Cracks, Pipe Breaks, Collapse
The "cracks" type of damage occurs primarily in rigid piping, whereby distinction is made between three main forms which can be the precursors for the pipe break and, finally, for the collapse of the sewer [ATVM143-1:1989].
- Longitudinal cracks;
- Lateral cracks;
- Cracks originating at a point (in some cases also with pipe fracture).
The causes of cracks are closely connected with their type, whereby the form of a crack, its dimension and its course allows conclusions on its cause. Thus it is possible that, on the one hand, one cause results in several cracks at different places and, on the other hand, one crack can have several causes.
Important criteria, which can provide information on the cause of the cracks are [Ruffe81] :
- Crack alterations over time (It can be seen from this whether the crack has come to rest or is still propagating);
- Depth of the crack (crack only superficial or extending throughout the whole component);
- Course of the crack (provides information on the direction of the force acting on it); and
- Mutual displacement of the edges of the crack (provides information on the direction of the force acting on it and the danger of failure).
Irrespective of the type of crack, the following are mentioned as causes [ATVM143-1:1989] :
- Non-adherence to the standards and regulations, e.g. EN 1610 [DINEN1610a], ATV-A 127E [ATVA127a].
- Damage to the pipes during transport, storage, laying, bedding, backfilling or compacting.
- Effects of wear.
In almost every case, the formation of cracks is a damage, the exception being some types of cracks in concrete and reinforced concrete.
Thus, according to [DIN4032b] for instance, small kerbs on the surface or irregularly running "spiders web" types of shrinkage cracks are not important for the utility of concrete pipes.
This is also the case for reinforced concrete pipes to DIN 4035 [DIN4035] whereby, in addition, cracks with a width of under 0.2 mm are permitted. For cast and steel pipes with cement mortar lining, cracks in the cement mortar lining of 1.5 mm are permitted, according to [DIN2614]. In the specifications of the German Concrete Association [DBV91] or the DVGW [DVGW88], a permissible width of crack for parting cracks with w = 0.15 mm [DBV91] or w = 0.20 mm [DVGW88] is specified.
For cement-bound materials one obviously assumes that cracks up to 0.2 mm width with water that forces its way or seeps in due to after-hydration of the cement particles or due to sinter processes, close automatically and tensionally [Mater82] [Mölle67] [Schäp86] [Manne77].
This process of "self healing" or "after sintering" is also taken into account in DIN 4033, Section 126.96.36.199 [DIN4033b] within the scope of the tightness testing when the time required for this is available and the pipe is kept filled up to the crown.
A summarised description of the present state of knowledge of this process is contained in [Stein95c].
The self-healing of cracks in concrete and reinforced concrete pipes is a chemical-physical process in which, the flowing water and the effective media, leads to the sealing of the cracks. It is recognised on visible structural components by a reduction of the soaking and, as a rule, by white lime flags or scabs at the dried concrete surface.
Only very recently has scientific knowledge been gained that explain the main causes of this phenomenon.
The most important factors in this, according to [Trost89] are:
- Clogging the crack by means of solid contents of the water;
- Clogging the crack by means of the concrete particles broken off by the crack;
- Formation of calcium carbonate;
- Hydration of the cement;
- Swelling of the concrete at the flanks of the crack.
The share of these influencing factors on self-healing is presently the subject of controversy in the literature, as the experiments in this respect do not provide clear answers. They do, however, confirm the possibility of applying a targeted self-healing under the conditions that the process itself and the chemical-physical procedures are well enough known that they can be used under real conditions.
It is characterised by a strongly falling curve at the start of the throughflow followed by an asymptotic part approaching the time axis. The flow thereby experiences a clear reduction. In some cases, the self-healing leads to a complete sealing of the crack [Clear82].