How to test for a good brick?

 

How to test for a good brick?

Drop a brick vertically from a height of 1 m. A good quality brick will not break.

Strike two bricks against each other. Good quality bricks will produce a clear ringing sound on contact.

Perfect Mix

For a 4"thick wall (partition walls in the middle of the house), keep mortar proportion as =1:4 • (cement: sand)

For a 9"wall (outer wall), keep mortar proportion as = 1:6 (cement: sand)

Good Practice

Begin work at the corners, first to a height of 3 or 4 layers with base extending in steps.

Place all bricks on their bed. The depression on top provides space for the mortar to bond well. Use line-string, plumb bob, and spirit level for checking alignment, and to keep vertical and horizontal lines straight.

Soak your bricks in water for 8 hours at least before use, else it will absorb moisture from mortar.

Building Foundation

Building Foundation

The foundation is the most critical part of any structure and most of the failure is probably due to faulty foundations rather than any other cause. The purpose of foundation is to transmit the anticipated loads safety to the soil.

Basic requirements:

·     To distribute the total load coming on the structure over a large bearing area so as to prevent it from any movement.

·     To load the bearing surface or area at a uniform rate so as to prevent any unequal or relative settlement.

·     To prevent the lateral movement of the structure.

·     To secure a level or firm natural bed, upon which to lay the courses of masonry and also support the structure.

·     To increase the suitability of the structure as a whole, so as to prevent it from overturning or sliding against such as wind, rain, frost etc.

Grading of Sand

Grading of Sand

According to the site of grains, sand is classified as fine, coarse and gravelly

·     Sand passing through a screen with clear opening of 1.5875mm is known as fine sand. It is generally used for masonry works.

·     Sand passing through a screen with clear openings of 7.62mm is known as gravely sand. It is generally used for plastering.

·     Sand passing through a screen with clear opening of 3.175mm is known as coarse sand. It is generally used for masonry work.

The Many Uses of Ready-Mix Concrete

The Many Uses of Ready-Mix Concrete

Concrete is a popular building material. You can see it in homebuilding sites, commercial building sites and government projects such as bridges and highways. It is easier to complete big projects like these by using ready-mix concrete. Completing projects on a tight deadline is possible and achieved in less than half the time it would when not using this product.

Ready-mix concrete comes from manufacturers and plants ready for delivery and mixing at work or project sites. With the use of a concrete mixer, one can get a precise mixture of concrete for the project. Using this type of cement material eliminates too much confusion at the work site. It also saves considerable amount of time because it requires less time to prepare than concrete prepared from scratch.

Cements, sands, aggregates (gravels) and water are the main contents of the mixture. It has the same ingredients as the ones that are not except that instead of carrying and mixing the raw materials at the site, the ready-mix arrives at the site on mixer trucks already pre-mixed and ready to use. Adding different additives and aggregates offsite and then delivering onsite for a different project based upon the specification of the customer is another feature of the ready-mix concrete. Different textures, finishes and colors are also available in ready-mix form.

Using ready-mix concrete eliminates having to carry and mixing the materials on site, which is a painstaking process. It eliminates errors that come with wrong measurement of water and the concrete materials. Using ready-mix saves time and effort. Big projects take less time to complete after pouring in the mixture using the transit mixer. The quality of the product is also much better than those that come in a non-ready mixed form.

Ready-mix concrete has a big potential in a lot of building projects. Aside from using it on big infrastructure projects such as building bridges, highways and huge buildings, we see this type of concrete as the choice of homeowners when building driveways, walkways made of concrete. Some homeowners choose concrete for their kitchen and bathroom countertops and as floorings. Stained concrete floors and counters provide a rustic look and patina adding more character to the interior of the home.

When using ready-mix concrete one must work with caution. One must ensure that there is enough space for the transit mixer. In addition, the location should be strong enough to handle the weight of the transit mixer and the concrete. The workers practice caution when working with the concrete mixer, by avoiding standing near the way of the mixer, especially when operating and pouring concrete. Accidents can and do happen which can endanger the well-being of the workers.

Characteristics of sand

Characteristics of sand

·     It should be chemically inert

·     It should be clean and coarse. It should be free from organic matter.

·     It should contain sharp, angular and durable grains.

·     It should not contain salts, which attract the moisture from atmosphere.

·  It should be well graded (i.e.) should contain particles of various sizes in suitable proportions.

Types of Cement

Types of Cement

In addition to ordinary cement, the following are the other varieties of cement.

