GRC Quality Control Series

Glassfibre Reinforced Concrete is a remarkable and complex composite capable of attaining incredible strengths and demonstrating astounding longevity. In fact, properly designed, manufactured and installed GRC products can expect a service life well in excess of 80 years. However, this exceptional lifespan is heavily reliant on following best practices in quality control throughout the production process.

Years of research and development in the GRC industry have led to a carefully determined set of guidelines that can be followed by manufacturers in order to produce GRC products that achieve the best performance capabilities.

GRC is well known for its incredible strength but this should not be taken for granted. Flexural and Tensile strength can be affected by a variety of factors which makes it important to test and monitor these properties throughout the production process.

To help those new to the industry understand the quality control processes followed by the world’s top GRC specialists, we’ve published a series on the different processes involved. We’ll be focusing on the ‘Flexural Strength Test’ in this instalment of the series.

Note: For more information on this test, please refer to the latest standards outlined in BS EN 1170:2024

Determination of Flexural Strength

Flexural and Tensile strength are the most important mechanical properties of GRC, unlike conventional concrete where compressive strength is the most important parameter.

A cube test is therefore not relevant for GRC which has led to a 4-point Flexural Test becoming the accepted standard.

Principle

The deflection of rectangular specimens of cured GRC is measured using four-point loading until failure occurs. The dimensions of the specimens are measured and the Limit Of Proportionality (LOP) & Modulus Of Rupture (MOR) are calculated from these measurements.

This four-point loading imposes pure bending forces over the middle third of the test specimen and is preferred to the three-point loading test where the stress is concentrated at the centre. This method is not applicable to thicknesses of more than 20mm.

Apparatus

Testing Equipment

• Capable of operating a constant crosshead rate, including a calibrated load measuring system, and capable of indicating the load to within +/- 1% of the actual applied load and of indicating deflections which do not exceed +/- 2% actual deflection

Bending Test Jig

• Comprising supports and loading rollers of 6mm minimum diameter and of 52mm length, with an adjustable distance between the supports to give the major and minor span distances given in Table 1, and designed so that the force applied to the specimen is perpendicular to the surface and is applied without eccentricity.

Silicon Carbide Saw (or other suitable equipment)

Test Specimen

Cut the specimens from a test board which is representative in composition and thickness of the cured GRC material.

Cut each specimen with a width of 50mm +/- 2mm and a length of between 20mm and 50mm greater than the major span dimension for the appropriate specimen thickness as given in Table 1. Prepare the rectangular specimens with parallel sides and mark them to identify the mould or trowel faces.

A minimum of four specimens should be cut from each board.

Table 1

Procedure

1. Prior to beginning the Flexural Strength Test, soak specimens in potable water at room temperature for a period of between 4 and 24 hours
2. Set the major and minor spans of the bending test jig to the appropriate values for the specimen thickness given in Table 1 and align the loading rollers and supports so that the axes of the cylindrical surface are parallel. Record the major span (L)
3. Remove the specimen from the water, wipe off surface droplets with absorbent cloth or paper and commence test within 2 minutes

4. Place the specimen in its correct attitude, symmetrically across the two parallel supports ensuring that the length of the test specimen is at right angles to the supports and that equal lengths of the specimen project beyond the rollers
5. Adjust the testing machine so that its crosshead speed corresponds with Table 1
6. Start the test machine and continue until sample failure
7. Record the load (W1) at the Limit Of Proportionality, i.e. the load at which the load deflection curve deviates from linearity and the maximum load (W2)
8. Measure and record the specimen thickness to the nearest 0.05mm at a minimum of three points and calculate their arithmetic mean (d) together with the specimen width (b) to the nearest 0.1mm at or near the failure location, taking care not to choose places where the specimen may have been expanded during the test
9. Disregard the results from any specimen that breaks outside the central third of the distance between the rollers and repeat the procedure with another specimen. Record the occurrence of such cases

From production equipment to raw materials, we’ve got the knowledge and expertise to answer your GRC enquiry

Calculation and Expression of The Results

Modern test machines are supplied with programmes to automatically calculate the results, but they can also be calculated manually using the following formulas:

Note:

• W1 = LOP Load (in N)
• W2 = Failure load (in N)
• L = Major Span (in mm)
• b = Specimen width (in mm)
• d = Arithmetic mean of three specimen thickness measurements (in mm)

Calculate and record the arithmetic mean of the results obtained from the LOP in N/mm²

1. For the specimens tested with their mould or trowel faces in contact with the major span supports
2. For the specimens tested with their mould or trowel faces in contact with the minor span supports
3. For all the specimens

Analysis

Typical Load Deflection Curve

Monitor and analyse the results to ensure compliance with the specification.

Through our wealth of knowledge and experience in the GRC industry, we can carry out quality control and testing, such as the Flexural Strength Test, on behalf of our customers to ensure that your products attain the highest levels of performance. Get in touch today to find out how we can help: info@power-sprays.co.uk

Or if you’d rather carry out testing in-house, have you seen or range of quality control equipment? Our Flexural Testing Machines feature automatic recognition of load cells, with an instant calibration check facility, a crosshead guidance system providing precise alignment and smooth running, as well as overload, overtravel and impact protection. Click here to download the brochure and find out more about this innovative piece of quality control equipment.