Half a century of reinforced concrete electric poles maintenance

Structure and Infrastructure Engineering Maintenance, Management, Life-Cycle Design and Performance

ISSN: 1573-2479 (Print) 1744-8980 (Online) Journal homepage: http://www.tandfonline.com/loi/nsie20

Half a century of reinforced concrete electric poles maintenance: inspection, field-testing, and performance assessment Romualdas Kliukas, Alfonsas Daniunas, Viktor Gribniak, Ona Lukoseviciene, Egidijus Vanagas & Andrius Patapavicius To cite this article: Romualdas Kliukas, Alfonsas Daniunas, Viktor Gribniak, Ona Lukoseviciene, Egidijus Vanagas & Andrius Patapavicius (2017): Half a century of reinforced concrete electric poles maintenance: inspection, field-testing, and performance assessment, Structure and Infrastructure Engineering To link to this article: https://doi.org/10.1080/15732479.2017.1402068

Published online: 21 Nov 2017.

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Date: 21 November 2017, At: 04:52

Structure and Infrastructure Engineering, 2017 https://doi.org/10.1080/15732479.2017.1402068

Half a century of reinforced concrete electric poles maintenance: inspection, fieldtesting, and performance assessment Romualdas Kliukasa, Alfonsas Daniunasb, Viktor Gribniakc, Ona Lukosevicienea, Egidijus Vanagasa and Andrius Patapaviciusa Department of Applied Mechanics, Vilnius Gediminas Technical University (VGTU), Vilnius, Lithuania; bDepartment of Steel and Composite Structures, VGTU, Vilnius, Lithuania; cLaboratory of Innovative Building Structures, VGTU, Vilnius, Lithuania

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a

ABSTRACT

Application of reinforced concrete (RC) poles as overhead electrical transmission line supports has become of great interest during recent years worldwide. The higher strength, longer life, and the potential to span longer distances than steel poles are the key reasons behind this tendency. A great variety of architectural shapes, relatively low maintenance costs, and high electrical resistance make RC a prominent alternative to steel. The drawbacks related to the application of RC poles include the high self-weight and vulnerability to damage. Self-weight might be reduced by prestressing the reinforcement and/or forming a tubular structure for the poles. However, both of these options might increase the vulnerability of the supports. Moreover, long-term processes (deterioration of concrete and corrosion of steel reinforcement) complicate the prediction of structural properties. This paper presents results of the everlasting inspection of the overhead electric power distribution system in Lithuania: technical state of more than 500 RC supports was assessed; selected poles were tested until failure. A specific point of this research is that most of the inspected supports were under maintenance since the middle of last century. The inspection results were used for developing the technical condition of RC pole evaluation scale proposed in this paper.

1. Introduction Reliability of power transmission networks is indispensable to modern society (Lin, Chang, & Fiondella, 2012). The customer interruption costs in Norway during the period 1995–2000 were about €90 million per year (Kjølle, Seljeseth, Heggset, & Trengereid, 2003) and in US about $30–$400 billion (Castillo, 2014). Overhead lines, comprising the highest number of equipment units, are the most vulnerable components in the distribution system (Bjarnadottir, Li, & Stewart, 2014; Dueñas-Osorio & Vemuru, 2009; Ryan, Stewart, Spencer, & Li, 2014; Zhou, Pahwa, & Yang, 2006). Almost half of the accidents can be associated with structural problems (Doukas, Karakosta, Flamos, & Psarras, 2011). In Norway, 40% of the power interruptions are related to the environment, 20% to technical failures and 5% are identified as a consequence of human errors (Kjølle et al., 2003). In Estonia, from 40 to 80% of events are related with accidents at the distribution network (Vinnal, Janson, Järvik, Kalda, & Sakkos, 2012). In Lithuania, 10% of accidents during the period from 1981 to 1999 were related to the failure of the supports (Kliukas, Vadlūga, & Kesiunas, 2003). In Ukraine, failure of the supports caused more than 30% of the accidents (Gorokhov, Bakayev, Nazim, Morgay, & Popov, 2010). Analysis of technical conditions of the