·     Acid Resistance Cement : This is consists of acid resistance aggregates such as quartz, quartzite’s, etc, additive such as sodium fluro silicate (Na2SiO6) and aqueous solution of sodium silicate. This is used for acid-resistant and heat resistant coating of installations of chemical Industry. By adding 0.5 percent of unseed oil or2 percent of ceresil, its resistance to water is increased and known as acid water resistant cement.

·     Blast Furnace Cement: For this cement slag as obtained from blast furnace in the manufacture of pig iron and it contains basic elements of cement, namely alumina, lime and silica. The properties of this cement are more or less the same as those of ordinary cement and prove to be economical as the slag, which is waste product, is used in its manufacture.

·     Colored Cement: Cement of desired color may be obtained by intimately mixing mineral pigments with ordinary cement. The amount of coloring may vary from 5 to 10 percent and strength of cement if it is exceeds 10 percent. Chromium oxide gives brown, red or yellow for different proportions. Colored cements are used for finishing of floors, external surfaces, artificial marble, windows.

·     Expanding Cement : This type of cement is produced by adding an expanding medium like sulpho – aluminate and a stabilizing agent to ordinary cement. Hence this cement expands where as other cement shrinks. Expanding cement is used for the construction of water retaining structures and also for repairing the damaged concrete surfaces.

·     High alumina Cement: This cement is produced by grinding clinkers formed by calcining bauxite and lime. The total content should not be less than 32 percent and the ratio by weight of alumina to lime should be between 0.85 and 1.30.

Advantages of artificial stones

Advantages of artificial stones

1.Cavities may be kept in artificial stones to convey pipes, electric wires etc.

2.Grooves can be kept in artificial stone while it is being cast which are useful for fixing various fittings.

3.It can cast in desired shape

4.It can be made in a single piece and hence trouble of getting large blocks of stone for lintels, beams etc is avoided.

5.It can be made stronger than natural stone

6.It is cheap and economical

7.It is more durable than natural stone

8.Natural bed is absent in artificial stones and hence, the question of taking precautions with respect to the natural bed of stones does not arise.

Qualities of a good building stone

The following are the qualities or requirements of a good building stone.

1. Crushing strength: For a good building stone, the crushing strength should be greater than l000kg per cm2.

2. Appearance: Good building stone should be a uniform color, and free from clay holes, spots of other color bands etc capable of preserving the color for longtime.

3. Durability: A good building stone should be durable. The factors like heat and cold alternative wet and dry, dissolved gases in rain, high wind velocity etc affect the durability.

4. Fracture: For good building stone its fracture should be sharp, even and clear.

5. Hardness: The hardness greater than 17, treated as hard used in road works. It is between 14 to 17, medium hardness, less 14 said be poor hardness.

6. Percentage wear: For a good building stone, the percentage wear should be equal to or less than 3 percent.

7. Resistance to fire: A good building stone be fire proof. Sandstone, Argillaceous stone resists fire quite well.

8. Specific gravity: For a good building stone the specific gravity should be greater than 8.7 or so.

9. Texture: A good building stone should have compact fine crystalline structure should be free from cavities, cracks or patches of stuff or loose material.

10. Water absorption: For a good building stone, the percentage absorption by weight after 24 hours should not exceed 0.60.

11. Seasoning: Stones should be well seasoned before putting into use. A period of about 6 to 12 months is considered to be sufficient for proper seasoning.

12. Toughness Index: Impact test, the value of toughness less than 13 – Not tough, between 13 and 19 – Moderate, greater than 19- high

Admixtures

Admixtures

Ready Mix Concrete is generally transported to different construction sites and delivered with the help of revolving type transits mixers. These sites are located at long distances and the concrete delivered is workable, plastic and flowable. Experience shows that slump loss takes place with certain types of cement and to prevent this superpplasticiser are used.

Ready Mix Concrete's often use admixtures formulated for special purposes lik e:

(a) Improvement of screeds, renders, mortars and concrete for increased water resistance and less risk - from aggressive materials,

( b) Integral water proofers,

 (c) Foamed - for light weight insulation. void filling and semi structural support,

(d) Water repelling for semi-dry block concrete,

(e) Microsillica for use in high performance concrete. The introduction of microsillica was observed to improve the pore solution chemistry of HPC. For higher replacement of cement by silica fumes, the concentration of both K+ and OH-ions are substantially removed. However, up to 20% replacement will not cause a drop in pH below that of saturated CaOH solution, which is approx.12.5. Even at 30%

replacement,  pH does not drop below 11.5, which is considered to be a threshold value for maintaining a good passivity of embedded steel.

