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Version 2010

Booklet 4

Formulation of sprayed concretes

Comité Technique Asquapro

Technical Committee

Booklet « Formulation» page 2 Version D 2010

Technical Committee


1 INTRODUCTION ................................................................................................................ 5

2 COMPOSITION OF CURRENT SPRAYED CONCRETES .......................................... 6

2.1 General points .................................................................................................................. 6

2.2 Composition of concretes sprayed by dry process ........................................................... 6 2.2.1 Issues .......................................................................................................................... 6

2.2.2 Grading curve of the aggregates ................................................................................. 8 Reference grading zone ...................................................................................... 8

Dimension of the biggest aggregate ................................................................... 8 Remarks on the ratio Sand/Gravels (S/G) .......................................................... 9

2.2.3 Optimization of fine elements .................................................................................... 9 2.2.4 Optimization parameters........................................................................................... 10 2.2.5 Consistency ............................................................................................................... 11

2.2.6 Additions .................................................................................................................. 11

2.2.7 Additives ................................................................................................................... 11

2.2.8 Fibres ........................................................................................................................ 12

2.3 Composition of concretes sprayed by wet process ......................................................... 13 2.3.1 Issues ........................................................................................................................ 13

2.3.2 Grading curve ........................................................................................................... 14 Reference grading zone .................................................................................... 14

Dimension of the biggest aggregate ................................................................. 14

Remarks on the S/G ratio .................................................................................. 14 2.3.3 Fine elements ............................................................................................................ 14 2.3.4 Optimization parameters........................................................................................... 15 2.3.5 Consistency ............................................................................................................... 15

2.3.6 Additions .................................................................................................................. 15

2.3.7 Additives ................................................................................................................... 15

2.3.8 Fibres ........................................................................................................................ 16

2.3.9 Water ........................................................................................................................ 16

3 COMPONENTS............................................................................................................... 17 3.1 Cement ............................................................................................................................ 17

3.1.1 Are they final or provisional works? ........................................................................ 17 3.1.2 Resistance aimed at 28 days? ................................................................................... 18 3.1.3 Requirements for short-term strength? ..................................................................... 18 3.1.4 Environment and required durability? ...................................................................... 19 3.1.5 Influence of climatic conditions? ............................................................................. 20 3.1.6 Reactivity of the aggregates? .................................................................................... 20 3.1.7 Aesthetic requirements? ........................................................................................... 20 3.1.8 Prestressed concrete - Refractory concrete ............................................................... 21

3.2 Aggregates for sprayed concretes and mortars ............................................................. 21 3.2.1. Fine gravels.............................................................................................................. 22

3.2.2. Sands ........................................................................................................................ 22

3.3 Fibres .............................................................................................................................. 22

3.3.1 Fibres content ........................................................................................................... 23 3.3.2 Precautions................................................................................................................ 23

3.4 Additives and additions for sprayed concrete ................................................................ 23 3.4.1 Definition, implementation and standardization ...................................................... 23 3.4.2 Which additive to use? ............................................................................................ 23

3.4.2.1. The accelerators ................................................................................... 24 3.4.2.2. Which product family? ........................................................................ 24

3.4.2.3. Which product in the family? .............................................................. 27

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3.4.2.4. Do not forget: ....................................................................................... 27 3.5 Water ............................................................................................................................... 27

4 CONCRETES ADAPTED TO SPECIFIC NEEDS ......................................................... 28

4.1 Concretes implemented in hot or cold weather ............................................................. 28 4.1.1 General points ........................................................................................................... 28 4.1.2 Consequences of heat ............................................................................................... 28

4.1.2.1 On fresh concrete (before setting) ........................................................ 28

4.1.2.2 On concrete at young age (after the setting has begun) ........................ 28

4.1.3 Rules to be retained in hot weather .......................................................................... 30 4.1.4 Rules to be retained in cold weather ......................................................................... 31

4.2 Concretes with fast setting and/or hardening ............................................................... 31 4.2.1 General points ........................................................................................................... 31 4.2.2 Needs ........................................................................................................................ 32

4.2.3 Classification ............................................................................................................ 32

4.2.4 How to obtain them? ................................................................................................. 32 4.2.5 Accelerators .............................................................................................................. 32

4.2.6 Quick-setting cements .............................................................................................. 33 4.2.7 Formulation rules ...................................................................................................... 34

4.2.7.1 general rules .......................................................................................... 34 4.2.7.2 proportionings ....................................................................................... 34 4.2.7.3 requirements ......................................................................................... 34

4.2.8 Role of the temperature .................................................................................... 34 4.3 freezing-thawing resistant concretes ............................................................................. 35 4.4 High-performance cementitious materials ................................................................... 35

5 SAND CONCRETES ......................................................................................................... 35

6 REFERENCES QUOTED IN THE BOOKLET “FORMULATION” .. ........................ 36

Technical Committee


11 IINNTTRROODDUUCCTTIIOONN

The best formulation will reach its full potential only if the concrete is sprayed by a qualified operator. The ASQUAPRO therefore recommends to require certified nozzlemen (please refer to section “training-certification” of website www.asquapro.com). This preliminary point is essential because implementation has a great influence on the performances of sprayed concrete, much more than for cast concretes. We will not come back to this subject in this document anymore.

The technology concerning the implementation of concrete by projection comprises four steps, each one having its own requirements with respect to the mixture:

1st step: manufacture of the mixture. It can be made in a factory (for dry mixtures), in an RMC plant or in-situ depending on the requirements relating to the site pace or to the precision of the composition to be made.

2nd step: machine run. The mixture must be formulated so as to flow easily in the machine.

3rd step: transfer of the mixture from the machine to the nozzle. The important point is that the mixture can go through the pipe without blocking while having adapted rheological properties at the exit of the nozzle.

4th step: projection of the mixture. It must guarantee on the one hand the good behaviour of the concrete on the substrate with the requested mechanical characteristics and on the other hand, the minimum of material losses.

For the wet process the formulation depends on pumpability. The methods used for the formulation of pumped cast concretes can be a good source of inspiration but it is necessary to integrate the constraints of fresh concrete behaviour on the substrate. The formulation must consider the length and diameter of the pipe.

On the other hand, for the dry process the formulation is mainly based on the experience.

In this document, you will not find any standard ready-mixed compositions which would be misleading for a neophyte reader. Indeed, the diversity of parameters (related to equipment, materials, projection or the site special features) and their influence on the formulation constitute a complex whole which must be taken into account for each case, in the search for a formulation adapted to the implementation and the awaited final characteristics.

The specifications about the materials and the composition are specified with the aim of being included in the specifications and articles of a S.T.S. (Special Technical Specifications) for sprayed works.

This booklet assumes that the choice of the projection method (dry or wet process) has already been made (see booklet “implementation”). The principles of composition for the two methods are developed separately (for an easier reading, the text about the wet process appears in blue). The composition of current use concretes will be broached first, then the components and finally the composition of sprayed concretes adapted to specific needs. The case of sand concretes is not treated here; please refer to the standard NF P 18-500 of June 1995, resulting from the SABLOCRETE national project.

