Useful Hints on Use of Cement Mortars in Masonry Work

yogendra borse

        The effect or implication of some simple but important factors which are usually ignored in Project specifications but have impact on the quality of the mortar and masonry job. 

Mortar

Cement mortar, a mixture of cement, sand and water depends for its quality on the properties of the cement sand and water that go into it. If the cement used is fresh and complies with I.S. 260, the sand satisfies the requirements of I.S. 383 and the water is reasonably clean and potable, i.e. the qualities of the mortar and the masonry built with it may be expected to depend essentially on the proportioning, production, placement which includes clean-up, rodding, consolidation and curing.

Proportioning

        Mortar proportion in field is generally based on volumetric batching such as 1:4, 1:6, 1:8 (by vol.) etc. The quantity of mixing water is not usually stipulated. A quantity of water depending on individual choice of workability (and this is usually that of mason) is added. The volume of the sand (the sand is often washed or is in a wet state as excavated from riverbeds) and the frequent change in the bulk density of the material due to bulking are generally neglected. Medium and fine sand bulk by 20% to 30% in the moisture range 3% to 8%. This affects the quality of mortar in two ways. Firstly, sand which is measured by volume is really less in quantity than that in the specified mix and secondly the extra water gets added to the mix. The former would lend to make the mix costly, and the latter would lower the strength. Added to this is the inaccurate (and generally less) quantity of cement. It is generally presumed that a bag of cement (50 kg.) would contain 0.034 cu.m (1.2 cft.) of cement. This is rarely true due to loss of some quantity of cement in transit. Further, when part-bag mixes are prepared, cement also is measured in boxes which can cause variation in the quantity of cement added. This along with other operational inaccuracies in the measurement of materials lead to large variations in quantity of mortar i.e. mix proportion and consistency as well as the quantity (Held) of mortar produced. The intended proportion will then no longer valid and so will be the strength of the mortar.

        It is emphasized that strength of the cement mortars is dependent on the water cement ratio, greater the quantity of water lower will be strength. Moisture in sand can vary, from nil to even 15%. Moisture content in sand can be determined in the field by using a measuring cylinder or a pycnometer (IS-2386-Part-II). In case of volume batching bulk density may be checked and sand content adjusted. About 6% to 8% moisture will be normally present in drained wet sands. A change in moisture content by 2% to 3% is not easily noticeable. 1% increase in moisture can increase slump by one inch or change the water cement ratio by 0.02 to 0.03 making it more sloppy and reduce compressive strength by 1.38-2.07 N/mm². Therefore, it is essential that adequate control is kept over the water cement ratio to ensure this at least the cement should be added by weight (short weight of the bag being made up) and a strict control is exercised over the quantity of mixing water after making due allowance of water already present in the sands. It may be mentioned that a change of 4.5 Kg. in the weight of a bag of cement can affect compressive strength to. the extent of 2.1 N/mm² to 4.2 N/mm² at 28 days and just one litre of water in a one bag batch can cause a change in strength of about 1.38-2.07 N/mm².

        Accuracy of mixing water must be to the nearest 250 c.c. Water meters are usually available with the mixer and may be relied upon for ensuring accurate batching of water. However, these meters also need to be frequently checked and calibrated if necessary.

    Usual practice of adding water by buckets should not be used as no adequate control on mixing water is then possible. Measuring cans used for petrol or oils are the next obvious choice.

Mixing

        To ensure proper control with the mortar mix, machine mixing should be adopted. For small jobs it is preferable to have small size mixers and it would be desirable to have these for almost all works on hand as it ensures uniformity of mixing. A mixing time of about 2 minutes after all the ingredients are loaded into the mixer has to be allowed. Remixing by manual means after unloading may be necessary in some cases, particularly when mortars are lean and the consistency is rather flowy.

        When machine mixing is adopted, cement should be added to the hopper on top of the batch material just before feeding the mixer. It should be noted that cement starts setting on contact with water. The sand being used in such cases is either wet or moist. If cement is added to the sand much earlier than required there is likelihood of the cement gradually setting and hardening to lumps. further if cement is added at the beginning of hold back (Material left out in the hopper) contains much of the cement thus making the batch deficient in cement. Often a full batch including cement, remains in the hopper, when there is lunch break or even a daybreak. Such a batch which contains partially or fully set cement when mixed later produces inferior quality mortar or concrete. Even in hand mixed batches, it is not unusual to see the cement Spread on sand long before the mix is required. This has to be avoided.

        It is always preferable to mix a number of small batches instead of producing a large batch. The quantity of mortar mixed should have direct relation with the number of masons working. When large batches are mixed, consumption may not be quick either due to deficiency in speed of handling or placing or due to breaks in the work. A loss is slump of about an inch within half hour is generally observed. This loss is often made good by working up the mix We. retempering or addition of water to bring up workability. This affects strength of mortar adversely and should be avoided. The loss in slump could be as great as 75 mm in 2 hours, indicating that the cement has set appreciably Disadvantages of using such a mortar by addition of water and working it up will be obvious and should be prohibited.

        In case of lean mortars (1:6 by volume and leaner) machine mixing may not always produce a homogenous mix. In such cases, cement frequently forms into small balls and lumps which may cause segregation of the paste. This could be due to moisture in sand, over-rapid addition of materials and improper or defective arrangement of blades in the mixer. The mortar cubes taken from such batches may show considerable variations in strength within the same batch. This would need to be watched. Remedy for such a situation would be mixing the mortar for a little longer time to ensure thorough mixing. Sometimes hand mixing may be preferred to ensure the homogeneity in such leaner mixes.

