Mortar Batch Proportions by Volume (ASTM C270)
ASTM C270 gives mortar as VOLUME PROPORTIONS of cementitious material and sand, and the batch scales from the cement.
Example
You enter
- Portland cement (volumes) 1
- Hydrated lime (volumes) 0.5
- Sand, times the cementitious total 2.5
- Batch size (bags of cement) 1
- Masonry unit compressive strength (psi, 0 to skip) 3000
- Mortar compressive strength (psi, 0 to skip) 1800
You get
- Cement cuft 1
- Lime cuft 0.5
- Cementitious cuft 1.5
- Sand cuft 3.75
- Sand min cuft 3.375
- Sand max cuft 4.5
- Lime bags 0.4
- Sand shovels 7.5
- Batch yield 5.25 cu ft before water
Details, formula, and sources
One bag of portland is one cubic foot, so a Type S at one part cement to half a part lime is a bag and about half a bag-equivalent of lime, with sand between 2.25 and 3 times the sum of the two. THE COUNTERINTUITIVE RULE IS THE IMPORTANT ONE: a stronger mortar is often a worse mortar. Mortar is meant to be the sacrificial element, so that movement and stress crack the joint -- which is repointable -- rather than the masonry unit, which is not. Repointing soft historic brick with a hard Type S or M mortar is a well documented way to destroy the brick, because the hard mortar transfers stress into the unit and the unit spalls. Type O, or a lime mortar, exists for exactly that case, and the strength comparison here is the check that catches it before the wall does. THE PROPORTION VERSUS PROPERTY DISTINCTION MATTERS AT THE MIXER. C270 allows a mortar to be specified either by proportion or by laboratory-tested properties, and the two are not interchangeable: a proportion-specified mortar is batched to the volumes and is not tested for strength at all. Field-tested mortar is tested to a different standard with different acceptance than laboratory mortar, and field results routinely come in below the laboratory values for the same mortar -- treating that as a failure is a common and expensive misunderstanding. SAND VOLUME IS THE TERM MOST OFTEN ABUSED, because it is measured by shovel rather than by box. The allowable range is broad, and running at the high end produces a harsh, unworkable mortar that masons then correct with water, which changes the water-cement ratio, the strength, the shrinkage and the bond all at once. The shovel count here is a working figure at about half a cubic foot per shovel of damp sand and is no substitute for a measured batch box. Proportions the reader supplies for the type in question. NO TYPE TABLE IS SHIPPED: the cement and lime volumes for M, S, N and O come from C270 itself, they differ between the portland-lime and the masonry-cement and mortar-cement systems, and a project specification can require something else entirely. It does not compute water, which is added to workability rather than to a ratio and is the mason's judgment; it does not address air content, retempering, board life, colour or admixtures; and it does not evaluate bond strength, water penetration, or freeze-thaw durability, which are properties of the assembly rather than of the batch. ASTM C270, the project specification, and the mason of record govern.
ASTM C270 proportions are BY VOLUME: the batch scales from the cement at one cubic foot and 94 lb per bag, with hydrated lime at about 1.25 cu ft and 50 lb per bag, and sand between 2.25 and 3.0 times the sum of the cementitious volumes. A working half cubic foot per shovel converts the sand to shovels.
The ASTM C270 proportion specification by name. NO TYPE TABLE IS SHIPPED: the cement and lime volumes for Types M, S, N and O come from C270 itself and differ between the portland-lime, masonry-cement and mortar-cement systems. Proportion-specified and property-specified mortars are not interchangeable, and field-tested mortar is tested to different acceptance than laboratory mortar. Water is not computed; it is added to workability. ASTM C270, the project specification, and the mason of record govern.
Three multiplications on proportions the reader takes from the specification; no C270 table is reproduced.
Estimate. AHJ and licensed professional govern.
Field names used by the API: cement_volumes, lime_volumes, sand_ratio, cement_bags, unit_strength_psi, mortar_strength_psi, cement_cuft, lime_cuft, cementitious_cuft, sand_cuft, sand_min_cuft, sand_max_cuft, lime_bags, sand_shovels, batch_yield_cuft
- Proportions are entered per type they differ between the portland-lime, masonry-cement and mortar-cement systemsASTM C270
- Water is not computed it is added to workability and is the mason's judgmentthe mason of record
- Shovel counts are a working figure about half a cubic foot of damp sand, no substitute for a batch boxthe project specification