CONVEYING After being discharged from the mixer the concrete shall be transported as rapidly as possible, which in no case shall exceed 30 minutes, to its the final position in the Works, by means that shall be approved by the Owner and which shall prevent segregation, loss or contamination of the ingredients. The containers that convey the concrete shall at all times be kept clean and free from hardened or partially hardened concrete.
Chutes or other equipment for conveying concrete shall be of
such size and design as to ensure a continuous flow of concrete at delivery and
without separation of ingredients, and shall be thoroughly cleaned before each
run. If use of pumping equivalent to convey concrete is selected, aluminum pipe
shall not be used. Pipe or other equipment where aluminum surfaces will be in
contact with plastic concrete shall not be used. A steel piping arrangement
utilizing a "Y" will be permitted provided a valve is installed at
the branch point to direct the flow into one branch at a time. Minimum pump
line size of 150 mm shall be provided for 20 mm and 40 mm maximum size aggregate
mixes. For mixes containing aggregate less than 20 mm maximum size, a smaller
line may be used.
Concrete shall not be allowed to drop freely more than 1.50
m. For drops which exceed 1.50 m use a hopper and drop chute.
PREPARATIONS FOR PLACING CONCRETE
Before concrete is placed, forms and reinforcing shall be
cleaned of dirt, debris. All necessary trenching, damning, draining and pumping
shall be done so that all concrete can be placed in the dry. Concrete shall not
be placed until forms and reinforcing have been inspected and approved by
Owner.
For preparation of construction joints, the provision
mentioned herein of this specification shall be strictly complied before fresh
concrete is placed on or against hardened new concrete or old concrete, thoroughly
clean surfaces of all laitance, soft or loose materials and deleterious
substances. The surfaces shall then be washed clean and thoroughly moistened.
Dry concrete surface shall be saturated for at least 24
hours. Immediately before fresh concrete is deposited, horizontal surfaces
shall be covered with cement mortar of same mix as concrete to be poured. Fresh
concrete shall then be placed before cement mortar has obtained its initial
set.
All reinforcing and embedded steel shall be washed with clean
fresh water using a high pressure jet to remove surface chloride contamination
immediately prior to placement of the concrete.
Washing shall be repeated if the steel is subsequently immersed in or contaminated by sea water.
Washing shall be repeated if the steel is subsequently immersed in or contaminated by sea water.
PLACING OF CONCRETE
The Owner shall be given 24 hours notice before any concrete
is poured in order that he may inspect the work in writing.
All concrete shall be placed in a continuous pour over the
area between construction joints in an uninterrupted operation in such manner
as to form a monolithic structure, the component parts of which are securely
bonded together. Do not deposit segregated concrete or concrete that has
partially hardened or been contaminated by foreign materials, nor use
retempered concrete. Time interval between placing of successive batches of
concrete shall not be greater than 30 minutes.
All concrete shall be deposited in forms within 1 hours
after introduction of mixing water to cement and aggregates unless otherwise
required.
All concrete in vertical members, such as columns, and
walls, shall be in place not less than 2 hours before any concrete is placed in
girders, beams or slabs to be supported thereon. Concrete for beams, girders,
brackets, column caps, and hounches shall be placed monolithic with floor
slabs.
Concrete shall be deposited in horizontal layers no deeper than 60 cm.
Concrete shall be deposited in horizontal layers no deeper than 60 cm.
Hot Weather Concreting
Hot weather concreting shall comply with ACI 305 and the
following :
Suitable means shall be provided to ensure that the
temperature of the concrete when it is placed in the work shall not exceed 32
0C.
Where necessary aggregate stockpiles shall be shielded from
direct sunshine and/or sprayed with fresh water especially when evaporation
rates are high. Due allowance shall be made for the extra water in the concrete
mix design. Mixing water shall be kept cool with insulated pipes and open
tanks, refrigerated or ice added as necessary, and cement shall be stored in
light colored silos or sheds.Where necessary the surfaces, including
reinforcement, against which concrete is to be placed shall be shielded from
direct sunshine and sprayed with fresh water to prevent excessive absorption of
water from the fresh concrete by the surface.
Concrete placing shall be completed as quickly and
efficiently as possible, and exposed surfaces shall be immediately covered to
reduce excessive evaporation of water from the concrete and conform to the
applicable requirement of ACI 301.
The Contractor shall present for the Owner's approval his
proposal for dealing with the following problems related to hot weather
concreting:
a) Reduced workability
b) Excessive plastic shrinkage
c) Rapid strength gain but possible low final strength
d) Rapid drying-out of concrete
e) Concrete mixing to counteract hot weather
f) Mixing water to counteract hot weather
Under water concrete shall placed in position by tremie tube specified in
Section 17 and contractor shall submit full detailing of the method proposed in
advance to the Owner for approval before starting concreting.
CONSOLIDATION OF CONCRETE
Concrete shall be considered by means of mechanical
vibrators, subject to the following minimum requirements :
a) Vibrators shall be electric or pneumatic power driven type and shall operate at frequency of 7.000 to 10.500 vibrations per minute for vibrating heads from 80 to 150 mm in diameter and from 8000 to 13500 vibrations per minute for vibrating heads of 30 to 60 mm or greater in diameter when fully immersed.
b) Intensity of vibration shall be such as to visibly affect as a mass of concrete of 250 mm slump over a radius of at least 450 mm.
c) The vibrators shall be inserted and withdrawn out of the concrete slowly. Application of vibrators shall be at points uniformly spaced and not farther apart than twice the radius over which the vibration is visibly effective. If requested by the Owner concrete shall be re-vibrated.
d) Vibration shall not be applied directly on through the reinforcement to sections of layers of concrete which have hardened to the degree that the concrete ceases to be plastic under vibration.
At least one spare vibrator shall be provided for each three
vibrators in use for immediate use in the placement.
Vibration shall continue until the concrete has been
compacted to its maximum practicable density, and is free from pockets, closes
saugly against all surfaces and is in perfect and complete contact with all
reinforcement bars and other metal embedded in the concrete. The manufacturer
of the vibrators and the total number of vibrators required shall be subject to
the approval of the Owner.
Duct banks and slab less than 150 mm thick shall
be consolidated by rodding or spading.