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Rehabilitation of manhole covers
Pergamon PII:
Trenchless Technol. Res., Vol. 14, No. 2, pp. 39--46, 1999 © 2000 Institute for Underground Infrastructure. Published by Elsevier Science Ltd All rights reserved Printed in Great Britain S0886-7798(00)00018-3 0886-7798/00/$ - see front matter
www.el sevier.com/locate/ttr
Rehabilitation of m a n h o l e covers C h r i s t i a n Falk Institut fiir Kanalisationstechnik (IKT), Exterbruch 1, 45886 Gelsenkirchen, Germany
The,,rehabilitation of manhole top sections forms part of drain maintenance work. In order to adjust and replace manhole top sections, considerable efforts are more necessary than ever because, in comparison with former times, especially high demands are placed on the evenness of the road surface which, as a result of the heavy traffic, is subjected to increased strains and stresses. This paper shows the restdts of a research programme. The target of the project was to evaluate substantial examinationsof different rehabilitation methods of manhole covers. The large number of damages to manhole covers shows that there are weak points in rehabilitation methods. To find these weak points was an additional point of the research programme. The examinations demonstrated the principal failures of a lot of rehabilitation methods. On the whole the mistakes were found to be the use of inappropriate materials and the methods used on site. Recommendations are made for improvement of the rehabilitation methods and to heighten the duration of the use of rehabilitated manhole covers. © 2000 Institute for Underground Infrastructure. Published by Elsevier Science Ltd. All rights reserved
1 INTRODUCTION
and drawing up suggestions for modifying them or producing new developments. Details of the research programme and the results achieved are given in this paper.
Several methods of rehabilitating manhole top sections exist; practical experience has shown that there are often considerable differences with regard to the service life of manhole top sections which have been rehabilitated using these different methods. Depending on the amount of wear and tear caused by traffic, rehabilitated manhole top sections frequently need to be rehabilitated again after as short a period as 1-2 years. ! A comparison and an evaluation of methods of rehabilitation in respect of the service life which can be achieved have so far not been produced; it was not possible to make quantitative statements about failure mechanisms and the suitability of the components and materials used. The Ministry for the Environment, Spatial Planning and Agriculture (MURL) in North-Rhine Westphalia commissioned the "Institut ftir Kanalisationstechnik" (the Institute for Underground Infrastructure) to carry out a research project involving comparing and evaluating existing methods of rehabilitation for manhole top sections
2 STATE OF TECHNOLOGY
2.1 General DIN EN 1242 defines a manhole top section as being the "upper part of a manhole or other area, consisting of a frame and top section and/or grid ..." (see Fig. 1). The following damage occurs especially frequently: (1) cracking of the top section, grid or frame; (2) wear to the frame and/or lid, in particular on the seat between the top section and the frame; (3) deviation of the position of the frame of the manhole top section; and (4) variations in the amount of settling between the roadway or pavement and the manhole top section. The most frequent cause of the two last-named 39
40
C. Falk J
J l
/
-2
5
1 2 3 4 5
Manhole top section Manhole frame Support ring Mortar joint Manhole collar
Fig. 1. Section through a manhole top section according to DIN EN 1241 or DIN 19549. 3
types of damage is the failure of the mortar joint between the frame of the manhole top section and the manhole collar, or possibly between the existing support rings as a result of a combination of dynamic loads caused by traffic passing over them and chemical/physical stresses caused, for example, by: • • • •
the effects of salt as used for thawing ice; variations in temperature; variations in moisture; stress caused by repeated frost and thawing.
Additional causes include the effects of atmospheric influences (temperature, rain) during installation or rehabilitation work, and construction faults. 1 The way of dealing with the first two or similar types of damage is by renewing the relevant parts; as a rule, the top section and the frame are replaced together. In all other or similar cases it is necessary to adjust the height of the manhole top section, unless the entire manhole has sunk and needs, for this reason, to be renewed or adjusted. In adjusting the height it is necessary to distinguish between the two different processes, i.e. with and without the removal of the relevant manhole top section.
Raising the level is carried out by underlaying the raised frame with a low-shrinkage, rapid-hardening special mortar based on cement or thermosetting resin, or with the aid of underlay wedges made of polymer concrete. With both methods, the total height between the manhole collar and the frame must not, according to ATV-A 241, 7 exceed 240 mm. To lower the level, a joint underneath the frame of the manhole top section is created by means of a mortiser. Mortising is possible in mortar, sewer bricks and concrete and reinforced concrete with low reinforcement levels, s The gap which remains after the frame has been lowered is then, as described above, filled with special mortar, or underlaying is carried out using polymer concrete wedges.
2.3 Height adjustment of manhole top sections with removal Height adjustment of manhole top sections with removal is always carried out when the surrounding surfacing has also been damaged by cracking, deformation etc. (see Fig. 2). Normally repair is carried out by the removal of the surrounding surface and sub-base and the damaged manhole top section, including any support rings where appropriate, and the subsequent refitting at the required level. Subsequently the surrounding sub-base and surface is restored (Fig. 3). A distinction is made between a circular repaving of the frame with bituminous joint sealant or the restoration of the bituminous surfacing tight up to the manhole top section.
