Pumice and Concrete: Pumice-crete Structures
Although concrete is an incredibly versatile construction material, it’s primarily known for its heavy-duty, cold-grey utilitarian role.
Pumice-Crete® is an innovative construction concept that combines the aggregate-binding properties of Portland cement with the lightweight foamed-stone characteristics of pumice aggregate to create a unique structural wall material with impressive insulative properties. The Pumice-Crete® mixture is cast into place using lightweight formwork, typically at a thickness of 18-24 inches and finished with a quality cementitious plaster inside and out, further enhancing the thermal performance of the thick honeycomb-like wall.
Pumice-Crete® walls are fireproof, noise-dampening, and economical in terms of placing and sourcing the material.
As the walls take on the look of the shaping forms, a wide range of architectural expressions are possible.
It’s important to understand that Pumice-Crete® is not a replacement for ordinary concrete in load-bearing footings, retaining walls, flatwork and other common uses, but rather is an economical, highly-insulative, cast-in-place wall system that stands as a cost-effective, efficient and performance-enhanced alternative to typical stick-built construction.
What are the benefits of Pumice-Crete® construction over the common stick-built approach? Primarily in materials cost savings. Compare the minimal cost of the formwork and bracing, the pumice-crete material itself, and the direct-application finishing plaster (inside and out) with the cost for the complex arrangement of hollow stud walls, insulation fill, rigid exterior-board and corner bracing, interior drywall, tape, texture and paint. Beyond the materials cost, consider the skilled labor costs involved with stick-built construction as well as the ongoing maintenance requirements for such a complex and fragile construction system. Finally, consider the rapid escalation of labor and material costs of stick-built construction if your architectural tastes go beyond a basic rectangular box.
When considered in those terms, pumice-crete is both economical and dead simple—the enduring strength, architectural style, and energy-efficient performance are INHERENT in the wall structure itself.
What is the cured strength of Pumice-Crete®? The carefully-graded pumice aggregate and cement mixture cures to approximately 400 PSI. The cement paste coats the 3/8 pumice aggregate and binds it together where it touches, leaving the spaces between open.
What is the weight of wet and cured Pumice-Crete®? Typical weights (depending on mix design and pumice grade used) range from 25 to 50 pounds per square foot. Once cured, dead load weights decrease slightly.
What about structural steel reinforcement? The thickness of the poured wall (18 to 24 inches) provides the necessary structural strength—making reinforcing steel unnecessary*. Window and door frame can be steel or framed from pressure-treated wood. A final, steel-reinforced bond beam locks the wall system envelope together and provides for roof attachment.
*Check with local building codes and consult with a structural engineer.
What kinds of roof systems can be used with Pumice-Crete® walls? Any roofing system that will work with a masonry wall. The bond beam at the top of the wall holds the anchor-bolts for the wood plates. Rebar can be extended out of the bond beam to tie into masonry or steel roof systems.
What special considerations are necessary to use Pumice-Crete®? Finished walls will be over twice as thick as conventional walls, so extended sills and jambs are needed unless the wall design calls for angles or curves that reduce the wall thickness at windows and door openings. Electrical and plumbing that absolutely must go into exterior walls need to be roughed-in before the pumice-crete is poured. Remember, the aesthetic quality of the finished walls in terms of being straight, plumb, and smoothly curved is entirely dependent on the quality of the formwork.
Where can I find additional information and expertise on building with Pumice-Crete®? Visit Scott MacHardy’s website (Pumice-Crete Building Systems of NM) at pumicecrete.com; also, a web search on “pumice-crete” turns up newspaper and magazine articles, books, videos, and websites from experienced pumice-crete material builders. Another Pumice-Crete builder with extensive experience is Ray Peters—he can be reached at firstname.lastname@example.org