ICS 91.080.30  
 
Reference number
DRS 539: 2023
© RSB 2023
  
 RWANDA 
STANDARD  DRS 
539 
 
First edition  
2023-mm-dd  
 
Method of test for masonry — 
Determination of flexural strength 
   
DRS 539: 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 539: 2023 
iii                ©RSB 2023- All rights reserved 
 Contents Page 
Foreword ............................................................................................................................................................. v  
1 Scope ...................................................................................................................................................... 1  
2 Normative references ............................................................................................................................ 1  
3 Principle ................................................................................................................................................. 1  
4 Terms and definitions ........................................................................................................................... 1  
5 Symbols (and abbreviated terms) ........................................................................................................ 1  
5.1 Definitions .............................................................................................................................................. 1  
5.2 Symbols .................................................................................................................................................. 1  
6 Materials ................................................................................................................................................. 2  
6.1 Masonry units ........................................................................................................................................ 2  
6.1.1 Conditioning of the units ...................................................................................................................... 2  
6.1.2 Testing .................................................................................................................................................... 2  
6.2 Mortar...................................................................................................................................................... 3  
7 Apparatus ............................................................................................................................................... 3  
8 Preparation of specimens .................................................................................................................... 3  
8.1 Masonry specimens .............................................................................................................................. 3  
8.2 Construction and curing of the specimens ........................................................................................ 4  
9 Procedure ............................................................................................................................................... 4  
9.1 Placing the specimens in the testing equipment ............................................................................... 4  
9.2 Loading ................................................................................................................................................... 4  
9.3 Measurements and observations ........................................................................................................ 6  
9.4 Replications ........................................................................................................................................... 7  
10 Calculations ........................................................................................................................................... 7  
11 Evaluation of results ............................................................................................................................. 7  
12 Test report .............................................................................................................................................. 8  
Annex A (normative)  Determine the compressive strength of a sample of masonry units .................... 10  
A.1 Procedures ........................................................................................................................................... 10  
A.1.1 Placing specimen in the testing machine ......................................................................................... 10  
A.1.2 Loading ................................................................................................................................................. 10  
A.2 Calculation and expression of results .............................................................................................. 11  
A.3 Evaluation of the results ..................................................................................................................... 11  
Annex B (normative)  Determination of moisture content of calcium silicate and autoclaved 
aerated concrete units ........................................................................................................................ 12  
B.1 Principle ............................................................................................................................................... 12  
B.2 Symbols ................................................................................................................................................ 12  
B.3 Apparatus ............................................................................................................................................. 12  
B.4 Preparation of specimens .................................................................................................................. 12  
B.5 Test procedure ..................................................................................................................................... 12  
B.6 Calculation and expression of results .............................................................................................. 12  
B.7 Evaluation of results ........................................................................................................................... 13  DRS 539: 2023 
©RSB 2023- All rights reserved                  iv 
 Annex C (normative)  Determination of consistence of fresh mortar (by flow table) ............................... 14  
C.1 Principle ............................................................................................................................................... 14  
C.2 Apparatus ............................................................................................................................................ 14  
C.3 Sampling, preparation and storage of test samples ....................................................................... 14  
C.4 Procedure ............................................................................................................................................ 15  
C.5 Calculation and expression of results .............................................................................................. 15  
Annex D (normative)  Determination of air content of fresh mortar ........................................................... 16  
D.1 Principle ............................................................................................................................................... 16  
D.2 Symbols ............................................................................................................................................... 16  
D.3 Apparatus ............................................................................................................................................ 16  
D.3.1 5.1 Apparatus for Method A — Pressure method ............................................................................ 16  
D.3.2 Apparatus for Method B — Alcohol method .................................................................................... 16  
D.3.3 Materials for method B —  Alcohol method ..................................................................................... 17  
D.4 Sampling, preparation and storage of test samples ....................................................................... 17  
D.5 Method A — Pressure method .......................................................................................................... 17  
D.5.1 Applicability ......................................................................................................................................... 17  
D.5.2 Operational principle .......................................................................................................................... 17  
D.5.3 Procedure ............................................................................................................................................ 18  
D.5.4 Calculation and expression of results .............................................................................................. 18  
D.6 Method B — Alcohol method ............................................................................................................. 18  
D.6.1 General ................................................................................................................................................. 18  
D.6.2 Procedure ............................................................................................................................................ 18  
D.6.3 Calculation and expression of results .............................................................................................. 19  
Annex E (normative)  Determination of flexural and compressive strength of hardened mortar ........... 20  
E.1 Principle ............................................................................................................................................... 20  
E.2 Definitions and symbols .................................................................................................................... 20  
E.2.1 Definitions ........................................................................................................................................... 20  
E.2.2 Symbols ............................................................................................................................................... 20  
E.3 Apparatus ............................................................................................................................................ 20  
E.4 Sampling .............................................................................................................................................. 21  
E.5 Preparation and storage of test specimens ..................................................................................... 21  
E.5.1 General ................................................................................................................................................. 21  
E.5.2 Preparation .......................................................................................................................................... 22  
E.5.3 Storage and curing conditions .......................................................................................................... 23  
E.6 Determination of flexural strength .................................................................................................... 23  
E.6.1 Apparatus ............................................................................................................................................ 23  
E.6.2 Procedure ............................................................................................................................................ 24  
E.6.3 Calculation and expression of results .............................................................................................. 25  
E.7 Determination of compressive strength ........................................................................................... 25  
E.7.1 Apparatus ............................................................................................................................................ 25  
E.7.2 Procedure ............................................................................................................................................ 26  
E.7.3 Calculation and expression of results .............................................................................................. 26  
 DRS 539: 2023 
v                ©RSB 2023- All rights reserved 
 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. 
DRS539 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 1052-2:1999 Methods of test for masonry — Part 2: Determination of flexural 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 DRS 539: 2023 
©RSB 2023- All rights reserved                  vi 
 Road Transport Development Agency (RTDA) 
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 539: 2023 
1                ©RSB 2023- All rights reserved 
 Method of test for masonry — Determination of flexural strength 
1 Scope 
This Draft Rwanda Standard specifies a method for determining the flexural strength of small masonry 
specimens for the two principal axes of loading. 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 edition cited applies. For undated references, 
the latest edition of the referenced document (including any amendments) applies. 
There are no normative references.  
3 Principle 
The flexural strength of masonry is derived from the strength of small specimens tested to destruction under 
four-point loading. The maximum load achieved is recorded. The characteristic value, calculated from the 
maximum stresses achieved by the samples is considered to be the flexural strength of the masonry 
4 Terms and definitions 
For the purposes of this standard, the terms and definitions given in DRS 546 apply. 
5 Symbols (and abbreviated terms) 
5.1 Definitions 
4.1.1 Masonry. An assemblage of masonry units laid in a specified pattern and jointed together with mortar. 
4.1.2 Flexural strength of masonry. The strength of masonry in pure bending assuming a linear 
stressdistribution of internal stresses. 
5.2 Symbols 
b is the height or width of a masonry specimen perpendicular to the direction of span, (mm) 
Fi, max is the maximum load applied to an individual masonry specimen, (N)  
©RSB 2023- All rights reserved                  2 
 fxi is the flexural strength of an individual masonry specimen, (N/mm2) fmean is the mean flexural 
strengthof the masonry specimens, (N/mm2) fxk is the characteristic flexural strength of masonry, 
(N/mm2) 
hu is the height of masonry unit, (mm) 
k is the numerical factor 
 
