Inscrivez-vous ou connectez-vous pour rejoindre votre communauté professionnelle.
Hello Team,
Sometimes specifiers and inspectors dictate the maximum free-fall distance of concrete because they believe limiting free fall is necessary to minimize concrete segregation. Usually they limit the free-fall distance to 3 to 5 ft (0.9 to 1.5 m), but occasionally the limit is as little as 2 ft (0.6 m). Neither ACI 301- 99, “Specifications for Structural Concrete,” nor ACI 318-02, “Building Code Requirements for Structural Concrete,” limit the maximum distance concrete can free fall. ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” states that “if forms are sufficiently open and clear so that the concrete is not disturbed in a vertical fall into place, direct discharge without the use of hoppers, trunks, or chutes is favorable.” ACI 301, 304, and 318, however, all require placing the concrete at or near its final position to avoid segregation due to flowing. At least four field studies have shown that free fall from great distances doesn’t reduce concrete quality:
Regards,
Saiyid
ACI's Guide for Measuring, Mixing, Transporting, and Placing Concrete (ACI 304R-89) in Section 5.4.1 gives some precautions for placing. The guide says equipment should be arranged so that the concrete has an unrestricted vertical drop to the point of the placement. The stream of the concrete shouldn't be separated by permitting it to fall freely over rods, spacers, reinforcement, or other embedded materials. If forms are sufficiently open and clear so that the concrete isn't disturbed in a vertical fall into place, direct discharge without the use of hoppers, trunks, or chutes is usually desirable.
PCA's Design and Control of Concrete Mixtures (13th edition, page 104) specifically addresses your question. This source says the height of free fall of concrete need not be limited unless a separation of coarse particles occurs (resulting in honeycomb). If honeycomb does occur, a limit of 3 to 4 feet may be adequate.
Do not drop the cocrete from higher that 2-3 feet.
it should be less than 1.5m
for more details visit : www.civilread.com
max free fall for the concrete should be less than 1.5 meter. practically some members (excessive steel columns) where placing boom space is not available a flexi canvas is used for the purpose to achieve lower hieght for placment. bleeding is not a segregation borne issue.
1.5 Meter
sagregation is sepration of course and fine aggregate in conerete mix and bleeding is water apear on concrete finsh surface
The Maximum height for placing concrete to prevent segregation is 1.5 meter.
Segregation is unbonding of constituents of concrete material and bleeding is deposition of water / mortar on the surface of placed concrete.
While not specifically noted in some documents, the general rule of thumb is not over 5'. The use of a converyor with a tube is the answer to any pours over 4'. This allows you to put the tube to within 4' of the bottom of any form and relase the concrete, then raise it as you pour, never allowing it to free fall more than 4'. It also allows you to position the concrete so it doesn't hit the rebar and ties on the way down and segregate.
Concrete placement operations are planned to allow for the free fall of concrete.
There are two heights that must be considered and treated differently. They are the overall free fall height of concrete and the lift height of concrete.
Concrete placement is recommended in 4 foot lifts. This does not mean that the concrete cannot be placed greater than 4 foot in height or free fall greater than 4’. The concrete is typically placed in a continuous operation in two to three lifts by making passes around the perimeter or section of the wall. Concrete is commonly placed to a maximum of 12 feet in height during one concrete pour in 4 foot lifts. This is standard practice and is not unusual in the ICF and concrete industry. The 4 foot lift recommendation has nothing to do with consolidation. This recommendation is used as a guideline for concrete form pressure to avoid exceeding the formwork’s capacity. The American Concrete Institute (ACI) does not directly address the height of concrete placement. The height of which the concrete is placed does not create honeycombing or voids.
The American Concrete Institute (ACI) does not directly address the height of concrete placement. Neither ACI 301, “Specifications for Structural Concrete,” nor ACI 318 “Building Code Requirements for Structural Concrete,” limits the maximum distance concrete can free fall. Field studies have shown that free fall from great distances does not reduce concrete quality or compressive strength.
In the specification notes to owner’s representative, ACI 336 states that recent research on free-fall concrete has confirmed that free fall does not cause segregation, at least for fall heights up to 60 feet.
The American Society of Concrete Contractors (ASCC) has written a Position Statement #17 addressing the free fall of concrete. At least four field studies have shown that free fall from great distances does not reduce concrete quality. The ASCC position statement goes on to state that although the field studies have been for caissons, the results should also apply to other structural elements such as walls, columns, and mat foundations.
ASCC references a 1994 FHWA study the provided test data leading the investigators to conclude that “the general expectation that concrete striking of the rebar cage will cause segregation or weakened concrete is invalid and they found “no segregation or strength differences between low and high slump concrete mixtures.”
Bruce Suprenant wrote and article titled “Free Fall of Concrete” in the June 2001 issue of Concrete International that summarizes many of the findings discussed in this technical brief.
For those who intend to restrict free-fall heights, they must consider that it does decrease concrete production rates, which increases owners’ costs without increasing concrete quality.
In summary, Fox Blocks recommends placing concrete generally up to a maximum of 12 feet in 4 foot lifts. This is standard practice and is not unusual in the ICF and concrete industry.
