ICS 91.080.30; 91.120.10  
 
Reference number
DRS 537: 2023
© RSB 2023
  
 RWANDA 
STANDARD  DRS 
537 
 
First edition  
2023-mm-dd  
 
Masonry units test methods — 
Determination of compressive strength 
   
DRS 537: 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 
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from RSB. 
Requests for permission to reproduce this document should be addressed to: 
Rwanda Standards Board 
P.O Box 7099 Kigali-Rwanda 
KK 15 Rd, 49 
Tel. +250 788303492 
Toll Free: 3250 
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw  DRS 537: 2023 
iii                ©RSB 2022- All rights reserved 
 Contents Page 
Foreword ............................................................................................................................................................ vi  
1 Scope ...................................................................................................................................................... 1  
2 Normative references ............................................................................................................................ 1  
3 Terms and definitions ........................................................................................................................... 1  
4 Symbols (and abbreviated terms) ........................................................................................................ 2  
5 Principle ................................................................................................................................................. 2  
6 Materials ................................................................................................................................................. 2  
7 Apparatus ............................................................................................................................................... 2  
8 Preparation of specimens .................................................................................................................... 3  
8.1 Sampling ................................................................................................................................................ 3  
8.2 Surface preparation ............................................................................................................................... 3  
8.2.1 General ................................................................................................................................................... 3  
8.2.2 Removal of tongues and grooves ........................................................................................................ 3  
8.2.3 Preparation of masonry units containing frogs and which are not to be capped .......................... 4  
8.2.4 Grinding .................................................................................................................................................. 4  
8.2.5 Capping .................................................................................................................................................. 4  
8.3 Conditioning of specimens before testing ......................................................................................... 5  
8.3.1 General ................................................................................................................................................... 5  
8.3.2 Conditioning to the air-dry condition .................................................................................................. 5  
8.3.3 Conditioning to the oven dry condition .............................................................................................. 6  
8.3.4 Conditioning to 6 % moisture content ................................................................................................ 6  
8.3.5 Conditioning by immersion .................................................................................................................. 6  
8.4 Loaded area ........................................................................................................................................... 7  
8.4.1 Gross area .............................................................................................................................................. 7  
8.4.2 Net loaded area of units containing frogs intended to be filled with mortar in practice ............... 7  
9 Procedure ............................................................................................................................................... 7  
9.1 lacing specimens in the testing machine ........................................................................................... 7  
9.2 Loading ................................................................................................................................................... 7  
10 Calculation and expression of results ................................................................................................ 8  
11 Evaluation of results ............................................................................................................................. 8  
Annex A (normative)  Determination of percentage area of voids in masonry units (by paper 
indentation) .......................................................................................................................................... 10  
A.1 General ................................................................................................................................................. 10  
A.2 Principle ............................................................................................................................................... 10  
A.3 Symbols ................................................................................................................................................ 10  
A.4 Materials ............................................................................................................................................... 10  
A.5 Apparatus ............................................................................................................................................. 10  
A.6 Preparation of specimens .................................................................................................................. 11  
A.6.1 Sampling .............................................................................................................................................. 11  
A.6.2 Surface treatment ................................................................................................................................ 11  
A.7 Test procedures ................................................................................................................................... 11  
A.7.1 Obtaining the indentations ................................................................................................................. 11  DRS 537: 2023 
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 A.7.2 Measurement ....................................................................................................................................... 11  
A.7.3 General ................................................................................................................................................. 11  
A.7.4 Measurement by geometry ................................................................................................................ 11  
A.7.5 Measurement by weight ..................................................................................................................... 11  
A.8 Expression of results ......................................................................................................................... 11  
A.9 Test report ........................................................................................................................................... 12  
Annex B (normative)  Determination of moisture content of calcium silicate and autoclaved 
aerated concrete units ....................................................................................................................... 13  
B.1 General ................................................................................................................................................. 13  
B.2 Principle ............................................................................................................................................... 13  
B.3 Symbols ............................................................................................................................................... 13  
B.4 Apparatus ............................................................................................................................................ 13  
B.5 Preparation of specimens .................................................................................................................. 13  
B.6 Test procedure .................................................................................................................................... 13  
B.7 Calculation and expression of results .............................................................................................. 14  