Technical Committee


22 CCOOMMPPOOSSIITTIIOONN OOFF CCUURRRREENNTT SSPPRRAAYYEEDD CCOONNCCRREETTEESS

2.1 General points

The composition of sprayed concretes must imperatively take into account the implementation and transfer modes specific to each projection method:

For the dry process, the dry or with low water content mixture must have a pulverulent consistency to allow the machine running and the transfer by compressed air until projection on the substrate. The compaction obtained by the speed at the exit of the nozzle allows the implementation of a firm consistency concrete and help it stick on the substrate, even without additive. On the other hand, this compaction generates rebounds and losses related to the impact force of the materials on the substrate. The creation of a layer in contact with the substrate and made of fine elements of the mixture, right from the start of the projection, prooves that projection by dry process creates a cohesive bond. Please remember that this process has no equivalent for projection by wet process. Knowing that it is mainly the quantity of cement present in this layer which determines the binding strength, a minimum proportion of cement must be respected during the development of the formula, without however being in surplus. For the wet process, the mixture is formulated to be pumpable, in the machine and until the end of the transfer pipe (see booklet “implementation”). The concrete behaviour on the substrate can be compared to “gluing” so the pumpability criterion is thus not sufficient and a restriction of formulation is that fresh concrete should not subside once in place. The use of a stiffener at the exit of the nozzle is therefore almost unavoidable. However this additive should not disturb the good placing of concrete, in particular the good coating of the frameworks, nor weaken the mechanical characteristics of concrete. The ASQUAPRO proposes to prohibit any use of stiffening additives containing alkaline silicates (aluminates are already prohibited) because, in addition to their harmful effect on concrete characteristics, they present serious hazards to health and environment. This prohibition must be clearly formulated in the writt en documents of the contracts.

2.2 Composition of concretes sprayed by dry process

2.2.1 Issues

The composition of a mixture intended for projection by dry process is limited to an optimization in order to reduce the losses during projection. The approach is simple because the composition accepts important variations as shown by a Swiss study carried out by Piétro TEICHERT. This study gathers and superimposes all the grading curves validated by sprayed concrete experts, experienced nozzlemen or some machine manufacturers. Then a very large body of curves is obtained (see graph below).

Technical Committee


These multiple possible approaches come from the fact that the projection method by dry process regulates the granular distribution of the concrete in place thanks to more or less important losses.

Based upon all these elements, one understands why the current recommendations are founded on empirical data and on-site observations.

The composition of the concrete actually implemented is different from that of the initial mixture because of the losses generated during projection. It is on this level that the development characteristic of a composition of concrete sprayed by dry process resides.

courbes g ranulai res d'un mé lang e r ée l proje té par voie s è che, du bé ton e t des per te s

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1 0

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1 0 0

Ta m i s (m m )

Tam

isat

s (%

)

lim it ein fé r ieure

lim it esup ér ieure

m é lan ge

bé t o n

p e r t es

0,16

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0,31

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0,63

1,25 2,5

5 8 10 12,5

16

Document from S. GEROMEY’s thesis

showing the granular comparison of the mixture before introduction in the machine and after projection, as well as that of the losses.

The traditional formulation methods of cast concretes do not include these losses and are thus not adapted.

The losses lead to a decrease of large aggregates and thus an increase of fine elements.

Thickness of the sprayed layer

Document from the book of C. Resse and Mr. Vénuat (1981) showing the enrichment of the cement content depending on the thickness of the sprayed layer (maximum cement content against the receiving surface)

Granular curves of a real mixture sprayed by dry process, concrete and losses

Lower limit

Upper limit

mixture

concrete

losses

sieve

In situ cement content

Towards the parement Towards the substrate

Recommended minimal thickness

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The composition is not the only factor influencing the quantity of losses; there are many other and variable parameters on a building site. They are linked to:

�� the mixture: nature and shape of the aggregates cleanliness and fillers content of sands

� the projection: flow and emission speed of the mixture (air pressure) water pressure at the nozzle

� the constraints of the building site: rigidity of the substrate thickness of the layer to be realised framework density

� the skill and experiment of the projection operators: distance nozzle/wall angle of incidence of the projection jet in relation with the substrate regularity of the movement water dosage

These parameters are controllable thanks to the nozzlemen’s skill and the adaptation of the projection equipment. The proportion of losses linked to the formulation can significantly be reduced, knowing that an estimated 15%-20% minimum losses on a vertical wall is necessary to obtain a compact concrete. It is judicious to work out an optimized composition. The low cost of a mixture should not be the only criteria for choice. A non-optimized mixture which generally goes through the machine will produce an important quantity of losses with the result that in the end the cost of concrete on the substrate will be very largely higher than the cost of the product introduced in the machine. We cannot save money by purchasing bad materials.

2.2.2 Grading curve of the aggregates

It is the first step of the formulation. The operation consists in defining the proportion of the various aggregates (sands and gravels) to obtain a grading curve in agreement with the grading zones which appear in the recommendations of AFTES [TOS 117], in standard NF P 95-102 and, since 2010, with the Asquapro grading zones given in the appendix to this booklet. These grading zones were modified to take account of the replacement of the 0.080 mm sieve by that of 0.063 mm in the European standards, and of the experience feedbacks over more than 30 years concerning the grading zones shape between the abscissas 0.5 and 1.5 mm.

Reference grading zone

These grading zones fix upper and lower limitations which result from know-how and experience. The granular characteristics of the available materials do not always allow it, however it is recommended to study the formulation to obtain a curve within the limits of the grading zone between the cut with the 0.315 sieve and the cut at: 6mm for Dmax = 8; 8 mm for Dmax = 10 mm; 10 mm for Dmax = 12.5 mm; 12.5 for Dmax = 16 mm and 16 mm for Dmax = 20 mm. The resultant grading curve must be the most continuous possible.

Dimension of the biggest aggregate

For the concrete sprayed by dry process, it is common to use a Dmax of 8-10 mm, even exceptionally up to 20 mm, which will be the absolute maximum.

The pipes diameter obliges to respect the rule of thumb: Dmax ≤ pipe D / 3 x 1.2 [Resse] to avoid a blockage.

The thickness to be sprayed and the density of frameworks are also to consider in the choice of Dmax.

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Remarks on the ratio Sand/Gravels (S/G)

The S/G weight ratio allows to define the grading curve of the aggregates mix.

The increase of the S/G ratio implies:

• less strengths • more shrinkage • less losses

The choice of the value of the S/G ratio is a compromise between the strengths and the losses, when cracking is not prejudicial. But this ratio is not sufficient and it is essential to study the grading curve of the aggregate mix.

Example of S/G according to the grading zones of standard NF P 95-102 and the AFTES recommendations.

Granular mix with max diameter equal to

S*/G Lower value of the ratio

Upper value of the ratio

8 mm 3.54 7.33 10 mm 3 6.6

12.5 mm 1.85 4.26 16 mm 1.22 2.22 20 mm 1 1.9

* Be careful: in this table, the limit sand/gravel is set at 5 mm whereas it is set at 4 mm according to standard NF EN 12 620. In addition to S/G, the value d/Dmax with d = Dmax /2 is relevant. Indeed the experience showed that by fixing it on the lower limit of the recommended grading zone, even slightly below it, we obtain a good passing of the pulverulent product in the machine and a good compaction while limiting the losses. This means that the sand of the granular mixture must also preferably be as close as possible to the lower part of the grading zone.

2.2.3 Optimization of fine elements

The denomination “fine elements” means any element with a size strictly under 80 µm: cement, fillers, silica fume, fly-ashes and sand fines.

The composition to be defined is that of the mixture before passing through the machine. The addition of cement brings a complement of fine elements smaller than 80µm.

These fine elements have an essential role in the implementation (see booklet “implementation”, paragraph 2.2.3.)

A strong percentage of fine elements reduces the losses by rebounds, increases the effects of shrinkage and reduces the concrete Young modulus. It presents a risk of clogging in the machine and the pipes.

The losses aim to control the fines proportioning: Thanks to the losses of coarser elements, a mixture initially under-proportioned in fines will become richer in fine elements during projection. On the other hand, if the initial mixture is already very rich in fines, the losses will be limited and there will be little fines enrichment. The cement representing the largest share of fine elements, the concrete sprayed by dry process cannot be underdosed in cement.

Technical Committee


The standard NF P 95-102 recommended a fine elements content ranging between 17 and 23%. This standard indicated a minimum cement proportioning of 280 kg/m3 and a percentage of 80 µm passing elements higher or equal to 17% by the weight of the mixture. These values were to be considered with caution because 280 kg/m3 represent only 13% for the cement share. The European standard imposes from now on a minimum of 300 kg/m3.

