Civil Engineering Materials-Construction Mortar
Chapter Five Construction Mortar
Catalogue
5.1 Introduction
5.2 The Composites of Mortar
5.3 Technical Property
5.4 Mix Design of Mortar
Basic requirements
Class hour
1 lecture
Contents
Construction mortar
The application and classification of construction mortar
Masonry mortar
Emphasis
Definition of workability and strength of masonry mortar
Index
Influencing factors of strength
Notice
To connect the study of mortar and concrete
To pay attention to the property differences and causes
§5.1 Introduction
It is much easier to lay blocks and bricks with mortar joints than to cut them to fit precisely with dry
joints. Today, mortar is usually made from sand, portland cement, and some additives, but, prior to the late
nineteenth century, mortar usually consisted of sand and lime. Since lime is water-soluble, the mortar joints
were a common source of weakness in stone and brick structures. Gypsum was used as mortar in Egypt,
and naturally occurring bitumen in Mesopotamia. In very important structures, the stone was sometimes
set in molten lead—for example, in the Aya Sofya, the great Byzantine church built in Istanbul in the sixth
century A. D.
Plastering has a similar purpose. It produces smooth surfaces on walls that would otherwise be rough.
Many plastered walls in important buildings were decorated with pictures from the time of Ancient Greece
to the eighteenth century. Fresco painting was done on wet lime plaster, and the paints dried and set with
the plaster.
Modern plaster is usually made with Portland cement, but gypsum plaster and lime plaster were used
until the early year of last century.
Cement mortar consists of one part of Portland cement mixed with three to four parts of sand. The
materials can be mixed on the building site, or the dry materials can be delivered premixed in paper bags.
Portland cement mortar is stronger than masonry cement mortar or cement-lime mortar. When Portland
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cement is used with additives, however, the resulting mortar is harsh and difficult to work. Furthermore,
any cracks that may form are liable to pass through the bricks or blocks instead of following the lines of
the mortar joints.
For this reason, Portland cement is ordinarily used with an additive to make it more plastic (that is,
improve its workability) and to increase its water retention. These goals can be achieved by the addition of
an inert filler to the cement in the factory; the resulting cement is called masonry cement. Masonry
cements are used particularly in the United States.
The alternative is to mix some hydrated lime with the Portland cement mortar. The hydrated lime is
usually added as a dry powder. Cement-lime mortars are particularly favored in Britain and Australia.
Cement is used for plastering both externally and internally. Gypsum is not suitable for exterior work,
and lime is gradually dissolved by rainwater and requires periodic renewal. Portland cement mortar is
therefore the most common material for exterior plastering. A small amount of lime is sometimes added to
produce a ‘fat’, easily workable plaster.
One coat is sufficient for cement plaster, but a second coat containing more cement and less sand than
the first coat is sometimes applied. Neat cement cannot be used for plastering, however, and it is therefore
not possible to obtain the same smooth finish as with gypsum plaster. Nevertheless, cement plaster is
commonly used also for internal work.
Brickwork, like squared masonry, originally relied on the rectangular shape and interlocking bond of the
bricks, the mortar merely taking up the irregularities in the mating surfaces and providing some adhesion.
Since modern mortar based on cement is of similar strength to the bricks themselves the interlocking bond
is of less importance than it was with weaker mortars. Nevertheless, interlocking bonds are still commonly
employed. Some bond patterns are used for decorative as well as structural purposes.
A strong mortar obviously has advantages in a heavily loaded wall. Thin layers of mortar are normally
in compression and, under high stress, are better able to resist being squeezed out from between the bricks,
with consequent damage to the edges of the latter. If there is any tension in the wall caused by wind loads
or eccentric vertical loads, then the tensile capacity of a strong mortar is also advantageous.
On the other hand, a soft mortar allows the wall to accommodate small movements without cracking.
Lime-rich mortar has the capacity to heal small cracks, since the lime never sets hard (it has practically no
tensile strength).
An important property of mortar is the ease with which it can be used by the bricklayer. Lime, or
plasticizers, or both are added to control its consistency so that it is soft enough to allow the brick to be
pressed into place and so that the overflow squeezed out of the joint is cohesive enough to be removed
cleanly with the trowel without marking the face of the wall.
The initial rate of absorption is a property of the brick to absorb water out of
the mortar. Too much absorption makes the mortar stiffen before the brick is
adjusted into position, whereas too little allows the brick to continue to settle
after it has been positioned.
