The mechanical strength properties of resin and grouts include determination of various properties similar to testing of rock and concrete. Based on suggested methods by various standards (British standard- BS 7861: Part 1: (1996), American Standards (ASTM- C579) (1991), (ASTM – D7012-13), South African Standard (SANS1534) (2004), and ISRM (International Society of Rock Mechanics (2007)), the appropriate evaluation of the resin strength properties normally include the determination of:

• • Uni-axial Compressive Strength
  • Elastic Modulus of Elasticity
  • Punch shear test

• Creep or Rheological Properties

UNI-AXIAL COMPRESSIVE STRENGTH

Traditionally resins are tested for compressive strength, using cube prism samples. Generally resin manufacturers (including Australia) tend to determine the UCS vales of the resin by compression testing of 40 mm `cubes. For Cementitious grouts the preferred samples size is 50 mm.  However both Chemical resin and cementitious grouts can also be tested using different sample sizes and shapes. The shape and size of samples used is dependent on the adopted standards and guidelines used and personal preference.  In Australia and other countries 40 mm cubes are used for testing resins while 50 mm size is used for cementitious grouts. The length to diameter ration of the cylindrical resins /grouts should vary between 2-2.5 as per various standards for testing cementitious products.

Figure 1 opposite shows the load /displacement profiles of one day old various shaped samples prepared from the same set time resin.   Also shown in Figure 2 opposite is the bar chart of the variation in average UCS values with changing sample shape and size of one day old cast samples. It is obvious the UCS values determined from various shaped samples differed with respect to the sample shape, size and to height /diameter (H/D) ratio. Typically the UCS values were highest for 40 mm cubes and lowest for 40 mm diameter cylindrical samples with H/D ratio of two. The quality and repeatability of the tested samples are evident. It is a well-known fact that the strength values obtained by testing cube samples tend to be on the higher values than the cylindrical samples (see Figure 3). Also, the strength values tend to vary significantly, irrespective of the sample shape and size as the samples are generally cast individually.

The variation in strength with cure time is shown in Figure 4 (Aziz, et al., 2014)

1. Aziz, N, Nemcik, J, Mirzaghorbanali, A, Foldi S, Joyce, D, Moslemi, A, Ghojavand, H, Ma S, Li, X and Rasekh, H (2014) Suggested methods for the preparation and testing of various properties of resins and grouts, in proceedings Coal Operators Conference (Coal 2014), Wollongong, February 12-14, ISBN 978 1 925100 02 0, pp163-176 (Eds. N Aziz, B Kinninmonth).
2. Aziz, N, Nemcik, J, Craig, P, and Hawker, R, ( 2014), Development of new procesure for the assessment of resin performance for improved encapsulated roof bolt installation in coal mines, ACARP Project C21011. 79 p

ELASTIC MODULUS OF ELASTICITY

The modulus of elasticity determination of resin as prescribed in BS 7861: part 1: 1996, recommends that a prism of H/D of 4 be subjected to a controlled compressive load. The axial and lateral strain are to be monitored by four strain gauges mounted on the samples, or by using other means of monitoring the axial and later deformation of the tested sample, such as linear variable differential transformers LVDTs, compressometers, optical devices or other suitable measuring devices. The tested sample is subjected to cyclic loading /unloading and the elastic modulus (E) is the mean of the three-secant moduli measure between two levels of the applied load.

Another method of determining the E value of resin can also be obtained from the straight line extrapolation of the 20-60 kN or 40-80 kN range of the load-displacement profile range of a 40 mm cube sample (Figure 5 opposite). However, E values by this method may not yield E values comparable to the recognised values as suggested by various standards for resins, grouts and rocks as the value of E for the 40 kN Load range will be equivalent to the sample compression. However the calculated value from this approach is markedly outside the values obtained from other more credited methods. The E-values are determined using the following mathematical relationships as:

Where;

E_t: tangent elastic modulus,

σ*: half of the peak stress value

u*: displacement at half of the peak stress

E_s: secant elastic modulus

E_r: 40 kN range elastic modulus

     du^/ : displacement at 40 kN load range (i.e. between 20 kN and 60 kN)

Figure 6 shows the comparison between E-values obtained through different ways of resin cubic samples with various cure times 7 to 21 days. E-values, for tangent and secant modulus, determined by the 40 kN applied load range (manufacturer recommended) are generally higher than those obtained from ISRM (International Society of Rock Mechanics) recommended methods. Also, E-values increased as the resin curing time increased from 7 to 21 days. (Aziz, et al,. 2014)

1. Aziz, N, Nemcik, J, Mirzaghorbanali, A, Foldi S, Joyce, D, Moslemi, A, Ghojavand, H, Ma S, Li, X and Rasekh, H (2014) Suggested methods for the preparation and testing of various properties of resins and grouts, in proceedings Coal Operators Conference (Coal 2014), Wollongong, February 12-14, ISBN 978 1 925100 02 0, pp163-176 (Eds. N Aziz, B Kinninmonth).

PUNCH SHEAR TEST

Table 1 lists various apparatus used for the testing of resins and composite material in shear (Aziz, et al., 2014). The testing for shear falls into two categories, direct and indirect methods. All listed methods are applicable for testing resins, but the resin characteristics, time and effort restrict their selection for any particular resin type.

Punch shear test method is most suited for testing resin. The punch shear box apparatus is shown in Figure 7. This methodology of shear strength determination is currently advocated by the South African Standard for testing of resins and grouts (SANS 1534:2004), and it has currently been used by various resin manufacturers in Australia.

Experience has shown that punch shear test is most suited for testing of resin particularly the fast setting resins. The test is carried out using a thin (3mm) disc-shaped specimen, which is slotted in the middle of the punch shear box (40 mm in diameter and 30 mm high) fitted with a hollow slot of the same diameter as the 12 .5 mm diameter punch as shown in  Figure 7. Full circle discs or a quarter circle segments can be used with this punch test apparatus.

The shearing strength is determined using:

t = F/ p DT

t: the shear strength of the tested sample

F: failure load

T: disc thickness

D: Punched disc diameter

Based on experience, the punch shear box test appears to be superior to other tests because of:

1. The ability to prepare a number of samples in a very short period of time and produce a number of samples form one resin mix, thus allowing repetition of the test results for confirmation.
2. It requires a small amount of resin preparation for testing; hence mixing time is not a problem.
3. It gives consistent results for different period of times, and
4. It is a fast testing method.

The punch shear box can be used as a suitable tool to assess the consistency and quality of the resin samples prepared for various strength properties (UCS, E). This can be achieved by a simple comparison of the shear strength values obtained from testing of 3 mm thick samples

(Aziz, et al,. 2014)

1. Aziz, N, Nemcik, J, Mirzaghorbanali, A, Foldi S, Joyce, D, Moslemi, A, Ghojavand, H, Ma S, Li, X and Rasekh, H (2014) Suggested methods for the preparation and testing of various properties of resins and grouts, in proceedings Coal Operators Conference (Coal 2014), Wollongong, February 12-14, ISBN 978 1 925100 02 0, pp163-176 (Eds. N Aziz, B Kinninmonth).