Mix Design
Proportion for密度 and durability
Mix design balances clay binder, sand skeleton, fibre, stabiliser, and water. Proportions are usually stated by volume on site, but serious work converts them to dry mass for repeatability. A typical adobe starting mix might be one part clay-rich soil to two parts sharp sand, plus a small straw fraction; rammed earth often uses the graded soil directly with 5–10% cement or lime. The aim is a dense, well-graded matrix, not the maximum clay content.
Fibre reduces plastic shrinkage cracking and improves toughness, but long straw creates planes of weakness and can rot if wet. Keep fibres short and evenly distributed. Water should bring the mix to optimum compaction moisture, not to a slurried consistency. Adding water for easier placement lowers density and strength.
Traps: measuring cement or lime by loose volume, adding clay to make the mix stickier, over-fibre to hide poor grading, and designing only for dry compressive strength while ignoring erosion and shrinkage.
Worked rule: make at least three trial mixes with clay contents differing by about 3 percentage points. Compact identical specimens at the same effort, cure them, and compare density, shrinkage, and wet strength. Choose the leanest mix that meets the specification. A drop test is only a field guide: a ball that crumbles is too dry, while one that slumps without holding its shape is too wet.