B.8 Evaluation of results .......................................................................................................................... 14  
B.9 Test report ........................................................................................................................................... 14  
Annex C (normative)  Determination of net and gross dry density of masonry units (except for 
natural stone) ...................................................................................................................................... 15  
C.1 General ................................................................................................................................................. 15  
C.2 Principle ............................................................................................................................................... 15  
C.3 Symbols ............................................................................................................................................... 15  
C.4 Apparatus ............................................................................................................................................ 16  
C.5 Sampling .............................................................................................................................................. 16  
C.6 Procedure ............................................................................................................................................ 16  
C.6.1 Determination of dry mass ................................................................................................................ 16  
C.6.2 Netdrydensity ...................................................................................................................................... 17  
C.6.3 Gross dry density ............................................................................................................................... 18  
C.7 Test report ........................................................................................................................................... 18  
Annex D (normative)  Determination of volume and percentage of voids and net volume of clay 
and calcium silicate masonry units by sand filling ......................................................................... 19  
D.1 General ................................................................................................................................................. 19  
D.2 Principle ............................................................................................................................................... 19  
D.3 Symbols ............................................................................................................................................... 19  
D.4 Materials .............................................................................................................................................. 19  
D.5 Apparatus ............................................................................................................................................ 19  
D.6 Preparation of specimens .................................................................................................................. 20  
D.6.1 Sampling ............................................................................................................................................ 20  
D.6.2 Surface treatment.............................................................................................................................. 20  
D.7 Test procedure .................................................................................................................................... 20  
D.7.1 Volume of voids .................................................................................................................................. 20  
D.7.2 Sand filling   procedure ...................................................................................................................... 20  
D.8 Calculation and expression of results .............................................................................................. 20  
D.9 Evaluation of results .......................................................................................................................... 21  
D.10 Test report ........................................................................................................................................... 21  
Annex E (normative)  Determination of net volume and percentage of voids of clay masonry units 
by hydrostatic weighing ..................................................................................................................... 22  
E.1 Principle ............................................................................................................................................... 22  
E.2 Symbols ............................................................................................................................................... 22  
E.3 Apparatus ............................................................................................................................................ 22  
E.4 Sampling .............................................................................................................................................. 23  
E.5 Test procedure .................................................................................................................................... 23  
E.6 Calculation and expression of  results ............................................................................................. 23  DRS 537: 2023 
v                ©RSB 2022- All rights reserved 
 E.7 9 Test report ......................................................................................................................................... 23  
 DRS 537: 2023 
©RSB 2023- All rights reserved                  vi 
 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. 
DRS537 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: 
1) EN 772-1:2011Methods of test for masonry units - Part 1: Determination of compressive strength 
2) EN 772-10, Methods of test for masonry units — Part 10: Determination of moisture content of calcium 
silicate and autoclaved aerated concrete units  
3) EN 772-2, Methods of test for masonry units — Part 2: Determination of percentage area of voids in 
aggregate concrete masonry units (by paper indentation)  
4) BS EN 772-13:2000, Methods of test for masonry units - Determination of net and gross dry density of 
masonry units (except for natural stone) 
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 DRS 537: 2023 
vii                ©RSB 2022- All rights reserved 
 Greenpack Africa Ltd 
Integrated Polytechnic Regional Centre (IPRC) — Musanze  
Mass Design Group 
NP Construction Company (NPCC) Ltd 
Road Transport Development Agency (RTDA) 
Rwanda Housing Authority 
Rwanda Inspectorate, Competition and Consumer Protection Authority (RICA) 
Rwanda Quarries Association (RQA) 
Rwanda Standards Board (RSB) – Secretariat 
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 537: 2023 
1                ©RSB 2022- All rights reserved 
 Masonry units test methods — Determination of compressive strength 
1 Scope 
This Draft Rwanda Standard specifies test methods for determining the compressive strength of masonry 
units. 
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 edition cited applies. For undated references, 
the latest edition of the referenced document (including any amendments) applies. 
DRS 515, Natural stone — Terminology and classification 
DRS 536, Methods of test for masonry units — Part 16: Determination of dimensions  
ISO 6507-1, Metallic materials— Vickers hardness test — Part 1: Test method  
RS 211-7, Mortar for masonry — Part 7: Determination of water absorption coefficient due to capillary action 
of hardened mortar  
3 Terms and definitions 
For the purposes of this standard, the following terms and definitions given inDRS 515 and ISO 6507-1and the 
following apply. 
3.1 
gross dry density 
the mass per unit gross volume after drying to constant mass 
3.1.2 
gross volume  
the volume  of the unit derived  from  the length, width and  height with a deduction of the volume of 
perforations, voids, recesses or indentations intended to be filled with mortar 
3.1.3 
net dry density   
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 the mass per unit net volume after drying to constant mass 
3.1.4 
net volume 
thegrossvolumeoftheunitlessthevolumeofanyperforationsorvoidsnotintendedtobefilled withmortar.  
4 Symbols (and abbreviated terms) 
fb  normalized compressive strength of masonry unit (N/mm2) 
d  shape factor multiplier used to convert the air-dry compressive strength of the masonry specimens to 
the normalised compressive strength . 
5 Principle 
The specimens, after preparation, when needed, are laid and centred on the platen of a compression testing 
machine. A uniformly distributed load is applied and increased continuously up to failure. 
6 Materials 
Sand with a maximum grain size of 1 mm.   
Cement   
7 Apparatus 
7.1 Testing machine that conforms to the requirements of Table 1. 
Table 1 — Requirements for testing machines  
 
