Fineness Modulus of Sand for Concrete

Fineness of fine aggregate is related to term fineness modulus; most commonly sand. Fineness modulus is a measure to define fineness of aggregate precisely it defines coarseness or fineness of aggregate to be used in concrete.

This can be defined as value that is derived by following expression:

F.M = (cumulative percentage retaining of aggregate)/100

The retaining means retaining on standard sieves.

Here we will discuss about ranges of fineness modulus of sand according to different standard.

IS 383:

Unfortunately this Indian Standard did not provide any specific limit of fineness modulus. Rather they divided sand in four zones:

a. Zone I

b. Zone II

c. Zone III

d. Zone IV

Where Zone I indicates a very coarse sand and Zone IV defines a sand very fine size. The intermediate zones are fall between them. They provided that, for mix proportioning for a structural concrete zone IV should be ignored and zone III are recommended.

USBR: 

USBR provided specific limits for fineness of sand these are:

F.M ≥2.5

F.M≤ 3.0

ASTM:

This standard provided that FM should be

F.M ≥2.3

F.M≤ 3.1

AASHTO:

They also provided same limit of FM as that of ASTM that is

F.M ≥2.3

F.M≤ 3.1

Dear reader we have just discussed about limits from fine aggregates in terms of fineness modulus, but there have some additional requirements about fine aggregates which we like to include in our next post according to AASTHO and ASTM.

Influence of Porosity and Absorption of Aggregate on Concrete Properties

Aggregates generally contain pores; these may be of various size and extent. Some aggregates have pores within solid where other have opening to the surface. Dear reader we will discuss about pore size of different rock aggregates and their extent in our next post. Here we want to explain influence of porosity, absorption in aggregate on the properties of concrete. The porosity, permeability and absorption of aggregate have influence on following properties of concrete.

a. Bond between aggregates and surrounding hydrates paste of cement.

b. Chemical stability of concrete

c. Resistance of concrete against freezing and thawing of concrete

d. Resistance of concrete against abrasion

e. Apparent specific gravity of aggregate itself.

As specific gravity is affected, for a specific mass of aggregate, concrete production also affected.

It is obvious that a porous aggregate produce concrete of unit weight, as that of light weight aggregates. Now-a-days lightweight aggregate concretes are used for structural member, which yield significant strength with special treatment.

But when absorption is concerned, it will effect water demand with in concrete and should be adjusted for water/cement ratio while proportioning mix design. In our next post we will discuss about pore sizes of natural aggregates. Please stay with us.

How to Differentiate Loess from Other Soil?

Dear reader in this post we will discuss about identification of loess soil from other soils. We have discussed about index properties of loess, which produce a basis to differentiate loess from other soil. The index properties we discussed are:

a. Specific gravity

b. Dry density

c. Liquid limit

d. Plastic limit etc.

This is a collapsible soil, so after determining collapse potential we can also identify it together with other properties. Collapse potential has already discussed in previous posts and more elaborate testing procedures will be published here very soon.

Loess has brownish yellow or light yellow color. We have discussed grain-size distribution of loess in previous posts. We know loess is mainly silts but contains some amount clay and sands also furnish with cementing action from calcite.

We are summarizing all index properties of loess in following table: 

We will discuss about permeability and shear strength of this soil in our upcoming posts.

Specification for Concrete in Pile Foundation (BNBC2006)

We have discussed about different concreting method for pile foundation. Now we will discuss about specification for concrete mixing and placing. Our aim is to produce workable concrete that remain cohesive during placing. Here we are providing some specification about concrete strength, cement content and slump as well.

For cast-in-situ or bored piles, the minimum strength should be 20 Mpa. But BNBC allowed 15 Mpa concrete with some conditions like

-maximum depth of pile is 6m

-concreting is done in underwater condition

-soil conditions surrounding concrete are non-aggressive i.e. favorable.

-higher strength is not a requirement for structural point of view.

Another requirement said that for tremie concreting in a small diameter pile which length doesn’t exceed 10m, the cement should be 350 Kg per cubic meter of concrete. Cement content have to be increased to 400 kg per cubic meter of concrete for deeper pile (>10 m) and/or larger diameter piles.

The 20 Mpa strength is correspond to 400 kg/m3 (minimum) of concrete and 15 Mpa strength is correspond to 350 400 kg/m3(minimum) of concrete.

A 10% excess cement should remain in mix than cement required in dry placement condition, when concreting is done under water. The coarse aggregate content should be as follows: Coarse aggregate = (1.5-2) X Fine Aggregate

The mix design is performed such that the slump of concrete remains in between (100-150) mm; these are minimum and maximum limit for slump. But when plasticizers are used, these slump may be adjusted which allows slump as much as 175 mm.

Concreting in Pile Foundation (BNBC2006)

Dear reader we have few idea about pile concreting; here we like to include BNBC requirements for concrete in piling work. We know that concreting can be done by tremie method and drop bottom bucket. Here we will learn about which method is suitable for what types of piling work.

