Settlement damage, uneven bedding, and poor re-excavation capability – these are common issues on sewer construction sites. Often, the cause lies in the backfill.

Temporarily Flowable Self-Compacting Backfill (TFSB) materials promise better performance – but which ones actually deliver?

We tested five different TFSB products in full-scale sewer trench simulations. The aim: to evaluate how these materials perform under realistic construction conditions.

The Approach: Large-Scale Testing, Real-Site Conditions

At the IKT’s large-scale test facility in Gelsenkirchen, we set up actual sewer construction scenarios, including pipes, manholes, trench shoring, and bedding layers.

We tested the materials for:

• Flowability and installation quality
• Walkability and early load-bearing capacity
• Re-excavatability after hardening
• Volume stability and risk of subsidence
• Environmental performance and recyclability

We used innovative inspection tools like the 🔗 MAC system for pipe-soil stiffness and a walkability test after 3 hours to assess real-life performance.

Key results for sewer construction:

• Not all flowable backfills are created equal: Some harden too much, while others remain unsuitable for overbuilding, e.g. for road construction, for up to 56 days.
• Re-excavation is a challenge – for good excavation, TSFB must not exceed a compressive strength of 0.3 N/mm² after 28 days.
• New test methods like the Mini-MAC and walkability test ensure practical and measurable evaluation.
• Environmental compliance, shrinkage, and recyclability vary widely between products.
• Two materials stood out: RSS® Flüssigboden and carbofill® showed the best balance between performance, usability, and long-term suitability.

Comparative Test Results: Two Materials Performed Best – Others Failed Key Criteria

We graded each material on a scale from 1.0 (“very good”) to 6.0 (“inadequate”).

Special emphasis was placed on re-excavation capability and early workability, both essential for practical use.

Top performers:
✅RSS® Flüssigboden – well-balanced, walkable, easy to re-excavate
✅ carbofill® – strong technical performance and consistent usability

We observed critical issues with:
❌ Terrapact© – hardened too much, impossible to excavate
❌ WBM-Flüssigboden® – excessive shrinkage, ammonia emissions, and safety concerns

Materials that failed key performance or environmental criteria were ruled out for sewer construction use.

 

Why This Matters for Sewer Network Owners and the Industry

Our study provides clear guidance for sewer network operators, planners, and contractors:
✔️ Backfill material can now be selected systematically and criteria-based
✔️ Supports long-term performance and maintainability
✔️ Promotes safer, more cost-effective, and more sustainable construction

Especially in urban environments, where tight schedules and complex infrastructures are the norm, choosing the right backfill is critical – not just technically, but also economically and environmentally.

Access the Full Study

The full paper is published in the journal:

Tunnelling and Underground Space Technology (Open Access)
Published: July 2025
Authors: Nicole Kimmling, Matteo Rubinato, Bert Bosseler et al.
🔗 Read the publication

🔗 Read also:
Liquid > Solid > Ready? Comparison of Five Flowable Backfills for Sewer Pipe Trenches

Acknowledgements

We would like to thank the members of the municipal steering committee for this project, who have closely accompanied our project and provided us with tremendous support.

The steering committee was composed of representatives from the cities:
Burscheid (Chair), Düsseldorf, Gladbeck, Hamm, Lünen, Mülheim, Oberhausen, Recklinghausen, Solingen, Troisdorf, Wuppertal

They also contributed to the financing of the project, as did the Ministry for the Environment, Nature Conservation and Transport of the State of North Rhine-Westphalia, Germany. We would like to express our sincere gratitude for this as well.

We would like to thank also the Cologne District Government, as well as Leibniz University of Hannover and Koblenz University of Applied Sciences for their support.

Contact

Prof. Dr.-Ing. habil. Bert Bosseler
E-Mail: bosseler@ikt.institute

Nicole Kimmling, M.Sc.
E-Mail: kimmling@ikt.institute

 

How to keep roots away from sewage pipes? IKT investigates protection systems in a long-term project. Researchers want to find out which protective measures are suitable. They have now dug up their worldwide unique in-situ test rig. The initial results are surprising.

At the start of the project, IKT laid wastewater pipes with various protection systems underground. They then planted trees on top.

The researchers left the roots to grow in peace for six years. They only checked on them from time to time. In August 2024, the time had come: they dug everything up to see how the roots had developed and how well the pipes were protected from them.

