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🏗️ Mortar, Concrete & Roads

How to lay brick and stone, pour and patch concrete, and keep a gravel road passable when no paving crew is ever coming: lime, cement-lime, earth and fire-clay mortars; concrete mixed by hand; what you can honestly build without store cement; why asphalt can't be made at home and how to patch what's left; and roads, ditches and culverts. Burning lime is in "Making lime" in the Natural Building guide. The main books are in Shelter-Building\Concrete-Masonry (Army FM 5-428 Concrete and Masonry; Gillmore's 1864 Limes, Hydraulic Cements and Mortars), Shelter-Building\Roads (FHWA Gravel Roads; Army FM 5-436 Paving and Surfacing) and Natural-Building\Plasters-Lime-Floors. Work out quantities with the Practical Math guide, the Calculator and the Unit Converter.

The short version

🧪 Lime for mortar: what's new beyond burning it

Stone, burning, slaking and the quicklime safety box are in "Making lime" in the Natural Building guide. Four more things matter once lime goes into mortar:

Make hydraulic lime from fat lime and clay ("twice-kilned"; Gillmore, pp. 92–93):

  1. Mix well-slaked lime with smooth potter's clay: about 10% clay gives "moderately hydraulic" lime.
  2. Roll balls 2–3 in across and dry them completely; damp clay stays inert.
  3. Burn at a bright red heat with a good draft. The balls can ride on top of an ordinary lime burn. Judge the heat with the heat-colour chart in the Blacksmithing guide.
  4. Grind to powder and keep it dry.

Pozzolans (the Roman trick). A pozzolan "is a substance which, when mixed with lime and water, hardens as a cement" (PA Quake Housing, p. 4): fine silica and alumina that react with slaked lime (Wood Handbook, p. 275).

Stretching scarce cement: in 1:4 or 1:5 mortar "at least one-third of the cement may be replaced by slaked lime without loss of strength" (Henley's, p. 694); lime up to 15% of the cement's weight also makes it more watertight (NBS Circular 30, p. 17).

📚 Go deeper: Gillmore, pp. 91–101 — four ways to make lime set under water; NBS Circular 30, pp. 11–18 — how lime mortar hardens; PA Testing Lime, p. 2 — field tests. Pictures: Hydraulic lime.

🧱 Mortar: mixes, sand and mixing

Type Cement : lime : sand Use it for
M 1 : ¼ : 3 Below ground against earth: foundations, retaining walls
S 1 : ½ : 4½ Walls that must resist sideways force; reinforced work in quake country
N 1 : 1 : 6 General work above ground; chimney brick (not firebrick)
O 1 : 2 : 9 Solid load-bearing walls that won't be soaked and frozen
K 1 : 3 : 11 Soft old brick
Lime 0 : 1 : 3 (putty) Soft brick and stone; what's left when cement runs out

By volume. M–O: FM 5-428, pp. 195–196; chimneys: USDA FB 1889, p. 8; K and lime: NPS Brief 2, p. 11; quake country: FEMA 232, p. 145 (the code bars Type N in grouted, reinforced masonry). Strength runs from 2,500 psi (M) to 75 psi (K) (NPS, p. 7); it is rarely the problem.

The one rule: softer than the brick. Harder mortar "will not 'give'", so the bricks crack and spall instead (NPS Brief 2, pp. 3–4). Granite can take strong mortar; soft brick wants lime (p. 7). Bagged masonry cement has no lime and "can damage historic masonry" (p. 6).

Sand and water: clean, free of salt and clay, rounded river sand packing best; it's 1 binder to 3 sand because good sand is about 30% air (NPS, p. 4). Two sizes of sand beat one (NBS Circular 30, pp. 14–15); screen it under ¼ in (Peace Corps Homes of Earth, p. 103). Salty water makes white bloom and weak mortar (FM 5-428, p. 196).

