6.4 Self-hardening slurries
Self-hardening slurries are generally used in the precast concrete diaphragm wall technique, for reinforced slurry walls and slurry cut-off walls. They serve as the supporting fluid during excavation, and, together with the fines from the natural ground, form the final, hardened material. The characteristics of the slurry must be suitable to ensure satisfactory performance during execution. Admixtures may have to be used to adjust workability during excavation and the insertion of elements, as well as setting time, taking into account the possible effects of temperature and chemical components of the soil and groundwater. The characteristics of the hardened material, as needed for the particular applications (e.g. permeability, strength and deformation properties) , together with testing methods, must be specified to satisfy the functional requirements of the wall.
Unless otherwise stated, the concrete used in cast in situ concrete diaphragm walls or in precast concrete diaphragm walls shall comply with ENV 206.
The following subclauses of this section apply to cast in situ concrete and deal only with the properties required for correct execution. The concrete shall be designed to avoid segregation during placing, to flow easily around the reinforcement, and when set, to provide a dense and watertight material. The specified properties of the hardened concrete, related to strength and durability, shall be compatible with the workability requirements.
In order to avoid segregation, the aggregates shall be well-graded. The maximum particle size shall not exceed 32 mm or 1/4 of the clear space between the longitudinal bars, whichever is the smaller.
In the case of a maximum aggregate size of 32 mm, the concrete mix shall have the following characteristics:
- sand content greater than 40 % by weight of the total aggregate;
- silt size particles in the concrete mix (including cement and other fine materials) between 400 kg/m3 and 550 kg/m3. These particles are those between 2 µm and 63 µm and they include cement and other fine materials.
The minimum cement content shall be related to the maximum aggregate size in accordance with Table 2.
Part of the cement can be replaced by additions such as pulverized fuel ash (PFA) or ground granulated blast furnace slag (GGBFS).
|Maximum grain size
|Minimum cement content
6.5.4 Water/cement ratio
The water/cement ratio should not exceed 0,6.
This value may have to be modified if additions are included in the mix.
In order to ensure that the concrete has the properties required for placing by concreting pipes, admixtures may be used as follows:
- water reducing and plasticizing admixtures including superplasticizers to avoid bleeding or segregation which might occur in some situations;
- retarding admixtures to prolong the workability as required for the duration of the concreting process and to cater for any interruptions.
6.5.6 Fresh concrete
The consistency of the fresh concrete just before concreting shall correspond to a slump value between 160 mm and 220 mm. A value between 180 mm and 210 mm is recommended.
The flow table test may be used instead of the slump test. In this case, the consistency of the fresh concrete just before placing shall correspond to a flow value between 520 mm and 630 mm. A value between 550 mm and 600 mm is recommended.
6.6 Plastic concrete or plastic mortar
Plastic concretes or mortars are used for cut-off walls when, in addition to low permeability, high deformability is required. Their constituent parts are:
- clay or bentonite;
- cement or another binder;
- well-graded aggregates;
- and possibly additions and admixtures.
The term "plastic mortar" applies when the size of the aggregates is limited to that of sand. The mix shall be designed in order to obtain the required deformability and permeability, together with adequate workability and strength.
Typical compositions for plastic concrete and plastic mortar using bentonite are given in annex A.
The reinforcement steel used in diaphragm walls shall comply with EN 10080. Unless special precautions are taken, other metallic elements used in cast in situ diaphragm walls, such as tubes, plates, connectors, etc., shall not be made of galvanized steel or other metals which may produce electrostatic effects resulting in a buildup of bentonite cake or may cause electrochemical corrosion of the reinforcement.