2.2 Height adjustment of manhole top sections without removal With this method of rehabilitation, both the manhole top section and the surrounding frame top section and sub-bases are retained. The work only involves raising or lowering the frame after it has been loosened by means of special mechanical or hydraulic machinery.4-6
Fig. 2. Cracks in the road surface as a result of damage to a manhole top section (City of DUsseldorf).
Rehabilitation of manhole covers
Fig. 3.
Height adjustment of manhole top sections with removal (City of DUsseldorf).
3 C O M P A R I S O N AND E V A L U A T I O N OF METHODS OF R E H A B I L I T A T I O N 3.1 Research
programme
In a comprehensive research programme, methods of rehabilitation were evaluated both in situ and in the laboratory with regard to their suitability for the permanent securing of the functioning of manhole top sections, i.e. the calculation of dynamic stresses caused by road traffic to the upper road surface or the manhole construction and their resistance to chemical/physical stresses. The tests concentrated on the area which connects the frame of the manhole top section to the manhole collar because, it is here that, as explained above, damage mainly occurs in practice. The following series of tests were carried out in the laboratory. • Simulation of dynamic traffic loads on manhole top sections (scale: 1:1) by a servohydraulic actuator (Fig. 4) (1) vertical load 400 kN (2) horizontal load 240 kN (caused by braking) (3) loading frequency 2-7 Hz (4) 1.000.000 load sequences (5) temperature 20°C • Stress caused by alternating frost and thawing according to Ref. 9 • Stress caused by salt used to thaw frost according to Ref. 9 • Compression resistance (static) (cf. Refs 10,11) • Dynamic strength tests (1) elastomer load transmission rings - - stress-amplitude o- = 0.6 N/mm 2
41
Fig. 4. Introduction of vertical and horizontal dynamic traffic loads onto a manhole top section by means of a fixed impulse generator (scale: 1:1).
- - loading frequency 10 Hz - - 720.000 load sequences - - temperature 20°C (2) cement mortar prisms - - stress-amplitude (start: 10% of the static strength; increase of the static strength by degrees of 10% at a time until breakage) In the in-situ tests, manhole top sections were rehabilitated in position. During the rehabilitation process, and after the rehabilitation had been completed, samples of the materials used were taken and examined in the laboratory. At the same time technical monitoring of possible damage to the manhole top section was carried out in the form of measurements of horizontal and/or vertical displacement.
3.2 Results
The bearing capacity tests carried out in the laboratory showed that, without exception, all of the reconditioning techniques were suited to enable the manhole covers to bear dynamic traffic loads from heavy-duty truck traffic and to transfer them into the manhole structure below, provided that the reconditioning works had been executed skilfully. No damage was observed. On the other hand, in-situ tests and laboratory experiments aimed at determining the resistance to chemical/physical strain of the employed materials revealed that, in many cases, their resistance to alternations between frost and thaw on account of
C. FaIk
42
those that have been produced and stored under laboratory conditions are comparatively represented. It became apparent that those test pieces that had been taken from the reconditioned manhole covers achieved by far the worst result, which allows the conclusion of detrimental effects during the execution of the construction works. It is not possible to directly transfer the results to the behaviour of reconditioned manhole covers or mortar joints in situ, as the storage conditions and the temperatures and frost-thaw alternations in situ significantly vary from the test conditions9 that the materials were exposed to during the tests in question. Yet, the suitability of mortars that present considerable deterioration after only a few frostthaw alternations--which showed during the tests--for employment in the reconditioning of manhole covers must be doubted. As shown above, the cause is not only the mortar recipe but, in many cases, also the processing of the mortar and the placing procedure in situ. In-situ tests that have been carried out with elastomer levelling rings, employed instead of the mortar joint customarily used, resulted in the fact that, during the test period in question, these presented only minor deformations, which decreased asymptotically in the course of time and, thus, were able
the application of unsuited recipes and/or their unskilled handling on site was insufficient. Figs 5 and 6 present the results of the tests as to frost-thaw alternations. Test pieces of the mortar compounds utilized, which were stored in water and a 3% NaC1 solution, were exposed to repeated frost-thaw alternations. The figures show the losses in volume of the test pieces owing to chipping-off caused by frost in proportion to the increasing number of frost-thaw alternations. Fig. 5 clearly reveals that all of the test pieces examined presented considerable deterioration, that is to say losses in volume between approximately 8 to 30%. Such damages inevitably result in a decrease of operability leading to a settling of the manhole cover. All of the test pieces examined had been taken from manhole covers that had been reconditioned in situ, after the reconditioning works had been terminated. The comparison of different test pieces from one single reconditioning procedure proved that not only the recipe of the mortar but also the method of placing and local boundary conditions had a considerable effect on the behaviour of the material. In Fig. 6, the results regarding the test pieces which have been taken during the reconditioning works, those that have been taken from the reconditioned manhole cover on the day of the test, and
Volume change [%] Frost- thaw altemation [-]
0 ~........ 14
28
-10
", ................. ~......
