3.2 What Can You Make with Pumice?

Once the pumice-concrete mixture consisting of pumice, cement and water has been properly prepared, it can be poured into various molds to produce different kinds of wall members, e.g. pumice-concrete tiles/ panels and reinforced pumice-concrete hollow-core planks (cf. Fig. 22).

Pumice concrete should not be used for making building members that will be exposed to heavy wear and tear, e.g. stairs, nor is it suitable for building members that are liable to have constant contact with moisture.

3.2.1 Pumice concrete

Lightweight pumice concrete is made in the same manner as normal-weight concrete, except that natural pumice takes the place of sand and gravel. To make pumice concrete from the basic materials pumice, cement and water, follow these steps;

- The first step after the raw pumice is delivered to the intended production site is to remove any humus and other impurities by screening or desilting as necessary.

- The second step is to establish the particle-size spectrum of the pumice material. To obtain a good pumice concrete, the particle -size distribution should be about 1-16 mm, i.e. the pumice should have roughly 40% particles measuring 1 - 3 mm in diameter, 25% particles measuring 3 -7 mm in diameter, and 35% particles measuring 7-16 mm in diameter.

Figure 22: Four pumice-concrete building members

If the particle-size distribution of the raw material does not approximately correspond to the above, it will have to be screened as shown in Figure 23.

Frequently, it will suffice to screen off the particles that are larger than 16 mm, perhaps replacing them with sand.

Figure 23: Screening the raw material

- The third step is to add cement and water to the pumice gravel to produce pumice concrete, preferably with the aid of an electric or diesel-powered mixer. If none is available, the concrete can be mixed just as well with a shovel on a clean base or in some kind of big tub (Fig. 24).

Figure 24: Hand-mixing system

How much cement and water are needed depends greatly on the physical condition of the pumice material, especially its inherent moisture and particle-size distribution. As a rule of thumb though, four parts pumice to one part cement and one part water is about right (Fig. 25).

Figure 25: Volume indication of quantities

Pumice concrete should be soil-moist, i.e. it should have no excess water. The moisture level is right if the mold surrounding the concrete can be removed immediately after compacting without having the shaped piece fall apart (Fig. 26).

Figure 26: Immediate removal of forms possible

3.2.2 Pumice concrete solid bricks/blocks

The least complicated kind of wall member for dolt-your-self production by people with little or no handicraft experience is the simple solid pumice brick (Fig. 27). The dimensions can be chosen at will, but adhering to a standard commercial brick format is recommended. If the bricks are to be used for repairing existing walls, they naturally should be of the same size as the bricks or blocks in the old masonry.

Figure 27: Pumice-concrete solid brick.

Elementary-type pumice-concrete bricks are best suited for use in filling out concrete skeleton structures, but are also good for putting up self-supporting walls. Particularly in areas where no loam or clay is found, pumice bricks serve well as alternative wall-building members -assuming, of course, that natural pumice is available (Fig. 28).

The production of pumice concrete solid blocks measuring 49 × 24 × 15 cm is described below. Such blocks are easy to make in a self-help situation.

First, make a simple wooden mold with inside dimensions corresponding to the desired block format (Fig. 29). Normal, smoothly planed boards or square timbers make good box-mold building material. In making the box mold, be sure that it will be easy to remove from the freshly compacted block, i.e. that it is either easy to take apart and put back together or has such smooth inside faces that the block slips out easily.

Figure 29: Wooden mold for pumice-concrete solid brick, including board

Place the box mold on a smooth, level base, or better yet on a smooth backing board. Try to have a large number of such boards on hand, depending on how many blocks are to be produced in a certain length of time.

Pour the pumice concrete into the mold(s) and compact it by tamping with a wooden or iron compactor (Figs. 30a and 30b). Smooth off the top with a lath (strike board). If the concrete is soil-moist, the box mold can be removed immediately after the concrete has been compacted (Figs. 30c and 30d). Clean it with water for immediate reuse. If the pumice concrete mixture is right, the freshly compacted block -the so-called "green compact" will not lose its shape, i.e. crumble or sag.

Figure 30: Forming pumice-concrete solid bricks

Give the green blocks four or five days to harden before stacking or otherwise handling them. Subsequently, they will require another four days of hardening before they can be transported. All in all, a curing time of 28 days, i.e. one month, is required before they can be placed.