CONTACT Viktor Gribniak

[email protected]

© 2017 Informa UK Limited, trading as Taylor & Francis Group

ARTICLE HISTORY

Received 28 March 2017 Revised 11 September 2017 Accepted 13 September 2017 KEYWORDS

Reinforced concrete; damage assessment; inspection; field-testing; deterioration; service life; non-destructive tests; corrosion

power transmission line of the South Grid in China (Yan et al., 2015) revealed that the most serious consequences are associated with damages to the poles. Failures of the distribution lines are mainly characterised by innate problems of the equipment, which often become evident with increasing age of the system (Willis & Schrieber, 2013). Equipment failure often occurs when deteriorating components interact with adverse conditions such as rain, extreme temperatures, tornadoes, ice or dust storms, and interaction with animals or vegetation (Brown, 2008). Trees unduly close to the overhead lines, during severe weather events, are often the cause of adverse interferences. Fires, particularly in the presence of combustible materials, are also responsible for failure accidents (Fisher, Stoliarov, & Keller, 2015). Han et al. (2009) pointed out the fact that both vegetation and fire-induced problems are closely related to the soil moisture level. Power transmission might be interrupted due to water penetration of the cable insulation during excessive raining (Bertling, Allan, & Eriksson, 2005). The power transmission infrastructure is also vulnerable to earthquake impacts (Oral & Dönmez, 2010). Environmental conditions are obviously beyond management controls. There is a possibility, however, that the power systems in severe weather conditions can transfer the energy better than


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structures located in regions with mild climate. Yu, Jamasb, and Pollitt (2009) pointed out that power outages in several major incidents were caused by maintenance problems. More than 70% of the main disturbances in North America cascade due to incorrect relay operations that in turn occurred due to undetected defects (North American Electric Reliability Corporation, 2014). Ageing infrastructure can be recognised as another important aspect affecting reliability of the power system (American Society of Civil Engineers, 2013). The number of major outages in North America has noticeably increased with time: from 76 in 2007 to 307 in 2011. Severe weather conditions and natural disasters reveal the vulnerability of the overhead lines, which are near the end of their service life (Gusavac, Nimrihter, & Geric Lj, 2008). Decisions on the time and type of maintenance actions depend on the importance of the line, i.e. the consequences of overhead line failure to the electric power system. Proactive maintenance (refurbishment, extension of service life, decommissioning) results in an increase of required investments, while delayed maintenance reduces the operation reliability and increases costs related to the increased number and duration of failures (Gusavac et al., 2008). Bjarnadottir et al. (2014) demonstrated that development of a proper mitigation strategy might theoretically reduce the maintenance expenses up to 20%. Hence, objective monitoring and assessment of the technical condition of the system components become essential for ensuring the reliability of the power transmission. Successful functioning of the maintenance system (with the main aim to optimise the maintenance costs) is impossible without a relevant information system (Gusavac, Nimrihter, Novakovic, & Savanović, 2003). Gusavac et al. (2008) proposed an evaluation and decision-making algorithm that accounts for financial indicators, failures, preventive checks, diagnostic checks, and repair of overhead power transmission lines. A similar approach has been applied for the analysis of the South Grid transmission system in China (Yan et al., 2015). The algorithm estimates technical conditions of the overhead transmission lines with the help of the operation data processing. Structural conditions of the overhead transmission lines can be assessed by considering the main components (Sheng, Jiang, & Zeng, 2007). The appropriate parameters could be acquired by daily inspection, on-line monitoring, preventive tests and on-line detection. However, the same parameter from different acquisition origins can lead to different evaluation results (Chen & Xia, 2010; Wang, 2007; Yang & Hao, 2011). The absence of a reliable evaluation system could be identified as the main cause of such inadequacy (Guo & Zhang, 2011).

Figure 1. Traditional lattice steel and ‘T-shaped’ pylons (Devine-Wright & Batel, 2013).

