The document proposes a new technology for filling voids created during tunnel excavation using lightweight cellular concrete (CLC) instead of traditional pea-gravel. This method aims to enhance the effectiveness of the tunneling process with lower costs and improved material characteristics, such as self-compaction and impermeability. The proposal also suggests that if accepted by TBM manufacturers, it could simplify the mixing and pumping systems used in mechanized tunneling projects.

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New technology for filling and clogging the void that is created in the back of the precast concrete segments
during the excavation phase of a tunnel using TBM-EPB
by Lamanna Luigi Franco [*]
a) - Introduction
As already illustrated in the year 2020, in a previous article of mine, with the title: << ... Indications on some
new solutions on how to do the clogging on the back of the lining made with prefabricated concrete segments
in a tunnel under construction and simultaneous resolution of the problem of water infiltration that occurs
through cracks that occur on the same precast segments ... >> and, given the production costs, which are
undergoing many "raw materials", indicated below, in this new memory, a possible alternative to the "Pel-
Gravel", which is normally used to create blockages, on the back of the coating of a tunnel during the
excavation phase to avoid uncontrolled failure of the lining.
This new type of intervention, which I will propose below, has very low costs.
Usually the lining, for this type of excavation, using TBM-EPB, is made with prefabricated concrete segments,
and through this memory, I would like to suggest a new technology and related methodology using, instead of
the "Pel-Gravel", of the lightweight cellular concrete / concrete CLC [Reported by ACI Committee 523].
What is very important is that even this type of proposed material must also be able to influence the interaction
between the support [which is the rock] and the excavation behavior along the tunnel layout.
Photo 1 – Pea-Gravel
Many illustrious colleagues have shown, over the years, using the "Pea-Gravel", that numerical simulation, with
continuous methods, have enormous shortcomings. In fact, these shortcomings, related to the instability of
the simulations of large deformations, limit the possibility of reproducing the transfer of the filling material.
Here is one of the reasons why the undersigned wants to bring to your attention. Lightweight cellular cement
/ concrete, known under other aspects and under the most varied trade names.
Therefore, becoming aware of this deficiency, I would like to suggest to all of you, technicians in the sector,
much more qualified than myself, to say if this proposal of mine could be a valid solution.
I believe it is very competitive, also because it can be applied directly in situ. I think it is very competitive, also
because it can be applied directly in situ. Furthermore, if the “TBM” manufacturers accept this new technology,
they can equip the TBM-EPB with a new and simple mixing and pumping system.
I would like to point out that, since the distant year 1978, I have made massive use of this particular technology
in various sectors of civil and infrastructural construction and, from experimental site tests, I have always been
able to see that, the use of cement / Lightweight cellular concrete has, among the many technical
characteristics, also that of being a final product of the Self-Compacting Concrete type as well as that of not
segregating itself [Non-Segregantis]. In fact, it has always been suggested by me to use this method, without
great success, while it has been used a lot in underground mines for the construction of "Fire Barriers".

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I would like to point out that, even today, this topic is treated as if it were a recent technology, in reality,
cement / lightweight cellular concrete has been experimented with since 1914 by Aylsworth and Dyer by
introducing air bubbles into a cement mixture using, initially, of aluminum powder, through chemical actions.
In reality, the beginning of its use in civil construction was and still is very widespread and bears the date of
large-scale use from the year 1923 while the patent was filed in 1931 by Erik Christian with U.S. Patent number
1.794.272.
Fig. 1 – Typicall cell structure of cellular concrete
Therefore, I would like to suggest that lightweight cellular cement / concrete can also be used normally during
the mechanized tunneling phase by TBM, both with single (SM) and double (DSM) shielding, where the precast
facing segments ( the so-called ashlars) are used as a support for the rock.
In particular, to fill the cavity between the wall of the rock excavation and the lining in precast concrete
"segments", replacing the "Pea-Gravel" and, if desired, if necessary, it can be safely injected into other cavities
that may arise present along the tunnel route.
Cement / cellular concrete CLC has always been indicated by sector technicians as a material that is obtained
by incorporating numerous small air bubbles [not communicating with each other], with a diameter between
0.3 and 1.5 mm, in a Portland cement paste, with or without the addition of fine aggregate.
Fig. 2 – Configuration of a tunnel type section where the injection / clogging in “Cellular Concrete” is clearly visible on the
back of the concrete segments. Its peculiarity is that it can fill every small void.
b) - Considerations on the experimentation of "lightweight cellular concrete / concrete" to be injected on the
back of the segments in the excavation of tunnels with mechanized cutters
I would like to examine and illustrate the technological aspects of lightweight cellular cement / concrete. In
tunnel construction technology, with mechanized systems, the diameter of the excavation has been built for
over 50 years, always greater than that of the final lining ring in concrete segments.
This cavity, as already introduced previously, is usually filled with a monogranular aggregate commonly called
"Pea-Gravel" with a size of about 15 mm, injected by means of a metering pump through the holes left in the

