Cement Mortar Lining - A method of refurbishment for the long-lasting preservation of the piping infrastructure

In this connection, all relevant people bear responsibility for preventing the increasing degradation of these assets by means of suitable measures and for assuring the long-term functioning of the infrastructure. The basic economic conditions must of course be taken into account as well as the technical potentials such as the selection of the most suitable refurbishing method guaranteeing the maximum possible service life. Against this background, it is logical that trenchless methods have increasingly gained in importance. “Trenchless refurbishment takes priority over replacement”, this has become the watchword of a large number of operators. And quite rightly: a costsaving refurbishment and repair means that the piping systems are refurbished in such a way that their planned service life will be achieved with great certainty. In addition to particular demands on the qualification of all the involved persons, the applied methods and products are also subject to specific requirements which are, for example, fulfilled by the cement mortar lining. This technique used for the refurbishing of buried cast iron and steel pipes is a standardised, technically mature, proven and recognised procedure.

Water conduits are a most important component of the national wealth. With the transportation of the drinking water they essentially contribute to secure our alimentation with this comestible. The trouble-free and economic operation of the mains is however guaranteed only when the pipeline systems are professionally installed, conscientiously maintained and, if need be, replaced or refurbished in due time with the appropriate methods. A great challenge for the public authorities and operators, engineers and planners. The supply and distribution lines have already suffered from lack of investment over the past years. In addition, the supply utilities see themselves exposed to further burdens as a result of increasing demands caused by the deregulation of the market.

The consequences: an excessively low rate of reinvestment and the concomitant obsolescence of the existing mains. In this connection, all relevant people bear responsibility for preventing the increasing degradation of these assets by means of suitable measures and for assuring the long-term functioning of the infrastructure. The basic economic conditions must of course be taken into account as well as the technical potentials such as the selection of the most appropriate method guaranteeing the maximum possible service life. A cost-saving refurbishment means that the piping systems are relined such as to guarantee that their planned service life will be achieved. In addition to particular demands on the qualification of all the involved persons, the applied methods and products are also subject to specific requirements. Namely requirements which are, for example, fulfilled by the cement mortar lining. The method of subsequently lining buried pipelines with cement mortar in order to protect them against corrosion and incrustation has been known in Germany for more than 50 years. Meanwhile this method is being used with great success worldwide, and in particular in eastern Europe it undergoes a real boom within the scope of the tender programmes for boosting the efficiency of pipelines. The refurbishment of buried cast iron and steel pipes with the cement mortar lining is a standardised, technically mature, proven and recognised procedure.

Fields of application

The internal lining of pipelines is a method preferentially used for the refurbishment of drinking water pipes made of ferromagnetic materials (cast iron, steel). This technique could however be successfully applied even for the lining of asbestos cement pipelines. Especially with regard to the discussion about loose asbestos cement fibres in the drinking water, a cement mortar lining offers a long-lasting protection preventing the intrusion of the fibres into the drinking water. A technically mature method which has also been successfully applied in long-distance heating pipelines and in sewer pipes. Here cement mortars hardened with plastic adjusted to the particular properties of the media – for instance the high temperature or the chemical aggressiveness – have been used.

Technical options

Four methods used for the lining of drinking water pipes with cement mortar are known, namely the:

• method of displacement
• method of injection
• method of centrifugal application
• method of lining by hand.

The method of displacement
At the beginning of the era of cement mortar lining the so-called method of displacement was the state of the art. Here the mortar is entered into buried water pipes and displaced against the pipe wall by means of a cylindrical body provided with a conic head, and then smoothed while tearing the displacement body through the reach by means of a winch. The outside diameter of the displacement body is smaller than the pipe about the layer thickness to be applied. But experience has shown that in many cases an even and well-sealed coating cannot be obtained with this method.