 
 
 
The testing machine shall have adequate capacity to crush all the test specimens, but the scale used shall be 
such that the failure 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 given in 9.2. The 
testing machine shall be equipped with two steel-bearing platens. The stiffness of the platens and the manner 
of load transfer shall be such that the deflection of the platen surfaces at failure load shall be less than 0,1 mm 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 
%  
2,0  ±2,0  ±0,4  DRS 537: 2023 
3                ©RSB 2022- All rights reserved 
 measured over 250 mm. The platens shall either be through hardened or the faces case hardened. The 
testing faces shall have a Vickers hardness of at least 600 HV when tested in accordance with ISO 6507-1. 
One platen of the machine shall be able to align freely with the specimens as contact is made, but shall be 
restrained by friction or other means from tilting during loading. The other platen shall be a plane non-tilting 
block. The bearing faces of both platens shall be larger than the size of the largest specimen to be tested. 
Where auxiliary platens are used, they shall be properly located and of the same hardness, stiffness and 
planeness as the main platens. The bearing surfaces of the platens shall not depart from a plane by more 
than 0,05 mm. 
7.2 Weighing instrument  capable of weighing specimens to an accuracy of 0,1 % of their mass. 
7.3 Sufficient stiff steel strips  for use on shell bedded or strip bedded ground units (See 9.1). 
8 Preparation of specimens 
8.1 Sampling 
The method of sampling shall be in accordance with the relevant product standard. The minimum number of 
specimens shall be six, but a larger minimum number may be specified in the product specification, in which 
case that larger number shall be used. In the case of large masonry units representative portions, e.g. cubes, 
may be cut from the masonry unit in different positions as stated in the relevant product standard (see also 
Note to 8.2.4). 
8.2 Surface preparation 
8.2.1 General 
Specimens shall be tested in the orientation specified, and this shall be stated in the test report. For certain 
forms of construction, it will be necessary to test the masonry units in more than one orientation. Where 
grinding in accordance with 8.2.4 significantly alters the contact surface or reduces the height significantly 
alternative approaches are required, see 8.2.4. Where it is not practicable to prepare clay HD units by grinding 
e.g. high strength units, the surface preparation may be carried out by capping in accordance with 8.2.5 
After the removal of any superfluous material, e.g. flashing from the production process, the faces of the 
specimen, whether a whole masonry unit or a piece cut from a larger unit (see 8.1 ), through which the load is 
to be applied, shall be plane to a tolerance of 0, 1 mm in any 100 mm and such that the top surface lies 
between two parallel planes which are parallel to the bottom surface, and not greater than 1 mm apart for 
every 100 mm. If the test faces of the masonry unit as manufactured, or the piece cut from a larger unit, fail to 
achieve this specification, then prepare the surfaces either by grinding (see 8.2.4) or by capping (see 8.2.5). 
Masonry units containing frogs, and not required to be capped, shall additionally be treated as given in 8.2.3. 
If the specimens contain grooves or tongues first prepare them as given in 8.2.2. Test specimens shall be 
prepared in accordance with Annex B. State the method of surface preparation in the test report.  
8.2.2 Removal of tongues and grooves 
Remove any tongues and/or grooves on the test faces of units prior to testing. If pieces are to be cut from 
larger units, arrange the cutting so that any tongues and/or grooves are eliminated.   
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 8.2.3 Preparation of masonry units containing frogs and which are not to be capped 
For masonry units with frogs that are assessed to have a net loaded area of more than 35 % of the bed face 
test them without removing or filling the frogs. Where the net loaded area of masonry units with frogs is less 
than or equal to 35 % of the gross area then the frogs shall be filled with mortar of the same kind as used for 
capping (see 8.2.5) and the curing shall be carried out in accordance with the requirements of 8.2.5.4. 
8.2.4 Grinding 
Grind the surfaces of the specimen until the requirement for planeness and parallelism given in 8.2.1 is 
achieved. However, if the masonry units have frogs, indented lettering, cavities, perforations, internal or 
external holes, leave these in place. If the grinding process would significantly alter the contact area of the 
tested faces then the capping procedure of 8.2.5 shall be followed. If the remaining height of the specimens 
after they are ground is less than 40 mm or the height /width ratio less than 0,4, then make up a composite 
specimen by placing one upon the other without using any mortar or binding material or separating layer(s) 
between them.  
NOTE Where a composite specimen is made up from more than one ground unit, this should be considered as a single  
specimen providing a single test result. Accordingly, a greater number of masonry units than that specifically 
stated in 2801 will be necessary in order to provide the required number of test results.  
8.2.5 Capping 
8.2.5.1 Capping of masonry units without voids or with voids unfilled  
Use a cement / sand capping mortar expected to attain a minimum compressive strength, when tested in 
accordance with RS 211, at the time of testing the specimens, at least that of the expected masonry unit 
strength or 30 N/mm2, whichever is the lesser. 
If necessary, e.g. for units with high water absorption characteristics, first moisten the surfaces to be capped. 
Bed each specimen on a smooth rigid plate of ground glass or stainless steel which does not depart from a 
true plane surface by more than 0, 1 mm in any 100 mm. A suitable method is as follows:  
Support the plate firmly with the machined face uppermost and level it in two directions at right angles using a 
spirit level. Coat the plate with a film of mould release oil or a sheet of thin paper or plastic film to prevent 
mortar adhering to the plate. Place a uniform layer of mortar about 5 mm thick on the plate about 25 mm 
longer than the unit and about 10 mm wider. Press one bed face of the specimen firmly into the layer so that 
the vertical axis of the specimen is perpendicular to the plane of the plate.  
Check this condition by using a square or spirit level held against each of the four vertical faces of the 
specimen in turn. Ensure that the thickness of the mortar bed is at least 3 mm over the whole area and that 
any cavity in the bed face normally filled when the masonry units are laid in the wall is completely filled with 
mortar. Do not fill cavities except for those that are intended to be filled in the construction. Trim off any 
surplus mortar flush with the sides of the masonry units.  
Cover the specimen and mortar with a damp cloth. Keep the cloth damp. When sufficiently hardened, examine 
the mortar bed. If free from defects such as a lack of compaction, lack of adhesion to the masonry unit and/or 
cracking, bed the second bed face in the same way as the first, using mortar made with materials drawn from 
the same batches of cement and sand and using the same mix proportions.  DRS 537: 2023 
5                ©RSB 2022- All rights reserved 
  After removing the specimen from the plate, check that the mortar bed is free from defects as before. Small 
holes may be made in the capping to drain water trapped in cavities, if necessary.  
8.2.5.2 Capping of masonry units to be face-shell bedded  
When masonry units are to be face-shell bedded in use and specimens are not to be prepared by grinding, 
bed each specimen in mortar as specified in 8.2.5.1 using the following procedure.  
Support and coat the plate as described in 8.2.5.1. Lay two parallel strips of mortar about 5 mm thick such that 
each strip is about 25 mm longer than the length of the unit and about 10 mm wider than the face shell.  
Press one bed face of the unit into the mortar such that the thickness of the mortar under the face-shells is at 
least 3 mm. Check that the vertical axis of the specimen is perpendicular to the plate using a square or vertical 
level check each vertical face.  
Trim off any surplus mortar. Store the specimen, examine and bed the second face in accordance with 8.2.5.1 
and 8.2.5.2 respectively.  
8.2.5.3 Capping of masonry units to be strip-bedded  