Is is the length of a masonry specimen in the direction of span, (mm) 
Iu is the length of masonry unit, (mm) 
I1 is the spacing of the outer bearings, (mm) 
I2 is the spacing of the inner bearings, (mm) 
n is the number of specimens 
s is the standard deviation of the log values 
tu is the width of masonry unit, (mm) 
 
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 units prior to laying. Measure the moisture content by mass of 
utoclaved aerated concrete and calcium silicate masonry units in accordance with Annex B . 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 method given in Annex A . For 
non-autoclaved concrete units determine the compressive strength at the time of testing the masonry 
specimens. DRS 539: 2023 
3                ©RSB 2023- All rights reserved 
 6.2 Mortar 
Take representative samples of fresh mortar from the mason's board to make mortar prisms, to determine the 
flow value in accordance with Annex C , and to determine the air content in accordance with Annex D . Use 
the prism specimens to determine the mean compressive strength at the time of testing of the masonry 
specimens. 
7 Apparatus 
A testing machine complying with the requirements given in table 1, and accommodating variations of plane. 
The testing machine 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. 
The bearings shall be designed to ensure that contact is provided over the full width of the masonry, for 
example by using a hollow rubber bolster of at least 7 mm wall thickness and a 10 mm bore containing an 8 
mm diameter steel rod. 
Table1:Requirementsfortestingmachines 
Maximum permissible 
repeatabilityofforcesas 
percentage of indicated force Maximum permissible 
meanerrorofforcesas 
percentageofindicated force Maximum permissible error of 
zero force as 
percentageofmaximum force of 
range 
2,0 ±2,0 ±0,4 
 
