Mix In Place Sidewalks

[manofstl]

I'm designing a site for a charter school. My client wants to use student help to build a walking trail using "mix-in-place" techniques. He want to use cement, in-situ soil, add water, and mix in place. Rough broom finish.

Client has heard of some cities building bike/walking trails with this technique. Pedestrian traffic only in this case, no bikes.

Anyone have experience with this, and can recommend a cement-soil-water mix ratio? Would rotor-tiller mixing suffice?

 

[civilperson]

Works great with sandy soils, (minus 200 less than 8%). Eight percent of cement by dry soil weight will give 800-1500 psi compressive strength. Works best with a compacted subgrade leveled and graded and then a submerged water cure of six or seven days after set.

 

[cvg]

With gravelly sands you could use perhaps only 5% cement. Amount of water should be just enough to make it moist.

doesn't work nearly as well with fine grained soils such as clayey or silty soils and they will require perhaps double the amount of cement to achieve similar strengths. Recommend taking a sample of the soil to a lab and asking them to do the soil cement mix design for you.

Contact Portland Cement Assoc.

http://www.cement.org/

and you can get quite a bit of information.

 

[concretemasonry]

It strictly depends on the free locally available materials and the climate (freeze/thaw). There is also the reflection back to the professional supervising the technical aspects.

From a purely technical standpoint, your cement contents will be between 5 and 10% if you have a reasonably granular soil and can compact the cement/sand mixture well. You are not dealing with normal concrete, but are in the realm of zero slump concrete that requires compaction and low cement contents to get stability. The closest similarity would be your classic proctor curve for soils where the maximum density possible (stability in your case) increases with moisture content until you reach a maximum.

It is a noble thought to get the students involved. It is also important to not allow them to think this is the level of quality that a professional feels is adequate for other concrete.

Dick

 

[manofstl]

Got it, and thanks guys. I have already revised the specs based on your input.

 

[Grubbyky]

I imagine your client has some picture in their head of a bunch of kids stomping around barefoot in concrete as if they were making mud bricks. I don't think this is a good idea. Some people's skin is hypersensitive to cement, and before the concrete is well mixed, there can be higher concentrations of hydrating cement which can be caustic. I'm not sure about the health concerns of fly ash, but I think it might contain some mercury and also would present the danger of silicosis.

I don't know if this is a valid concern, but at least check out the MSDS for the components you specify. If you do use students to mix the concrete, at least have a good amount of vinegar to wash them off with.