When masonry units are to be strip-bedded, the procedure specified in 8.2.5.2 shall be followed except that 
the capping mortar shall be applied to all of the surfaces to be bedded in use.  
8.2.5.4 Storage of capped specimens  
Cure the specimens under sacks kept damp throughout the curing period or store in a conditioning chamber at 
a relative humidity of greater than 90 %. Cure for a period sufficient to ensure the mortar achieves the 
minimum strength specified in8.2.5.1. 
8.3 Conditioning of specimens before testing 
8.3.1 General 
Specimens shall be conditioned using a prescribed regime of moisture conditions or to a prescribed moisture 
condition as appropriate. The method of conditioning shall be as described in this clause. The method shall be 
as specified in Annex B. In all cases, except for conditioning by immersion, free air circulation around each 
specimen shall be ensured during conditioning.  
8.3.2 Conditioning to the air-dry condition 
The condition air dry will be achieved in accordance with one of the following procedures:  
8.3.2.1 Store the specimens for at least 14 d in the laboratory at:  
a) Temperature               ≥ 15ºC  
b) Relative humidity         ≥ 65%   
©RSB 2023- All rights reserved                  6 
 The specimens may be tested before 14 d if constant mass has been reached.  
Constant mass shall be considered to have been reached if, during the drying process in subsequent 
weighing’s with not less than a 24 h interval, the loss in mass between two determinations is less than 0,2 % 
of the total mass.  
8.3.2.2 Dry the specimens   at 105 °C ± 5 °C for at least 24 h and cool at room temperature   for at 
least 4 h.  
8.3.3 Conditioning to the oven dry condition 
The condition oven dry will be achieved in accordance with one of the following procedures:  
a) Dry the specimens   at 105 °C ± 5 °C to constant mass.  Constant   mass shall be considered   to have 
beenreached if, during the drying process in subsequent   weighing’s   with not less than a 24 h interval, the 
loss in mass between  two  determinations    is less than  0,2 % of the total  mass.  Allow the specimens   to 
cool to ambient temperature   before testing. 
b) Dry the specimens   at 70 °C ± 5 °C to constant mass.   Constant   mass shall be considered   to have 
been reached  if, during  the  drying  process  in subsequent   weighing’s   with  not less than  a 24 h  interval,  
the  loss in  mass  between  two  determinations    is less than  0,2 % of the total  mass.  After drying and prior 
to testing store the specimens   at 20 °C ± 2 °C until thermal equilibrium   is reached.   After that, perform the 
test within 24 h.  
8.3.4 Conditioning to 6 % moisture content 
Condition the specimens   to a moisture content of 6 % ± 2 % by mass as follows:  
Calculate the dry mass ofthe unit from thevolume, which shall be calculated   from the dimensions determined 
as in DRS 536 andthe drydensity determined   in accordance   with Annex C.  The mass of the specimen   at 
the time of testing shall be the dry mass multiplied by 1.06.   Dry the specimens   at a temperature   not 
exceeding 50 °C until this mass is attained with an accuracy of ± 0.2 % of the dry mass.  
After conditioning   to the 6 % moisture content and prior to testing, store the specimens   at room temperature 
for at least 5 h.  
Check and record the mass of the specimen just before testing.  
Determine the moisture content in accordance with Annex B. 
8.3.5 Conditioning by immersion 
Immerse thespecimens inwater at a temperature of 20 °C ± 5 °C   for aminimum periodof 15h and 
subsequently allow them to drain for 15 min to 20 min.  DRS 537: 2023 
7                ©RSB 2022- All rights reserved 
 8.4 Loaded area 
8.4.1 Gross area 
The gross area of the loaded surface shall be calculated in square millimetres by multiplying the length by the 
width of each specimen determined in accordance with DRS 536. Where units are to be tested with the 
compressive force other than normal to the bed face, then the gross area shall be calculated similarly but 
using the width and height or length and height as appropriate.  
8.4.2 Net loaded area of units containing frogs intended to be filled with mortar in practice 
Where the net loaded area of masonry units with a frog which is intended to be filled with mortar in practice 
(see also 7.2.3) is not less than 35 % of the gross area then the compressive strength shall be calculated on 
the basis of the net loaded surface of the frogged bed face. Where the net loaded area of masonry units with a 
frog is less than 35 % of the gross area then the compressive strength shall be calculated on the basis of the 
gross area of the masonry unit. In the case of units with frogs in both bed faces the net loaded area to be used 
shall be the smaller of the two.  
Where the frog is of a regular shape, determine the frog area of each specimen by simple measurement and 
the principles of geometry. For rectangular frogs, determine the frog area of each specimen by measuring the 
length and width at the outer perimeter of the frog, using a rigid steel rule. Measurements shall be made to the 
nearest 1 mm. Calculate the net loaded area for each specimen as the difference between the gross area of 
the bed face and the frog area.  
Alternatively, and for units possessing a non-regular frog shape, the net loaded area may be determined by a 
paper indentation method following the principles of Annex A. 
9 Procedure 
9.1 lacing specimens in the testing machine 
Wipe the bearing surfaces of the testing machine (6.1) clean and remove any loose grit from the bed faces of 
the specimen. Align the specimen carefully with the centre of the ball-seated platen, so that a uniform seating 
is obtained.  Units with a single frog shall be placed with the frog uppermost. Units with a frog in both bed 
faces shall be placed with the larger frog uppermost.  
Do not use any packing material except for units intended to be face-shell bedded or strip-bedded and which 
have been prepared by grinding. In such cases position four stiff steel strips (6.3), the same width as the face• 
shells and 50 mm longer, two at the top and two at the bottom, overlapping equally at each end.  
9.2 Loading 
Initially, use any convenient rate of loading but, when about half the expected maximum load has been 
applied, adjust the rate so that the maximum load is reached in not less than approximately 1 min.  Table 2 is 
given as a guide to choosing appropriate loading rates.  
Record the maximum load achieved.    
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 Table 2 — Loading rate  
Expected compressive strength  
(N/mm2)  Loading rate (N/mm2)/s  
 < 10  0.05  
11 to 20  0.15 
21 to 40  0.3  
41 to 80   0.6  
> 80  1.0  
NOTE With some specimens, the applied load may fluctuate several times before maximum failure load is reached. This 
will be indicated by a reduction in load as the specimen yields followed by an increase to a new maximum as loading is 
continued. This temporary reduction may occur several times before the specimen finally fails.  
10 Calculation and expression of results 
 Calculate the strength of each specimen by dividing the maximum load achieved by its loaded area, which is 
the gross area for units intended to be laid on a full bed of mortar, to be face shell or strip bedded and those 
containing frogs not intended to be filled in practice, or in accordance with 7.4.2 in other cases, and express it 
to the nearest 0, 1 N/mm2.  
11 Evaluation of results 
The test report shall contain the following information:  
a) number, title and date of issue of this Kenya Standard;  
b) name of the organisation that carried out the sampling and the method used;  
c) date of testing;  
d) type, origin and designation of the masonry unit by reference to product standard;  
e) number of specimens in the sample;  
f) date of receipt of the specimens in the testing laboratory;  
g) a sketch of the specimen, if needed, showing the extent of the loaded area and the height and the 
orientation of loading;  
h) method of conditioning;  
i) for those units conditioned to 6 %, the moisture content at the time of testing;  
j) method of surface preparation used;  DRS 537: 2023 
9                ©RSB 2022- All rights reserved 
 k) failure load in N, and measured dimensions in mm of each specimen;  
l) strength of the specimens in N/mm2, to the nearest 0, 1 N/mm2, and the coefficient of variation of the 
sample to the nearest 0.1 %;  
m) compressive strength of the sample in N/mm2, to the nearest 0, 1 N/mm2;  
n) required the value of the normalised compressive strength (see Annex A) in N/mm2 to the nearest 0,1 
N/mm2;  
o) remarks, if any.  
 