Mixing by hand (FM 5-428, pp. 196–197; NPS, p. 10; photos):

  1. Measure every part with the same bucket; sand damp and loose.
  2. Cement-lime: mix the dry parts first, add half the water, mix 5 minutes, then the rest a little at a time.
  3. Lime putty: put the sand into the putty. Beating it with a mallet or ax handle and chopping with a hoe makes it work better. Lime and sand alone keep "indefinitely" covered damp; once cement goes in, they don't.
  4. Use it in time: remix mortar that stiffened by drying, but throw out any that has begun to set; use it within 2½ hours above 80 °F, 3½ below (FM 5-428). Repointing old walls, NPS says 30 minutes to 1½ hours, no added water.
  5. Never use mortar that froze; add no "antifreeze" (FM 5-428, p. 197).

Wet the bricks, not the blocks: wet clay brick that drinks fast; never wet concrete blocks (FM 5-428, p. 194). Earth blocks: if 20 drops of water in a 1-in circle vanish in under 1½ minutes, wet them (Peace Corps, p. 104).

Earth, chimney and firebrick mortars:

🪨 Laying stone and repointing old joints

Rubble stone walls (FM 5-428, pp. 236–238): biggest stones in the footing, as long as it is wide, in about 2 in of mortar; each stone on its broadest face; wet porous stones first; pack tight and fill gaps with small stones and mortar; one bonding stone through the wall every 6–10 sq ft; stagger the joints; nothing bigger than two people can handle easily. Quake caution: "double-faced walls, without tie stones and with rubble infill should be strictly avoided" (PA Quake Housing, p. 4), and the house code "essentially prohibits the use of rubble stone masonry" (FEMA 232, p. 124). Use stone for footings, low garden walls and terraces, and build houses of braced timber (rule of thumb).

Repointing loose joints (NPS Brief 2; FM 5-428, p. 284):

  1. Stop the water first: fix leaking roofs and gutters (NPS, p. 2). Work with the wall at 40–95 °F, in shade (p. 7).
  2. Rake out with hammer and chisel to 2–2½ times the joint's width, about ½–1 in for brick; grinders chew brick edges (p. 9; right and wrong).
  3. Pick the mix: scrape old mortar with a screwdriver, then a cold chisel; scratch the brick with a fingernail. The new mortar must not be harder than the brick (p. 14). FM 5-428's Type S suits hard modern brick only.
  4. Brush out dust and dampen the joint, no standing water (p. 10). For cement-lime, prehydrate: wet the dry mix just enough to hold a ball, let it stand 1–2 hours, remix stiff; it shrinks much less (FM 5-428, p. 284).
  5. Pack ¼-in layers, each "thumb-print hard" before the next; tool the last at thumb-print hardness (NPS, p. 11). Brush off smears with bristle or nylon, never wire, after it dries but before it sets, about 1–2 hours (p. 12).
  6. Mist it and keep it under burlap 3 days (p. 11). Try a 3 × 3 ft test patch first (p. 9).

Two cut-through views of a mortar joint between bricks: wrong, raked out unevenly with the brick edges chipped by a tool; right, raked square to an even depth of about 1 in with the brick edges undamaged Rake the old mortar out square and even, without chipping the brick edges (NPS Brief 2, p. 8).

🧮 Concrete: what it is and the mixes

Concrete is cement, sand, gravel and water. "The amount of water used to mix these ingredients is by far the most important factor in determining the final strength" (Wells, p. 234). Beginners use too much, and once it sets "there is no simple non-destructive test" of strength (VITA, p. 284).

Cement : sand : gravel Use Source
1 : 2 : 3 Small jobs and repairs; 6 gal water a sack; a sack makes about 5 cu ft VITA, pp. 291–292; FM 5-428, p. 116
1 : 2 : 4 Water tanks, reinforced floors, walks, fence posts Brace & Mayne, p. 263
1 : 2½ : 5 Foundation walls p. 273
1 : 3 : 4 House footings, 5–7 gal water a sack Peace Corps, p. 71
1 : 3 : 6 / 1 : 4 : 8 Cellar and barn floors, walks / big heavy foundations Brace & Mayne, p. 263
1 : 1 : 3 Water troughs (or 1 cement : 3 bank-run gravel) p. 279
1 : 3 : 2 Thin toilet slab (more sand than gravel) Hesperian, p. 20

For ordinary work "1:2:4, 1:2:3, 1:2.5:5 … all work equally well" (Wells, p. 234). Water per 94-lb sack: 5 gallons for floors that take hard wear; 6 for anything watertight (tanks, septic tanks, driveways); 7 for footings and foundation walls (VITA, p. 290). Count water already in wet sand (USDA AH 73, p. 10).