 

Roots obstruct drains
Sewage network operators are constantly struggling with root ingrowth in sewage pipes. Obstacles to drainage can form and even cause complete blockages. These then have to be laboriously milled out. In the worst case, the only solution is expensive excavation. Structural protection measures are designed to prevent precisely this, so that trees and pipes can coexist in harmony in cramped urban spaces.

Material mix in the experimental setup
IKT researchers therefore want to find out which passive measures are suitable for protecting pipes. Six years ago, they set up a worldwide unique in-situ test stand for this purpose. At a depth of 1.20 metres, they laid two 30-metre-long DN 150 and DN 300 sewer pipes in parallel. On top they planted five fast-growing poplars, each five metres apart.

To see how roots interact with different pipe systems, they chose pipes made of concrete, PVC, PP and GRP. The total of 15 pipe connections are standard push-in joints, shrink sleeves, transition sleeves and an experimental bentonite tape.

What works against root ingrowth?
In the bedding area around the trees, the IKT researchers installed eight root barriers, such as foils and vertical panels from various manufacturers, as well as two mineral encapsulations.

That was six years ago. The poplars are now ten metres tall and in great shape. Time to see how the roots have grown and whether the protective measures are working.

Digging like archaeologists
The IKT researchers took archaeological care when excavating the roots of their trees so as not to damage the roots.

They meticulously documented how the roots had grown. Their painstaking work took a whole week with the help of hand shovels, suction excavators and compressed air lances.

Roots surprise researchers
Visually alone, there was something unexpected: even thick roots branched out like an ancient Roman trident when they encountered resistance. They followed the relatively loose soil space around the sewage pipes. They looked for their way and found it. They bypassed the built-in obstacles – successfully at first glance.

Some roots penetrated deeper into the earth along the vertical protective plates, only to grow upwards again behind the obstacle. Others bypassed the protection systems sideways. All of them grew towards the bedding zone of the pipes, probably because the soil is less compacted there and they can advance more easily than in the natural soil space.

Of the 15 pipe connections, 13 withstood the roots, two did not. The connections between the vertical protective plates were also not impenetrable in all cases. The roots snaked through here too.

Back to the lab
Now it’s back to the lab, where the researchers will investigate exactly what the roots have done. The final results of this research project are expected in early 2025 – we will report back.

Thanks to the Netherlands
The IKT root test stand is located in a new development area in the city of Almere, near Amsterdam. It was set up in the winter of 2018/2019 and has now been dismantled.

We would like to thank the city of Almere and the Dutch foundation RIONED for funding the project to the tune of 205,000 euros. Prof Dr Thomas Stützel, Director Emeritus of the Biological Garden at Ruhr University Bochum, supported us with his impressive wealth of knowledge as a biologist and root expert.

 

Photo gallery: Excavation of the IKT tree root test field
in Almere/Netherlands

Click on pictures for full view

 

Aging pressure sewer pipes, just like gravity sewers, require renovation. But which methods are best suited for the job? What are the pros and cons of each? The latest IKT comparative product test on pressure sewer liners offers valuable insights.

Pressure sewers are critical and sensitive elements of urban wastewater infrastructure. Many are ageing so sewer network owners are increasingly having to deal with their rehabilitation and there are different methods available on the market for this.

Consequently, the neutral, independent and not for profit IKT Institute for Underground Infrastructure, in Germany, has been examining rehabilitation solutions in an extensive comparative product test. Over a three-year period an evaluation project was undertaken on behalf of six municipal network operators from Bottrop, Bremen, Burscheid, Iserlohn, Cologne and Voerde and two regional water associations, the Emschergenossenschaft and the Wupperverband.

The project was supported by the district government of Münster and the State Office for Nature, Environment and Consumer Protection of North Rhine-Westphalia (LANUV). It was financed jointly by the NRW Ministry of the Environment and the eight network operators.

These organisations formed a steering group that determined the pressure sewer damage scenarios to be remedied, the testing programme and the evaluation of the results. IKT developed the test concept, set up the test rigs in its large 1:1 scale test pit and carried out the testing.

