Pump Mix for Concrete Wall Construction 6

[sybie99]

Hi Guys

I have a case where the contractor would like to use a concrete pump to pour a 4m high wall in one go. The structure is water retaining (as per BS8007, the British Code for water retaining structures). I know that a pump mix is very different to a normal mix, it has a high slump, possibly plastisizers as well. I am no expert in the design of a typical pump mix, but my question is whether there are any problems that could occur when using a pump mix in walls (250mm thick wall). In my experience pumped concrete is generally used to construct slabs, not walls. Seeing that the wall needs to be water retaining, could there be issues with shrinkage cracking or any other potential problems stemming from the use of a pump mix?

Thanks

Seb

 

[JAE]

For water retaining structures:

1. Keep the water/cement ratio low - 0.36 to 0.45
2. Plasticizers don't add to the w/c ratio
3. Use aggregates (coarse) that are 1 to 1 1/2" (25 to 40 mm) max. size. Don't use small aggregates without the larger sizes.
4. Here in the US - the ACI 350 requires a higher amount of horizontal reinforcement to control crack width. Don't know what standard you would design to but there should be an equivalent one.
5. Don't use higher strength concrete. Higher cement (richer) mixes shrink more.
6. ACI 350 also provides for control joints or expansion joints in walls at various distances and this spacing affects the required amount of horizontal reinforcement.
7. The wall pour sequence can be planned out ahead of time to minimize shrinkage affects on the walls...i.e. placing the corners last.

 

[Ron]

JAE's list is on target as usual. The mix design is key. JAE noted the coarse aggregate size that should be used...this is perhaps the most critical...first, because that will reduce shrinkage, since less cement paste will be necessary to cover the surface of the aggregate, and secondly, it will force the use of a pump that has the capability of pumping larger aggregate.

Do not allow the use of a "typical" pump mix with small aggregate. The strength will likely be higher, but the shrinkage will be relatively large.

 

[HSIII]

...not to mention jamming of the pump when the wrong mix/wrong pump size is used.

 

[msquared48]

As a side note here, if your wall has two layers of reinforcing, which probably does for a wall of that height, and considering rebar clearances, it might be well to stick with the 1" (25mm) aggregate max considering a 1.5" clearance at any non-soil face. Higher clearances would allow larger aggregate. Otherwise, you will get rock pockets.

If, perchance, the wall has only a single layer of reinforcing, then there should be no problem, considering an 8" minimum thickness of wall.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

For ACI 350, the required clearance for rebar in walls with water is 2" (50 mm) so the 1 1/2" (40 mm) aggregate may be OK for that.

 

[sybie99]

Very good information, thanks guys, that really gives me something to work with when checking the mix design.

 

[msquared48]

You're right... I was thinking 315, but 350 is more appropriate here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Agree with others that mix design is the key. Not a mix design guru myself, but my experience is that a good mix for pumping usually results in high quality concrete in place, so I look on the concrete pump as a quality control device. Probably because not just any technician will tackle a pump mix...too much risk involved if it doesn't work.

 

[msquared48]

318...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[henri2]

With regards to minimizing drying shrinkage cracking, a 1-1/2 inch nominal maximum size aggregate ( MSA) mix will perform better than mixes with smaller MSA for reasons elucidated by Ron earlier. However, pumping a 1-1/2 inch MSA mix could be a major challenge if the right type of pumping system is not selected.

While several factors affect choice of inside pump diameter, none is more important than the MSA.

In many areas 1-1/2 inch MSA mixes are not pumped but are "chuted". So it is not uncommon for a concrete pumping company (that is accustomed to pumping 1 and 3/4 inch mixes) to have problems when a 1-1/2 inch mix is ordered by the concrete contractor. Most are simply caught of guard, “Gee wiz (plus some other colorful vocabulary), we only have a 4-inch pump here to pump the mud!!!”

As a rule of thumb, the inside diameter of the pump and placing line should be 3 to 4 times the MSA. While a 4-inch diameter pump will work fine with a well proportioned 1-inch MSA mix, chances of it working with a 1-1/2 MSA mix are slimmer. A 6-inch diameter line should work fine with a 1-1/2inch mix.

 

[rscassar]

5. Don't use higher strength concrete. Higher cement (richer) mixes shrink more.

I'm not sure this is entirely correct. The concrete design code which I use (Australian Standard AS3600-2009) gives smaller shrinkage strains for higher strength concretes. I can post a excerpt if you would like.

 

[Zambo]

JAE,

the BS Standard is 8007:1987, I think this will limit the crack width to 0.2mm.

I'm not sure about point 2. Plasticisers should help to keep the w/c ratio low while improving the workability of the concrete. But I believe all liquids, including plasticisers, are included in the calculation of w/c ratio.

As has been mentioned an important factor is finding enough space in the rebar to get the pump discharge low down inside the formwork

 

[hokie66]

I have always understood that it is not actually higher amounts of cement or a higher w/c ratio that promotes more shrinkage, but rather the physical quantity of water. So water reducing admixtures help to control shrinkage by reducing the water requirement. The fact that less water also increases strength is not a disadvantage. For water retaining structures, it would be unusual to require high strength concrete, normally considered to be concrete above 6000 psi or 40 MPa.

 

[JAE]

Zambo - you may be correct that all liquids may be included in the w/c ratio, but a teaspoon of plasticizer does much more than water to increase slump. You may be incorrect too - not sure about it but it is a WATER/cement ratio.

I just got off a project/study where a very rich mix added to the shrinkage - in this case it was autogenous shrinkage, not drying shrinkage alone. Rich mixes definitely can add to shrinkage problems.

 

[BAretired]

JAE,

What is the effect on the concrete of using too much plasticizer? Are there any negative effects?

BA

 

[Ron]

Agree again with JAE...rich mixes (high cement content) cause more shrinkage.

For a typical concrete mix, say with a water-cement ratio of 0.5, only about 50 to 60 percent of that water is required for hydration of the cement. The remainder is "water of convenience" or water to aid placement. While that remaining water is very important, it is also the water that partially evaporates from the concrete. As this water goes away, the resulting drying causes shrinkage.

The water used in the hydration reaction is a part of the autogenous shrinkage that JAE noted. Autogenous shrinkage is a result of the chemical reactions that take place during hydration and curing. In a simplistic manner, the resulting chemical compounds take up less space than the elements and compounds present before hydration. This shrinkage component is typically less than drying shrinkage, but does increase with richer mixes, simply because there is a greater volume of reactant (the cement).

hokie66 is right in that the "bulk" water is what causes most of the shrinkage, as just explained. As he also noted, the placement can be enhanced by reducing some of this water by using admixtures, with the side effect of increasing the strength because of the lower resulting water-cement ratio.

Mix designs are a somewhat delicate dance (concrete in a tu-tu?) of placement, performance and strength. Achieving all three is not usually done by accident!

As for the effect of too much plasticizer....the two most common problems are set times and segregation. The set time can be affected either up or down, depending on the admixture and the dwell time of the plasticizer can be a sudden shock during placement...I've seen concrete go from a high slump to stiff in a matter of minutes.

 

[JAE]

BAretired: I don't know about too much plastixizer...ref: Ron above.

Ron: concrete in a tu-tu? You should be a writer - a flair for words and a lot of imagination!