Checking the wet mix:

🪣 Sand, gravel and water for concrete

🏋️ Mixing, forms, steel and placing by hand

Hand mixing (FM 5-428, p. 117; VITA, pp. 293–294):

  1. On a clean, tight, wetted platform, spread the sand, the cement over it, then the gravel.
  2. Turn it dry at least 3 times, till it's one colour.
  3. Make a hollow, add water slowly, and turn at least 3 more times.
  4. Get it into the forms within 20 minutes. One person mixes about a cubic yard an hour; rinse tools at day's end (Wells, p. 247).

Forms:

Steel: concrete resists squeezing but breaks when stretched; steel is the opposite (Wells, pp. 243–245).

Placing: don't let it fall more than 3–5 ft (FM 5-428, p. 11). Rod it and tap the forms, but too much tamping sends the stones to the bottom (Wells, p. 248). New on old: clean, damp, some stones showing, no puddles (FM 5-428, p. 132); after a stop of more than a few hours, "clean, score, and wet the top" (AH 73, p. 11). Finish with a wooden float for grip; trowel too much and it "will not wear well" (Brace & Mayne, p. 270).

💧 Curing, cold and heat

Concrete hardens by a slow reaction with water, not by drying. Once dry it stops, and it "cannot be re-wetted in the field to re-start the hardening process" (Wells, p. 249).

Kept damp for Strength it finally reaches (Wells, p. 249)
Not at all About 60% of what it should
3 days About 80%
7 days Almost 100%, and more as long as it stays moist

Kept moist a month, it's "about twice as strong" as open-air cured (VITA, p. 299).

Cold (FM 5-428, pp. 152–158): protect concrete whenever it's 40 °F or colder; thin work (under 12 in) should be 55 °F going in. Heat the water (max 180 °F) and if needed the sand and gravel (max 150 °F), and mix those first, then the cement. Never pour on frozen ground. Keep it above freezing at least 3 days: concrete that freezes young is permanently weakened, and homemade concrete (not air-entrained) is hurt more easily. Cover it with straw (AH 73, p. 12). No "antifreeze", and no unvented fuel heater in a heated tent: carbon monoxide (pp. 156, 158). For mortar, warm the sand and water and protect the work from frost (NPS, p. 6).

Hot (FM 5-428, pp. 149–152): keep concrete under 90 °F; hot, dry wind is worst. Shade the materials, use cold water, sprinkle the gravel, wet the forms, steel and ground just before pouring, work early or at night, and loosen the forms soon and keep it wet. In this valley, pour in spring and early fall when you can (hard winter frosts, hot dry summers) (rule of thumb).

🏡 Footings, slabs, posts and troughs

Top: a wall footing drawn in section, twice as wide as the wall and as deep as the wall is thick. Bottom: a square concrete footing with a pedestal and a steel pin for a wood post Sizing a wall footing, and a post footing with its pedestal (USDA AH 73, p. 12).

How much to mix (Wells, pp. 237–238): finished volume × 1.5 = dry materials, plus 10%, split by the parts; a sack is 1 cu ft. A 10 × 10 ft slab 4 in thick is 33 cu ft; × 1.5 + 10% = 55 cu ft of 1:2:3 = about 9 sacks of cement, 18 cu ft of sand, 28 of gravel, and about 55 gallons of water (arithmetic). VITA's 5 cu ft a sack gives 7 sacks, so 9 leaves a margin. It weighs about 4,800 lb at 144 lb per cu ft (see the Practical Math guide) (arithmetic).

🩹 Patching and repairing concrete

🏛️ Concrete without store cement: the honest picture

Portland cement can't be made on a homestead. A precise, finely ground mix of limestone and clay must be burned to a white heat where it starts to melt; "The ordinary temperature of lime-kilns would be far too low" (Gillmore, p. 69). Natural cement comes from limestone that happens to hold the right clay (Brace & Mayne, p. 262); only a test burn shows if any local rock does.

