What is the Significance of Modulus Elasticity of Concrete?

The definition and graphical interpretation of modulus of elasticity is common to all. We are concerned here, what is the significance of this in concrete engineering. The significance of it for materials that are homogeneous, defines inter-atomic forces of bond.

For such materials the microstructure is not important i.e. independent of microstructure properties. The concrete is not the same material as it is a heterogeneous material which has multiple phases in microstructure.

The typical range of modulus of elasticity of concrete under compression lies between (14X10³– 40X10³) Mpa. In FPS unit this lies between (2X10⁶ – 6X10⁶) psi.

In structural design, the practical significance of elastic limit depicts the fact which represents the allowable maximum stress of that material before undergoing permanent deformation.

The theoretical concept is the slope up to straight line portion of Stress-strain (σ-ε) plot i.e. within the region of elastic deformation.

Thus the higher modulus of elasticity of a material represents a stiffer one. This is a measure for resistance against elastic deformation. So the engineer should know this value of concrete as it have influence on rigidity of design member.

Dear reader we will discuss modulus of elasticity of concrete in respect of microstructural point of view and how microstructure influence modulus of elasticity.

What is Aggregate in Concrete Engineering? ACI C125

Dear reader we have discussed many aspects of aggregates and some advance application of aggregates; but we have not defined yet aggregate. Let’s try to define them.

This definition is quoted from ACI C125 and ACI committee 116. ACI 125 provides standard definition of terms related to concrete and as well as concrete aggregate. ACI committee 116 provides a glossary of terms in the field of cement and concrete technology.

Aggregate is a granular material which is mixed with cementing medium or compounds to produce either mortar or concrete. The usual examples are

-sand

-gravel

-crushed stone

-crushed blast-furnace slag

-or demolition or construction waste

Depending on size we can group aggregate as coarse aggregate and the fine aggregate. Coarse aggregate are those that have particle size greater than 4.75 mm (or No. 4 standard sieve).

On the other hand fine aggregate are those that have particle size range > 75µm to <4.75 mm. 75µm is stand for retaining on No.200 sieve.

But some aggregate containing substances that can check hydration process, deviate hydration products and some produce expansion that results serious problem to concrete and compromising durability.


We have provided deleterious substances that can remain in aggregate, their effect and volume fraction to limit harmful reaction, workability or durability problems.

Energy effciency of concrete

Now we are very much concern about environment pollution especially with carbon emission. During industrial revolution, unplanned burning of coal produce huge amount of greenhouse gas and later we also continue the emission by better managing fossil fuel but using in large amount. Now global worming is burning question especially for low lying country.

Now-a-days, conscious people look for energy consumption of materials they used per unit volume. Our material is here concrete. With many advantages of concrete, energy efficiency of it, is also appreciated with rest. Can you imagine? each cubic meter of steel production seek 300 GJ energy while concrete need only 3.4 GJ for each cubic meter.

Major energy consumption and related cost for concrete involved with cement production. In case of reinforced concrete steel also considered. The energy requirement of both materials have specified above. Steel used as reinforcement have greater energy consumption but used as a mere percentage of concrete. Say in column only (2-3)% steel used [limit (1-8)%], in other sections like slab this percentage is also quite low.

Again above materials are industrial by-product which are not the primary product of industry i.e. in industrial process, energy is consumed to produce those product but unexpected in the production process. Thus using these materials we also using wasted energy if they left unused.

Now another consideration can also be include here, as energy efficiency, is its excellent thermal properties (thermal conductivity 3 w/m.k). Regarding thermal conductivity we have published many posts and also some about fire attack and damage in this blog and many will be posted soon.

Factors Influencing Aggregate Characteristics for Concrete

The characteristics that decide whether an aggregate produce good, workable, strong as well as durable concrete, were discussed in previous post. These characteristics are derived from three factors as follows:

a. Mineral composition of rock from which aggregate are derived whether from it or from a weathered form of rock. Thus geologic process of rock formation is important.

b. As stated above rock may be weathered to form modified rock. These modification or degree of modification of rock depend on exposure conditions under which they are subjected before mining work.

c. The equipment used to extract rock and production of required grade and size of aggregate.