The experiment shows that the percentage of 63 µm passing elements must vary between 21 and 23% and that the share of cement should not be lower than 17%. This corresponds to a minimum cement proportioning which can vary (depending on the respective cement and aggregates densities) between 330 and 380 kg for the quantity of dry material necessary to obtain 1m3 of compacted concrete, without losses. This corresponds approximately to 165 - 180 kg of cement per ton of dry mixture.

Graph showing the AFNOR grading zone 0/8 mm (grey curves), the curve of the dry mixture (taken out from a big bag) and the curve of the concrete in place according to the analysis of the fresh concrete

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mailles des tamis en mm

TA

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après projection

mélange sec

fuseau AFNOR

Repair works after the fire in the Channel tunnel in 1997

Analysis made on site on the dry mixture and on the samplings of fresh concrete carried out for the external control (at that time the 0.080 sieve was used).

2.2.4 Optimization parameters

Based upon the information given in the previous paragraphs and the experience, the formulation parameters can be defined for the optimization of the bond strength, the limitation of shrinkage and the characteristics in compression:

• Fine elements < 63 µm: ranging between 21 to 22% with a cement proportioning from 375 to 400 kg/m3 and 3 to 4% of fines

• Fineness modulus of the sandy share 0.16 - 5mm of the mixture: ranging between 3.2 and 3.5. • For this sandy share, the value of the mass between two sieves of the set must not exceed 20 to

25%. All this mean to create the most continuous possible curve.

The current formulations are carried out from local aggregates resources: The sands do not always show the above optimized characteristics. Generally we do our best to correct the sands curve. These values of fineness moduli are not an obligation but they are necessary to obtain the strengths required for concretes with a class ≥ 40 MPa.

After projection

AFNOR grading zones

Dry mixture

Sieves mesh size – in mm

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The specifications retained for the choice of cements and of aggregates criteria are indicated at paragraph 3 “Components”.

2.2.5 Consistency

The mixture thus formulated comprising sand, gravels, cement and possibly additives has a pulverulent consistency1 which requires to take into account apparent volume and mixture density criteria.

These criteria vary depending on the water content of the pulverulent mixture:

� for a dehydrated mixture, such as products distributed in bags, big-bags or silos, the apparent density is about 1,750 kg/m3, that is to say a bulking factor of 1.25.

� for a mixture slightly humidified by the water content of the aggregates, the apparent density is about 1,450kg/m3, that is to say a bulking factor of 1.5.

These parameters are not without influence:

� The spraying machines outputs are indicated in relation to the volume capacity of the mechanical device for the distribution of the mixture; it is an apparent volume which is given and which passes through the machine. � For the mixtures delivered from a ready-mixed concrete plant, the equivalent of the weight of the mixed mixture for 1m3 of concrete will occupy 1.5m3 in the tank of the mixer truck. It is an apparent volume which is delivered. If you order 4m3, you must have approximately 6 m3 of mixture on site for a water content ranging from 3% to 4%.

2.2.6 Additions

The attention of the formulator is drawn to the aftereffects of the addition of products like silica fumes or fillers (risk of flaw on the reinforcements coating) during projection.

Moreover, the use of silica fume increases the shrinkage leading to surface cracking (see booklet “implementation”).

If needed, please refer to the document of national application of the standard NF EN 206-1 which specifies all the standards relating to the additions.

2.2.7 Additives

The additives are components which are used with a proportioning ≤ 5% of the cement weight. Beyond 5% we talk about additions for cast concretes.

The standard NF EN 206-1 refers to the standard EN 934-2.

For sprayed concretes, the standard NF EN 934-5 applies.

In general, additives are not often used for projection by dry process. They can however be used for particular applications in which one wishes to increase the thickness of the layers sprayed in a single pass (projection on ceilings or vaults). See paragraph 3.4 « Components ».

1 The term « consistency » is not used here with the same meaning as in paragraph 4.2.1 of the standard NF EN 206-1, in which it is specified that there is no subsidence class corresponding to concretes with a "humid soil" consistency intended to be compacted thanks to a particular process (here, projection).

Technical Committee


2.2.8 Fibres

The composition of a mixture with fibres must take account of the fibre losses during projection, which leads to an overdose depending on the type of fibres. This type of composition can be approved only by a study.

See paragraph 3.4 “Components” and appendix A of the standard NF EN 14 487-1.

2.2.9 Water

The water being added at the projection nozzle, it is only taken into account depending on the manufacturing process of the mixture:

• Dry industrial mix ready for use, the water is not taken into account at the time of the formulation, the aggregates having a water content lower than 0.5%, • Mixture from ready-mix plant or on-site plant, the water content is limited to 5% of the aggregates (percentage by weight).

The water added at the nozzle must have the same qualities as the mixing water of a cast concrete. It must thus comply with the standard NF EN 1008 “mixing water for concrete”

Technical Committee


2.3 Composition of concretes sprayed by wet process

The text is in blue to avoid confusions with the dry process.

2.3.1 Issues

The projection of concrete by wet process must reconcile the discrepancy that characterizes this technique: the pumpable plastic mixture must be almost instantaneously stiffened to be able to hold on the substrate. Generally the mixtures are formulated from the local aggregate resources. All the work of formulation consists in optimizing the curve of the granular composition and in making a judicious use of additives.

The formulation of a mixture to be sprayed by wet process must take account of several criteria:

• pumpability of the mixture in the machine, in the transfer pipe and in the nozzle (in spite of the low diameter and the interactions of the air and the stiffening agent),

• projectability of the mixture by limiting the losses,

• sticking of the concrete on the substrate,

• Required characteristics considering the use of a stiffener or an accelerator which can lead to losses of mechanical resistances in the medium and long term, even in the worst cases reduce the concrete to dust after 20 years!

The most problematic point remains the pumpability of the mixture. Any difficulty during the pumping can generate important disorders in the pace of works; the blockages being difficult to evacuate, they generally require much handling and can have consequences on the quality in place. So the composition must carefully be studied and its reproducibility perfectly assured.

The traditional methods of formulation of cast concretes are adapted for concretes sprayed by wet process. They should nevertheless be supplemented to take account of the concrete behaviour (sticking) on the substrate which requires the quasi obligatory use of a stiffening additive or a setting activator.

There are also losses (about 10 to 15%) but they do not modify the final composition of the concrete.

The parameters influencing the pumpability are many and variable on a building site. They are linked to:

- the mixture:

composition, fines content nature and shape of the aggregates additives type and quantity of the fibres used,

- the transport:

distance pipe diameter pipe layout

- the environment of the building site and the ambient temperature

Technical Committee


The parameters influencing the behaviour on the substrate are many and variable on a building site. They are linked to:

- the projection: flow and emission speed of the mixture (air pressure) choice of the type of stiffener and introduction process

- the constraints of the building site: rigidity of the substrate thickness of the layer to be realised framework density

- the skill and experience of the workmen, nozzlemen and/or robot pilot: distance nozzle/wall angle of incidence of the projection jet in relation with the substrate regularity of the movement

These parameters are controllable by an elaborate composition requiring the use of additives and additions, by the skill of the operators and the adaptation of the projection equipment.

If the use of the necessary stiffening additives is not controlled, it has a direct repercussion on the final quality of the concrete in terms of resistance and durability.

2.3.2 Grading curve

The approach is comparable with the one described for the dry process.

Reference grading zone

The inscription of the obtained grading curve is done as indicated for the dry process.

Dimension of the biggest aggregate

For the concrete sprayed by wet process, it is common to use Dmax from 8 to 12mm, 16 mm being regarded as a maximum. The choice of Dmax depends on the type of machine used and the possibilities of local resources. The thickness to be sprayed and the frameworks density must also be considered for the choice of Dmax.

