Spacing of Vertical Joints in Insitu Concrete Basement Walls

[FinnB]

I have always used a rule of thumb that vertical joints in poured insitu concrete basement walls should be spaced at a distance between 3 and 5 times the height of the wall.

For example when a basement retaining wall is 10 feet high we would locate vertical construction joints in walls at 30ft(10x3) to 50ft(10x5 spacing.

Does anyone know if there is guidance in a code or document that describes the maximum spacing of verticl joints in basement walls?

 

[Den32]

I typically use about 2 times the height of the wall (except for short walls). 3 to 5 times (50 feet) seems far unless your horizontal reinforcing is enough to limit cracking.

Check ACI 224 for controlling cracking in concrete.

 

[hokie66]

From observation, cracks will form at about 10' centres if a wall is restrained by a previously poured footing. Whether or not these cracks are of concern, and whether or not they need to be controlled by more reinforcement, is a matter to be considered in each instance.

 

[miecz]

Makes me wonder what would happen if the footing was not continuous, but had a 1 inch compressible filler every ten feet.

 

[hokie66]

I think the wall would still crack at that point due to the stress riser created by the joint in the footing. You can't win.

 

[FinnB]

hokie66 (Structural) 11 Feb 11 17:44

From observation, cracks will form at about 10' centres if a wall is restrained by a previously poured footing. Whether or not these cracks are of concern, and whether or not they need to be controlled by more reinforcement, is a matter to be considered in each instance.

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I assume you mean cracks will form at 10 foot centres when there is little or no reinforcing in the wall.

For a rebar reinforced retaining basement wall I would have no problem with joints at 30 feet or more. Drying shrinkage is what I would be most concerned with. Once temperature is not an issue rebar reinforced walls (10 to 12" thick, 10' high) are regularly poured up to 50 feet in length in the UK and Ireland.

 

[hokie66]

No, I meant they crack whether or not there is reinforcement, due to drying shrinkage of the wall after the restraining footing has already done its shrinking. The more area of reinforcement of smaller diameter bars, the closer held the cracks will be. I wasn't suggesting joints at 10' centres or at any other spacing, just pointing out what I have observed over the years.

 

[FinnB]

hokie66

I understand now where you were coming from in relation to the restraint that the already poured footing provides and it's a good point.

Going back to my experience with basements I have not seen cracking at 10 foot spacing. General we would use 5/8in vertical bars and 3/8in horizontal bars in a 10in thick RC wall for heights up to 10 foot. Where I work there is generally a lot of moisture in the air and its rare for temperatures to rise above 60 degrees. I suppose this helps the curing process. We have no problems pouring walls 30 feet or more in length that have very little or no cracking.

I was more looking for a reference that recommended pour lengths for walls to reduce cracking. I was asked to look at another consulting engineers design for a basement wall that has cracked with the resut it is leaking water. I haven't yet found a comprehensive document on this in the British Codes. It is easy enough to design for crack widths of between 0.1 to 0.3 mm but there is little guidance on recomended wall pour lengths.

 

[hokie66]

The British standard for water resisting structures (I forget the number at the moment) is the basis for most other water retaining codes. From memory, it assumes that there are no joints, but rather provides adequate reinforcement to control the cracking so that leakage is prevented. Reinforcement in the range of 0.5 to 0.6% Ag is required. If a wall is not specifically designed to be waterproof, then a membrane is required on the water side.

 

[FinnB]

The British standard water retaining structure is BS 8007. I am very familiar with this code. Crack witdths are limited to 0.1 or 0.2mm with the main criteria effecting crack widths being the tension from bending in the face that is cracking, the cover to the reinforcing and the spacing of bars. When drawing up the rebar it is necessary to stagger rebar laps to avoid cracking apperaing along a lap line. It is possible to have joints which can be waterproofed using waterstop either of the pvc dumbbell/rearguard type or by something like hydrophilic bentointe rope. I recently designed an inlet bay for a canal and used hydrophilic bentonite rope in joints.

BS8007 doesn't give guidance on the max length of wall pours. I can't find a code reference for the length of wall pours. There are a few unofficial references that suggest wall pours should be 20 feet for a standard 10 inch thick wall 10 feet high.

 

[hokie66]

That's what I was saying. If you follow the code, you don't need contraction joints. Most 50 metre Olympic pools have no movement joints.

 

[ATSE]

In the US, the concrete code for buildings is ACI 318, and for environmental structures is ACI 350.
In ACI 350, joint spacing is a function of horiz reinf area.
For 30 foot joint spacing using fy=60ksi steel, minimum horiz area = 0.004.
That is, (2) #5 bars per 12" vertical spacing for a 12" wide wall.
Various ACI commentaries note crack control for environmental structures is intended to limit cracking to 0.10mm (0.004").
Agree with previous comment that drying shrinkage is the primary problem when temperature swings are controlled.
I always define construction joint locations on the dwgs, and they are typically 40ft apart, keeping at least 4ft away from corners and Tees.

 

[FinnB]

Thanks for that ATSE. To the British Standard 0.004 is the minimun steel area for horizontal steel for any concrete wall. Does ACI350 give guidance on joints in walls longer than 30 foot.

I need to get my hands on copies of ACI318 and 350. I use British Standards generally but I did use ACI318 some years ago for punching shear in a heavily loaded slab as it gives guidance on how to use structural steel beams in a concrete slab to resist shear over column heads.

 

[FinnB]

hokie66

There are different views on how to pour Olympic pools. I worked on an Olympics pool 7 or 8 year ago and we used a chequer pattern for the floor of the pool with water stop at the joints. It worked ok but was a lot of work for the contractor.

Pouring a floor 50m by 25m in one hit is a 375m3 (490 cubic yards) pour which is quite big for a thin floor slab around a foot deep.

I think two pours on an Olympic pool makes sense.

Remember designing a small pool where tiles were laid over the construction joints which didn't work out very well.