-, ....
....~.. Adjusting mortar system 1 (RM 1) (Mean value from 4 test pieces)
-20
\\
--o-Viscous mortar system 1 (FM 1) (Mean value from 2 test pieces)
,.
.~,-~.
-30
Viscous mortar system 2 (FM 2) (Mean value from 2 test pieces)
\\ \
.....~
Deteriorated mortar (Mean value from 6 test
-40
\
\
pieces)
.\ \
\
\
-50
Remarks: 1 test piece of viscous mortar system 1 (FM 1) \ \ \
disintegrated after 14 frost-thaw alternations 1 test piece of the viscous mortar system 2 (FM 2) disintegrated after 14 frost-thaw alternations 1 test piece of the deteriorated mortar disintegrated after 14 frost-thaw altemations
-60
-70
.......................................................................................................................................................................
Test piece from the reconditioned manhole cover taken on the day of the rehabilitation Fig. 5. Results of the tests on frost-thaw alternations - - comparison of different mortar compounds.
Rehabilitation of manhole covers
43
Volume change[%] 10
0
;~~'"~'~"-,~,,,,~ 7
~ - J - - _ -10
14 ~
" ~ .............
:6] Frost-___tthaw-altemation[-]
~
......... ~........
....
I
.,,,~..,. ~ , - . . . ~
'................
~ "~-~.~..~.~,
~
Test piece produced during reconditioning as a prism, storage in the manhole (Mean value from 3 test pieces)
-20
Test piece from the reconditioned manhole cover, taken on the day of the test (Mean value from 2 test pieces)
N
-30
\
\ \ \o,
-40
\
- ~ - Production and storage of the test pieces in the laboratory (Mean value from 2 test pieces)
\\ \
-50
\ \
\
N \
\
-60
-70
Remarks: 1 test piece from the reconditioned manhole cover disintegratedafter 14 frost-thaw alternations
\ "e
............................................................................................................................................
Viscous mortar system 2 (FM 2), Manhole W 4
Fig. 6. Results of the tests on frost-thaw alternations - - comparison of test pieces that have been prepared and stored in different ways.
to withstand the strain from traffic as well as biological and chemical/physical stresses (cf. Figs 7 and 8).
4 NEW CONCEPTS FOR THE R E H A B I L I T A T I N G OF M A N H O L E TOP SECTIONS 4.1 Improvement of traditional manhole arrangements On account of the weak points of customary mortars utilized to bed the frame of the manhole cover, resulting from their unsuited recipes and/or unskilled handling in situ, the developments described and discussed below aim to replace these mortar joints. One of the possible solutions would be the employment of elastomer levelling rings. The tests revealed that these are able to bear dynamic traffic loads on condition that suited materials and degree of hardness are chosen (cf. Fig. 8). Their considerably reduced modulus of elasticity in comparison with standard types of mortar means that peaks in the dynamic stresses on manhole top sections and the manhole can be reduced. Other methods for the re-laying and rehabilitation of manhole top sections, in the case of height
adjustment coupled with removal, aim to reduce the load transmitted through the frame to the support rings and manhole cover by distributing these loads to the adjacent sub-base of the road construction. These systems differ • in the structural design of the load distribution, and • in the size of the ground area of the sub-base used for load distribution. Examples of this are the replacement of compensating rings and corresponding intermediate joint layers by a mix-in-situ concrete or asphalt concrete sub-base between the manhole collar and the road surface layer. The manhole top section or the frame lie on this sub-base by gravitational force (Fig. 9). The newly developed manhole top section represents a further variant within the framework of the above-mentioned research project. 12 This is a special height-adjustable frame in connection with a steel spring element. This manhole top section can adapt itself to differing cambers or heights of the upper surface of the road; there is no need for support rings and mortar joints for height compensation. The steel spring element reduces the peak of the dynamic traffic loads and in consequence the stress placed on the manhole top section and the manhole. Its damping characteristics are adjustable (Fig. 10).
44
C. F a l k Mean settlement [ram[ 1,20
I1•
Arrangement of the reference marks (Top view) Traffic direction
1,00
0,80
..y
f
J
0,60 Manhole structure 0,40
0,20
e. Elastomer levelling rings Manhole W9 II Elastomer levelling rings Manhole W8
0,0C
50
100
150
200
250
Time [days]
Fig. 7. Time controlled deformation behaviour of elastomer load transmission rings placed in situ. 0,400 --WA --WA
1 Max 1 Min
0,~so _, . . . . L .,-..~--
~ ....... ~,,.~r.;.=..L~.;,.
/
0,300
"~'!