8 Preparation of specimens 
8.1 Masonry specimens 
For each of the two principal axes of loading use at least five specimens according to figure 1 having the sizes 
given in table 2. The size of the masonry specimens shall be chosen so that the distance between the inner 
and outer bearings shall be not less than the thickness of the masonry specimen. The thickness of the 
specimen shall be equal to tu unless otherwise specified. 
Table2:Specimensizesfortestingtheflexuralstrengthofmasonry 
Direction hu 
(mm) b 
(mm) Additional conditions 
Flexuralstrengthforaplaneof failure 
parallel to the bed joints any ≥400and 
≥1,5Iu minimum2bedjointswithin I2 
Flexural strength for a plane of failure 
perpendicular to the bed joints ≤250 ≥240and 
≥3hu minimum 1 head joint every 
course within I2  
©RSB 2023- All rights reserved                  4 
 >250 ≥1000 minimum1bedjointandminimum 
1headjointeverycoursewithin I2 
 
 
 
8.2 Construction and curing of the specimens 
Build the specimens within 30 min after completion of the conditioning of the units, using mortar mixed not 
more than one hour beforehand unless the mortar is designed to be used over a more prolonged period. 
Construct the specimens to the bond specified. Do not allow the work to be interrupted before completion. 
Immediately after building, pre-compress each specimen using a uniformly distributed mass to give a vertical 
stress between 25 · 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 mortar prevent the test specimens from drying out during 
the curing period by close covering with polyethylene sheet , andmaintain the specimens undisturbed until 
testing unless otherwise specified. Test each specimen at an age of 28 days ± 1 day, unless otherwise 
specified, and determine the compressive strength of the mortar at the same age, following EN 1015-11. For 
lime-based mortars an alternative curing regime and period may be necessary and this should be specified. 
9 Procedure 
9.1 Placing the specimens in the testing equipment 
Test the masonry specimen in the vertical attitude under four-point loading (see figure 1). The distance 
between the outer bearings and the end of the specimen shall be greater than or equal to 50 mm. The 
distance between the inner bearings may be varied to suit the format of the masonry but shall be 04 to 0,6 
times the spacing of the outer bearings. The inner bearings shall be located so that they are, as far as 
practicable, midway between the nearest mortar joints which are parallel to the bearings. 
Ensure that the base of each masonry specimen is free from frictional restraint, for example by setting it on 
two layers of polytetrafluoroethylene with grease between them or on ball, needle or roller bearings. 
9.2 Loading 
Increase the flexural stress at a rate between 003 N/mm2/min and 03 N/mm2/min. DRS 539: 2023 
5                ©RSB 2023- All rights reserved  
 
 
 
©RSB 2023- All rights reserved                  6  
9.3 Measurements and observations 
Record the following: 
- age of non-autoclaved concrete units, 
- the dimensions of the cross section of the specimen parallel to the bearings, to the nearest 1 mm, 
- the spacing of the outer and of the inner bearings, in mm, 
- the maximum load Fi,max to the nearest 10 N. Discard any result where failure does not occur between the 
inner bearings, 
- the length of time from the start of loading until the maximum load is achieved, 
- crack patterns. 
DRS 539: 2023 
7                ©RSB 2023- All rights reserved 9.4 Replications 
If less than five results are obtained in which failure occurs between the inner bearings, further tests shall be 
carried out until five valid results are obtained. 
10 Calculations 
 
11 Evaluation of results 
 
 
©RSB 2023- All rights reserved                  8 
 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) number of specimens tested in each direction; 
d) date of building the specimens; 
e) curing conditions (time, temperature, humidity); 
f) date of testing the specimens; 
g) description of the specimens, including dimensions, number of courses, bonding pattern and spacing of 
the bearings; 
h) description of the masonry units and the mortar (to include also details of the mortar mixing procedure, 
flow value, air content and compressive strength), preferably consisting of the appropriate test reports, or of 
extracts taken from these reports; 
i) age of non-autoclaved concrete units at the time of testing the masonry; 
j) moisture content by mass of autoclaved aerated concrete and calcium silicate units or , for other types of 
unit, the method of conditioning prior to the time of laying; 
k) maximum load reached by the test specimens; 
l) the length of time from the start of loading until the maximum load is achieved; 
m) mean compressive strength of the masonry units in N/mm2 to the nearest 001 N/mm2 
n) and the coefficient of variation; 
o) mean compressive strength of the mortar in N/mm2 to the nearest 001 N/mm2 and the coefficient of 
variation, at 28 days ± 1 day; 
p) individual values for the flexural strengths, of the masonry specimens in N/mm2 to the nearest 001 
N/mm2, with notes on any unusual form of failure ; 
q) mean and characteristic flexural strength of masonry in N/mm2 to the nearest 001 N/mm2; 
r) statistical treatment of the results where relevant; DRS 539: 2023 
9                ©RSB 2023- All rights reserved 
 s) crack patterns; 
t) remarks, if any.  
©RSB 2023- All rights reserved                  10 Annex A 
(normative)  
 
