KENYA STANDARD            DKS 2801-1:2019 
ICS 
  
 
 
  
 
  
 
 
 
  
 
  
Specification for Masonry units 
 
Part 1: 
 Clay Masonry Units  
 Public Review Draft April 2019  
 
 
 
 
  
 
  
 
  
 
    
 
  
 
  
 
© KEBS 2019 First Edition, 2019 
 DKS 2801-1:2019  
ii  © KEBS 2019 — All rights reserved  
  KENYA STANDARD                                       DKS 2801-1:2019 
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 2801-1:2019  
iv  © KEBS 2019 — All rights reserved  
 
 
  
 
  
 
 
  
 
Specification for Masonry units 
 
Part 1: 
 
Clay Masonry Units  
 
 
 
  
 
 
  
 
  
 
  
 
  
 
  
 
  
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 
  KENYA STANDARD                                       DKS 2801-1:2019 
ICS  
© KEBS 2019 — All rights reserved   v  
Foreword 
 
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.  
The standard has been developed to replace KS 02-547:1983, Spec ification for burnt clay building blocks and 
KS 02-300: 1983, Specification for burnt building bricks. This standard is by all means more comprehensive 
and captures the development in the clay industry over the year s. This standard may not give specific 
dimensions for the clay units but would give values of other pa rameters to be attained in the test methods. 
 During the development of this standard, reference was made to the following document: 
 
BS EN 771-1:2011+A1:2015, EN 771, Specification for masonry uni ts consists of: Part 1: Clay 
masonry units. 
 
KS 02-547:1983, Specification for burnt clay building blocks 
 KS 02-300: 1983, Specification for burnt building bricks 
 
Acknowledgement is hereby made for the assistance received from  these sources. 
  
KENYA STANDARD           DKS 2801-1:2019 
 
 
© KEBS 2019 — All rights reserved   1  
Specification for Masonry units 
 
Part 1:  
Clay Masonry Units  
 
 
1 Scope and field of application 
 
This Kenya Standard specifies the characteristics and performan ce requirements for masonry units 
manufactured from clay for which the main intended uses are pro tected or unprotected masonry structure (see 
definitions 3.3  and  3.4)  (e.g.  Facing and rendered masonry,  loadbearing or non-loadbearing masonry 
structures, including internal linings and partitions, for buil ding and civil engineering).  
 
NOTE For control purposes, regardless of the use or the compres sive strength, that is Protected Units (P) or Category II Units  
respectively, the properties of Unprotected Units (U) and Categ ory I Units shall apply. 
 
It defines the performance related to e.g.  Dimensional toleran ces, strength, density measured according to 
the corresponding test methods contained in separate Kenyan Sta ndards. 
 
This Kenya Standard does not specify standard sizes for clay ma sonry units, nor does it specify standard work 
dimensions, angles and radii of specially shaped clay masonry u nits. This    Kenyan Standard does not include 
method of measurement of angles an d radii characteristics of sp ecially shaped clay masonry units. 
 
This Kenya Standard does not cover requirements for the followi ng: units for paving, flue liners and storey 
height clay masonry units and clay masonry units with an incorp orated thermal insulation material bonded to 
the faces of the unit susceptible to be exposed to fire.  It do es, however, include clay masonry units for external 
chimney masonry 
 
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. 
 DKS 2802-1 Masonry units — Test Methods — Part 1:  Determinatio n of compressive strength. 
 
DKS 2802-2 Masonry units — Test Methods — Part 2: Determination  of net volume and percentage of voids 
of clay masonry units by hydrostatic weighing. 
 
DKS 2802-3 Masonry units — Test Methods — Part 3: Determination  of the active soluble salts content of clay 
masonry units. 
 
DKS 2802-4 Masonry units — Test M ethods — Part 4: Determination  of water absorption of clay masonry 
damp proof course units by boiling in water.  
DKS 2802-5 Masonry units — Test Methods — Part 5: Determination  of volume and percentage of voids and 
net volume of clay and calcium silicate masonry units by sand f illing. 
 DKS 2802-6 Masonry units — Test Methods —  Part 6: Determinatio n of water absorption of aggregate 
concrete, autoclaved aerated concrete, manufactured stone and n atural stone masonry units due to capillary 
action and the initial rate of water absorption of clay masonry  units. 
  
2  © KEBS 2019 — All rights reserved DKS 2802-7 Masonry units — Test Methods — Part 7: Determination  of net and gross dry density of masonry 
units (except for natural stone). 
 
DKS 2802-8 Masonry units — Test Methods — Part 8: Determination  of dimensions. 
 
 
DKS 2802-9 Masonry units — Test Methods — Part 9: Determination  of moisture expansion of large 
horizontally perforated clay masonry units. 
 
DKS 2802-10 Masonry units — Test Methods — Part 10: Determinati on of flatness of faces of aggregate 
concrete, manufactured stone and natural stone masonry units.  
DKS 2802-11 Masonry units — Test Methods — Part 11: Determinati on of water absorption of clay and calcium 
silicate masonry units by cold water absorption.  
DKS 2802-12 Masonry units — Test Methods — Part 12: Determinati on of initial shear strength. 
 DKS 2803 Masonry and masonry products —   Methods for determini ng thermal properties. 
 
DKS 2804-1 Fire classification of construction products and bui lding elements — Part 1:   Classification using 
data from reaction to fire tests.  
KS ISO 12572, hydrothermal performance of building materials an d products -    Determination of water v apour 
transmission properties.   
3 Term and definitions 
 
For the purposes of this document, the following terms and defi nitions apply. 
 
NOTE Annex B to this Kenya Standard is informative and gives de scriptions of such matters as applications, exposure and durabi lity. 
 
3.1 masonry unit preformed component intended for use in masonry construction 
 
3.2 clay masonry unit masonry unit made from clay or other argillaceous materials wit h or without sand, fuel or other additives fired 
at a sufficiently high temperat ure to achieve a  ceramic bond 
 
3.3 protected masonry 
masonry which is protected against water penetration and is not  in contact with soil and ground water 
 
Note 1 to entry:   
it can either be masonry in external walls which is protected, (e.g. by a layer of suitable render or by cladding), 
or it can be the inner leaf of a cavity wall or it can be an in ternal wall. it may or may not be loadbearing. 
 
3.4 unprotected masonry 
masonry which may be exposed to rain, freeze/thaw and/or may be  in contact with soil and ground water 
without a suitable protection 
 
Note 1 to entry: It can either be masonry in external walls which is fully unpro tected, or which is intended to be provided by a 
limited protection (e.g. by a th in layer of render). It may or may not be loadbearing. 
 
