Covering further processing of manufactured earth construction products and their testing for suitability for use in construction

To achieve the maximum strength of molded soil blocks, a certain holding time (drying) is required. At the same time, the correct implementation of drying minimizes the possibility of defects at the final stage of production of the soil blocks.

With accelerated drying, when the block is left to dry in the open air, under the influence of solar radiation and wind, the surface of the block will quickly lose its moisture and shrink. This may cause cracks to appear on the block surface.

Therefore, the drying conditions must ensure a gradual decrease in humidity throughout the entire amount of the molded block. Meanwhile, if cement is used to stabilize the blocks, it is necessary that the moisture remains inside the block body for several days. This is necessary for complete hydration of the cement.

One method of keeping a block wet is simply to put the blocks in a plastic bag. Reasonable care must be taken not to break off the corners, as they have little strength during the drying period. After filling the bag with blocks, its open end must be closed to preserve free moisture.

The second method, no less simple, is that freshly formed blocks are laid out in a row on a non-hygroscopic surface and covered with sheets (plastic, grass mats, burlap, etc.) to prevent moisture evaporation.

After two or three days, depending on the temperature, the blocks (especially those stabilized with cement) will gain sufficient strength for their re-laying. If lime was used as a stabilizer, the blocks should be left to dry for approximately seven days. After pre-drying, the blocks can be removed from under the protective coating and stacked in a stack, which is also desirable to cover with some kind of covering. The stack of blocks and the covering should be periodically sprayed with water.

When the blocks are stacked, the top layer should always be moistened and covered, while the lower layers are air-dried and reach maximum strength.

The duration of drying for maximum strength varies depending on the type of soil and, more importantly, depending on the type of stabilizer used. When stabilizing with cement, it is recommended to hold the blocks for at least three weeks. The drying period for lime stabilization should be at least four weeks.

The soil blocks must be completely dried before they are used for construction.

The strength of the soil blocks depends on several factors, including type of soil used, amount of stabilizing agent applied, quality of compaction, drying method, and strength test method. Tests show that you can expect a difference between dry and wet compression strengths of 50 % or more.

The minimum compressive strength of the soil blocks in the air-dry state must be at least 25 kg/cm2. The soil blocks are subjected to tests similar to those to bricks and concrete blocks. The most accurate results can be achieved in the laboratory, but using the services of the construction laboratory is expensive and often not fast.

Therefore, it is most advisable to test the first samples of the soil blocks on the construction site. These tests will assess the suitability of the block as a wall element.

Usually, the ground blocks are tested after 28 days, although the same tests can be performed earlier, for example, after 7, 14, 21 days in order to determine the increase in strength over time.

It is recommended to keep accurate records during testing. They should include the composition of the soil mixture, blocks production method, size, time since the sample released, and maximum breaking load in the dry state.

The following simple tests can be performed on the construction site.

Only stabilized (water-resistant) soil blocks are subjected to the wet-dry cycle tests. They are tested by random selecting of five dried soil blocks and submerging them completely in water for 12 hours. Then they are taken out of the water and left to dry in the sun during the day. This procedure of soaking and drying blocks is repeated 10 times. The total duration of 10 cycles is approximately one and half two weeks.

Examination of the samples should show whether everything is in order with the soil used or the stabilizing agent used. For example, the blocks can break into pieces, crack, crumble, and even explode, indicating that the mixture must be changed or, in the worst case, another ground must be found. Therefore, it is recommended to produce the first few batches of blocks with different amounts of stabilizer in order to determine whether the difficulties encountered can be solved by using a suitable amount of the stabilizer. If problems persist, other soil mixes and stabilizers should be tested.

The water absorption test can be performed in combination with a wet - dry cycle. Each block is weighed before being submerged for the first time and after being in the water for 12 hours each block is weighed again.

Experience shows that if the block contains less than 15 % of the -absorbed water, it has good strength and will last a long time.

The wet - dry density test is performed as follows. Immediately after the production of the soil block, its density is determined in the freshly formed state. To do this, the block is weighed and its dimensions are measured, and then its volume and density are sequentially calculated.

After 28 days before the "wet-dry" cycle test, the block is weighed again and its dimensions are measured to determine its density in a dry, fully dried state.

Normally, the block has a specific density in the freshly formed state in the limits of 1800-2200 kg/m3. The minimum dry specific density of the block is 1700-2000 kg/m3. If the measured wet and dry density is less than the minimum specified above, the mixture must be adjusted.

The latter recommendations are valid provided that no additives were -used in the soil mixture to reduce the density of the soil, such as sawdust, expanded sand, and similar materials. The sound test is that if, after the required 28-day drying period, two blocks are knocked together and a good "ringing" sound is heard, it means that the blocks are sufficiently strong and resistant to weather conditions.

Test for compressive strength. A wet block of the first class must withstand a compression force of 2.3 MPa (35 MPa x 0.65, where the first value is the compressive strength in the dry state, and the second is the softening coefficient). A load of 69000 N (6.9 tons) is required to test a block with a length of 250 mm and a width of 120 mm. If such a load cannot be provided on the construction site, it can be reduced by testing a smaller cubic sample with sides of 100 mm. The load for testing such a block will be 2.3 tons. Such load can be achieved on the construction site using a customized device, the scheme of which is shown in the figure. The test block on this device must be cut off from the normal-sized ground block.

The scheme of the customized device for blocks testing for strength.

If a more powerful press is available, the total breaking pressure by crushing the blocks on the press must be determined.

The compressive strength of the block is defined as the quotient of the values of the breaking load and the surface area to be affected.

These tests give only approximate results. If more accurate estimates are needed, the blocks should be subjected to laboratory tests, possibly far from the construction site and at a high cost. Therefore, a higher level of accuracy should be justified in making the decision to perform such tests.

When using the capabilities of the laboratory, it is recommended to determine the strength of the soil blocks in dry and wet conditions. At least 10 units must be tested for the same production conditions.