Testing of Concrete

Testing of Concrete

The results of concrete tests are used as the basis for deciding whether the delivered concrete is in accordance with the specification, if the reported results are below the compliance loyal, doubt is cast on the quality of the supplied concrete and the long term durability of the structure. If the investigation indicates that the sampling, preparation and or storage of the test specimens have not been in accordance with the required standard procedures, then the result will be invalid. Further valuable management time, better devoted to other management functions, will have been wasted.

It is, therefore, in everyone's interest that sampling and testing is done correctly so that the results provide a valid basis for logical decision making. Where the contractor is required to perform on-site concrete testing, it is imperative that suitable test facilities are available, that the appropriate equipment is calibrated and that the staff has been fully trained in the relevant test techniques, one can look the factors that affect the price compliance test, (i.e.) the compressive strength test.

As  a result is the average of the compressive test specimens made for the same sample of concrete. Individual variation should not be more than 15 Percent of the average. If more. The results of the average. If more, the results of the sample are invalid. This is because differences of this magnitude indicate poor sampling, cube making, curing or crushing. It is, therefore essential that correct test facilities are provided and the people with responsibility for sampling and testing concrete are suitably trained.

Slump test for predicting the workability of concrete is done at batching plant and working site. The main problem in the production of ready mixed concrete is maintaining the workability of the mix right to the time of placing. Concrete stiffens with time and the stiffening may also be aggravated by prolonged mixing and by high temperature. With the use of retarding admixtures, the time limit can be extended to 3 or even 4 hours, The United States Bureau of Reclamation provides for an extension of 3 to 6 hours in the time of contact between cement and wet aggregate in transport prior to mixing.  The general requirement is that concrete shall be discharged from the truck-mixer within 2 h of the time of loading. However, a longer period may be permitted if retarding admixtures are used or in cool humid weather or when chilled concrete is produced.

Sampling of Concrete

Sampling of Concrete

Critical decisions, often involving very high potential costs, are made on the basis of concrete test results. Correct sampling is paramount to the validity of these test results but is an aspect of testing that is frequently overlooked and often carried out by untrained people. It is therefore essential that the sampling is done correctly and is representative of the concrete delivered.

After the truck-mixer has re-mixed its delivery on site allow at least the first one-third of a m3 of concrete to be discharged prior to taking any samples. Take at least 4 incremental samples from the remainder of the load avoiding sampling the last cubic meter of concrete. Thoroughly re-mix this composite sample either on a mixing tray or in the sampling bucket and proceed with the required testing.

Describe the recommended sampling methods for ready mixed concrete in British code. Using a standard scoop, this can collect about 5kg of normal weight concrete. Each load of concrete to be tested should be nominally divided into a number of scoopfuls.

The Standard method: To ensure that the concrete is representative of the whole load is standard sample consists of scoopfuls taken from at least four different parts of the load and collected in buckets. The scoopfuls should be taken at equally spaced intervals; the scoop being passed through the whole width and thickness of the stream in a single movement. The first and the last 1/6th

portion of the discharge should be disregarded as unrepresentative. This is then thoroughly re-mixed on a non-absorbent surface before carrying out any individual test. This operation is necessary to even out any variation between individual scoopfuls and to counteract any segregation that may have occurred in transporting the sample from the sampling point to the testing area.

The Alter native Method: An alternative method of sampling concrete for slump testing from

a truck-mixer before the majority of the load has been discharged is permitted. This enables the concrete to be tested before being placed. When this alternate method is used, an initial discharge of at least 0.3 m3 is made before a sample of six scoopfuls is collected from the

moving stream; The sample is then r e-mixed on a non-absorbent surface and split into two equal parts. Each part is then tested or slump, with the average of the two tests recorded as the test result. This method of sampling is only applicable to the slump test. Concrete sampled by this method must not be used to make cubes for compliance testing, as it will produce erroneous results.

