The characteristics of good building stones

 Stone is a naturally available building material, which has been form the early age of civilization.  It is available in from in the form of rock, which is cut to the required size and shape and used as building block. Stone has been used to build small residential building to large places and temples.

Stones should possess fine – grained texture. It should be free from soft patches, flows and cracks

The strength of stone should be ascertained if it is to be subjected to any excessive or unusual stresses.

Shade of the stone should be uniform and pleasing.  Freshly broken surface should be bright and cracks

The stone should be strong, hard and durable. the hardness of stone is often of importance, especially if it is to be subjected to a considerable amount of wear and friction, as in pavements. 

The specific gravity should not be less than 2.5

It should possess good resistance to weathering

It should be easily workable before seasoning.the appearance of stone is often a matter of importance, especially in the face work of conspicuous buildings.In order that the appearance may be preserved, a good weathering stone should of course be selected, free from flaws, clayholes, etc.

It should have good resistance to smoke, fire and acidic atmosphere

It  should not absorb more than 5% of water by weight, when kept immersed in water for 24 hrs.

In attrition test, the stone should not show wear of more 2%

The weight of a stone for building has occasionally to be considered. In marine engineering works it is often advisable to use heavy stones to resist the force of the sea. A light stone would be best adapted for arches, while heavy stones would add to the stability of retaining walls.

It should become hard after seasoning. This seasoning gets rid of the moisture, sometimes called "quarry sap," which is to be found in all stone when freshly quarried.

Toughness index should be more than 19

Fiber Reinforced Concrete

Fiber reinforced concrete is concrete containing fibrous material which increases its structural integrity. Fiber reinforced cements and concretes are firmly established as construction materials. 

Since the early 1960’s extensive research and developments have been carried out with FRC materials leading to a wide range of practical applications. In recent years, a great deal has been leaned regarding the limitations of some FRC materials and, particular, the importance of good design. The universities and the construction industry world-wide are blessed with experts in all facets of FRC theory and applications. Is there anybody bold enough to ask the experts, individually or or collectively

Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers. Within these different fibers that character of fiber reinforced concrete changes with varying concretes, fiber materials, geometries, distribution, orientation and densities.

It has very good compressive strength but comparatively little tensile strength, which makes it likely to crack under many conditions. Cracking leads to further damage.  Fiber reinforced concrete is less likely to crack than standard concrete.

Advantages of Fiber reinforced concrete

Corrosion resistance

Light weight

High longitudinal strength

improve freeze-thaw resistance

High fatigue endurance (varies with type of reinforcing fiber and bar)

Magnetic transparency

Low thermal and electric conductivity

Limitations of Fiber reinforced concrete

Low modules of elasticity

Low share strength

Reduce durability in moist, acid/salt, and alkaline environments

Low transverse strength

High coefficient of thermal expansion perpendicular to the fibers relative to concrete

Fire resistance can be less than adequate, depending on the type of matrix used for producing frp bars

Chemical Admixture

Modern concretes almost always possess additives, either in the mineral form or chemical form. Particularly, chemical admixtures such as water reducers and set controllers are invariably used to enhance the properties of freshand hardened concrete.

 A 'Chemical Admixture' is any chemical additive to the concrete mixture that enhances the properties of concrete in the fresh or hardened state. This does not include paints or coatings. ACI 116R [2000] defines the term admixture as 'a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing'. A number of types of chemical admixtures are used for concrete [Ramachandran, 2002; Rixom and Mailvaganam, 1999]. The general purpose chemicals include those that reduce the water demand for a given workability (called 'water reducers'), those entraining air in the concrete for providing resistance to freezing and thawing action (called 'air entrainers'), and those chemicals that control the setting time and strength gain rate of concrete (called 'accelerators' and 'retarders'). Apart from these chemicals, there are others for special purposes - namely, viscosity modifying agents, shrinkage reducing chemicals, corrosion inhibiting admixtures, and alkali-silica reaction mitigating admixtures.

A water reducing chemical, as the name implies,is used to reduce the water content of a concrete mixture while maintaining a constant workability. The resultant effect of the reduced water content is the increased strength and durability of concrete. However, water reducers may also be employed to 'plasticize' the concrete, i.e. make concrete flowable

In this case, the water content (or water to cement ratio) is held constant, and the addition of the admix- tures makes the concrete flow better, while the compressive strength (which is a function of the water to cement ratio), is not affected. Another use of water reducers is to lower the amount of cement (since water is proportionately reduced) without affecting both strength and workability. This makes the concrete cheaper and environmentally friendly, as less cement is consumed.

The Masterbuilder - January 2009