Remarks on the S/G ratio

This ratio is not sufficient to characterize a ‘wet process’ composition. It is essential to study the grading curve of the aggregate mix by taking account of the pumpability criteria and the obtaining of an optimum compactedness. The determination of the S/G ratio can be made thanks to the “Baron Lesage” method but while judiciously intervening on the proportioning of the fine elements.

2.3.3 Fine elements

The composition to be defined is that of the mixture before passing through the machine. The addition of cement brings a complement of fine elements smaller than 63 µm.

The denomination “fine elements” means all the elements of size lower than 63 µm: cement, fillers, silica fume, fly-ashes and sand fines.

Technical Committee


These fine elements have an essential role during the implementation (see booklet “implementation”, paragraph 2.2.3)

The experiment for a common concrete shows that the minimum cement proportion in the dry mixture should not be lower than 400 kg/m3. A proportioning lower than 400kg/m3 can be used but this requires a more elaborate formulation concerning the additions of fine elements.

2.3.4 Optimization parameters

Based upon the previous paragraphs and experience, optimization parameters can be defined:

• Cement minimum proportion of 400 kg/m3

• Fineness modulus of the sandy share 0.16 - 5mm of the mixture inferior to 2.5 mm

• For this sandy share, the value of the mass between two sieves of the set must not exceed 20

to 25%. All this mean to create the most continuous possible curve.

Chronology of the approach to obtain the composition:

• Study of the S/G ratio to obtain the optimum compactedness,

• Define the proportion of fine elements (cement, filler, sand fines) for pumpability.

2.3.5 Consistency

Pumpability involves selecting a consistency corresponding to a slump category from S3 to S4. The consistencies of the slump categories S1 and S2 are very difficult to use. It must also be adapted to the type of pump and to the exit diameter. To be pumpable, the concrete must have a greasy aspect and must not sweat nor have segregation. In any case, plasticizers or superplasticizers even retarding agents or setting inhibitors (stabilizers) and air-entraining agents will be used.

At the exit of the nozzle, it is advisable to correct the plasticity which has become excessive by introducing a stiffener-setting accelerator.

2.3.6 Additions

Unlike the dry process, the use of additions is useful to optimize the granular skeleton. Please refer to the national document concerning the application of the standard NF EN 206-1. This document specifies all the standards relating to the additions.

2.3.7 Additives

The use of additives is almost always necessary to develop a composition for the wet process. The additives are components which are used with a proportioning ≤ 5% of the cement weight. Beyond 5% we speak about additions. In this case, they are classified in the standard NF EN 206-1 in the heading “Other additions”. These additions can be added to the concrete - in agreement with the user - to confer it special properties. These other additions must be the subject of justifying studies. The standard that relates to sprayed concrete (EN 934-5) authorizes a proportioning > 5%.

See the table, chapter 3.4 “Additives and additions for sprayed concrete”.

Technical Committee


2.3.8 Fibres

The fibres influence the pumpability by increasing the mixture cohesion. The mixture cohesion must be put right by adapting the quantity of the fine elements “paste” and introducing additives to “break” the cohesion.

2.3.9 Water

As for the dry process, the water must have the same qualities as the mixing water of a cast concrete. It must thus comply with the standard NF EN 1008 “mixing water for concrete”.

Effective water includes:

• The mixing water introduced during the mixing process

• Part of the water brought by the aggregates

• The water contained in the liquid additives introduced during the mixing process

• Do not forget the water brought by the introduction of liquid additives at the nozzle.

The water will determine the resistance by the W/C ratio. The process is comparable with that of cast concretes.

It is necessary to adjust the consistency by using plasticizers or superplasticizers rather than with a water addition.

Technical Committee


33 CCOOMMPPOONNEENNTTSS

3.1 Cement

It is specified that the use of cements is standardised in standard NF EN 197-1. The attention is drawn to the fact that this standardisation requires to take into account the added annotations kept for a use on the French territory (annotation “sea set” for example. In case of use in other countries of the European Community it is appropriate to check that the cement composition is compatible with the environment class of the works; in our example of sea works, the clinker percentage and the C3A content. To choose the cement best adapted to your application, it is first necessary to answer the following questions:

1) Are they final or provisional works? Will a provisional phase be necessary (particular cases of water inflows, frozen zones, other constraints related to the substrate…)? 2) Which is the strength aimed at 28 days? 3) Are there requirements of short-term strength? If so, what are they? 4) Which durability is aimed? In which environment? 5) During which season the works will be carried out? 6) Are the aggregates potentially reactive with respect to the alkali-reaction? 7) Which are the aesthetic requirements (colour, others)? 8) Is it about a particular application? (prestressed concrete? refractory concrete?)

Note: the projection method (wet or dry process) does not directly influence the choice of the cement. Please remember that class 32.5 is seldom enough for sprayed concretes.

Here is how your answers to these questions will guide you towards the best adapted cement:

3.1.1 Are they final or provisional works?

For final works, standardized cements are compulsory for public procurements because they present maximum durability guaranties. For provisional works or phases, standardized cements shall be used in priority. Combined with additives, they cover a very wide range of use. Only particular constraints such as frozen or very wet substrates, or works in a drawdown zone, lead to the use of non-standardized cements. Non-standardized cements are:

• special cements that meet specific needs when standardized cements, even with additives, do not have the required performances: generally they are used for provisional works before a final phase; they are elaborate cements with a Quality Assurance Plan, European Technical Approvals and an EC marking.

• But sometimes they are also badly labelled cements (unknown source, unclear characteristics…). They do not present any quality guarantee and so are never used in civil engineering works.

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3.1.2 Resistance aimed at 28 days?

The answer to this question largely defines the cement class to use.

We generally use a cement class 52.5 for the wet process starting from class C35/45 and for the dry process starting from class C40/50. For lower strengths, a cement class 42.5 is used.

To a certain extent, we can counterbalance a lower class cement with a higher proportioning. However, this approach does not always make it possible to obtain a fines content compatible with the “projectability” of the mixture by dry process (range of the standard NF P 95-102 between 17 and 23% of elements smaller than 80 µm – see paragraph “formulation”).

3.1.3 Requirements for short-term strength?

“Short term” strength requirements - up to 24 hours - are indexed in the standard NF EN 14487-1 (table 8) in three classes: J1, J2 and J3 (see graphic below). The curves A, B, C determine the limit between these classes. J1 is the field ranging between the curves A and B. J2 lies between B and C. J3 is above the curve C.

We first define the required short-terme strength class. A given composition leads to different short-term strength classes depending on the concrete temperature which is a key parameter of the hardening kinetics at young ages.

The tables below show, in dry process and then in wet process, how to choose a type of cement allowing to reach the desired level of short-term strength:

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Dry process

Fresh concrete temperature

J1 J2 J3

5°C to 15°C CEM I 52.5 and accelerator dosed at less than 5% or Quick cement type Portland cement without gypsym2

CEM I 52.5 and accelerator dosed at more than 5%3 or special quick cements

Quick special cements

or

CEM I 52.5 and accelerator dosed at more than 5% above 10°C

15°C to 25°C CEM I 52.5 CEM I 52.5 and accelerator dosed at less than 5% or Quick cement type Portland cement without gypsym

CEM I 52.5 and accelerator dosed at more than 5% or Quick special cements

25°C to 35°C CEM I 52.5 CEM I 52.5 CEM I 52.,5 and accelerator dosed at less than 5%

Note: cements CEM I 52.5 R facilitate obtaining high strengths at young age but increase shrinkage cracking.

Wet process

Fresh concrete température

J1 J2 J3

5°C to 15°C Concrete made of CEM I 52.5 R and an accelerator dosed at more than 5%

Concrete made of CEM I 52.5 R and an accelerator dosed at more than 5%

performances currently impossible to obtain

15°C to 25°C Concrete made of CEM I 52.5R and an accelerator dosed at less than 5%

Concrete made of CEM I 52.5R and an accelerator dosed at more than 5%

performances currently impossible to obtain

25°C to 35°C Concrete made of CEM I 52.5 R and an accelerator dosed at less than 5%



3.1.4 Environment and required durability?

Please refer to the environment classes of standard NF EN 206-1.

