Tuesday, March 25, 2014

Trowel Works 01-Mortar and Concrete

Mortar is the material used in bedding, jointing and pointing bricks and blocks in masonry walling

Constituents of mortar - Mortar is composed of:-
·         Binding material (cement, lime, clay, gypsum, plaster of Paris)
·         Fine aggregate
·         Water

Additives are sometimes mixed to mortar to improve its properties.

Other forms - Fine aggregates mixed with binding material and water also forms some important building finishes such as Plaster, Rendering.

Materials for mortar - General purpose mortar can consist of Sand, Ordinary Portland cement, Water, Plasticizer, Retarder, Accelerator, Pigments or coloring agents and Hydrated lime. Hydrated lime was once commonly used as a binder for mortar but it is rarely used today.

Requirements of a mortar mix – A good mortar mix should have:
·         Adequate compressive strength.
·         Adequate bond strength between mortar and bricks.
·         Durability – resistance to frost and chemical attack.
·         Joints sealed against wind-driven rain.
·         An attractive appearance.
The ability of the mortar to meet these requirements will depend upon the Materials specified for the mix, Workmanship of the bricklayer, Protection of the materials and brickwork against adverse weather.
Mix Proportions - Before mortar can be mixed, the ingredients have to be measured in their correct proportions. The ingredients can be measured by:  Volume or by Weight. Volume mixing can be carried out by hand or by machine, while weight mixing can only be carried out by machine.
Methods of Volume mixing
Materials calculated by volume should be accurately gauged or measured into the correct quantities for each specified mix. The methods used are- Measuring by shovel, Measuring by bucket (or pan) and Measuring by gauge box
Standard Gauge Box

The internal dimensions of a gauge box should be:-
400 x 350 x 250 mm = 0.035 m³
0.035 m³ is the volume of a bag of cement weighing 50 kg.
Mixing by Weight
By using a weight batch mixer, the weight of the aggregate is recorded as it is shoveled into the hopper. This is a more accurate method of batching materials than any of the previously described methods. The materials can be loaded into the hopper while the previously batched materials are being mixed. The weight of materials can be read on a dial by the operator who controls the weight, based on previous calculations for the constituent weights for the required mix. Water is added in liters (1 liter weighing 1kg), after calculating the requirement.
Mixing time - Mixing time depends on the method of mixing - Machine Mixing or Hand Mixing.
Machine Mixing - In general terms, the time of machine mixing should be between two and three minutes. On no account should the mix be allowed to stay in the machine longer because the materials will start to segregate

A concrete and mortar mixer

Cleaning the mixer – It is very important to clean the mixer after day’s work to prevent remnants of mortar or concrete setting and sticking into the inner parts of the mixer. Un-cleaned mixers should not be left overnight. After mixing some sand, coarse aggregate and water should be loaded into the mixer and the mixer turned for about 5 min. This will remove all cement/mortar/concrete stuck onto the blades and the drum thus maintaining a clean machine.
After cleaning the machine should be stored under cover on an elevated level to protect from elements and provide drainage.

Hand mixing
This should take as long as required to ensure that all the particles have been completely integrated together. A high degree of agitation is required when hand-mixing, to ensure that any added plasticizer etc has the necessary effect. Mixed mortar should never be ‘re-tempered’ or ‘knocked up’ with added water because this dilutes the cement/lime element of the mortar which will result in a weaker mix. Mortar is a mixture of the following materials in different combinations: 

                  Sand + Cement + Water
                  Sand + Lime + water
                  Sand + Lime + Cement + Water
                  Sand + Cement + Plasticizer + Water

Typical Mortar Mixes
In work sites, mortar is possibly the least understood and most abused material on the building site. The design strength of the mortar should be determined mainly by the strength of the brick or block to be bedded in it. The mortar strength should roughly match that of the brick or block and in no case should it be stronger than it.

Mortar will work more easily if the mix contains lime. The more lime the mix contains, the more workable it will be. If the mortar contains cement, it will stiffen more quickly, therefore the more cement within the mix the quicker it will stiffen and set. When mortar of high strength is required, the mix should be composed of cement and sand only.

Design of Mortar Mixes
Mortar mixes are designed to suit certain building requirements. The chart below shows the different mixes and their designated uses which range from 1 to 4. As the mixes progress through the designations of 1 to 4, they become progressively weaker, but with higher lime content they become more tolerant to structural movement.

Cement, lime with sand
Masonry Cement, sand
Cement, sand with plasticizer
Intended use

Class A engineering bricks and dense concrete blocks
1:21/2 to 31/2
1:3 or 4
Class B engineering bricks
1:1:5 or 6
1:4 or 5
1:5 to 6
Bricks and blocks below DPC level
1:2:8 or 9
1:51/2 to 61/2
1:7 to 8
Bricks and blocks above DPC level

Concrete mixes - Concrete mixes are identified in two methods:

1.      By the proportions in which the raw materials are mixed – 1:2:4 (20mm). (1 part cement + 2 parts Fine Aggregate + 4 parts Coarse Aggregate. 20mm denotes the maximum particle size of coarse aggregate). Identifying concrete by this method has disadvantages.

The mix proportion does not specify required strength of concrete and has no control over the quality of concrete.
To overcome this modern day concrete mixes are identified by strength.

2.      Concrete mix by strength - Mixes are identified by the strength which the concrete should possess on setting. These mixes are called Graded Concrete mixes.
Example s –
Grade 10 concrete             (strength 10 mpa)
Grade 15 concrete             (strength 15 mpa)
Grade 20 concrete etc.      (strength 20 mpa)

Concrete mixes and material requirements for a 50kg bag of cement:

Quantities per 50 kg bag of cement

Fine aggregate (m³)
Coarse aggregate (m³
App water content (liters)
Very strong water tight concrete
Reinforced concrete for floor slabs, columns, beams etc
Foundations, screeds


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