3.5 P unit 
clay masonry unit for use in protected masonry. 
 
3.6 U unit 
clay masonry unit for use in unprotected masonry  
 
3.7 coordinating size 
size of the coordinating space allocated to a masonry unit incl uding allowances for joints and tolerances DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   3 3.8 work size 
size of a masonry unit specified for its manufacture, to which the actual size conforms within permissible 
deviations 
 
3.9 actual size size of a masonry unit as measured 
 
3.10 regular-shaped masonry unit masonry unit with an overall rectangular parallelepiped shape 
 
3.11 specially shaped masonry unit 
masonry unit which is not a rectangular parallelepiped  
3.12 accessory unit 
masonry unit which is shaped to provide a particular function, e.g.  to complete the geometry of the masonry 
 
3.13 interlocking features 
shaped matched projections and indentations on masonry units  
example  tongue and groove systems.  3.14 vertical perforation 
formed void that passes completely through a masonry unit perpe ndicular to the bed face 
 3.15 horizontal perforation 
formed void that passes completely through a masonry unit paral lel to the bed face 
 3.16 cell 
formed void that does not pass through a masonry unit 
 
3.17 frog 
depression formed in one or both bed faces of a unit 
 
3.18 recess 
depression  or indentation in one or mor e surfaces of a masonry unit 
 
example mortar pocket, rendering keyway, grip hole.  3.19 grip hole 
hole in a masonry unit to enable it to be more readily grasped and lifted by hands or machine 
 3.20 shell 
peripheral material between a perforation and the surface of a masonry unit 
 3.21 web 
solid material between the perforations in a masonry unit 
 3.22 declared value 
value that a manufacturer is confident in achieving, bearing in  mind the precision of test and the variability of 
the manufacturing process  3.23 mean compressive strength of masonry units 
arithmetic mean of the compressive strengths of masonry units 
 3.24 normalized compressive strength 
compressive strength of masonry units converted to the air dry compressive strength of an equivalent 100 mm 
wide x 100 mm high masonry unit 
 
NOTE See the procedure given in DKS 2802-1. 
 
3.25 damp proof course unit  
4  © KEBS 2019 — All rights reserved clay masonry unit which, when laid in two courses with broken b ond in a strong cementitious mortar, will resist 
rising damp in masonry 
 
3.26 high precision clay masonry unit 
clay masonry unit with small dimensional tolerance especially i n unit height 
 
3.27 vertically perforated or hollow clay masonry unit 
clay masonry unit with one or more formed voids that pass compl etely through a masonry unit perpendicular 
to the bed face 
 
3.28 horizontally perforated or hollow clay masonry unit 
clay masonry unit with one or more formed voids that pass compl etely through a masonry unit parallel to the 
bed face 
 
3.29 clay masonry unit for concrete or mortar infill clay masonry unit with special perforation suitable for concret e or mortar infill 
 
3.30 clay masonry unit for masonry panels clay masonry unit suitable for production of reinforced masonry  or masonry storey height panels with vertical 
channels for mortar or concrete infill 
 
3.31 clay masonry subject to severe exposure 
masonry or elements of masonry which, under end use conditions,  a re  sub jected  to  sa tu ratio n w i th  w a ter 
(driving  rain,  ground  water)  combined  with  frequent  free ze/thaw-cycling,  due  to  climatic  conditions  and 
absence of protective features 
 
3.32 clay masonry subject to moderate exposure masonry or elements of masonry which, under end use conditions,  are exposed to moisture and freeze/thaw• 
cycling, excluding constructions subjected to severe exposure 
 
3.33 clay masonry subject to passive exposure 
masonry or elements of masonry which, under end use conditions,  are not intended to be exposed to moisture 
and freezing conditions 
 
3.34 category I masonry units units with a declared compressive strength with a probability o f failure to reach it not exceeding 5 % 
 
NOTE This may be determined via the mean or characteristic valu e. 
 
3.35 category ii masonry units 
units not intended to comply with the level of confidence of ca tegory i   units 
 
3.36 combined thickness of webs and shells 
sum of the thicknesses of the sh ells and webs from one face or header of a masonry unit to the opposite face 
or header respectively along whichever path, via the formed voi ds, gives the smallest value, expressed as a 
percentage of the unit width or length respectively  3.37 product group 
products from one manufacturer having common values for one or more characteristics 
 3.38 consignment shipment from the supplier 
 
3.39 product type set of representative performance  levels or classes of a const ruction  product,  in   relation  to  its essential 
characteristics,  manufactured using a  given combination of ra w materials or other  elements  in a  specific 
production process. 
 
4 Materials and manufacture 
 
See 3.2, 8.3.4 and 8.3.5. DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   5 5 Requirements for clay masonry units 
 
5.1 General 
 
The requirements and properties specified in this standard shal l be defined in terms of the test methods and 
other procedures referred to in this Kenya Standard. 
 
NOTE It should be noted that the test methods are not usually a pplicable to specially shaped and accessory units as defined in  3.11 
and 3.12.  
 
In general, for specially shaped clay masonry units it will be satisfactory to declare the properties as that 
determined on units of a rectangular parallelepiped shape and o f the same product type which has been 
subjected to a similar m anufacturing process. 
 
The conformity criteria given in the following sub clauses rela te to product type determination (see 8.2) and, 
when relevant, to consignment testing (see Annex A).  For the c ompressive strength of Category I units use a 
50 % fractile (p = 0,50) for m ean values and a confidence level  of 95 %. 
 
For production    e v a l u a t i o n ,  t h e  m a nu f a c tu r e r  s h a l l  d e f i n e  t h e  c o n f o r m i t y  c r i teria in the factory production 
control documentation (see 8.3). 
 
The manufacturer   shall declare whether the unit fulfils the r equirements for P units (see Figure 2) or 
U units (see Figure 3). 
 
5.2 P or U units sizes 
 The standard Sizes of units (P or U) shall conform to Table 1. The format of walling  blocks shall be designated 
in terms of their coordinating sizes, which with the exception of Widths include the thickness of a mortar joint. 
This is taken for the purpose of this standard as equal to 10 m m. 
 
