Experience of watertight construction joints in concrete / Ways of avoiding joints

[JackieMoone]

Hi

Has anyone had experience of producing large continuous concrete structures that must be kept water tight?

My concerns are primarily with cracks caused by plastic shrinkage. If I wanted to produce a continuous concrete element 30m long (i.e. a wall) without any major cracking would precasting the element in smaller sections and joining them together by an insitu concrete pour over exposed reinforcement be a reliable way of avoiding plastic shrinkage issues?

Or has anyone had experience using admixtures to prevent cracking in large continuous elements?


Any advice / thoughts welcome

 

[JedClampett]

Watertight (this depends on definition) walls can be constructed. Precast is not the way to do it. The way to do it is to cast concrete in place using joints at proper spacing (30 ft maximum), reinforce adequately (around .0050 minimum total, if there is no bending or other stress affects, more is very possible), use low water to cement concrete (.45 w/c), and use good joint details. Plastic shrinkage should not be an issue if concrete is cured correctly. Long term shrinkage is a bigger problem and is partially mitigated by allowing enough time between pours (14 days is ideal). The contractor must be "encouraged" to vibrate concrete, especially at the wall bottoms. Tie holes should be repaired using proper materials and skilled labor.
As far as admixtures, there's no magic bullet for good techniques.
You need to get a copy of ACI 350 and follow its recommendations.

 

[bimr]

Wastewater tanks are commonly constructed to much larger dimensions. Follow the procedures listed by JedClampett and there should not be any problems.

 

[miningman]

About 18 years ago there was a new admixture on the market called Eclipse. This was marketed as something that would reduce shrinkage cracking. It was so new ( and not cheap) that there was no way for me to evaluate the salesman's claims. However I was about to make a mass pour underground ( 4m x 4m x 4m) where prevention of cracking was important. I paid the premium, and no cracks occurred. About two years later I was involved in a similiar project where I wasnt the responsible engineer. Eclipse wasnt used and cracking did occur. Evaluate these comments for your own circumstances.

 

[hokie66]

Jed's advice is good.

It is important to get the terminology right. "Plastic shrinkage" occurs while the concrete is still plastic, i.e. before it hardens. I think drying shrinkage due to restraint is the main issue here. Cracks form vertically at about 10' centers if the base restrains the walls, and these cracks can't be avoided, but can be controlled by an adequate amount of reinforcement. My minimum for water retaining structures is 0.006Ag, and I like to add a few bars near the bottom to better control the restraint crack initiation.

There are successful specialist builders of water tanks who use precast panels and posttension them together. Not for the uninitiated.