ICS 91.100.30  
 
Reference number  
DRS 266: 2021  
© RSB 2021  
  
 RWANDA 
STANDARD  DRS  
266  
 
Second  edition  
2021-mm-dd 
 
Concrete building blocks — Specification  
 
   
DRS 266: 2021  
©RSB 2021  - 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  
 
 
 
 
 
 
 
 
© RSB 2021  
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 R SB. 
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 266: 2021  
iii                ©RSB 2021 - All rights reserved  
 Contents  Page 
Concrete building blocks — Specification  ................................ ................................ ..............................  i 
1 Scope  ................................ ................................ ................................ ................................ .............  1 
2 Normative references  ................................ ................................ ................................ ...................  1 
3 Terms and definitions  ................................ ................................ ................................ ..................  1 
4 Requirements  ................................ ................................ ................................ ................................  2 
5 Marking  ................................ ................................ ................................ ................................ ..........  5 
6 Certificates  ................................ ................................ ................................ ................................ .... 5 
7 Sampling and conformity  ................................ ................................ ................................ ............  5 
8 Inspection  ................................ ................................ ................................ ................................ ...... 6 
 (normative)  Method of measuring dimensions  ................................ ................................ ...... 7 
 (normative)  Method of testing block density  ................................ ................................ .........  8 
 (normative)  Moisture content  ................................ ................................ ................................ .. 9 
 (normative)  Methods of testing compressive strength ................................ .......................  10 
 (normative)  Determination of transverse breaking load  ................................ .....................  11 
 (normative)  Methods of testing drying shrinkage  ................................ ...............................  12 
 (normative)  Wetting expansion  ................................ ................................ .............................  13 
 DRS 266: 2021  
©RSB 2021  - All rights reserved                  iv 
 Foreword  
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda 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 S tandards adopted 
by Technical committ ees are ratifie d by me mbers of RSB Board of Director s for publication and gazettment as 
Rwanda Standards.  
RS 266 was prepared by Technical Committee RSB/TC 009, Building m aterials and civil en gineering . 
In the preparation of this standard, reference was made to the follo wing standard s: 
1) IS 2185 -1: 2005, Concrete masonry units — Specification — Part 1: Hollow and solid concrete blocks   
2) MS 71: 2000, Concrete building blocks — Specification  
The assistance de rived from the above source is hereby acknowledged with thanks.  
This second edition cancels and replaces the first edition (RS  266: 2015) of which has been technically revised.  
Committee membership   
The following organizations were represented on the Technical Committee on Building materials and civil 
engineering  (RSB/TC 009) in the preparation of this standard.  
B. J Construction Ltd  
Bridge to Prosperity  
City of Kigali  
Green Pact Africa  
MASS Design  
NPD Ltd  
Rwanda Housing Authority (RHA)  
Rwanda Inspectorate , Competition  and Consumer Protection Authority (RICA)  
Standards for Sustainability (SfS)  
University of Rwanda/College of Science and Technology (UR -CST)  
Rwanda  Standards Board  (RSB) – Secretariat  
 DRS 266: 2021  
1                ©RSB 2021 - All rights reserved  
 Concrete building blocks — Specification  
1 Scope  
This Draft Rwanda Standard specifies  the requirements,  sampling and test methods  for concrete building 
blocks.  
It applies to solid, hollow or cellular concrete building blocks including aerated blocks.  
2 Normative references  
The following referenced documents are indispensable for the application 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.   