Batching and Mixing Plants

Batching and Mixing Plants

The principal functional elements of every stationary concrete production Plant comprises of the following:

·     Storage of materials - Silos, containers and bins

·     Batching arrangement

·     Measuring and recording equipment

·     Mixing equipment

·     Control systems

·     Electrical, hydraulic and pneumatic drives
·     Conveying systems (belt / screw conveyors)

Ready Mix Concrete

Ready Mix Concrete

Ready Mix Concrete is a specialized material in which the cement aggregates and other ingredients are weigh-batched at a plant in a central mixer or truck mixer, before delivery to the construction site in a condition ready for placing by the builder. Thus, `fresh' concrete is manufactured in a plant away from the construction site and transported within the requisite journey time. The RMC supplier  provides two services, firstly one of processing the materials for making fresh concrete and secondly, of transporting a product within a short time.

Sometimes Materials such as water and some varieties of admixtures can be transit-mixed (also known as Transit Mixture), that is they can be added to the concrete at the jobsite after it has been batched to ensure that the specified properties are attained before placement. Here materials are batched at a central plant and are completely mixed in the Batching Plant or partially mixed intransit. Transit-mixing keeps the water separate from the cement and aggregates and allows the concrete to be mixed immediately before placement at the construction site (Dry Concrete). This method avoids the problems of premature hardening and slump loss that result from potential delays in transportation or placement of central-mixed concrete.

Additionally, transit-mixing allows concrete to be hauled to construction sites further away from the plant. There are several types of RMC plants varying in type of mixing and capacity of concrete production.

Concrete Admixtures and the Environment

Concrete Admixtures and the Environment

Concrete admixtures are liquid or powder additives. They are added to the concrete mix in small quantities to meet specific requirements:

·     To increase the durability

·     To improve the workability

·     To change the setting or hardening reaction of the cement

The effect of admixtures is always to improve the concrete. In quantity terms, super plasticizers (high range water reducers) and plasticizers (water reducers) as a group make up about 80% of all of the admixtures used today.

How much do concrete admixtures leach, biodegrade or release fumes?

Super plasticizers should be non-toxic, water-soluble and biodegradable.

Tests on pulverized concrete specimens show that small quantities of super plasticizers and their decomposition products are leachable in principle. However, the materials degrade well and do not cause any relevant ground water pollution. Even under the most extreme conditions, only small quantities of organic carbon leaches into the water.

·     Conclusion of test: The air is not polluted by super plasticizers.

To summarize: How environment-friendly are super plasticizers?

Concrete admixtures are appropriate for their application and when correctly used are harmless to man, animals and the environment.

The technical benefits of super plasticizers for clients and construction professionals outweigh the occurrence of low, controllable emissions during use. Concrete admixtures merit being rated environment-friendly because they create negligible air, soil or ground water pollution.

See the following publications:

·     “Environmental Compatibility of Concrete Admixtures” Report by the Association of Swiss Concrete Admixtures Manufacturers (FSHBZ)

·     EU Project ANACAD Analysis and Results of Concrete Admixtures in Wastewater Final report BMG Engineering AG Zürich

Concrete Curing

Concrete Curing

For high durability, concrete must not only be “strong” but also impermeable, especially in the areas near the surface. The lower the porosity and the denser the hardened cement paste, the higher the resistance to external influences, stresses and attack. To achieve this in the hardened concrete, measures have to be taken to protect the fresh concrete, particularly from

·     Premature drying due to wind, sun, low humidity etc.

·     Extreme temperatures (cold, heat) and damaging rapid temperature changes

·     Rain

·     Thermal and physical shock

·     Chemical attack

·     Mechanical stress

Protection from premature drying is necessary so that the strength development of the concrete is not affected by water removal. The consequences of too early water loss are:

·     Low strength in the parts near the surface

·     Tendency to dusting

·     Higher water permeability

·     Reduced weather resistance

·     Low resistance to chemical attack

·     Occurrence of early age shrinkage cracks

·     Increased risk of all forms of shrinkage cracking

Steel Fiber reinforced Sprayed Concrete

Steel Fiber reinforced Sprayed Concrete

Definition

Steel fiber reinforced sprayed concrete, like conventionally reinforced sprayed concrete, consists of cement, aggregates, water and steel. By using and adding steel fibers, the sprayed concrete is homogeneously reinforced. 

Reasons for using steel fiber reinforced sprayed concrete:

·     Saving on the costs for installation of steel mesh reinforcement

·     Reduction in slump due to higher early strengths

·     Elimination of “spray shadow” when spraying onto reinforcing mesh

·     Due to its homogeneity, steel fiber reinforced sprayed concrete can withstand forces of various kinds in various directions at any point on its cross-section. 