Determine the compressive strength of a sample of masonry units 
A.1 Procedures 
A.1.1 Placing specimen in the testing machine 
 
A.1.2 Loading 
 
Table 3 — Loading rate 
 
DRS 539: 2023 
11                ©RSB 2023- All rights reserved  
A.2 Calculation and expression of results 
 
A.3 Evaluation of the results 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  12 
 Annex B 
(normative)  
 
Determination of moisture content of calcium silicate and autoclaved 
aerated concrete units 
B.1 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.2 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.3 Apparatus 
5.1 Ventilated oven, capable of maintaining a temperature of (105 ± 5) 8C. 
5.2 Weighing instrument, capable of weighing specimens to an accuracy of at least 0,1 % of their mass. 
B.4 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.5 Test procedure 
Before drying, weigh the test specimens (mo,s). Dry the test specimens at a temperature of (105 ± 5) 8C, to 
constant mass. 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 less than 0,2 % of the total mass. 
After drying to constant mass, weigh the specimens (mdry,s). 
B.6 Calculation and expression of results 
Calculate the moisture content (ws) 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 %: DRS 539: 2023 
13                ©RSB 2023- All rights reserved  
B.7 Evaluation of results 
Calculate the mean value of the moisture content of the specimens to the nearest 1 % 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  14 
 Annex C 
(normative)  
 
Determination of consistence of fresh mortar (by flow table) 
C.1 Principle 
The flow value is measured by the mean diameter of a test sample of the fresh mortar which has been  placed 
on a defined flow table disc by means of a defined mould, and given a number of vertical impacts by raising 
the flow table and allowing it to fall freely through a given height. 
C.2 Apparatus 
4.1 A flow table, as described in Annex A, consisting of the following main parts: 
— stand;  
— rigid table plate and disc; 
— horizontal shaft and lifting cam;  
— lifting spindle. 
4.2 A truncated conical mould, made of stainless steel or brass, 60 mm ± 0.5 mm in height and with internal 
diameter of 100 mm ± 0.5 mm at the bottom and 70 mm ± 0.5 mm at the top. The inside surface and the 
edges of the mould are smooth. The planes containing the top and bottom edges are at right angles to its axis. 
The minimum thickness of the mould wall is 2.0 mm. 
4.3 A tamper, consisting of a rigid, non-absorptive rod of circular cross-section, approximately 40 mm in  
diameter and approximately 200 mm long. The tamping face is flat and at right angles to the length of the 
tamper. The mass of the tamper is 0. 250 kg±0.015 kg. 
4.4 Calipers, capable of measuring diameters up to 300 mm with an accuracy of 1 mm.  
4.5 Trowel.  
4.6 Palette knife.  
C.3 Sampling, preparation and storage of test samples 
The fresh mortar for this test shall have a minimum volume of 1.5 l and shall be obtained by reduction of the 
bulk test sample using a sample divider or by quartering. Ready to use mortars (factory-made wet mortars 
which are retarded), and pre-batched air-lime/sand wet mortars when not gauged with hydraulic binders, shall 
be tested within their specified workable life. Mortars that are made from dry constituents and water shall be 
mixedunless otherwise specified. DRS 539: 2023 
15                ©RSB 2023- All rights reserved 
 The length of the mixing period shall be measured from the moment all the constituents are introduced into 
the mixer. 
Before testing, the batch shall be gently stirred by hand using a trowel or palette knife in 5 – 10 seconds to 
counteract any false setting etc., but without any additional mixing of the batch. 
Any deviation from the mixing procedure shall be noted.  
Two test samples shall be tested. 
C.4 Procedure 
Before each test, wipe the disc and the inner surface and edges of the mould clean with a damp cloth  and let 
it dry. If the table has not been used within the last 24 h, operate it for ten revolutions before use. 
Place the mould centrally on the disc of the flow table and introduce the mortar in two layers, each  layer being 
compacted by at least 10 short strokes of the tamper to ensure uniform filling of the mould. 
During filling, hold the mould firmly on the disc, using one hand. 
Skim off the excess mortar with a palette knife and wipe the free area of disc clean and dry, being especially 
careful to remove any water from around the bottom edge of the mould. After approximately 15 s, slowly raise 
the mould vertically and spread out the mortar on the disc by jolting the flow table 15 times at a  constant 
frequency of approximately one per second. 
Measure the diameter of the mortar in two directions at right angles to one another using calipers.  State the 
results in mm to the nearest mm. 
C.5 Calculation and expression of results 
Calculate the mean value of the two measurements. This mean value is the flow value for the test sample. If 
the individual flow values from the two test samples deviate from their mean value by less than 10 %, use this 
mean value as the flow value of the mortar. If the two individual flow values deviate from their mean value by 
more than 10 %, repeat the test using further mortar from the reduced bulk test sampleand if the results 
deviate from the mean value by less than 10 % use the mean value from the repeat test as the flow value of 
the mortar. If the results differ by more than 10 % consider the  measurements unsatisfactory and take fresh 
test samples from the bulk test sample or laboratory prepared mortar and repeat the test. 
   
