Cement Mortar for masonry - materials, mixing and technical details - CIVIL ENGINEERING OBJECTIVE

Cement Mortar for masonry – materials, mixing and technical details

Cement Mortars
Desirable properties of mortars for use in masonry are:
(a)        Workability
(b)        Water retentivity
(c)        Rate of stiffing
(d)        Strength
(e)        Resistance to rain penetration
(f)        Durability


MATERIALS
Water
  • Water used for mixing and curing shall be clean and free from injurious quantities of alkalies, acids, oils, salts, sugar, organic materials, vegetable growth or other substance that may be deleterious to bricks, stone, concrete or steel. potable water is generally considered satisfactory for mixing. The Ph value of water shall be not less than 6. The following concentrations represent the maximum permissible values: (of deleterious materials in water).
  • The physical and chemical properties of ground water shall be tested along with soil investigation and if the water is not found conforming to the requirements of IS 456-2000, the tender documents shall clearly specify that the contractor has to arrange good quality water for construction indicating the source.
  • Water found satisfactory for mixing is also suitable for curing. However, water used for curing shall not produce any objectionable stain or unsightly deposit on the surface.
  • Sea water shall not be used for mixing or curing
  • Water from each source shall be tested before the commencement of the work and thereafter once in every three months till the completion of the work. In case of ground water, testing shall also be done for different points of drawdown. Water from each source shall be got tested during the dry season before monsoon and again after monsoon.
Cement
The following cements are used for preparing the Mortars
(a)        33 grade ordinary Portland cement conforming to IS 269.
(b)        43 grade ordinary Portland cement conforming to IS 8112.
(c)        53 grade ordinary Portland cement conforming to IS 12269.
(d)        Rapid hardening Portland cement conforming to IS 8041.
(e)        Portland slag cement conforming to IS 455.
(f)        Portland Pozzolana cement (flyash based) conforming to IS 1489 (Part 1).
(g)        Portland Pozzolona cement (calcined clay based) conforming to IS 1489 (part 2).
(h)        Hydrophobic cement conforming to IS 8043
(i)         Low heat Portland cement conforming to IS 12600.
(j)         Sulphate resisting Portland cement conforming to IS 12330
(k)        White cement conforming to IS 8042
Compressive Strength: Compressive strength requirement of each type of cement for various grades when tested in accordance with IS 4031 (part 6) shall be as under:
Sample
Strength in N/mm2 not less than for
Age at testing
Gr. 33
Gr.43
Gr. 53
72 + 1 hr
16
23
27
168 + 2 hrs
22
33
37
672 + 4 hrs
33
43
53


