Fiber cement flat sheets are made of an inorganic hydraulic binder or a calcium silicate binder fanned by the chemical reaction of a calcareous and siliceous material reinforced by organic fibers and inorganic synthetic fibers.
An acceptance test aims to establish whether a batch of products conforms to a given specification. The tests shall be carried out on samples drawn either from continuous production or from a delivered consignment.
Contents:
Sampling of Test Specimen
In a consignment, all the sheets of identical size, category, and type, with even thickness and manufactured under similar conditions shall be grouped to constitute a lot. The number of sheets to be selected from the lot shall be as per the table given below:
| 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 shall be two square specimens of 250 mm x 250 mm per sheet.
- The test specimens must be cut from the same part of the sheet. A possible layout for cutting a sample is shown in the figure below.
- Conditioning for equilibrium strength is carried by placing the test specimens for seven days in an ambient condition and in such a manner that all faces are adequately ventilated. Record the temperature and relative humidity.
- Conditioning for wet strength is carried out by immersing the test specimens in water at ambient temperature for 24hrs. The test specimens shall be tested immediately upon removal from the water.
- The test apparatus 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.
- The distance between the two supports shall be 215 mm and 18 times the thickness of square and rectangular specimens, respectively.
Test procedure
- Arrange the test piece with the underside against the supports and load the test piece along its centerline with the help of the loading bar.
- The specimen is loaded such that breakage occurs within 10s to 30s.
- A constant deflection rate is preferred; where this facility is not available, a regular rate of loading is acceptable.
- The thickness is measured at two points for smooth specimens and four points for textural specimens along the section of breakage.
- For square specimens, the broken sample is re-assembled and submitted for a second bending test.
- The line of load application for the second test shall be at right angles to that of the first test.
- The thickness is measured as given in Step-4.
- For rectangular test specimens, the strengths in the two directions are obtained by testing each of the appropriate specimens.
Calculation
The modulus of rupture, Rf in MPa, is given by the formula:
Where,
P is the breaking load in N
l is the distance between axes of supports in mm
b is the width of the test piece, in mm; and
e is the average thickness of the test piece
2. Water Impermeability Test
Specimen Preparation
- Three test specimens shall be cut, that is, one from each of the three sheets.
- Sheets used to provide specimens for other type-tests may be used, or other sheets may be taken.
- The dimension of the specimen shall be at least 600 mm x 500 mm except for narrow products, where the dimensions shall be 600 mm x the maximum possible width available.
- A suitable frame to be sealed on top of the specimen. The dimension of the frame shall be at least 550 mm x 450 mm. A narrow frame of the same length shall be used for narrow products.
- The specimens shall be kept in a controlled environment for at least 7 days at ambient temperature.
Procedure
- Place and seal the frame on top of the face of the specimen and fill it with water to a height of 20 mm above the face of the sheet.
- Place the specimens at ambient conditions and record the temperature and relative humidity.
- The duration of the test shall be 24hrs.
Result Interpretation
Examine the underface after 24hrs and verify that it conforms to the requirement:
The traces of moisture may appear on the underside of the sheet during the water impermeability test, but in no instance shall there be the formation of drops of water.
3. Frost Resistance Test
Specimen Preparation
- Ten sets of paired specimens shall be cut from 5 different sample sheets as carried out for the bending test.
- The apparatus is the same as the bending test. In addition to the apparatus used in the bending test, cooling units shall be used, which are filled with test pieces, to ensure a temperature of -20°C, 2hrs after starting freezing.
- The freezing chamber is equipped with an air circulation unit.
Test Procedure
- The test specimen is immersed in water at ambient temperature until the difference of mass between two consecutive weighings spaced 24hrs apart is less than 0.5%.
- The sample is submitted to 50 freeze-thaw cycles consisting of:
- Cooling air to -20°C ± 2°C in not less than 1h and not more than 2hrs.
- Thawing in water at ambient temperature for 2hrs and not more than 3hrs. Then freezing shall recommence.
- The relative humidity and temperature shall be measured.
- Each freeze-thaw cycle will have a minimum cycle time of 4hrs and a maximum of 6hrs.
- The temperature specified above refers to the freezing cavity.
- At the end of this period, place the specimens in a laboratory atmosphere for 7 days.
- Examine the specimens with the naked eye to detect possible cracks, delamination, or other defects, and record any observations.
Result Interpretation
For each pair of specimens, i (i =1 to 10), calculate the individual ratio r1 as follows:
Where,
Rf1 is the modulus of rupture of the ith test specimen after 50 freeze-thaw cycles.
Rfc1 is the modulus of rupture of the ith reference test specimen.
Calculate the average r and standard deviation, S, of the individual ratios r1. Calculate the 95 percent lower confidence limit, L1 of the average ratio, as follows:
L1=r-0.58s
For frost-resistant application sheets, after 50 freeze-thaw cycles, the limit of the average ratio r shall not be less than 0.75.
Read More:
Fiber Cement Flat Sheets: Types, Characteristics, Tests, and Advantages