Our aim is to produce and place concrete in such way that it precludes segregation in bored or cast-in-situ piling operation. Here we will use deposit instead of placing of concrete, as in this case this word seems perfect.

A continuous deposition of concrete is expected until brought to desire level. Top surface of concrete should be kept level (up to practical precision) and seam formation is avoided. The concreting method depends on diameter of bore hole. For a large diameter borehole, concrete depositions can be done either by

-tremie method

-drop bottom bucket

For a small diameter hole drop bottom bucket method is ignored and tremie method is adopted.

Now we like to include some slump value to have workable concrete that won’t get segregated:

In case of under-reamed piles, concrete should have slump of 100 mm.

In case of providing concrete in water free condition in borehole, the slump should be 150 mm.

In case of tremie concreting slumps of concrete are kept within (125-150) mm.

Dear reader there has some requirements of cement content and strength of concrete for different piling conditions and way to concreting, which we will learn in our next post.

Strength Test for Differentiating Silt from Clay Soil

We have already discussed about importance of index properties of soil and also field identification test for differentiating one types of soil from other. Say a soil of clayey consistency can be set under unconfined compressive strength apparatus but a sandy soil is not suitable for this apparatus and should use direct shear test or other suitable apparatus for sandy soil. We will discuss about four tests for differentiating silt form clay; these are:

a. Shaking test

b. Strength test

c. Rolling test

d. Dispersion test

Of these shaking test has been discussed in our last post. We will learn about strength test here.

Strength test:

The sample to be tested is prepared to for briquette; then it is left for drying. After drying, it is tested by breaking and if it broken easily, we can say it is silt. When the sample is of clay soil, it takes some effort to break this.

Another observation can be made, if the loose material of surface of briquette can be dusted off, it is silt.

While a clay sample cannot be easily dusted off.

Moreover moist clay soil provides flowing identification appearance:

a. When pressed within finger, it produce a soapy feel

b. It shows sticky feeling

c. It dries slowly

In our upcoming test we will discuss about dispersion test and rolling test used to differentiate these soils. Please stay with us.

Field Identification of Silt from Clay Soil by Shaking Test

Dear reader we have discussed about many methods to differentiate one type of soil from other and some methods are still to come. We have, mainly, so far, discussed about filed identification of soils. Here in this post we will discuss about some simple tests to differentiate silt from clay soils.

We can place soil particles under microscope only in laboratory which is a time consuming and also costly and difficult task. So for rough estimation about project involved soil (say for tender purpose) to have rough idea about cost and time of construction (as an example related to consolidation) and sometimes for determining way of examination and testing program, we depend on index properties of soil. The test like shaking test is easier but rough than even index properties, but useful in some cases.

Shaking test:

Here a portion material is placed in the palm and shaken-

If silt-

A shiny appearance is observed as water comes out to surface.

When kneaded, the shiny appearance disappeared as moisture in surface is re-entered into soil.

If clay-

The moisture cannot move out easily which doesn’t provide shinny look, remaining soil dark as in previous state.

But if there have some soil in clay, the shine appears relative slow rate which gives a rough estimation of amount of silt in soil particles.

Collapsible Soils Other Than Loess

We have discussed about collapsible soil, loess in our previous many posts; here we will learn about collapsible soil other than loess. Sometimes poorly or loosely consolidated soil shows collapse potential. As an example we can include about deposits that are formed by mud flows and flash floods.

These deposits after drying out looks stable soil; but collapse under change in load and moisture condition. Many residual soils also behave like collapsible soil.

Residual soils are formed due to weathering of rock but remains in place of their origin. This phenomenon is occurred when weathering rate of rock is greater than transportation of weathered materials. In this regard wind, water and gravity contribute large share of residual soil to form, not transporting them.

Thus this soil retains many properties of its parent rock. Unlike transported soil, residual soil may have variable engineering properties from one place to another which depends on

a. Parent rock

b. Climatic condition during formation of residual soil.

As residual soils are involved in many engineering constructions, either as foundation support material or construction material, we have to predict their properties, including collapse potential, which is our concern in this post.

The weathering action results soils having wide range of grain size distribution while formation of residual soil. Colloidal and soluble materials, by weathering action, leached out to produce large void ratio and render an unstable structure. Residual soils produced from parent granites, are found in large parts of Zimbabwe and South Africa.

Causes of Cementing Properties of Loess Soil

The most naturally forming collapsible soils are wind transported (Aeolian). They are

a. Loess deposit

b. Aeolic deposits

c. Volcanic dust depositions

Generally fine particles of sand or silts and sometimes both are blown to form these deposits. The main properties of collapsible soil are:

a. Low unit weight

b. High void ratio

c. Generally cohesionless or sometimes have slight cohesion.

Dear reader we have discussed about clay contamination of loess soil in our previous post. The silt or silt size particles have a coating of clay around them. These clay particles render a stable condition under unsaturated state. Dear reader we have discussed about stability of slope and cut of loess soil in our previous post.

