Blocks

Blocks

Concrete Masonry Unit (CMU) , also called concrete block, cement block, and foundation block – is a large rectangular brick used in construction.

Concrete blocks are made from cast concrete, i.e. Portland cement and aggregate, usually sandand fine gravel for high-density blocks. 

Lower density blocks may use industrial wastes as an aggregate. 

Blocks can be laid more quickly than bricks because they are larger than bricks. However, the larger sizes also means that less versatility in laying especially when building up ends or corners and also laying to curves.

Uses of Blocks

When built in tandem with concrete columns and tie beams and reinforced with rebar, block is a very common building material for the load-bearing walls of buildings, in what is termed "concrete block structure" (CBS) construction. 

American suburban houses typically employ a concrete foundation and slab with a concrete block wall on the perimeter. Large buildings typically use copious amounts of concrete block; for even larger buildings, concrete block supplements steel I-beams. Tilt-wall construction, however, is replacing CBS for some large structures. 

Sizes and Structures

Concrete blocks may be produced with hollow centres to reduce weight or improve insulation. The use of blockwork allows structures to be built in the traditional masonry style with layers (or courses) of staggered blocks. 

Blocks come in many sizes. 

In the US, with an R-Value of 1.11 the most common nominal size is 16×8×8 in (410×200×200 mm); the actual size is usually about ³⁄₈ in (9.5 mm) smaller to allow for mortar joints. 

In Ireland and the UK, blocks are usually 440×215×100 mm (17×8.5×3.9 in) excluding mortar joints. 

In New Zealand, blocks are usually 390×190×190 mm (15×7.5×7.5 in) excluding mortar joints. 

IGNEOUS ROCKS- GRANITE AND CAST BASALT

Igneous rocks

Igneous rocks are the oldest, having been formed by the solidification of the molten core of the earth.  They form about 95 % of the earth crust. Depending whether solidification occurred slowly within the earth’s crust or rapidly at the surface, the igneous rocks are defined as plutonic or volcanic respectively. In the plutonic rocks, slow cooling from the molten state allowed large crystals to grow which are characteristic of the granites. Volcanic rock such as basalt is fine-grained and individual crystals cannot be distinguished by the eyes.

Granite

Most granites are hard and dense, and thus from highly durable for building materials, virtually impermeable to water, resistant to impact damage and stable within industrial environments. Granites may be flamed to a peeled off surface, produced by the differential expansion of the various crystalline constituents. Nowadays, many buildings have combined the polished and flamed material to create interesting contrast in the depth of color and texture. A wide variety of colors including black, blue, green, red, yellow, and brown are imported from foreign countries. Because of the high cost of quarrying and finishing granite, it’s frequently used as cladding materials or alternatively cast directly onto concrete cladding units.

HOW TO CUT AND POLISH GRANITE 

KITCHEN WITH GRANITE FINISHES

Cast basalt

Basalt is a fine-grained stone nearly as hard as granite. It can be melted at 2400 ⁰ C and cast into tile units which are deep steel grey in color. A slightly patterned surface can be created by swirling the molten basalt within the mould. Annealing in a furnace produces a hard virtually maintenance-free shinny textured surface flecked with the shades of green, red and bronze. Larger cast units for worktops, in either a honed or polished finish, can be cut to size.

BASALT WALL AND FLOOR TILES

SAWN AND FLAMED BASALT

STONE

The term stone refers to natural rocks after their removal from the earth’s crust. The significance of stone as a building material is illustrated by widespread prehistoric evidence and its sophisticated use in the early civilizations of the world, including the Egyptians, the Incas if Peru, and the Mayans of Central America. 

Types of Building Stone

-Building stone, also called dimension stone, derives from one of three naturally occurring rock types:

1.  Igneous 

- Hard and non-porous rock formed from the slow or quick cooling of molten magma. The best examples are granite and cast basalt.

GRANITE SAMPLES

CAST BASALT TILES

2.  Sedimentary

 - Soft and fairly porous rock formed from deposits of eroded pre-existing rock that settled in layers mostly on sea beds, and became compacted. The best examples are sandstone and limestone.

SAND STONES

LIMESTONE

3.  Metamorphic

 - Hard and non-porous rock formed from pre-existing rock that has been altered by intense heat or pressure. The best examples are marble and slate.