I
Positionofthedlsplaem'~nt
measur~llentInstNlmen~ion
0,250
I
E
horizontalload
0 200
0,150
~sha~ 0,100 500
1000
1500
2000
2500
3000
3500
time [rain]
Fig. 8. Pressure on a rehabilitated manhole top section with a dynamic vertical load - - asymptotic decay of the plastic vertical deformations up to a load of 5 × 105 load alternation.1
4.2 Separation of cover and manhole The new conception of a manhole cover is based on the principle of separating the cover from the manhole with regard to load transfer, the traffic loads thus being no longer directly transferred to the manhole. Accordingly, the frame of the manhole cover will be connected w i t h the taper shaft ring by a short pipe, which serves for sealing this zone and which is integrated within the manhole in
such a way as to prevent vertical forces from being transferred (Fig. 11). This type of manhole cover hence differs in principle from the manhole cover according to DIN EN 1242 and DIN 19549. 3 Thus, the weakest point of customary manhole covers as far as load transfer between cover and manhole is concerned has been eliminated by this new conception. The developments presented will yet have to furnish proof of their suitability in situ over a pro-
Rehabilitation of manhole covers Manhole top section !
45
longed period of several years. The test phase of 1 year effected in the course of the research project was too short to allow assessment of the long-term behaviour with sufficient certainty.
5 S U M M A R Y AND O U T L O O K With the tests presented here, for the first time an evaluation of the methods of rehabilitation of manhole top sections is made available. The limitations to their use, possible mistakes in carrying out the rehabilitation, and the causes of the often very limited period of use of rehabilitated manhole top sections of only a few years, were ascertained. The principal causes of premature failure were found to be the freeze-thaw performance of the mortar, and methods used on site for the mortar processing and laying. Alternative solutions were developed which, as far as possible, avoid the above-mentioned limitations to the installation or rehabilitation methods. These included
Fig. 9. Underlaying of the frame of the manhole top section with poured asphalt.
m a n h o l e toD section
• elastomer rings; • distribution of loads directly to the sub-base; • a newly developed height-adjustable frame incorporating steel spring elements; and • a new concept of separating the cover and the manhole.
steel spring e l e m e n t
However, proof of their long-term suitability has still to be provided by means of an in-situ test programme over several years. For the first time the studies showed considerable differences in the different methods of rehabilitation for manhole top sections, and form the basis for the selection of suitable methods of rehabilitation from economic and technical considerations,
m a n h o l e collar
Fig. 10. New conception of a manhole cover: a combination of a height adjustable manhole cover (PASSAVANT, Aarbergen) with a steel spring element to cushion the dynamic traffic loads (sketch).
manholeframe ~ ~ ~ ~ ~ ~ ] ~ j c o n c m t e
climbing iron is
I~ I
I
I
ring or reinforcedconcretering
~
I ruing
Fig. 11. New conception of a manhole cover: separation of cover and shaft (sketch).
46
C. Falk
and therefore the economical rehabilitation of this important element of the sewerage networks.
REFERENCES 1. Stein, D., Falk, C. and Liebscher, M., Abschlul3bericht Forschungsvorhaben--Sanierung schadhafter Schachtabdeckungen. Institut ftir Kanalisationstechnik (IKT), November 1995, unver6ffentlicht. 2. DIN EN 124, Aufstitze und Abdeckungen fiir Verkehrsfltichen, Baugrundsatze, PrUfungen, Kennzeichnungen, Gtitetiberwachung, August 1994. 3. DIN 19549, Schtichte fiir erdverlegte Abwasserkantile und -leitungen, Allgemeine Anforderungen und Priifungen, February 1989. 4. Existing Sewer Evaluation and Rehabilitation, ASCE Manuals on Engineering Practice No./WPCF Manual of Practice FD-6, ASCE and WPCF, New York and Washington, 1983. 5. Inspector Handbook for Sewer Collection System Rehabilitation, The National Association of Sewer Services Companies (NASSCO), Winter Park, FL, 1985.
6. Operation and Maintenance of Wastewater Collection Systems. Manual of Practice No. 7, Water Pollution Control Federation, Washington, 1985. 7. ATV A 241, Bauwerke der Ortsentwtisserung, Empfehlungen und Hinweise, March 1994. 8. N. N. Firmeninformation, W. Loos, R6thenbach. 9. Deutscher Ausschuss fur Stahlbeton, Priifung von Beton, Empfehlungen und Hinweise als Ergtinzung zur DIN 1048, erarbeitet vom Arbeitsausschuss DIN 1048, zusammengestellt von Norbert Bunke, Heft 422, Beutb Verlag GmbH, Berlin, 1991. 10. DIN 196 Teil 1, Priifverfahren f~r Zement, Bestimmung der Festigkeit, March 1990. 11. DIN 18555 Teil 3, Priifung von MOrteln mit Mineralischen Bindemitteln, FestmOrtel, Bestimmung der Biegefestigkeit, Druckfestigkeit und Rohdichte, September 1982. 12. Stein, D. and Falk, C., Abschlufibericht Forschungsvorhaben: Einsteigschtichte fiir Abwasserkantile--Entwicklung eines neuen Bau- und Sanierungsverfahrens fiir die Verbindung des Rahmens der Schachtabdeckung zum Schachthals. Institut fur Kanalisationstechnik (IKT), January 1998, unver6ffentlicht.