Compaction

        Quite often due to improper rodding or due to some deficiency in the mortar, the mortar in the masonry may be porous. Such voids reduce the strength about 5%, voids can bring down strength as much as 30%. Proper placing and compaction of the mortar is equally important as the quality of materials and proportioning of the mixes etc.

        A rough check on quality early in the life of the jobs is to take a sample of the 24 hours old mortar from the masonry and check its density. In properly produced and placed mortars, it may range from 2002.3 to 2162.5 Kg/m³ depending on the specific gravity of the ingredients and mix proportions. If the density as determined is far from the theoretical density (which should include about 5% of entrapped air) the cause of the variation may be investigated, and recurrence avoided. It is also a good practice to check the density of mortar, specimens after 24 hours.

Curing

        The versatility of cement is due to its strength development from the time of its coming in contact with water. For adequate development of strength, presence of moisture is essential, and hydration of cement is hindered if saturation falls below 80%. It is therefore essential to prevent the loss of water from the mortar. This is where the importance of curing comes in. An idea of the effect of inadequate curing can be had from the following table: -

Period of Curing	Strength at end of wet curing	28 days Strength development (Proportionate)
28 days	100	100
First 14 days		92
First 7 days		85
Nil (In Lab air)		60

Note: The remaining period of curing in laboratory air at relative humidity 60% in shade.

        In the field jobs it is rarely that a 60% relative humidity is maintained and as such an improperly cured job in the field would adversely affect the strength.

        Often the cement is used on works after it is stored for a considerable time. The deterioration in quality of cement due to prolonged storage is indicated by loss in strength as given below This deterioration presumes storage under good condition.

Period of Stoarage	% loss in strength
Fresh	Nil
3 months	15-20%
6 months	20-30%
1 year	40-50%

        Cement should therefore be used on work with "first-in-first out" principle i.e. that received first in godown should be used first.

    When cement is improperly stored greater deterioration would result. This deterioration is mainly due to slow hydration of cement due to moisture in the air. Cement is therefore liable to deterioration fast in moist climates. It would therefore be advisable not to store cement across the monsoon period in copious rainfall areas if this is possible. Otherwise, special precautions in the godowns such as lit charcoal stoves during wet days in monsoon will be required to effectively bring down the humidity, normally this would not be practicable. It is good practice to test the cement that is stored over long periods before it is issued on jobs.

Strength

        The strength of specific mortar proportion greatly depends on its consistency. In the normal working conditions, the required consistency may vary with the type of materials used and the method of laying (i.e. hand laid or vibrated). A well proportionated mortar of 75 mm slump (80-85% flow) would give plastic consistency and is well suited for hand laid masonry. For routine jobs however the requirement of consistency is often varied by the mason from the point of view of facility of water for him. He may be prone to add extra water than required for this purpose which should not be permitted.

        When properly mixed, placed and cured, the strength of well-proportioned mortars can be expected to have the following: -

Proportion of Cement to Sand by Volume	28 days compressive strength in N/mm2
1:3	12-20 N/mm2
1:4	8-12 N/mm2
1:5	5-8 N/mm2
1:6	3-5 N/mm2

        The strength of mortar cubes cannot reflect strength of the mortar in the masonry. Obviously, the latter would depend on factors such as extent of compaction, exposure and curing conditions, size effect etc. The test cubes would only reveal the information regarding quality of ingredients used, their potential in strength development under laboratory condition and uniformity in production.

Other Points

        Besides the above, the following points may be kept in mind to ensure good masonry construction.

1) The largest stones should be used in the foundations. Stones should be laid with the broadest sides on bed. Stratified stones should be laid on their natural bed and normal to the direction of load. Porous stones should be soaked before use so that they may not draw water of the mortar. Only clean stones should be used, and cleaning shall be done in an area outside the zone of the placement.

2) Courses should be perpendicular to the direction of load. Walls of masonry should have sufficient lateral bond and should be achieved by the use of plenty of bond stones, headers or through stones at regular intervals. The specifications specify one header or through stone at every 10 Sq.m of area. When thickness of masonry is la larger than 60 mm line of through stones overlapping by at least 150 mm may be provided.

3) The voids and joints in the masonry should be properly filled with mortar using adequate number of spalls and wedge stones of proper sizes. size Two stones should have contact with each other only through a medium of mortar.

4) It is a good practice to check - at random, vacant spaces hollows and camouflaged cover by poking the joints with a bullet pointed steel rod during and after construction of masonry. This check is possible on one hour to 24 hours old masonry, depending on the strength of the mortar. Such a check can bring out the deficiencies at an early stage so that they can be remedied in time. This can save a lot of botherations at a later date.

5) Before the commencement of the day's work, the inspector should lightly knock at random, the stones laid on the previous day with a 5 kg. hammer on the face or top layer of masonry to verify if it has had the adequate bond and proper placement with the rest of the masonry. Those stones giving a dull hollow sound (due to being not properly bedded or being concave, thin or flat) should be got removed. Flat stones easily come off by levering with the mason's rod.

6) Particular care must be taken to ensure cleanup of the area of placement and the general cleanliness in the area. This may be particularly attended to when fresh work for a season is started.

7) Whenever cement pointing is specified, care should be taken to rake out the joints 15-20 mm deep before the mortar had hardened. Once the mortar is hardened, raking is hardly feasible. The joints should be cleaned and wetted with copious water before application of mortar for pointing.