Binder Hardens Good for Not for
Fat lime Slowly, from the outside, in moist air Mortar, plaster, floors and fill in dry places Wet ground, under water, early loads (Gillmore, p. 76)
Hydraulic lime or lime + pozzolan Under water, slowly Footings and linings that stay wet Work that dries out in the air (Henley's, p. 694)
Earth stabilised with lime Over weeks Blocks (cure 37 days), floors Wet bases (PA Soil Blocks, p. 12)

Lime concrete (hardens only in air):

  1. About 1 hydrated lime : 3 sand : 6 coarse stone (3–4 in), what a lime kiln used as fill under its floor (PA Shaft Kiln, p. 2).
  2. Slake the lime fully and make it into mortar with the sand before adding stone; hot quicklime concrete "has grown into disrepute" (Gillmore, p. 230).
  3. Use a bit more mortar than there are gaps in the stone (pail test above; p. 229).
  4. Ram it in layers about 6 in thick until a thin film of water shows under the rammer (p. 231).
  5. Keep it damp for weeks; don't load it early (rule of thumb).

Lime-pozzolan concrete: the same steps with the lime-pozzolan or hydraulic-lime mortar from the first section. It's the only homemade kind that hardens in wet ground. Before trusting a footing to it, cure a few test blocks damp for a month and break them beside a store-cement block (rule of thumb). Save store cement for what only cement does: wells and cisterns, toilet slabs, footings in wet ground, and repairs (rule of thumb).

📚 Go deeper: Gillmore, ch. 7, pp. 229–262 — concrete with lime and cement. Pictures: Roman concrete.

🛢️ Asphalt, tar and pitch: what you can and can't do

What asphalt is: "a natural or man-made by-product of petroleum distillation"; natural asphalt comes from distant pits such as Trinidad's, and road tar from coal-gas and coke works (FM 5-436, p. 15). There's neither here. Paving asphalt is solid at room temperature and must be heated to 250–350 °F (p. 20); "cutbacks" are asphalt thinned with gasoline, kerosene or diesel and catch fire easily; emulsions are asphalt in soapy water and won't burn (pp. 16, 21).

Wood tar and pine tar: a charcoal retort gives about 50 kg of thick tar per tonne of hardwood, "useful … as a wood preservation agent and caulking compound"; from softwood it's Stockholm tar (FAO Charcoal, pp. 31–32). Use it to caulk seams and protect posts and handles. It isn't a road binder: no book here uses it for roads, it softens in summer sun, and a road would need far more than a household could make (rule of thumb). It "cannot be allowed to escape into streams" (same page). Pine-pitch glue is in the Make-Do Household guide.

Using what's left:

Four steps of a pothole repair drawn in section: trim the hole square with straight sides; replace and tamp the base; prime the bottom and sides; fill with patch in layers of 3 inches or less The pothole patch above, step by step (FM 5-436, p. 120).

Hot asphalt is a fire and burn job (FM 5-436, pp. 100–101): never heat it near buildings or anything that burns; keep a fire extinguisher by; stand upwind; no smoking within 50 ft; long sleeves, cuffless pants, heavy gloves, boots. A dense yellow vapour means it's overheated "to the extent that a spark could cause an explosion": put the fire out and get everyone away. Don't heat asphalt over 400 °F or coal-tar pitch over 375 °F (FM 5-426, pp. 128–129); hot bitumen brings "burns and smoke-poisoning" (Biogas Digest, p. 24). No kettles or open fires June to October (rule of thumb). Dusty road? Never spread used engine oil (FHWA, p. 89); water it, or work a little clay into the gravel.

🛣️ Gravel roads and driveways

"The three most important things … are drainage, drainage, and drainage!" (FHWA, p. 56). Even pavement fails without base and drainage (p. 131); a drained gravel road is the plan.

Crown, shoulder and ditch

A gravel road cut across. Right: a straight crown falling ½ in per foot to level shoulders and flat-bottomed ditches with gentle sides. Wrong: a rounded crown that holds puddles, and a ridge of gravel at the edge that traps water on the road The shape that sheds water, and the shape that holds it (a diagram drawn for this guide).