In recent years, the application of reinforced concrete (RC) poles as supports of overhead power transmission lines has become of increased interest to engineers and the managing authorities worldwide. A great variety of architectural shapes, relatively long lifecycle and low maintenance costs, together with high electrical resistance make the RC a prominent alternative to steel. The drawbacks related to the application of RC poles include high self-weight (increases transportation and construction expenses) and damage vulnerability. The former problem could be solved by prestressing the reinforcement and/or forming a tubular structure for the supports, but it might have an opposite effect on the damage resistance. Time-dependent effects (i.e. deterioration of the concrete and corrosion of the steel reinforcement) might lead to the loss of service properties and, confronting worst-case scenario, uncertain failure of RC supports. To improve the quality of the concrete, a spun-column technology can be used. During the second half of the past century, the centrifuged concrete elements were introduced as poles of the power lines (Shalaby, Fouad, & Albanese, 2011). The structure of the centrifuged RC poles is useful for improving the public acceptance of the overhead power transmission lines, i.e. fitting pylon into rural landscapes (Cohen, Reichl, & Schmidthaler, 2014). The centrifuged concrete might also be a prominent alternative for tubular steel profiles in ‘T-shaped’ pylons (Figure 1), which represent a new design concept of the supports. It will be used in the construction of new power lines in England and Wales, which currently contain 88,000 lattice pylons (DevineWright & Batel, 2013). This study is focused on the structural behaviour of RC supports of the overhead power transmission system. The key motivation behind this study is the ageing electrical transmission infrastructure for which restoration funding is often not sufficient. The paper reports results of transient (everlasting) inspection of the existing power distribution system in Lithuania. Most of the considered poles were maintained for a period compatible with the design-life of such structural elements; selected poles were tested until failure. With reference to the collected data, the influence of identified defects on the load-bearing capacity of the supports was evaluated; an assessment scale of technical condition of RC supports has been proposed.

2. Inspection of the supports RC poles have been used in Lithuania since 1958 for high (>1 kV) and since 1961 for low voltage power lines. In 2003, the 126,900 km power system contained 2.3 million supports. In


STRUCTURE AND INFRASTRUCTURE ENGINEERING

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Table 1. Technical conditions of the overhead line supports (Kliukas et al., 2003).

2.1. Study area

Period Deficient poles, % Scheduled repairs, %

The most general features of the Lithuanian climate are related to the geographical location. To the west of Lithuania lies the Baltic Sea and the Atlantic Ocean and only small land areas: Scandinavian and Jutland peninsulas and islands. To the east of Lithuania, several thousand kilometres stretching the Eurasian continent. Although Lithuanian territory is in a coastal region, the climate is not typical for such a region. The average annual fluctuation amplitude of air temperature ranges from 19 to 20 °C (at the seaside) up to 23–24 °C (in the eastern part). The average annual number of days of crossing over 0 °C (freeze–thaw cycles) is 120. The climate can be assessed as moderately cold, with snowy winters (Leonovič, Laurinavičius, & Čygas, 2014). Precipitation is distributed mostly evenly throughout the year with an increased intensity during the warm period. The annual mean precipitation amount is 675 mm. The first snow appears on average in late October and disappears in late March. The maximum snow cover is 10 cm to 35 cm. Steady frost is formed in December and remains until April. The frost depth ranges from 80 cm to 120 cm. The coldest month average temperature is below −3 °C, and the warmest does not exceed 22 °C. For at least four months, the average temperature is higher than 10 °C. Such climate is characteristic of the Middle East of Europe. In the western part of Lithuania, the climate is defined as moderately warm: the average temperature of the coldest month is higher than −3 °C. This climate is prevalent in the Western Europe. Southwestern and southern winds are mainly dominant in the fall and winter with repeatability of 20–25%. In spring and summer, northwestern and western winds are prevailing. The average wind speed is 5.5 m/s (on the seaside). An increase of the wind speed above 15 m/s is considered as dangerous.