There have also another cementing material that also renders cohesion to loess; this is calcite. It may occur in loess in two processes:

a. Leached into loess deposit from above

b. Some capillary water may contain this in dissolved form and left in loess when they get evaporated.

An observation made by Bandyopadhyay in 1983 that loess found in Kansas shows their cohesive properties mainly due to montmorillonite clay. Whereas calcite remains in loess in such fine state that it dispersed uniformly act as filler material, rather cementing material.

So, calcite provides inter-granular support to loess, as secondary materials, together with clay which provides primary support. Illite also acts for this purpose with montmorillonite but its contribution is small.

Contamination of Silt Soil with Clay

We have already heard that pure silt soil have no particles that have attraction and shows inert property. But the actual condition is not the same, in most cases, it shows identical properties, not in same degree, of clay.

They frequently show cohesion like clay which is due to the impurities, sometimes called contamination of silt with clay. When a foundation bed has clay particles, a foundation engineer becomes conscious about problems associated with settlement and strength. Being similar properties, silt is also a suspicious soil.

Throughout last two posts, we have discussed about differentiating silt from other soils and from sands. We will discuss about differentiating various types of soil from each other in this blog.

Here our concern is clay but organic materials also render cementation properties to silt which will be discussed in upcoming posts.

The degree of cohesion rendered by clay depends on followings:

a. Grain size of silt

b. Type of mineral present in clay

Depending of above factors a mere (5-8) percent of clay content render a deposit of silt to behave like clay with sufficient cohesion.

When higher percentage of clay is found in clay or based on visual effects, sometimes silt is called clay, especially in foundation construction personnel.

Environmental Change Done by Silt Soil

We have learned about loess soil which is formed by transporting silt to new form resulting new deposit with the help of wind. Silt can be transported long distance as dust due to its particle size (fine enough to blow). Not only with wind, it also transported with water.

Clay and silt are responsible to produce turbidity property in water; when silt is found to pollute water, the phenomenon is called siltation. Silt is carried to ocean with water current or stream.

The excess amount of silt is harmful to water quality which can impact aquatic environment. In case of construction project where excess amount of silt disposal, discharge or any form of silt release can impair water quality, we use silt fence. Dear reader we will discuss about silt fence in details in our upcoming posts.

Silt deposition often enriches and makes fertile soil. The deposition we meant is due to flood which we observed in ancient civilization (Egyptian).

Sometimes silt deposition forms new land between water bodies due to formation of obstruction against water current (the prominent example is Bangladesh). In last 50 years, Noakhali district in Bangladesh gained over 73 square kilometer land.

The causes of presence of silt in rural river are

a. Erosion due to plowing of agricultural field

b. Clearfelling of forest

c. Due to converting forest to agricultural land.

The main source in case of urban river is construction activity disturbing soil.

Differentiate Silt from Sand Soil in Field

Dear reader our post length is shrinking as we are discussing more specific term and trying to differentiate one term from other.

We have already have learned about sand and silt in our previous post. Here we are providing field identification method to differentiate silt from sand in field before sample reaching to laboratory.

Particles size difference between two types of soil has discussed in last post. It is very difficult to distinguish fines and from silt by normal visual examination. Silt may appear little darker than sand in color.

But we can include dispersion test which can differentiate two types of soil. In this test, a spoonful soil sample is poured into a jar filled with water.

Two soils perform clearly distinguishable setting phenomena. If sample contains sand, it takes only one to two minutes to settle down. But if it is silt, a setting time may extend up to 15 minutes to an hour. But there have a similarity in both soil samples; this is, ultimately nothing is found in the suspension.

In case of sand individual grain can be detected as granular shape and dry state they are free flowing. Silt has some impurities of sand and clay which may be 20% of it. At dry state, it seems to be

a. Cloudy

b. Can be easily pulverized to powder

c. Have feel of soft flour

A simple identification with squeeze and ribbon of sample can be used: 

How to Differentiate Silt from Sand?

Clay and silt have drawn attention from foundation engineers as they produce most problems related to strength and foundation settlements. Here our concern is silt; we have discussed many aspects of clay in previous posts.

Silt has mineral origin of feldspar and quartz. It has moderate specific surface area having a typical plasticity, non sticky feel. Sand is a granular material has mineral particles and fine fragments of rock. Though it has variable composition, in most cases, silica as quartz is the main constituents of it. This sand is very resistant to weathering because of hardness and chemical inertness of constituent minerals.

At dry state silt has floury feel but when get wet it results slippery feel.

The grain size of silt and sand produces the principle difference. We are providing a table below which lists grain size of both types of soil particles.

0.625mm is the demarcation line between sand and silt (ISO 14688). The upper limit of silt is (0.074 -0.05) mm probably 0.625mm is defined from the average of both.

Particle of silts is smaller than above value and go down up to 0.004mm. Sand shows opposite trend but upper limit is variable according to different standard.

Silts without having contamination are inert i.e. there have no particle that have attraction potential. Contamination term is used as silts often remain in contaminated state. We will discuss about this in our next post.