Warning : Currently, the way the requirements are expressed is changing: instead of an obligation of means (not imposed, or less and less), it is a performance requirement which is increasingly demanded. It is a logical evolution that it is useful to continue. Nevertheless we are only at the beginning of this process and today it is impossible to guarantee a 120 years durability, as that is

2 Fréquemment utilisé en Allemagne 3 Attention à la durabilité si trop fort dosage en accélérateur

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sometimes required… The first production of cement goes back only to 1842 and the beginning of the rise of industrial cement goes only back to the years 1880!

In this paragraph about the choice of cement, let us insist on the fact that the durability does not depend only on cement but on formulation (interactions between all the components) and on the implementation of concrete. The choice of an adapted cement cannot guarantee, under no circumstances, the durability of all the concrete material. However the concept of a minimal proportioning still remains a durability criterion.

In the spirit of the “performance oriented way” quoted above, accelerated life tests on the selected concrete formula are necessary. The difficulties lie not only in the representativeness of the tests but especially in the way from “accelerated time” to “real time”. This knowledge will get sharpened thanks to increasing feedbacks. Now all you need is a little patience!

3.1.5 Influence of climatic conditions?

Please refer to the paragraph “Adaptation to specific needs - hot weather/cold weather” of the present document.

In the case of strength requirements at young age, please refer to the tables of paragraph 3.1.3 which take into account the temperature influence.

3.1.6 Reactivity of the aggregates?

If the use of potentially reactive aggregates with respect to the alkali reaction is considered, the choice of a cement with a low alkali content may be necessary to fulfil the balance of all the alkali [Recommendations for the prevention of disorders due to the alkali reaction, 1994]. However these recommendations should be treated with caution. They are applicable for the wet process because after projection the concrete keeps its initial manufacture composition. For the dry process, the layer in contact with the substrate is strongly enriched in cement and does not contain aggregates. It should not thus present any phenomenon of alkali-silica reaction. For the rest, it can be envisaged to take into account an average cement enrichment (up to 20%) to calculate the alkali balance. For all that relates to the accelerated tests, it is necessary to saw the test-tubes in boxes and to pay great attention to the reliability of the positioning of the measurement studs.

3.1.7 Aesthetic requirements?

To obtain a facing with a uniform aspect (colour or texture), it is necessary to implement a final layer (thickness 1 to 2 cm) whose function is only aesthetic. This final layer falls under the coatings technique. For clear colours, it is possible to carry out this layer with a sprayed concrete containing white or clear cement not improved.

It is more difficult to guarantee a given colour for sprayed concrete than for cast concrete. It is even more difficult in the dry process than in the wet process. The structural role of concrete can be distinguished, without any requirement for colour, and a coating used to meet aesthetic needs.

In a general way, the cement strongly affects the final aesthetics of the concrete like all fines. For sprayed concrete, its influence mainly concerns the colour.

To obtain a clear colour concrete, a clear cement should be used. A clear filler added to a dark cement is not enough.

White cements are very seldom used for sprayed concretes. They are nevertheless essential to repair white concrete constructions (for example: the Ministry of Finance in Bercy, the Hertzian tower of

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Sainte Baume) or to carry out works with a very clear coloured concrete (Caisse d’Epargne of Bordeaux-Mériadec) or tunnels intrados (tunnel in Sisteron).

The addition of colourants is rather frequent (embankments, cliffs). It does not modify the choice of cement as the colourant is an ordinary inactive filler. If dark colours are chosen, it is almost impossible to guarantee the uniformity of the final aspect.

The aesthetic defects such as efflorescences do not exclusively come from cement but from the whole of the concrete formula and the variations of the atmospheric humidity.

3.1.8 Prestressed concrete - Refractory concrete

For the use of concrete together with post tensioning frameworks, it is recommended to choose a formulation with aggregates whose quality standards are imperatively all of class A indexed A (see specifications below). For sands, the cleanliness criterion must be characterized by a value of sand equivalent higher than 85. For the use of sprayed concrete in a site subjected to strong temperatures (cement kiln for example), it is imperative to use a mixture formulated with cements and “refractory” aggregates; these formulations are worked out by suppliers specialized in refractory materials.

3.2 Aggregates for sprayed concretes and mortars

The aggregates specifications are those of the standards NF EN 12-620 (aggregates for concretes), NF EN 13-139 (aggregates for mortars) and XP P 18-545.

In this last standard, the aggregates are codified by letters (A, B, C and D). Moreover, these codes are indexed. It is advisable to specify the codes and indexes corresponding to the project features in the Special Technical Specifications according to the environment classes, (see chapter 10 of the standard XP P 18-545).

To take account of the implementation mode, the code to be specified on the order for aggregates for sprayed concrete is A: all the characteristics must be indexed A.

For the concretes with a strength class higher or equal to C45/55, the following values are to be specified:

Specified value (Vss) at 90% if more than 14 results

Fineness modulus MFA limit values Tolerances on Vs

2.4 ≤ ΜF ≤ 3.2 E 0.6 (± 0.3)

Absorption of water ABA Vss ≤ 2.5 Mechanical strength Los Angeles coefficient LAA

LA ≤ 30%

Flatness coefficient D≤10mm AA

D≥10 mm AA (NF P 18-561)

FI ≤ 25% FI ≤ 20%

Cleanliness of sands SE SE ≥ 85

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3.2.1. Fine gravels

The code to be specified is GrA. The attention is drawn to the incidence of the aggregates angularity, characterized by the flatness coefficient which is now only measured on alluvial or marine materials as defined by the standard XP P 18-545. For the formulation of a mixture that will be sprayed, it is advisable to check that the fine gravels resulting from crushed materials are close to the values indicated in the foregoing table. For the wet process If crushed fine gravels are used, it is necessary to improve the formulation with an additive for pumping.

For the dry process If dry and porous fine gravels are used, there is a risk the connection paste/aggregates will get weaker because of the absorption of water. This point justifies the fine gravels water absorption criterion lower than 2.5.

3.2.2. Sands

Due to the importance of sands cleanliness, the value required for the equivalent of visual sand must be higher or equal to 85. This value is more severe than that required in the standard XP P18-545 but it determines the quality standards of sprayed concrete as well as of a good passing in the machine. It is essential for strengths ≥ to 40 MPa.

3.3 Fibres

The fibres used in sprayed concretes and mortars are the same type as those employed for cast concretes and mortars.

The specific aspects of the fibres characteristics are not included in this document, so please refer to standard NF EN 14889-1 for steel fibres and to standard NF EN 14889-2 for polymeric fibres, as well as to the manufacturers’ technical documents. Please note that it was not possible to write “metal fibres” instead of “steel fibres” in the European standard because of the presence of this denomination in a former European standard still in use today. Nevertheless, the “sprayed concrete” working group admitted the use of the standard NF EN 14889-1 for amorphous cast iron fibres quoted hereafter (amorphous cast iron contains, just like steel: iron, carbon but with a higher percentage, and chromium).

Fibres are used in response to various expectations:

� Anti-shrinkage cracking (steel or synthetic fibres)

� Improvement of the behaviour under bending stress (rather steel fibres, even some synthetic

fibres subject to confirmation of the ductile aspect of the break)

� Impact resistances (steel fibres)

� Corrosion resistance (synthetic or amorphous cast iron fibres; however, please note that

corrosion is superficial for traditional steel fibres and it is only a matter of aesthetic aspect)

� Limitation of the flaking off phenomena in case of fire (monofilament polypropylenes fibres,

diameter < 50 µm).

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3.3.1 Fibres content

The fibre proportioning in the starting mixture must take account of the impoverishment rate related to the projection mode, namely, as an indication for steel fibres:

� 30 to 50% for the dry process,

� 5 to 20% for the wet process.