MARBLE SAMPLES

SLATES

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Positions of bricks

Stretcher: a brick laid horizontally, flat with the long side of the brick exposed on the outer face of a wall
Header: a brick laid flat with the short end of the brick exposed
Soldier: a brick laid vertically with the narrow ("stretcher") side exposed
Sailor: a brick laid vertically with the broad side exposed
Rowlock or Bull Header: a brick laid on the long, narrow side with the small or "header" side exposed
Shiner: a brick laid on the long narrow side with the broad side exposed

Bricks bonding patterns

The bond of the brickwork in which the bricks are placed in a pattern of headers and stretchers gives the wall strength and stability.





English bond is made up of alternating courses of stretchers and headers. This produces a solid wall that is a full brick in depth.  English bond is easy to lay and is the strongest bond for a one-brick-thick wall.

Flemish bond is created by alternately laying headers and stretchers in a single course.  The next course is laid so that a header lies in the middle of the stretcher course below.

Stretcher bond is one of the most common bonds.  It is easy to lay with little waste and composed entirely of stretchers set in rows, offset by half a brick.





Another two types of bricks bonding patterns

Header bond is created by rows of headers, only displaced by half a brick on each row. This bond is often use to create curved brickwork.

Stack bond is a pattern made up of rows of stretchers with each stretcher centred on the stretcher below it.  All joints run vertically down the entire wall.







There are two videos further information for the understanding of  bricks......

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The brick manufacturing process

The process of making bricks consist of following steps: raw material preparation, the forming process, setting and drying and firing.

Raw material preparation

• Extration of the raw material from quarry and its transportation to storage area, by conveyor belt or road transport

• remove top soil and unsuitable burden

• screen raw material to remove any rocks

• ground into powder by crushers or rollers with further screening to remove small particles.

crushing

storage area

Gathering raw material

screening

pug mill

Forming process

• handmade brick-----a soft mixture is forced through an extruder, cut into slugs and conveyed to work stations. The slugs are then individually picked up, rolled in sand and thrown into a pre-sanded wooden mold by a worker. Excess raw material is removed by a wire and endless belt. As the filled mold boxes continue on their journey, they are mechanically bumped on their ends to loosen the brick from the mold prior to dumping.
•  soft mud process------made from shale or clay material that is pugged or mixed with considerable water and placed in a machine that presses the wet mix into molds previously sanded. The mold boxes are then bumped and dumped. A variety of sands are used to keep the brick from sticking in the molds and to affect different textures and colors of the final product.
• extruded wire-cut process-------- The shale and clay materials for extruded brick are mixed with a moderate amount of water. This mixture is forced by means of an auger through a die having the shape of the brick. Prior to entering the die, the material passes through a vacuum changer that reduces the amount of air in the mix resulting in a denser, more homogeneous product. It is here that core holes are placed in the column. The core holes are needed to reduce the mass for firing and the weight for future handling. The column that is produced by the extruder is cut by wires to make individual brick. Scratching, scraping, rolling or sanding the surface of the column as it exits the die produce multiple textures on the face of the brick.

  

  Handbrick making

  
  

extruder

cutter for slice brick

  

clay in mold box

handbrick shape

sliced extruder birck

                                            











Drying & Firing

• Drying----- the bricks are hand or mechanically set onto kiln cars. Prior to entering the kiln, the unfired or green brick must be properly dried. This is an extremely important part of the manufacturing process. Moisture in the brick must be limited at this time to prevent scumming and certain mechanical defects from occurring when the brick is subjected to the intense heat of the kiln. Generally, the drying process is done by placing the green brick in enclosed dryers which utilize excess heat from the cooling kiln.

• Firing------green brick pass through the long length of the kiln on a continuous procession of cars moving on rails, much like a small railroad train. The continuous tunnel kiln employs a combination of vertical and horizontal drafts. The preheating, burning and cooling is done in zones varying in temperatures up to 2,000 degrees. In this type of kiln, closer temperature control is possible and less handling of the green brick results in better quality products.When the green brick enters the kiln, the manufacturer determines the type of firing required to produce the desired range of color. If a range of clear burn color is wanted, a straight burn from start to finish produces the desired result. The only color variations being the extremes of dark color nearest the fire or in the crown on the kiln and the light color at the bottom where the brick have the lowest temperature. The remainder of the brick will have medium tones. However, if the kiln is designated to be flashed, the dampers are regulated at the end of the burn to cut off the air and subject the brick to a reducing fire, thus bringing out the blacks, blues, browns, etc. which make up a flashed range.

hand setting

dryer

firing brick

fire firing

fire tunnel kiln

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Workability Tests on Concrete

Workability measurement methods
1.Slump test  - simplest and crudest test
The apparatus used for doing slump test are Slump cone and Tamping rod. 