Trenchless Technol. Res., Vol. 14, No. 2, pp. 39--46, 1999 © 2000 Institute for Underground Infrastructure. Published by Elsevier Science Ltd All rights reserved Printed in Great Britain S0886-7798(00)00018-3 0886-7798/00/$ - see front matter
www.el sevier.com/locate/ttr
Rehabilitation of m a n h o l e covers C h r i s t i a n Falk Institut fiir Kanalisationstechnik (IKT), Exterbruch 1, 45886 Gelsenkirchen, Germany
The,,rehabilitation of manhole top sections forms part of drain maintenance work. In order to adjust and replace manhole top sections, considerable efforts are more necessary than ever because, in comparison with former times, especially high demands are placed on the evenness of the road surface which, as a result of the heavy traffic, is subjected to increased strains and stresses. This paper shows the restdts of a research programme. The target of the project was to evaluate substantial examinationsof different rehabilitation methods of manhole covers. The large number of damages to manhole covers shows that there are weak points in rehabilitation methods. To find these weak points was an additional point of the research programme. The examinations demonstrated the principal failures of a lot of rehabilitation methods. On the whole the mistakes were found to be the use of inappropriate materials and the methods used on site. Recommendations are made for improvement of the rehabilitation methods and to heighten the duration of the use of rehabilitated manhole covers. © 2000 Institute for Underground Infrastructure. Published by Elsevier Science Ltd. All rights reserved
1 INTRODUCTION
and drawing up suggestions for modifying them or producing new developments. Details of the research programme and the results achieved are given in this paper.
Several methods of rehabilitating manhole top sections exist; practical experience has shown that there are often considerable differences with regard to the service life of manhole top sections which have been rehabilitated using these different methods. Depending on the amount of wear and tear caused by traffic, rehabilitated manhole top sections frequently need to be rehabilitated again after as short a period as 1-2 years. ! A comparison and an evaluation of methods of rehabilitation in respect of the service life which can be achieved have so far not been produced; it was not possible to make quantitative statements about failure mechanisms and the suitability of the components and materials used. The Ministry for the Environment, Spatial Planning and Agriculture (MURL) in North-Rhine Westphalia commissioned the "Institut ftir Kanalisationstechnik" (the Institute for Underground Infrastructure) to carry out a research project involving comparing and evaluating existing methods of rehabilitation for manhole top sections
2 STATE OF TECHNOLOGY
2.1 General DIN EN 1242 defines a manhole top section as being the "upper part of a manhole or other area, consisting of a frame and top section and/or grid ..." (see Fig. 1). The following damage occurs especially frequently: (1) cracking of the top section, grid or frame; (2) wear to the frame and/or lid, in particular on the seat between the top section and the frame; (3) deviation of the position of the frame of the manhole top section; and (4) variations in the amount of settling between the roadway or pavement and the manhole top section. The most frequent cause of the two last-named 39
40
C. Falk J
J l
/
-2
5
1 2 3 4 5
Manhole top section Manhole frame Support ring Mortar joint Manhole collar
Fig. 1. Section through a manhole top section according to DIN EN 1241 or DIN 19549. 3
types of damage is the failure of the mortar joint between the frame of the manhole top section and the manhole collar, or possibly between the existing support rings as a result of a combination of dynamic loads caused by traffic passing over them and chemical/physical stresses caused, for example, by: • • • •
the effects of salt as used for thawing ice; variations in temperature; variations in moisture; stress caused by repeated frost and thawing.
Additional causes include the effects of atmospheric influences (temperature, rain) during installation or rehabilitation work, and construction faults. 1 The way of dealing with the first two or similar types of damage is by renewing the relevant parts; as a rule, the top section and the frame are replaced together. In all other or similar cases it is necessary to adjust the height of the manhole top section, unless the entire manhole has sunk and needs, for this reason, to be renewed or adjusted. In adjusting the height it is necessary to distinguish between the two different processes, i.e. with and without the removal of the relevant manhole top section.
Raising the level is carried out by underlaying the raised frame with a low-shrinkage, rapid-hardening special mortar based on cement or thermosetting resin, or with the aid of underlay wedges made of polymer concrete. With both methods, the total height between the manhole collar and the frame must not, according to ATV-A 241, 7 exceed 240 mm. To lower the level, a joint underneath the frame of the manhole top section is created by means of a mortiser. Mortising is possible in mortar, sewer bricks and concrete and reinforced concrete with low reinforcement levels, s The gap which remains after the frame has been lowered is then, as described above, filled with special mortar, or underlaying is carried out using polymer concrete wedges.
2.3 Height adjustment of manhole top sections with removal Height adjustment of manhole top sections with removal is always carried out when the surrounding surfacing has also been damaged by cracking, deformation etc. (see Fig. 2). Normally repair is carried out by the removal of the surrounding surface and sub-base and the damaged manhole top section, including any support rings where appropriate, and the subsequent refitting at the required level. Subsequently the surrounding sub-base and surface is restored (Fig. 3). A distinction is made between a circular repaving of the frame with bituminous joint sealant or the restoration of the bituminous surfacing tight up to the manhole top section.