The method of injection
The coating with the method of injection takes place by injecting cement mortar into an ring-shaped gap. The ring-shaped gap is formed by inserting into the pipeline a formwork which is removed as soon as the required minimum stability has been reached. The method of injection with cement mortar provides the use of a hose formwork which is widened either pneumatically or hydraulically. The thus produced ring-shaped gap is filled with cement mortar. After the hardening of the mortar (usually 13–20 hours after the filling), the hose formwork can be removed. The spacer net on the hose used for the centring and the fixing remains inside the coating.

The methods of centrifugal application
Nowadays the different centrifugal application techniques belong to the most frequently applied operation methods. These techniques use fast rotating centrifugal heads throwing the cement mortar against the inner pipe wall. The centrifugal application method is sub-divided into two groups depending on the machines which are used:

The centrifugal application with pneumatically driven centrifuges in non-accessible pipelines
Pipelines as from the dimension DN 80 can be lined by means of pneumatically driven centrifuges over a pipe length of up to 150 m. The cement mortar is fed by pumps via high pressure hoses to an extremely fast rotating centrifugal head (Figure 2) and then thrown against the wall by the centrifugal force. The result is a highly compacted uniform mortar lining of several millimetres.

The centrifugal application with electrically driven centrifuges in accessible pipelines
Manned self-propelled machines are used (Figure 3) for the lining of pipelines as from the dimension DN 800. The machine operator operates an engine, secured by an insulating transformer and a low voltage, working according to the preceding principle and producing a layer thickness of up to 1.5 mm in one work cycle. One advantage: the sections can be extended up to 700 m.

The method of lining by hand
Pipe parts in accessible pipelines which cannot be lined mechanically, for example, shaped parts must be coated by hand. Here it must be observed that the applied cement mortar has the same consistency and meets the same requirements than the mortar used for the mechanical lining.

The construction operation

Civil works where pressure pipes are lined with cement mortar are basically to be divided into three groups: civil engineering, pipe construction as well as cleaning and refurbishment. Usually it is not necessary to excavate the construction pits deeper than 50 cm under the bottom of pipe. The structure used for securing the pit serves at the same time as counter bearing for the machines needed for the refurbishment. The distance between the construction pits depends on the discontinuities due to the disassembly of fittings and on the applied machinery and equipment. In the inner-city area, mostly characterised by small pipe lines within a nominal diameter range of DN 600, the next discontinuity is usually reached with a refurbishment length of up to 150 m. As to transportation pipelines on an open terrain, the distance chosen between the construction pits can be up to 700 m, in case a largesize pipe technique with nominal widths > DN 600 is applied. The long distances are possible as the section to be refurbished is reduced by half, when self-propelled engines can be used and when up to 350 m can be lined with the centrifugal application method at each cycle length. A transition, which might occur in the middle of the section, is reworked by hand.

In a first work step the pipelines are put out of service and separated. Then a thorough cleaning takes place. For this purpose, a first rope connection is either blown with compressed air into the pipe by means of a small parachute or – in large pipelines – the rope connection is made by means of a self-propelled device. As soon as a steel rope with a suitable dimension has been inserted into pipeline, the cleaning with scrapers and discs can take place. All the loose and slightly adhering incrustations or corrosion products are to be removed. In small pipes the so-called scraper pigs and disc pigs necessary for the cleaning are moved by means of winches with a tensile strength of up to 6 tons (Figure 4).

Winches with a tensile strength of 20 tons are used in large pipelines. The frequency of the cleaning sequences depends on the degree of pollution. If need be, they must be repeated several times. In case of a formation of graphitic corrosion pockets, a cleaning with steel brushes is compulsory, but it is not necessary to obtain a blank metallic surface.

Besides the mechanical cleaning, a hydraulic cleaning with rotating nozzles at a maximum pressure is also possible. Because of the high water demand and the complex disposal, the application of this cleaning method is indeed very rare.

As soon as the cleaning is finished, the mechanical lining takes place ( Figure 5 ). When pneumatically driven centrifugal engines are used, the centring on the skids is most important, and when electrically driven and self-propelled centrifugal machines are used, the modification of the axial width must be strictly observed, in order to consequently guarantee the positioning of the centrifugal head in the centre-line of the pipe.

