Retaining wall joints 2

[Enhineyero]

Hi all, would like to ask a pretty basic questions about the retaining wall joints. There are three joints used in concrete retaining wall construction 1. Construction joint 2. Expansion Joint 3. Contraction joint.

Now, I know that construction joint is formed when casting of a wall cannot be done in one go. The two other joints got me a bit confused , From what I understand reinforcing steel bars prevents shrinkage cracks, why then the need for the expansion and contraction joint?

Thanks in advance.

 

[darthsoilsguy2]

reinforcing steel doesn't prevent shrinkage cracks... it does keep them from opening wide up. old construction saying: 'there are 2 types of concrete... concrete that cracked and concrete that is going to crack' a lot of people refer to contraction joints as control joints. The only thing that prevents shrinkage cracks is freedom-of-movement from casting to cured (with the rare exception of perfectly designed/executed shrinkage compensating concrete)

 

[Teguci]

Consider a 300 feet long wall that is freshly cast. Over time, that wall will experience volume change along its length due to temperature change, shrinkage and possibly creep (for P/T). If you run the numbers, you'll get something like 2" length change over the course of time just from shrinkage. Under an imaginary no restraint condition (or under a P/T systems that overcomes silly restraints like column stiffnesses), the wall could slip towards its center of mass and not form any cracks. Or, a more likely situation where a foundation wall is restrained by a bearing surface, the wall will break at semi-regular distances. If you provide weak planes (construction, contraction or expansion joints) in the wall at 20 ft (or less) intervals, you'll have a 1/16th to an 1/8th separation at each joint that can easily be sealed and looks pretty. If you only want to cast 20 ft (or less) of wall at a time, then you are good to go. If the contractor wants to make money though, you will need to give him a detail to create a weak plane under a continuous cast operation (contraction joint).

For a free standing wall, it is often appropriate to match an expansion joint in the building sitting on it. For a below grade wall cast against earth, it is probably more appropriate to consider the foundation wall as monolithic and provide for the buildings expansion as starting from the top of the wall. Some reasons - waterproofing expansion joints below grade is a bit more difficult. Contraction but not expansion (assuming a fully below grade wall) may occur due to long term settlement of surrounding soils causing the joint to slowly close over time and not match the superstructures behavior.

If you have a wall that will not be exposed to site and is well waterproofed on the exterior, I say let it crack and feel free to ignore the shrinkage cracks - let everyone know its going to happen though.

 

[kipfoot]

what darthsoilsguy said bears repeating: Reinforcing steel does not prevent shrinkage cracks.

Expansion joints are a global consideration for large buildings that I don't think are related to your issue. It seems that you're asking about contraction joints. This link might be helpful:

Joints in Poured Walls

 

[Enhineyero]

Great insight guys. Do we still put these joints if the walls are short say 70ft long wall? I've seen these joints in retaining walls that are not within 300ft mark.