Table 1 — Standard dimensions for (P or U) units 
 
Length (mm) Height (mm) Width (mm) 
Nominal size Work size Nominal size Work size Nominal size Work s ize
200 190 225 225 75 65 
225 215 300 300 100 90 
300 290 400 400 150 140 
    2 0 0  1 9 0  
    2 2 5  2 1 5  
    3 0 0  2 9 0  
 
5.3 U units: intended to be  used in unprotected masonry 
 
5.3.1 Dimensions and tolerances (U units) 
 
5.3.1.1. Dimensions (U units) 
 The dimensions of a clay masonry unit shall be declared by the manufacturer in mm for length, width, and 
height, in that order (see Figure 1).  They shall be given in t erms of work size. 
 
NOTE In addition the co-ordinating size may be given. 
  
6  © KEBS 2019 — All rights reserved  
 
Figure 1 — Dimensions and surfaces 
  
DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   7 For units of special shape that fit within the envelope of a re ctangular parallelepiped shape, the length, width 
and height are those for the tightest envelope.  For other shap es these dimensions are defined in 5.3.2. 
 
NOTE  In addition the co-ordinating size may be given. 
 
The measurement procedure shall be in accordance with DKS 2802- 8. 
 
5.3.1.2 Dimensional tolerances (U units) 
 
5.3.1.2.1 Tolerances (U units) 
 
The manufacturer shall also declare which of the tolerance cate gories for mean values in 5.3.1.2.2 the clay 
masonry units fulfil.  
When relevant to the uses for which the unit is placed on the m arket, the manufacturer shall also declare which 
of the range categories in 5.3. 1.2.3 a given consignment of the  clay masonry units fulfils. 
 
NOTE This additional declaration may be made for example in rel ation to: 
 
a) achievement of the required accuracy (planarity, bonds and t hin layer joints) of the masonry; 
 
b) Use of detailed project drawings to achieve these requiremen ts. 
 
5.3.1.2.2 Tolerances of the mean value (U units) 
 
When clay  masonry  units  are  sampled  from  a  consignment  in accordance  with  Annex A  and tested  in 
accordance with DKS 2802-8, the difference for all dimensions b etween the declared value and the mean 
value derived from measurements of the test sample shall be not  greater than the declared one of the following 
categories, where the value shall be rounded to the whole mm: 
 
5.3.1.2.3 Range (U units) 
 When declared and regular-shaped clay masonry units are sampled  from a consignment in accordance with 
Annex A and tested in accordance with DKS 2802-8, the maximum r ange for any given dimension (i.e. the 
difference between the largest and smallest determined dimensio ns on individual units) to be found within the 
test sample shall be within the declared one of the three categ ories indicated below, where the value shall be 
rounded to the whole mm:  
 
 
5.3.1.2.4 Flatness of bed faces (U units) 
 When clay masonry units are intended to  be used  with  thin  l ayer  mortar,  the  manufacturer  shall declare  
the maximum  deviation  from  flatness  of the bed faces. 
 When regular-shaped clay masonry units  are  sampled  from  a c onsignment in accordance   with  Annex   A 
and tested   in  accordance with DKS 2802-10,   the  deviation   from  flatness   of  the  bed  faces   shall  not  
exceed   the declared  value.  
 
 
 
 
8  © KEBS 2019 — All rights reserved 5.3.1.2.5 Plane parallelis m of bed faces (U units) 
 
When clay masonry units are intended to be used with thin layer  mortar, the manufacturer shall declare the 
maximum deviation from plane parallelism of the bed faces. 
 
When regular-shaped clay masonry units are sampled from a consi gnment in accordance with Annex A and 
tested in   accordance with DKS 2802-8, the deviation from plan e parallelism shall not exceed the declared 
value. 
 
5.3.2 Configuration (U units) 
 
 
Figure 2 —Examples of U units 
 
  
DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   9  
5.3.3 General (P units) 
 
 
 
Figure 3 — Examples of P units 
 
When relevant to the uses, for which clay masonry U units are p laced on the market, the configuration shall 
be declared. The  declaration   may  be  made  by  reference   to  one  or  another   of  the  groups   defined   
in EN 1996-1-1     or  EN 1996-1-2  and/or  it may  include  one  or more  items  such  as those  i n the  following   
list,   as relevant: 
 a) Shape and  features,   including  the  direction   of  perfo rations   (by  means  of a dra wing   or  illustration,     
when relevant); 
 b) volume  of all formed  voids  as a percentage   of the lengt h  x  width x  height of the unit; 
 
c) volume of the largest of any formed voids as a percentage of  the length x width x height of the unit; 
 
d) volume of grip holes as a percentage of the length x width x   height of the unit; 
 
e) thickness of webs; 
 f) thickness of shells; 
 
g) combined thickness  of webs and shells from face to face;  
h) combined thickness of webs and shells from header to header;  
 i) area of voids on a bed face as a percentage of the length x  width of the unit; 
 
 
10  © KEBS 2019 — All rights reserved j) Total volume of frogs as a percentage of the length x width x height of the unit. 
 
NOTE The header of clay masonry units may have interlocking fea tures, e.g.  Mortar pockets, tongue and groove systems. The fac e 
of clay masonry units may have a surface-profile (rendering key ways). 
 
For units of special shape the dimensions to be considered as l ength, width and height of the unit shall be 
given. 
 Each declared value shall be stated as either an upper limit or  a lower limit or, alternatively, given as a range 
of values.  When clay masonry units are sampled from a consignm ent in accordance with Annex A and tested 
in accordance with DKS 2802-8, DKS 2802-5 and DKS 2802-2, the m ean value derived from measurements 
of the test sample shall be within the range or limit declared.  
 
5.3.4 Density (U units) 
 
5.3.4.1. Gross dry density (U units) 
 
When relevant to the uses for which the unit is placed on the m arket and in all cases for masonry units intended 
to be used in elements subject to acoustic requirements, the gr oss dry density of clay masonry units shall be 
declared by the manufacturer. 
 
NOTE 1 The manufacturer may also provide information to show ho w the declared gross dry density fits into a national classific ation 
system.  
When the gross dry density is declared, the manufacturer shall also declare which of the deviation categories 
in 5.3.3.3   applies. This    dec laration does not apply to uni ts of special shape.  
 
For solid units with or without c ells (e.g. frogs or recesses i ntended to be filled with mortar), the declaration of 
the gross dry density and its dev iation may be taken equal to t he net dry density a ccording to 5.3.3.2. 
 
NOTE 2 This declaration may be made for evaluation of: 
 a) loading; 
 
b) airborne sound insulation;  
c) resistance to fire; 
 d) Thermal insulation. 
 
In addition, the manufacturer may declare the minimum and maxim um individual values of gross dry density. 
 