1975–1980 1981–1985 1986–1990 1991–1995 1996–2000 6.76 8.59 16.2 25.5 34.2 1.18

1.17

1.08

1.04

1.91

general, RC supports are used for the 0.38 kV overhead lines. The total length of these lines is 68,200 km, supported by 760,000 poles, which have been in maintenance for over 25 years (that represents a theoretical service life of the poles). The supports are mainly made of prestressed concrete. According to the technical regulations for operation of the electrical network, a 0.38–10 kV power transmission line could be repaired every 12 years (Kliukas et al., 2003). The respective values are presented in Table 1 along with the inspection results. A noticeable increase of the deficient supports after 20–30 years of the exploitation is evident. In 2001, the power transmission system in Lithuania was split into two low- and high-voltage sub-systems. The underground cables are replacing the low-voltage overhead lines. In almost all European countries, underground cables are used mainly in urban areas, as well as in the countryside, where ecological or historical sites need to be preserved. In 2003, 95% of high-voltage (220–400 kV) infrastructure of 17 European countries (that consisted of 220,000 km of power lines in total) were overhead lines (Doukas et al., 2011). In Lithuania, the high-voltage power system (that consists of 6,700 km of overhead lines, with 60% of the components older than 40 years and with an average age of the RC supports equal to 33 years) is managed by replacing the deteriorated elements. The managing company reported that on average 222 RC poles were replaced each year during 2001–2011 (LITGRID, 2013). Figure 2 depicts the structure of the replaced supports separated by age group. The technical conditions of the power distribution system in Lithuania are similar to the situation in Latvia and Estonia (Vinnal et al., 2012), Ukraine (Gorokhov et al., 2010), Serbia (Gusavac et al., 2008; Hakala & Bjelic, 2016), Poland (Jaskólski, 2016), and the Check Republic and Slovakia (Rečka & Ščasný, 2016). For instance, almost 40% of the power units in Poland are over 40 years old and more than 15% are maintained for over 50 years (Polish Information & Foreign Investment Agency, 2013). A major part of the 7,000 km of the overhead power transmission system in UK was built between 1955 and 1970; some circuits are reaching middle age (Aggarwal, Johns, Jayasinghe, & Su, 2000). 3.5% Percentage of the replaced poles 3.0% 2.5% 2.0% 1.5% 1.0% 0.5% 0.0%

Age group

10 20years 20 30 years 30 40 years 40 50 years 50 60 years

Figure 2. Percentage of the replaced RC poles within the age groups (LITGRID, 2013).

2.2. Research plan This study addresses the assessment of the quality of the existing overhead power transmission system and the identification of major problems related with long-term exploitation of RC supports. It is known that deterioration of the concrete and corrosion of the steel reinforcement might lead to the loss of service properties of RC structures. Cracking and damage of the cover concrete might accelerate the corrosion process and reduce the overall resistance of the supports. Field-testing is the most reliable way of evaluating the actual resistance of the poles. Unfortunately, such assessment results in failure of the tested elements. After replacing the destroyed support with a new one, the resultant resistance of the renewed system remains unknown. Thus, the field-test results must be extrapolated for estimating reliability of the power system. Such extrapolation, unreliable in principle, must include additional information about the technical state of the elements. Additional information can be accumulated with the help of non-destructive approaches, which might involve elaborate control procedures (Dérobert, Lataste, Balayssac, & Laurens, 2017; Guler, Yavuz, Taymuş, & Korkut, 2017, Tsioulou, Lampropoulos, & Paschalis, 2017; Zhang et al., 2016). Reliability of the non-destructive procedures is not as high as of the direct tests though it might be sufficient for development of robust predictive models (Benyahia, Ghrici, Kenai, Breysse, & Sbartai, 2017; Farrar

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Figure 3. Damages observed in RC supports of the overhead power transmission lines.