Additional notes:

·     The cement content may have to be increased because the fines content of steel fiber reinforced sprayed concrete must be higher than in standard wet sprayed concrete, to anchor the fibers.

·     Adding Silica fume helps to achieve the target values of the sprayed concrete because it also improves anchorage of the fibers.

·     improves the pump ability considerably.

·     The minimum diameter of the pump line should be at least double the maximum fiber length.

Density of hardened Concrete

Uses of Cement

Principle

The standard describes a method to determine the density of hardened concrete.

The density is calculated from the mass (weight) and volume, which are obtained from a hardened concrete test specimen.

Test specimens

Test specimens with a minimum volume of 1 liter are required. If the nominal size of the maximum aggregate particle is over 25 mm, the minimum volume of the specimen must be over 50 D3, when D is the maximum aggregate particle size.

Example: Maximum particle size of 32 mm requires a minimum volume of 1.64 liters.)

Determining the mass

The standard specifies 3 conditions under which the mass of the specimen can be determined:

·     As a delivered sample

·     Water saturated sample

·     Sample dried in warming cupboard (to constant mass)

Determining the volume

The standard specifies 3 methods to determine the volume of a specimen:

·     By displacement of water (reference method)

·     By calculation from the actual measured masses

·     By calculation from checked specified masses (for cubes)

Determining the volume by displacement of water is the most accurate method and the only one suitable for specimens of irregular design.

Test result

The density is calculated from the specimen mass obtained and its volume:

D=m/V       D=density in kg/m³

              m=mass of specimen at time of test in kg

              V=volume determined by the relevant method in m³

The result should be given to the nearest 10 kg/m³.

Development of Hydration Heat

Development of Hydration Heat

When mixed with water, cement begins to react chemically. This is called hydration of the cement.

The chemical process of hardening is the foundation for the formation of the hardened cement paste and therefore of the concrete. The chemical reaction with the mixing water produces new compounds from the clinker materials —->hydration.

Viewing under an electron microscope shows three distinct phases of the hydration process, which is strongly exothermal, i.e. energy is released in the form of heat.

Hydration phase 1

Generally up to 4 to 6 hours after production

The gypsum in the plastic cement paste binds the tricalcium aluminates (C3 Al) to form trisulphate (ettringite), a water-insoluble layer which initially inhibits the conversion process of the other components. The gypsum addition of 2–5% therefore has a retarding effect.

The longer “needles” which are created in this phase bind the separate cement particles together, causing the concrete to stiffen.

Hydration phase 2

Generally between 4 to 6 hours after production and up to one day

After a few hours comes the start of vigorous hydration of the clinker materials, particularly the tricalcium silicate (Ca3Si), with the formation of intertwined long-fiber calcium silicate hydrate crystals which further consolidate the structure.

 Hydration phase 3

From about one day

The structure and microstructure of the cement matrix are initially still open. As hydration progresses, the interstices are filled with other hydration products and the strength is further increased.

Concrete Sulphate Resistance

Concrete Sulphate Resistance

Water containing sulphate sometimes occurs in soil or is dissolved in ground water and can attack the hardened concrete.

Process

Water containing sulphate combines with the tricalcium aluminate (C3 Al) in the cement to form ettringite (also thaumasite under certain conditions), which leads to increases in volume and to high internal pressure in the concrete structure and therefore cracking and spalling occurs.

Measures

Concrete structure as impermeable as possible

Low water/cement ratio

Use cement with a minimum tricalcium aluminate (C3 Al) content

Curing suited to the structure

Note: Clarification of specific requirements is essential for every project. Limiting values for exposure classification of chemical attack from natural soil and ground water:

Concrete strength

Concrete strength

Many factors influence the rate at which the strength of concrete increases after mixing. Some of these are discussed below. First, though a couple of definitions will be useful:

The process of strength growth is called 'hardening.' This is often confused with 'setting' but setting and hardening are not the same.

Setting is the stiffening of the concrete after it has been placed. A concrete can be 'set' in that it is no longer fluid, but it may still be very weak; you may not be able to walk on it, for example. Setting is due to early-stage calcium silicate hydrate formation and to ettringite formation. The terms 'initial set' and 'final set' are arbitrary definitions of early and later set; there are laboratory procedures for determining these using weighted needles penetrating into cement paste.

Hardening is the process of strength growth and may continue for weeks or months after the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement continues to hydrate.

The rate at which concrete sets is independent of the rate at which it hardens. Rapid-hardening cement may have similar setting times to ordinary Portland cement.