5.3.4.2. Net dry density (U units) 
 When relevant to the uses for which the unit is placed on the m arket, the net dry density of clay masonry units 
shall be declared by the manufacturer.  
NOTE The manufacturer may also provide information to show how the declared net dry density fits into a national classificatio n 
system. 
 
When the net dry density is declared the manufacturer shall als o declare which of the deviation categories in 
5.3.3.3 Applies. This declaration does not apply to units of sp ecial shape. 
 
In addition, the manufacturer may declare the minimum and maxim um individual values of net dry density. 
 
5.3.4.3. Tolerances (U units) 
 
When clay  masonry  units are  sampled  from  a  consignment  i n accordance  with  Annex A  and  tested  in 
accordance with  DKS 2802-7, the mean gross and net dry density  derived from  measurements of the test 
sample  shall  not differ from the  manufacturer's  declared dr y  density  by more than  one of the following 
categories: 
 a) D1:       10 % 
 DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   11 b) D2:       5 % 
or 
 
c) Dm: a deviation in % as a whole number declared by the manuf acturer (may be wider or closer 
than the other categories). 
 
5.3.5 Compressive strength (U units) 
 
When relevant to the uses for which the unit is placed on the m arket and in all cases for masonry units intended 
to be used in elements subject to structural requirements, the mean compressive strength shall   be declared 
by the   manufacturer.  When   relevant, the manufacturer shall  also declare the   normalized compressive 
strength. In general, for specially shaped clay masonry units t he compressive strength may be declared as 
that determined on units of a rectangular parallelepiped shape which have been subjected to a similar 
manufacturing process. If the shape of a specially shaped clay masonry unit has a function in resisting load 
this shall be taken into account by testing representative samp les of whole units but the declaration   of strength 
is based upon loading the largest rectangular area normal to th e load expected in the brickwork. 
  
NOTE 1  DKS 2802-1 gives instructions on how to convert the uni t compressive strength into the normalized compressive strength . 
 
In addition, the manufacturer sh all declare whether the clay ma sonry unit is Category I. 
 When the clay masonry units are sampled from a consignment in a ccordance with Annex A and tested in 
accordance with DKS 2802-1, then: 
 a) mean compressive strength of the specified number of clay ma sonry units from a consignment shall  
be not less than the declared compressive strength; 
 b) Individual strengths of specimens measured within the test s ample shall be not less than   80 % of the 
declared value. 
 
The declaration shall relate to and indicate the orientation(s)  of the clay masonry uni ts as tested, the methods 
of bedding the units and whether any voids present are intended  to be fully filled with mortar. 
 
NOTE 2 The manufacturer may provide information to show how the  declared compressive strength fits into a national classificat ion 
system. 
 NOTE 3 For tapered units special provisions apply, see DKS 2802 -1. 
 
5.3.5.1 Compressive strength values ( U units) 
 
When tested in accordance with DKS 2802-1, the compressive stre ngth for clay units for non-load bearing 
blocks shall not be less than 3N/mm2 and the compressive strength for clay units for load bearing b locks shall 
not be less than 7N/mm2. 
 
Table 2 — Clay masonry units — Strength classes 
 
Designation Grade MIN. Compressive strength 
(N/mm2) 
Engineering Units U - A 48 
Category I Units (Severe conditions) U - B 28 
Category I Units load bearing U – C 7 
Category I Units non – load bearing U - D 3 
 
 5.3.6 Thermal properties (U units) 
 
When   relevant to the uses for which the units are placed on t he market and in all cases for masonry units 
intended to be used in elements subject to thermal insulation r equirements, the manufacturer shall provide the  
10,dry,unit  -value   and the determination  model as prescribed in DKS 280 3 or alternatively give net dry 
density  or gross dry dens ity and configuration. 
 
In addition another fractile may be used.  If so the used fract ile shall be provided together with the additional 
10,dry,unit  -value. 
 
When   the  clay  masonry  units  are  sampled  in accordance  with  Annex A  and tested  in accordance  with 
DKS 2803 following the model provided, then the obtained A-valu e of the specified number of clay masonry 
value. 
 
When relevant to the use for which the units are placed on the market the value of specific heat capacity given 
in DKS 2803 may be provided. 
 
5.3.7 Durability (U units) 
 
The  freeze/thaw  resistance  category  of  a  clay  masonry  u nit  shall  be  declared  by the  manufacturer  by 
reference to its applicability to masonry or elements subjected  to passive, moderate, and severe exposure 
(see B.3). 
 
- FO -   Passive exposure 
 
- F1 -   Moderate 
 
- F2-    Severe 
 Until a Kenyan Method of test is available, the freeze/thaw res istance shall be evaluated and declared to the 
provisions valid in the intended place of use of the units. 
 
5.3.8 Water absorption (U units) 
 
5.3.8.1 External element (U units) 
 
For clay masonry units intended to be used in external elements  with the face of the unit being exposed the 
water absorption of a consignment shall be declared by the manu facturer. When clay masonry units are 
sampled from the consignment in accordance with Annex A and tes ted in accordance with DKS 2802-11, the 
mean water absorption of the specified number of clay masonry u nits shall be not greater than the declared 
water absorption. 
 
5.3.8.2 Damp proof courses (U units) 
 The water absorption of clay masonry units intended to be used in damp proof courses shall be declared by 
the manufacturer. When clay masonry units are sampled from a co nsignment in accordance with Annex A and 
tested in accordance with DKS 2802-4, the mean water absorption  of the specified number of clay masonry 
units shall be not greater than th e declared water absorption. 
 NOTE         There is no general relationship between water abs orption and compressive strength or durability. 
 
5.3.9 Initial   rate of water  absorption (U units) 
 
When relevant for the intended use the range of the initial rat e of water absorption of a consignment of clay 
masonry units shall be declared by the manufacturer. If so and when clay masonry units are sampled from the 
consignment in accordance with Annex A and tested in accordance  with DKS 2802-6 using an immersion 
period of (60 ± 2) s the mean ini tial rate of water absorption of the specified number of clay masonry units shall 
fall within the declared range of the initial rate of water abs orption. 
 
5.3.10 Active soluble salts content (U units) 
 
When the intended use of the product only provides limited prot ection (e.g. thin layer of render) or the product 
in the intended use is exposed to weather the content of active  water soluble salts shall be declared by the DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   13 manufacturer on the basis of the categories given in Table 3. W hen clay masonry units are sampled from the 
consignment in accordance with Annex A and tested in accordance  with DKS 2802-3, the content of water 
soluble salts shall not be greater than the declared active sol uble salts content. 
 
