BCDC 2 (283 ) DTZS  
i 
 
DRAFT TANZANIA STANDARD  
Concrete blocks (masonry units)  Specification  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TANZANIA BUREAU OF STANDARDS  
 
©TBS 2022            third  edition  
 
i                  
 
 
This draft Tanzania Standard was published under the authority of the Board of Directors of Tanzania 
Bureau of Standards on yyy -mm-dd. Tanzania Bureau of Standards (TBS) is the statutory national 
standards body for Tanzania established under  the Standards Act No. 3 of 1975, repealed and replaced 
by the Standards Act No. 2 of 2009. The Building and Construction Divisional Standards Committee 
(BCDC), under whose supervision this Tanzania Standard was prepared, consists of representatives 
from t he following organizations:  
*College of Engineering and Technology, University of Dar er Salaam  
 Tanzania Commission for Science and Technology (COSTECH)  
 Ministry of Works and Transportation (MoWT)  
 National Housing Corporation (NHC)  
 Contractors Registration Board (CRB)  
 Ardhi University (ARU)  
 Jeshi la Kujenga Taifa (JKT)  
 *National Estates and Designing Consultancy Company Ltd (NEDCO)  
 Architects and Quantity Surveyors Registration Board (AQRB)  
 Institution of Engineers Tanzania (IE T) 
 *National Construction Council (NCC)  
 Engineers Registration Board (ERB)  
 
The organizations marked with an asterisk (*) in the above list, together with the following were directly 
represented on the Technical Committee entrusted with the preparation o f this draft Tanzania Standard:  
 