The layer thicknesses are regulated by the DVGW-worksheet W 343 (Table 1).

The utilisation of trailing truing devices is refrained from as far as possible, as these may trouble the structure of the mortar. The micro particles of cement clay and water are drawn upward just as in normal plastering procedures, but they do not similarly ameliorate the coefficients of the surface finish.

Applied materials

The substances coming into contact with the drinking water must correspond with the provisions of the food law § 31. That is the reason why only purified and fire-dried silica sands are used, which comply with the requirements according to DIN 4226-1. A grading curve as described in Figure 6 offers a perfect protection against a rebound and guarantees good compression and bending strength.

Cements such as PZ 35 F corresponding to DIN EN 197-1 or DIN 1164 are preferred. Additives or grinding aids are not allowed. The added water must have the quality of drinking water and the mortar must be pressuremixed with high precision in a ratio of 1:1 with a water cement coefficient of at most 0.35.

Chemical combinations

The protection effect of the lining essentially consists in the lagging of the pipe wall by the mortar layer. The low porosity hardly admits any diffusion of the water. Chemical processes with a long-time effect, which withdraw molecules and generate connections with a bigger volume, cement the residual pores thus increasing the service life. This procedure is subsumed under the term of passive corrosion protection by mechanical lagging.

In addition, there is an active protection provided by the interaction of water diffused into the interface between the cement mortar and the iron. The cement mortar undergoes a chemical combination with the iron of the pipe, comparable to the process of the reinforcement iron in the steel construction. It causes the formation of an anchoring layer – a slow and innocuous structural change of the mortar layer. The interaction of the lining with the flowing water has a positive effect on cracks produced from mechanical stress. A dicalcium silicate facilitates a “self-healing" of cracks thanks to a new granulation without impairing the material or the coating.

A cement mortar lining protects against corrosion and avoids incrustations. Furthermore, it has a static effect as the layer between iron and cement mortar evolves into a compound layer. An action which can be statistically proven, as the frequency of pipe burst of lined pipes has considerably decreased.

Hydraulics

Factory-lined pipes have a k-coefficient of 0.1. By means of pressure and flow measurement network operators have ascertained coefficients up to k = 0.04 for subsequently refurbished pipelines. Incrusted pipelines showed coefficients of k = 5 to k = 40. These measurements have shown that subsequent cement mortar linings re-establish the full capacity of the pipeline. To some extent there are even flow measurements with better coefficients than in new cast iron pipelines. The reason here are the low turbulences of the centrifugal application around the sleeves.

Quality assurance

The quality assurance in accessible pipelines is unproblematic. The lining can be controlled by spot-sampling. If need be, the concerned spots are reworked by hand.

As to the lining of small pipelines with a nominal internal diameter range < DN 600, it is quite different. Here the quality assurance begins with the selection of the machinery and the equipment. Not only the exact mixing ratio of silica sand, water and cement is relevant, but the application of compulsory mixers is likewise obligatory. And the so-called gypsy winch is as well an important tool for a high-quality execution of the lining. Thus a uniform tensile velocity for the steel rope, reeved several times around rolls and capstan heads, is guaranteed - independent of the needed force. In addition, the application of a condensate collector at the compressor is required. It prevents an icing of the fast rotating centrifugal wheel. And an out-coming water condensate might impair the stiffness of the mortar. Oil filters must also be utilised as they prevent that the oil, carried along in the air in form of an organic substance, pollutes the inside of the lining. The layer thickness of the cement mortar lining is determined with the so-called cut-in gauge at the fresh mortar at the beginning as well as at the end of the refurbished section. A continuous tensile velocity yields an even layer thickness at a constant mortar output. The tensile velocity is measured by an automatic tension diagram at the winch. So evidence of the layer thickness can be indirectly provided.

The use of measuring devices with magnetic induction mounted on carrying systems has been rejected because of the great number of disturbing factors on the continuous measuring method. Nowadays an acceptance of the cleaning and an acceptance of the cement mortar lining by means of a TVcamera (self-supervision) is common practice.