Table 3 — Active soluble salts content categories 
 
 
 
NOTE See B.4 for further information.  5.3.11 Moisture movement (U units)  
In countries where there is a requirement for moisture movement , it shall be evaluated and declared to the 
provisions valid in the intended place of use of the units. 
 
5.3.12 Reaction to fire (U units)  
 
For units intended to be used in elements subject to fire requi rements the manufacturer shall declare the 
reaction to fire classification of the masonry unit.  
For masonry units containing s 1,0 % by mass or volume (whichev er is the most onerous) of homogeneously 
distributed organic materials the declaration may be fire Class  A 1   without the need to test. 
 
Masonry units containing > 1,0 %  by mass or volume (whichever is  the most onerous)  of homogeneously 
distributed  organic  materials  shall  be classified  in   acc ordance with  DKS 2804-1 and the  appropriate fire 
classification declared. 
 
NOTE Attention  is  drawn  to  the  Commission  Decision  96/60 3/EC  as  amended  by  the  Commission   
Decision 2000/605/EC in which non-combustible masonry units con taining not more than 1,0  % by mass or  
volume (whichever   is the more onerous) of homogeneously distr ibuted organic materials are classified as 
reaction to fire Class  A1 without testing. 
 
5.3.13 Water vapour permeability (U units) 
 
For units intended to be used in external elements, the manufac turer shall provide information on  the  water 
vapour  permeability through  the water  vapour  diffusion  coe fficient tabulated  values  given   in  DKS 2803 
or determined in accordance with KS ISO 12572. 
 
5.3.14 Bond strength (U units) 
 
5.3.14.1 General (U units) 
 
For clay masonry units intended to be used in elements subjecte d to structural requirements the bond strength 
of the unit in combination with mortar shall be declared in ter ms of the characteristic initial shear strength in 
accordance with DKS 2802-12. The declaration may be made either  on the basis of fixed values as in 5.2.14.2 
or tests as in 5.2.14.3.     The manufacturer shall declare whe ther the value of bond strength has been obtained 
from the fixed values or from test. 
 
NOTE In most cases it is expected that the use of fixed values will be sufficient. 
 
5.3.14.2 Declaration based  o n fixed  values (U units) 
 
When no declaration is made under 5.3.13.3, the characteristic initial shear strength of the unit in combination 
with mortar may be declared by re ference to EN 998-2:2010, Anne x C. 
 
5.3.14.3 Declaration   ba sed  on tests  (U units) 
 
14  © KEBS 2019 — All rights reserved  
The characteristic initial shear strength of the unit in combin ation with one or more specific type of mortar 
specified in accordance with EN 998-2 may be declared based on tests on clay masonry units sampled from 
a consignment in accordance with Annex A and tested in accordan ce with Specification for mortar for masonry 
2802-12. The characteristic initial shear strength shall not be  less than the declared value. 
 
NOTE  Bond strength depends on the mortar, the masonry unit and  the workmanship. 
 
5.3.15 Dangerous substances 
 
National   regulations   on  dangerous  substances  may  requir e verification  and  declaration  on  release,  
and sometimes content,  when const ruction products covered by t his standard are placed on those markets. 
 
In the absence of Kenyan harmonized test methods, verification and declaration on release/content should be 
done taking into account national provisions in the place of us e. 
 
6 Description, designation, designation code and classification  of clay masonry 
units 
 
6.1 Description and designation 
 
6.1.1 P units 
 
A description and designation of a clay masonry unit shall comp rise at least the following: 
 
a) The number and date of issue of this Kenya Standard; 
 
b) Type of unit (see 5.1); 
 
c) Dimensions and tolerances (mean value) (see 5.2.1); 
 
d) Gross dry density and tolerances (see 5.2.3). 
 
e) Compressive strength (see 5.2.4); 
 
When relevant to the uses for which the unit is placed on the m arket, the description and designation shall 
include: 
 
f) Configuration (see 5.2.2); 
 g) Tolerances (range) (see 5.2.1); 
 
h) Net dry density and tolerances (see 5.2.3);  
i)  Thermal properties (see 5.2.5); 
 k) Category of active soluble salts 15) content G (see 5.2.8); 
 
l) Moisture movement and its basis (see 5.2.9); 
 
m)  Reaction to fire (see 5.2.10); 
 n) Water vapour permeability (see 5.2.11   ); 
 
o) Bond strength (see 5.2.12);  
p) Dangerous substances (see 5.2.13).  
 
6.1.2 U units 
 DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   15 A description and designation of a clay masonry unit shall comp rise at least the following: 
 
a) The number and date of issue of this Kenyan Standard; 
 
b) Type of unit (see 5.1); 
 
c) Dimensions and tolerances (mean value) (see 5.3.1); 
 
d) Compressive strength (see 5.3.4); 
 
When relevant to the uses for which the unit is placed on the m arket, the description and designation   shall 
include: 
 
e) Configuration (see 5.3.2); 
 
f) Tolerances (range) (see 5.3.1); 
 
g) Gross and net dry density and tolerances (see 5.3.3); 
 
h) Water absorption (see 5.3.7); 
 
i) Initial rate of water absorption (see 5.3.8); 
 
j) Thermal properties (see 5.3.5); 
 
I) category of active soluble salts content (see 5.3.9); 
 
m) Moisture movement and its basis (see 5.3.10); 
 
n) Reaction to fire (see 5.3.11); 
 
o) Water vapour permeability (see 5.3.12); 
 p) Bond strength (see 5.3.13); 
 
q) Dangerous substances (see 5.3.14).   
 
6.2 Classification 
 Specification of the properties of clay masonry units may be gi ven by reference to classification systems 
provided those systems are based only on single properties incl uded in this standard and do not themselves 
constitute a barrier to trade. 
 This does not remove the requirement that all manufacturers cla iming compliance with this standard shall state 
declared values of the properties of their products when requir ed. 
 
7 Marking 
 The following particulars shall be clearly marked on one of the  following: the units, the packaging, delivery note 
or any certificate supplied with the clay masonry units: 
 
a) Name, trademark, or other means of identification of the man ufacturer of the unit; 
 
b) Means of identifying the clay masonry units and relating the m to their description and designation. 
 