Dar es salaam Institute of Technology (DIT)  
Tanzania National Roads Agency (TANROADS)  
Small Industries Development Organization (SIDO)  
Tanzania Bureau of Standards  
P O Box 9524  
Dar es Salaam  
Tel: +255 (22) 2450206/2450949/ 2450298  
Fax: +255 22 2450298  
E-mail: info@tbs.go.tz  
Website: www.tbs.go.tz   
  ii 0.National foreword  
The Tanzania Bureau of Standards is the statutory national standards body for Tanzania, established 
under the act.No.3 of 1975, amended by act.No.2 of 2009  
This Draft  Tanzania Standard is being prepared by the Masonry Technical Committee, under the 
supervision of the Building and Construction Divisional Standards Committee (BCDC)  
This Third  Edition  cancels  and replaces  the second  Edition  (TZS  283:2002 ) which  has been  technically  
revised  
In the preparation of this Tanzania Standard, reference was made to  Indian Standard IS 2185 (Part 1) – 
Concrete Masonry Units - specification. 
  1 1 Scope  
This Tanzania Draft standard specifies requirements of  the following conc rete m asonry building  units 
which are used in the construction  of load-bearing  and partition  walls 
a) Hollow (open and closed cavity) load bearing concrete blocks,  
b) Hollow (open and closed cavity) non -load bearing concrete blocks, and  
c) Solid load -bearing concrete blocks and non -load bearing concrete blocks.  
2 References  
For the purpose of this Tanzania Standard, the following references shall apply:  
TZS 58 (Part 2):  1980, Aggregates - Natural aggregates for Concrete -Specification.  
TZS 62 (Part 4): 1980, Concrete - Methods of testing concrete for strength.  
TZS 727 (Part 1): 2002, Cement - Composition, specifications and conformity criteria for common cements  
TZS 687:2002,    Masonry - Vocabulary.  
TZS 849:2005 , Building limes -   Specification  
TZS 4: 1979, Rounding off numerical values  
3 Terminology  
For the purpose of this Tanzania Standard, the following definition shall apply ; 
3.1 Block -   A concrete masonr y unit, either hollow (open or closed cavity), or solid (other than units used 
for bonding, such as a half block).  
3.2 Block Density -    The density calculated by dividing the mass of a block by the overall volume, 
including holes of cavities and end recesses.  
3.3 Drying Shrinkage -   The difference between the length of specimen which has been immersed in water 
and then subsequently dried to constant length, all under specified conditions; expressed as a 
percentage of the dry length of the specimen.  
3.4 Face Shells -   The two outer plates of the hollow concrete block. These are connected together by 
webs.  
3.5 Gross Area -   The total area occupied by a block on its bedding face, including areas of the cavities 
and end recesses.  
3.6 Height -   The vertical dimension of  the exposed face of a block, excluding any tongue or other device 
designed to provide mechanical keying.  
3.7 Hollow. (Open or Closed Cavity) Block -    A block having one or more large holes or a cavity which 
either pass through the block (open cavity) or do not effectively pass through the block (closed cavity) 
and having the solid material between 50 and 75 percent of the total volume of the block calculated from 
the overall dimensions.  
3.8 Length   -    The horizontal dimension of the exposed face  of a block, excluding any tongue or other 
device designed to provide mechanical keying.  
3.9 Moisture Movement -   The difference between the length of the specimen when dried to constant 
length and when subsequently immersed in water, all under specified conditions, expre ssed as a 
percentage of the dry length of the specimen.  
3.10 Webs -    The solid sections of the hollow concrete blocks which connect the face shells.  
3.11 Width -   The external dimension of a block at the bedding plane, measured at right angles to the length 
and h eight of the block. 
2 3.12 Frog - Depression formed in one or both bed faces of a unit, the total volume of which does not exceed 
25% of the gross volume of the unit.  
4 Requirements  
4.1 Dimensions and Tolerances  
Concrete masonry building units shall be made in sizes and shapes to fit different construction needs. 
They include stretcher, comer, double comer or pier, jamb, header, bull nose, and partition block, and 
concrete floor units.  
4.1.1  Concrete block - Hollow (open or closed cavity) or solid shall be referred to by it s nominal 
dimensions. The term nominal m eans that the dimension exclude  the thickness of the mortar joint.  
Actual dimensions (length and depth only) shall be 10 mm short of the nominal dimensions.  
4.1.2  The nominal dimensions of concrete block shall be as foll ows: 
Length 400, 450,500 or 600 mm . 
Height 100 , 125, 150  or 200 mm .  
Width 50, 75, 100, 150, 200, 230, 250 or 300 mm.  
In addition, block   shall be manufactured in half lengths of 200, 250, 230 or 300 mm to correspond 
to the full lengths. Full length and half -length  U-blocks may also be manufactured for the purposes 
of band and lintels.  
The nominal dimensions of the units are so designed that taking account   of the thickness   or 
mortar joints, they will produce wall lengths and heights which will conform to the pri nciples of 
modular co -ordination.  
4.1.3  Blocks of sizes other than those specified in 4.2.1 may also be used by mutual agreement 
between the purchaser   and the supplier. In the case of special concrete masonry units such as 
wall blocks and ornamental blocks, th e specified sizes may not necessarily apply.  
4.1.4  The variation in the length of the units shall not be more than ± 5 mm and variation in height and 
width of units, not more than ± 3 mm.  
4.1.5  Face shells and webs shall increase -in thickness from the bottom to the to p of the unit. Depending 
upon the core moulds used, 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 i n Table 1.  
Table   1   Minimum Face Shell and  Web Thickness  
S/n Nominal block 
width (mm , min ) 
 Face shell 
thickness  
(mm, min ) Thickness of 
web (mm , 
min) Total  web 
thickness per 
course in any 
200mm length 
of wall.  
i 100 or less          25 25 25 
ii 100-150 25 25 30 
ii 150-200               30 25 30 
iv Over200  35 30 38  
  3 4.1.6  Subject to the tolerances specified in 4.4.1 and provisions of 4.5.1 the faces of masonry units 
shall be flat and rectangular, opposite faces shall be parallel, and all angles shall  be square. The 
bedding surfaces shall be at right angles to the faces of the blocks.  
4.1.7  Blocks with Special Faces  
Blocks with special faces shall be manufactured and supplied as agreed upon between the 
supplier and the purchaser.  
4.2 Classification  
4.3.1  Hollow (Open and Closed Cavity) and solid Concrete Block  
The hollow (open and closed cavity)  and solid  concrete blocks shal l conform to the following four  
grades:  
a) Grad e A - These  are used  as load bearing units  and shall have a minimum  block  density 
of 1500  kg/m3 These  shall be manufactured for minimum  average  compressive  strengths 
of 3.5, 4.5, 5.5, 7.0, 8.5, 10.0, 12.5 and 15.0  N/mm2 respectively  at 28 days  (see Table  
2). 
b) Grade B - These are also used as load bearing units and shall have a block density 
between 1100 kg/m3 and 1500 kg/m3 These shall be manufactured for minimum 
average compressive strengths of 2.0, 3.0 and 5.0 N/mm2 respectively at 28 days (see 
Table 2).  
c) Grade C – These grades of concrete blocks are used as non -bearing units and shall 
have a block density of less than 1500 kg/m3 but not less than 1000 kg/m3. These shall 
be manufactured for minimum average compressive strength of 1.5N/mm2 at 28 days.  
(see table 2)  
d) Solid Concrete Block – Grade D  
The solid concrete blocks are used as load bearing units and shall have a block density 
not less than 1800 kg/m3. These shall be manufactured for minimum average 
compressive strength of 4.0 and 5.0N/mm2 re-spectively (see Table 2).   
4  
Table 2 -compressive strength and Density of concrete block.  
 
