KENYA STANDARD           DKS 2802-1:2019 
ICS  
  
 
 
  
 
  
 
 
 
  
 
  
Masonry units — Methods of test  
 
Part 1:  
Determination of compressive strength 
 
Public Review Draft April 2019 
 
 
 
 
 
  
 
   
 
  
 
   
 
  
 
  
 
© KEBS 2019 First Edition, 2019 
 DKS 2802-1:2019  
ii  © KEBS 2019 — All rights reserved  
  KENYA STANDARD                                       DKS 2802-1:2019 
ICS  
© KEBS 2019 — All ri ghts reserved   iii TECHNICAL COMMITTEE REPRESENTATION 
 
The following organizations were represented on the Technical C ommittee:  
 
National Housing Corporation 
Kenya Clay Products Ltd. 
Coast Clay Works Ltd. 
Consumer Information Network 
University of Nairobi  
Kenya Industrial Research & Development Institute 
Architectural Association of Kenya 
M&O Consulting Engineers 
Kenya Association of Manufacturers, 
Lake Basin Development Authority. Kenya National Federation of Jua Kali Association 
Kenya Bureau of Standards — Secretariat 
 
 
 
 
REVISION OF KENYA STANDARDS 
 
In order to keep abreast of progress in industry, Kenya Standar ds shall be regularly reviewed.  Suggestions 
for improvements to published standards, addressed to the Manag ing Director, Kenya Bureau of Standards, 
are welcome. 
  
 
 
© Kenya Bureau of Standards, 2019 
 
Copyright. Users are reminded that by virtue of Section 25 of the Copyright Act, Cap. 130 of 2001  of the Laws of Kenya, copyri ght subsists 
in all Kenya Standards and except as provided under Section 25 of this Act, no Kenya Standard produced by Kenya Bureau of Stand ards 
may be reproduced, stored in a retrieval system in any form or transmitted by any means without prior permission in writing fro m the 
Managing Director.  
  DKS 2802-1:2019  
iv  © KEBS 2019 — All rights reserved  
 
 
  
 
  
 
 
  
 
Masonry units — Methods of test  
 
Part 1: 
 
Determination of com pressive strength 
 
 
 
  
 
 
  
 
  
 
  
 
  
 
  
 
  
 
 
KENYA BUREAU OF S TANDARDS (KEBS) 
 
Head Office: P.O. Box 54974, Nairobi-00200, Tel.: (+254  020) 605490, 602350 , Fax: (+254 020)  604031 
E-Mail: info@kebs.org, Web:http://www.kebs.org  
 
Coast Region    Lake Region     Rift Valley Region  
P.O. Box 99376, Mombasa-80100  P.O. Box 2949, Kisumu-40100       P.O. Box 2138, Nakuru-20100 
Tel.: (+254  041) 229563, 230939/40 Tel.: (+254 057) 23549, 223 96       Tel.: (+254 051) 210553, 210555  
Fax: (+254  041) 229448   Fax: (+254 057) 21814 
 
Foreword 
 KENYA STANDARD                                       DKS 2802-1:2019 
ICS  
© KEBS 2019 — All rights reserved   v This Kenya Standard was prepared by the Clay and Clay Products Technical Committee under the guidance 
of the Standards Projects Committ ee and in accordance with the procedures of the Kenya Bureau of 
Standards. 
 
During the development of this standard, reference was made to the following document: 
 
BS EN 772-1:2011+A1:2015 Methods of test for masonry units - Pa rt 1: Determination of 
Determination of compressive strength. 
 Acknowledgement is hereby made for the assistance received from  this source. 
  
KENYA STANDARD      DKS 2802-1:2019 
 
 
© KEBS 2019 — All rights reserved   1  
Masonry units — Me thods of test  
 
Part 1:  
Determination of compressive strength
 
 
  
1 Scope  
 
This Kenya Standard specifies a m ethod for determining the comp ressive strength of masonry units. 
 
2 Normative references 
 The following referenced documents are indispensable for the ap plication of this document.  For dated 
references, only the edition cited applies.  For undated refere nces, the latest edition of the referenced 
document (including any amendments) applies. 
 