©RSB 2023- All rights reserved                  16 
 Annex D 
(normative)  
 
Determination of air content of fresh mortar 
D.1 Principle 
A volume of mortar is placed into a specified measuring vessel. Water is introduced on top of the mortar 
surface, and by means of applied air-pressure or the use of an alcohol-water mix water is forced into the 
mortar displacing air from within any pores. The water level falls and reflects the volume of air displaced from 
the mortar. 
D.2 Symbols 
Lair content of mortar (%); 
Vm,iinitial volume of mortar (ml); 
Vm,ffinal volume of mortar plus alcohol (ml). 
D.3 Apparatus 
D.3.1 5.1 Apparatus for Method A — Pressure method 
D.1.1 Sample container and cover assembly, comprising a metal bowl (sample container) with a capacity of 
approximately 1 litre, fitted with a metal cover assembly to which is attached a sealed air chamber (pressure 
chamber). A pressure gauge for measurement of the applied air pressure is connected to it. 
D.1.2 Tamper, consisting of a rigid, non-absorptive rod of circular cross-section, approximately 40 mm in 
diameter and approximately 200 mm long. The tamping face is flat and at right angles to the length of the 
tamper. The mass of the tamper is 0.250 kg ± 0.015 kg. 
D.1.3 Trowel. 
D.1.4 Palette knife. 
D.3.2 Apparatus for Method B — Alcohol method 
D.2.1 Graduated measuring cylinder, with a capacityof 500 ml and approximately 50 mm in diameter. 
D.2.2 Rubber bung, fitting the measuring cylinder. 
D.2.3 Funnel, fitting the measuring cylinder. DRS 539: 2023 
17                ©RSB 2023- All rights reserved 
 D.2.4 Trowel. 
D.3.3 Materials for method B —  Alcohol method 
Mixture of ethyl alcohol, 60 % by volume, and water,40 % by volume. 
D.4 Sampling, preparation and storage oftest samples 
The fresh mortar for this test shall have a minimumvolume of 1.5 1 or at least 1.5 times the quantity neededto 
perform the test, whichever is the greater, and shalleither be obtained by reduction of the bulk test 
sampleusing a sample divider or by quarteringor by preparation from dry constituents and water inthe 
laboratory. The flow value of the mortar in the bulktest sample shall be determined and reported. 
Laboratory mixed samples shall be brought to a definedflow value before testing. 
Ready to use mortars (factory-made wet mortars whichare retarded), and pre-batched air-lime/sand 
wetmortars when not gauged with hydraulic binders, shallbe tested within their specified workable life. 
The length of mixing period shall be measured fromthe moment all the constituents are introduced into 
themixer. 
Before testing, the batch shall be gently stirred by handusing a trowel or palette knife in 5 to 10 s to counteract 
any false setting etc., butwithout any additional mixing of the batch. 
Any deviation from the mixing procedure shall be noted. 
Two test samples shall be tested. 
D.5 Method A — Pressure method 
D.5.1 Applicability 
This method shall only be used for mortars withdeclared air contents of less than 20 %. 
D.5.2 Operational principle 
A specified pressure is applied in the closed apparatus bymeans of a hand pump or compressed air line 
through avalve connection at the top of the apparatus. The coverassembly is fitted with air valves, air 
bleedervalves, and valves for bleeding off water or throughwhich water may be introduced as necessary for 
theparticular meter design. The cover has machined smoothinterior surfaces contoured to provide an air 
space abovethe top level of the sample container. 
1) This information is contained in the certificate of sampling. 
The operational principle of this meter consists ofequalizing a known volume of air at a known pressurein a 
sealed air chamber with the unknown volume ofair in the mortar sample container, the two chambersbeing  
©RSB 2023- All rights reserved                  18 
 connected through an air valve. The lowered airpressure in the air chamber reflects the air content ofthe 
mortar sample and is read from the dial on thepressure gauge, which is calibrated in terms ofpercentage of air 
for the observed pressure at whichequalization takes place. 
D.5.3 Procedure 
Fill the bowl completely with mortar in fourapproximately equal layers, each layer beingcompacted by 10 short 
strokes with the tamper (5.1.2),evenly distributed to give a flat mortar surface. 
Using the palette knife skim off the excess mortar,leaving the mortar surface plane and level with the topedge 
of the bowl. 
Wipe the outside of the bowl clean and dry and clampthe cover securely in place on the bowl. 
Close the main air valve between the air chamber andthe sample container (see annex A). Fill the air 
spaceunder the cover and above the mortar with waterthrough valve A, keeping valve B open until all the 
airabove the mortar surface is expelled. 
Pump air into the sealed air chamber until a stabilized pressure is reached equal to that determined in the 
calibration test (see annex A). Close both valves A and B and open the valve between the air chamber and 
the sample container. When equilibrium is reached read the air content from the calibrated pressure gauge or 
from a calibration curve. Record this value to the nearest 0.1 %. 
D.5.4 Calculation and expression of results 
Calculate the air content as the mean value from the two individual values of each mortar test sample rounded 
to the nearest 0.5 %. If the two individual values deviate more than 10 % from their mean value, repeat the 
test with two additional test samples taken from the reduced bulk test sample, in which case use the individual 
and mean values of the two additional samples. 
D.6 Method B — Alcohol method 
D.6.1 General 
This method shall only be used for mortars with declared air contents of 20 % or more. 
D.6.2 Procedure 
Fill the measuring cylinder (5.2.1) with approximately 200 ml of mortar using the funnel (5.2.3), taking care to 
avoid creating voids in the mortar. Tap the measuring cylinder to level the mortar surface and record the 
mortar volume, V m,i, to the nearest ml. Pour the mixture of alcohol and water carefully into the measuring 
cylinder up to the 500 ml mark. 
Seal the measuring cylinder with the rubber bung and invert 20 times to obtain a complete dispersion of the 
mortar in the alcohol-water mixture. Let the mixture settle for 5 min and read the resulting surface level, V m,f, 
to the nearest ml. Repeat this process until two consecutive readings do not differ by more than 1 ml. DRS 539: 2023 
19                ©RSB 2023- All rights reserved D.6.3 Calculation and expression of results 
Calculate the air content, L, of each mortar sample from the following formula to the nearest 0.1 %. 
 
