ICS 91.080.30  
 
Reference number
DRS 538: 2023
© RSB 2023
  
 RWANDA 
STANDARD  DRS 
538 
 
First edition  
2023-mm-dd  
 
Masonry units test methods — 
Determination of initial shear strength 
   
DRS 538: 2023 
©RSB 2023- All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition 
 
 
 
 
 
 
 
 
© RSB2023 
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ePortal: www.portal.rsb.gov.rw  DRS 538: 2023 
iii                ©RSB 2023- All rights reserved 
 Contents Page 
Foreword ............................................................................................................................................................ iv  
1 Scope ...................................................................................................................................................... 1  
2 Normative references ............................................................................................................................ 1  
3 Principle ................................................................................................................................................. 1  
4 Terms and definitions ........................................................................................................................... 1  
5 Symbols (and abbreviated terms) ........................................................................................................ 2  
6 Materials ................................................................................................................................................. 3  
6.1 Masonry units ........................................................................................................................................ 3  
6.1.1 Conditioning of the units ...................................................................................................................... 3  
6.1.2 Testing .................................................................................................................................................... 3  
6.2 Mortar...................................................................................................................................................... 3  
7 Apparatus ............................................................................................................................................... 3  
8 Preparation and curing of specimens ................................................................................................. 4  
8.1 Preparation of masonry specimens .................................................................................................... 4  
8.2 Curing and conditioning of the specimens ........................................................................................ 5  
9 Procedure ............................................................................................................................................... 5  
9.1 Placing the specimens in the testing machine .................................................................................. 5  
9.2 Loading ................................................................................................................................................... 7  
9.2.1 Procedure A ........................................................................................................................................... 7  
9.2.2 Procedure B ........................................................................................................................................... 7  
9.2.3 Loading rate ........................................................................................................................................... 8  
9.3 Measurements and observations ........................................................................................................ 8  
9.4 Replications ........................................................................................................................................... 8  
10 Calculations ........................................................................................................................................... 9  
11 Evaluation of results ............................................................................................................................. 9  
11.1 Procedure A ........................................................................................................................................... 9  
11.2 Procedure B ......................................................................................................................................... 10  
11.2.1 General ................................................................................................................................................. 10  
11.2.2 Simple method ..................................................................................................................................... 10  
11.2.3 Statistical method ................................................................................................................................ 10  
12 Test report ............................................................................................................................................ 12  
Annex A (normative)  Types of Failure ........................................................................................................... 13  
 DRS 538: 2023 
©RSB 2023- All rights reserved                  iv 
 Foreword 
Rwanda Standardsarepreparedby Technical Committees and approved by Rwanda Standards Board (RSB) 
Board of Directors in accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards. 
The main task of technical committees is to prepare national standards. Final Draft Rwanda Standards 
adopted by Technical committees are ratified by members of RSB Board of Directors for publication and 
gazettment as Rwanda Standards. 
DRS538 was prepared by Technical Committee RSB/TC 9, Civil engineering and Building materials.  
In the preparation of this standard, reference was made to the following standard: 
BS EN 1052-3:2002: Methods of test for masonry — Part 3: Determination of initial shear strength 
The assistance derived from the above source is hereby acknowledged with thanks. 
Committee membership  
The following organizations were represented on the Technical Committee on Civil engineering and Building 
materials (RSB/TC 9) in the preparation of this standard. 
A+Construction Group Ltd 
Africeramics Ltd 
Consultants Engineers Group (CEG) Ltd 