2. Compacting factor test
Compaction Factor is the ratio of the weight of partially compacted concrete to the weight of the concrete when fully compacted in the same mould. The weight of partially compacted concrete in relation to its fully compacted state is a reasonably good indication of the workability of concrete. 

3. Vebe test
The Vebe time test is a more scientific test for workability than the Slump Test, in that it measures the work needed to compact the concrete. The freshly mixed concrete is packed into a similar cone to that used for the slump test. The cone stands within a special container on a platform, which is vibrated at a standard rate, after the cone has been lifted off the concrete. The time taken for the concrete to be compacted is measured. Vebe times range from 1 second for runny concrete to more than 12 seconds for stiff concrete. Unlike the slump test, the Vebe time test gives useful results for stiff concretes. 

4. Flow table test
The flow table test or flow test is a method to determine the consistence of fresh concrete 

The below video will provide you with a further understanding. Click on it!!!

http://www.youtube.com/watch?v=_peigT6McIM 

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The properties of hardened concrete

Hardened concrete has a number of properties, including:

1) Mechanical strength, in particular compressive strength. 
The strength of normal concrete varies between 25 and 40 MPa. Above 50 MPa, the term High Performance Concrete is used (50 MPa corresponds to a force of 50 tonnes acting on a square with sides of ten centimetres).

2) Durability. 

Durability refers to the resistance to the forces of environment such as weathering, chemical attack and fire. Freezing and thawing result in adverse stresses which develop due to the presence of water in the pores of the concrete. Generally, dense and strong concretes have better durability in extreme weather conditions. The cube-crushing strength alone is not a reliable guide to the quality and durability of concrete. It should have an adequate cement content and a low water-cement ratio. Concrete is extremely resistant to the physico-chemical attack emanating from the environment (frost, rain  atmospheric pollution, etc...) It is particularly well-suited for structures exposed to demanding and extreme conditions.

3) Porosity and density. 
These properties are responsible for the first two. The denser (or the less porous) the concrete the better its performance and the greater its durability. 
The density of concrete is increased by optimizing the dimensions and packing of the aggregate and reducing the water content.

4) Fire resistance.

5) Thermal and acoustic insulation properties.

6) Impact resistance.

7) Impermeability

This is the resistance of the concrete to the flow of water through the pores. Excess water during concreting leaves a large number of continuous pores leading to permeability. Since permeability reduces the durability of concrete, it should be kept very low by using low water-cement ratio, dense and well-graded aggregates, proper compaction and continuous curing at low temperature conditions. The cement content used should be sufficient to provide adequate workability with a low water cement ratio and with the available compaction method.

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Fresh Concrete 

Following are the important properties of fresh concrete Setting Workability Bleeding and Segregation Bleeding Segregation Hydration Air Entrainment

1. Setting of Concrete

The hardening of concrete before its hydration is known as setting of concrete. OR

The hardening of concrete before it gains strength. OR

The transition process of changing of concrete from plastic state to hardened state. Setting of concrete is based or related to the setting of cement paste. Thus cement properties greatly affect the setting time.

Factors affecting setting:

Following are the factors that affect the setting of concrete.

1. Water Cement ratio
2. Suitable Temperature
3. Cement content
4. Type of Cement
5. Fineness of Cement
6. Relative Humidity
7. Admixtures
8. Type and amount of Aggregate

2. Workability of Concrete

Workability is often referred to as the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation.

OR

The internal work done required to overcome the frictional forces between concrete ingredients for full compaction. It is obvious that no single test can evaluate all these factors. In fact, most of these cannot be easily assessed even though some standard tests have been established to evaluate them under specific conditions.

In the case of concrete, consistence is sometimes taken to mean the degree of wetness; within limits, wet concretes are more workable than dry concrete, but concrete of same consistence may vary in workability.

Because the strength of concrete is adversely and significantly affected by the presence of voids in the compacted mass, it is vital to achieve a maximum possible density. This requires sufficient workability for virtually full compaction to be possible using a reasonable amount of work under the given conditions. Presence of voids in concrete reduces the density and greatly reduces the strength: 5% of voids can lower the strength by as much as 30%.

Slump Test can be used to find out the workability of concrete. View Procedure of Slump Test

Factors affecting concrete workability:

1. Water-Cement ratio
2. Amount and type of Aggregate
3. Amount and type of Cement
4. Weather conditions
   1. Temperature
   2. Wind
5. Chemical Admixtures
6. Sand to Aggregate ratio

3(a). Concrete Bleeding 

Bleeding in concrete is sometimes referred as water gain. It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thin members like roof slab or road slabs and when concrete is placed in sunny weather show excessive bleeding.

