I have a concrete tank that needs a leak test done. The leak test will be done after its already been backfilled and the tank is 21m by 21m and holds about 3000m3 of water. Wall thickness ranges from 600mm at the bottom to 450mm at the top (7.1m tall).  My question is once the tank is filled up, the concrete will absorb some of the water.  How long should I wait before I start my actual measurement of change in water height. How long should I give the concrete to absorb the bulk of what it will absorb?    
 

Here is a specification for the test:

A. Upon completion, the tank shall be tested to determine watertightness. The tank shall be filled with potable water to the maximum level.

The test shall consist of measuring the liquid level over the next 24 hours to determine if any change has occurred. If a change is observed and exceeds the maximum allowance, the test shall be extended to a total of five days. If at the end of five days the average daily change has not exceeded the maximum allowance, the test shall be considered satisfactory.

B. The liquid volume loss for a period of 24 hours shall not exceed one-twentieth of one percent of the tank capacity, 0.0005 x tank volume. If the liquid volume loss exceeds this amount, it shall be considered excessive, and the tank shall be repaired and retested.

C. Damp spots will not be permitted at any location on the tank wall. Damp spots are defined as spots where moisture can be picked up on a dry hand. All such areas shall be repaired as necessary.

D. Damp spots or standing water on the footing may occur upon tank filling and are permissible within the allowable volume loss. Measurable flow in this area is not permissible and must be corrected.

Per AWWA, the allowable leakage rate is 0.1 percent of the water volume in 24 hour.

You can always specify higher requirements than the minimum standards.

This issue was previously discussed:

http://www.eng-tips.com/viewthread.cfm?qid=222702&page=15  

Thanks bimr,

Would you be able to provide me a link to this specification and also does the AWWA allowable leaked rate of 0.1 include concrete absorption? Shouldn't concrete absorption vary for different concrete mixes and other factors?  

What about evaporation??

You will need to allow the tank to stabilize for three (3) days while the water is absorbed into the concrete.

http://www.mwua.org/Presentations/MalenfantAPR07.pdf

You also have to allow for evaporation and rain.

http://www.cement.org/bookstore/profile.asp?itemid=LT186

@bimr: I am not sure if AWWA provides a standard or a guideline for concrete water tanks. Would you please provide the standard number for the allowable leakage rate is 0.1 percent of the water volume in 24 hour?

@jqeng: Please note that there is maximum rate of a filling for a water tank that is being filled for the first time. I have the uniform rate of not greater than 2 m in 24 h in back of my head.

Getting to your questions, the stabilizing period depends on the maximum design crack width and the time needed for the healing. I remember it was 7 days for a crack width of 0.1 mm according to BS. there is also a limit for the maximum drop in level during this 7-day test based on the full tank depth. I suggest to take a look to the latest BS standard. BTW, there is also a separate test for the roof.
 

@bimr: The link you gave quoted the ACI 350.01-01 Clause 2.3.5. I have the ACI 350.1-06 and could not find the wording from this clause anywhere do you know what the new clause in the 06 version is?

@waterpipe: thanks for your response, as a note Im new and I didnt think that my first post worked so I reposted when I found this thread! Turns out they both worked.  

The ANSI/AWWA D110-04 (Wire- and Strand-Wound, Circular, Prestressed Concrete Water Tanks) and the ANSI/AWWA D115-95 (CIRCULAR PRESTRESSED CONCRETE WATER TANKS)have leakage rates specified. The net liquid loss for a period of 24 hr shall not exceed 0.05 of 1 percent of the tank capacity.
 

• http://files.engineering.com/getfile.aspx?folder=c99713d8-e983-49dd-b8ed-2e

Is the purpose of the tank for potable water storage?

I worked on a project where inadequate concrete consolidation resulted in significant segregration (you could see through the walls after the forms were removed. The contractor used epoxy grout repairs and tried testing with this method to check repairs.

Wave action was an issue to record surface level changes. We tried a stilling well but still could not get a "steady" level.

We excavated a few areas and found leaks where water squirted out of pinhole leaks. Additional repairs were required.

Depending on the purpose of the tank and future maintenance costs - you might consider coating the tank.

Thank you bimr.
I was thinking of ordinary reinforced concrete water tanks (normally rectangular), which is not by AWWA. But I see no reason to apply the acceptance of the leakage for a prestressed concrete water tanks to the ordinary one.
 

waterpipe; Consider that the prestressed tank walls are typically thinner than standard cast-in-place concrete, that quality concrete construction should be impermeable, that wall segments for the ANSI/AWWA D115-95 standard are also cast-in-place concrete, and that the application is also for storing water. Yhere should be no reason not to use the same AWWA leakage rate.

http://www.durastortank.com/tabid/1583/Default.aspx

Thanks bimr.

Sorry but I meant:
" I see no reason NOT to apply the acceptance of the leakage for a prestressed concrete water tanks to the ordinary one."

but I've missed the NOT, for the same reason I've missed "covered" before "by AWWA" in my respond! So I fully agree with you on applicability of the leakage rate.

Well, this could be the Friday afternoon "allergy", which means I have to leave the office for a happy hour! I do it in 10 minutes.

Have a nice weekend.