Fiber cement flat sheets are composed of either an inorganic hydraulic binder or a calcium silicate binder, which is formed through the chemical reaction of a calcareous and siliceous material. These binders are then reinforced by both organic and inorganic synthetic fibers.
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To ensure that a batch of these products meets a particular specification, an acceptance test must be performed. This test involves taking samples from either the ongoing production process or a delivered consignment, and using them to determine whether the batch conforms to the given specifications.
Sampling of Test Specimen
In a consignment, sheets that are of identical size, category, and type, and have been manufactured under similar conditions, shall be grouped together to form a lot. The lot shall be comprised of sheets with even thickness. In order to determine the number of sheets to be selected from the lot, the table provided below shall be followed.
| Size of the Lot | Sample Size |
| ≤ 150 | 3 |
| 151 to 180 | 3 |
| 181 to 500 | 4 |
| 501 to 1200 | 5 |
| 1201 to 3200 | 7 |
| 3201 to 10000 | 10 |
Test on Fiber Cement Flat Sheet
1. Bending Strength Test
Specimen Preparation
The test specimens required for the testing process should be square in shape and measure 250 mm x 250 mm per sheet. It is important to ensure that the specimens are cut from the same part of the sheet to ensure consistency in the testing. A suggested layout for cutting the samples can be seen in the accompanying figure.
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Conditioning for equilibrium strength involves placing the test specimens in an ambient condition with adequate ventilation for a period of seven days. During this time, the temperature and relative humidity of the environment are recorded.
On the other hand, conditioning for wet strength is carried out by immersing the test specimens in water at ambient temperature for a duration of 24 hours. After removal from the water, the test specimens are immediately tested.
The test apparatus used for conducting the strength test includes a bending test machine, two parallel supports (one rigid and one self-aligning), a loading bar identical to the two supports, and a micrometer, as shown in the figure below.
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The distance between the two supports is required to be 215 mm and it should be 18 times the thickness of square and rectangular specimens, respectively.
Test procedure
To load the test piece, place the underside against the supports and use the loading bar to apply pressure along the centerline. The goal is for the specimen to break within 10 to 30 seconds. It’s preferred to use a constant deflection rate, but if that’s not possible, a regular rate of loading is acceptable. When measuring thickness, two points should be measured for smooth specimens, and four points for textural specimens along the section where the specimen breaks.
For square specimens, a second bending test is performed by re-assembling the broken sample and submitting it for testing again. However, the line of load application for the second test must be perpendicular to the first test. The thickness should still be measured as described in step 4.
For rectangular test specimens, the strengths in both directions are obtained by testing the appropriate specimens separately.
Calculation
The modulus of rupture, denoted as Rf and measured in megapascals (MPa), can be calculated using a specific formula. To do so, several variables must be taken into account. Firstly, the breaking load of the test piece must be measured in newtons (N) and denoted as P. Additionally, the distance between the axes of supports, represented by l and measured in millimeters (mm), must also be considered. The width of the test piece, denoted as b and measured in mm, is another variable that must be included in the formula. Finally, the average thickness of the test piece, denoted as e, must also be taken into account. By plugging these variables into the formula for calculating Rf, the modulus of rupture in MPa can be determined.
2. Water Impermeability Test
Specimen Preparation
In order to conduct certain types of tests, it is necessary to obtain specimens from sheets. For this purpose, three specimens must be cut, with one being taken from each of the three sheets. If sheets have already been used to provide specimens for other tests, they may be used again. Alternatively, different sheets may be chosen.
The dimensions of the specimen must be no less than 600 mm x 500 mm, except in the case of narrow products, where the dimensions must be 600 mm x the maximum width available. To seal the specimen, a suitable frame must be used. The frame should have dimensions of at least 550 mm x 450 mm. However, for narrow products, a narrow frame with the same length should be used.
After obtaining the specimens and sealing them in frames, they must be placed in a controlled environment for a minimum of 7 days. This should be done at ambient temperature.
Procedure
To conduct a test on a specimen, first, place the frame on top of the face of the specimen and ensure that it is sealed properly. Next, fill the frame with water to a height of 20 mm above the face of the sheet. Once the water is added, place the specimens in ambient conditions and record the temperature and relative humidity. It is important to note that the duration of the test should be 24 hours. During this time, the specimen will be exposed to the ambient conditions, and the temperature and humidity will be monitored to ensure that the test is conducted properly. By following these steps, accurate results can be obtained regarding the properties of the specimen being tested.
Result Interpretation
After 24 hours, the underface must be examined to ensure that it meets a specific requirement. During the water impermeability test, it is possible for moisture to leave traces on the underside of the sheet. However, it is crucial to verify that there are no drops of water formed in any instance. Therefore, it is essential to conduct a thorough examination of the underface to ensure that it conforms to this requirement. If drops of water are present, the sheet may not be deemed impermeable, and further action may be necessary.
3. Frost Resistance Test
Specimen Preparation
For the purpose of conducting a freezing test, ten sets of paired specimens will be extracted from five distinct sample sheets, using the same method as the one applied in the bending test. The apparatus that will be utilized for the freezing test is identical to that of the bending test, but it will require cooling units. These cooling units will be filled with the test pieces, and their function will be to maintain a temperature of -20°C. It is imperative that the temperature reaches this level two hours after the start of the freezing process.
The freezing chamber will be equipped with an air circulation unit to facilitate the freezing process. The goal of this test is to evaluate the behavior of the specimens under freezing conditions, and to determine the effect that this has on their properties. To ensure accurate results, it is essential that the samples are prepared and tested according to the specified methodology. The freezing test will provide valuable information that can be used to optimize the performance of the materials in question under different temperature conditions.
Test Procedure
The test involves immersing a specimen in water at ambient temperature and monitoring the difference in mass between two consecutive weighings, which must be less than 0.5%. Once this threshold is met, the sample is subjected to 50 freeze-thaw cycles, each of which involves cooling air to -20°C ± 2°C within a period of 1 to 2 hours, followed by thawing in water at ambient temperature for a duration of 2 to 3 hours, after which the freezing process is repeated. The relative humidity and temperature are monitored throughout this process. Each freeze-thaw cycle must last between 4 and 6 hours, with the specified temperature referring to the freezing cavity.
After the 50 cycles are completed, the specimens are left in a laboratory atmosphere for 7 days. During this time, the samples are visually inspected for any possible cracks, delamination, or other defects, and any observations are recorded.
Result Interpretation
To assess the frost resistance of fiber cement flat sheets, we need to determine the modulus of rupture for both test specimens and reference specimens. We will compare the two values to calculate the individual ratio (r1) for each pair of specimens. We will repeat this process ten times for ten pairs of specimens.
We will then find the average ratio (r) and standard deviation (S) of the individual ratios (r1). Using these values, we will calculate the 95% lower confidence limit (L1) of the average ratio, which will be determined by subtracting 0.58 times the standard deviation (S) from the average ratio (r).
For fiber cement flat sheets to be considered suitable for frost-resistant applications, the average ratio (r) after 50 freeze-thaw cycles must not be less than 0.75.