Costs

A calculation comparing the laying of new pipes and the refurbishment of pipes can indeed be made in a particular case, but with respect to the generally valid statements it does not bring satisfying results. There are too many influencing factors. The age and the condition of the pipeline, the soil conditions, the location in the traffic area, the depth, the action of ground water and the service life make a comparison of the values very difficult.

Internal comparative calculations show that the costs for a refurbishment amount to less than 40 % of the costs for a replacement. With an increasing cross-section of the pipe, this ratio considerably gains in favour of the refurbishment with a cement mortar lining.

Prospects

In future, plastics will also be increasingly used for the internal lining of pipelines. A consortium with the participation of leading German cement mortar lining firms has already done a good preliminary job. Typical fields of application are pipelines, which are subject to considerable chemical or physical corrosion. Here pipelines in the industry or raw water transporting lines must be mentioned, where the inner wall may be damaged due to the abrasion of small particles (sands) existing in the raw water.

Epoxy resins have already been in use for several years. For the refurbishment of house water connections, the application by spraying has taken place since the beginning of the nineties. It is only a question of time until these products and others will find their way into other piping systems or parts of piping systems. The call persists on system operators, engineers and planners to defend with all the means at their disposal the preservation and the protection of the infrastructure of the pipelines. The long lasting stagnation of investments has led to a considerable accumulation in the refurbishment sector. The consequences must be born by all of us: a further degradation of the underground public assets. This is the reason why regular investments are necessary. They contribute in the long run to stop the degradation of assets. The precondition for this is the observance of unitary standards of the quality assurance, the constructional engineering and execution and the operation as well as the permanently continuing development of proved and tested refurbishment methods and the formation and further training of the personnel. In addition, it is obligatory in public tenders for refurbishment to take account of the minimum requirements as defined in the DVGW-regulations and the RSV-data sheet.

A refurbishment of a drinking water conduit with a cement mortar lining is an excellent possibility to be able to attain these goals. At the moment, this procedure undergoes a real renaissance. A great number of refurbishment measures in Germany have already been successfully executed. It has also been possible to realise even large-scale projects – for example in East-European countries.

A German consortium, consisting of Heitkamp Rohrbau GmbH, Herne, and Diringer & Scheidel Rohrsanierung GmbH & Co. KG, Mannheim, has been awarded with a superlative refurbishment project. By order of the responsible water and waste water authorities “Vilnius Vandenys”, 50 km of the drinking water mains have been lined with cement mortar in Vilnius, the capital of Lithuania. In co-operation with a local building company, the sections have been put out of service one by one and the pipes have been opened. After a careful mechanical cleaning the pipes have been lined on the inside with cement mortar by means of pneumatically or electrically driven centrifuges in dependence of the nominal widths. The result: in the future, damage by corrosion as well as incrustations will be prevented. In addition, the pipelines now have much better hydraulic properties and a turbidity of water through rust is stopped.

The procedure has become accepted by the customers as being a reasonable alternative to the replacement and other procedures. A trend which does also become apparent in other countries: international network operators are more and more convinced that the lining of drinking water conduits with cement mortar executed by a professional enterprise certified according to the DVGWstandard R4 is an ideal method of refurbishment (Figure 7).

Literature

[1] Kunzler, R; Schwenk,. Änderungen der Wasserparameter bei Kontakt von Trinkwasser mit frischem Zementmörtel, 1986
[2] DIN 2614
[3] DIN 2880 „Anwendung von Zementmörtel-Auskleidung für Gussrohre, Stahlrohre und Formstücke“ (1999-01)
[4] RSV: 50 Jahre Rohrleitungssanierung mit Zementmörtelauskleidung in Deutschland, RSVSonderdruck 2006
[5] Technische Regel, Arbeitsblatt W 343, April 2005, DVGW Regelwerk
[6] Röscher, R. u.a.: Sanierung städtischer Wasserversorgungsnetze, Verlag Bauwesen, 2000