The dimensions 50 × 25 × 12 cm and 30 × 24 × 11.5 cm make a good choice for commercial-scale production of handstruck blocks/bricks, because one and the same kind of block/brick can be used for putting up a 30 cm thick wall, a 24 cm thick wall or an 11.5 cm thick wall.

3.2.3 Pumice concrete cavity blocks

With a little practice and skill, pumice concrete cavity blocks are also easy to make in small quantities. The size of the wooden mold is more or less a question of personal preference, but a 49 × 24 × 15 cm format with two cavities is recommended (Fig. 31). With a view to facilitating placement of the blocks, it is advisable to leave the cavities open at one end only. That way, the mortar is easier to distribute around the supporting surface without having it fall into the cavities (Fig. 32). Since the blocks are supposed to be removed from the molds immediately after they are compacted (so that the wooden molds are immediately available for reuse), the inside of the molds should be made as smooth as possible. Some sort of sheet metal lining serves exceptionally well. Considering the hand-made nature of the finished blocks, either round plastic tubing or blocks of wood would be the best choice for use as cores for forming the cavities, since both are easy to twist out of the green product without damaging the cavities.

Figure 31: Wooden mold for two-cavity blocks

The production of concrete cavity blocks requires careful work to avoid damaging the corners and edges of the blocks when the molds are removed. The main things to watch for are that the pumice concrete is neither too dry nor too wet and that it is carefully compacted.

Figure 32: Spreading mortar on a cavity block

Follow this procedure for making two -cavity pumice-concrete blocks:

- Place the wooden mold on a support (wooden board).
- Cover the bottom of the mold with about 2 cm of pumice concrete (Fig. 33a).
- Put the core pattern (for plastic tubes or wooden blocks) on the mold (Fig. 33b).
- Insert the tubes or blocks for the cavities.
- Remove the pattern.
- Fill the remainder of the mold with pumice concrete and compact it well (Figs. 33c and 33d).
- Then, slowly and carefully pull the plastic tubes or wooden blocks out of the mold and remove the mold itself (Fig. 33e).

Figure 33: Forming pumice-concrete two-cavity blocks

Leave the block on the board to dry for 4 -5 days. Then stack the blocks to harden for another 4 days. After a total of 28 days, the blocks will have cured sufficiently for transportation and use. Handle the blocks with care, because they break more easily than solid blocks.

If you wish to produce large numbers of cavity blocks, use either steel molds instead of wooden molds or, better, a simple hand -operated mechanical press that compacts the blocks and ejects them from the molds.

Figure 36: Filling the corners with concrete and reinforcing bars

Since cavity blocks have relatively thin walls (approx. 2 - 3 cm), the pumice concrete should have a maximum particle size of about 10 mm, i.e. any fraction above 10 mm will have to be screened out of the pumice gravel prior to mixing the concrete. Screening can be accomplished using simple wire screens with mesh sizes of 10 mm (approx. 3/8") and 7 mm (approx. 1/4"). The recommended mixing ratio reads:

2 parts pumice, 1-6 mm in diameter
2 parts pumice, 6 -10 mm in diameter 1 part (Portland) cement.

The advantage of cavity blocks is that they weigh less than solid blocks/bricks, which also means that less pumice concrete (and, hence, less cement) is consumed in making enough blocks for a wall of a given size. An additional advantage is that the cavities situated at the corners of the house can be filled with concrete and reinforcing bars to yield a strong framework' which can be very important in areas subject to earthquakes (Fig. 36). To do so, ram the reinforcing bar (or some other round tool) through the block bottoms to get wall-length cavities at the corners (Fig. 37).

Figure 37

Pumice concrete cavity blocks are useful above all else for filling out skeleton structures, but they are also suitable for making load-bearing walls. Different house-building systems based on cavity blocks are discussed in Chapter 4.3.

3.2.4 Pumice wall panels

How self-help builders with little or no training can use pumice to make simple wall panels measuring 100 × 50 × 5 cm or 100 × 50 × 7 cm is described below. Such panels can be used in any of several time-tested special-purpose house-building systems.