8 Assessment  and verification of constancy of performance(AVCP ) 
 
8.1 General 
  
16  © KEBS 2019 — All rights reserved The  manufacturer shall demonstrate  compliance  for  his  prod uct with  the  requirements  of this Kenya 
Standard and with the declared 15) performance for the product properties by carrying out both: 
 
a) product type  determination (see  8.2), which  can  be physi cal testing,  calculation,  reference to 
tabulated values or combin ations of these methods; 
 
b) Factory production control (see 8.3).  
Alternative methods of test to the reference methods specified in this Kenyan Standard may be adopted except 
for the product type determination tests and in case o' dispute , provided that these alternative methods satisfy 
the following:   
a) A correlation can be shown to exist between the results from  the reference method and those from the 
alternative method; or  
b) A safe relationship can be demonstrated when using the alter native method compared to the reference 
methods and  
c The information on which the relationship is based is availab le. 
 
8.2 Product type determination 
 After completion of the development of a new product type and b efore placing on the market, appropriate 
product type determination shall be carried out to confirm that  the properties predicted from the development 
meet the requirements of this standard and the performance of t he characteristics to be declared for the 
product. 
 Whenever a major change in the source, blend, or nature of raw materials occurs, or when there is a change 
in processing conditions, leading to what the manufacturer cons iders will constitute a new product type being 
produced, the appropriate product type determination shall be r epeated.   
 
The manufacturer may define product groups. The product group m ay differ according to the characteristics in 
question.  
In the product type determination process, a manufacturer may t ake into consideration already existing results. 
 A manufacturer may use the product type determination results d etermined by someone else (e.g.   another 
manufacturer or a research, technology and development service provider) to justify  his   own declaration of 
performance regarding a product that is manufactured according to the same design and with raw materials, 
constituents and manufacturing methods of the same kind, provid ed that permission is  given by the owner of 
the results, and the results are valid for both products. 
 
The tests to be conducted shall be the tests or calculations as  described in Table A.2 for the properties selected 
from the following list relevant to the manufacturer's declarat ion for the product type's intended   use: 
 
a) dimensions; 
 
b) dimensional tolerances, including flatness and plane paralle lism of bed faces; 
 
c) configuration; 
 
d) dry density and tolerances;  
 
e) compressive strength;  
 
f) thermal properties;  
 
g) freeze/thaw resistance;  
 
h) water absorption; 
 DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   17 i) initial rate of water absorption;  
 
j) active soluble  salts content;  
 
k) moisture movement; 
 
l) reaction to fire; 
 
m) Bond strength. 
 
NOTE Sampling for product type determination tests shall be in accordance with Annex A. The results of the 
product type determination shall be recorded.  
 
8.3 Factory production control 
 
8.3.1 General 
 
The manufacturer shall establish, document and maintain a facto ry production control system to enable 
continuing conformity with this Kenyan Standard and the declare d performance of the characteristics of the 
products placed on the market. 
 The factory production control system may consist of procedures  related to the process only (full process 
control and consequently no finished product testing, i.e. 8.3. 6 does not apply), to finished products only 
(consequently no process control, i.e.  8.3.5 does not apply) o r any  combination  of  both.  Consequently 
conformity criteria depend on the individual factory production  procedures. 
 
As appropriate, the responsibility, authority and interrelation  of all personnel who manage, perform and verify 
work affecting the quality of masonry unit products shall be es tablished. 
 
The factory production control system shall describe the contro l procedure of production, the regular checks 
by the manufacturer and his testing, depending on the combinati on of the procedures related to process control 
and/or finished product testing. Controls and tests may include  the characteristics of raw materials and finished 
products, the procedure of production, the production equipment  or the production machines, the test 
equipment or the testing instruments and the marking of the pro duct. 
 
The test results shall be recorded.  
Actions to be taken when the control test values or criteria do  not meet those specified should be documented 
by the manufacturer. 
 
8.3.2 Testing  and measuring equipment 
 
All relevant weighing, measuring and testing equipment, that ha s an influence on the declared values, shall be 
verified and regularly inspected. 
 
8.3.3 Production equipment 
 When the factory production control system includes process con trol procedures, all production equipment, 
that forms part of these procedures and has an influence on the  declared values shall be regularly inspected. 
 8.3.4 Raw materials 
 
As appropriate, the manufacturer shall define the acceptance cr iteria of raw materials and the procedures 
operated to ensure that these are met. 
 
8.3.5 Production process 
 
As appropriate, the relevant features of the production process es shall be defined giving the frequency of the 
manufacturer’s checks together with the required criteria. Acti ons to be taken when the criteria are not achieved 
shall be specified by the manufacturer.  
18  © KEBS 2019 — All rights reserved  
8.3.6 Finished  product  testing 
 
As appropriate, the factory production control system shall inc orporate a sampling plan and the frequency of 
testing of the finished product. The results of sampling and te sting shall be recorded. 
 
The sample shall be representative of the production.  
Depending   on the corrective measure, nonconformities may resu lt in higher frequencies of testing than the 
ones used. 
 
8.3.7 Statistical techniques  
When reasonably practicable and applicable, the results of chec ks and testing shall be interpreted by means 
of statistical techniques, by attributes or by variables, to ve rify the product characteristics and to determine if 
the production conforms to the compliance criteria and the prod uct conforms to the declared values. 
 
NOTE One method of satisfying this conformity criterion is to u se the approach given in ISO 12491. 
 
8.3.8 Marking and stock control of products 
 
The marking and stock control shall be documented. Individual p roducts or/and a defined quantity of products 
(e.g. a consignment of products) should be identifiable and tra ceable. 
 
8.3.9 Traceability 
 
As appropriate, systems of traceability shall be given in the f actory production control system. 
 8.3.10 Non-conforming products 
 
The     procedure for dealing with nonconforming products shall  be documented.  Products that   do not conform 
to the requirements or the performance of the product type shal l be segregated and marked accordingly. 
However, these products may be reassessed by the manufacturer a nd assigned to another product type.  
The manufacturer shall take action to avoid a reoccurrence of t he nonconformity. 
 
  DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   19 Annex A 
(normative) 
 
Sampling for product type determ ination tests and for independe nt testing of 
consignments. 
 
 
A.1 General 
 
This sampling procedure shall apply for product type determinat ion tests and in the event that there is a 
requirement for an assessme nt of product compliance. 
 
Only those characteristics declared by the manufacturer shall b e assessed by this procedure. 
 
The required number of clay masonry units to determine complian ce with the specification shall be sampled 
from a consignment of masonry units not more than 20 m3 (see fi gure A.1).  
 
A.2 Sampling procedure 
 
A.2.1 General 
  
NOTE The choice of the method of sampling will normally be dict ated by the physical form of the consignment in question. 
 