Six liners in the comparative product test

The steering committee selected the following lining technologies for the comparative product test:

Close-fit linier method:

• Compact Pipe (Wavin GmbH)
• egeLiner (egeplast international GmbH)

Cured in Place Pipe (CIPP) liner process:

• Esders HPS Liner (Esders Pipeline Service GmbH)
• Nordiflow WPE (NordiTube Technologies SE)
• SaniPipe (AMEX Sanivar AG)
• Starliner Structure-S (Karl Weiss Technologies GmbH)

Remediation task

For each liner system, the test setup consisted of a DN200 steel pipe with damage patterns such as holes, leaky connections, point loads, transverse and longitudinal cracks, ovalisation and incrustations. This realistically depicted the damage that network operators find in their pipes.

Class A liner systems

The central issue was whether the liners are suitable as Class A products. A Class A liner must be able to withstand internal and external stresses on its own, regardless of the condition of the host pipe. The stress testing programme conducted as part of the IKT comparative product test went well beyond the regular warranty period of five years in order to consider the entire useful life.

Test program and evaluation scheme

The test programme consists of three phases: The first phase depicted the regular, normal operation of a rehabilitated pressure pipe. Internal water pressures of between 2 and 6 bar were applied at different flow speeds.

In the second phase, the degeneration of the host pipe was simulated over a prolonged period of time. For this purpose, some of the damage scenarios in the host old pipe were worsened in order to simulate progressive damage development and the resulting changes in external influences on the liner. Test pressures and flow rates remained the same as in the first phase.

Finally, the third phase served to simulate additional, non-every day and extraordinary loads on the liner that may occur over the course of its useful life. These included high-pressure cleaning at 80 bar, abrasive substances, the rapid switching on and off of the pump or elevated groundwater levels, such as those that occur when pipes pass under rivers.

Evaluation criteria

The overall grades for the liner systems are made up of the four main criteria of tightness, stability, operational safety and quality assurance. These are divided into nine sub-criteria. The range of grades lies between VERY GOOD (1.0) and INADEQUATE (6.0).

Watertightness criterion (weighting 45%)

The main weak points found in the four CIPP liner processes were the end connections to the host pipe, there were leaks. In contrast, the PE flange and electrofusion sleeve connections of the close-fit liners were reliably watertight.

The close-fit systems Compact Pipe and egeLiner proved to be watertight after renovation. In contrast, the picture for the CIPP liner end connections was very different: Nordiflow and SaniPipe each had to be reworked once to make them all watertight, whilst the Starline end connections had to be reworked twice in order to get them tight. The Esders HPS liner remained leaky even after the connections had been repaired twice and thus it failed on this criterion.

Stability criterion (weighting 25%)

 

The stability (load-bearing capacity of the structure) was classified by the steering committee as a central “KO” (failure) criterion. It was gratifying that five of the six liner systems tested passed this criterion with GOOD or SATISFACTORY grades. They show no or only minor abnormalities such as local wrinkling.

The SaniPipe liner failed this criterion as it collapsed under external pressure. The reason for this was insufficient fabrication of the liner, which took place without static proof. It was therefore not a Class A liner and consequently received the overall rating INADEQUATE, regardless of performance against all other criteria.

Operational performance criterion (weighting 15%)

This examined to what extent the liners can withstand normal operating conditions such as pressure fluctuations, abrasion, static pressures and high-pressure cleaning.

 

Here, Compact Pipe and egeLiner scored a GOOD grade. Also, they did not form folds and obstacles along their length. However, their installation leads to a hydraulic loss of 6% compared to unlined pipe this was in the middle range. The Nordiflow and Starline liners both had folds larger than 6 mm in the installed liners sheets and thus achieved a SATISFACTORY score. However, the hydraulic power loss of the Nordiflow was particularly high at 8%, whilst the Starline was the lowest at 3%.

With high-pressure cleaning, the Compact Pipe and egeLiner close-fit products achieved a VERY GOOD rating. Nordiflow and Starline withstood this operational stress SATISFACTORILY. On the other hand, Esders and SaniPipe failed this criterion because holes and delaminations occurred. Chemical loads did not have a negative effect on any liner system.

Quality assurance criterion (weighting 15%)

Although all the manufacturers provided an installation procedure manual, some of them have significant deficits in training, test certificates and external and internal monitoring. In addition, the installed Esders and SaniPipe liners each had a continuous longitudinal fold along the length and SaniPipe had design defects, which led to a devaluation of the grade.

Overall result and conclusion

The IKT comparative product test “Renovation process for sewage pressure pipes – Class A liner” confirmed that it is possible to achieve good renovation results. However, there are major differences in the performance of the six rehabilitation technologies examined, which are reflected in the test results awarded. One of the six liners could not qualify as a Class A liner.