EN 771-1, Specification for masonry units — Part 1:  Clay mason ry units 
 
EN 771-2, Specification for masonry units — Part 2: Calcium sil icate masonry units 
 EN 771-3, Specification for  masonry units — Part  3:  Aggregat e  concrete  masonry  units  (Dense  and 
lightweight aggregates) 
 
EN 771-4, Specification for masonry units — Part 4: Autoclaved aerated concrete masonry units 
 
EN 771-5, Specification for masonry units — Part 5: Manufacture d stone masonry units 
 EN 771-6, Specification for masonry units — Part 6: Natural sto ne masonry units 
 
EN 772-2, Methods of test for masonry units — Part 2: Determina tion of percentage area of voids in aggregate 
concrete masonry units (by paper indentation) 
 
EN 772-10, Methods of test for masonry units — Part 10: Determi nation of moisture content of calcium silicate 
and autoclaved aerated concrete units 
 
DKS 2802-7, Methods of test for masonry units — Part 13: Determ ination of net and gross dry density of 
masonry units (except for natural stone) 
 
DKS 2802-8, Methods of test for masonry units — Part 16: Determ ination of dimensions 
 
EN 1015-11, Methods of test for mortar for masonry — Part 11: D etermination of flexural and compressive 
strength of hardened mortar 
 EN ISO 6507-1, Metallic material s—  Vickers hardness test — Par t 1: Test method  
 
3 Principle 
 The specimens, after preparation, when needed, are laid and cen tred on the platen of a compression testing 
machine. A uniformly distributed load is applied and increased continuously up to failure. 
 
  
2  © KEBS 2019 — All rights reserved  
4 Symbols 
 
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 Materials 
 
Sand with a maximum grain size of 1 mm.  
 
Cement  
 
6 Apparatus 
 
6.1 Testing machine  that conforms to the requirements of Table 1. 
 
Table 1 — Requirements for testing machines 
 
Maximum permissible 
repeatability of forces as 
percentage of Indicated force 
% Maximum permissible mean error 
of forces as percentage of 
Indicated force 
% Maximum permissible error of 
zero force as  percentage of 
maximum force of range 
% 
   
2,0 ±2,0 ±0,4 
 
The testing machine shall have adequate capacity to crush all t he 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 lo ad to be applied at the rate given in 8.2. The 
testing machine shall be equipped with two steel-bearing platen s. The stiffness of the platens and the manner 
of load transfer shall be such that the deflection of the plate n surfaces at failure load shall be less than 0,1 mm 
measured over 250 mm. The platens shall either be through harde ned or the faces case hardened. The testing 
faces  shall  have  a Vickers  hardness of  at least  600  HV w hen  tested  in   accordance  with EN ISO 6507-
1. 
 
One platen of the machine shall be able to align freely with th e specimens as contact is made, but shall be 
restrained by friction or other means from tilting during loadi ng. The other platen shall be a plane non-tilting 
block. The bearing faces of both platens shall be larger than t he size of the largest specimen to be tested. 
Where auxiliary platens are used, they shall be properly locate d and of the same hardness, stiffness and 
planeness as the main platens.  The bearing surfaces of the pla tens shall not depart from a plane by more 
than 0,05 mm.  6.2 Weighing instrument capable of weighing specimens to an accuracy of 0,1   % of thei r mass.  
 
6.3 Sufficient stiff steel strips for  use on shell bedded or strip bedded ground units (See 8.1).  
 
7 Preparation of specimens 
 
7.1 Sampling 
 
The method of sampling shall be in accordance with the relevant  part of DKS 2801. The minimum number of 
specimens shall be six, but a larger minimum number may be spec ified in the product specification, in which 
case that larger number shall be used.   In the case of large m asonry units representative  portions, e.g. cubes, 
may be cut from the masonry unit in different positions as stat ed in the relevant part of DKS 2801 (see also 
Note to 7.2.4). 
  
 DKS 2802-1:2019  
© KEBS 2019 — All rights reserved   3  
7.2 Surface preparation 
 
7.2.1 General  
 
Specimens shall be tested in the orientation specified, and thi s 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 7.2.4 significantly alters the cont act surface or reduces the height significantly 
alternative approaches are required,  see 7.2.4.  Where it is n ot practicable to prepare clay HD units by grinding 
e.g.  high strength units,  the surface preparation may be carr ied out by capping in accordance with 7.2.5 
 After the removal of any superfluous material, e.g. flashing fr om the production process, the faces of the 
specimen, whether a whole masonry unit or a  piece cut from a l arger unit (see 7.1 ), 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 s urface, and not greater than 1   mm apart for 
every  100 mm. If the test faces of the masonry unit as manufac tured, or the piece cut from a larger unit, fail 
to achieve this specification, then prepare the surfaces either  by grinding  (see 7.2.4)  or by capping (see 
7.2.5). 
 
