Rammed Earth: Mix Design

1. Rammed Earth: Mix Design Will Hunter ECI 281a Term Paper Presentation

2. Overview • Brief History • Materials • Construction Methods • Stabilization • Problems? (Courtesy Rammed Earth Development, 2004 [1])

3. Origins & History • Related to Adobe, cob, and brick construction • Dates back 10000 years [2] • Found on all continents except Antarctica [2] (Courtesy Takeuchi Nozomu, 2006 [2]) (Courtesy Takeuchi Nozomu, 2006 [2])

4. Materials • Historically 70% sand, 30% clay [3] • Modern construction • Well graded • Quarry fines • Optional stabilizer • As little organics as possible (1-2%) [4] • Prepare at optimum moisture content • 200 tons of soil for a typical home Typical particle-size distribution for rammed earth soils. The names of the soils correspond to their composition in terms of sand, gravel, and clay (ie 712 has 7 parts sand, 1 part gravel, and 2 parts clay) [4]

5. Construction Methods [3] • Constitutive soils thoroughly mixed and moistened • Buckets, loaders, rototillers • Soil laid in forms (similar to concrete) • Numerous form styles • Compacted using impact methods • Done monolithically in lifts of ~8” (~4-5” compacted) • Maximum compaction indicated by “ringing sound” • Typically tied together with concrete bond beam at top Pneumatic backfill tamper (Courtesy Kehm Equipment, Inc. [5])

6. Stabilization • Unstabilized Rammed Earth • Relatively high compressive strength ( up to 800 psi) [6] • Water-soluble • No tensile strength • Basis for Stabilization • Higher strengths required in seismically active areas • Waterproofing in wet areas • Stabilizing Agents [8]: • Portland cement • Lime • Asphalt emulsion • Fly ash

7. Stabilization (cont.) • Most common stabilizing agent: Portland cement • Essentially becomes well-compacted soil-cement • Cement content: anywhere from 3-15% by weight • Optimum content ~6-9% depending on soil type [7] • Benefits include higher strength and greater resistance to moisture ingress / damage Unstabilized RE sample (at left) compared to a 6% cement stabilized RE sample (right) after a 1 hour absorption test [7]

8. Rammed Earth Use in California: Concerns • Seismic Performance • Bond beams • Cement stabilization • Semi-cold joint between lifts • Waterproofing / Moisture Ingress • Clay content • Variations in density

9. Water Resistance • Unstabilized RE is essentially a water-soluble material • High clay content (up to 30%) – swelling / expansion / cracking [8]? • Kaolinite: low expansion potential, suitable for use • Montmorillonite, Bentonite: high expansion potential, unsuitable

10. Water Resistance (cont.) • Factors in Moisture Ingress [7] • Capillary suction • Pressure differential • Surface finish • SSA / % Clay – Proposed indicator of granular stabilization • 3.35 Ratio [9] Mass of water absorbed due to capillary suction versus the square root of time [9] SSA & 3.35 Ratio for different soil mixes [9]

11. Factors Governing Mix Recipes • Soil particle-size distribution is the single biggest factor in the ultimate properties of a RE wall • Fines Content • Silt: undesirable • Addition of Cement? • Increases fines content • Optimum moisture content for compaction compared to that for cement hydration • Excess cement = essentially silt (bad) • Strength Requirements (Seismic Zone?) • Aesthetics

12. Local soils versus imported? Quarry fines typically used Nun’s Canyon Quarry locally [8] Mix typically chosen by architects, contractors Aesthetics Moisture content Proper mixing Field tests (thread test, ribbon test, soil ball test [3]) Mix Recipes (cont.) Soil ball test [6]

13. Additional Concerns • No building codes • Typically falls under “unreinforced masonry” [6] • Submittal to building department… how? • Openings in walls? • Lintels • Construction strength versus ultimate strength • 30 psi immediately, up to 300 psi (unstabilized) [6] • Bond strength between reinforcing/earth • Uniform mix, uniform water content Compaction using pneumatic backfill tamper [1]

14. References • Rammed Earth Development. Rammed Earth Development. 2004. 02 Dec. 2006 <http://www.rammedearth.com/> • Takeuchi, Nozomu. The Western China. 2006. 03 Dec. 2006 <http://www-es.s.chiba-u.ac.jp/~takeuchi/China.html> • Easton, David. The Rammed Earth House. Vermont: Chelsea Green Publishing Company, 1996. • Hall, Matthew, and Youcef Djerbib. “Rammed Earth Sample Production: Context, Recommendations, and Consistency.” Construction and Building Materials 18 (2004): 281-286. • Kehm Equipment, Inc. Model 131. 2001. 02 Dec. 2006 <http://www.kehm.com/apt_5214_backfill_tamper.htm> • McHenry, Paul Graham Jr., and May, Gerald W. Adobe and Rammed Earth Buildings: Design and Construction. New York: John Wiley & Sons, Inc., 1984. • Hall, Matthew, and Youcef Djerbib. “Moisture Ingress in Rammed Earth: Part 2 – The Effect of Particle-Size Distribution on the Absorption of Static Pressure-Driven Water.” Construction and Building Materials 20 (2006): 374-383. • King, Bruce. Buildings of Earth and Straw: Structural Design for Rammed Earth and Straw-Bale Architecture. California: Ecological Design Press, 1996. • Hall, Matthew, and Youcef Djerbib. “Moisture Ingress in Rammed Earth: Part 1 – The Effect of Soil Particle-Size Distribution on the Rate of Capillary Suction.” Construction and Building Materials 18 (2004): 269-280.