Passed:

• Compact Pipe (Wavin) GOOD (1.8)
• egeLiner (egeplast international) GOOD (1.8)
• Nordiflow W PE (NordiTube Technologies) SATISFACTORY (2.6)
• Starline Structure-S (Karl Weiss Technologies) SATISFACTORY (2.6)

Failed:

• Esders HPS Liner (Esders Pipeline Service) DEFICIENT (5.3)
• SaniPipe (Amex Sanivar) INADEQUATE (6.0)

Results at a glance and complete report (English)

The further deterioration of the condition of the host pipe over time was found to have no effect on the success of the rehabilitation. This applied in particular to signs of corrosion such as simulated pitting and point loads. Only in one case did the complete loss of the supporting host pipe lead to liner failure under external water pressure.

All six of the liner systems could be installed through the four 15° bends included in the rig. Three of the liners were even able to install through a further 30° bend.

The systems were able to withstand normal operating conditions such as pressure fluctuations, abrasion and static pressure without any problems. However, there are clear limits to high-pressure cleaning and holes and delamination can occur here. Chemical stresses did not affect the tightness of the liner.

All liner systems lead to hydraulic performance losses in the pressure sewer the highest up to 8% at the top. The internal diameter was reduced by more than 20% in some places in some liners. Wrinkles >6 mm could be seen on all the CIPP liners. In contrast, the close-fit liners showed no creasing whatsoever, but there was clear ovalisation in the bends.

IKT has joined with six other urban drainage labs to form Co-UDlabs, a new international “Consortium of Urban Drainage laboratories”. Together we are enabling access to our research facilities, providing training and undertaking common research activities with funding by the EU, and will build long term collaboration in urban drainage research. IKT will host a free workshop on optimising urban drainage assets.

Background to Co-UDlabs

Existing Urban Drainage Systems are ageing, but they are critical for protecting public health, reducing pollution impacts and urban flooding risks. The overall aim of Co-UDlabs project is integrate research and innovation activities in the field of Urban Drainage Systems (UDS) to address pressing public health, flood risks and environmental challenges.

Blog post: IKT joined EU’s Co-UDlabs project

Join our workshop to find out more

IKT is hosting one of Co-UDlabs first activities, a workshop over the mornings of 3rd and 4th November 2021 on identifying good practice and research for optimising the performance of urban drainage assets and improving their resilience to climate change and sustainability. Attendance is free, the agenda is available online and there is a simple registration.

About Co-UDlabs

The consortium is coordinated by Universidade da Coruña (Spain) and comprises 9 partners from 7 European countries. Between us we are making available 17 unique “field scale” urban drainage experimental facilities, providing innovation, collaboration and high-level training opportunities.

How to engage with Co-UDlabs

1. Take advantage of the “Transnational Access” to conduct your research in the facilities
   The project will be enhancing scientific and technical progress in the urban water sector through experiments carried out in 17 unique “field scale” urban drainage experimental facilities of seven research infrastructures: we aim to provide a total number of 29 accesses, with around 1080 days of granted access to the facilities, involving 141 different research users. Click here for more information
2. Join our network and networking activities
   To receive regular information you can complete our contact form. Activities are planned to consolidate the European community of urban drainage researchers, innovators and utilities and to contribute to create a culture of cooperation with the main actors working in UD field. Find out more about our networking activities
3. Engage with the training available
   Co-UDlabs supports education and training in UDS through seminars, advanced workshops, PhD courses, webinars and online videos.
4. Learn from the Joint Research Activities being undertaken by Co-UDlabs
   A set of three Joint Research Activities will support services provided by the different facilities through the transnational access and also to facilitate progress in the urban drainage discipline by the transfer of new technologies, procedures and best practices:
   • JRA1 Smart sensing and monitoring in urban drainage
   • JRA2 Evaluation of assets deterioration in urban drainage systems
   • JRA3 Improving resilience and sustainability in urban drainage solutions

For more information visit the Co-UDlabs website

Contact the IKT Co-UDlabs Team

• Thomas Brüggemann
  T: +49 (0) 209 17806-18
  E: brueggemann@ikt.institute
• Marcel Goerke
  T: +49 (0) 209 17806-34
  E: goerke@ikt.institute
• Iain Naismith
  T: +44 (0) 7983 605219
  E:naismith@ikt.institute

 

Around the world sewer network owners are struggling to address the options for replacement or rehabilitation of ageing pressure sewers. Often the preferred solution is to dig them up and replace them. Rehabilitation solutions using trenchless technology tend to be used only when it is too difficult to excavate, but sewer network owners are interested in using such technology more often. First, they need to fully understand it.