3.3.2 Precautions

Some fibres are incompatible under alkaline conditions. It is the case of fibreglass and polyester fibres whose protection of the edges or of the fracture surfaces can create a problem.

3.4 Additives and additions for sprayed concrete

3.4.1 Definition, implementation and standardization

For cast concretes, the additives are components which are used with a proportioning ≤ 5% of the cement weight. Beyond 5% we talk about additions. For sprayed concretes, we can go beyond 5% for specific additives (see NF EN 934-5).

The additives and additions are either liquids or powders. In any case, the proportioning is controlled during the implementation as well as any other component of the concrete formula.

They are implemented:

during the fabrication of premixed products and delivered on site in bags, big-bags or in silos (pulverulent additives)

or during the mixing in a ready-mix plant (pulverulent or liquid)

or during projection, at the exit of the pipe.

In all the cases it is necessary to check their homogeneous distribution in the concrete.

Concrete “traditional” additives are the subject of the standard NF EN 934-2. There is a standard (NF EN 934-5) which applies to sprayed concrete additives. This standard introduces in particular elements concerning the alkalinity and the acidity of the products.

3.4.2 Which additive to use?

The question is:

“Which is (are) the characteristic(s) of fresh or hardened concrete I wish to bring or modify?”

The choice of an additive or an addition is initially defined in term of product family: plasticizer, superplasticizer, air-entraining agent, accelerator, stabilizer, water-repellent, siliceous or calcareous addition, etc.

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3.4.2.1. The accelerators

They can be classified in two families depending on their action: Family 1: in the short run (in a few tens of minutes, even a few hours): they are the hardening accelerators. They accelerate the speed at which the concrete hardens in the short-term by accelerating the formation of hydrates. Family 2: in the very short term: they are the stiffeners (instantaneous action) and setting accelerators (action in a few minutes). The stiffeners accelerate the “stiffening” on contact with the substrate. After the stiffening, the setting accelerators make it possible to implement a new projection pass after a few minutes on the previous layer which has begun to set. This way, it is possible to spray a thick layer in several steps in the same projection station. The additives for sprayed concrete (see NF EN 934-5) generally cumulate the functions of stiffener and setting accelerator. It should be made sure that they do not damage longer-term strength or durability. It is thus important to well define the moment at which the acceleration must be obtained, whatever the denomination used: accelerators, stiffeners, activators or other: when the concrete arrives on the substrate or to be able to put a strain on the material more quickly? It is specified that if certain hardening accelerators can have an impact on the setting time, on the contrary setting accelerators do not accelerate the rise of the strength. The setting accelerators antifreeze function should not be taken into account because projection must be carried out on a substrate whose temperature is higher than +5°C (except in specific cases).

The products containing aluminates and sodium silicates are generally considered “alkaline”.

Aluminates are prohibited in France.

The ASQUAPRO proposes to prohibit the use of silicates, because:

� they present important health risks for users and the environment, � they strongly damage the mechanical strengths of concretes in the medium and long

term, in case of an excessive dose from 3% to 5% depending on the products, � they bring additional alkalis (alkalis ratio with respect to the alkali-reaction).

It is important to indicate this prohibition of alk aline accelerators in the special technical specifications in order to prevent some companies from making their costing on the basis of these inexpensive products. The “non-alkaline” products cover today all the possible range of use.

3.4.2.2. Which product family?

The following tables allow to determine the products family to be used depending on the characteristics of fresh or hardened concrete that one would like to bring or modify.

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Desired characteristics

Projection mode

Additives and additions

Desired effect Negative effect and possible fixing

Evaluation method

Pumpability WP Plasticizers Superplasticizers

Increase plasticity with constant water content

Can modify the setting time. Can entrain air. Requires the use of a setting accelerator.

Measurement of the pumping pressure

Setting retardant Allows to maintain the concrete workability depending on the proportioning

Stops the setting. Requires the use of an activator

Check the plasticity

Improvement of strengths Behaviour / holding Behaviour / holding

WP WP WP

Plasticizers Superplasticizers

With a constant water content, increases plasticity. Requires the use of a stiffener

Can modify the setting time Can entrain air

Measurement of the implemented thickness

Air-entraining admixture

Increases the volumetric efficiency

Reduction of mechanical strengths


Setting accelerators Reduces the implementation time. Increases the thickness implemented

Possible reduction of strengths Defects in the frameworks coating


Silica fumes

Increases the thixotropy and concrete cohesion

Tendency to flocculate : fix with a superplasticizer. Increases shrinkage


DP

Stiffeners Reduces the implementation time. Modifies the rebound and compactedness. Increases the implemented thickness

Possible reduction of strengths Defects in the frameworks coating


Silica fumes Increases the thixotropy and concrete cohesion

Tendency to flocculate


Increase of initial strengths

WP

Hardening accelerators

The effect depends on temperature. It may be interesting to couple them with a superplasticizer.

Can reduce the strengths in a longer term perspective. Increases shrinkage. Can accelerate the beginning of setting

Measurement of mechanical strengths at young age

DP Hardening accelerators

Effects depends on temperature

Can reduce the strengths in a longer term perspective. Increases shrinkage. Can accelerate the beginning of setting

Measurement of mechanical strengths at young age

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Desired characteristics

Projection mode

Additives and additions

Desired effect Negative effect and possible fixing

Evaluation method

Increase of mechanical strengths

WP


Reduction of water content with constant workability

Can modify the setting time. Can entrain air.

Measurement of mechanical strengths

Silica fumes Puzzolanic reaction and increase of compactedness

Tendency to flocculate (fix with a superplasticizer)

Measurement of mechanical strengths

Decrease of shrinkage

WP


Reduction of water content with constant workability


Measurement of shrinkage

Increase of durability

WP


Reduction of water content with constant workability. Improvement of compactedness


Measurement of porosity

Silica fumes Increase compactedness, resistance to abrasion, some aggressive environnements and to the alcali reaction

Tendency to flocculate (fix with a superplasticizer) Increases shrinkage


DP Silica fumes Increase compactedness, resistance to abrasion, some aggressive environnements and to the alcali reaction

Tendency to flocculate (fix with a superplasticizer) Increases shrinkage


Resistance to the effects of freezing

WP

Air-entraining admixture

Increases the volumetric efficiency. formation of micro air bubbles in the mixture

Reduction of mechanical strengths

Measurement of average distance between the bubbles, on samplings of hardened concrete


Reduction of water content with constant workability Improvement of compactedness.


Spall test

Watertightness WP

Waterproofing compound

Obstruction of capillary network

Measurement of permeability to pressurised water


Reduction of water content with constant workability Improvement of compactedness.

Can modify the setting time. Can entrain air


DP Waterproofing compound

Obstruction of capillary network


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Note: When the table does not present a heading concerning the dry process for a given characteristic, the desired effect can be obtained by modifying the concrete formulation (grading, cement proportion, etc). It is always preferable to start by adapting the formulation to the desired characteristics.

3.4.2.3. Which product in the family?

Once the family is chosen, it is appropriate to select the adapted product among the range of products proposed by the manufacturers.

The questions that demand answers are:

Which are the side effects, acceptable or not, for this specific application? Which are the impacts on hygiene and safety (dust, aerosol effects…)? Which are the storage conditions of the products? (storage life, frost protection,…) Is the product easy to use (easy proportioning, consequence of a dosage error…)?

3.4.2.4. Do not forget:

A good additive never counterbalances a bad concrete mix formulation. It is used to improve some of its characteristics or performances.

When an additive is used, it is considered as an integral part of the concrete formulation and its compatibility with the other components must be checked.

The technical and safety specifications of the products must be read.

Additives must not be mixed together without asking for the opinion of the supplier and/or a preliminary test.

3.5 Water

The criteria are the same as for cast concrete. The only thing that differs is the moment of introduction depending on the projection mode.

It must thus comply with the standard NF EN 1008 “mixing water for concrete”.