D&D Resources Ltd 
Dutureheza Ltd 
Enabel Rwanda 
Greenpack Africa Ltd 
Integrated Polytechnic Regional Centre (IPRC) - Musanze  
Mass Design Group 
NP Construction Company (NPCC) Ltd 
Road Transport Development Agency (RTDA) DRS 538: 2023 
v                ©RSB 2023- All rights reserved 
 Rwanda Housing Authority (RHA) 
Rwanda Inspectorate, Competition and Consumer Protection Authority (RICA) 
Rwanda Quarries Association (RQA) 
SKAT Consult  
St Joseph Engineering Company (SJEC) Ltd 
Standards for Sustainability (SfS) 
Stonecraft Industries Ltd 
University of Rwanda - College of Science and Technology (UR - CST) 
Rwanda Standards Board (RSB) – Secretariat 
  DRS 538: 2023 
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  DRS 538: 2023 
1                ©RSB 2023- All rights reserved 
 Methods for testing masonry — Determination of initial shear strength 
1 Scope 
This Draft Rwanda Standard specifies a method for determining the in plane initial shear strength of horizontal 
bed joints in masonry using a specimen tested in shear.  
Guidance is given on the preparation of the specimens, the conditioning required before testing, the testing 
machine, the method of test, the method of calculation and the contents of the test report. 
2 Normative references 
The following documents are referred to in the text in such a way that some or all of their content constitutes 
requirements of this document. For dated references, only the editioncited applies. For undated references, 
the latest edition of the referenced document (including any amendments) applies. 
DRS 536Methods of test for masonry units — Determination of dimensions  
RS 108, Mortar for masonry — Specification. 
3 Principle 
The initial shear strength of masonry is derived from the strength of small masonry specimens tested to 
destruction. The specimens are tested in shear under four-point load.  
Four different failure modes are considered to give valid results.  
Two procedures, A and B are included. Procedure A involves testing specimens at different precompressions 
and the initial shear strength is defined by a linear regression curve to zero prestress. Procedure B involves 
testing specimens at zero precompression and determining a characteristic initial shear strength from a simple 
or a statistical consideration of the results." 
4 Terms and definitions 
For the purposes of this standard, the following the terms and definitions given in DRS 546 and the following 
apply. 
4.1. 
masonry 
assemblage of masonry units laid in a specified pattern and jointed together with mortar  
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 4.2 
shear strength of masonry 
strength of masonry subjected to shear forces 
5 Symbols (and abbreviated terms) 
Ai    is the cross-sectional area of a specimen parallel to the bed joints, in square millimetres (mm2) 
e  distance between centre lines of the mortar bed and the loading roller, in millimetres (mm) 
fvoi    is the shear strength of an individual sample, in Newton per square millimetres (N/mm2) 
fpi    is the precompressive stress of an individual sample, in Newton per square millimetres (N/mm2) 
fvo    is the mean initial shear strength, in Newton per square millimetres (N/mm2) 
fvko is the characteristic initial shear strength, in Newton per square millimetres (N/mm2) 
F     is the representation of the force applied to the specimen, in Newton (N) 
Fi,max is the maximum shear load, in Newton (N) 
Fpi    is the precompressive force, in Newton (N) 
h1 and h2 are the heights of cut units, in millimetres (mm) 
hu    is the height of the units according to DRS 536 , in millimetres (mm) 
ls    is the length of specimen, in millimetres (mm) 
lu    is the length of the units according to DRS 536 , in millimetres (mm) 
tbj    is the thickness of the bed joint, in millimetres in millimetres (mm) 
ts    is the thickness of the steel loading plates, in millimetres (mm) 
α    is the angle of internal friction, in degrees 
αk    is the characteristic angle of internal friction, in degrees 
 DRS 538: 2023 
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 6 Materials 
6.1 Masonry units 
6.1.1 Conditioning of the units 
The conditioning of masonry units shall be as specified: 
Record the method of conditioning the masonry units prior to laying.  Measure the moisture content by mass 
of autoclaved aerated concrete and calcium silicate units in accordance with Annex B of DRS 539 .  Record 
the age of non-autoclaved concrete units at the time of testing the masonry specimens. 
6.1.2 Testing 
Determine the compressive strength of a sample of masonry units, using the test method given in Annex A of 
DRS 539. 
For non-autoclaved concrete units determine the compressive strength at the time of testing the masonry 
specimens. 
6.2 Mortar 
The mortar, its mixing procedure and its flow value shall conform with the requirements ofRS 108-, unless 
otherwise specified, and these shall be reported in the test report. 
Take representative samples of fresh mortar from the mason’s board to make mortar prism specimens, to 
determine the flow value in accordance with Annex C of DRS 539 and to determine the air content in 
accordance withAnnex D of DRS 5397.  Use the prism specimens to determine the mean compressive 
strength at the time of testing the masonry specimens in accordance with Annex E of DRS 539. 
7 Apparatus 
The testing machines used to apply the shear loads and precompression shall comply with the requirements 
given in Table 1. 
The testing machine to apply the shear loads shall have adequate capacity but the scale used shall be such 
that the ultimate load on the specimen exceeds one fifth of the full-scale reading. The machine shall be 
provided with a load pacer or equivalent means to enable the load to be applied at the rate specified. 
Table 1 - Requirements for testing machines 
Maximum permissible 
repeatability of forces as 
percentage of indicated 
force Maximum permissible mean 
error of forces as percentage 
of indicated force Maximum permissible error of 
zero force as percentage of 
maximum force of range  
©RSB 2023- All rights reserved                  4 2.0 ± 2.0 ± 0.4 
Apparatus  capable of measuring the cross-sectional area of the specimens to an accuracy of 1 %. 
8 Preparation and curing of specimens 
8.1 Preparation of masonry specimens 
Prepare type I specimens according to Table 2 and Figure 1. If hu> 200 mm, type II specimens may be used. 
Where for practical purposes it is necessary to cut units, ensure that the faces of the unit to be mortared are 
representative of the unit as a whole. 
 