These mean rock formation,description and classification of minerals and rocks and factors involved in industrial processing are of great importance.

Consider silicious rocks like

-cobbles

-gravel

-sand

They are generated by geological process of weathering. Though they formed from parent hard rocks-minerals but weathering process makes them smooth and round surface are available in these aggregate.

Laminated limestone, shales and sandstones have tendency to produce flaky and elongated fragments while crushing predominantly when jaw crushers is used for crushing to provide workable sized aggregate. 

Important Characteristics of Aggregate Significant in Concrete Mix Design

We know aggregate is comparatively cheap component of concrete mix design and usually assumed they will not go to reaction in presence of water.

Thus we can treat them as inert filler. But this inertness is rendered by selecting suitable aggregate. In this blog we will discuss about source characteristics, application of aggregate especially special aggregate (from waste materials, artificial aggregate, recycled aggregate, rock origin of aggregate etc.)

Being inert particles we can not ignore its importance in concrete proportioning. Now-a-days, the conventional view of inertness is of burning question, with the increase awareness of roles played by them influencing many significant properties of final product concrete.

The significant characteristics among many for proportioning concrete mix design includes:

a. Porosity

b. Moisture absorption

c. Grading or size distribution

d. Shape of aggregate

e. Surface texture of aggregate

f. Elastic modulus

g. Crushing strength

h. Type and form of deleterious substances exist in aggregate

Source of Fine Materials in Aggregate for Concrete

Dear reader we have already learnt that fine materials are of crusher dust and silt except clay. We have learnt about clay in concrete the previous post; here we concern about other two fines.

Silt size lies between (2-60) µm. They are occurred due to weathering process, so they occur naturally in aggregate.

Crusher dust is produced during pulverizing of rock to form crushed stone. This is also a fine material; sometimes may produce due to crushing of gravel to form fine aggregates.

Properly installed processing plant usually washout such materials. But some materials remain in well-bonded condition which cannot be removed easily.

All materials on the surface of aggregates are not harmful. If they are in well-boned, chemically stable condition and do not produce any deleterious effect, these fines are permitted, though there may have shrinkage effect.

When they reacts chemically with cement, even though they are physically stable, will lead to severe problems within concrete. Silt and dust from crusher may also form coating like clay as discussed in previous post. They may be as loose particles not in bonded condition to coarse aggregate. Both forms are detrimental to concrete when remain in excessive quantities.

Becoming Durability of Concrete a Central Feature

This topic actually covers significance of durability of concrete. In designing concrete structure once only mechanical properties of concrete in relation to initial coat was considered. We know what are the mechanical properties of concrete? But in recent years we have to concentrate on durability ; here we will learn about these facts.

At first we will start with socio-economic implications. The structures which were constructed having less concern of durability, before reaching its service life, suffer severe or considerable deterioration.

This fact was observed and treated seriously when cost of replacement and repair of  concrete structures to rehabilitate them found to be significant portion of total budget of construction.

Let us consider an industrially developed country, where maintenance and repair cost of existing structures is reached 40 % of total cost of overall development and maintenance work. That is only 60% budget is remaining for new construction.

Thus engineers are bound to be conscious about durability due to increase in cost of replacement and increasing emphasis on cost to produce longevity of structures rather than initial cost.

Again durability of structures or materials is closely related to ecology. 

Conservation of energy and material is common feature that are considered very frequently in application and development of different technology. We know concrete constructed of fine aggregate, coarse aggregate, binding materials (most commonly cement) and some admixtures or additives in the presence of water.

In case of fine aggregate artificial sand and crushed glass particles are using to conserve natural sand. The coarse aggregates are also replaced by some extent or fully by glass particles, industrial by-products and recycled products.

Aggregate from crushed concrete is included in ASTM and ACI code providing some provisions. The replacement of aggregate by glass, by-products or crushed concrete always associated with some unexpected reactions while hydration which are mitigated by necessary modification by adding flyash, ASR controlling admixtures and pozzolanic modification of aggregate (say fineness).