2.2 Height adjustment of manhole top sections without removal With this method of rehabilitation, both the manhole top section and the surrounding frame top section and sub-bases are retained. The work only involves raising or lowering the frame after it has been loosened by means of special mechanical or hydraulic machinery.4-6
Fig. 2. Cracks in the road surface as a result of damage to a manhole top section (City of DUsseldorf).
Rehabilitation of manhole covers
Fig. 3.
Height adjustment of manhole top sections with removal (City of DUsseldorf).
3 C O M P A R I S O N AND E V A L U A T I O N OF METHODS OF R E H A B I L I T A T I O N 3.1 Research
programme
In a comprehensive research programme, methods of rehabilitation were evaluated both in situ and in the laboratory with regard to their suitability for the permanent securing of the functioning of manhole top sections, i.e. the calculation of dynamic stresses caused by road traffic to the upper road surface or the manhole construction and their resistance to chemical/physical stresses. The tests concentrated on the area which connects the frame of the manhole top section to the manhole collar because, it is here that, as explained above, damage mainly occurs in practice. The following series of tests were carried out in the laboratory. • Simulation of dynamic traffic loads on manhole top sections (scale: 1:1) by a servohydraulic actuator (Fig. 4) (1) vertical load 400 kN (2) horizontal load 240 kN (caused by braking) (3) loading frequency 2-7 Hz (4) 1.000.000 load sequences (5) temperature 20°C • Stress caused by alternating frost and thawing according to Ref. 9 • Stress caused by salt used to thaw frost according to Ref. 9 • Compression resistance (static) (cf. Refs 10,11) • Dynamic strength tests (1) elastomer load transmission rings - - stress-amplitude o- = 0.6 N/mm 2
41
Fig. 4. Introduction of vertical and horizontal dynamic traffic loads onto a manhole top section by means of a fixed impulse generator (scale: 1:1).
- - loading frequency 10 Hz - - 720.000 load sequences - - temperature 20°C (2) cement mortar prisms - - stress-amplitude (start: 10% of the static strength; increase of the static strength by degrees of 10% at a time until breakage) In the in-situ tests, manhole top sections were rehabilitated in position. During the rehabilitation process, and after the rehabilitation had been completed, samples of the materials used were taken and examined in the laboratory. At the same time technical monitoring of possible damage to the manhole top section was carried out in the form of measurements of horizontal and/or vertical displacement.
3.2 Results
The bearing capacity tests carried out in the laboratory showed that, without exception, all of the reconditioning techniques were suited to enable the manhole covers to bear dynamic traffic loads from heavy-duty truck traffic and to transfer them into the manhole structure below, provided that the reconditioning works had been executed skilfully. No damage was observed. On the other hand, in-situ tests and laboratory experiments aimed at determining the resistance to chemical/physical strain of the employed materials revealed that, in many cases, their resistance to alternations between frost and thaw on account of
C. FaIk
42
those that have been produced and stored under laboratory conditions are comparatively represented. It became apparent that those test pieces that had been taken from the reconditioned manhole covers achieved by far the worst result, which allows the conclusion of detrimental effects during the execution of the construction works. It is not possible to directly transfer the results to the behaviour of reconditioned manhole covers or mortar joints in situ, as the storage conditions and the temperatures and frost-thaw alternations in situ significantly vary from the test conditions9 that the materials were exposed to during the tests in question. Yet, the suitability of mortars that present considerable deterioration after only a few frostthaw alternations--which showed during the tests--for employment in the reconditioning of manhole covers must be doubted. As shown above, the cause is not only the mortar recipe but, in many cases, also the processing of the mortar and the placing procedure in situ. In-situ tests that have been carried out with elastomer levelling rings, employed instead of the mortar joint customarily used, resulted in the fact that, during the test period in question, these presented only minor deformations, which decreased asymptotically in the course of time and, thus, were able
the application of unsuited recipes and/or their unskilled handling on site was insufficient. Figs 5 and 6 present the results of the tests as to frost-thaw alternations. Test pieces of the mortar compounds utilized, which were stored in water and a 3% NaC1 solution, were exposed to repeated frost-thaw alternations. The figures show the losses in volume of the test pieces owing to chipping-off caused by frost in proportion to the increasing number of frost-thaw alternations. Fig. 5 clearly reveals that all of the test pieces examined presented considerable deterioration, that is to say losses in volume between approximately 8 to 30%. Such damages inevitably result in a decrease of operability leading to a settling of the manhole cover. All of the test pieces examined had been taken from manhole covers that had been reconditioned in situ, after the reconditioning works had been terminated. The comparison of different test pieces from one single reconditioning procedure proved that not only the recipe of the mortar but also the method of placing and local boundary conditions had a considerable effect on the behaviour of the material. In Fig. 6, the results regarding the test pieces which have been taken during the reconditioning works, those that have been taken from the reconditioned manhole cover on the day of the test, and
Volume change [%] Frost- thaw altemation [-]
0 ~........ 14
28
-10
", ................. ~......