Type  Grade  Density of 
block kg/m3 Minimum 
average 
Compressive 
Strength of 
Units N/mm2 Minimum 
Compressive 
Strength of 
Individual Units, 
N/mm2 
Hollow (open and closed 
cavity) load bearing unit  A(3.5)  Not less 
than 1500  3.5 2.8 
A(4.5) 4.5 3.6 
A(5.5) 5.5 4.4 
A(7.0 ) 7.0 5.6 
A(8.5) 8.5 7.0 
A(10.0 ) 10.0 8.0 
A(12 .5) 12.5 10.0 
A(15.0 ) 15.0 12.0 
 B(3.5)  Less than 
1500 but 
not less 
than 1100  
 
  
3.5 2.8 
B(5.0 ) 5.0 4.0 
Hollow (open and closed 
cavity) non-load bearing 
unit C(1.5)  Less than 
1500 but  
not less 
than 1100  1.5 1.2 
Solid load bearing unit  D(5.0)  Not less 
than 1800  5.0 4.0 
D(4.0) 4.0 3.2 
 
4.3 Surface Texture and Finish  
4.3.1  Concrete masonry units can be given a variety of surface textures ranging from a very fine close 
texture to a coarse open texture by the proper selection, grading, and proportioning of aggregates 
at the time of manufacture.  Textures   may also be developed   by treating the face of the units 
while still green by wire brushing or combing, slightly eroding the surface by playin g a fine spray 
of water upon it, and by splitting  (split block).   Colour   may be   introduced by incorporating non -
fading mineral pigments in the facing concrete, or by applying a coloured cement grout or paint to 
the face of the units soon after they are  removed from the moulds. Selected   coloured aggregates   
may also be used in the facing and exposed by washing with water or dilute hydrochloric acid 
followed by thorough washing   with water to ensure no traces of acid are left on the surface.   
  5  
4.3.2  Wall mad e concrete masonry may not require plaster in case of unimportant buildings in low 
rainfall areas; two or three coats of a cement paint being sufficient to render it resistant to rain 
water. If, however, it is intended to plaster concrete masonry, the bloc k shall have a sufficiently 
rough surface to afford a good key to the plaster.  Water proofing admixtures may be used for 
preparing the plaster.  
4.4 Compressive Strength  
The minimum compressive strength at 28 days being the average of eight   units, and the 
minimum compressive strength at 28 days of individual units, when tested in the manner 
described in Annex D shall be as prescribed in Table 2.  Materials . 
4.5 Drying shrinkage  
The average drying shrinkage of units, determined in accordance with 6.7 , shall not exceed 0.06% for normal 
shrinkage units and 0.08% for high shrinkage units . 
4.6 Moisture Movement  
The moisture movement (average of three blocks) shall not exceed 0.09% . 
4.7 Water Absorption  
The water absorption, being the average of three units, when determined in accordance with 6.10  not be 
more than 10 percent by mass.  
4.8 Expansion on re -wetting  
The average expansion on re -wetting, determined in accordance with 6.8 , shall not exceed the actual 
value obtained for drying shrinkage by more than 0.02%  
4.9 Squareness  
In the case of units of overall height exceeding 90 mm, any out of squareness of a unit, determined in 
accordance with 6.5, shall not exceed 2 mm.  
5 Marking  
Each consignment of units shall be accompanied by dispatch or consignment note in which 
the followin g information is given:  
a). The manufacture’s name, trade name or trade mark;  
b). The date of manufacture or related code number;  
c). Whether solid or hollow units;  
d). The work size;  
e). The nominal compressive strength (as given in column 4 of table 2);  
f). The average drying shrinkage;  
g). In the case of face units having a coloured finish, the colour  
 