The main merit of the relatively small format is that it makes the panels relatively light and accordingly easy to produce, haul and handle -just right for do-it-yourselfers. A panel width of 50 cm and length of 100 cm combine well for a 2.00-m wall height, and openings for doors and windows can be made by simply leaving out a number of panels at the appropriate places.

To make such pumice-concrete panels, proceed as follows:

Make a simple wooden box mold out of 5-8 cm thick boards. If a large number of panels are needed, it would be a good idea to make several identical molds. That way, the panels can be stacked to save space. The long sides of the panels are supposed to be grooved. To make the grooves, use strips of wooden trim or plastic tubing (Fig. 38a). Later on, when the panels are being placed, the grooves must be filled with mortar to obtain strong joints. For details on wall construction with pumice-concrete panels, refer to Chapter 4.4.

Figure 38: Forming pumice-concrete panels

The panel-making area must be absolutely level. Each panel should have its own support made of smooth sheet-metal or wood. If nothing else is available, smooth paper or plastic sheeting can be laid out under each mold/panel, as long as the ground is perfectly level.

Considering the size of the panel, it would be a good idea, but not absolutely necessary, to include some form of iron reinforcement consisting of, say, a lattice arrangement of 10 mm (3/8") reinforcing bars sized to match the panels' dimensions. Any panel that will be subject to bending stress (sag), though, should have at least two such bars running lengthwise with several bends/curves (Fig. 38b).

Figure 39: Mold for hollow-core planks

For poring the panels, prepare a soil-moist pumice-gravel concrete, consisting of four parts pumice gravel to one part cement, and fill the wooden frame with it as described in Chapter 3.2.1. Place the reinforcing lattice such that it "floats" at the center of the panel; smooth the surface of the panel with a strike board or trowel (Figs. 38c and 38d). Leave the panels on the ground to set and harden for about five days, after which they can be handle and stacked. Then give them 25 days to cure prior to transportation and placement. In loading the panels for transportation, be sure to protect them against impact and bending, i.e. it is better to arrange them in an upright position instead of laying them flat.

If the floor in question will not be subject to heavy loads, reinforced pumice-concrete cavity planks can be used in place of the reinforced hollow girders (cf. Fig. 40, Chp. 3.2.5). Planks up to 10 cm thick, however, can only be used to span not more than than 3.0 m. In homes of simple construction, however, such reinforced planks can serve well, as long as careful attention is given to reinforcement, installation and handling, in addition to clarification of the acceptable span width with the aid of a stress analyst (structural engineer).

Figure 40: Forming pumice-concrete hollow-core planks

The prime use for such simple building panels is for filling in skeleton structures, although they can just as well be used for repairing existing walls and building new houses. Consider for example the house described in Chapter 4.4. It consists of a skeleton made of channel-section steel into which the panels are inserted. An alternative example consists of a load-bearing wooden framework and inserted panels.

Figure 40 (2)

Figure 40 (3)

3.2.5 Reinforced pumice-concrete hollow -core planks

Compared to the simple type of panel de scribed in the preceding Chapter, it takes somewhat more skill, tools and technical equipment to produce reinforced pumice -concrete hollow-core planks. Consequently, this approach is more suitable for collective self-help building projects than for individual homes. Since easy handling of building members is an important criterion in connection with self-help building projects, care should be taken to avoid making excessively large planks that could not be carried by hand. A maximum length of 250 cm and a maximum width of 50 cm are recommended. The planks used in the model homes discussed in Chapter 4.5 measure 220 × 50 × 10 cm. Planks of that size are just small enough to be carried and placed by four workers.

A relatively large area is needed for producing hollow-core planks. Especially the casting area has to be absolutely level, hard-wearing and easy to clean. The plank molds should be made of solid wood, because they will have to be used repeatedly (Fig. 39). Longitudinal cavities are necessary to save weight. To make them, place plastic tubes or steel pipes in the molds and pull them out after the planks have been compacted. To cast the planks, place the fully assembled wooden molds on a perfectly smooth and level floor panel or on ground covered with plastic sheeting. Alternatively, the floor panel can be coated with used oil before the molds are filled. Soil-moist pumice concrete prepared as described in Chapter 3.2.1 should be used for making the planks.