-, ....
....~.. Adjusting mortar system 1 (RM 1) (Mean value from 4 test pieces)
-20
\\
--o-Viscous mortar system 1 (FM 1) (Mean value from 2 test pieces)
,.
.~,-~.
-30
Viscous mortar system 2 (FM 2) (Mean value from 2 test pieces)
\\ \
.....~
Deteriorated mortar (Mean value from 6 test
-40
\
\
pieces)
.\ \
\
\
-50
Remarks: 1 test piece of viscous mortar system 1 (FM 1) \ \ \
disintegrated after 14 frost-thaw alternations 1 test piece of the viscous mortar system 2 (FM 2) disintegrated after 14 frost-thaw alternations 1 test piece of the deteriorated mortar disintegrated after 14 frost-thaw altemations
-60
-70
.......................................................................................................................................................................
Test piece from the reconditioned manhole cover taken on the day of the rehabilitation Fig. 5. Results of the tests on frost-thaw alternations - - comparison of different mortar compounds.
Rehabilitation of manhole covers
43
Volume change[%] 10
0
;~~'"~'~"-,~,,,,~ 7
~ - J - - _ -10
14 ~
" ~ .............
:6] Frost-___tthaw-altemation[-]
~
......... ~........
....
I
.,,,~..,. ~ , - . . . ~
'................
~ "~-~.~..~.~,
~
Test piece produced during reconditioning as a prism, storage in the manhole (Mean value from 3 test pieces)
-20
Test piece from the reconditioned manhole cover, taken on the day of the test (Mean value from 2 test pieces)
N
-30
\
\ \ \o,
-40
\
- ~ - Production and storage of the test pieces in the laboratory (Mean value from 2 test pieces)
\\ \
-50
\ \
\
N \
\
-60
-70
Remarks: 1 test piece from the reconditioned manhole cover disintegratedafter 14 frost-thaw alternations
\ "e
............................................................................................................................................
Viscous mortar system 2 (FM 2), Manhole W 4
Fig. 6. Results of the tests on frost-thaw alternations - - comparison of test pieces that have been prepared and stored in different ways.
to withstand the strain from traffic as well as biological and chemical/physical stresses (cf. Figs 7 and 8).
4 NEW CONCEPTS FOR THE R E H A B I L I T A T I N G OF M A N H O L E TOP SECTIONS 4.1 Improvement of traditional manhole arrangements On account of the weak points of customary mortars utilized to bed the frame of the manhole cover, resulting from their unsuited recipes and/or unskilled handling in situ, the developments described and discussed below aim to replace these mortar joints. One of the possible solutions would be the employment of elastomer levelling rings. The tests revealed that these are able to bear dynamic traffic loads on condition that suited materials and degree of hardness are chosen (cf. Fig. 8). Their considerably reduced modulus of elasticity in comparison with standard types of mortar means that peaks in the dynamic stresses on manhole top sections and the manhole can be reduced. Other methods for the re-laying and rehabilitation of manhole top sections, in the case of height
adjustment coupled with removal, aim to reduce the load transmitted through the frame to the support rings and manhole cover by distributing these loads to the adjacent sub-base of the road construction. These systems differ • in the structural design of the load distribution, and • in the size of the ground area of the sub-base used for load distribution. Examples of this are the replacement of compensating rings and corresponding intermediate joint layers by a mix-in-situ concrete or asphalt concrete sub-base between the manhole collar and the road surface layer. The manhole top section or the frame lie on this sub-base by gravitational force (Fig. 9). The newly developed manhole top section represents a further variant within the framework of the above-mentioned research project. 12 This is a special height-adjustable frame in connection with a steel spring element. This manhole top section can adapt itself to differing cambers or heights of the upper surface of the road; there is no need for support rings and mortar joints for height compensation. The steel spring element reduces the peak of the dynamic traffic loads and in consequence the stress placed on the manhole top section and the manhole. Its damping characteristics are adjustable (Fig. 10).
44
C. F a l k Mean settlement [ram[ 1,20
I1•
Arrangement of the reference marks (Top view) Traffic direction
1,00
0,80
..y
f
J
0,60 Manhole structure 0,40
0,20
e. Elastomer levelling rings Manhole W9 II Elastomer levelling rings Manhole W8
0,0C
50
100
150
200
250
Time [days]
Fig. 7. Time controlled deformation behaviour of elastomer load transmission rings placed in situ. 0,400 --WA --WA
1 Max 1 Min
0,~so _, . . . . L .,-..~--
~ ....... ~,,.~r.;.=..L~.;,.
/
0,300
"~'!