6 Inspection, sampling and methods of test  
6.1 Inspection  
Inspect  each  unit in the sample  for compliance  with the requirements  of 4.1 and 4.2. 
6 6.2 Sampling test specimen.  
6.2.1  A statistical representative sample of masonry units required for test purposes shall be randomly 
selected to the mutual satisfaction of the manufacturer, supplier and the purchaser.  
6.2.2  The blocks required for carrying out the tests laid down in this standard    shall be taken a sample 
of 20 blocks  shall be taken from every lot/consignment of 5000 blocks or part thereof   from the same 
grade. size   and same batch of manufacture.  
6.2.3  The required   number of blocks shall be taken at regular    intervals    during the loading of the 
vehicle or the   unloading    of the vehicle depending   on whether sample is to be taken before delivery 
or after delivery. When this is not practicable, the sample shall   be taken from the stack in which case 
the required   number of blocks shall be taken at random from across the top of the stacks, the sides 
accessible and from the interior of the. Stacks by opening trenches from the top.  
6.2.4  The sample of blocks shall   be marked for future identification of the consignment it represents. 
The blocks shall be kept under cover and protected from extreme conditions of temperature, relative 
humidity and wind until they are required for test. The tests shall be undertaken as soon as practicable 
after the sample has been taken.  
6.3 Number  of Tests  
6.3.1  All the 20 blocks shall be checked for dimensions and inspected for visual defects.  
6.3.2  Out of the 20 blocks, 3 blocks shall be subjected to the test for block density, 8 blocks to the test 
for compressive strength, 3 blocks to the test for water abs orption, and 3 blocks to the test for drying 
shrinkage and later to the test for moisture movement. The remaining 3 blocks shall be reserved 
for retest for drying shrinkage and moisture movement, if a need arises.  
6.4 Test for dimension  
6.4.1  Apparatus  
6.4.1.1  Overall dime nsions 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 not less than 15 mm and not more than 25 mm in length.  
6.4.2  Specimens  
Twenty full size units shall be measured for length, width and height. Cored units shall also be 
measured for minimum thickness of face shells and webs.  
NOTE -    These specimens shall be used for another test also.  
6.4.3  Measurement and report  
6.4.3.1  Individual measurements of the dimensions of each unit shall be read to the nearest division of 
the scale or caliper and the average recorded.  
 
6.4.3.2  Length shall be measured on the longitudinal center line of each face, width across the top and 
bottom bearing -surfaces at midlength, and height on both faces at midlength.  Face -shell thickness and 
web thickness shall be measured at the thinnest point of each such element -15 mm above the mortar -
bed plane.  Where oppo -site face -shells differ in thickness by less th an 3 mm, their measurements shall 
be averaged. Sash grooves, dummy joints, and similar details shall be disregarded in the measurements.  
 
