THE RESEARCH OF REINFORCED CEMENT LINER FOR RECONSTRUCTION OF WATER SUPPLY AND SEWER PIPES

The research is focused on the investigation of the features of mortar liner reinforced with composite mesh, which may be one of the possible options in case of one possible method of trenchless reconstruction of water and sewer pipes. The research had two goals: to study the strength under static loading of a mortar liner reinforced with a composite mesh frame and to define the maximum depth of underground location of a coating element, which is of significant importance if a host pipe cannot bear external loads any longer. Within the research, two specimens of coating with the length of 1000 mm and diameter of 800 mm were tested. The compression results showed that for both specimens the loss of bearing capacity occurred to the fluidity of composite reinforcement under the load of ap proximately 30 kN. After the strength test, a calculation was made to evaluate the maximum depth of liner location in case if a host pipe fully exhausts its bearing capacity. The results of the calculation showed that the maximum height of the soil layer above the crown of a pipe is 2.8 to 3.2 m depending on the type and features of the soil.


INTRODUCTION
Reliability and environmental safety are the main requirements applied to modern wastewater disposal systems. In general, reliability may be considered as the feature of an object to perform specified functions and to maintain the values of operational parameters in time and within the specified limits corresponding to the specified modes and conditions of use [1]. Reliability in general unites and includes several single properties: durability, maintainability, operational controllability and safety, etc. [2]. A drainage system from the viewpoint of reliability can be considered either as an entire system or as a combination of its certain elements: pumping stations, sewage treatment plants, gravity networks, and pressure sewer pipelines [3]. Some water pipelines have been in service for decades, which may result in significant wear of pressure and gravity pipelines, can increase the risk of accidents, and therefore, strongly affect their reliability [4]. The selection reconstruction approach conventionally depends on the failure mechanism and host pipe condition, including the degree of corrosion, structural deterioration, and hydraulic capacity loss [5]. Compared with traditional open-cut methods, trenchless reconstruction of water and sewer pipes has certain advantages: cost-efficiency of being cost-effective, less disturbance to the ground (traffic and residents), less environmental impact, and fast construction [4,6]. The main direction of the trenchless restoration of underground pipelines considers the application of internal protective coatings (linings, shells, jackets, membranes, inserts, etc.), which may cover the host pipe along the entire length or individual defects. Among the variety of trenchless methods of water supply and drainage networks restoration, the following approaches can be pointed out: • spray in place (SIP) is the application of spraying coatings on the inner surface. One of the possible options is a cement mortar liner or a cement coating sprayed on the surface of the inner walls of the existing pipe [7,8]; • cured in place pipe (CIPP) rehabilitation of the pipelines through a flexible (thermosetting) liner, which is inserted into an existing pipe [9]; • sliplining is the method of pipe reconstruction when a new pipe is inserted into a deteriorated host pipe [10,11]; • modified sliplining is a method of rehabilitation of host pipe with a spiral wound [12]; • lining with inserted flexible polymeric hose [13]. Each of the listed methods of restoration has certain specific features, which determine the scope of their application to restore water supply and drainage networks. The choice and further implementation of a particular method require a detailed preliminary inspection to investigate the condition of the pipeline, its dimensions, the type of transported medium, the surrounding underground infrastructure, the type of soil, the presence of groundwater, etc. [14,15]. Among the well-known methods of rehabilitation, cement spraying can be considered as one of the oldest with an experience of implementation since the early 1900s. The main advantages of cement liner are the coating strength and evident anticorrosion properties [16]. The use of mortar liner should be considered, first, as anti-corrosive insulation of the inner surface of pipelines. A special property of the cement-based coating provides passive and active effects. The passive effect is achieved due to the mechanical insulation of the pipe walls with a strong protective layer, and the active effect is based on the formation of a saturated solution of calcium hydroxide with pH = 12.6 on the interface of the ce- T ment coating and the pipe wall. Under these conditions, low-alloy steel does not corrode [17]. At the same time, the mortar coating has the property of self-treatment. It consists of the fact that cracks and crevices that may occur during the application and setting of the solution are self-sealed both due to the swelling of the material and the calcareous deposits released in the form of calcium carbonate [18]. While remaining in demand today, mortar liners are being gradually replaced with the new polymer of composite materials. One of the options for the improvement of mortar coatings is their reinforcement to increase strength characteristics. Application of reinforced coating may be especially reasoned if the partially deteriorated host pipeline turns to full deterioration and loses its strength [19,20]. The research had two goals.
The first was to study the strength under static loading of a coating element made of a cement-sand base and reinforced with a composite mesh frame. The obtained bearing capacity of a coating element may play a significant role if the host pipeline cannot provide load-bearing capacity (coating becomes an in-dependent pipeline), or maybe considered as additional strength of combined structure (host pipe+coating). The second goal was to define the maximum depth of under-ground location of a coating element, which is of significant importance if a host pipe cannot bear external loads any longer. Thus, both research goals complement each other.

Specimen
There were two specimens (Figs.1 and 2) manufactured for the research to imitate the cylindrical segment of inner cement coating of a host pipeline. The mortar liner specimen was reinforced with mesh frame of composite fiber ( fig. 3 a, b).