I
Positionofthedlsplaem'~nt
measur~llentInstNlmen~ion
0,250
I
E
horizontalload
0 200
0,150
~sha~ 0,100 500
1000
1500
2000
2500
3000
3500
time [rain]
Fig. 8. Pressure on a rehabilitated manhole top section with a dynamic vertical load - - asymptotic decay of the plastic vertical deformations up to a load of 5 × 105 load alternation.1
4.2 Separation of cover and manhole The new conception of a manhole cover is based on the principle of separating the cover from the manhole with regard to load transfer, the traffic loads thus being no longer directly transferred to the manhole. Accordingly, the frame of the manhole cover will be connected w i t h the taper shaft ring by a short pipe, which serves for sealing this zone and which is integrated within the manhole in
such a way as to prevent vertical forces from being transferred (Fig. 11). This type of manhole cover hence differs in principle from the manhole cover according to DIN EN 1242 and DIN 19549. 3 Thus, the weakest point of customary manhole covers as far as load transfer between cover and manhole is concerned has been eliminated by this new conception. The developments presented will yet have to furnish proof of their suitability in situ over a pro-
Rehabilitation of manhole covers Manhole top section !
45
longed period of several years. The test phase of 1 year effected in the course of the research project was too short to allow assessment of the long-term behaviour with sufficient certainty.
5 S U M M A R Y AND O U T L O O K With the tests presented here, for the first time an evaluation of the methods of rehabilitation of manhole top sections is made available. The limitations to their use, possible mistakes in carrying out the rehabilitation, and the causes of the often very limited period of use of rehabilitated manhole top sections of only a few years, were ascertained. The principal causes of premature failure were found to be the freeze-thaw performance of the mortar, and methods used on site for the mortar processing and laying. Alternative solutions were developed which, as far as possible, avoid the above-mentioned limitations to the installation or rehabilitation methods. These included
Fig. 9. Underlaying of the frame of the manhole top section with poured asphalt.
m a n h o l e toD section
• elastomer rings; • distribution of loads directly to the sub-base; • a newly developed height-adjustable frame incorporating steel spring elements; and • a new concept of separating the cover and the manhole.
steel spring e l e m e n t
However, proof of their long-term suitability has still to be provided by means of an in-situ test programme over several years. For the first time the studies showed considerable differences in the different methods of rehabilitation for manhole top sections, and form the basis for the selection of suitable methods of rehabilitation from economic and technical considerations,
m a n h o l e collar
Fig. 10. New conception of a manhole cover: a combination of a height adjustable manhole cover (PASSAVANT, Aarbergen) with a steel spring element to cushion the dynamic traffic loads (sketch).
manholeframe ~ ~ ~ ~ ~ ~ ] ~ j c o n c m t e
climbing iron is
I~ I
I
I
ring or reinforcedconcretering
~
I ruing
Fig. 11. New conception of a manhole cover: separation of cover and shaft (sketch).
46
C. Falk
and therefore the economical rehabilitation of this important element of the sewerage networks.
REFERENCES 1. Stein, D., Falk, C. and Liebscher, M., Abschlul3bericht Forschungsvorhaben--Sanierung schadhafter Schachtabdeckungen. Institut ftir Kanalisationstechnik (IKT), November 1995, unver6ffentlicht. 2. DIN EN 124, Aufstitze und Abdeckungen fiir Verkehrsfltichen, Baugrundsatze, PrUfungen, Kennzeichnungen, Gtitetiberwachung, August 1994. 3. DIN 19549, Schtichte fiir erdverlegte Abwasserkantile und -leitungen, Allgemeine Anforderungen und Priifungen, February 1989. 4. Existing Sewer Evaluation and Rehabilitation, ASCE Manuals on Engineering Practice No./WPCF Manual of Practice FD-6, ASCE and WPCF, New York and Washington, 1983. 5. Inspector Handbook for Sewer Collection System Rehabilitation, The National Association of Sewer Services Companies (NASSCO), Winter Park, FL, 1985.
6. Operation and Maintenance of Wastewater Collection Systems. Manual of Practice No. 7, Water Pollution Control Federation, Washington, 1985. 7. ATV A 241, Bauwerke der Ortsentwtisserung, Empfehlungen und Hinweise, March 1994. 8. N. N. Firmeninformation, W. Loos, R6thenbach. 9. Deutscher Ausschuss fur Stahlbeton, Priifung von Beton, Empfehlungen und Hinweise als Ergtinzung zur DIN 1048, erarbeitet vom Arbeitsausschuss DIN 1048, zusammengestellt von Norbert Bunke, Heft 422, Beutb Verlag GmbH, Berlin, 1991. 10. DIN 196 Teil 1, Priifverfahren f~r Zement, Bestimmung der Festigkeit, March 1990. 11. DIN 18555 Teil 3, Priifung von MOrteln mit Mineralischen Bindemitteln, FestmOrtel, Bestimmung der Biegefestigkeit, Druckfestigkeit und Rohdichte, September 1982. 12. Stein, D. and Falk, C., Abschlufibericht Forschungsvorhaben: Einsteigschtichte fiir Abwasserkantile--Entwicklung eines neuen Bau- und Sanierungsverfahrens fiir die Verbindung des Rahmens der Schachtabdeckung zum Schachthals. Institut fur Kanalisationstechnik (IKT), January 1998, unver6ffentlicht.