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single segment of prefabricated lining and subsequently this is clogged for by means of a cement-based
injection.
Usually the consumption of this two-component system of mixture to be injected can vary from 2.5 to 3.5 liters
per m3.
In fact, our proposal provides for the following processing:
- total elimination of the mono-granular aggregate commonly called "Pea-Gravel";
- filling of the cavity, by injection / clogging, to ensure the complete filling of the void, with cement / cellular
concrete of the light type (with low density values) which would allow, instead, a simplified and short-lasting
processing cycle compared to technique used up to now;
- this system has the advantage of being pumped, with special machines that guarantee uniformity and
consistency of the characteristics, and of filling the cavities between the profile of the excavated tunnel and
the segments installed;
- this type of product does not require an accelerating additive and it could be possible to avoid making the
cement / light cellular concrete flow back from the lining towards the rear of the TBM-EPB machine, and would
also give rapid stabilization to the prefabricated segments;
- however, also this process is limited to the rear part of the cutter (tail) with the creation of a sealing ring
made of a grease, commonly called "tail grease" [Tail Sealant];
- the cellular cement / concrete has characteristics such as to make grouting of the joints between the segments
unnecessary;
- it also has good homogeneity, almost zero shrinkage and high impermeability.
Depending on our resistance objective to be achieved, our mixture, through its composition, can be used to
obtain a density that can vary from 300 kg per m3 to 600 kg per m3 [using only water, cement and foaming
additive].
However, the strength characteristics depend on the density, the Water / Cement Ratio, the content and type
of cement and the size of the air bubbles that appear to be present in the foam.
Tab. 1 – Compressive strength values that cellular concrete can achieve light with Portland 52.5 cement mixes only
The modulus of elasticity is 3 to 30 times smaller than that of normal concrete, depending on the density.
For the above filling, if you need to have a higher compressive strength, with verification tests on cylindrical
specimens at 28 days, between 4 and 6 Mpa and a density of 1.300 kg per m3 corresponding to a final density
more than 1180 kg per m3, in this case a lightweight cellular cement / concrete must be made with sand mixes
according to the indications in Table 2 below [using only water, cement, foaming additive and sand].

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Tab. 2 – Compressive strength values that cellular concrete can achieve light with sand mixes: Portland 52.5 cement
Thanks to its very fluid consistency and thixotropic behavior, the lightweight cellular concrete / concrete is self-
compacting and can simply be pumped into the back of the segments, even if the vacuum is very narrow and
irregular, as long as it is not flooded with water. No precautions are needed for ripening. In fact, although the
shrinkage is greater than in normal concretes, the reduced loss of moisture that occurs in the excavations,
contributes to inducing a widespread micro-cracking, unless one operates in very porous or permeable dry
soils.
c) - General characteristics of the foaming additive
To make the foam, a special aerator is required which, through an air compressor incorporated in it, provides
the air necessary for the formation of the foam additive. The capacity of the aerator can vary from 150 liters

