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Rammed Earth: Mix Design. Will Hunter ECI 281a Term Paper Presentation. Overview. Brief History Materials Construction Methods Stabilization Problems?. (Courtesy Rammed Earth Development, 2004 [1]). Origins & History. Related to Adobe, cob, and brick construction

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rammed earth mix design

Rammed Earth: Mix Design

Will Hunter

ECI 281a

Term Paper Presentation

overview
Overview
  • Brief History
  • Materials
  • Construction Methods
  • Stabilization
  • Problems?

(Courtesy Rammed Earth Development, 2004 [1])

origins history
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])

materials
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]

construction methods 3
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])

stabilization
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
stabilization cont
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]

rammed earth use in california concerns
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
water resistance
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
water resistance cont
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]

factors governing mix recipes
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
mix recipes cont
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]

additional concerns
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]

references
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.
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