10.003  Free Falling Concrete

Where in the UBC does it state the maximum drop height of concrete? The closest code sections I can find in 1905.10 that just states to deposit the concrete as close as possible to final position.

-- Unknown

11/?/05 - Response prepared by Greg Ruf, Managing Engineer - San Francisco Bay Area Operations, Krazan & Associates

The short answer to your question is that there is no UBC limitation on the maximum drop height of concrete during placement. Although factually there is no UBC requirement to limit concrete drop height, there are implied practical limits. The Code refers to the issue of concrete segregation during conveying and depositing of concrete. In referencing UBC Section 1905.10 you correctly cited the Code but left out an important element of the Code provision. Section 1905.10 states "Concrete shall be deposited as nearly as practical in its final position to avoid segregation [emphasis added] due to rehandling or flowing." Section 1905.9.2 states "Conveying equipment shall be capable of providing a supply of concrete at site of placement without separation of ingredients...". These two provisions show intent to maintain concrete quality to the point of final deposit by preventing or avoiding separation of materials. Experience has shown that free falling of concrete through close spaces with obstructions, such as reinforcing steel and embeds, can cause segregation of concrete. Based on this experience and the absence of specific limits on free-fall drop heights in the UBC, a specific provision was added to Title 24 of the California Building Code. CBC Section 1905A.10.9 states "In depositing concrete in columns, walls or thin sections of considerable height, concrete shall be placed in a manner that will prevent segregation... unless otherwise approved by the enforcement agency, the unconfined vertical drop of concrete... to the placement surface shall not be greater than 6 feet (1829 mm)." The American Concrete Institute also addresses placement of concrete in several publications, including ACI 304R-00. ACI 304R-00 states that "... if forms are sufficiently open and clear so that concrete is not disturbed in a vertical fall into place, direct discharge without the use of hoppers, trunks, or chutes is favorable." In summary, though not specifically limited, drop height has been shown to have practical limits based on the conditions where the concrete is being placed. The drop height should be limited to that where concrete quality can be maintained and segregation is prevented.

COMMENTS
Historically, drop heights of 10-20 feet have been referenced as the maximum allowable. Studies of the impacts of free-fall placement of concrete in large diameter drilled cast-in-place piers (also referred to as cast-in-drilled-hole caissons) up to 150 feet deep indicate concrete can free fall great distances without appreciable problems. Because of this, reference to maximum drop heights has been removed from many current specifications, including those of the Federal Highway Administration (FHWA). The trend towards removal of controlling concrete drop height based on these studies and the advocacy of less control on concrete placement techniques by contractor-based organizations may not be well founded for concrete placement in structures other than large uncongested structural elements. It should be understood that the studies conducted involved large diameter piers or caissons, which allowed for minimal impact with reinforcing steel. Though the FHWA has been quoted as stating that "the general expectation that (concrete) striking of the rebar cage will cause segregation or weakening of the concrete is invalid," it is important to recognize that the dynamics of the concrete falling into place, even when striking rebar in a large diameter caisson, are very different than in a close space, such as a wall, thin section or small diameter column. The studies cited by contractor-based organizations, such as the American Society of Concrete Contractors, advocating unrestricted fall heights for concrete, are all based on large diameter caissons. They do not refer to studies of more restrictive structural elements, such as walls and smaller diameter caissons or columns, though they are advocates of applying the unrestricted free-fall practice to other structural elements, including walls and columns.

The reader may wish to closely examine position paper #17 from American Society of Concrete Contractors, which does note than "Concrete placing operations are often planned to allow for the free fall of concrete. This planning must also consider any segregation that might occur when the concrete free falls into place." The paper may be viewed at www.ccagc.org/tech_info.php or purchased online at www.ascconline.org.