  
©RSB 2023- All rights reserved                  10 
 Annex A 
(normative)  
 
Determination of percentage area of voids in masonry units (by paper 
indentation) 
A.1 General 
This Annex gives a method of determining the percentage area of voids in masonry units. 
A.2 Principle 
After preparation of the specimens thick pieces of paper are impressed on the test surfaces1) of the masonry 
units and the indentation measured to give the percentage area of voids. 
A.3 Symbols 
Av      is the total cross-sectional area of voids (mm2) 
Au     is the cross-sectional area of masonry unit (mm2) 
Mpv   is the mass of paper cut to the shape of the void (g) 
Mpu   is the mass of paper cut to the shape of the masonry unit (g) 
A.4 Materials 
Thick, stiff sheets of paper, the dimensions of each piece of paper exceeding the dimensions of the masonry 
unit by at least 20 mm in both directions. 
A.5 Apparatus 
A.5.1  Compression testing machine. 
A.5.2   Planimeter. 
A.5.3   Weighing instrument, capable of weighing to an accuracy of 0,1g. DRS 537: 2023 
11                ©RSB 2022- All rights reserved 
 A.6 Preparation of specimens 
A.6.1 Sampling 
The minimum number of specimens shall be six, but a larger minimum number may be specified in the 
product specification, in which case that larger number shall be used. 
A.6.2 Surface treatment 
Remove any roughness on the test face(s) of the masonry unit. 
A.7 Test procedures 
A.7.1 Obtaining the indentations 
Place one sheet of paper on the bottom platen of a compression testing machine (A.5.1). Place the masonry 
unit on the paper, with the voids running vertically, and then, if required to test two faces simultaneously, place 
a second sheet of paper on the top face of the masonry unit. Apply a load of approximately 3 kN. Mark the 
outline on both sheets of paper of the indentations that have been made. 
A.7.2 Measurement 
A.7.3 General 
Measure the areas of the cross-section of the voids and the masonry unit in accordance with A.7.2.2 or the 
weights of the corresponding areas of paper in accordance with A.7.2.3. 
A.7.4 Measurement by geometry 
For each sheet of paper determine the total cross-sectional area of the voids Av and of the masonry unit Au 
and express them to the nearest 10 mm2 the planimeter (C.5.2).  
A.7.5 Measurement by weight 
For each sheet of paper cut out the area representing the area of the masonry unit and weigh it to the nearest 
0.1g. Record the weight Mpu. 
From the sheet of paper representing the area of the masonry unit cut out the areas representing the voids 
and weigh these to the nearest 0.2g. Record the weight M pv. 
A.8 Expression of results 
Determine the percentage area of voids by: 
Geometry:  ஺ೡ
஺ೠ 𝑥 100      (1)  
©RSB 2023- All rights reserved                  12 
 Or by weight: ெ೛ೡ
ெ೛ೠ 𝑥 100     (2) 
for the paper applied to each bed face of the masonry unit. 
Determine the mean percentage area of voids, if required, to the nearest 1 %. 
A.9 Test report 
The test report shall contain the following information: 
a) the number, title and date of issue of this Standard; 
b) the name of the organization that carried out the sampling and the method used; 
c) the date of testing; 
d) the type, origin and designation of the masonry unit by reference to the product standard; 
e) the number of specimens in the sample; 
f) a reference to the method used (i.e.: by geometry or by weight); 
g) the percentage area of voids of each specimen and the mean value, if required, to the nearest 1 %; 
h) remarks, if any. DRS 537: 2023 
13                ©RSB 2022- All rights reserved 
 
Annex B 
(normative)  
 
Determination of moisture content of calcium silicate and autoclaved 
aerated concrete units 
B.1 General 
This Annex gives a method for determining the moisture content of calcium silicate and autoclaved aerated 
concrete masonry units. 
B.2 Principle 
After drying to constant mass, the moisture content is calculated as the ratio of the loss of mass during drying 
to the mass after drying. 
B.3 Symbols 
ws          is the moisture content, in percentage by mass;  
mo,s    is the mass  of specimen before  drying, in grams;  
mdry,s  is the mass  of specimen after  drying, in grams 
B.4 Apparatus 
5.1  Ventilated oven, capable of maintaining a temperature of (105±5) oC. 
5.2  Weighing instrument, capable of weighing specimens to an accuracy of at least 0.1 % of their mass. 
B.5 Preparation of specimens 
Take a representative sample of no fewer than six units.  For units with lengths ≥ 500 mm and/or heights ≥300 
mm, take no fewer than six representative portions cut from at least three units. 
 