Masonry units containing frogs, and not required to be capped, shall additionally be treated as given in 7.2.3. 
 
If the specimens contain grooves or tongues first prepare them as given in 7.2.2. Test specimens shall be 
prepared in accordance with Annex B. 
 
State the method of surface pre paration in the test report. 
 
7.2.2  Removal of tongues and grooves 
 
Remove any tongues and/or grooves on the test faces of units pr ior to testing.  If pieces are to be cut from 
larger units, arrange the cutting so that any tongues and/or gr ooves are eliminated. 
 
7.2.3  Preparation of masonry uni ts containing frogs and which are not to be capped 
 
For masonry units with frogs that are assessed to have a net lo aded 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 7.2.5) and the curi ng shall be carried out in  acc ordance with the requirements of 7.2.5.4. 
 
7.2.4  Grinding 
 
Grind the surfaces of the specimen until the requirement for pl aneness and parallelism given in 7.2.1 is 
achieved.  However, if the masonry units have frogs, indented l ettering, cavities, perforations, internal or 
external holes, leave these in place. If the grinding process w ould significantly alter the contact area of the 
tested faces then the capping procedure of 7.2.5 shall be follo wed. If the remaining height of the specimens 
after they are ground is less than 40 mm or the height /width r atio 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 groun d 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 te st results. 
 
7.2.5 Capping 
 
7.2.5.1 Capping of masonry units without voids or with voids un filled 
 
Use a cement / sand capping mortar expected to attain a minimum  compressive strength, when tested in 
accordance with EN 1015-11, at the time of testing the specimen s, at least that of the expected masonry unit 
strength or 30 N/mm2, whichever is the lesser. 
  
4  © KEBS 2019 — All rights reserved If necessary, e.g. for units with high water absorption charact eristics, first moisten the surfaces to be capped. 
Bed each specimen on a smooth rigid plate of ground glass or st ainless steel which does not depart from a 
true plane surface by more than 0, 1   mm in any 100 mm.  A sui table method is as follows: 
 Support the plate firmly with the machined face uppermost and l evel it in two directions at right angles using a 
spirit level. Coat the plate with a film of mould release oil o r a sheet of thin paper or plastic film to prevent 
mortar adhering to the plate. Place a uniform layer of mortar a bout 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 i s perpendicular to the plane of the plate. 
 
 Check this condition by using a square or spirit level held ag ainst each of the four vertical faces of the 
specimen in turn. Ensure that the thickness of the mortar bed i s at least 3 mm over the whole area and that 
any cavity in the bed face normally filled when the masonry uni ts 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 compacti on, 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 prop ortions. 
  After removing the specimen from the plate, check that the mor tar bed is free from defects as before. Small 
holes may be made in the capping to drain water trapped in cavi ties, if necessary. 
 
7.2.5.2 Capping of masonry units to be face-shell bedded 
 
When masonry units are to be face-shell bedded in use and speci mens are not to be prepared by grinding, 
bed each specimen in mortar as spe cified in 7.2.5.1 using the f ollowing procedure. 
 
Support and coat the plate as described in 7.2.5.1. Lay two par allel strips of mortar about 5 mm thick such that 
each strip is about 25 mm longer than the length of the unit an d about 10 mm wider than the face shell. 
 
Press one bed face of the unit into the mortar such that the th ickness of the mortar under the face-shells is at 
least 3 mm. Check that the vertical axis of the specimen is per pendicular to the plate  using a square or vertical 
level check   each vertical face. 
 
Trim off any surplus mortar. Store the specimen, examine and be d the second face in accordance with 7.2.5.1 
and 7.2.5.2 respectively. 
 
7.2.5.3 Capping of masonry units to be strip-bedded 
 
When masonry units are to be strip-bedded, the procedure specif ied in 7.2.5.2 shall be followed except that 
the capping mortar shall be applied to all of the surfaces to b e bedded in use. 
 
7.2.5.4 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 perio d sufficient to ensure the mortar achieves the 
minimum strength specified in 7.2.5.1.  
 