Lining in Large Diameter Pipes

IKT offered sewer network owners in other countries the opportunity to create partner projects alongside its current 2-year comparative evaluation of pressure sewer rehabilitation technologies and a group of seven UK and Irish sewer network owners are participating. They are being kept up to date with progress on the main project here in Gelsenkirchen and are able to contribute ideas and comments about the pressure sewer damage scenarios and test rig development. However, the Partner Project has some additional objectives of its own, which will also make useful contributions to the overall study of rising main rehabilitation. These include understanding how lining can be used in larger diameter pipes, new techniques for access, survey and cleaning, and considering how pressure sewers can be designed in future so that it will be easier to repair and rehabilitate them.

Another partner project is being undertaken with the City of Arnhem, The Netherlands, to examine pressure sewer inspection techniques.

IKT research projects are always made publicly available, so at the end of the research, having this partner project will ensure that the outputs are translated into English and so made available to a wide international audience.

Read more about the current IKT Compare of lining systems for pressure sewers (in English)

Contacts

• UK: Dr Iain Naismith
  Project Manager
  phone: +44 (0) 7983 605219
  email: naismith@ikt.institute
• Dipl.-Ing. Markus Gillar
  IKT Senior Research Fellow
  phone: +49 209 17806-46
  email: gillar@ikt.institute

 

We have evaluated with sewer network owners the suitability of “Flowable Backfill” for use sewer trench backfilling in an IKT-Compare evaluation of this technology. Five products were compared and all were found to flow well into 1:1 scale excavation pits in our large-scale test facility – around pipes and manholes shafts and into every corner of the simulated trenches. However, during the subsequent performance tests it became increasingly clear that only three suppliers had actually installed “Flowable Backfill”. One product more or less concreted over the test pit, another material took far too long to set and posed a risk to working safety. The results have now been made publicly available.

IKT-Compare evaluation of Flowable Backfill in Sewer Construction
Results table in English (PDF)
Product test report in German (PDF)

Flowable backfill is also known as „Controlled Low-Strength Material“ (CLSM) or „Temporarily Flowable, Self-compacting Backfill Material“ (TFSB, or ZFSV in Germany). These products can be used to fill a pipe trench quickly, without the need for compaction equipment. They have the potential to flow into every corner of the trench, to bed the pipes optimally and to then harden to such an extent that you can build a pavement or road on top. And, if you need to access the pipe again later, the material can be dug out again – in the ideal case.

Advantages and risks of using Flowable Backfill

The demands on the material are high. What if the supplier does not quite meet the optimum performance? What if after some time the road surface sinks, or in 20 years’ time you can no longer get to the pipe because the once-liquid soil is as hard as concrete?

It is not surprising that Flowable Backfills are popular. But, the enthusiasm of clients for their benefits often obscures awareness of the risks. IKT, as a neutral and independent institute, has considered these, working with with eleven committed wastewater network operators, to undertake a major comparative product test of the technology. So, in future wastewater network operators will know what to look out for when they order a Flowable Backfill.

Can a product deliver all the desired properties?

Five systems from five suppliers were tested under realistic, reproducible conditions in a large-scale test facility. After long and intensive preparation, the product test team spent a year testing, measuring, observing, evaluating and reporting. Now the results of this new IKT-Compare product test “Flowable Backfill in Sewer Construction”, funded by the NRW Environment Ministry, are available. The testing has basically confirmed the performance advantages of Flowable Backfill that network operators appreciate, but only if the formula and installation is correct.

It was found that all five materials were able to fill the trenches completely without voids – even when shoring removal was simulated after backfilling. The testers rated the pipe bedding provided by the products very positively, and a contribution to protection of pipes from tree roots can be expected from all the products. However, two Flowable Backfills failed to meet all criteria set by the steering committee (made up of wastewater network owners), as being essential for their use in sewer construction. One material hardened to such an extent that the required re-excavation capability was not achieved, another took far too long to achieve the strength required for building over, and also exhibited risks for re-excavating of the material and an unacceptable ammonia load during excavation.

