settlement cracking (PSeC)

Plastic settlement cracks occur while concrete is still in its plastic state, during the process of settlement. These cracks typically start to occur after the concrete has been cast till the concrete is no longer plastic, which generally corresponds with the initial setting time (Owens, 2009 & Combrinck & Boshoff, 2012).

Plastic settlement cracks can be distinguished from plastic shrinkage cracks by the characteristic crack pattern that typically mirrors the form of restraint that the concrete element is subjected to, for example steel reinforcing or a change in element section as shown in Figure 3.8 (Cement, Concrete & Aggregates Australia, 2005). Plastic settlement cracks can easily develop further by acting as weak zones for crack widening caused by subsequent volume changes related to plastic, drying, thermal, and chemical shrinkages (Holt & Leivo, 2004).

This section first discusses the mechanisms and development of PSeC. After this the interaction between PSeC and PShC is described, and finally, the methods to reduce PSeC occurrence and severity will be presented.

3.2.1 Mechanisms and development of PSeC

Plastic settlement cracks do not occur as a result of capillary pressure build-up or air entry, as is the case with plastic shrinkage cracks. Rather, it occurs as a product of a combination of vertical settlement and vertical restraint, resulting in differential settlement.

After fresh concrete has been cast, the effect of gravity causes the vertical settlement of the material. Bleed water moves to the surface while solid particles settle downwards. If no vertical restraint is present it merely results in a slightly lowered concrete surface. However, if the concrete is locally restraint from settling, while the adjacent concrete continues to settle, it gives rise to the development of stresses in the weak material. If these stresses exceed the strength of the fresh concrete, cracking or separation will occur in the material over the source of restraint (Mehta & Monteiro, 2014).

The differential settlement of fresh concrete around a reinforcing bar results in three distinctive defects, as shown in Figure 3.9. The first are the angled cracks that originate from the sides of the reinforcing bar to the concrete surface, as depicted in Figure 3.9 b), and referred to as shear cracks. The origin of these cracks corresponds with the border between the respective settlement zones. While these shear cracks are a result of tensile stresses, as shown by the rotated block with principal stresses in Figure 3.9 b), the source of these tensile stresses are shear stresses produced by the mechanism of differential settlement. The second defect is the formation of a tensile crack on the surface that propagates downwards to the reinforcing bar. These cracks are a product of direct tensile stresses, as shown in Figure 3.9 b), that also originate as a result of the differential settlement between the concrete at the sides of the reinforcing bar and the concrete above the reinforcing bar. The last defect is the development of water pockets under the reinforcing bar. This happens as a result of the settlement of concrete below the reinforcing bar and these water pockets form voids at a later stage (Combrinck & Boshoff, 2014).

It should be noted that restraint is essential for the occurrence of both types of early age cracking. Plastic shrinkage cracks require horisontal restraint for cracks to occur while plastic settlement cracks require vertical restraint (Combrinck & Boshoff, 2012a)

Figure 3.9: a) Respective settlement zones caused by reinforcing bar. b) Distinctive defects caused by differential settlement. c) Distinctive defects at initial setting time. d) Distinctive defects after final setting

time (Combrinck & Boshoff, 2014)

Since differential settlement is the mechanism leading to the occurrence of plastic settlement cracking, concrete mixes that produce minimal settlement are less prone to the occurrence of these cracks. Bleeding is traditionally a good indication of settlement as a large amount of bleeding is usually indicative of a corresponding amount of settlement (NRMCA, 1998). However, it was found that low bleed concrete mixes that utilised a large amount of fine particles produced greater differential settlement than their high bleed counterparts. This is because these types of low bleed mixes lead to higher paste mobility and capillary pressure build-up and therefore also PShC. This causes an increase in PSeC that can only be ascribed to the vertical component of capillary pressure build-up that leads to an additional amount of vertical settlement (Combrinck & Boshoff, 2012a).

3.2.2 Interaction between PSeC and PShC

Combrinck & Boshoff (2012) carried out a study to determine the influence of restraint on early age cracking as well as the relationship between PSeC and PShC. These tests were conducted under conditions that were conducive to the development of both plastic PSeC and PShC. It was found that the first cracks occurred considerably earlier than the initial setting time and air entry. It is thus conclusive that the cracking was induced by differential settlement and not capillary pressure. Vertical plastic settlement as well as horisontal plastic shrinkage was responsible for the substantial crack growth up to the initial setting time. This was substantiated by the fact that the settlement was still occurring before the initial setting

a) b)

time had occurred and capillary pressure build-up as well as air entry, leading to plastic shrinkage, was also witnessed during this time. The bulk of the cracking occurred long before it was expected due to the prior formation of plastic settlement cracks (Combrinck & Boshoff, 2012a).

It was determined that under environmental conditions with high evaporation rates, the occurrence of plastic settlement cracks could force the early and unanticipated development of the bulk of all early age cracking before the initial setting time had occurred. This “crack jump behaviour” is due to a combination of the following factors (Combrinck & Boshoff, 2012a):

 The plastic state of early age concrete,

 The practically fully developed capillary pressure build-up at the time of crack occurrence,

 The global pressure relaxation across the entire concrete element once the crack occurs, leading to all additional deformation being localised at the cracking position,

 The combined effect of PSeC and PShC working in agreement to open the crack further in both the vertical and horisontal direction.

3.2.3 Preventing and mitigating PSeC

The following measures can be taken to decrease the likelihood of PSeC occurrence and its severity (NRMCA, 1998):

 Use mixes with low settlement characteristics. Lower slumps and good cohesiveness are usually indications of this. Caution should however be exercised not to use an excessive fines content to achieve this as this will increase the vertical component of capillary pressure, leading to increased differential settlement (Combrinck & Boshoff, 2012a),

 Increase the ratio of concrete cover to reinforcement bar diameter. This will decrease the restraining effect of the reinforcement on the concrete. One can either achieve this by increasing the cover or decreasing the reinforcement bar diameter,

 Wetting the subgrade or formwork prior to casing to reduce the amount of water absorbed and subsequently removed from the concrete,

 Ensure that all formwork is installed accurately and securely so that no movement will take place during the casting of the concrete,

 Compact concrete sufficiently and ensure that prompt and proper curing takes place,

 Make use of sequential placement. Ensure that concrete is first cast into deep sections, like columns, and allowed to settle prior to casting and compacting of top

sections, like slabs. This should be done with care as not to cause cold joints (Kwak & Ha, 2006).

 Re-vibrating of the concrete just prior to its initial setting time in order to close the crack formed by PSeC (Kwak & Ha, 2006).

Although the addition of a low volume of synthetic microfibres reduces the occurrence and severity of PShC, it shows no significant influence on PSeC. PSeC usually occurs before the initial setting time of concrete. This is the time at which the interfacial shear bond stress between the microfibres and the concrete matrix start to develop. As a result, no significant additional resistance to PSeC is provided to the concrete with the addition of microfibres. This has been proven under environmental conditions conducive to a high evaporation rates and is still to be confirmed under other environmental conditions (Combrinck & Boshoff, 2012a).