6.4.3.3  The report shall show the average length, width and height of each specimen, and the minimum 
face shell and web thickn ess and total web thickness in 200 mm length of walling per course as an 
average for the 20 specimens.   
6.5 Test for squareness   
In the case of units of overall height exceeding 90 mm, test both ends and both sides of each unit as follows:     
Use an engineer's square having a blade of length at least equal to the height of the unit and so place the inner 
face of the stock against each bed -face of the unit in tum and at right angles to the face being tested that the 
  7  blade touches the face.  If i t is not in contact with both edges of the face being tested, take the distance (measured 
to the nearest one millimetre) of the blade from the edge furthest from the adjacent to the stock as the out -of- 
squareness of the face.  Check for compliance with th e requirements in 4.9.                              
6.6 Compressive strength test.  
6.6.1  Apparatus  
6.6.1.1  Testing machine  
A testing machine of sufficient capacity for the test and equipped with a means of providing the 
rate of loading of 15 Mpa/min ± 3 MPa/min (as calculated using the area of the bed -face (see 
6.6.2)   of the unit being tested) and in which the bearing faces of both platens are at least as 
large as (and preferably larger than} the bed -faces of the unit being tested, and do not depart from 
plane by more than 0.05mm.  
6.6.1.2  Capping table  
A table or bench that has a non -absorbent surface which is plane to within 0.15mm,  is level in two 
directions at right angles to each other and is sufficiently rigid and so supported as to prevent it 
from being measurably de flected during the capping process.  
6.6.1.3  Measuring device  
A measuring device capable of measuring a reduction in the height of the unit under test to the 
nearest 0.5 mm.  
6.6.2  Area of bed -face 
Measure to the nearest one millimeter the length and width of the bed-faces of each unit,  calculate 
the overall area of each of these faces, and use the smaller of the two areas for the calculation of 
the compressive strength.  
6.6.3  Preparation of test specimens  
6.4.1  Cap each  bed-face of each  unit (see 6.4) in the following  manner:  
 
Mix a quantity  of capping  mortar,  consisting  of one part (by mass) of  Portland  cement  to 
one part  (by mass ) of calcined  gypsum  (plaster  of paris) , with enough  water  to allow  the 
mortar  to spread evenly.  
 
6.4.2  Completely re -fill with capping mortar any frog(s)  in the solid units.  
 
6.4.3  Coat the surface of capping table with appropriate release material (e.g. a film of oil or a sheet of 
absorbent paper) and spread a quantity of capping mortar on the coated surface.   Place one of 
the bed -faces on the mortar and, whil e maintaining the vertical faces of  the unit at right angle s to 
the surface of the table, so press the unit down firmly, with a single movement into the mortar as 
to ensure that the average thickness of the cap does not exceed  5mm.  Trim the extruded morta r 
flush with the vertical faces of the unit. Where the cap covers the cavity or cavities in a hollow 
unit, make several small holes in the cap through which water can drain.  
 
NOTE -   Do not patch caps after they have set, but remove imperfect  caps and rep lace with sound ones.  
 
 
6.4.4  As soon as the cap has hardened sufficiently, slide the unit from the surface of the table 
and repeat the capping process on the opposite side of the unit, ensuring that the two  capped 
faces are parallel to each other by levelling the hardened capped face in two directions (at right 
angles to each other) by means of a spirit level.  As soon as the second cap has hardened 
sufficiently, slide the unit from the surface of t he table, leave it to stand for 2 hours, and then, 
before testi ng, immerse it in clean water  maintained at a temperature of 22°c - 25 °C, for a period 
of at least 24 hours. 
8 6.6.4  Procedure  
6.6.4.1  Test each unit within 5 minutes of the time of its removal from the water.  
 
6.6.4.2  Wipe  clean  the bearing  surface  of the platens of  the testing  machine  and remove  any 
loose  sand or  other  material.  
 
6.6.4.3  So position  the unit in the testing  machine  that it rests  on one capped  face with its 
centre  of gravity over  the centre  of the tower  platen  of the machine .    Do not use packing pieces  
between  units  and the platens.  
 
6.6.4.4  Record, in Ne wtons, the maximum failing load  
 
NOTE -  The maximum  failing  load is reached  when : 
 
a). no greater  load can be sustained  by the unit, or 
b). the height  of the unit is reduced  by 5% of its original  
height , whichever occurs  first . 
6.6.5  calculation.  
6.6.5.1.  Calculate, to the nearest 0.01 MPa, the compressive strength of each unit as follows:  
 
Compressive strength, Mpa  = Failure  load ,N
Area  of load −face ,mm2 
 
Then calculate the average compressive strength , to the same accuracy as the individual  results, as the 
arithmetic mean of the individual values of the 5 or 12 units, as relevant (see table 2).  
 