Calculate the mean value of the two measurements tothe nearest 0.5 %. If the two individual air contentvalues 
deviate from their mean value by less than 10 %,use the mean value as the air content of the mortar. Ifthe two 
individual values deviate from their meanvalue by more than 10 %, repeat the test using furthermortar from 
the reduced bulk test sample(see clause 6) and if the results deviate from theirmean value by less than 10 % 
use the mean value fromthe repeat test as the air content of the mortar. If theresults differ by more than 10 % 
consider themeasurements unsatisfactory and take fresh testsamples from the bulk test sample or 
laboratoryprepared mortar and repeat the test. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  20 
 Annex E 
(normative)  
 
Determination of flexural and compressive strength of hardened mortar 
E.1 Principle 
The flexural strength of mortar is determined by three point loading of hardened moulded mortar prism 
specimens to failure. The compressive strength of the mortar is determined on the two parts resulting from the 
flexural strength test. Where the flexural strength is not required, the parts for compressive strength testing 
can be produced from the prisms in any way which does not lead to these parts being damaged. 
E.2 Definitions and symbols 
E.2.1 Definitions 
air-lime1) 
limes mainly consisting of calcium oxide or hydroxide which slowly harden in air by reacting with atmospheric 
carbon dioxide. Generally, they do not harden under water as they have no hydraulic properties 
E.2.2 Symbols 
F  is the maximum load applied to the specimen, in Newtons (N). 
l   is the distance between the axes of the support rollers, in millimetres (mm). 
b is the width of specimen in millimetres (mm). 
d is the depth of the specimen in millimetres (mm). 
E.3 Apparatus 
E.3.1 Metal moulds, consisting of an open frame of removable walls forming three compartments 
whenassembled (see Figure 1 for typical design and annex A for a detailed description). 
E.3.2 A tamper, consisting of a rigid, non-absorptive rod of square cross-section, each side of which is 12 mm 
± 1 mm. The tamping face is flat and at right angles to the length of the tamper. The mass of the tamper is 50 
g ± 1 g. 
E.3.3 Storage chambers, capable of maintaining a temperature of 20 8C ± 2 8C and a relative humidity of 95 
% ± 5 % or 65 % ± 5 %. 
E3.4 A clamp, enabling the assembled mould frame to be kept together at right angles. DRS 539: 2023 
21                ©RSB 2023- All rights reserved 
 E.3.5 White cotton gauze, four sheets each with a size of approximately 150 mm 3 175 mm. 
E.3.6 Absorbent filter paper, with a specific mass of 200 g/m2 ± 20 g/m2 and water absorption capacity of 160 
g/m2 ± 20 g/m2; twelve sheets each with a size of approximately 150 mm ± 175 mm. 
E.3.7 Polyethylene bags, capable of containing the steel moulds. 
E.3.8 Two glass plates, of sufficient area to cover the steel mould. 
E.3.9 A palette knife. 
E.3.10 A grid, with webs of triangular section providing point contact support for storing and curing the 
specimens. 
E.3.11 A trowel. 
E.4 Sampling 
The fresh mortar for this test shall have a minimum volume of 1.5 1 or at least 1.5 times the quantity needed 
to perform the test, whichever is the greater, and shall be obtained either by reduction of the bulk test sample 
using a sample divider or by quartering, or by preparation from dry constituents and water in the laboratory. 
The flow value of the mortar in the bulk test sample shall be determined in accordance with Annex C and 
reported. 
Laboratory mixed samples shall before testing be brought to a defined flow value.Ready to use mortars 
(factory-made wet mortars whichare retarded), and pre-batched air-lime/sand wetmortars when not gauged 
with hydraulic binders, shallbe tested within their specified workable life. 
The length of mixing period shall be measured fromthe moment all constituents are introduced into themixer. 
Before testing, the batch shall be gently stirred by handusing a trowel or palette knife in 5 s to 10 s 
tocounteract any false setting etc., but without anyadditional mixing of the batch. 
Any deviation from the mixing procedure shall benoted. 