B.6 Test procedure 
Before drying, weigh the test specimens (m o,s). 
Dry the test specimens at a temperature of (105±5) oC,to constant mass.  Constant mass is reached if, 
duringthe drying process, in two subsequent weighings with a 24 h interval, the loss in mass between the two  
©RSB 2023- All rights reserved                  14 
 determinations is less than 0.2 % of the total mass.After drying to constant mass, weigh the specimens 
(mdry,s). 
B.7 Calculation and expression of results 
Calculate the moisture content (w s) of the specimen from the ratio of the loss in mass during drying to the dry 
mass, expressed as a percentage to the nearest 0,5 %. 
𝑤௦= 𝑚௢,௦− 𝑚ௗ௥௬,௦
𝑚ௗ௥௬,௦ 𝑥 100 
B.8 Evaluation of results 
Calculate the mean value of the moisture content of the specimens to the nearest 1 %. 
B.9 Test report 
The test report shall contain the following information: 
a) the number, title and date of issue of this Standard; 
b) a description of the specimens to the relevant product standard; 
c) the method of sampling, and by which organization; 
d) the date of delivery of the specimens; 
e) the date of preparation of the specimen and the date of testing; 
f) the number of specimens in the sample, and whether these are whole units or representative portions 
thereof; 
g) the individual values of moisture content to the nearest 0,5 % and the mean value of moisture content to 
the nearest 1 %; 
h) remarks, if any. DRS 537: 2023 
15                ©RSB 2022- All rights reserved 
 Annex C 
(normative)  
 
Determination of net and gross dry density of masonry units (except for 
natural stone) 
C.1 General 
This Annex gives a method amethodofdeterminingthe 
netandgrossdrydensityofmasonry units(exceptfornaturalstonemasonry units). 
C.2 Principle 
After drying to constant mass and calculating the net and gross volume, the net and gross dry density of the 
masonry units are calculated. 
C.3 Symbols 
Grams 
lu is the unit length, (mm)  
wu is the unit width, (mm)  
hu is the unit height, (mm)  
mo,u is the mass of a whole unit specimen prior to drying, (g)  
mo,p,tot is the combined mass of a group of three representative portions of a whole unit specimen prior to 
drying, (g) 
mdry,p          is the mass of one individual representative   portion of the three portions of a whole unit specimen 
after drying to constant mass, (g)  
mdry,p,tot is the combined mass  of a group of three representative portions of a whole unit specimen after 
drying to constant mass, (g)  
mdry,u is the mass of a whole unit specimen  (or corresponding  whole unit where representative portions 
have been taken) after drying to constant mass, (g)  
wp is the moisture content of a group of three representative portions of a whole unit specimen  
vn,u  is the net volume of unit, (mm3)  
©RSB 2023- All rights reserved                  16 
 vg,u  is the gross volume of unit, (mm3)  
vg,p  is the gross volume of parts of unit, (mm3)  
ρn,u  is the net dry density, (kg/m3)  
ρn,p  is  the net dry density of parts of unit, (kg/m3)  
ρg,u  is the gross dry density, (kg/m3) 
C.4 Apparatus 
4.1  Ventilated oven, capable of maintaining a temperature of 105 °C ± 5 °C for use with clay, calcium 
silicate and autoclaved aerated concrete units. 
4.2        Ventilated oven, capable of maintaining a temperature of 70 °C ± 5 °C for use with aggregate 
concrete and manufactured stone units. 
4.3        Weighing instrument, capable of weighing the whole unit or portions of it to an accuracy of 0.1 % of 
their mass. 
C.5 Sampling 
The method of sampling shall be in accordance with the product standard. The minimum number of 
specimens shall be six, but a larger minimum number may be specified in the product specification, in which 
case that larger number shall be used. 
C.6 Procedure 
C.6.1 Determination of dry mass 
C.6.1.1 Whole unit specimens 
Dry the test specimens to constant mass m dry,u  in a ventilated oven at a temperature  of 70 °C ± 5  °C (6.2) for 
aggregate concrete and manufactured stone masonry  units or 105 °C ± 5 °C (6.1) for clay, calcium  silicate 
and autoclaved aerated concrete masonry units. Constant mass is reached, if during the drying process in two 
subsequent weighings with a 24 h interval, the loss in mass between the two determinations is not more than 
0.2 % of the total mass. Record the mass m dry,u. 
C.6.1.2 Portions of whole unit specimens 
Where representative portions are used, first weigh each whole unit specimen and record the mass 
(mo,u).·Then, obtain three representative solid portions,  i.e. containing no enclosed or exposed formed voids, 
of at least 100 g from each whole unit specimen and weigh each group of three together. Record the mass of 
each group ( mo,p,tot). 
 DRS 537: 2023 
17                ©RSB 2022- All rights reserved 
 Dry each group of three portions to constant mass following the procedure of C.6.1.1 above. Record the mass 
of each group ( mdry,p,tot). 
Calculate the moisture content of each group of three portions using equation (1): 
𝑊௣= ௠೚,೛,೟೚೟ష ೘೏ೝ೤,೛,೟೚೟
௠೏ೝ೤,೛,೟೚೟    (1) 
Calculate the corresponding dry mass of the whole unit (m dry,u) using equation (2): 
𝑚ௗ௥௬,௨= ௠೚,ೠ
௟ା௪೛     (2)  
C.6.2 Netdry density 
C.6.2.1 Wholeunits 
Determinethenetvolumeusingoneofthefollowingmethodsas appropriate;  
a) Forclaymasonryunits,byweighingin waterasindicatedinAnnex E 
b) Forcalciumsilicatemasonryunits,byusingsandfillingasindicatedinAnnex D 
c) For manufactured stone and solid aggregateconcretemasonry units,bysubtracting thevolumeofallrecesses  
orindentations,  determined  usingasuitablemeasuring  device, fromthevolume (lengthxwidthxheight ) 
determined inaccordance with DRS 536.Express thenetvolume Vn,utothenearest 104mm3. 
d)  Foraggregate 
concretemasonryunitswithformedvoids,bythemethodofc)above,but inadditionsubtractingthevolumeofallvoids.E
xpressthenetvolume Vn,utothenearest104mm3. 
Determinethenetdrydensityofthemasonryunit(ρ n,u)usingequation (3); 
𝜌𝑛.𝑢= 𝑚𝑑𝑟𝑦,𝑢
𝑉𝑛,𝑢 𝑥 106 (𝑘𝑔
𝑚3)     (3) 
Calculate the net dry density of each whole unit specimen to the nearest 5 kg/m3 for densities  up  to 1000 
kg/m3    and to the nearest  10 kg/m3    for densities  over  1000 kg/m3. Calculate the mean of the net dry 
densities of the specimens. 
C.6.2.2 Representative portions of whole unit specimens 
Determine the net dry density of the parts as follows: 
a) Determine the dry mass of each portion of the whole unit specimen mdry,p by drying according to 
C.6.1.2; 
b) Determine the volume of the parts Vg,p by measuring length, width  and height  to the nearest 1 mm; 
c) Calculate the net dry density ρn,p to the nearest 5 kg/m3 using equation (4): Comment [SN1]: This reference is not 
available 
Comment [AK2]: make this Annex E.  
©RSB 2023- All rights reserved                  18 
 𝜌௡,௨= ௠೏ೝ೤,೛
௏೒,೛ 𝑥 10଺(௞௚
௠య)     (4) 
Define the net dry density of each whole unit specimen (from which the portions were obtained) as the mean 
value of the net dry density of the three representative portions.  Express the net dry density of the whole unit  
specimens   to the nearest  5 kg/m3  for densities   up to  1000 kg/m3    and to the   nearest 10 kg/m3  for 
densities over  1000 kg/m3.  Calculate the mean of the net dry densities of the whole unit specimens. 
C.6.3 Gross dry density 
Calculate  the gross volume  Vg,u  of the unit from the unit dimensions  length, width and height  in 
accordance with DRS 536 with a subtraction   of the volume  of perforations,   voids, recesses  or indentations   
intended to be filled with mortar determined by measurement by any suitable method with the same accuracy 
of measurement as incorporated  in DRS 536. To find the gross dry density of the unit (ρg,u) [see equation 
(5)], divide the dry mass ( mdry,u) obtained according to C.6.1 by the gross volume ( Vg,u) of the unit: 
𝜌ீ,௨= ௠೏ೝ೤,೛
௏೒,ೠ 𝑥 10଺(௞௚
௠య)     (5) 
Express the gross dry density of each whole unit specimen to the nearest 5 kg/m3 for densities up to 1000 
kg/m3    and to the nearest   10 kg/m3  for densities   over  1000 kg/m3.  Calculate the mean of the gross dry 
densities of the specimens. 
C.7 Test report 
The test report shall contain the following information: 
a) the number, title and date of issue of this Standard; 
c)   a description of the masonry unit to the relevant product standard 
d)   the method of sampling and by which organization; 
e)   date of receipt of the specimens by the test laboratory; 
f)    the date of testing the specimens; 
g)   the number of specimens in sample and whether these are whole units or representative portions thereof; 
h)   the individual and mean values of net dry density; 
i)    the individual and mean values of gross dry density; and  
j)    remarks, if any. DRS 537: 2023 
19                ©RSB 2022- All rights reserved 
 Annex D 
(normative)  
 