6.6.5.2.  Record, to the nearest 0.1 MPa, the individual and average compressive strengths of 
the units and check for  compliance  with the relevant  requirement  of 4.4. 
 
6.7 Drying shrinkage.  
6.7.1  Apparatus.  
6.7.1.1  Oven  
A forced -draught drying  oven  capable  of maintaining a temperature of 50°C -55 °C and a relative  
humidity  of 15 % -25%,  and of size such  as to allow  a free air space of width  at least  20 mm around  
each test  specimen . 
 
 
NOTE -    This relative humidity may be maintained by the inclusion  of trays of saturated  calcium  
chloride  solution. The trays  should  provide  a total exposed  area of at least  1m2 per 1m3 of volume  
of oven  and should  contain  sufficient  solid calcium chloride  to show  above  the surface  of the 
solution  throughout  the test. 
6.7.1.2   Gauge  
A gauge capable of measuring the gauge length (see 6.7 .4) of the test specimens to an accuracy of 
0.002 mm  
 
6.7.1.3  Steel balls, of nominal diameter 6 mm  
6.7.1.4  Desiccator  
A desiccator that is large enough to accommodate the test specimens and that contains a saturated 
solution of calcium chloride (see the NOTE in 6.7.1.1 above).   
  9 6.7.2  Preparation of test specimens.  
6.7.2.1.  Cut from each unit (see 6.2) a test specimen of length (paraffel to the length of the unit), 
approximately 200 mm and cross -section at least 70mm x 25 mm.  
 
6.7.2.2.  Using an epoxy resin compound, so fix a steel ball into the centre of each end of each specimen 
that half of the surface of the ball protrudes.  
 
6.7.2.3.  After the resin has set sufficiently for the specimen to be handled, clean the exposed surfaces of 
the steel balls and grease them to prevent corrosion.  
6.7.2.4.  Use the outer extremities of the balls as reference points.  
6.7.3  Procedure.  
6.7.3.1  Immerse the specimens in clean water maintained at temperature of 22oC – 25oC, for a period 
of 4 days . 
6.7.3.2  Remove the specimens from the water and wipe off any excess water with a damp cloth, wipe the 
grease from the steel balls, and immedi ately measure the length between the reference points 
(measurement A).  
6.7.3.3  After measuring the distance, re -grease the steel balls and dry the specimens in the drying oven 
for a period of at least 48 hours. (Do not place wet specimens in the oven together with  partially dried 
specimens).  
6.7.3.4  Remove specimens from the oven, allow them to cool to a temperature of 22°C -25°C in the 
desiccator, wipe the grease from the steel balls and again measure the length between the reference 
points.  
6.7.3.5  Repeat this drying and cooling  procedure (but using drying periods of 24 hours) until the  difference 
between consecutive measurements is less than 0.004 mm  
 
6.7.3.6  Take the final reading as the dry length  (measurement B ). 
 
6.7.3.7  Calculate the drying shrinkage of each specimen (as given in 6.6.4), and record the arithmetic 
mean of the six individual results as the average drying shrinkage of the units and check for 
compliance with the relevant requirement of 4.7.  
 