The most costly element of concrete is cement which is also partially replaced by different pozzolanic materials and industrial by-product (say BFS). So we have some idea about conservation of concrete materials. We can conserve materials also by providing longer service life of concrete structure, which is achieved by durability considerations; thus furnishes an ecological step.

Now concrete is using in many challenging environment having industrial exposure, hostile marine environment and as container of corrosive chemicals or pressurized gas vessels. Concrete pipes are using in many purposes of industrial and municipal activity. This exposure extremity also forced designer to be conscious about durability.

Failure of Oil drilling Platform and Emphasis on Concrete Durability

The most widely used material  for construction of dwelling, industrial and mining structures is Concrete. With wise change in ingredients it shows wide range of changes in its properties.

But steel is also a widely accepted materials and from very ancient times it has been using in different purposes. We also take help of steel structures even in concrete construction both as permanent or temporary purposes.

In water logged site or different offshore construction and drilling, the use steel for different purposes are very usual and essential. Due to advantage of prefabrication, like long span beam and pipes, it is adopted in almost all construction conducted above and under water.

We have discussed piling, drilling work in our many posts which also include application of steel. With the advantage of prefabrication, it saves times of construction.

Steel is stronger than concrete both under compression and tension loading but have much susceptibility to deterioration especially under such hostile environment.

So designers are choosing concrete instead of steel considering durability. This fact was notices in 1980; platform constructed with steel for oil drilling purpose failed suddenly at Stavanger coast (North Sea) which took 123 lives.

This followed by another failure of such platform near Newfoundland, Norway which also cost 64 lives. This sudden failures all over the world resulted casualties, economic losses and also interruption of services.

Therefore concrete was chosen to flight against hostile environment due to ease of modification of its properties with some simple and wise techniques.

Why are Environmental References Required to Define Concrete Durability?

At first we like to provide definition of durability. According to the ACI Committee 201:

Durability is the ability of Portland cement concrete to resist different weathering action, abrasion, chemical attack or any sort of deterioration process.

In brief concrete that keep its original state under the exposure of probable or expected service environment is called durable concrete. The original states means:

a. Form

b. Quality

c. Serviceability

So, exposures are important in defining durability. The several exposures may act simultaneously around concrete member. A synonym of durability is longevity or long service life.

In this world we have no material that is durable inherently. In respect of chemical reaction, one material can be durable (like gold) but it may be very weak in physical causes of deterioration. We have discussed about physical causes of concrete deterioration in last post.

So a material or structure made of such materials is considered to exhaust its service life when required properties of serviceability under given exposure suffered deterioration of such extent that the continuation of use is considered uneconomical or unsafe.


So durability of concrete under certain set of exposures may not mean durability in other set. That means, we have to include some general reference of environment when certifying a material durable.

What are the Causes of Concrete Deterioration?

Form beginning of concrete construction, it is the strength that controls the design of concrete members. But now-a-days the interest on the durability of concrete is governing and in many structures the durability is the primary criterion of design.

With the development of concrete technology, application of concrete in various purposes that can be subjected to severe effect of environment and manmade physical exposures is increasing.

Here we will discuss about agent that can render deterioration to concrete. Let us consider physical effects; these are 

a. Surface wear like concrete highway pavement, industrial floors etc.

b. Salts reached in various ways in concrete can result cracking under crystallization

c. Temperature extremity like fire and frost action. We have discussed many posts about explosion resistant and fire resistant concrete and also necessary retrofitting of such deteriorated or destructed concrete members.

Regarding chemical effects on concrete we can include:

a. Leached cement paste due to acidic solution

b. Expansive reaction which can include 

1. Reaction due to sulfate attack

2. Alkalis-aggregates or alkalis-silica reaction

3. Expansion due to corrosion of steel embedded in concrete as reinforcement

But the main culprit is water not only for concrete but also in almost all solids, especially having porous structure and concrete is the ideal materials that include various forms of pores with mere change in mixing, placing, curing and also variation of ingredients.

In such porous structure, the ease to penetration of water the more the concrete susceptible to deterioration. So porosity (especially interconnected pores) determines the rate of its deterioration.