Page 170 - Water Standards
P. 170

allowable overbreak amount or overfilling concrete in col⁃
                                lapsed zones. When a concrete pump is used for pouring
                                the arch concrete, the additional pressure of pumped con⁃
                                crete shall be considered in the formwork load calculation.
                             3) The formwork stripping time shall be in accordance with
                                the regulations in Article 2   7   6 of this specification. Be⁃
                                sides, factors like the stability of the surrounding rock,
                                section shape, span  and  exterior  loads  shall  be
                                considered. The time of form stripping can be shortened
                                appropriately in case of being tested.
                           7  Concrete placement shall meet the following requirements:
                              1 ) Placement   method.  Pumping  shall  be  selected
                                   preferentially for the placement of the concrete for side
                                   and roof arches. The placement of bottom arch concrete
                                   can be done directly by a carrier vehicle or belt convey⁃
                                   or.
                             2) The pouring of pumped concrete shall be in accordance
                                   with the regulations in Article 2   7   31 of this specifica⁃
                                   tion.
                             3) When transporting concrete by a belt conveyor, the maxi⁃
                                   mum dip angle shall not exceed the values listed in Ta⁃
                                   ble 3   5   3   For mixing proportion, the sand ratio shall
                                   be appropriately increased, the maximum grain size
                                   should not exceed 80mm, and the cement content can
                                                3
                                   be 50-75kg / m lower than that for pumping. The run⁃
                                   ning speed of the belt conveyor should not be larger
                                   than 1-1   2m/ s.
                             4) Attention shall be paid to the following items in concrete
                                   placement: the pouring of side arches shall be done
                                   layer by layer, and both sides shall rise symmetrically
                                                                            1 6 3
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