Five participants at the start

The five systems tested achieved grades varying from GOOD (1.9) to INADEQUATE (6.0) (on a scale from 1 to 6) – see link to results table below. In terms of quality, there are significant differences between the individual materials:

• Carbofill from Thomas Zement GmbH & Co KG – GOOD (score 1.9)
• RSS Flüssigboden from FiFB Research Institute for Flowable Backfill GmbH – GOOD (score 1.9)
• TerraFlow from Heidelberger Beton GmbH – SATISFACTORY (score 3.4)
• Terrapact by Holcim Beton und Betonwaren GmbH – INADEQUATE – cannot be used in sewer construction due to very poor re-excavation properties
• WBM-Flüssigboden by WBM-Flüssigboden GmbH – INADEQUATE – cannot be used in sewer construction due to slow hardening, poor recyclability and high ammonia values

Three reached the finish line, two fell by the wayside

The winner with the grade GOOD (1.9) was Carbofill from the manufacturer Thomas Zement, which only has a weakness worth mentioning in the test of walkability after filling. In second place, very close behind, comes the RSS liquid floor from the FiFB (Forschungsinstitut für Flüssigboden) research institute, also with a GOOD (1.9) rating. The material only had difficulties with the filling of the shoring removal simulation. TerraFlow from Heidelberger Beton received a low score for “re-excavation capability”, a devaluation of 1.0 grade points, but is otherwise on a par with the best performers. Overall, TerraFlow achieved a SATISFACTORY (3.4) and thus third place.

The Terrapact material from Holcim Beton und Betonwaren could only be removed from the test trench with great effort and heavy equipment. The result was an INADEQUATE in the criterion of re-excavation capability, and thus was found to be not suitable for use in sewer construction. The WBM-Flüssigboden material, on the other hand, had a problem with the criterion of hardening. It could only have been built over after considerably more than 28 days. In addition, the MAK value for ammonia was exceeded for this product (MAK = maximum allowable workplace concentration) and the recyclability was also inadequate. The verdict of the testers: INADEQUATE, not applicable in sewer construction.

Much effort was expended in order to provide users with valuable insights

For this IKT-Compare product test “Flowable Backfill in Sewer Construction”, the test hall team divided the Institute’s 15-metre long, six-metre wide and six-metre deep large-scale test stand using 25 millimetre thick steel plates to create five test chambers. Exactly the same construction of concrete manhole shafts, plastic manholes shafts and shoring boxes, as well as main, transverse and longitudinal pipes were installed in each chamber. The man-hours for installation alone added up to many weeks. Added to this is the time spent by IKT structural engineer Dr Mark Klameth on the calculations for the installations. A total of 30 tonnes of steel were used. Then the suppliers came and pumped their Flowable Backfills into the compartments and the evaluations could begin.

Testing confirmed that Flowable Backfill has many advantages

All five materials flowed well into the trenches – a pleasure to watch as, like liquid lava (only not so hot), they glided elegantly into every corner, enclosed the pipes and manhole shafts, and filled the test stand up to the top. Satisfied faces were also to be seen on the construction sites that the evaluation team visited to gather in-situ impressions and experiences.

Extensive test programme

The extensive testing programme began during the installation of the Flowable Backfills: the product test team determined the consistency of the backfill, among other things, by means of a spreader gauge, checked the shear strength with a visco-balance, measured the pH value, filled the containers for a 3-segment cylinder test – which shows whether the suspension separates over time – and took samples for cube pressure testing, which determines the modulus of elasticity after seven days. This was followed by walkability tests, load plate compression tests and the cube compression tests mentioned above. In further testing during the use and disposal phases, the product testers examined the pipe bedding and the backfilling of the shoring removal simulation. The results of eluate tests allowed the scientists to assess the environmental compatibility of the materials. And the steering committee members evaluated the ability of the soil to be removed using a spade by picking up the spade themselves and comparing the materials directly.

With exceptions, Flowable Backfill can be built over within 7 days

A minimum value of 45 MN/mm² achieved in the load plate compression test is decisive for determining if hardened Flowable Backfill can be built over. The measured EV2 values (modulus of elasticity) reached this value within the first week for four of the five products tested. However, extremely high EV2 values can also indicate that the final strength of the flooring may be too high for re-excavation.

In addition to the laboratory tests at IKT, the Flowable Backfills were also installed in trenches at a test site in Burscheid. Here and during further construction site investigations, the basic handling of the materials under practical conditions was recorded and any differences with the installations witnessed in the IKT large test facility was also checked. These observations were considered as part of the scoring for quality assurance.

