10.003 Free Falling Concrete
|Q||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.
|A||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.
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.