 
 
Table 2 - Dimensions and type of shear test specimens  
Unit length  Specimen type and dimensions  
lu  
mm  Type according to Figure 1  Dimensions mm  
DRS 538: 2023 
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 ≤  300  I  ls = lu 
>  300  I  300 < l s< 350  
≤  300  II  h1 = 200 ls = lu 
>  300  II  h1 = 200  
300 < l s< 350 
 
Build the specimens within 30 min after completion of the conditioning of the units. Use mortar mixed not more 
than 1 h beforehand unless it is designed to be used over a more prolonged period. 
 
The bearing surfaces of the masonry units shall be wiped clean of any adherent dust. The lower unit shall be 
laid on a clean level surface. The next unit shall be laid so that a final mortar joint thickness of 8 mm to 15 
mm, representative of masonry with conventional mortar joints, or of 1 mm to 3 mm, representative of 
masonry with thin layer mortar joints, is attained. The masonry unit shall be checked for linear alignment and 
level using a set-square and spirit level. Excess mortar shall then be struck off with a trowel. In the case of 
Type I specimens according to Figure 1, the procedure for the second unit shall be repeated for the top unit. 
8.2 Curing and conditioning of the specimens 
Immediately after building, pre-compress each specimen by an uniformly distributed mass to give a vertical 
stress between 2,0 × 10-3 N/mm2 and 5,0 × 10-3 N/mm2 
Then cure the specimens and maintain them undisturbed until testing. For other than lime based mortars, 
prevent the test specimens from drying out during the curing period by close covering with polyethylene sheet, 
and maintain the specimens undisturbed until testing, unless otherwise specified. Test each specimen at an 
age of 28 d ± 1 d, unless otherwise specified for lime based mortars, and determine the compressive strength 
of the mortar at the same age following EN 1015-11. 
9 Procedure 
9.1 Placing the specimens in the testing machine 
Support the end units of each specimen in the test apparatus in accordance with Figure 2. For this, use pieces 
of steel at least 12 mm thick, with an appropriate capping, if necessary, to ensure good contact. The diameter 
of the roller bearings shall be 12 mm with a length of at least the width of the unit. 
Apply the load through a ball hinge placed in the centre of the top central steel plate.  
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DRS 538: 2023 
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9.2 Loading 
9.2.1 Procedure A 
Test at least three specimens at each of three precompression loads. For units with compressive 
strengthsgreater than 10 N/mm2use precompression loads that give approximately 0.2 N/mm2 0.6 N/mm2 
and1.0 N/mm2 For units with compressive strength less than 10 N/mm2use precompression loads that 
giveapproximately 0.1 N/mm2, 0.3 N/mm2 and 0.5 N/mm2. The precompression load shall be kept within ± 2 
%of the initial value. The pre-compression shall be applied according to Figure 3. 
The stiffness of the loading beams that are used for the precompression, shall be sufficient to ensure an 
equally distributed stress. If the platens of the machine are shorter than the length of the specimen lu, loading 
beams may be used. These shall have a length equal to the length of the specimen l u and a depth greater 
thanor equal to the length beyond the edge of the plate. 
9.2.2 Procedure B 
Test at least six specimens at zero precompression.  
 
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 9.2.3 Loading rate 
Increase the shear stress at a rate between 0,1 N/(mm2/min) and 0,4 N/(mm2/min) 
9.3 Measurements and observations 
Record the following: 
 the age of non-autoclaved concrete units 
 the cross-sectional area Ai of the specimens parallel to the shear force with an accuracy of 1 % 
 the maximum load Fi,max 
 the precompression load Fpi for procedure A 
 the type of failure (see annex A). 
9.4 Replications 
If failure is by: 
 shear failure in the unit parallel with the bed joint (see Figure A.3) or; 
 crushing or splitting of the units (see Figure A.4), then; 
 
either 
 further specimens may be tested until shear failures of the types shown in Figure A.1 or Figure A.2 have 
been achieved for each precompression level (Procedure A) or six times (Procedure B) or alternatively; 
 the result may be used as a lower bound to the shear strength for each precompression level. 
Lower bound results should not be used in the evaluation of results in clause 10. If necessary, an alternative 
precompression may be needed for Procedure A so that sufficient failures are achieved. 
 