& Worden, 2007; Fiore & Marano, 2017; Goyal & Pabla, 2016; Szilágyi, Borosnyói, & Zsigovics, 2014). The evaluation of the actual technical state might be performed using elaborate prediction procedures and/or simplified engineering methodologies (often based on different evaluation scales). This study is concentrated on the simplified engineering approach. The previous study (Kliukas et al., 2003) has shown that damage and loss of the cover concrete as well as the longitudinal cracks are the main causes of structural degradation (Figure 3). Mainly, it is a result of the corrosion of the reinforcement (Portland Cement Association, 2002), which in turn, is dependent on the quality (density) of the concrete and the cover depth. Dynamic load and thawing-freezing cycles may accelerate the cover damaging process (Ožbolt, Oršanić, & Balabanić, 2017). The cover becomes incapable of resisting penetration of the aggressive ions thus activating corrosion of the reinforcement. Electrical leakage further accelerates the corrosion. The concrete structure must be as dense as possible. It allows to not only protect reinforcement from corrosion, but also to increase the freeze–thaw resistance of the concrete. Density of the concrete is closely related to the water-to-cement (W/C) ratio. The strength, density and resistance of the centrifuged poles are noticeably higher than the respective characteristics of the vibrated supports. Several factors can be mentioned in this regard. Centrifugation changes the composition and structure of the concrete matrix. Depending on the centrifugal speed, from 20 to 40% of the excess water is removed from the concrete. This process could also help to remove clay particles (found in mixtures with insufficiently clean aggregates) dispersed in the water increasing the quality of the concrete. The quality of the outer surface of the concrete increases as well. The vibrated supports often have demonstrated a high scatter of the strength assessments. Specific differences could be related with the characteristics of the bottom and the top surfaces (associated with the pouring position of the formworks). Van Mier and Van Vliet (2003), Gribniak et al. (2010), Szilágyi et al. (2014), and other researchers reported similar results. The study is based on the results gathered through visual observation and technical measurements as well as field and laboratory tests. The following devices were employed: a microscope for crack measurements (0.05 mm precision); Schmidt hammer type N; and Profometer 3 – electro-magnetic induction device for localisation of steel reinforcement and identification of its diameter and cover. Geometry parameters were investigated

using 10 m measuring tape (with 1 cm graduation), ruler with 1 mm gradation, sliding callipers (with 0.1 mm accuracy), and plumb-line (for verifying vertical position of the supports). The visual inspection was focused on the existence and orientation of the cracks, assessing the intensity of the cracking pattern, the quality of the surface and quantification of visible corrosion products, integrity of the cover concrete, localising the damaged areas, and evaluating corrosion of the naked reinforcement (i.e. identification of corrosion concentrations and, if possibly, cross-section losses). The corrosion assessment was based on the scale proposed by Kliukas and Vadlūga (1999). It separated the exploited supports into three categories: • Poles without any visible corrosion signs and damage of the cover. • Corrosion products are visible at the concrete surface; there is no significant damage of the concrete cover; a slight mechanical impact does not separate the cover, the impact sound is not changing along the concrete surface. • Evident corrosion of the naked transverse reinforcement (corrosion concentrations, corrosion-induced reduction of the cross-section area and/or separation of transverse and longitudinal bars). Only poles belonging to the first two categories were selected for field-testing. The technical conditions of the exploited supports of the third category were further monitored for assessing the degradation tendencies. The technical monitoring was aimed at determining the strength of the concrete (non-destructive testing), cover depth, the diameter of the reinforcement (using electro-magnetic induction device), geometrical properties of the poles including residual deformations and deviation from the vertical position, and crack widths. The experimental part (field-tests) is dedicated to the assessment of the load-bearing capacity of the supports. After the tests, mechanical properties of the components (concrete strength as well as strength and deformation modulus of the reinforcement) and corrosion degree of the reinforcement samples were determined in a laboratory. The latter parameter was related with the weight losses (Alcantara de, Silva de, Guimarães, & Pereira, 2015). The laboratory corrosion tests were used for verifying the absence of significant damage of the reinforcement in the poles selected in the field-testing (i.e. the supports without evident corrosion-induced surface deterioration).

