Slip Form Concrete and Hempcrete

Hello Everyone,

I have been reading the Permies forum for awhile now and I was finally compelled to make a post since I couldn't find any information on this idea. It is all theoretical speculation on my part, as I am nowhere close to being able to build my own house anytime soon but I want to grow my knowledge base for when I do have the opportunity. I am also very interested in general in sustainable building techniques and green building.

So essentially I am asking about two topics, slip form concrete and hempcrete and how people felt about each one. I was also wondering if it was possible to combine the two as it would seem like they would be compatible. I really like the idea of slip form concrete as a way to build a very permanent and low maintenance structure, it would at least seem to be more durable and rot-resistant than many other techniques. Portland cement obviously isn't the most green or sustainable of items but in my opinion it is kind of like using an trac-hoe to build a dam or swales, something that will last a long time and need few inputs in the future.

I had two ideas for using the hempcrete along with slip form techniques, one being you could frame the house in hempcrete blocks and then use slipform to cover both the inside and outside of the blocks. Or better yet is hempcrete sturdy enough to use as a substitute for portland cement? and if so would the stones in the slipform compromise the insulating properties of the hempcrete?

Thanks for any opinions or information on these two techniques.

*edit* One last question, would it be feasible to incorporate a rocket mass heater into the design of the slipform itself, I would think that the thermal mass of the house would also help to store and radiate heat.

Daniel

Hello Daniel,

Welcome to Permies!!!

I will try to briefly cover each question/comment and this is a very vast subject.

slip form concrete and hempcrete and how people felt about each one.

If the Hempcrete is lime or cob based, I support the practice. I do not support the portland industry at all, as little as possible.

I was also wondering if it was possible to combine the two as it would seem like they would be compatible.

Hempcrete is permeable (breaths) to moisture diffusion. Portland based products are not in general and hold onto moisture which contributes to other issues such as molds.

I really like the idea of slip form concrete as a way to build a very permanent and low maintenance structure, it would at least seem to be more durable and rot-resistant than many other techniques.

Ordinary portland cements (OPC as it's called in the industry) is not as infallible as you would think. Look at our highways and bridges. Do some research on the negative aspects of portland.

Portland cement obviously isn't the most green or sustainable of items...

No, not by a long shot.

...could frame the house in hempcrete blocks and then use slipform to cover both the inside and outside of the blocks.

Yes, in theory, but would that be the best use of resources for the area you build. Look to the vernacular in an area, if you want to know what the most enduring architectural form is.

...is hempcrete sturdy enough to use as a substitute for portland cement?

Depends on the mix ratios, application, and load requirements placed on it from the wall and roof diaphragms.

would the stones in the slipform compromise the insulating properties of the hempcrete?

Yes

would it be feasible to incorporate a rocket mass heater into the design of the slipform itself, I would think that the thermal mass of the house would also help to store and radiate heat.

Remember Daniel, whenever you incorporate a component into the overall infrastructure of a building, you inhibit serviceability and augmentation of that component. When designing, think modulare whenever possible and/or applicable. Bricks and timbers in a timber frame are modular components that can be serviced much easier than many of their counterparts use in the modernity of contemporary architecture.

Regards,

jay

Jay,

Thank you for your very thoughtful response, I hadn't thought of the serviceability of such a design, that is an excellent point. I am still researching the different kinds of building techniques and you are right I should do more research into the most enduring architectural form in my area (Northeast United States)

I was essentially trying to figure out if a hemp/ lime based mortar mixture would be sturdy enough for a home, after reading your response I think perhaps slip form would still be an appropriate medium for a basement and foundation with metal bracers sunk into the mortar for posts to be bolted on to act as the house frame. I am more just curious and I don't have any upcoming plans for building it's just all very fascinating to learn.

I am however still just learning and I really appreciate you taking the time to answer my questions, I am definitely going to do more research on the RMH forum to further expand my knowledge on how they are currently implemented effectively.

Regards,

Daniel

I have built a few walls with standard cement and a slip form. For the smaller wall we used sheets of 3/4 plywood split down the middle They were backed by 2 rows of 2x4. As spacers in between we used 1/2 pvc pipe and 3/8" all thread to pull everything together. It was fairly easy to move the form. The bottom run was of course the full 2 ft height. From there on up you only added about 16 inches per lift. Window forms can be temporarily screwed to the plywood to keep them from moving under load. Most of the pvc pipes can be recovered without damage. On one side a person with a hardwood punch drives them out. On the other to protect the green concrete a second person holds a backer weight with a hole just a little larger than the pipe so it can come out without tearing the green concrete. Of course you leave the upper pipes till the forms are pulled off the next tiers pour. We tried oiling, graphiting and wrapping the pipes in paper. The paper actually made getting the pipes out worse. It apparently absorbed moisture and hung on to the pipe harder. Graphite didn't stay very well during the pour and oil didn't seem to make much difference but was better than nothing. The plain pipe will drive out several times without protection but it gets more scored with each go and harder to drive out. One I would try for future is wrap the pipe in paper and then in plastic wrap to keep it dry. For the tall wall we used full sheets and 3 rows of 2x4 bolted the forms with the all thread. That one 3 rows wasn't enough and the forms bulged in places making a wavy wall. So I would double up the 2x4s for each row and use pieces of 5 or 6 in channel iron washers to trap them in place. I would also make 3 or 4 cross bars between them so the forms were supported both directions. For the building walls we concrete patched the outside of the hole while the concrete was still green and spray foam filled the rest. For the cross support wall put into the cistern when it began to collapse those cross holes didn't matter and they were left open.