Cave stone, also known as “travertine”, is mainly composed of calcium carbonate. The environment in which cave stones are formed is mainly the seabed, at least at the bottom of the Great Lake. Commercially valuable cave stone mines require uniform materials with beautiful patterns. Only in environments with very weak hydrodynamic forces such as the seabed or the bottom of large lakes, can underground hot springs flow smoothly to the surrounding areas, and the calcium carbonate in them can be evenly precipitated and form beautiful patterns. One of the main defects of commercial cave stones is the bending pattern, which is the result of disturbance caused by the eruption of hot springs.
The mainstream color of cave stones is light beige, as well as yellow cave stones, coffee cave stones, ultra white cave stones, red cave stones, and gray cave stones. Cave stones make people feel gentle and rich in texture, promoting the decorative buildings to often have a strong cultural and historical charm, thus being favored by architects; The characteristics of cave stones are low strength, loose texture, easy accumulation of water in the holes, poor freeze-thaw resistance, and weather resistance. Some low-strength cave stones and processed stone slabs even break and break during transportation. In view of this, considering the safety function of the curtain wall, the cave stone curtain wall should also have a certain scope of application and certain usage restrictions.
01. Cave stones should not be used on the exterior walls of schools, kindergartens, daycare centers, hospitals, or buildings inhabited by the elderly.
02. Cave stones should not be used on the exterior walls of buildings with high personnel mobility, such as shopping malls, train stations, airports, exhibition halls, theaters, and office buildings.
03. Cave stones should not be used on the exterior wall facing the road at a T-junction.
04. Cave stones should not be used on the exterior walls of high-rise and super high-rise buildings.
05. Cave stones should not be used in areas with severe pollution, acid rain environments, or marine environments.
06. Cave stones should not be used in cold areas, severe cold areas, or areas with significant changes in annual temperature differences.
07. When using cave stones on exterior walls, the individual area of the cave stones should be smaller than the area of granite; And the long and short sides should be determined based on the texture of the cave stone.
08. The connection form between the cave stone and the steel skeleton should consider the disadvantage of the weak edge strength of the cave stone.
09. The water absorption rate of cave stones should be lower than that of granite.
10. When using cave stones on external walls, the bending strength of each batch of cave stones should be checked, and unqualified ones are strictly prohibited from use.
11. When using cave stones on external walls, certain strengthening measures should be taken to ensure the safety of cave stone curtain walls.
12. When using cave stones on super high-rise exterior walls, experts should be organized to conduct feasibility studies. Only when feasible can they be used and reliable safety measures be taken.
For those who truly want to use cave stones, there are certain requirements:
Due to the poor material properties of the cave stone itself, it is generally not recommended for use in curtain walls. If the architectural design is unavoidable, at least the following minimum requirements should be considered:
1. Each batch of stone slabs used for curtain walls should undergo bending strength tests, and their test values should meet the following requirements:
When the height of the curtain wall does not exceed 80m, the average test value shall not be less than 5N/mm ²， The minimum test value shall not be less than 4 N/mm ²； When the height of the curtain wall exceeds 80m, the average test value shall not be less than 6N/mm ²， The minimum test value shall not be less than 4.5N/mm ²； A stone slab subjected to unidirectional force should meet the above requirements in the main direction of force; A stone slab subjected to bidirectional force should meet the above requirements in both directions of force.
2. The holes in the cave stone should not be too dense, and the diameter of the holes should not be greater than 3mm, and there should be no transparent holes.
3. The water absorption rate should not exceed 6%, and after applying a waterproof surface layer, it should not exceed 1%.
4. The freeze-thaw coefficient should not be less than 0.8 and should not be less than 0.6.
Test requirements for cave stone application
Due to significant differences in the material properties of cave stones from different origins, basic material properties tests should be conducted before determining the use of market-based ore veins.
1. Bending test
When determining the mineral vein, select stone specimens for the dry bending test and saturated bending test, which are the basic indicators of material properties. The saturated bending strength is beneficial for understanding the sensitivity of stone to water. During the experiment, clean stone specimens should be used, and reinforcement and composite layers should not be attached.
2. Water absorption rate
Stones that have not undergone surface waterproofing treatment and those that have undergone waterproofing treatment should be tested. After waterproofing treatment, the water absorption rate of the stone should not exceed 1%.
3. Freeze-thaw coefficient
The freeze-thaw coefficient refers to the ratio of the flexural strength after a certain number of freeze-thaw cycles to the flexural strength without freeze-thaw cycles. It determines whether this type of stone can be applied in cold regions. This test shall be carried out in areas where negative temperatures may occur. Only after confirming through basic material properties tests that these basic conditions are met, can this type of cave stone be processed into panel panels for use in curtain wall engineering.
The processed stone panel should also meet the following basic requirements:
1. Plate thickness
The bending test shows that the bending strength of panel stone with a larger thickness is lower than that of stone with a smaller thickness. However, due to the fact that the load-bearing capacity of the board is directly proportional to the square of the thickness, the load-bearing capacity of thick stone is still greater than that of thin stone. The minimum thickness of the cave stone board can be determined by the standard value of bending strength fk: when fk ≥ 8.0MPa, the minimum thickness is 35mm; When 4.0 ≤ fk ＜ 8.0MPa, the minimum thickness is 40mm. When this value is less than the minimum experimental value, use the minimum experimental value. The allowable deviation for sheet thickness is+2mm, -0mm, and negative deviation is not allowed.
2. Sheet size
The strength of the cave stone is low, so the size of the board should not be too large, and should generally be controlled within 1.0 square meters. It is not advisable to use slender strip-shaped stones, as these stone slabs are easily broken during transportation and installation. The conversion ratio of stone should preferably be within 1:2 and should not exceed 1:3
3. Surface treatment
The water absorption of cave stones is very high, and waterproof coatings should be used to reduce their water absorption to less than 1%. The stone curtain wall adopts adhesive board joints, and the stone board can only be used for large surface waterproofing on the outer surface; When using open board joints, the stone should be waterproof on six sides. Hole stones with large and numerous holes should be sealed.
