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SECTION III. RUBBLE

RUBBLE STONE MASONRY

8-35. You can use rubble stone masonry (see Figure 8-24 below) for walls--both above and below ground--and for bridge abutments, particularly when form lumber or masonry units are not available. You can lay up rubble masonry with or without mortar, but for strength and stability use mortar. There are two types of rubble-stone masonry: random and coursed.

 

Figure 8-24. Rubble stone masonry

RANDON STONE MASONRY

8-36. This type is the crudest of all types of stonework. It does not require laying the stone in courses (see Figure 8-24 above), but each layer must contain bonding stones that extend through the wall (as shown in Figure 8-25) to tie the walls together. Make the bed joints horizontal for stability; the build or head joints can run in any direction.

Figure 8-25. Bonding stones extend through a rubble stone masonry wall

COURSED RUBBLE MASONRY

8-37. This type contains roughly squared stones laid in nearly continuous horizontal bed joints as shown in Figure 8-24.

RANDOM RUBBLE MASONRY MATERIALS

8-38. The two main random rubble masonry materials are stones and mortar. Some of the more common suitable stones are limestone, sandstone, granite, and slate.

STONES

8-39. Use stones that are strong, durable, and cheap for random rubble masonry. Durability and strength depend on the stone's chemical composition and physical structure. Use unsquared stones or fieldstones from nearby ledges or quarries. No stones should be larger than what two persons can handle easily. The larger the variety of sizes you select, the less mortar you need.

MORTAR

8-40. Table 7-1 gives the proportions of the portland-cement-lime mortar mixture to use with random rubble masonry. Mortar made with ordinary portland cement stains most types of stone. To prevent staining, substitute nonstaining white portland cement. Lime usually does not stain the stone.

LAYING RUBBLE STONE MASONRY

8-41. The quality of workmanship affects the economy, durability, and strength of a rubble stone masonry wall more than any other factor. Lay out the wall--

By eye. If the wall does not have to be exactly plumb and true to line, lay it out by eye without using a level and line. This requires frequent sighting.

By line. If the wall must be exactly plumb and true to line, erect wood corner posts to serve as corner leads, and lay the stone with a line. Remember that some parts of the stone will extend farther away from the line than other parts. Do not try to lay the stone in level courses.

RULES FOR LAYING

8-42. Lay each stone on its broadest face. If appearance is important, place the larger stones in the lower courses. Lay stones of increasingly smaller sizes as you build to the top of the wall.

Moistening. Moisten porous stones before placing them in mortar to prevent water absorption from the mortar, thereby weakening the bond.

Packing and filling. Pack adjoining stones as tightly as practicable, completely filling any spaces between them with smaller stones and mortar.

Removing. If removing a stone after placing it on the mortar bed, lift it clear and reset it.

FOOTINGS

8-43. Because a footing is always larger than the wall itself, use the largest stones in the footing to give it greater strength and lessen the risk of unequal settlement. Select footing stones as long as the footing is wide, if possible. Lay them in a mortar bed about 2 inches deep, and fill all the spaces between them with smaller stones and mortar.

BED JOINTS

8-44. Bed-joint thickness varies with the stone you use. Spread enough mortar on top of the lower course stone to completely fill the space between it and the stone you are placing. Take care not to spread mortar too far ahead of the stonelaying.

HEAD JOINTS OR BUILDS

8-45. Form the head joints before the bed joint mortar sets up. After laying three or four stones, make the head joints by slushing the small spaces with mortar and filling the larger spaces with both small stones and mortar.

BONDING STONES

8-46. Be sure to use one bonding stone for every 6 to 10 square feet of wall. Bonding stones pass all the way through the wall as shown in Figure 8-26. Offset each head joint from adjacent head joints above and below it as much as possible (see Figure 8-26) to bond the wall together and make it stronger.

 

Figure 8-26. Bonding stones extend through a rubble stone masonry wall

David L. Heiserman, Editor

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Revised: June 06, 2015