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4.1 Concretes implemented in hot or cold weather

4.1.1 General points

The precautions to take to obtain quality sprayed concretes, even in hot or cold weather, follow the same rules as for cast concretes. They must be taken into account for the concrete formulation and its implementation.

One of the characteristics of sprayed concrete is that it is used in average thickness (10-20 cm), seldom protected by a formwork and seldom as a mass concrete. In consequence, sprayed concretes are more sensitive to the outside temperatures: these of the substrate and of the outdoor air.

But at young age, the cement hydration speed, therefore the concrete hardening speed, is practically proportional to its temperature and the concrete durability is played at very first age since it is at this time that it has a maximum vulnerability to the desiccation phenomenon.

4.1.2 Consequences of heat

4.1.2.1 On fresh concrete (before setting)

The modifications of the cement hydration kinetics involve:

� a faster setting,

� a loss of rheology.

The concrete manoeuvrability deteriorates more quickly. It can lead to difficulties in the pumping and in the implementation of concrete spread by wet process.

A faster evaporation of the mixing water causes:

� a faster stiffening,

� and thus a risk of inopportune water addition which would be harmful to the concrete.

As an example, on a plastic mixture for a projection by wet process, a slump (Abrams cone) of about 10 cm measured on a fresh concrete at 20°C will fall to approximately 4 to 5 cm if the temperature rises up to 35°C. On a wet pulverulent mixture for a projection by dry process, the time of use, from mixing to projection, will be shortened from 2 hours for a temperature of 20°C to 45 minutes if the temperature rises up to 35°C.

4.1.2.2 On concrete at young age (after the setting has begun)

An acceleration of the concrete hardening process due to a rise in temperature leads to: � an increase of the strengths at young age,

� but a mineralogical structure little organised and thus lower strengths at 28 days (the optimization of long-term strengths being obtained with a treatment at 13°C).

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A faster water evaporation starting from a temperature > 35°C, an hygrometry < 65% and a wind speed > 20 km/h leads to:

� an increase in porosity and permeability of the concrete surface layer,

� a deterioration of the interface paste-aggregate,

� a lower protection of the frameworks (carbonation of the surface layer which is more porous),

� a weakening of the mechanical properties,

� a weakening of the connection by grip,

� an imperfect hydration of cement,

� a crack due to plastic shrinkage.

Relative humidity (%) Concrete temperature (°C)

Air temperature (°C)

Evaporation speed

Evaporation speed In kg/m²/h

Air speed –km/h)

Example : Air temperature: 25°C Relative humidity: 50% Concrete temperature: 27°C Air speed: 24 km/h

� Evaporation speed: # 1.2 kg/m²/h

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4.1.3 Rules to be retained in hot weather

FORMULATION RULES Cement: − use cements with low hydration heat, − limit the use of class R cements (too much reactive) Aggregates: − clean and protected from the sun during storage − avoid aggregates with important porosity − in the case of projection by dry process, make sure the aggregates temperature is not too high after their drying. Mixing water: − for the dry process, no specific recommendations − for the wet process, apply the BPE indications − never add water upon arrival on the building site Additives: − for the dry process, no specific recommendations − for the wet process, use setting retardants. The plasticizers have a side effect: they limit desiccation

IMPLEMENTATION RULES

DRY PROCESS

Dry pulverulent products: storage temperature lower than 50° C Wet pulverulent products: Limit the time of use between the mixing and the implementation, depending on the temperature: − temperature ≤ 20°C: time of use up to 2 hours − from 20°C to 25°C: time of use up to 1.5 hour − from 25°C to 30°C: time of use up to 1 hour − from 30°C to 35°C: time of use up to 45 minutes − beyond 35°C: postpone projection

WET PROCESS Plastic products, ready mixed mixture: − If the formulation does not take into account the temperature constraints, apply the same rules as for the wet pulverulent mixture. − If the formulation takes into account the addition of retardants, the time of use must be validated by suitability tests.

PROTECTION AGAINST DESICCATION (DP and WP)

− The most effective action is humidification during one week (absolute minimum 72 hours), by

regular water sprayings. − Treatment products can be used or wet geotextiles installed, possibly supplemented by plastic

covers to avoid desiccation (depending on the evaporation speed, the effectiveness of the treatment products can be limited).

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4.1.4 Rules to be retained in cold weather

It is recalled that the temperature of the substrate must be higher than 5°C. This is particularly important for thin coatings.

Exceptionally, for urgency reasons and for coatings of ground protection, it can be envisaged to work at lower temperatures by taking into account the following recommendations, knowing that there will be a delay in the setting and hardening.

For a substrate temperature between 0°C and 5°C:

FORMULATION RULES Cement: − use cements with high hydration heat, strong initial strengths or fast special cements − increase the cement proportion, Water: Heat it Additives: Water reducer, accelerator. Aggregates: Heat them Ready-for-use dry mix: Heat it

IMPLEMENTATION RULES − increase of the mixing time − Quick transport and implementation, − Take care of the concrete treatment but the treatment by watering is prohibited. It must be replaced

by the installation of a plastic film covered with an insulating material such as rockwool or any other thermal protection.

4.2 Concretes with fast setting and/or hardening

4.2.1 General points

The sprayed concrete technology is well adapted for the implementation of fast setting and fast hardening concretes. As soon as it is sprayed in a sufficient thickness, the setting can begin. The setting is immediately followed by the concrete hardening.

The setting phenomenon appears right after the loss of manoeuvrability. It corresponds to a change of state, a “coagulation”, causing the stiffening of the concrete. It is a transitory episode.

The hardening phenomenon corresponds to the formation and the precipitation of hydrated minerals. It hardens the cement matrix. The resulting strengths are then measurable and increase over time.

For concrete sprayed by wet process, the successive operations of mixing, transport and pumping impose time constraints which must be taken into account in the choice of the cement. The accelerator being added at the nozzle, there is no problem of premature stiffening of the concrete.

As concrete sprayed by dry process does not have these manoeuvrability constraints over time, the accelerator can be added either at the end of the nozzle in liquid, or in powder in the dry mixture to be sprayed at the moment of its introduction into the machine.

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4.2.2 Needs

The fast setting and fast hardening mixtures for projection are very useful:

� to stick on a wet and/or dripping substrate without subsiding, � for a fast return to service of the treated structure, � to quickly apply a great thickness without subsidence � to secure a wall, � at low temperatures, to reach a minimum strength before the arrival of freezing, � to increase the work pace.

4.2.3 Classification

Please refer to table 8 of standard NF EN 14 487-1.

4.2.4 How to obtain them?

We have the choice between using a Portland cement setting accelerator or using a fast setting and hardening cement.

4.2.5 Accelerators

They are used:

• Either in the same proportion as additives (< 5%),

• Or in the same proportion as additions, between 5 and 10% of the cement weight (see standard NF EN 934-5).

In any case, they must be regarded as full components of concrete.

It is important to take into account the dosing mode of the accelerator. A liquid product is added at the nozzle, considering its volume. This way, it will not be as precise and reliable as a powder accelerator which will be weighed at the factory to be added to a ready-for-use dry concrete.

There are many accelerators with various compositions. Do not forget that the first accelerator is heat. It is possible to distinguish the main groups:

• calcium chlorides,

• alkaline bases and their salts: KOH, NaOH and sodium silicates and aluminates,

• calcium nitrates and nitrites,

• calcium formate,

• aluminium derivatives: hydroxides, aluminium sulphates and reactive aluminates.

By modifying the hydration process, these additions can have an impact on the durability of concrete: • for example, the calcium chloride will form a calcium mono-chloroaluminate which corrodes steels. This is why the use of calcium chlorides is highly regulated, • the hardening kinetics can be modified. If alkaline bases and their salts are used, the strengths of the concrete at 28 days can be weaker than those of the control concretes without accelerator. Do not forget that some products are dangerous for people and the environment (sodium silicates and aluminates). The latest-generation non-alkaline accelerators do not have these defects.