Quite OK: Quality assurance by contractors

In addition to the system tests of the Flowable Backfills in the test set-ups, the testers also examined the quality assurance provisions by the manufacturers. With few exceptions, the scores in this area show that the suppliers are making great efforts to ensure quality. But there is still a bit of room for improvement.

Special attention to meeting key performance criteria

The steering committee of the network operators defined early on the performance characteristics which must be met at all costs in order to ensure that a Flowable Backfill can be used in sewer construction. These concerned: the flowability in the installation phase, the build over capability in the utilisation phase and the re-excavation capability and potential for recycling in the disposal phase. Two out of five products proved to be defect-free in tests against all these criteria: Carbofill and RSS Flüssigboden. The remaining products, on the other hand, revealed visible or even serious defects, which in two cases led to the IKT-Compare test verdict “Inadequate”.

Quality assurance in the mixing plant is extremely important

In the test, the suppliers were asked to offer the products in such a way that their composition was clearly defined and that it was also possible to re-order them for future installations. Accordingly, the suppliers were free to choose the soil material to be used. In at least one case, however, an unsuitable soil material (organic matter, ammonia release, poor build over capability) was used in the performance tests. The network operators represented in the steering committee see the suppliers/manufacturers as responsible here for reliably determining the quality of the substances and soils used in their product and making this information transparent. Soil management and quality assurance in the mixing plant where the Flowable Backfill is produced are therefore of particular importance.

Conflicting objectives: need to firm up quickly, but not be too firm for later re-excavation

Obviously, for many suppliers there is a conflict of objectives between rapid walkability and buildability and the later re-excavation capability (using a spade) of the material. Only one provider (RSS Flüssigboden) succeeded in fulfilling these criteria in the system tests with very good or good results throughout. The four remaining suppliers showed clear deficiencies in at least one of these criteria in the test.

Pulling out the shoring can lead to massive surface fractures

Pert of the evaluation involved the removal of simulated shoring from the test trenches. In individual cases, pulling the shoring led to massive surface fractures, which can lead to uncertainty at the construction site about the success of the backfilling process and necesitate. However, in the testing, the observed cases did not show any effects on the load situation in the subsoil. Where fractures did occur, the manufacturers are required to adapt the materials accordingly or to specify suitable times for pulling the shoring.

Quality assurance on the construction site

The test results show that there are important assessment criteria that should be checked in construction site practice in the course of internal and external monitoring of a Flowable Backfill installation. This applies in particular to the flowability, walkability, build over ability, and bedding properties as well as the lifting of shoring. Furthermore, the manufacturer should provide the composition of the materials used, including the soil material, in a transparent and verifiable manner. It may also be appropriate to check the homogeneity of deliveries to site, i.e. the extent to which variations in properties between batches can be observed.

In the run-up to this product test, 16 manufacturers of Flowable Backfill were researched who offer their products for use in sewer construction. However, only six suppliers were in a position to offer a nationwide service such that the product could be used in North Rhine-Westphalia for both the test fields in Burscheid and in the performance tests at IKT in Gelsenkirchen. Moreover, one of these products was not pumpable at the time of the award of the contract, so ultimately only five products were used and compared in the system tests. Consequently, an expansion of the delivery areas for more products and also new and further developments of products are desirable. IKT testing could then underpin their quality.

Possibility for retesting

Not been involved in this product test? Your material can do that too? Then there is an opportunity to prove it in follow-on testing – the same tests under the same conditions with the same evaluation criteria. At the end there is a score and a place in the results table. And if you want, you can also get a seal.
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Improving the image of Flowable Backfill

The IKT-Compare product test shows that in principle, Flowable Backfill is already a suitable technology for sewer construction. Not for all products, but compositions can be optimised, and at some point the ideal case will become the norm.

IKT-Compare product test “Flowable Backfill in Sewer Construction”
to the results table of this product test (in English) (PDF)
Download the product test report (in German) (PDF)

Contact persons

Dipl.-Ing. (FH) Serdar Ulutaş, MBA
Head of IKT Product Testing
phone: +49 209 17806-32
e-mail: ulutas@ikt.institute
Dr Iain Naismith
IKT Project Manager UK and International
phone: +44 7983 605219
e-mail: naismith@ikt.institute