6.7.4  Calculation.  Calculate the drying shrinkage of each specimen as follows:  
Dryin g shrinkage, % =  measurement  A−measurement  B
gauge  length x 100  
10 Where the gauge length is the drying length (measurement rounded off to the nearest one 
millimetre) minus 12mm  
Record the individual results to the nearest 0.001% and average result to the nearest 0.01% . 
6.8 Expansion on re -wetting test  
Apparatus  
Gauge as in 6.7.1.2  
6.8.1  Test specimens.  
Use the six test specimens previously used in the test for drying shrinkage (see 6.6).  
6.8.2  Procedure.  
After completion of the test f or drying shrinkage given in 6.7 , re-grease the steel balls and immerse the 
test specimens in clean water maintained at a temperature of 22°C -25°C, for a period of 4 days.  
6.8.3  Remove the specimens from the water and wipe off any excess water with a damp cloth, wipe the 
grease from the steel ba lls and immediately measure the length between the reference points 
(measurement A).  Regard this as the final wet length.  
6.8.4  Use the dry length obtained in 6.7 .3   as measurement B.  
6.8.5  calculate the expansion on re -wetting of each specimen (as given in 6.7.4),  and the arithmetic 
mean of the six individual results as the average expansion on re -wetting of the units and check for 
compliance with the requirement of 4.8.  
6.8.6    Calculation  
Calculate the expansion on re -wetting of each specimen as follows:  
 
Expansion on re-wetting, % = mearement  A−measurement  B
gauge  length x 100 
 
where the gauge length is the dry length (measureme nt B rounded off to the nearest  one millimeter) 
minus 12 mm.  
Record the individual results to the nearest 0.001% and the average results to the nearest 0.01%.  
6.9 Method for the determination of block density  
6.9.1  Procedure  
Three blocks taken at random from the samples selected, shall be dried to constant mass in a 
suitable oven heated  to approximately  100oC. After cooling the blocks to room temperature, the 
dimensions of each block shall be measured in centimetres (to the nearest millimetre) and the 
overall volume computed in cubic centimetres.  The block shall then be weighed in kilograms (to 
the nearest  10g) and the density of  each block calculated  as fo llows:  
     
Density  = mass  of block ,in kg
volume  of specimen ,in cm3 ×106 kg/m3 
 
6.9.2  The average for the three blocks shall be taken as the average density.  
 
6.10 Method for the  determination of water absorption  
6.10.1  Apparatus  
6.10.1.1  The balance used shall be sensitive to within 0.5 percent of the mass of the smallest 
specimen tested.  
6.10.1.2  Three full -size   units shall be used. 
  11 6.10.2  Procedure  
 
6.10.2.1  Saturation  
The test specimens shall be  completely immersed in water at  room  ·temperature for 24 h.   The specimens 
shall then be weighed, while  suspended by  a metal wire and completely    submerged    in water. They   
shall   be removed   from the water and  allowed to  drain for  one minute by  placing them on  a 10 mm or  
coarser   wire mesh,  visible  surface water  being  removed with a damp cloth and immediately weighe d.     
6.10.2.2   Drying  
Subsequent to saturation, all specimens shall be dried in a ventilated oven at 100  oC to 115 oC for not 
less than 24 h and until two successive weighing at intervals of 2 h show an increment of loss not 
greater than 0.2 percent of the last previously determined mass of the specimen.  
 
6.10.3  Calculation and report  
Calculate the water absorption as follows:  
 
Water absorption, kg/m3 = 𝐴−𝐵
𝐴 −𝐶 × 1000  
 
Water absorption, percent = 𝐴−𝐵
𝐵 ×100 
 
Where  
A= wet mass of units, in kg;  
B= dry mass of units, in kg; and  
    C= suspended immersed mass of units, in kg.  
 
6.10.4  Report  
Report all results separately for each unit and as the average for the three units.   
12 ANNEX A  
(normative)  
Typical materials recommended for use in the manufacture of concrete masonry 
units . 
 
A.1 Cement   
Cement used in manufacturing blocks shall complying with TZS 727  
A.2  Aggregates    
The aggregates used in the manufacture of blocks at the mixer or the mixing platform shall be clean and free 
from deleterious matter and shall conform to the requirements of TZS 2202 . 
A.3 Water   
The water used in the manufacture of concrete m asonry units shall be free from matter harmful to concrete or 
reinforcement, or matter likely to cause efflorescence in the units.  
A.3 Additives or Admixtures   
Additives or admixtures may be added either as additives to the cement during manufacture, or as  admixtures 
to the concrete mix. Additives or admixtures used in the manufacture of concrete masonry units may be:  
a) accelerating, water reducing, air -entraining and super plasticizer  
b) Waterproofing agents  
c) Colouring pigments.