foundation system of burj al arab

What is the Significance of Modulus of Subgrade Reaction of Soil?

Modulus of subgrade reaction is an important term that is frequently used in structural analysis of components of foundation. This is used to design continuous footings, rafts and different types of piles. Modulus of subgrade reaction is a relationship between pressure and associated deflection of soil. Plate load test can produce these data. A δ Vs σ plot is produced during testing; the plot is found non-linear in most cases. Whether tangent or secant lines are used to determine slope (i.e. Ks). The basic equation is
Modulus of subgrade reaction-1


Δσ=increment of soil pressure 
Δδ=respective change in deformation or settlement

Factor Controlling Workability of Concrete

Concrete in fresh state is moldable mix which can be placed with reasonable amount of compaction and finished with selected means. The two qualifying words (adjective), reasonable and selected are actually refer to workability. A concrete is said to be workable when internal and external friction can be overcome with reasonable amount of effort exerted by compaction which indicates ease in placing, compaction and finishing as well. The factors that control workability or in simple word that offers lubrication effect on fresh concrete by reducing friction to ensure easy compaction are as below:

• Water content
• Aggregate size
• Aggregate shape and their surface texture
• Grading of aggregate
• Application of admixture
• Mix proportions

Besides these we can explain influencing factors on workability based on only three terms

• Water/cement ratio
• Aggregate/cement ratio
• Water content

Some can also include external factor like temperature, air flow etc. Again cement content and aggregate grinding has also influence on initial workability and change in workability of mix.

Water content:

Water content in fresh concrete has significant influence on workability of concrete. This is the main factor that defines fluidity of concrete, the higher the value for a given volume of concrete , the higher the fluidity of concrete (expressed as kg (or liters) per cubic meter of fresh concrete). As it is often misunderstood fluidity as workability, an inexperienced supervision frequently depends on inclusion of more water in concrete to improve workability.

This practice becomes very common as it is the one of the simplest way that can be adopted at site. It should be noted that addition of water is the last option to improve workability of concrete (applicable even to uncontrolled concrete). In case of controlled concrete, arbitrary increase in water content is not permitted. To keep water/cement ration of the mix unchanged with the addition of water, cement content also have to be increased to meet the design strength of hardened concrete.

Aggregate size:

A large size of aggregate will offer least amount of surface area for given volume of concrete. Thus the surface area available to be wetted by water will be less, again less amount of paste or matrix will be required to lubricate surface of aggregate. Thus less matrix and less amount of water is required to overcome internal friction between particles. For particular amount of water and paste, larger size of aggregate will offer higher workability, but within some limit.

Aggregate shape and their surface texture:

Shape of aggregates has good influence on workability. Flaky or elongated and angular aggregate make mix very harsh as compared to cubical or round shape aggregate. Rounded shaped aggregates contribute better than others, as for a given volume (or weight) such aggregate will have least surface area then flaky or angular aggregate. They also can be packed with least voids and by nature a round shape particles exert least frictional resistance. This is why sand and gravel collected from river bed offer greater workability them sand and aggregate derived from crushed stone.

Very low water/cement ratio is essential in production of high performance and high strength concrete. When water/cement ratio as low as 0.25, all the factors, may be minor, has to be taken into account and in that case shape of aggregate is a vital factor in present construction industry. It should be remembered that natural source of sand will be exhausted very soon or become very costly and there will be no choice to choose manufactured sand.