Determination of volume and percentage of voids and net volume of clay 
and calcium silicate masonry units by sand filling 
D.1 General 
This Annex gives a method of determining the volume and percentage of voids and net volume of calcium 
silicate masonry units containing cells, frogs and holes. 
D.2 Principle 
The principal of this test is to measure the volume of voids in a masonry unit by measuring the volume of sand 
required to fill them. 
D.3 Symbols 
Vn,u is the net volume of each unit (104 mm3), 
Vs,u is the volume of voids (104 mm3), 
Vg,u is the gross volume of each unit (104 mm3), 
D.4 Materials  
Dry graded sand with a grain size above 0.5 mm and passing a 1 mm test sieve (D.5.4). 
D.5 Apparatus 
5.1 Thin sheet of foam rubber, or other resilient material. 
5.2 Rigid plate, slightly larger than the unit. 
5.3 Glass measuringcylinder, graduated in ml. 
5.4 Two test sieves, one with 0.5 mm apertures and one with 1 mm apertures.  
©RSB 2023- All rights reserved                  20 
 
D.6 Preparation of specimens 
D.6.1 Sampling  
The method of sampling shall be in accordance with the relevant product standard. The minimum number of 
specimens shall be six, but a larger minimum number may be specified in the product specification, in which 
case that larger number shall be used. 
D.6.2 Surface treatment  
Remove any superfluous material adhering to the unit as a result of the manufacturing process before 
commencing the test. 
 
 
D.7 Test procedure 
D.7.1 Volume of voids 
Place the air-dry unit on the thin sheet of foam rubber (D.5.1) or other resilient material with the open end of 
voids uppermost. When the unit has voids or recesses in the ends that need to be measured, close the ends 
with the rigid plate (D.5.2). 
D.7.2 Sand filling   procedure 
Fill the glass measuring cylinder (D.5.3) accurately with the dry sand and record the volume. Fill the voids with 
the sand by pouring from the cylinder, refilling it if required. Keep the cylinder lip as close as possible to the 
top of the void, pour steadily and then strike off level. 
Return to the cylinder any sand struck off and note the total volume of sand ( Vs,u) used to the nearest 104 
mm3. 
D.8 Calculation and expression of results 
Calculate the gross volume of the unit ( Vg,u)   by multiplying the length ( lu) by the height  (hu) and the width 
(Wu) of the unit measured  in accordance  with DRS 536. 
Vg,u = Iu x Wu x Hu   (1) 
Calculate the net volume ( Vn,u)   of each unit to the nearest  104 mm3, if required. 
Vn,u = Vg,u – Vs,u  (2) 
Calculate the net volume ( Vn,u)   of each unit to the nearest 104 mm3, if required. DRS 537: 2023 
21                ©RSB 2022- All rights reserved 
 𝑉௡,௨= ௏ೞ,ೠ
௏ೞ,ೠ 𝑥 100   (3) 
D.9 Evaluation of results 
Calculate the mean value of the water absorption of the specimens to the nearest 0.1 %. 
D.10 Test report 
The test report shall contain the following information: 
a) the number, title and date of issue of this Standard; 
b) a description of the specimens to the relevant product standard; 
c) the method of sampling, and by which organization; 
e) the date of preparation of the specimen and the date of testing; 
g) the number of specimens in sample; 
h) the net volume of each specimen and the mean value to the nearest 104 mm3; 
i) the percentage of voids of each specimen and the mean value, if required, to the nearest 1%; and 
h) remarks, if any. 
 