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equal to a production of about 2,000 liters of foam up to 500 liters equal to a production of about 7,000 liters
of foam that can be dispensed from a minimum of 300 liters per minute up to a maximum 600 liters per minute.
In fact, the foam is formed with 50 liters of water for each liter of single-component liquid additive (about 2%
by weight) based on special surfactants, of which the best is still of animal origin [meat processing waste] and
vegetable [therefore a Green product]. The introduction of the foam into a mixture brings about 70 - 80 liters
of water per m3 to have a dough density equal to 350 kg per m3 and 20 ÷ 30 liters of water per m3 to have a
dough density equal to 1,600 kg per m3.
The aerating additive is presented as a dark brown liquid with the following main technical characteristics (not
intended for the uses provided for by Directive 2004/42 / EC) and being able, through a physical action, to act
on fresh cement by replacing with air bubbles a part of fine sand in the mixture:
- chemical composition: peptones and peptics obtained by hydrolysis of animal and vegetable
proteins
- specific weight at + 20°C 1.12 ± 0.03 kg / l
- solubility in water: total
- viscosity: 17 ± 5 cst + 20°C
30 ± 10 cst 0° C
75 ± 20 cst – 10°C
- freezing temperature: - 15 ° C
Peptones are amino acid chains joined to lipids (in low concentrations) and ash and have a pH between 6.8 and
7.1 while peptides are a class of chemical compounds, whose molecules have a molecular weight of less than
5,000 daltons, consisting of an extremely variable chain of amino acids joined together through a peptide (or
carboamide) bond. The shortest peptides are the dipeptides, consisting of two amino acids joined by a single
peptide bond; polypeptides consisting of a few units are commonly referred to as oligopeptides.
In addition to these particular products, to obtain “aerated” cements with bubbles obtained by physical action,
other synthetic substances can be used: such as natural fatty acid soaps, some petroleum-derived sulphates,
etc.
The density of the preformed foam varies between 40 and 65 kg / m3 and this typical formulation of surfactants,
from my experience, I can confirm that it is able to produce and keep stable the air cells inside the cement
mixture, obtaining physical actions imposed during mixing, pumping, laying and setting of the cement /
lightened cellular concrete.
The air bubbles have a diameter that is generally between 20 and 200 µ; the distance between them varies
between 5 and 200 µ, while the density can be between 100,000 and 400,000/m3.
The ASTM-C-796 and ASTM-C-869 standards provide standard methods for laboratory measurement of
performance specifically formulated for the production of a foam, ideal for use in the production of an air
bubble foam, non-communicating between them, for the production of our lightweight cellular cement /
concrete.

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This additive, to make the foam for our lightweight cellular cement / concrete, is different from the one used
to make SCC concretes, that is a Self Compacting Concrete (SCC) or a Self-Leveling Concrete or SLC which is a
cement conglomerate which, in addition to having a very high fluidity, in the fresh state, also has a high
resistance to segregation.
Thanks to its rheological properties, SCC concrete completely fills the formworks, eliminating macro-voids and
excess air, obtained by chemical action, inside the cement matrix.
This avoids the onset of macro-defects in concrete which are the cause of the reduction of its mechanical
properties and its degree of durability.
Given the confusion that is often made on this topic, I must remember that an SCC concrete is very different
from a lightweight cellular concrete / concrete, as it has different qualities.
I repeat, since in all these years I have seen that there is a lot of confusion on this topic, I would like to illustrate
the different qualities below.
In particular, SCC concrete must have:
• a smaller diameter of the coarse aggregate (16-20 mm) and a smaller volume of coarse aggregate
(approximately 280-350 l / m3): among other things to reduce blocking;
• a high content (about 500-600 kg / m3) of fine materials (<100 µm) made up of both reactive powders, such
as cement and type II mineral additions with low development of hydration heat (fly ash, silica smoke , etc.),
which from non-reactive powders (type I mineral additions) such as inert fillers (finely ground limestone, etc.)
and very fine aggregates: increases the cohesion of the conglomerate and therefore increases its resistance to
segregation;
• an adequate Water / Dust volume ratio (0.80-1.20): an excessive value would make the mixture too fluid
(risk of segregation) while a too low value would make the fresh mix too viscous and therefore not very mobile
and difficult to pump;
• a suitable Water / Cement weight ratio (W / C ≤ 0.5): to also satisfy the resistance class and exposure class
requirements defined in UNI EN 206-1. However, this relationship must be related to the previous one;
• use of high dosages of superplasticizing additives (latest generation carboxylated ethers) to ensure the
correct fluidity of the mixture;
• use of viscosity modifying additives (VMA): to ensure a suitable viscosity to the mix.
Super-plasticising additives and VMAs must be used in the right proportions in order to ensure the mixture has
the right balance between fluidity and cohesion.
While the physical properties of the foam are those shown in Table 3 below.
Density (gr/cm3) Appearance pH
1,12 Light brown 6,8  2
Tab. 3 - Physical properties of foam additives
d) - Characteristics of the cement
The type of cement that I have usually mistakenly seen used is the white CEM I 52.5 R. This type of cement is
not needed to inject / block the back of the prefabricated segments. In fact, the chemical properties of this