RS EAS 18 -1, Cement — Composition, specifications and conformity criteria for common cements  
3 Terms and definitions  
For the purposes of this stand ard, the following terms and definitions apply.  
3.1 
block  
masonry unit either hollow or solid exceeding the s ize of a brick in any dimension.  
3.2 
brick  
mason ry unit that does not exceed 300  mm in length, 130mm in width, and 1 20 mm in thickness.  
3.2  
lot  
not more than 1 000 blocks of the same type ,manufacturer, submitted at any one time for inspection and test  
3.3  
work size  
size of block specified for manufacture to which the actual size should conform subject to specified tolerance  
3.4  
cross area  
area occupied by block on its bedding face  
3.5  
masonry unit  
pre-formed component intended for use in masonry construction   
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 3.6  
actual dimension (of the product)  
dimension found by measurement (on the finished product).   
4 Requirements  
4.1 Classification of blocks  
4.1.1  Type A  
Dense aggregate solid or hollow concrete blocks of density not less than 1 500 kg/m3. This type of blocks is 
suitable for general use in building as load -bearing walls.  
4.1.2  Type B  
Dense aggregate hollow concrete or light aggregate, solid or hol low concrete blocks of density less than 1 500 
kg/m3. Dense aggregate blocks of this class are suitable for general use in building as load -bearing walls.   
4.1.3  Type C  
Light aggregate, solid or hollow concrete blocks of density less than 1 500 kg/m3 for use i n internal non -load 
bearing walls and partitions (including panels in framed construction).  
4.2 Materials  
4.2.1  Cement  
Cement used in the manufacture of blocks shall conform to RS EAS 18 -1.  
4.2.2  Aggregates  
The aggregates at the mixer shall be acceptably clean and fr ee from deleterious matter and shall all pass 
through a sieve of nominal aperture size 19 mm.  
4.2.3  Water  
Water used for mixing shall be free from matter harmful to concrete and its reinforcement, and also free from 
substances which would impair the concrete b lock performance.  
4.2.4  Additives or admixture  
Additives or admixture which do not have deleterious effects and which do not affect adversely the properties 
or durability, or both, of the block with the passage of time may be used.  
4.3 Finishes  
4.3.1  Appearance  
Blocks shall be free from cracks and other defects which would interfere with the proper laying of the blocks or 
impair with strength, durability or appearance of the construction.  
4.3.2  Form  
Subject to the tolerances given in 4.3.5, the face of the blocks sha ll be flat and rectangular. Opposite faces shall 
be parallel and all areas shall be square.  DRS 266: 2021  
3                ©RSB 2021 - All rights reserved  
 4.3.3  Surface texture  
When intended for use with rendering or plastering, the surface texture of the blocks shall be such as to provide 
a suitable key for rendering.  
4.3.4  Dimensions  
4.3.4.1  Concrete block, hollow (open or closed cavity) or solid shall be referred to by its nominal 
dimensions . The term nominal means that the dimension includes the thickness of the mortar joint. Actual 
dimensions (length and height only) shall be 10 mm short of the nominal dimensions. The nominal dimensions 
of blocks are the following:  
a) length: 400 mm, 500 mm o r 600 mm;  
b) height: 100 mm or 200 mm ; and  
c) width: 100 mm, 150 mm, 200 mm, 250 mm or 300 mm.  
4.3.4.2  The nominal dimensions of the units are so designed that taking account of the thickness or mortar 
joints, they shall produce wall lengths and heights whic h shall conform to the principles of modular coordination.  
4.3.4.3  Blocks of sizes other than those specified in 4.3.4.1  may also be used by mutual agreement 
between the purchaser and the supplier. In the case of special concrete masonry units such as scr een wall 
blocks and ornamental blocks, the specified sizes may not necessarily apply.  
4.3.5  Tolerances  
The dimensions of the blocks shall be measured in accordance with annex A and may vary from the work size 
as follows:  
a) the maximum variation in the length of  the individual block shall not be more than ± 5 mm and the maximum 
variation in height and width of the unit, not more than ± 3 mm.  
b) face shells and webs shall increase in thickness from the bottom to the top of the unit. Depending upon the 
core moulds us ed, the face shells and webs shall be flared and tapered or straight tapered, the former 
providing a wider surface for mortar. The thickness of the face shell and web shall be not less than the 
values given in Table 1, as appropriate.  
Table 1 — Minimum fa ce shell and web thicknesses  
