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Re[2]: Long cracks in top slab

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Calvin, it appears you are saying here that the 70 cm slab concrete was 
placed in two 35 cm thick halves, 24 hrs apart. If so, is it possible there 
is a horizontal cold joint, which then would not give you a monolithic 70 cm 
slab structurally.


______________________________ Reply Separator _________________________________
Subject: Re: Long cracks in top slab
Author:  seaint(--nospam--at)seaint.org at Internet
Date:    01/09/2001 5:50 PM


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.

Calvin
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<DIV>
<DIV>Javier, Hasan, Shashi, Nick, Harold, Gil, </DIV>
<DIV>&nbsp;</DIV>
<DIV>Thanks for concerning about my problem.</DIV>
<DIV>&nbsp;</DIV>
<DIV>Sorry the incomplete information I provided.</DIV>
<DIV>&nbsp;</DIV>
<DIV>The project is in Taiwan to build an underground parking structure in a 
primary school, exactly under the sports ground.&nbsp;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.</DIV>
<DIV>My&nbsp;project is&nbsp;under a school sports ground, which results 1.0 
ton/m^2&nbsp;&nbsp;(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.</DIV>
<DIV>&nbsp;</DIV>
<DIV>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&nbsp;slab horizontally. It took about 24 hours to pour the

bottom half concrete&nbsp;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.&nbsp;It took another 24 hours to pour the upper half 
concrete&nbsp;continuously from one end to the other end. </DIV>
<DIV>&nbsp;</DIV>
<DIV>
<DIV>The reinforcements provided in the slab and wall are continuous without 
cutoff on both faces and both directions.</DIV>
<DIV>For B1 floor slab: #6@8" (G60) top and bottom &amp; each way.</DIV>
<DIV>For top slab: #6@8" (G60) top and bottom &amp; each way, with additional 
top reinforcements on top of every interior columns.</DIV>
<DIV>For exterior wall: #6@8" (G60) each face &amp; each way.</DIV>
<DIV>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.</DIV>
<DIV>&nbsp;</DIV>
<DIV>For the underground MRT (Mass Rapid Transit) system here, we built lot of 
underground stations. Each station is about 250m&nbsp;length, 17m&nbsp;net 
width, 11m&nbsp;net height, with 4m~6m backfill on top slab. The station is 
composed of four thick slab elements with thickness about 4'~6',&nbsp;just 
like&nbsp;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&nbsp;backfilled. This is why I 
didn't design any expansion joint for my parking structure project.</DIV>
<DIV>&nbsp;</DIV>
<DIV>Calvin</DIV></DIV></DIV></BODY></HTML>