First, pour a 2 or 3 cm thick layer of pumice concrete into the properly prepared mold and carefully tamp it with a broad hand-held compactor. Even better results can be achieved with a roller, e.g. a steel pipe filled with concrete (Figs., 40a and 40b). Try to get the surface as level as possible. Next, insert the pipes or tubing through the holes in the short ends of the molds (Fig. 40 c). Place thin reinforcing rods (do not forget to have them ready) between the core tubes/ pipes (Fig. 40d). Now, fill out the interspaces with a second layer of pumice concrete that just barely covers the pipes/tubes. Again, carefully tamp the concrete with a broad compactor (or use a roller). Then, pour the third and last layer of pumice concrete, compact it, and strike off the surface with a straightedge lath, subsequently smoothing it over with a trowel (Figs. 40e and 40f). Now, carefully twist the pipes/tubes out of the mold and remove the mold from the green plank. Leave the planks on their bases to set and harden for about seven days. After that, they will be durable enough for lifting and carrying. They must be transported in an upright position (as opposed to Lying flat) and will require a total of 28 days curing prior to use (Fig. 41). With a view to achieving uniform quality, the planks should, if possible, be prepared in series in a small production installation. That, in turn' will require the availability of several identical molds and pumice concrete of uniform quality.

Reinforced pumice-concrete hollow-core planks serve well as filler members in various types of frame construction. A simple model house made of load-bearing hollow-core planks is described in Chapter 4.5.

3.2.6 Special-purpose pumice-concrete building members and their applications

Channel blocks can be very useful (Fig. 42) as form blocks for peripheral tie beams, as lintels for doors and windows, and as filler blocks for anchoring steel door hinges, wall ties, etc. (Fig. 43).

Figure 43: Channel form block in a tie-beam configuration

Channel blocks are made in much the same manner as cavity blocks, except that the core (block of wood) is not placed at the center, but flush with one side of the mold. The walls of channel blocks should be at least 3 or 4 cm thick to make them strong enough to cope with the pressures that arise in connection with pouring and compacting the pumice concrete.

Closed, square hollow blocks serve primarily as form blocks for columns and as chimney blocks (Fig. 44). The blocks must be carefully aligned during placement, or there will be danger that the concrete could push them out of line, resulting in a crooked column. Such blocks serve well as chimney blocks if the clear cross section measures at least 10 × 10 cm and the walls are at least 5 cm thick.

Figure 44: Chimmey block

Naturally, attention should be paid to dimensional accuracy in fabricating the blocks in order to obtain straight, well -functioning chimneys.

Figure 45: Masonry corner with channel block serving as support fromwork

Fine-grained pumice concrete can also be used to make diverse kinds of vent blocks that provide through-wall ventilation without letting in sizable vemin or other uninvited guests (Fig. 46). Such blocks also serve as ornaments and in the construction of ventilated store-rooms. They are made in a manner similar to that used for producing cavity blocks, as described in Chapter 3.2.3. However, we recommend not trying to make blocks of very complicated shape, because pumice blocks are never as smooth as those made of normal-weight concrete.

Figure 46: Vent block

Yet another application for pumice-concrete blocks are intermediate floors. So-called pumice-concrete "hollow floor fillers" can be used in constructing ribbed floors (Fig. 47), e.g. when there is a shortage of form - work material, since such floors consist exclusively of prefabricated members.

The load-bearing beams, i.e. "lattice girders with concrete flanges" are suspended between the walls in a carefully aligned arrangement, with spacing to accommodate the hollow floor fillers. Then the fillers are placed side by side on the concrete flanges of the lattice girders. Check the visible under -face, the seating, the end blocks, etc. and install any supplementary reinforcement that may be considered necessary. After that, place a 5 cm-thick layer of pumice concrete over the fillers. The main function of the pumice in such floors is to minimize concrete consumption and reduce the weight burden in the tensioned zones of the floor.

With the requisite accuracy of static analysis, orderly installation and a small-scale industrial production mode, self-help groups can manufacture so-called "beam floors with pumice-concrete hollow-core plank fillers". The precast hollow planks should measure about 30 × 30 cm, with a length of 3-4 m, and have structural-iron reinforcement in their tension zone. They are placed side by side and then filled with concrete. This yields a very sturdy floor that will carry relatively heavy loads, depending on the span width, reinforcement, and the thick-ness of the pumice-concrete hollow girders.