7.3 Conditioning of specimens before testing 
 
7.3.1 General 
 Specimens shall be conditioned using a prescribed   regime of m oisture 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  condit ioning   by immersion, free  air  circulation around  
each specimen   shall  be ensured  during  conditioning. 
  7.3.2 Conditioning to the air dry condition DKS 2802-1:2019  
© KEBS 2019 — All rights reserved   5  
The condition air dry will be achieved in accordance with one o f the following procedures: 
 
7.3.2.1 Store the specimens  for at least  14 d in the laborato ry at: 
  
a) Temperature               ≥ 15ºC 
 
b) Relative humidity         ≥ 65% 
  
The specimens may be tested before 14 d if constant mass has be en reached. 
 Constant mass shall be considered to  have  been  reached if, d uring  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.  
7.3.2.2 Dry the specimens   at 105 °C ± 5 °C for at least 24 h and cool at room temperature   for at least 4 h. 
  7.3.3  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.  Constan t   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    i s less than  0,2 % of the total  mass.  Allow 
the specimens   to cool to ambie nt temperature   before testing . 
 
b) Dry the specimens   at 70 °C ± 5 °C to constant mass.   Constan t   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. 
  7.3.4  Conditioning to 6 % moisture content  
 
Condition the specimens   to a m oisture content of 6 % ± 2 % by  mass as follows: 
 
Calculate  the dry mass  of the  unit  from  the volume,  which   shall  be calculated   from  the dimensions 
determined as in DKS 2802-8  and the  dry density  determined   in accordance   with  DKS 2802-7.  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 w ith an accuracy of ± 0,2 % of the dry mass. 
 After conditioning   to the 6 % moisture content and prior to t esting, 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 EN 772-10.  
7.3.5  Conditioning by immersion 
 
Immerse  the specimens  in water  at  a temperature of  20 °C ±   5 °C   for  a minimum  period of  15 h  and 
subsequently allow them to drain for 15 min to 20 min. 
 
7.4 Loaded area 
 
7.4.1  Gross area 
 
  
6  © KEBS 2019 — All rights reserved The gross area of the loaded surface shall be calculated in squ are millimetres by multi plying the length by the 
width of each specimen determined in accordance with DKS 2802-8 . Where units are to be tested with the 
compressive force other than normal to the bed face, then the g ross area shall be calculated similarly but using 
the width and height or lengt h and height as appropriate. 
 7.4.2  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 t he compressive strength shall be calculated on 
the basis of the net loaded surface of the frogged bed face. Wh ere 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 f rogs 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 o f each specimen by simple measurement and 
the principles of geometry. For rectangular frogs, determine th e frog area of each specimen by measuring the 
length and width at the outer perimeter of the frog, using a ri gid steel rule. Measurements shall be made to the 
nearest 1 mm. Calculate the net loaded area for each specimen a s the difference between the gross area of 
the bed face and the frog area. 
 
Alternatively, and for units possessing a non-regular frog shap e, the net loaded area may be determined by a 
paper indentation method following the principles of EN 772-2. 
 
8 Procedure 
 
8.1 Placing specimens in the testing machine 
 
Wipe the bearing surfaces of the testing machine (6.1) clean an d remove any loose grit from the bed faces of 
the specimen. Align the specimen c arefully with the centre of t he 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 sti ff steel strips (6.3), the same width as the face• 
shells and 50 mm longer, two a t the top and two at the bottom, overlapping equally at each end. 
 
8.2 Loading 
 Initially, use any convenient rate of loading but, when about h alf 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.   
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 t imes before maximum failure load is reached. 
This will be indicated by a reduction in load as the specimen y ields followed by an increase to a new maximum as loading 
is continued. This temporary reduction may occur several times before the specimen finally fails. 
 
 DKS 2802-1:2019  
© KEBS 2019 — All rights reserved   7 9 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 m ortar, to be face shell or strip bedded and those 
containing frogs not intended to be filled in practice, or in a ccordance with 7.4.2 in other cases, and express it 
to the nearest 0, 1 N/mm2. 
 
10 Evaluation of results 
 
Calculate the compressive strength as the mean value of the str ength of the individual specimens to the 
nearest 0, 1 N/mm2. 
 
Calculate the coefficient of variation of the sample. 
 
11 Test report 
 
The test report shall contain the following information:  
a) number, title and date of issue of this Kenya Standard; 
b) name of the organisation tha t carried out the sampling and t he method used; 
c) date of testing; 
d) type, origin and designation of the masonry unit by referenc e to DKS 2801; 
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; 
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 (s ee Annex A) in N/mm2 to the nearest 0,1 
N/mm2; 
o) remarks, if any.