Test sequence
The tests were carried out in accordance with the requirements of standard of Russian Federation "GOST 6482-2011 "Reinforced concrete non-pressure pipes. Specifications" (Fig.5) [21]. The experiment procedure was very similar to procedure in [22], which is typical for strength research, e.g. research of De La Fuente et al. [23] and Park et al [24].
The following sequence of experiment was applied: 1. A cylindrical specimen of reinforced cement coating was placed horizontally on two wooden beams laid parallel to the longitudinal axis of the pipe on a fixed base (the power floor of the laboratory). A wooden beam was laid along the crown of the pipe with a steel traverse installed on it. Dimensions (cross section) of a wooden beam was 80x80 mm. 2. A leveling layer of cement-sand mortar was laid under the upper and lower beams to provide even load to the specimens. 3. Loading of the specimen was carried out gradually with the step of 5 kN/m = 500 kg/m, using hydraulic jacks equipped with displacement and load sensors. The load increase was carried out evenly for 2-3 minutes, after which it was maintained for 10 minutes. The maximum possible load that was applied to the specimens was 33 kN/m. 4. The loading stage corresponding to the moment of cracking was determined by visual inspection (using a Brinell microscope). After the crack formation load, the crack formation was monitored and the crack opening value was measured with a measurement error of 0.01 mm).

5.
Loading was applied until the bearing capacity of the specimen was exhausted, characterized by one of the following states: • fluidity of composite reinforcement, characterized by a crack opening width of more than 1.5 mm in pipes with a double frame and more than 2 mm in pipes with a single frame; • crushing of concrete from compression; • rupture of spiral fittings; • delamination of the pipe wall (separation of fittings in a pipe top or tray).  Tables 2 and 3 show the summary of the displacement based on data of sensors installed on specimens #1 and #2, respectively. According to the testing method, the maximum load perceived by the specimen was estimated by the crack opening width. Tables 4 and 5 show data on the width of the cracks, which were formed because of the loading of specimen #1. As can be seen, for specimen #1, the load value at which the specimen lost its bearing capacity was 31 kN.   Note: Here and in the tables 5-7: 1. The value of "X/Y" = "crack opening width "X" mm /crack length "Y" mm". 2. The value of the opening at a distributed load with the symbol "*" is indicated after exposure under load for 10 minutes in accordance with the loading procedure.   Tables 6 and 7 show data on the width of the cracks, which were formed because of the loading of specimen #2. The maximum load was equal to 30 kN for specimen #2.  Note: I'* is a crack located along the circumference of the pipe.  Obtained results correspond to existing practice, when a crack load and ultimate load relate to each other with a safety factor of 1.5 [25]. Following the test report, the permissible vertical load at which both tested specimens retained their bearing capacity (after being loaded for 10 minutes) was F = 30 kN. The specimens' strength test is based on a standard for reinforced concrete pipes [21]. This document prescribes the division of pipes into , which are applied for rehabilitation of partially deteriorated pipes; it is a matter of certain practical interest to estimate the maximum height of the soil layer above the crown of the pipe. If a host pipe fully deteriorates with the exhaustion of bearing capacity, the loads are perceived directly by the inner coating. Based on the dependencies presented in [26] and [27], the permissible vertical pressure on the pipe (coating) can be estimated as follows: (1) where: L is length of pipe (coating); L was equal to 1 m within this research; y is the maximum vertical deformation (6.31 mm, see table 3); di is the average inner diameter of the pipe/coating (see table 1 The normative vertical pressure of the soil Py, may be determined using the equation [28]: where: h is the height of the soil layer above the crown of the pipe [m]; y n is the normative specific weight of the soil [kN/m 3 ]; Cy is the vertical pressure coefficient, which is determined according to the following equation [28]: where: d is the outer diameter of the coating element [m]; B is a width of the pipe foundation: where: φ n is the normative angle of internal friction of the soil [27]; τ n is the coefficient of the normative horizontal (lateral) pressure of the soil, determined by the formula [28]: where: a is the distance between the basement of the trench (undisturbed soil layer) and the crown of the coating element. For further calculation, a is assumed to be equal to 1 m (a sum of pipe foundation thickness (0.2 m) and outer diameter of the coating element (0.8 m)); s is the coefficient, which is equal to 1.0 when a pipe lays on soil foundation. Based on the proposed sequence, a comparative calculation of the maximum height of the soil layer above the crown of the coating element for 4 different soils (sand, sandy loam, loam, clay) will be carried out. According to the reference data from [29], the following characteristics are accepted for calculation ( Table 8). The calculation was made for the Dfb region according to Köppen climate classification [30].  [32,33], or the joint work of the coating and the host pipe [7,34,35]. Within these researches, reinforced concrete pipes can normally withstand significantly higher loads than those obtained in our case, as far as the investigated coating element is not a pipe, but is sprayed in place directly to the restored pipe, which may certainly affect its strength characteristics.

CONCLUSIONS
The performed research allowing making the following outputs: 1. The research was aimed at a strength test of two specimens of sprayed-in-place cement (mortar liner) reinforced with composite fibers under the three-axis compression. 2. The compression results showed that for both specimens the loss of bearing capacity occurred to the fluidity of composite reinforcement under the load of approximately 30 kN. The behavior of specimens under load corresponds to the common relation of crack load to ultimate load with the factor of 1.5.
3. Obtained value of load approximately corresponds to the lowest class of reinforced concrete pipes according to the review of several standards. 4. Analysis of obtained results showed that research of stand-alone liner without a host pipe is not common, however, available results may be considered as similar under comparable test conditions. 5. After the strength test, a calculation was made to evaluate the maximum depth of liner location in case if a host pipe fully exhausts its bearing capacity. The results of the calculation showed that the maximum height of the soil layer above the crown of a pipe is 2.8 to 3.2 m depending on the type and features of the soil.