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Whatever the products used, it is imperative to follow the advice given by the suppliers concerning compatibility with the type of cement and use.

4.2.6 Quick-setting cements

- characteristics of cements used They are more adapted to the dry process; they have a specific mineralogical composition which will offer, after hydration, a quick setting and hardening without using accelerators. Rapidity is brought by:

• minerals such as some calcium aluminates and sulfoaluminates in varying proportions. The other minerals are more traditional (tri- or di-calcium silicates), • a higher fineness than current Portland cements, • the absence of gypsum which, as a setting regulator, runs counter to rapidity.

We distinguish:

• quick-setting cements as such:

Ciment Prompt Naturel (CNP PM - NF P 15-314) Vicalpes Chronolith Surpracem Dorocem, etc…

• aluminous cements (NF EN 14647), They quickly harden (as of 4 to 5 hours) but have a classic setting time. A minimum proportioning (400 kg/m3) as well as maximum W/C ratio (0.4) must be used to optimize the durability of this kind of concrete. Indeed, under certain conditions the conversion phenomenon which is a transformation of the hydrates can involve a decrease of strengths in the relatively long term.

• Underwater cements (NF EN 14647)

• mixtures made of Portland cements with aluminous cements quickly set in a certain proportion of each one of them. To obtain a progressive and continuous increase of the strength, it is necessary to use an additive.

• Portland cement clinkers without gypsum addition. The gypsum regulating role is not required any more.

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4.2.7 Formulation rules

4.2.7.1 general rules

The fast setting concretes follow the same formulation rules as Portland cement concretes with regard to grain size. Only the accelerator or the quick-setting cement proportion differs from traditional sprayed concretes’. It is recommended to contact the suppliers to obtain more precise information on accelerator or quick-setting cement proportioning.

4.2.7.2 proportionings

The proportioning of a quick-setting cement depends on the strengths at young age as well as those at 28 days.

If an accelerator is used, the initial strengths are related to its proportioning. The strengths at 28 days depend on the type of accelerator used, on its interaction with the type of cement and on the cement proportioning. The impact of an accelerator will be different between cement CEM I 52.5 and CEM II or III.

As an indication, the table below summarises the maximum effectiveness of the various solutions with current proportioning, according to the temperature, by recalling the above classification:

Fresh concrete temperature Concrete made of CEM I with accelerator

Concrete made of fast setting cement

Additive < 5%

Additive > 5%

Portland without gypsum

Other fast setting cements

5 to 15°C J1 J 2 J 1 J 3 15 to 25°C J2 J3 J 2 J 3 25 to 35°C J3 J3 J 3 J 3

4.2.7.3 requirements

The choice of the strength class requested at young age will depend on the performances sought for this concrete.

For example, the strength can be:

• from approximately 1 MPa at a few minutes for just an anti-slump effect, • to more than 10 MPa at 3 hours if we want to quickly put a strain on this concrete.

It is very important to consider the long-term durability of this kind of concrete right from the beginning of the formulation, and to choose the best adapted cement or accelerator.

4.2.8 Role of the temperature

The formulation of these concretes must take account of the ambient temperature during the application. Like all concretes, their setting time and their hardening kinetics will be related to:

• the temperature of each ingredient (cement, sand, water etc…), • the temperature of the substrate, • the ambient temperature.

Technical Committee


With the same level of requirements concerning the strength, a fast setting concrete implemented during the summer needs an accelerator proportion less important than in winter. This is also why the performance of a fast setting concrete is tested in cold temperature.

On the contrary, concretes containing fast-setting cement need a retarding agent in hot weather. It is necessary to avoid a too fast setting before the end of compaction by hammering of the gravels. An excessively fast setting can lead to a lower compactedness and thus to also lower strengths.

4.3 freezing-thawing resistant concretes

For concrete sprayed by wet process, the traditional recommendations of standard NF EN 206-1 (table F1 in appendix F) and the recommendations for the durability of hardened concretes subjected to freezing published by the LCPC in December 2003, are applicable but one knows little practical cases of application considering the fields of application of the wet process. For concrete sprayed by dry process, the implementation principle makes it possible to obtain air contents close to those indicated in the table F1 of standard NF EN 206-1 [Gérômey]. Feedbacks show that an excellent durability can be obtained even in very cold areas, without using air-entraining agents whose control would be difficult in the dry process.

4.4 High-performance cementitious materials

With regard to the wet process, the formulation of such a concrete requires the same rigour as for high-performance traditional concretes. For “dry process” concretes, it is imperative to use code A index A aggregates.

The attention is drawn to the damping effect of the silica fume particles which will lead to an addition of water. This water will be difficult to compensate by the addition of superplasticizers to obtain a reduction of the equivalent water/binder ratio.

55 SSAANNDD CCOONNCCRREETTEESS

These concretes are the subject of the standard NF P 18-500 and of a guide which were published after the tests organised as part of the national project SABROCRETE.

They are called “sand concrete” and not mortar because their cement dosage is similar to that of a traditional concrete. They do not contain fine gravels. They are formulated from a 0-4 mm sand and a high proportion of fillers.

They are spread only by wet process.

Their field of use is related to their aptitude for being transferred on long distances and to the few losses they generate. This is particularly interesting in small sanitation galleries.

Technical Committee


66 RREEFFEERREENNCCEESS QQUUOOTTEEDD IINN TTHHEE BBOOOOKKLLEETT ““ FFOORRMMUULLAATTIIOONN””

AFNOR standard NF P 95-102 2002 Engineering structures – Repair and strengthening by sprayed concrete NF P 18-545 2008 Aggregates: Defining elements, conformity and coding NF P 15-314 1993 Prompt natural cement NF P 18-500 1995 Sand concrete European standards approved by AFNOR (NF EN) NF EN 14487-1 2006 sprayed concrete: Definitions, specifications and conformity NF EN 14487-2 2006 Sprayed concrete: Execution NF EN 206-1 2002 Concrete: Specification, performance, production and conformity NF EN 197-1 2001 Current cements: composition, specifications and conformity NF EN 12-620 2008 Aggregates for concrete NF EN 13-139 2003 Aggregates for mortars NF EN 1008 2003 Mixing water for concrete NF EN 14647 2006 Calcium aluminate cement: composition, specifications and conformity NF EN 14 889-1 2006 Steel fibres NF EN 14 889-2 2006 Polymeric fibres NF EN 934-2 1998 Admixtures for concrete: definitions, requirements, conformity

and marking NF EN 934-5 2005 Admixtures for sprayed concretes: definitions, requirements

and conformity NF EN 1504-3 2006 Products and systems for structural and non-structural repairs

Recommendations

AFTES 1993 Technology and implementation of sprayed concrete TOS n°117 supplement AFTES 1994 Technology and implementation of fibre reinforced sprayed concrete TOS n°126 LCPC Dec. 2003 Durability of hardened concretes subjected to freezing ASQUAPRO 1997 Prevention, hygiene and safety during the projection of mortars and concretes

Booklets of the ASQUAPRO Guide Booklet « Implementation of sprayed concrete » (2007)

Bibliography RESSE C. and VENUAT M., 1981 « Projection des mortiers, bétons et plâtres » (projection of mortars, concretes and plaster), techniques and applications, construction and civil engineering. Published by the authors TEICHERT P., 1991 « Béton projeté » (sprayed concrete) – Published by LAICH SA, Avegno, Switzerland PROJET NATIONAL SABLOCRETE, 1994, Presses de l’ENPC GEROMEY S., 2003 Evaluation des paramètres d’obtention de la qualité des bétons projetés utilisés dans les soutènements provisoires, des revêtements définitifs et des renforcements d’ouvrages (Evaluation of the parameters to obtain quality sprayed concretes used in provisional supports, final coatings and reinforcements of works.) Scientific thesis: National institute of Applied Sciences of Lyon.

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