Selection of Construction Materials, Biogas Plant Construction Manual Fixed-dome Digester

Selection of Construction Materials

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If the materials used in the plant construction such as cement, sand, aggregate etc. are not of good quality, the quality of the plant will be poor even if the design and workmanship are excellent.  A brief description regarding the specifications for some of the construction materials is provided below to assist with selection of the best quality materials.  The list of construction materials is given in Table 3.1.

Table 3.1. Biogas Plant Construction Materials

3.1  Cement

The cement to use in the plant construction must be of high quality Portland cement from a brand with a known reputation.  It must be fresh, without lumps and stored in a dry place.  Bags of cement should never be stacked directly on the floor or against the walls to protect the cement from absorbing moisture before use.

3.2  Sand

Sand for construction purpose must be clean.  Dirty sand has a very negative effect on the strength of the structure.  If the sand contains 3% or more impurities by volume, it must be washed.  The quantity of impurities especially mud in the sand can be determined by a simple test using a bottle and clean water.  For the test, the bottle is half-filled with sand, filled with clean water, and then stirred vigorously.  Allow the bottle to sit stationary to allow the sand to settle.  The particles of sand will settle first while mud particles will settle last.  After 20-25 minutes, compare the thickness of the mud layer to the sand inside the bottle are; the percent of mud should be less than 3% of the overall volume.  Course and granular sand can be used for concrete work however fine sand is necessary for plastering work.

3.3  Gravel

Gravel size should not be too big or too small.  Individual gravel diameter should not be greater than 25% of the thickness of concrete product where it is used.  As the slabs and the top of the dome are not greater than 8 cm (3″) thick, gravel should not be larger than 2 cm (0.75″) in size.  Furthermore, the gravel must be clean.  If it is dirty, it should be washed with clean water.

3.4  Water

Water is mainly used for preparing the mortar for masonry, concrete and plastering work.  It is also used to soak bricks/stones before using them.  Water is also used for washing sand and aggregates.  It is advised not to use water from ponds and irrigation canals for these purposes, as t is usually too dirty.  Dirty water has an adverse effect the strength of the structure; hence,
water to be used must be clean.

3.5  Bricks

Bricks must be of the best quality locally available.  When hitting two bricks together, the sound must be crisp or clean.  They must be well baked and regular in shape.  Before use, bricks must be soaked for few minutes in clean water.  This will prevent the bricks from soaking moisture from the mortar after laid in place.

3.6  Cobble Stones

If cobble-sized stones, 7.5-30 cm (3-12”) in diameter are used for masonry work, they must be clean, solid and of good quality.  Cobbles should be washed if they are dirty.

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Biogas Plant Design

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