All dimensions in mm  
Nominal block width  Face shell thickness  Thickness of web  
100 or less   25  25  
Over 100 to 150  25  25  
Over 150 to 200  30  25  
Over 200  35  30  
4.4 Density  
The block density of type A  block whether solid, hollow, or cellular, shall be not less than 1 500 kg/m3 when 
determined in the manner described in Annex B and similarly the density of type B and C shall not exceed 1 
500 kg/m3.   
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 4.5 Moisture content  
When determined by the method descri bed in Annex C, the moisture content of blocks at time of delivery to the 
purchaser shall not exceed 40 % of the total water absorption of the blocks .  
4.6 Strength  
4.6.1  Compressive strength  
When tested in accordance with Annex D, the average compressive strength  of an individual block (other than 
type C) shall comply with the values given in Table 2 appropriate to the type and designation of the block 
specified.  
Table 2 — Compressive strength of class A and B blocks  
Block type  Grade  Minimum compressive strength  
Average of 10 blocks  
(N/mm2)  Lowest individual blocks  
(N/mm2)  
A  A(5)  5.0  4.0  
A(7) 7.0 6.0 
A(10)  10.0  8.2  
A(20)  20.0  16.5  
A(28)  28.0  22.0  
B  B(3.5)  3.5  2.8  
 B(5)  5.0  4.0  
C  C(3)  3.0  2.4  
C(4)  4.0  3.2  
4.6.2  Breaking load  
For type C blocks, the average transverse breaking load of the sample and the corresponding lowest transverse 
breaking load of any individual block thereof, when determined in accordance with Annex E, shall be not less 
than the values appropriate to the si ze given in Table 3.  
Table 3 — Transverse breaking load for type C blocks  
Work size of block  
(mm)  Minimum transverse breaking load  
(N)  
Height  Width  Average of 6 blocks  Lowest individual 
blocks  
190  90  1745  1396  
215  100  2435  1948  
4.6.3  Other strengths  
When blocks of type A or type B of other strengths are required, they shall comply with the requirements of this 
standard except that the purchaser and the supplier shall agree on the values for the average compressive DRS 266: 2021  
5                ©RSB 2021 - All rights reserved  
 strength. The streng th for the lowest individual block shall be not less than 80 % of the agreed average 
compressive strength.  
4.7 Drying shrinkage  
The average drying shrinkage determined in accordance with Annex F, shall not exceed the following:  
a) Type A: 0.05 %, maximum;  
b) Type B: 0.07 %, maximum; and  
c) Type C: 0.08 %, maximum  
4.8 Wetting expansion  
The average value of the wetting expansion of the sample shall not exceed the appropriate value given for 
drying shrinkage in 4.7, when determined in accordance with Annex G, by more th an 0.02 %.  
4.9 Curing  
The blocks, unless steam cured, shall be continuously dump cured for at least seven days, followed by a 
minimum drying and maturing period of two weeks before dispatch.  
5 Marking  
5.1  Each block shall bear an approved mark by which the manufacturer can be identified.  
5.2 Each consignment of blocks shall be accompanied by a dispatch or delivery note in which the following 
information shall be included:  
a) manufacturer’s name;  
b) size, type and designation of blocks; and  
c) date of manufacture of the blocks.  
6 Certificates  
The supplier shall satisfy by regular periodical testing that, at the time of delivery, the blocks comply with the 
requirements of this standard and, if required, the supplie r shall provide to the purchaser a certificate to this 
effect.  
7 Sampling and conformity  
7.1 Sampling  
Sampling shall be carried out only on blocks ready for dispatch. The purchaser may take as samples from every 
lot (every 1 000 blocks or part thereof) the nu mber of blocks required for carrying out the tests (see Table 4).The 
samples so drawn shall be deemed to represent the lot.  
Table 4 — Number of blocks required for each test  
Test to be performed  Number of blocks required  
Type A or B  Type C   
©RSB 2021  - All rights reserved                  6 
 Visual inspection and checking dimensions1  (10)  (10)  
Determination of density  3  3  
Determination of moisture content  3  3  
Determination of drying shrinkage and/or wetting expansion  4  4  
Determination of strength  10  5  
Total blocks required for full sets of tests  20  15  
1 Sample available for other test   
7.2 Conformity  
A lot shall be deemed to conform to the requirements of this standard if, after inspection and testing of the 
sample taken in accordance with 7.1  no defective is found.  
8 Inspection  
Visual examination of the sample drawn in accordance with 7.1 for compliance with the requirements of 4.3 
shall be done.  
 
