Unless otherwise stated in this European Standard, the materials used shall comply with other relevant European Standards.
Bentonite is used in supporting fluids, either as a bentonite suspension or as an addition to polymers. It is also used as a constituent part of self-hardening slurries.
Bentonite is a clay containing mainly the mineral montmorillonite or similar minerals. A distinction should be made between calcium bentonite, natural sodium bentonite and activated bentonite, which is a sodium bentonite produced from natural calcium bentonite by ion exchange.
Bentonite used in bentonite suspensions shall not contain harmful constituents in such quantities as may be detrimental to reinforcement or concrete.
The chemical and mineralogical composition of the bentonite shall be supplied.
The type of cement to be used shall take into account the aggressiveness of the soil and groundwater. Cement for concrete shall comply with 4.1 of ENV 206.
6.3 Supporting fluids
This subclause covers only bentonite suspensions, polymer solutions and self-hardening slurries.
6.3.1 Bentonite suspensions
A bentonite suspension shall be prepared with either natural or activated sodium bentonite.
In certain cases, e.g. when the density of the suspension has to be increased, suitable inert materials may be added.
A self-hardening slurry may be prepared with calcium bentonite or natural or activated sodium bentonite. Under normal circumstances, the bentonite suspension shall meet the conditions shown in Table 1. The values in Table 1 may be modified in special circumstances, for example in the case of:
- soils or rock with high permeability or cavities where loss of bentonite may occur;
- high piezometric levels (artesian conditions);
- very soft soils;
- salt water conditions.
|Fresh||Ready for re-use||Before concreting|
|Density in g/ml||< 1,10||< 1,25||< 1,15|
|Marsh value in s||32 to 50||32 to 60||32 to 50|
|Fluid loss in ml||< 30||<50||n.a.|
|pH||7 to 11||7 to 12||n.a.|
|Sand content in %||n.a.||n.a.||< 4|
|Filter cake in mm||< 3||< 6||n.a.|
|n.a.: not applicable|
At the stage "before concreting", an upper limit value between 4 % and 6 % for sand content may be used in special cases (e.g. non-load bearing walls, unreinforced walls).
In order to keep the sand particles in suspension and to reduce penetration into the ground, it is necessary to have a bentonite suspension with sufficient gel strength.
When deemed to be necessary, the gel strength can be checked by using rotational viscometers or other suitable equipment.
The Marsh value, the fluid loss, the sand content and the filter cake can be measured, for example, using the tests described in the American Petroleum Institute document Recommended Practice Standard Procedure for Field Testing Water-Based Drilling Fluids (reference: American Petroleum Institute Recommended Practice 13B-1, June 1, 1990).
The Marsh value is the time required for a volume of 946 ml to flow through the orifice of the cone. A volume of 1 000 ml may be used, but in this case, the Marsh values given in Table 1 shall be adjusted.
The duration of the fluid loss test may be reduced to 7,5 min for routine control tests. However, in this case, the values for fluid loss and filter cake shall be adjusted. The fluid loss for the 7,5 min test will be approximately half of the value obtained in the 30 min test.
NOTE Sand content is the percentage by volume, and not by weight, of particles larger than 74 µm, which is the mesh size of the sand content set.
6.3.2 Polymer solutions
Polymer solutions, possibly with the addition of bentonite, may be used as supporting fluids on the basis of previous experience in similar or worse geotechnical conditions, or after full-scale trial trenches on the site. A reasonable extrapolation of previous experience may be made with the support of laboratory tests and theoretical analysis.