Re: Long cracks in top slab[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: Re: Long cracks in top slab
- From: "Calvin Chang" <ccpe(--nospam--at)ms38.hinet.net>
- Date: Tue, 9 Jan 2001 17:50:13 +0800
Javier, Hasan, Shashi, Nick, Harold, Gil,
Thanks for concerning about my problem.
Sorry the incomplete information I provided.
The project is in Taiwan to build an underground parking structure in a primary school, exactly under the sports ground. Parking is a big problem here. The demand is 5,000,000 cars, the supply is 1,000,000 cars, if I recall correctly. School sports ground and park are common choice for large underground parking structures. 50cm backfill is needed for grass on top of top slab if the structure is built under the sports ground. 200cm backfill is needed for tree on top of top slab if the structure is built in a park.
My project is under a school sports ground, which results 1.0 ton/m^2 (200 psf) backfill dead load and 0.5 ton/m^2 (100psf) live load, a total of 1.5 ton/m^2 (300psf) loading on top slab, not including the weight of top slab.
The outdoor temperature is about 15~25 degree Celsius during the construction of top slab. According to the description of site engineer, the contractor poured the concrete of top slab along the long direction (112m). They divided the 70 cm top slab horizontally. It took about 24 hours to pour the bottom half concrete continuously from one end to the other end. Immediately after the bottom half was poured, they turned to the original point to pour the upper half. It took another 24 hours to pour the upper half concrete continuously from one end to the other end.
The reinforcements provided in the slab and wall are continuous without cutoff on both faces and both directions.
For B1 floor slab: #6@8" (G60) top and bottom & each way.
For top slab: #6@8" (G60) top and bottom & each way, with additional top reinforcements on top of every interior columns.
For exterior wall: #6@8" (G60) each face & each way.
The reinforcements provided in top slab are about twice the minimun temperature reinforcements in both directions per ACI code. Even twice the temperature reinforcement, cracks happened. Improper pouring, mix design, rebar design and curing will result significant cracks, as I learned from all of you.
For the underground MRT (Mass Rapid Transit) system here, we built lot of underground stations. Each station is about 250m length, 17m net width, 11m net height, with 4m~6m backfill on top slab. The station is composed of four thick slab elements with thickness about 4'~6', just like an underground culvert. We tend to design no expansion joint along the 250m station for the recent ten years, since it is underground and should have less temperature effect once the top slab is backfilled. This is why I didn't design any expansion joint for my parking structure project.
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- From: Sprague, Harold O.
- RE: Long cracks in top slab
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