 DRS 266: 2021  
7                ©RSB 2021 - All rights reserved  
 
 
(normative)  
 
Method of measuring dimensions  
 
A.1 Test specimen  
Ten blocks taken at random from the  sample drawn in accordance with 9.1  
A.2 Apparatus  
Overall dimensions shall be measured with a steel scale graduated in 1 mm divisions. Face shell and web 
thickness shall be measured with a caliper  rule graduated in 0.5 mm divisions and having parallel jaws neither 
not less than 15 mm nor not more than 25 mm in length.  
A.3 Procedures  
Using apparatus stated in A.2, measure to the nearest 1 mm the length, height and thickness of each test 
specimen. Recor d for each block seven individual measurements.  
A.4 Calculations  
Calculate to the nearest 1 mm the average of the seven measurements of each block. Take these averages to 
be the dimensions of the block, to be used for determination of gross volume and gross area.  
   
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(normative)  
 
Method of testing block density  
 
B.1 Test specimens  
Select at random three blocks from the sample drawn in accordance with 7.1.  
B.2 Calculations  
B.2.1  Measure the mass of each block as received to the nearest 50 g.  
B.2.2   Calculate the gross volume of each of the three to the nearest 1 000 mm3 in accordance with A.4. Then 
calculate the density of each block to the nearest 10 kg/m3.  
Density =𝑀𝑎𝑠𝑠  (𝑎𝑠 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑 ),𝑘𝑔
𝐺𝑟𝑜𝑠𝑠  𝑣𝑜𝑙𝑢𝑚𝑒 ,cm3×106 
B.2.3  Report the mean density of the 3 blocks to the nearest 10kg/m3 as the block density as received.  
B.3 Alternative  
B.3.1  If the as -received density exceeds the specified block density, determine the oven –dry block density 
by drying the 3 blocks for 24 h in a ventilated oven having t he temperature controlled at 105 0C ± 3 0C.  
B.3.2  Cool the blocks for 6 h to 8 h and measure their mass. Repeat the process of drying and cooling until 
the loss in mass for one cycle does not exceed 50 g.  
B.3.3  Calculate the oven -dry block density as in B.2 using the oven -dry mass and record the results to the 
nearest 10 kg/m3 as oven -dry density.  
 DRS 266: 2021  
9                ©RSB 2021 - All rights reserved  
 
 
(normative)  
 
Moisture content  
C.1 Test specimens  
Three whole blocks taken at random from the samples selected for test shall be used.  
C.2 Apparatus  
A weigh balance sensitive to within 10 g, drying oven capable of maintaining 105 o C ± 3 o C; and a water tank.  
C.3 Procedure  
Determine the mass as received, A, or each of the test specimens to the nearest 10 g. Then completely immerse 
the test specimens at room temperature for 24 h, after which period they shall be wiped dry using a damp cloth 
and their mass, B, determined immediately. Subsequent to saturation all specimens shall be dried to constant 
mass, C, in ventilated oven maintained at a temperature of  105 o C ± 3 oC.  
C.4 Calculation  
C.4.1   Calculate the water absorption and moisture content as follows:  
Total absorption, %  =𝐵−𝐶
𝐶×100 
Moisture content ,%  = 𝐴−𝐶
𝐶×100 
where  
A mass as received;  
B saturated mass of unit; and  
C Oven -drying mass of unit.  
C.4.2  Report all the results separately for each block and also the mean of the 3 blocks, to the nearest 0.5 %.  
  
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(normative)  
 
Methods of testing compressive strength  
D.1 Test specimens  
For the determination of compressive strength, take ten blocks, previously measured in accordance with Annex 
A. Normally, the specimens are whole blocks. Where the blocks are claimed to have such a high compressive 
strength that the capacity of the machine  is likely to be exceeded, the blocks may be cut as far as possible 
symmetrically about an axis perpendicular to the original length. Discard the remainder of the block.  
D.2 Apparatus  
Compressive strength testing machine.  
D.3 Procedure  
D.3.1  Each block shall b e immersed in water maintained between 25 o C ± 2 oC for a period of 24 h, and then 
bedded with flat layers of mortar composed of cement and sand in the ratio of one to one by weight, on both 
bed faces. The mortar bed shall at no point be less than 5 mm cl ear of the block, and shall be true to the square. 
All cavities shall be completely filled with the mortar. Surplus mortar shall be trimmed off flush with the sides of 
the block. A minimum period of 24 h shall elapse before the bedding of the opposite bed -face.During this period, 
the block shall be covered with a damp sack. Another period of 24 h shall elapse before the cubes are immersed 
in water. The period of immersion shall be such that the mortar shall have a compressive strength of not less 
than 28.0N /mm2.  
D.3.2  When the mortar strength has attained the required value, the blocks shall be tested, each separately, 
on the testing machine. The load shall be applied exactly, manually or automatically without chock at a rate 
within the range of 0.2N/mm2 to 0.4N/mm2 per second until no greater load can be sustained.  
D.4 Calculation  
D.4.1  Record the maximum load in Newtons carried by the specimen. During the test, calculate the 
compressive strength of the block as follows:  
Compressive strength in N/mm2= 𝑀𝑎𝑥 𝑖𝑚𝑢𝑚  𝑙𝑜𝑎𝑑 ,𝑁
𝐺𝑟𝑜𝑠𝑠  𝑎𝑟𝑒𝑎 ,𝑚𝑚2   
D.4.2  This is reported to the nearest 0.05N/mm2 for blocks with an average compressive strength less than 
7.0 N/mm2 and to the nearest 0.1N/mm2 for blocks with an average compressive strength of 7.0 N/mm2 or more.  
D.4.3  The arithmetic mean of the compressive strength of the ten blocks is taken as the average compressive 
strength of the sample and is reported to the same accuracy as the individual results.  DRS 266: 2021  
11                ©RSB 2021 - All rights reserved  
 
