## 1 Introduction  
The importance of water supply networks is huge for humanity. For this reason, obtaining simple but effective solutions for engineering practice, at an acceptable cost price, is an important objective for the designer and justifies the theme studied in this paper. The use of pipes is widespread in the transport of liquids, especially for that of oil and gas, but also water needed by human communities. For example, for oil and gas transport alone, data from 2014 indicate a total pipeline length of approximately 3,500,000 km built into 120 countries of the world [1]. The pipe, as the main element of a supply network strictly necessary for the transport of water, was made, for more than two millennia, out of traditional materials that were common at the time in those communities where they were used (stone, ceramics, wood, metals, non-metals, etc.). Constructive variants of pipes, of different shapes and sizes and made of different materials, have been used by many and various civilizations that have followed one another throughout history, and countless artifacts can illustrate this development. Water supply networks experienced great development, especially in the second half of the twentieth century when science and technology allowed for the emergence and production of plastics, which became the main materials of the pipes used in water supply networks. The most suitable and current plastic material used to produce the pipes needed for water transport in modern water-supply networks has been proven to be HDPE. The pipes and fittings obtained are characterized by a long service life, low maintenance costs, and their remarkable special strength and low weight. The HDPE pipes have many advantages in comparison to the classic ones used so far. Listing some of them ([2,3]): they allow a long service life; they do not need cathodic protection systems; there is no need for additional coating--internal or external--necessary for anti-corrosive protection; maintenance costs are low; the quality of the inner surface, very smooth, offers special hydraulic properties, which are maintained during the service; assembling them is easy, saving time on assembly; insured sealing is very good when required under high pressure; the pipe joints allow for angular deviations, thus making possible the changes of direction, without the use of special fittings; the weight is reduced when compared with concrete or metal pipes; the costs are reduced, thus ensuring a fast and cheap transport; large pipe lengths can be achieved, which is why few connections are required and thus can be installed quickly; the materials used are thermally stable, resulting that their properties will not change during temperature changes; the pipes that are marketed are odorless, tasteless, non-toxic and stable under the action of chemical agents or meteorological factors; aging is, in the case of these pipes, a very slow process; pipes made of polyethylene can be used in an assembly for a period of 50 years, if the pressures and temperatures recommended by the manufacturer are observed in service. The behavior of HDPE material has been intensively studied in the last decades and, besides the reference work in the field [4], in recent years there have been intense studies to improve this material [5, 6, 7]. In [8] it is studied how the type of fiber can improve the mechanical properties of WPCs. It is observed that the addition of continuous fibers can increase the flexural, tensile and impact strength of the material by up to 83.1%, 47.3% and 713.4%, respectively. An analysis of the damping ratio shows that the interfacial bonding of glass-yarn reinforced WPCs has improved properties. It results that the addition of continuous fibers to WPCs, even if the volume ratio is low, recommends promoting their use as load-bearing engineered materials.