A.2.2   Random sampling 
 
Whenever possible, the  random  sampling  method  shall  be  us ed,  in  which  every  masonry  unit  in  the 
consignment    has an equal chance of being selected for the sa mple. The appropriate number of masonry 
units shall   be selected at random from positions throughout t he consignment without any consideration being 
given to the condition   or qua lity of those selected except th at units damaged in transit shall not be selected. 
 
NOTE In practice, random sampling is  normally  only  convenien t  either  when  the  masonry  units  forming  the consignment are 
being moved in a loose (unpacked) form from one place to anothe r or when they have been split into a large  number of small st acks, e.g. 
on scaffolding awaiting laying. 
 
A.2.3 Representative sampling 
 
A.2.3.1   General 
 When random sampling is impracticable or not convenient, e.g. w hen the masonry units form a large stack or 
stacks with ready access to only a limited number, a representa tive sampling procedure shall be used. 
 
A.2.3.2   Sampli ng from a stack 
 The consignment shall be divided into at least six real or imag inary sections, each of a similar size. An equal 
number of not more than four clay masonry units shall be select ed at random from within each section  in order 
to give the required number without any consideration being giv en to the condition or quality of those selected 
except that units damaged in  tr ansit shall not be selected. 
 
NOTE 1    It will be necessary to remove some sections of the s tack or stacks in order to gain access to masonry units within the body of 
such stacks when taking samples. 
 NOTE 2    Sampling from a stack may not be satisfactory when te sting for active soluble sulfates because contamination from th e ground 
or other sources may occur. 
 A.2.3.3      Sampling from a consignment formed of banded packs  
 
At least six packs shall be selected at random from a consignme nt. The band around one blade or slice in 
each pack shall be removed and an equal number of not more than  four units shall be sampled at random from 
within each of the broken blades or slices in order to give the  required number without any consideration being 
given to the condition or quality of those selected except that  units damaged in transit shall   not be selected. 
  
20  © KEBS 2019 — All rights reserved A.2.4   Dividing the sample 
 
When the sample is to provide clay masonry units for more than one test, the total number shall be collected 
together and then divided by taking masonry units at random fro m within the total sample to form each 
successive sub-sample. 
 
The exact number of clay masonry units required for the test(s)  as given in Table A.1   and Table A.2 shall be 
taken at random from those sampled from the consignment by one of the methods given in A.2.3. 
 
A.2.5 Number of units required for testing 
 
The sample   size for each test shall be in accordance with Tab le A.1. 
 
Table A.1 – Number of units required for a test 
 
Property Clause 
Number Test Method (s)Number of unitsa 
Dimensions  5.2.1 DKS 2802:8 10 
Flatness of bed faces 5.2.1.2.4 DKS 2802:10 3 
Plane parallelism of bed 
faces 5.2.1.2.5 DKS 2802:8 3 
 
Configuration  
5.2.2 DKS 2802:8 
DKS 2802:2 DKS 2802:510 
Gross dry density  5.2.4.1 DKS 2802:7 10 
Net dry density 5.2.4.2 DKS 2802:7 10 
Compressive strength 5.1 and 5.2.5 DKS 2802:1 10 
Thermal resistance 5.2.6 DKS 2803 - 
Water vapour permeability 5.2.13 DKS 2803 or 
KS ISO 12572 
Freeze / thaw resistance 5.2.7 b c 
Water absorption 5.2.8 DKS 2802:11(external 
elements) 
DKS 2802:4(damp proof 
course units)  
Initial rate of water absorption 5.2.9 DKS 2802:6  
Moisture movement 5.2.11 b c 
Active soluble salts content 5.2.10 DKS 2802:3 10 
Reaction to fire 5.2.12 DKS 2804:1 - 
Bond strength 5.2.14 DKS 2802:12 
 
Procedure A 
Procedure BType I 
specimen 
27 
18 Type II 
specimen
18 
12 
Dangerous substances 5.2.15 National provision in place of 
use of unit 
National provisions 
a  if appropriate e.g when the un its are not affected by a test procedure, the same test units may be 
used for different test.  
b  Testing according to provisions valid in the intended place o f use of the units. 
 
c  The used provisions give the number to be tested. 
 
 
 
  DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   21 Annex B 
(informative) 
  
Additional information 
 
B.1 Use of clay masonry units 
 
Clay masonry units are used for a large variety of applications , and each requires performance levels to be 
specified. Some applications are traditional and the related sp ecification is laid down in standards or traditional 
rules for good practice. These rules often have a local charact er, due to climate, building traditions, locally 
available materials, maintenance traditions, etc. Other applica tions might be new and non-traditional, and 
formulation   of the performance levels for materials and execu tion are in that case the responsibility of the 
designer. 
 
Particular applications are:   
a) Common masonry 
 Masonry used outside or inside having no claim to an attractive  appearance.  It may or may not be load bearing. 
 
b) Protected masonry  
Masonry which is protected against water penetration. It can ei ther be masonry in external walls which is 
protected by a suitable layer of render or by a cladding or it can be the inner leaf of a cavity wall or it can be 
an internal wall.  It may or may not be loadbearing. 
 
c) Rendered/plastered masonry 
 Masonry used outside or inside which will be rendered/plastered . It may or may not be loadbearing. 
 
d) Thermal insulating masonry  
Masonry which in itself significantly contributes to the therma l insulation function of an external wall. The 
masonry consists mainly of highly perforated clay masonry units .  It may or may not be loadbearing. 
 
e) Facing  masonry 
 Masonry used outside or inside that is intended to have an attr active appearance. It is constructed from 
attractive masonry units using a standard of workmanship and mo rtar joint finish appropriate to the masonry 
unit type.  It may or may not be loadbearing.  
f) Civil  engineering masonry 
 Masonry used in civil engineering works, e.g.  drainage works, earth  retaining  walls,  etc.,  in which masonry  
units with a high level of durability and compressive strength and a low level of water absorption are sometimes 
used.  g) Structural  masonry 
 
Masonry   used outside or inside that resists loads other than its self-weight. This expression is usually used 
in situations where the structural design of the masonry has be en carried out by an appropriately qualified 
person. Structural masonry may be facing, common, rendered, or civil engineering masonry. Structural 
masonry may also be earthquake resistant.  
In connection with the applicat ions cited above, the masonry ha s such performances as: 
 a) fire resistance;    
22  © KEBS 2019 — All rights reserved  
b) sound insulation;  
 
c) thermal insulation  
Which, if needed, requires additional specifications to be made . 
 Masonry units should be sufficiently durable to resist local ex posure conditions so as to maintain the structural 
and operational integrit y of the building. 
 
