Shrinkage Cracks in a Shotcrete Retaining Wall - Steel Requirements

[palmahouse]

I am evaluating whether additional reinforcing steel is required in a shotcrete retaining wall (with soil nails) to reduce shrinkage cracks. Overexcavation was required to construct the wall. The average overexcavation depth (thickness) is about 4 inches, and is up to about 16 inches at some locations. The wall design calls for an 8-inch-thick wall reinforced with two layers of 4 x 4 W4 x W4 mesh and continuous horizontal bars behind the bearing plates at each soil nail row. Soil nails are spaced every 5 vertical feet.

Because of the overexcavation, the as-built wall thickness is 12 inches average, and up to 24 inches at some areas.

The plans call for additional continuous horizontal rebars to reduce shrinkage cracks depending on the as-built wall thickness (because of overexcavation) as follows:

wall thickness = 10" - #3 @ 16" o.c.
12" - #3 @ 12" o.c.
14" - #3 @ 14" o.c.
16" - #3 @ 8" o.c.

I think the additional steel for shrinkage crack reduction is based on ACI guidelines for minimum steel area as a percentage of reinforced concrete area for a slab (but not sure).

Shotcrete and mesh is placed as followes:

1) backfill shotcrete in overexcavations and out to first mesh layer

2) hang first mesh layer

3) shotcrete to second mesh layer

4) hang second mesh layer

5) shotcrete to wall face and rod finish


I am uncertain of the effect that phasing of shotcrete placement has on shrinkage cracking, and whether the additional steel is necessary (and where it should be placed).

Please provide input on whether additional steel is required as scheduled (and whether the phasing of shotcrete precludes the need for it), and if so, whether the amount called for is reasonable (or not enough). Also, where should the additional steel be placed?

The AASHTO manual for soil nail wall design does not address design of additional steel to reduce shrinkage cracks.

 

[Ron]

Additional steel will not likely decrease shrinkage cracking. Looks like you're above the minimum and to think that rebar will control all cracking is a fallacy.

 

[hokie66]

Concrete does not crack because it shrinks, but rather because it is restrained when it wants to shrink. In a shotcrete wall, the irregular earth provides restraint. In a layered shotcreting operation with some delay between layers, the first layer restrains the second and so on. Thus, vertical cracking will occur at fairly close spacings, maybe 6 to 10 ft. These cracks can be controlled by reinforcing to a certain extent. The amount of reinforcing to use depends on what degree of crack control you need for your application. Generally, the amount should vary from about .25% of the concrete area for a minor degree of crack control to .60% for a strong degree of crack control.

 

[BigH]

Any reason not to use steel fibre??

 

[palmahouse]

Thank you all very much for your input -especially hokie66

I now have a better understanding of the cracking mechanism. We have observed vertical shrinkage cracks in these walls.

I expect then, that one factor is that the outside of the concrete dries faster, shrinks faster, and is un-restrained. In contrast, the inside dries slower, shrinks slower, and is contrained. I figure these factors result in tensile stresses on the wall face, curling of the concrete "slab," and the subsequent cracking. I then figure cracking in the vertical direction is more likely to occur because the wall has less bending resistance in the in the horizontal direction, so it curls in the horizontal direction.

I figure one function of a granular layer below a slab-on-grade floor is to make the drying more uniform and cause less curl (but not to provide less restraint).

I figure adding more horizontal steel on the outside of the wall face is intended to provide more tensile resistance, and therefore, more "restraint" from shrinkage on the outside of the concrete as well as bending stiffness to resist horizontal curl, and therefore, reduce vertical cracks I expect this additional steel, in this case, would do the most work if placed on the outer layer of WWF - about 2 inches from the wall face (and before the final shotcrete is placed).

Am I on the right track?

A few more questions and requests for input:

1) How long will it take the curl to occur? I am not certain what the w/c ratio is (but probably typical for a soil nail wall).

2). Is exposure to the sun, both in the short term (during construction) and the long term a factor to consider?

3). Where can I find published information about the 0.25 to 0.6 percent area guideline (or other guidelines that seem to be a good fit for this structure and construction sequencing)?

4). In the real world, is there little value to trying to control cracking with additional steel (as Ron suggests)?

5) Fiber will improve strength and durability, but, will it help control the shrinkage cracking?

 

[hokie66]

palmahouse,

You are on the right crack, but the mechanism of cracking in the restrained layers does not involve curling. The restraint force is at the inside of the new layer, so the crack initiates there. But you want to limit the cracking near the face, so the reinforcement should be placed with normal cover requirements. Curling sometimes occurs in slabs on ground with joints, causing the ends of the sections to lift slightly, resulting in tension stress in the top of the slab. As you don't have to worry about gravity causing tensile stress of this type in vertical elements, curling doesn't really apply to walls.

You get less horizontal cracks in walls because the dimension is less than the horizontal, the restraint is less, and in this case gravity assists.

The w/c ratio for shotcrete is low, so this is in your favor. Drying shrinkage for thin elements, say a 4" floor slab, occurs quite quickly, and you probably get 80% in 6 months. For thick elements, it takes years.

Exposure to sun can be an issue, but wind is more of a problem. And if you are worried about the sun, the temperature may be high. The main thing here is the temperature of the concrete when placed, which should be as low as possible.

The percentages of crack control reinforcement I gave you are from the Australian Concrete Structures Standard AS3600. Other codes probably have similar clauses.

The value of controlling shrinkage cracking depends entirely on the use of your structure. If it is to be water retaining, then controlling the crack width is critical. In this case, there are other more detailed standards for crack width control. If the wall needs to be waterproof against groundwater from outside, then you should have a membrane on the outside surface. If for aesthetics, ask the architect.

Steel fibres greatly improve toughness and abrasion resistance for industrial floors. They also increase to a degree the tensile strength of the concrete. But for controlling shrinkage crack width, deformed bars are the answer. Plastic fibres = sales pitch.