5.1.1 Definition
Mortar is formed by mixing and hardening binding material, fine aggregate,
admixtures and water according to stated ratio.
Construction mortar is regarded as concrete with sand rate of 100%.
5.1.2 Applications
Fig.5.1.1 mortar bonding block material
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Civil Engineering Materials-Construction Mortar
Construction mortar can be used to bonding block material (Fig.5.1.1), jointing material for pipes and
board, surface plastering, anti-corrosion or waterproof treatment.
5.1.3 Classifications
1. Classified by binding material:
Cement Mortar
Lime Mortar
Mix Mortar
2. Classified by applications:
Masonry Mortar
Surface Mortar
Especial Mortar
§5.2 The Composites of Mortar
The composites of mortar do not include coarse aggregate, while other materials are similar to concrete.
1. Cement
(1)Cement types
The six common kinds of cement can all be used in mixing masonry mortar. But for special uses such as
component adapter, structure reinforcement, crack mending, it can use expansion cement.
(2)Strength grade
Low strength grade cement is usually used. However, right amount of mixed material can be added
when the grade is too high.
2. Sand
Sand used in mortar is different from the sand of concrete.
(1) Limitations for maximum of sand particle diameter
①Rubble masonry: Dmax<1/4～1/5 thickness of mortar layer
②Brickwork: medium sand Dmax≯2.5mm
③Smooth flour-milling jointing: fine sand
(2)Limitations for clay content: In order to ensure mortar quality, especially high strength mortar，the
following limitations for clay quantity of sand needed to be noticed:
①Impurity quantity of clay: 5%
②Impurity quantity of clay: 10%
Furthermore, aggregate can be formed with manufactured sand, hilly sand,
furnace slag and so on. The technical index of mortar can be fixed according
to experience or experiment.
3. Lime
In order to save cement and improve workability, proper amount of lime
or clay can be added. But Lime must be made into lime paste (consistency
120mm) first, but quicklime powder or slaked lime powder can sometimes
be straightly used.
§5.3 Technical Property
Fig.5.3.1 consistometer of mortar
Technical properties of mortar: mobility and water retentivity of fresh mortar; strength of hardened
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Civil Engineering Materials-Construction Mortar
mortar; durability. In this chapter, we will mainly introduce mobility (consistency), water retentivity,
strength.
5.3.1 Mobility (Consistency)
1. Definition
Mobility is the property of flowing of fresh mortar under the influence of gravitation and by the external
force.
2. Influencing factors
It is similar to effective factors of concrete workability and it is related to water dosage, binding material
quantity and property of aggregate.
3. Testing
(1) Index: the sinking degree
(2) Apparatus: consistometer of mortar (Fig.5.3.1)
(3) Method
Put the fresh mortar in taper canister evenly under conoid, turn zero, and loosen setscrew, then the
conoid will sink into mortar in 10 seconds. The sinking depth of conoid is the sinking degree. The deeper it
is, the greater the mobility is.
4. Choices of mobility
(1) Kinds of masonry
(2) Construction condition
(3) Climate condition
Choices of the mobility of mortar refers to Tab.5.3.1. E.g.: porous masonry, xerothermic climate, great
mobility of mortar
Table 5.3.1 Mobility of Construction Mortar (the sinking degree: mm)
Kind of Masonry
Dry climate
Cold climate
Plaster project
Mechanical
Construction
Manual
operation
Fired normal
brickwork
80-90
80-90
The base
80-90
110-120
Stonework
70-80
70-80
Substrate
70-80
70-80
Normal concrete
hollow block
40-50
40-50
Face-layer
70-80
90-100
Lightweight
aggregate concrete
block
30-40
30-40
Plaster-slurry
face-layer
－
90-120
5.3.2 Water Retentivity
1. Definition
Water retentivity is the ability of keeping water, or the inseparability of the composites of flesh mortar.
Mortar with poor water retentivity can easily bleed, laminate, or segregate, resulting in bonding
improperly and influencing the normal hardening as well as decreasing the strength of mortar.
2. Influencing factors
(1) Type and quantity of binding material
(2) Property of aggregate
(3) Water quantity
(4) Admixture, etc.
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3. Testing
Index: layering degree
Apparatus: Apparatus of mortar layering degree
Methods: Put fresh mortar (the sinking degree 1) in layering degree cylinder, stew 30min,get rid of the
top 2/3 mortar, then gauge the sinking degree of surplus mortar (the sinking degree 2)
Layering degree= the sinking degree 1-the sinking degree 2
4. Evaluation of water retentivity
Evaluation of water retentivity is shown inTab.5.3.2.