B.2 Durability 
 
Kenyan codes of practice have not yet been prepared dealing wit h architectural design and workmanship, 
encompassing the specification and use of masonry units to ensu re that satisfactory durability in service in the 
finished masonry is achieved. Until these codes become availabl e, it has been found necessary to attach this 
annex which relates the grades specified for such properties as  freeze/thaw resistance and soluble sulfates 
content to service conditions, including the degree of exposure  and risk of saturation. 
 
B.3 Freeze/thaw   resistance 
 
B.3.1 General 
 
When making a choice as to which level of freeze/thaw resistanc e of a masonry unit should be specified for 
particular elements of clay masonry, it is necessary to assess the likely degree of exposure to which the units 
are to be subjected, including the protection against saturatio n of the masonry construction. 
 The exposure (severe, moderate, and passive) expresses the risk  of the masonry being exposed to a high 
water content coincident with cycles of freezing and thawing, d ue to local climatic conditions in combination 
with the design of the construction. The factors forming part o f the exposure evaluation are temperature and 
moisture conditions as well as the occurrence of any aggressive  substances.  In the evaluation it is necessary 
to take account of local or national experiences. 
 
The influence of various surface coatings (plastering, painting ) should be evaluated particularly in connection 
with thin layer coatings. A surface coating may result in areas  with typical temperate coastal climates   in 
tightened demands for the construction. 
 The examples given in the following are only indicative to help  the user to choose the appropriate materials 
and are not exhaustive. 
 
B.3.2  Masonry subjected to severe exposure 
 In the following, examples are giv en for masonry or masonry ele ments subjected to severe exposure: 
 
a) unrendered masonry near to external ground level (approximat ely two courses above and  below) 
where saturation with freezing can occur; 
 
b) unrendered parapets where saturation with freezing can occur , e.g.  where the parapet is not provided 
with an effective coping; 
 
c) unrendered external chimney masonry where saturation with fr eezing can occur; 
 
d) cappings, copings, and sills in areas where freezing conditi ons can occur; 
 
e) freestanding boundary and screen walls where saturation with  freezing can occur, for example if the 
wall is not provided with an effective coping; 
 
f) earth retaining walls where saturation with freezing will oc cur for example where the wall has not been 
provided with  an effective copi ng or a water proofing treatmen t on the retaining face. 
  DKS 2801-1:2019  
© KEBS 2019 — All rights reserved   23  
B.3.3  Masonry subjected to moderate exposure 
 
In the following, suitable measur es to prevent saturation of th e masonry are given: 
 
a) protection to wall heads by roof overhangs or copings; 
 
b) projecting throated sills; 
 
c) damp proof courses at the top or base of walls. 
 
B.3.4  Masonry subjected to passive exposure 
 In the following, examples are giv en for masonry or masonry ele ments subjected to passive exposure: 
 
a) masonry in external walls if provided with suitable protecti on, the extent of which depends on climatic 
conditions: in some parts of Europe, experience suggests that a   thick layer of suitable external render 
provides  such a protection; in situations where there  is risk  of wetting accompanied  by freezing, the 
protection should take the form of an impermeable cladding; 
 
b) masonry in internal walls and the inner leaves of cavity wal ls. 
 
B.4 Sulfate action on mortars and plasters 
 
Sulfate attack on masonry mortars is principally caused by the reaction between sulfate in solution and the 
tricalcium aluminate  (C3A) constituent of Portland cement whic h forms calcium sulfoaluminate (or ettringite). 
This reaction only occurs if there is an appreciable C3A conten t, found in ordinary Portland cement. The risk 
is greatly reduced by the use of sulfate resisting cement, wher e the C3A content is limited. 
 
Sulfate  attack  occurs  only  if there  is  a  considerable  a mount  of water  movement  through  the  masonry. 
Diffusion  alone  will  not carry  sufficient  amounts  of  sul fate to the  hydrated cement  in the  mortar. Water 
movement may occur by percolation of water through the masonry under the action of gravity, such as in free 
standing  walls,  or below clay  masonry  unit sills where  eff ective  damp  proof courses  have  not been 
provided. Movement of water may also be brought about by evapor ation and capillary action, for example, 
through   retaining walls which are not waterproofed on the ret aining face, or in external walls between ground 
level and the damp proof course.  
The specification of the level of soluble sulfates content of c lay masonry units with the appropriate grade of 
mortar is a complex matter, whi ch may be dealt with in national  design codes. 
 The risk of saturation can be deduced from the exposure categor ies mentioned in relation to freeze/thaw 
resistance in this annex. 
 The requirement   to declare the active soluble salts content c ategory given  in 5.3.10 is intended to ensure 
that under particular   service conditions, damage to the mason ry units,  mortar or render (if any) will not occur. 
The three  categories given in Table 3    specify the maximum c ontent of water soluble sulfates (those of 
sodium, potassium  and   magnesium) for use in service conditio ns where the risks of saturation differ, ranging 
from expectations  of prolonged saturation  (S2 with  ordinary  Portland cement  in the  mortar or S1   with  
sulfate resisting cement  in the masonry mortar or render under coat), normal exposure to the weather where 
the masonry is protected by the design detailing of the buildin g (S1) to completely dry (SO).  In addition, soluble 
 
Sulfates, especially magnesium sulfate may cause damage to the units themselves, an effect sometimes 
known as "cryptoefflorescence", and for this reason, separate r equirements for magnesium are given. 
 
For masonry completely protected against water penetration, cat egory SO is assumed to be appropriate. That 
means no requirement for the content of active soluble salts is  necessary. 
 
  
  
24  © KEBS 2019 — All rights reserved 8.5 Efflorescence and staining 
 
The appearance of efflorescence in a building is a result of we t masonry drying out and may be due to 
excessive wetting during construction, or due to inadequate pro tection and design detailing, allowing water to 
percolate through parts of the completed construction. In addit ion, soluble materials from the mortar or from 
any adjacent concrete may contribute to the amount of staining and efflorescence observed in practice. 
 
8.6 General Guidance on the appearance of facing clay masonry u nits 
 
The appearance of masonry units and the assessment should be th e subject of purchase contract.  The 
requirement will vary according to the use to which the masonry  units are to be put, and their inherent 
characteristics, e.g. common facing and handmade should be take n into account. As a guide special attention 
should be paid to deep or extensive cracks, damage to edges and  corners, pebbles and to expansive particles 
of lime.