E.5 Preparation and storage of testspecimens 
E.5.1 General 
The test specimens shall be prisms160 mm 3 40 mm 3 40 mm. Three specimens shall beprovided. For the 
compressive strength test, break theprisms into two halves to provide six half prisms.  
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 E.5.2 Preparation 
E.5.2.1 General 
Prepare mortars based on hydraulic binders (retardedor not retarded), and air-lime/cement mortars withmass 
of air-lime not exceeding 50 % of the total bindermass, in accordance with E 5.2.2. 
Prepare mortars based on air-lime, and air-lime/cementmortars with cement mass not exceeding 50 % of 
thetotal binder mass, in accordance with E.5.2.3. 
Preparation and storage conditions are given inTable 4. 
Prepare three specimens for testing at an age of28 days, or more if retarding agents are incorporated inthe 
mortar, unless otherwise specified. 
Clean the moulds and lubricate the internal faces ofthe assembled moulds with a thin layer of mineral oilto 
prevent adhesion of the mortar. 
E.5.2.2 Mortars with hydraulic binders, andair-lime/cement mortars with mass of air-lime 
notexceeding 50 % of the total binder mass 
Fill the mould with mortar in two approximately equallayers, each layer being compacted by 25 strokes of 
thetamper. 
Skim off the excess mortar with a palette knife, leavingthe mortar surface plane and level with the top of 
themould. Then store the mould as described in E 5.3. 
E.5.2.3 Mortars based on air-lime, andair-lime/cement mortars with cement mass notexceeding 50 % 
of the total binder mass 
Place the assembled mould frame, clamped together atright angles, on a glass plate on which two layers ofdry 
white cotton gauze have been placed. Fill themould with mortar in two approximately equal layers,each layer 
being compacted by 25 strokes of thetamper. 
Skim off the excess mortar with a palette knife leavingthe mortar surface plane and level with the top of 
themould. 
Place two layers of white cotton gauze tightly on themortar surface. Place six layers of absorbent filterpaper 
on top of the gauze. 
Cover the absorbent filter paper with a glass plate andturn the mould upside down keeping the glass plates 
atthe bottom and top firmly attached to the mould. 
Carefully remove the glass plate from the top of theinverted mould, place six layers of absorbent filterpaper on 
the exposed gauze and re-cover with theglass plate on top. 
Re-invert the mould back to its upright position andplace it on a fixed table and load with mass 
ofapproximately 5 kg. DRS 539: 2023 
23                ©RSB 2023- All rights reserved After 3 h remove the load and the glass plate. Discardthe absorbent filter paper and the gauze on top of 
themould, and re-cover with the glass plate on top. Invertthe mould, keeping the glass plates at the bottom 
andthe top firmly attached to the mould. Remove the glassplate from the top of the inverted mould and 
discardthe absorbent filter paper and the gauze. Then storethe mould as described in E.5.3. 
E.5.3 Storage and curing conditions 
Place the mould in a humidity chamber or in sealedpolyethylene bags. Then after the period given in 
Table 1 remove the specimens from the mould andsubsequently store them on the grid with triangularsection 
webs under the conditions also described inTable 4. 
Table 4 — Preparation and conditions of storing specimens 
 
E.6 Determination of flexural strength 
E.6.1 Apparatus 
A testing machine, capable of applying the load at a rate specified in E 6.2. The machine shall comply with the 
requirements in Table 2. The machine shall have two steel supporting rollers of length between 45 mm and 50 
mm and 10 mm ± 0.5 mm diameter, spaced 100.0 mm ± 0.5 mm apart, and a third steel roller of the same 
length and diameter located centrally between the support rollers (Figure 2). The three vertical planes through 
the axes of the three rollers shall be parallel and remain parallel, equidistant normal to the direction of the 
prism under test. One of the supporting rollers and the loading roller shall be capable of tilting slightly to allow 
a uniform distribution of the load over the width of the prism without subjecting it to any torsional stresses. 
 