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cement are indicatively those reported in Table 4) below. The density of white cement, which many use
erroneously for this particular type of application, is measured at 3.06 gr / cm³ and its surface is 4950 cm² / gr.
Si02 Al203 Fe203 Ca0 Mg0 S03 Na20 K20
20,6 3,35 4,9 65,9 1,30 2,90 0,30 0,45
Tab. 4. Chemical properties of White Cement I 52,5 R
However, the cement to be used does not necessarily have to be white, but of the Portland type [CEM. TYPE I
52.5 R] however also other types of cement can be used, as long as their resistance class corresponds to that
required for the particular use, and satisfy the requirements of ASTM-C-150 (Portland cement), C-595 (mixed
concrete) or C-1157 (hydraulic concrete). Mixed cements include cement containing combinations of Portland
cement, pozzolan, slag, other hydraulic cement, or a combination of these as required by ACI-523.1R-06. I point
out that blended cement may result in lower rates of early strength development and suggest testing for use
in specific applications. Type III or HE high strength cement produces concrete with higher rates of initial
strength development but which are of no use for our particular use.
Therefore, the natural structure of light cellular concrete / concrete requires fresh and finely ground cements
to allow a homogeneous and uniform production of spherical cementitious films around the air cell to ensure
a perfect three-dimensional network regularly distributed in the mass. Therefore, the use of old cements, that
is, those that have absorbed moisture, resulting in the formation of lumps, do not adapt well to the success of
the aforementioned structure with a consequent decrease in strength and lengthening of the hardening times.
e) - Quantity of water
To make the cement / lightweight cellular concrete, the average amount of water used must be 50 liters per
100 kg of cement, plus 15 liters of water introduced into themix in the form of foam, therefore the final average
A / Ce ratio is of 0.65.
f) - Aggregates
In addition to fine sand, low density lightweight cellular cement / concrete may include aggregates such as
vermiculite or perlite as the lightweight material, which must meet the requirements of ASTM-C-332, Group 1
to reduce slump, and maintain moisture. in particular climatic regions.
However, any initiative for the use of particular proposed aggregates should always be tested to know the
properties, in particular the pumpability and compatibility in the test mixes. But even this at the moment is not
the object of our memory.
g) - Conclusion
Therefore the production of lightweight cellular cement / concrete turns out to be an economical building
material due to its ease of application and its lightness. In this brief illustration, the specific bulk density of the
cement / lightweight cellular concrete to be injected behind the precast segments, as clogging, was indicated
by me as 250 kg / m3, while a compressive strength was measured around 0.75 - 0.85 MPa while, its thermal
conductivity coefficient, which is of little importance for the object of the present report, is indicated in about
0.075 W / mK.

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Tab. 5 – Consumption of sand and cement – Kg/m3
I would like to point out that as the dry density increases, the compressive strength also increases. As the raw
materials used in the production of the lightweight cellular cement / concrete are non-flammable, they would
not exhibit combustible properties during a fire, should the final product come into contact with open flames.
Furthermore, I would like to point out that even today, I feel proposed to use fly ash instead of sand, and they
disagree because fly ash does not bring any advantage but slightly increases the dry density of the lightweight
cellular cement / concrete that although fine, light aggregates can have a varied influence, due to their different
characteristics.
While to obtain light cellular cement / concrete, i.e. aerated concrete, it must be emphasized that the
quantities of aerating additive necessary to obtain a certain percentage of air bubbles in the concrete depend
on various factors, such as the nature and type of cement, the dosage , any sand, the type of mixing, etc. etc.
recalling in conclusion that in hardened lightweight cellular cement / concrete, the air cavities present, in
addition to replacing a part of fine aggregate in the mixture, improve the impermeability on the back of the
concrete segments, thus preventing water from spreading by capillarity inside the gallery.
[*] Luigi Franco, LAMANNA
Independent Technical Consultant in the sector of Tunnelling, Mining and Underground Technology
President of the Fondazione Internazionale di Centro Studi e Ricerche, ONG
132, via dei Serpenti, 00184 ROMA, Italy, U.E.
Email: lamannaluigifranco1@gmail.com