Culverts and water bars

Culvert rules (FM 5-34, p. 270): inlet at or below the ditch bottom; pipe out at least 2 ft beyond the foot of the fill; bedded on a layer at least a tenth of the pipe's width deep; slope 2–4% (never under 0.5%); a headwall at the inlet and rock at the outlet so the water can't dig. Make it a little steeper than the ditch so silt flushes, with upright rods at the inlet to stop branches (WHO, p. 128; right and wrong). Pipes can be "made cheaply from used oil drums by cutting out the bottoms" (WHO, p. 129), or a box of durable logs (USDA FB 1660, pp. 24–25; woods that last: Shelter guide). Size for the worst storm: Cave Junction once got 8.12 in of rain in a day (Soil Survey, p. 5); FM 5-34 sizes culverts for up to 100 acres of runoff (pp. 268–269). Inspect them after every heavy storm (FM 10-67-1, p. 212); plugged culverts "cause serious damage when water flows over roads" (FHWA, p. 61). Mark weight limits on bridges and culverts, and keep the driveway clear 20 ft wide and 13 ft 6 in high for fire engines (OSFM, p. 18).

Four culverts under a road, seen from the side: one laid on the grade of the flow (correct); one with its upstream end too high, leaving standing water; one laid too flat, filling with sand and gravel; one with its downstream end too low, leaving standing water Lay a culvert on the grade of the stream, not too high, too low or too flat (WHO, p. 129).

Water bars turn water off a sloping road before it cuts gullies; the Soil Survey recommends them here (p. 93). No book on the drive gives spacing, so (rule of thumb): dig a shallow trench about 30° across the road and pile the soil on its downhill side as a firm, rounded hump vehicles can cross; let it empty onto rock, brush or firm ground, never loose fill; space them closer the steeper the road, every 50 ft or less on a steep pitch; walk the road in the first big storm and add one wherever water starts to cut.

A sloping road seen from above with two water bars crossing it at about 30°, catching water running down the road and spilling it onto rocks at the side; and a cut along the road showing the shallow trench and the rounded hump on its downhill side Water bars turn water off the road onto firm ground (a diagram drawn for this guide from the rule of thumb above).

Potholes, ruts and washboard

Good gravel, and where to find it

Corduroy, log roads and stone roads

Three cross-sections of corduroy road: 6–8 in logs laid side by side between curb logs held by pickets; the same on log stringers; and heavy corduroy on 10–12 in sleepers Corduroy roads in cross-section (FM 5-34, p. 64).

⛑️ Safety: burns, dust, lifting and failing forms

⚠️ Old advice in the library to ignore

Book says Do this instead
Asbestos mortar, asbestos with 10% lime (Henley's, p. 481) Never. Fire clay for heat (FM 5-428, p. 279)
Litharge (lead oxide) cement with sand, quicklime and linseed oil (Lee's, p. 186) Lead poisons. Use lime or cement mortar
Salt in the mix water against frost, "about one pound of salt to a sack" (Brace & Mayne, p. 266) No antifreeze; warm the materials and cover with straw (FM 5-428, p. 156)
Calcium chloride to speed setting, said not to lessen strength (Wells, p. 237; VITA, p. 299) Never with steel; FM 5-428 bars it from reinforced masonry (p. 197)
Salt water for mixing, only 15% weaker (VITA, pp. 285–286) "Seawater can corrode the steel" (FM 5-428, p. 17); never with steel
Lime concrete will "soon grow as hard as rock" (Lee's, p. 186) It hardens slowly, only in air (Gillmore, p. 76)
Making cutbacks by thinning asphalt with gasoline (FM 5-436, p. 99) "A greater danger of fire or explosion" (same page). Use emulsion or cold patch
Priming tar joints with benzol (FM 5-436, p. 178) Benzol is benzene, which causes cancer (rule of thumb). Skip it
Muriatic acid washes for mortar stains (FM 5-428, p. 284) Scrape with a putty knife or chisel (same page), then scrub with water and a stiff brush (p. 285)
Hard portland or masonry-cement mortar on soft old brick (NPS Brief 2, p. 6) Mortar softer than the brick
Stairs: riser + tread = 15 in (Brace & Mayne, p. 273) 17–19 in (Practical Math guide)

🖼️ Pictures & diagrams

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