 
 
 
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Figure 2 — Flexural strength test 
 
Table 5 — Requirements for testing machine 
 
E.6.2 Procedure 
E.6.2.1 Preparation  
Test the specimen at 28 days after casting, or more if retarding agents are incorporated in the mortar, unless 
otherwise specified, and immediately after removing from the storage atmosphere. Wipe the bearing surfaces 
DRS 539: 2023 
25                ©RSB 2023- All rights reserved of the roller and the sides of specimen with a clean cloth to remove any loose grit or other material. Place the 
specimen with one of its faces (which has been cast against the steel of the mould) on the supporting rollers. 
E.6.2.2 Loading  
Apply the load without shock at a uniform rate in the range 10 N/s to 50 N/s so that failure occurs within a 
period of 30 s to 90 s. NOTE A loading rate at the lower end of the permitted range may need to be used for 
the lower strength mortars. Record the maximum load applied, in N. Return the broken specimen to the 
storage chamber and keep it there if required, for compressive strength measurements.  
E.6.3 Calculation and expression of results 
Calculate the flexural strength, f, in N/mm using the following equation: 
 
b and d (see 4.2) may be taken as the internal moulddimensions. 
Record the flexural strength of each specimen to thenearest 0.05 N/mm2. Calculate the mean to the 
nearest0.1 N/mm2. 
Record age of test specimen and age at demoulding. 
E.7 Determination of compressive strength 
E.7.1 Apparatus 
a) A testing machine , capable of applying the load at a rate specified in E 7.2.2. The machine shall comply 
with the requirements in Table 2. The upper machine platen shall be able to align freely as contact is made 
with the specimen, but the platens shall be restrained from tilting with respect to one another during loading. 
b) Two bearing plates, made of tungsten carbide or of steel of surface hardness at least 600 HV Vickers 
hardness value in accordance with ISO 6507-1. 
The plates shall be 40.0 mm long340,0 mm ± 0.1 mm wide and 10 mm thick. The dimensional tolerance for 
the width shall be based on the average of four symmetrically placed measurements. The flatness tolerance 
for the contact faces shall be 0.01 mm. 
c) Compression jig, used to facilitate the location of the bearing plates. The base plate of the jig shall be of 
hardened and tempered tool steel and the faces shall have a flatness tolerance of 0.01 mm. A device to 
provide positive centring on the lower platen of the testing machine shall be provided. Hardened and 
tempered silver steel pillars shall be symmetrically placed about the centring device so that the gap in one 
direction is the nominal width of the prism plus 0.3 mm and in the other direction is the nominal width of the 
prism plus 0.8 mm. The top face of the base plate shall be marked with an arrow in the direction of the greater 
distance between the pillars to indicate the direction of the long axis of the bearing plates. 
 
©RSB 2023- All rights reserved                  26 
 E.7.2 Procedure 
E.7.2.1 Preparation 
Test the specimen at 28 days after casting, or more if retarding agents are incorporated in the mortar, unless 
otherwise specified, and immediately on removing from the storage atmosphere or after the flexural strength 
test. Remove any loose grit or other material from the sides of the specimen as cast. Wipe the bearing surface 
of the testing machine, and the bearing plates and jig, with a clean cloth and place the specimen in the 
machine in such a manner that the load is applied to one of its faces (which has been cast against the steel of 
the mould) Arrange the prism so that the cast end is 16 mm ± 0.1 mm from the nearer edge of the platens or 
bearing plates. Discard any specimens that do not provide a cube of solid material between the top and 
bottom platens or bearing plates. Carefully align the specimen so that the load is being applied to the whole 
width of the faces in contact with the platens. When using the bearing plates and jig, place one bearing plate 
on the upper surface of the jig with its long axis parallel to the indicating arrow, ensuring that it makes close 
contact over the whole surface. Place the specimen in the jig, between the pillars, with its long axis 
perpendicular to the arrow and place the other bearing plate on top of the specimen parallel to the lower 
bearing plate. Carefully centre the compression jig assembly on the lower platen of the test machine. 
E.7.2.2 Loading 
E.7.3 Calculation and expression of results 
Calculate the strength as the maximum load carried by the specimen divided by its cross-sectional area. 
Record the strength of each specimen to the nearest 0.05 N/mm2. Calculate the mean to the nearest 0.1 
N/mm2. 
Record the age of specimens and the age at demoulding.  
 DRS 539: 2023 
27                ©RSB 2023- All rights reserved 
  DRS 539: 2023 
 
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