Due to bleeding, water comes up and accumulates at the surface. Sometimes, along with this water, certain quantity of cement also comes to the surface. When the surface is worked up with the trowel, the aggregate goes down and the cement and water come up to the top surface. This formation of cement paste at the surface is known as “Laitance”. In such a case, the top surface of slabs and pavements will not have good wearing quality. This laitance formed on roads produces dust in summer and mud in rainy season.

Water while traversing from bottom to top, makes continuous channels. If the water cement ratio used is more than 0.7, the bleeding channels will remain continuous and un segmented. These continuous bleeding channels are often responsible for causing permeability of the concrete structures. While the mixing water is in the process of coming up, it may be intercepted by aggregates. The bleeding water is likely to accumulate below the aggregate. This accumulation of water creates water voids and reduces the bond between the aggregates and the paste.

The above aspect is more pronounced in the case of flaky aggregate. Similarly, the water that accumulates below the reinforcing bars reduces the bond between the reinforcement and the concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste due to bleeding can be remedied by re vibration of concrete. The formation of laitance and the consequent bad effect can be reduced by delayed finishing operations.

Bleeding rate increases with time up to about one hour or so and thereafter the rate decreases but continues more or less till the final setting time of cement.

Prevention of Bleeding in concrete

• Bleeding can be reduced by proper proportioning and uniform and complete mixing.
• Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse.
• Air-entraining agent is very effective in reducing the bleeding.
• Bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.

The bleeding is not completely harmful if the rate of evaporation of water from the surface is equal to the rate of bleeding. Removal of water, after it had played its role in providing workability, from the body of concrete by way of bleeding will do good to the concrete.

Early bleeding when the concrete mass is fully plastic, may not cause much harm, because concrete being in a fully plastic condition at that stage, will get subsided and compacted. It is the delayed bleeding, when the concrete has lost its plasticity, which causes undue harm to the concrete. Controlled re vibration may be adopted to overcome the bad effect of bleeding.

3(b). Segregation in concrete

Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture. There are considerable differences in the sizes and specific gravities of the constituent ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall apart.

Segregation may be of three types

1. Coarse aggregate separating out or settling down from the rest of the matrix.
2. Paste separating away from coarse aggregate.
3. Water separating out from the rest of the material being a material of lowest specific gravity.

A well made concrete, taking into consideration various parameters such as grading, size, shape and surface texture of aggregate with optimum quantity of waters makes a cohesive mix. Such concrete will not exhibit any tendency for segregation. The cohesive and fatty characteristics of matrix do not allow the aggregate to fall apart, at the same time; the matrix itself is sufficiently contained by the aggregate. Similarly, water also does not find it easy to move out freely from the rest of the ingredients.

The conditions favorable for segregation are:

1. Badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates
2. Insufficiently mixed concrete with excess water content
3. Dropping of concrete from heights as in the case of placing concrete in column concreting
4. When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades
5. Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete

Vibration of concrete is one of the important methods of compaction. It should be remembered that only comparatively dry mix should be vibrated. It too wet a mix is excessively vibrated; it is likely that the concrete gets segregated. It should also be remembered that vibration is continued just for required time for optimum results. If the vibration is continued for a long time, particularly, in too wet a mix, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matrix.

4. Hydration in concrete

Concrete derives its strength by the hydration of cement particles. The hydration of cement is not a momentary action but a process continuing for long time. Of course, the rate of hydration is fast to start with, but continues over a very long time at a decreasing rate In the field and in actual work, even a higher water/cement ratio is used, since the concrete is open to atmosphere, the water used in the concrete evaporates and the water available in the concrete will not be sufficient for effective hydration to take place particularly in the top layer.

If the hydration is to continue, extra water must be added to refill the loss of water on account of absorption and evaporation. Therefore, the curing can be considered as creation of a favorable environment during the early period for uninterrupted hydration. The desirable conditions are, a suitable temperature and ample moisture.

Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of view of volume stability. Jeat of hydration of concrete may also shrinkage in concrete, thus producing cracks. If the heat generated is removed by some means, the adverse effect due to the generation of heat can be reduced. This can be done by a thorough water curing.

5. Air Entrainment

Air entrainment reduces the density of concrete and consequently reduces the strength. Air entrainment is used to produce a number of effects in both the plastic and the hardened concrete. These include:

1. Resistance to freeze–thaw action in the hardened concrete.
2. Increased cohesion, reducing the tendency to bleed and segregation in the plastic concrete.
3. Compaction of low workability mixes including semi-dry concrete.
4. Stability of extruded concrete.
5. Cohesion and handling properties in bedding mortars.