Properties of Mortar
Properties of mortar which are sought for use in masonry are: workability, water retentivity, rate of stiffening, strength, resistance to rain penetration and durability. These properties have been discussed below explaining their effect on masonry. Choice of masonry mortar is governed by several considerations such as
- Type of masonry unit and its properties,
- Degree of exposure to weather and environments,
- Strength requirements, etc.
Workability is the property of mortar which enables it to be spread and applied to masonry unit with ease. It also facilitates proper filling of joints in masonry. A good mortar would hang from the trowel and will flow down readily when lightly jerked. This property of mortar depends on properties of various ingredients used for making mortar and on the method of mixing adopted.
As a general rule, a mud mortar prepared from fine clay soil has better workability than one prepared from sandy soil and a lime mortar has a better workability than cement mortar.
Lime when used in the form of putty, gives better workability than when used in dry hydrated form. Also grinding of lime mortar in a mortar mill, results in improved workability.
When using dry hydrated lime in mortar, it is desirable to soak lime in water before mixing with sand in order to improve its workability. When mortar is made by mixing dry hydrated lime and sand without pre-soaking of lime, workability can be improved somewhat, by keeping the mixed mortar in a covered heap for about 12 hours before use. This process, known as maturing, allows particles to swell up time to time.
A mortar made from well graded sand has better workability than one made from ungraded sand.
Cement mortar made with ungraded coarse sand has poor workability, particularly when mix is lean and sand used is angular. Workability of such a mortar can be improved by either adding lime or chemicals known as plasticizers.
To some extent workability depends upon consistency of mortar which is measured by recording depth of penetration of a standard cone as detailed in IS 2250 : 1981. That Standard recommends following values of depth of penetration for different purposes:
For laying walls with solid bricks – – 90-130 mm
For laying perforated bricks — 70-80 mm
For filling cavities — 130-150 mm
As a general principle, when joints are thin or units have high suction, consistency should be more and when units are heavy and have low suction, consistency should be less.
A good craftsman adjusts the consistency of mortar by varying the quantity of water through his experience.
Composite cement-lime mortars are well known for their good workability and have some other desirable properties.
2. Water Retentivity
Most of the masonry units have normally appreciable suction, depending on their porosity and moisture content and they begin to suck moisture from mortar as soon as these come in contact with mortar. If units draw out too much moisture from the mortar rapidly, the latter is unable to gain adequate strength, when gain of strength is dependent on the process of hydration in mortar. Thus, when binder used is Portland cement or hydraulic lime, it is necessary that mortar should not part with its moisture readily by suction-that is mortar should have good water retentivity.
As a general rule, lime mortar and cement-lime mortar have good water retentivity while plain cement mortar made with coarse ungraded sand has low water retentivity.
Water retentivity of cement mortar is improved by the addition of hydrated lime or finely ground limestone or chemical compounds known as plasticizers. Generally speaking, mortars having good workability have also good water retentivity.
A standard test for determination of water retentivity in masonry mortars is given in IS 2250 : 1981. In accordance with that standard, water retentivity of masonry mortar should not be less than 70 percent. It may be clarified that property of water retentivity in masonry mortars is important mainly when masonry units have high rate of suction-as for example, common burnt clay brick and concrete block. In case of engineering brick and hard stone, which have low suction, high water retentivity of mortar does not have much advantage. In case of common brick which has water absorption of about 20 percent, suction rate of units is reduced by pre-soaking or pre-wetting of the units.
In case of concrete blocks and such other units, which have very high shrinkage rate, pre-soaking or prolonged pre-wetting is likely to result in extensive cracking of masonry due to drying shrinkage and therefore pre-wetting has to be done on a restricted scale and a mortar with high water retentivity (85 percent or more) should be chosen for such masonry.
3. Rate of Stiffening
Stiffening of mortar in masonry is caused either by loss of moisture or by the setting action of binder used in the mortar or by both. Most of the moisture lost is absorbed into the masonry unit but some evaporates into the atmosphere. A mud mortar stiffens only by loss of moisture and there is no setting action of its clay. A lime-sand mortar made from non-hydraulic lime (limes of grade C and D) also stiffens in early stages by loss of moisture but it has also very mild and slow setting action due to carbonation. A cement mortar stiffens mainly through setting action of cement.
Behaviour of a cement-lime mortar is in-between that of lime mortar and cement mortar. It is necessary that mortar should have sufficiently high rate of initial stiffening so that construction work could proceed at a reasonable pace.
If rate of stiffening is too low, mortar, due to its plasticity will get compressed and squeezed out, as the work proceeds, due to self-load of masonry, thus resulting in variation in thickness of joints and distortion of masonry.
On the other hand, if rate of stiffening is too rapid, it will result in cracking masonry as the unavoidable shrinkage in units due to drying and slight settlements in foundation due to loads, cannot then be accommodated within the mortar joints.
In cold regions, when nights are frosty, it is important that mortar should stiffen rapidly enough so that it is not damaged by frost by formation of ice crystals within the body of mortar. For this reason, as a general rule, cement mortar should not be leaner than 1:5 and cement lime mortar leaner than 1: 1/2 : 4.5.
In addition, some further precautions like preventing masonry units, sand and water from getting too cold, use of warm water for mixing of mortar, use of calcium chloride as an accelerator in cement mortar, covering the freshly laid masonry with tarpaulins at the close of the day’s work, etc. should be taken.