 
 
 
 
 
 
  
©RSB 2023- All rights reserved                  22 
 Annex E 
(normative)  
 
Determination of net volume and percentage of voids of clay masonry 
units by hydrostatic weighing 
E.1 Principle 
The principle of this test  is to obtain  the net volume  of the masonry unit by weighing  in air and weighing  in 
water  and to subtract this from the gross  volume, obtained by measurement of its dimensions, to obtain 
The volume of voids. 
E.2 Symbols 
Mwu   is the apparent mass of the specimen in water (g). 
Mau    is the mass  of the specimen in air (g). 
lu        is the length of the specimen (mm). 
wu      is the width of the specimen (mm). 
hu       is the height of the specimen (mm). 
Vgu    is the gross volume  of the specimen (mm3). 
 Vvu    is the volume of voids in the specimen (mm3). 
Vnu     is the net volume of the specimen (mm3). 
rw     is the density of water  (g/mm3) (approx. 0,001 g/mm3). 
E.3 Apparatus 
5.1   Tank,  with adequate capacity to submerge a whole  masonry unit. 
5.2   Weighing instrument,  capable of weighing  whole masonry units  with an accuracy of at least 0,1 % of 
their mass  when  dry. 
 
 23        E.4 Sampling  
The method of sampling shall be in accordance with 
shall be six, bu t a larger minimum number may be specified in the product specification, in which case that 
larger number shall be used. 
E.5 Test procedure 
Measure the length  (lu), width  (wu)  and height  (hu)  of the specimen in accordance with 
Immerse the specim en in water  (5.1) for at least 1 hour. When the apparent masses (Mwu), measured by two 
successive weighings  at 30 min intervals, differ by less than 0,2 % remove  the specimen from the water and 
record the result  of the second weighing  as the apparent 
towel  and determine the weight (Mau) immediately.
E.6 Calculation and expression of  results
Calculate the net volume  of the specimen (Vnu)  by subtracting the mass  of the specimen obtained by 
weighing  it un der  water  from that  obtained when weighing  it in air (Mau 2 Mwu) and dividing it by the 
density  of water  (rw). Express the net 
Determine the mean value of net volume determinations to the nearest 104 
Calculate the gross volume of the specimen (Vgu) bymultiplying the length (lu) by the height (hu) andwidth 
(wu) of the specimen measured in accordancewith
E.7 9 Test report  
The test report shall contain the following 
         ©RSB 2022The method of sampling shall be in accordance with product standards . The minimum number of specimens 
t a larger minimum number may be specified in the product specification, in which case that 
length  (lu), width  (wu)  and height  (hu)  of the specimen in accordance with 
en in water  (5.1) for at least 1 hour. When the apparent masses (Mwu), measured by two 
successive weighings  at 30 min intervals, differ by less than 0,2 % remove  the specimen from the water and 
record the result  of the second weighing  as the apparent mass  (Mwu).  Remove surface
towel  and determine the weight (Mau) immediately.  
Calculation and expression of  results  
Calculate the net volume  of the specimen (Vnu)  by subtracting the mass  of the specimen obtained by 
der  water  from that  obtained when weighing  it in air (Mau 2 Mwu) and dividing it by the 
density  of water  (rw). Express the net volume of the specimen to the nearest 104 mm3  as
Determine the mean value of net volume determinations to the nearest 104 mm3.
Calculate the gross volume of the specimen (Vgu) bymultiplying the length (lu) by the height (hu) andwidth 
(wu) of the specimen measured in accordancewith  DRS 533 to the nearest 104 mm
 
The test report shall contain the following information: 
DRS 537: 2023 
2022- All rights reserved 
 . The minimum number of specimens 
t a larger minimum number may be specified in the product specification, in which case that 
length  (lu), width  (wu)  and height  (hu)  of the specimen in accordance with DRS 533 . 
en in water  (5.1) for at least 1 hour. When the apparent masses (Mwu), measured by two 
successive weighings  at 30 min intervals, differ by less than 0,2 % remove  the specimen from the water and 
Remove surface  water  with a damp  
Calculate the net volume  of the specimen (Vnu)  by subtracting the mass  of the specimen obtained by 
der  water  from that  obtained when weighing  it in air (Mau 2 Mwu) and dividing it by the 
the specimen to the nearest 104 mm3  as  
mm3. 
Calculate the gross volume of the specimen (Vgu) bymultiplying the length (lu) by the height (hu) andwidth 
to the nearest 104 mm3.  
©RSB 2023- All rights reserved                  24 
 a) the number, title and date of issue of this Rwanda Standard; 
b) the name of the organization that carried out thesampling and the method used; 
c) the date of testing; 
d) the type, origin and designation of the masonryunit  
e) the number of specimens in sample; 
f) the net volume of each specimen and the meanvalue to the nearest 104 mm3; 
g) the percentage of voids of each specimen and themean value, if required, to the nearest 1 %; 
h) remarks, if any 
 
 
 
 
 DRS 537: 2023 
 
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