 
(normative)  
 
Determination of transverse breakin g load  
E.1 Test specimen  
Select at random five blocks from among the ten blocks used for measurement of dimensions in accordance 
with Annex A.  
E.2 Procedure  
E.2.1  Immerse the blocks for at least 16 h in water at room temperature. Remove each block from the wate r 
and allow to drain for 30 min to 45 min and then test.  
E.2.2  Place each block centrally on the cylindrical support bearers with the bedding faces perpendicular to 
the plane of the bearers. Apply the load without the shock and increase continuously at a rate such that the 
extreme fibre stress increases at approximately 1.5N/mm2 per minute, until failure occurs.  
E.2.3  Record the maximum load carried by the block during the test to the nearest 25N.The arithmetic mean 
of the five recorded loads is taken as the transverse breaking load of the sample and is reported to the nearest 
25N together with the lowest individual block breaking load.  
 
 
 
 
 
 
 
 
 
 
  
©RSB 2021  - All rights reserved                  12 
 
 
(normative)  
 
Methods of testing drying shrinkage  
F.1 Test specimen  
For the determination of drying shrinkage, take four whole blocks at random from the sample taken and in 
accordance with 9.1. Cut a test specimen of length approximately 150 mm and cross -section as near as is 
practically to 75 mm × 75 mm.  
F.2 Apparatus  
Micro meter gauge, or suitable dial gauge reading accurately to 0.005 mm: drying oven capable of maintaining 
105 oC ± 3 oC and a water tank.  
F.3 Procedure  
F.3.1  Using an apoxy -resin compound or other suitable agent, fix a 6 mm diameter steel ball into the centre 
of each specimen so that half of the surface of the ball protrudes. After the resin has set sufficiently clean the 
exposed surface of the steel balls and grease them to prevent corrosion. Use the outer extremities of the balls 
as reference points.  
F.3.2  Completely immerse the specimen in clean water at a temperature of 25 oC ± 2 oC for four days. Remove 
the specimen from the water, immediately wipe the grease from the steel balls and measure the length, dry the 
specimens in the oven for at least 48 h.  
F.3.3 Do not place wet specimens in the oven together with partially dried specimens. Cool the specimens 
for at least four hours in the desicator and again measure the length.  
F.3.4  Repeat this drying and cooling procedure, using drying periods of 24 h, until  the difference between 
consecutive measurements is less than 0.005 mm. Regard the final reading as the dry length.  
F.4 Calculation  
Calculate the drying shrinkage of each specimen as the difference between the original wet measurement and 
the dry measurement  expressed as a percentage of the dry length. Determine the average value for four 
specimens and report this as the drying shrinkage of the sample expressed to the nearest 0.005 %.  
 DRS 266: 2021  
13                ©RSB 2021 - All rights reserved  
 
 
(normative)  
 
Wetting expansion  
G.1 Test specimen  
Use the test specimen used for shrinkage test given in Annex F.  
G.2 Apparatus  
As given in F.2   
G.3 Procedure  
G.3.1  Unless the test is carried out immediately after drying shrinkage test, then the test specimen shall be 
kept in the desiccator  for a period not ex ceeding 3 days.  
G.3.2  The specimens are first tested for drying shrinkage in accordance with Annex F and the measurement 
recorded. Then coat the stainless steel balls with lubricating grease and immerse the specimen in water being 
maintained at 25 oC ± 2 oC. Immediately after the removal of the specimen from the water, wipe the grease from 
the balls and measure the length of each specimen as described for determining the original wet length.  
G.4 Calculations  
Calculate the wetting expansion of each specimen a nd the final wet measurement expressed as a percentage 
of the length. Report the average value of the wetting expansion to the nearest 0.005 %.   
©RSB 2021  - All rights reserved                  14 
 Bibliography  
RS 266: 2015, Concrete building blocks — Specification  
 
 
 
 DRS 266: 2021  
 
Price based on 1 3 pages  
©RSB 2021 - All rights reserved