A mortar gains strength, to a small extent by loss of moisture that is by drying action as in mud mortar and non-hydraulic lime mortar, but mainly by setting action of its cementitious content, namely lime and cement. In case of lime mortar made from non-hydraulic lime, which sets through carbonation, gain of strength is very slow.
In case of cement mortar or lime mortar made from hydraulic lime, gain of strength is due to hydration and is comparatively rapid.
From structural considerations it is necessary that masonry should attain the requisite strength by the time loads are imposed on it. With that in view, 28-day strength of a mortar is taken into consideration.
As stated earlier, mud mortar stiffens only by loss of moisture and its constituents, namely soil has no setting action. It softens again on absorbing moisture and is easily eroded by rain. It has, therefore, very low strength and poor durability. For this reason, mud mortar is considered suitable only for use in superstructure of temporary or semi-permanent buildings with very light loadings. When mud mortar is used in brick or stone masonry, basic stress in masonry should be limited to 0.2 N/mm2 and to prevent erosion due to rain, external face of walls should be protected either by lime/cement pointing or some form of non-erodible plaster. Mud mortar should not be used in moist or wet situations for example foundations of a wall. This mortar is also not suitable for use in areas infected with white ants.
Strength of masonry depends on strength of mortar. It is however, to be kept in mind that undue importance should not be attached to strength of mortar at the cost of other properties of mortar.
Mortar need not, therefore, be stronger than what is necessary from consideration of strength of masonry, and it should possess other desirable properties. High strength mortar has an advantage only in case of high strength units, and heavy loads.
Ordinarily in buildings designed as per provisions of Design Standard Codes, slenderness ratio of load bearing elements is restricted so that due to over-loading, failure of masonry would take place by tensile splitting of masonry and not by buckling. Therefore, bond between mortar and masonry is more important than compressive strength of mortar. Use of composite cement-lime mortar, because of its better bond strength, gives a stronger masonry than that with plain cement mortar, even though plain cement mortar may have higher compressive -strength.
5. Resistance to Rain Penetration
Rain water penetrates a masonry wall by three different modes, namely:
- Through pores of masonry units,
- Through pores of mortar, and
- Through cracks between units and mortar.
It has been found that rain penetration through units and mortar is not very significant and main source of rain penetration is through cracks in masonry.
Moreover, rain penetration is much more through wide cracks, even if few in number, than through thin cracks which may be more in number.
These cracks are mostly caused by shrinkage of units and mortar on drying, thermal movement of units and mortar and inevitable slight settlement to which every building is subject. Thus, from the view point of rain penetration, bonding property of mortar is of great importance. It has been observed that if mortar is not very strong, if it gains strength slowly, and if it has good bond with units, movement of units due to shrinkage, temperature variations and settlement of foundation get accommodated to a great extent within the mortar and cracks are, therefore, thin and evenly distributed. As a result, masonry has much better resistance to rain penetration.
A composite cement-lime mortar possesses practically all the above mentioned desirable qualities. In this mortar relative proportion of cement and lime is varied to suit the strength requirement of masonry and shrinkage coefficient of units.
For units having high shrinkage for example, concrete block, lime content should be ample. Mixes of composite mortars in common use are 1 : 1/2 : 4.5, 1 : 1 : 6, 1 : 2 : 9 and 1 : 3 : 12. Of these 4 mixes 1: 1 : 6 mix is in more common use since it has reasonably good strength and also, imparts to masonry, adequate resistance to rain penetration.
Deterioration in mortar takes place due to:
- Frost action before the mortar has gained sufficient strength, and repeated cycles of freezing and thawing,
- Prolonged chemical action between soluble sulphates present either in burnt clay bricks or in soil in contact with masonry in foundation, and
- Ingress of moisture through cracks into the body of the masonry and consequent repeated cycles of wetting and drying over a number of years and crystallization of salts.
For protection against frost damage, and repeated cycles of freezing and thawing, it is necessary that mortar should gain strength rapidly, it should be dense and should have good ultimate strength. lt should therefore, contain adequate proportion of portland cement, and sand should be well graded.
Since lime mortar is slow in setting, and does not have much ultimate strength, its use is not suited when there is early frost hazard or when masonry is likely to be subjected to repeated cycles of freezing and thawing.
Use of an air-entraining admixture in cement mortar 1 : 5 or 1: 6 considerably improves its resistance to frost action and repeated cycles of freezing and thawing.
For protection against sulphate attack, a rich cement mortar (1 : 4 mix or better) or composite cement-lime mortar 1 : 1/2: 4.5 using ordinary portland cement should be used when only moderate protection is needed and rich cement mortar (1 : 4 or better) with sulphate resisting cement should be provided when sulphate attack is expected to be severe.
It should be borne in mind that if masonry in any situation remains generally dry, sulphates, even if present in brick or sand in excessive quantity, cannot cause much damage.
To ensure durability of mortar against weathering action due to repeated cycles of wetting and drying of masonry (for example, parapets) in exposed situations, mortar should be dense and moderately strong. For this mortar should be either 1 cement: 5 sand or 1 cement: 1 lime: 6 sand using well graded sand.
Further, properties of mortar should match the type of unit used in masonry, so that there are no wide cracks in masonry. For example, when using units having high shrinkage, such as concrete blocks, cement-lime mortar should be used since this mortar, being slow in gaining strength, permits volumetric changes in units within the mortar joints without occurrence of wide cracks and has good resistance to rain penetration. Use of some air-entraining admixture in cement mortar also improves its durability quite appreciably.
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