Tab.5.3.2 Evaluation of Water Retentivity
Layering degree
(mm)
Water retentivity
<10
Rich water retentivity, excess of binding material, extra-fine sand, great
shrinkage, useless
>30
Poor water retentivity, easy segregation, useless
10～20
Good
5.3.3 Strength
In order to assure the strength of whole structure, there are certain requirements for the strength of
masonry mortar of foundation, walls, columns etc.
1.Compressive strength average f2（design strength）and strength grade:
(1) Compressive strength average f（design
strength）: The pressure of cube specimen (the length of 70.7),
2
cured in standard condition, aged of 28, has 85% strength guaranty.
Notice: the standard condition of hydraulic binding material: 24± 5℃, Rh>90% curing 28d in humidity,
20±3℃, humidity Rn=60～80%.
(2) Strength grade: According to f2, mortar can be divided to 6 strength grades: M2.5、M5、M7.5、M10、
M15 和 M20.
(3) Choice of grade: Consider the force in size, environment and importance of project more. Common
grades are M2.5～M10, and M10 for significant structure.
(4) Stirring meter of mortar is shown in Fig.5.3.2.
Fig.5.3.2 stirring meter of mortar
2. Strength formula and influencing factors
(1)The major differences between the influencing factors of concrete are:
① lacking coarse aggregate;
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② Setting, hardening and the process of increasing strength are influenced by hygrophanous beds, that is
to say, different mortar strengths are for different beds.
(2)Strength formula
①dense substrate ( imhygrophanous beds, e.g.: stone)
Like concrete, strength lies on cement strength and W/C
Formula:
fmo=Afce(C/W－B)
fce- 28d survey compressive strength of cement (N/mm2 or Mpa)
fmo - 28 day strength of mortar (N/mm2 or Mpa)
A,B-empirical coefficient,(according to testing statistic data).
C/W-water cement ratio
②Porous substrate (hygrophanous beds)
Formula:
fmo=A·fce·Qc/1000+B
Qc-cement quantity in mortar (Kg/m³)
fce-survey cement strength
A,B-empirical coefficient
③Empirical coefficient A,B can be chosen from Tab.5.3.2.
Tab.5.3.2
Empirical coefficient A, B
A
B
Cement Mix Mortar
1.50
-4.25
Cement Mortar
1.03
3.5
④Explanation
Mortar strength formula is much immature, changes caused by constructional materials quantity,
material properties in different areas, mixing methods, and can be modified according to local experience
and data.
5.3.4 Durability
The adhesion between mortar and substrate is good with small shrinkage distortion. Thus good
durability is produced.
When freezing and thawing, mortar also needs to have certain frost resistance.
Masonry mortar with request for the number of freezing and thawing, quality loss rate is no more than
5% and compressive strength is no more than 25%.
§5.4 Mix Design of Mortar
There are two methods to design mix of mortar: consult table–based on experience; calculation–consult
with JGJ98-2000.
1.Calculate steps
(1) Trial Strength
fm,o=fm,k+0.645σ
fmo- trial strength of mortar
fmk- design strength grade of mortar
σ- Strength standard deviation of Mortar
(2) Cement Quantity:
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QC=1000（fm,o-β）/αfce
Qc—Cement quantity of mortar per cubic metre, with accuracy of 1kg
fce—Survey strength of cement, with accuracy of 0.1MPa
α、β－α＝3.03, β ＝-15.09
(3)Fix admixture quantity
Workability of mortar is good when cement quantity >350 while the water retentivity is poor when the
cement quantity is low; thus admixtures are needed
QD= QA-QC
QD－admixture quantity of mortar per cubic metre, with accuracy of 1kg.
QC－cement quantity of mortar per cubic metre, with accuracy of 1kg.
QA－cement quantity of mortar and admixture quantity of mortar per cubic metre, better between 300～
500
(4) Fix sand quantity (fix the below table)
Tab.5.4.1 Fix quantity of sand
water quantity of sand
1～3
0
>3
Mortar sand quantity per cubic meter
volume (m3)
1
0.92
1.1～1.2
weight (kg)
ρos×1
ρos×0.92
(1.1～1.2)ρos
ρos- apparent density of Sand
(5) Fix the water quantity
The water quantity is fixed by experiments according to the sinking degree.
Explanation: The above calculated mix ratio is fixed by adaptation of workability and strength.
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