 
 
 DRS 538: 2023 
9                ©RSB 2023- All rights reserved 10 Calculations 
For each specimen calculate the shear strength and for Procedure A " the precompression stress to the 
nearest 0,01 N/mm2using the following equations:  
 
Fvoi is the shear strength of an individual sample (N/mm2);  
fpi   is the precompressive stress of an individual sample (N/mm2);  
Fi, max is the maximum shear force (N);  
Fpi  is the precompressive force (N);  
Ai  is the cross-sectional area of a specimen parallel to the bed joints (mm2)."  
 
11 Evaluation of results 
11.1 Procedure A 
Plot a graph of the individual shear strength fvoi against the normal compressive stress fpi as shown in Figure 
4. Plot the line determined from a linear regression of the points. Record the mean initial shear strength f vo  at 
zero normal stress to the nearest 0,01 N/mm2. Obtain this from the intercept of the line with the vertical axis. 
Record also the angle of internal friction to the nearest degree, from the slope of the line. 
 
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The characteristic value of the initial shear strength is f vok where f vok = 0,8f vo and the characteristic angle of 
internal friction from tan α k = 0,8 tan α. 
11.2 Procedure B 
11.2.1 General 
Calculate the mean initial shear strength fvo to the nearest 0,01 N/mm2.  
The characteristic initial shear strength may be calculated using 10.2.2 or 10.2.3.  
11.2.2 Simple method 
Thecharacteristic shear strength, fvok, shall be calculated as:  
fvok = 0,8 ×fvo 
or fvok shall be taken as the lowest individual result whichever is the lower, and shall be given to the nearest 
0,01 N/mm2.  
11.2.3 Statistical method 
Calculate for each individual bond strength fvo1, fvo2 ………. fvon the values of Y1, Y2 ………Yn 
DRS 538: 2023 
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Where: 
s is the standard deviation of the n log values;  
k is a function of n given in Table 3;  
n is the number of individual values (normally 6);  
Y is log 10 of the initial shear strength, fvo. 
Calculate the characteristic initial shear to the nearest 0,01 N/mm2 
Table 3 - Relationship between n and k 
n  k  
6  2,18  
7  2,08  
8  2,01  
9  1,96  
10  1,92  
11  1,89  
12  1,89  
20  1,77  
Take the characteristic initial shear strength to be fvko = anti log 10 (Yc) N/mm2 to the nearest 0.01 N/mm2.  
NOTE  The characteristic value derived is based upon a 95 % confidence level. 
 
 
 
 
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 12 Test report 
The test report shall contain the following information: 
a) the number, title and date of issue of this Rwanda  Standard; 
b) name of the testing laboratory; 
c) the Test Procedure used, A or B; " 
d) date of building and number of " specimens;  
e) curing conditions (e.g., time, temperature, humidity); 
f) date of testing the specimens; 
g) description of the specimens including dimensions; 
h) descriptions of the masonry units and the mortar, preferably consisting of the appropriate testreports, 
securely attached, or of extracts taken from these reports; 
i) age of non-autoclaved concrete units at the time of testing the specimens; 
j) type of mortar and the mixing procedure of the mortar; 
k) the method of conditioning the units prior to the time of laying and for autoclaved aeratedconcrete and 
calcium silicate units the moisture content by mass; 
l) maximum load reached by the test specimens; 
m) mean compressive strength of the masonry units in N/mm2 to the nearest 0,01 N/mm2 and thecoefficient 
of variation; 
n) mean compressive strength of the mortar in N/mm2 to the nearest 0,01 N/mm2 and thecoefficient of 
variation;  
o) individual values for the shear strength and for Procedure A precompression stress for each specimen in 
N/mm2to the nearest 0,01 N/mm2and the description of the failure mechanism of each specimen and whether 
any lower bound values have been recorded;  
p) mean and characteristic initial shear strength in N/mm 2to the nearest 0,01 N/mm2 and in the case of 
Procedure B whether the simple or statistical method has been used; " 
q) if Procedure A is used the angle of internal friction and characteristic angle of internal friction;  
r) remarks, if any. DRS 538: 2023 
13                ©RSB 2023- All rights reserved Annex A 
(normative)  
 
Types of Failure 
 
 
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 Bibliography 
[1] EN 772-20, Methods of test for masonry units - Part 20: Determination of flatness of faces of 
aggregate concrete, manufactured stone and natural stone masonry units. 
 DRS 538: 2023 
 
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