STRUCTURE AND INFRASTRUCTURE ENGINEERING


2.3. Classifying the observed defects The present analysis of technical state of the existing RC supports is based on observation and test results of more than 500 poles collected since 1993–2009. During this period, the engineering inspections were carried out every three-four years. The study also involves the outcomes of the in situ tests carried out in 1979–1980 (Kliukas & Vadlūga, 1999). The investigated supports were made of vibrated or centrifuged concrete with prestressed and/or ordinary reinforcement. The cross-section shape of the poles was a rectangle (with a long periphery perpendicular to the direction of the power-line) or circle (centrifuged poles). The design service life of the supports was equal to 25 years. It is worth mentioning, however, that the poles were produced by following different design regulations. Thus, the target concrete strength of the vibrated supports varied from 20 MPa to 30 MPa, while this parameter was equal to 40–50 MPa for the centrifuged poles; the cover depth of 10–15 mm and 15–20 mm was assumed for the ordinary and prestressed reinforcement bars, respectively. Cross-section dimensions of the supports were also different. The dimensions of the cross-section at the ground level of the supports made of vibrated concrete varied from 165 × 150 mm to 277 × 185 mm; the diameter of the spun-poles varied from 300 mm to 560 mm with the wall thickness of 50–60 mm. Some of the vibrated poled were I-shaped. The characteristic damages are shown in Figure 4. Summary of the identified defects is given in Table 2 that presents the percentage of the damaged poles. The defects are summarised separately for the supports made of vibrated and centrifuged concrete. In total, 275 vibrated supports and 247 spun-poles were inspected. It can be observed that the structure of the vibrated concrete is evidently vulnerable for all identified types of the defects (with exception of the longitudinal cracks with total length lc exceeding 1 m). The appearance of the longitudinal cracks is attributed to a specific technology of the spun concrete. The defects of the poles can be categorised as follows: • Transverse cracks appeared during production (e.g. the centrifugal joint), construction, or exploitation period. • Loss of the cover concrete and large crack opening (above 0.5 mm) related with the corrosion of the transverse reinforcement. • Longitudinal cracks appeared during the production or exploitation period.

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• The washing out of the cement in the centrifugal longitudinal joint and the consequential corrosion of the spiral (transverse) reinforcement. • Insufficient width of the hollow section concrete wall. • Corrosion of the reinforcement due to insufficient cover caused by production inaccuracies, construction or exploitation accidents. • Although the average strength of the concrete fulfilled design requirements, the assessment of lower capacity specimens gave very low values compared to the design strength. • Damage of the hollow section wall caused by the corrosion of the reinforcement. • Damage of the compressive concrete at the crossbar location that induced corrosion of the reinforcement. • Welding of the main reinforcement in the critical section (near the support). • Unacceptable sparse distribution (or absence) of the transverse reinforcement. • High residual deformations of the crossbars (due to an extreme load). • Deviation from the vertical position. The main sources of the aforementioned damages are the following: • Degradation of the concrete and corrosion of reinforcement. • Insufficient cover concrete. • Insufficient resistance of the concrete to the moisture ingress (low density, high absorption and water permeability). • Imperfect thermal conditions (during production). • Incorrect concrete mixture (high W/C ratio, low cement content, etc.). • Aggressive environmental conditions. • Accidental effects. • Brittle failure due to corrosion (characteristic of highgrade steel). In 1999, a registered collapse of a RC support was associated with corrosion cracking of one of the four prestressed reinforcement bars made of thermally enhanced steel. It was a result of the stress concentration caused by corrosion. While this effect is of marginal importance for mild steel, this process often leads to a dangerous outcome (brittle failure) for elements with high-grade steel reinforcement. Thermal enhancing/hardening increases brittleness of the steel.

Figure 4. Defects characteristic of the vibrated supports: (a) corrosion of the reinforcement; (b) the corrosion-induced spalling of the concrete cover; (c) longitudinal cracking; (d) transversal cracking; and (e) improper position of the reinforcement.

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Table 2. Summary of the defects identified during the engineering inspection.


Support type No 1

Type of the defect Cover depth, c, does not match the design value, cd1

2

Cover concrete is continuously lost in the zone of the length lc

3

The total length of longitudinal cracks is equal to lw

4 5

Transverse cracks Corrosion of the reinforcement

6 7 8

The concrete strength is below the design value Cross-section geometry does not match design requirements Type and/or diameter of bar reinforcement do not match design requirements

Characteristic parameter c = (0.75...0.99)⋅cd c = (0.50...0.75)⋅cd c