Setting Time: Setting time of cement of any type of any grade when tested by Vicat apparatus method described in IS 4031 shall conform to the following requirement:
(a)        Initial setting time: Not less than 30 minutes
(b)        Final setting time: Not more than 600 minutes
Stacking and Storage: Cement in bags shall be stored and stacked in a shed which is dry, leak proof and as moisture-proof as possible. Flooring of the shed shall consist of the two layers of dry bricks laid on well consolidated earth to avoid contact of cement bags with the floor. Stacking shall be done about 150 to 200 mm clear above the floor using wooden planks. Cement bags shall be stacked at least 450 mm clear off the walls and in rows of two bags leaving in a space of at least 600 mm between two consecutive rows. In each row the cement bags shall be kept close together so as to reduce air circulation. Stacking shall not be more than 10 bags high to avoid lumping under pressure. In stacks more than 8 bags high, the cement bags shall be arranged in header and stretcher fashion i.e. alternately lengthwise and crosswise so as to tie the stacks together and minimize the danger of toppling over.
Different types of cement shall be stacked and stored separately.
Cement bags shall be stacked in a manner to facilitate their removal and use in the order in which they are received.
For extra safety during monsoon, or when cement is expected to be stored for an unusually long period, each stack shall be completely enclosed by a water proofing membrane, such as polyethylene, which shall cover the top of the stack. Care shall be taken to see that the water proofing membrane is not damaged at any time during use.
Fine Aggregate
Aggregate most of which passes through 4.75 mm IS sieve is known as fine aggregate. Fine aggregate shall consist of natural sand, crushed stone sand, crushed gravel sand stone dust or marble dust, fly ash and broken brick (Burnt clay). It shall be hard, durable, chemically inert, clean and free from adherent coatings, organic matter etc. and shall not contain any appreciable amount of clay balls or pellets and harmful impurities e.g. iron pyrites, alkalies, salts, coal, mica, shale or similar laminated materials in such form or in such quantities as to cause corrosion of metal or affect adversely the hardening, the strength, the durability or the appearance of mortar, plaster or concrete. The sum of the percentages of all deleterious material shall not exceed 5%.
Silt Content: The maximum quantity of silt in sand as determined by the method prescribed shall not exceed 8%.
Fine aggregate containing more than allowable percentage of silt shall be washed as many times as directed by Engineer-in-charge so as to bring the silt content within allowable limits for which nothing extra shall be paid.
Grading: On the basis of particle size, fine aggregate is graded in to four zones. The grading when determined in accordance with the procedure prescribed in Appendix ‘B’ of Chapter 3 shall be within the limits given in Table 3.1 below. Where the grading falls outside the limits of any particular grading zone of sieves, other than 600 micron IS sieve, by a total amount not exceeding 5 per cent, it shall be regarded as falling within that grading zone.
Fine Aggregates
IS Sieve
Percentage passing for
Grading Zone I
Grading Zone II
Grading Zone III
Grading Zone IV
10 mm
100
100
100
100
4.75 mm
90-100
90-100
90-100
95-100
2.36 mm
60-95
75-100
85-100
95-100
1.18 mm
30-70
55-90
75-100
90-100
600 microns
15-34
35-59
60-79
80-100
300 microns
5-20
8-30
12-40
15-50
150 microns
0-10
0-10
0-10
0-15
PREPARATION OF MORTARS AND ITS GRADE
Grade of Masonry Mortar
The grade of masonry mortar will be defined by its compressive strength in N/mm2 at the age of 28 days as determined by the standard procedure detailed in IS 2250.
For proportioning the ingredients by volume, the conversion of weight into volume shall be made on the following basis:
(a) Burnt Clay Pozzolana        860 Kg/cum
(b) Coarse Sand (dry) 1280 kg/cum
(c) Fine sand (dry)      1600 kg/ cum
(d) Fly Ash      590 kg/ cum
Proportioning: Proportioning on weight basis shall be preferred taking into account specific gravity of sand and moisture content. Boxes of suitable size shall be prepared to facilitate proportioning on weight basis. Cement bag weighting 50 kg shall be taken as 0.035 cubic meter. Other ingredients in specified proportion shall be measured using boxes of size 40 x 35 x 25 cm. Sand shall be measured on the basis of its dry volume in the case of volumetric proportioning.
Mixing
 (a) Mechanical Mixing: Cement and sand in the specified proportions shall be mixed dry thoroughly in a mixer. Water shall then be added gradually and wet mixing continued for at least three minutes. only the required quantity of water shall be added which will produce mortar of workable consistency but not stiff paste. Only the quantity of mortar, which can be used within 30 minutes of its mixing shall be prepared at a time. Mixer shall be cleaned with water each time before suspending the work.
(b) Hand Mixing: The measured quantity of sand shall be leveled on a clean masonry platform and cement bags emptied on top. The cement and sand shall be thoroughly mixed dry by being turned over and over, backwards and forwards, several times till the mixture is of a uniform color. The quantity of dry mix which can be used within 30 minutes shall then be mixed in a masonry trough with just sufficient quantity of water to bring the mortar to a stiff paste of necessary working consistency.
Precautions: mortar shall be used as soon as possible after mixing and before it begins to set, and in any case within half hour, after the water is added to the dry mixture.