Organic Content, Acid and Shine Test for Clay Soil

Dear reader throughout our last few posts we were discussing about field identification of different types of soils. In this regard, it have to keep in mind that filed identification tests of soil do not produce accurate idea about soil sample and an experienced engineer cannot predict right idea. He must have through knowledge about the geology of the project.

Organic content identification:

Organic content in soil is carefully handled as it renders peculiar behavior to soil that cannot be explained with conventional soil mechanics. We have discussed about few aspects of organic soil in our previous post.

The organic content in soil is determined in laboratory by monitoring difference in liquid limit before and after oven drying. But we have no facility to check these in filed. So have to depend on color and odor of soil.

Moist and fresh organic soil generally has a distinguishable odor for decomposition of organic matter. The odor can be enhanced by heating soil sample. Besides it have distinguishable color; generally they are of dark color. Such soil is called organic clay.

Acid test:

Dear reader we have discussed about collapsible behavior of loess soil in our previous post. We learnt the dry strength of loess soil in previous post. We learnt the dry strength of loess is the contribution of clay and calcite. Under saturation, they lose cementing properties provided by above mentioned two constituents. So calcium carbonate (calcite) is important constituents to indentify such soil to examine its dry strength.

In this test, dilute hydrochloric acid is used to identify calcite in clay soil. A strong reaction is observed when there have calcite content, otherwise we can conclude that colloidal clay render cementing properties.

Shine test:

This test is used to determine highly plastic clay. If a lump of soil in slightly moist or dry soil is sliced with knife, a shiny sliced surface indicates very plastic clay.

In case of dull surface, the sample is taken as silt or low plastic clay.

Rolling Test and Dispersion Test for Differentiating Silt from Clay Soil

In our previous posts we have learnt about shaking test and strength test for differentiating silt from clay. Now we will discuss about rolling test and dispersion test. We have already learnt about methods for differentiating silt from sand. These two tests we will discuss are very simple test and can provide an rough idea about presence of silt and clay, in soil sample.

Rolling Test:

In this test, a thread of about 3 millimeter diameter is tried to make with moist sample.

If soil sample is silt-

Crumbling and disintegration is observed while making thread, in a word, the faulty thread is formed.

If soil sample is clay-

Thread can be made easily without disintegrating and any other fault. A thread of about 300 millimeter in length can be produced.

The most interesting is that the thread can support self weight without disintegration even when make it suspended holding its ends. This test is also known as toughness test.

Dispersion test:

In this test a jar is filled with water and a spoonful sample is poured into water from top.

It this is silt-

The soil particles take about 15 mins to an hour to settle completely.

If this is clay-

A suspension is formed, that will hold its previous position up to hours to even days, but no flocculation is observed.

Why should Curing of Concrete Member be Started Immediately after Compaction? (BS-8110)

Concrete member, after properly placing, have to undergo curing. This term means providing a barrier against loosing moisture from concrete until an adequate temperature regime is maintained. The objective is to prevent formation of temperature gradient of high degree within the respective concrete.

A sometimes special type of cement or admixture is used to facilitate gaining strength of concrete. One of them is super sulfated cement; in this case, the gaining of early strength is remarkably hampered under lower temperature.

So concrete produced with super sulfated cement is greatly affected by improper and inadequate curing. The minimum required curing period for such concrete and normal concrete will be discussed in next post. Here we are discussing these topics according to BS 8110.

Here our concern is starting time of curing after pacing. The requirement is immediate after compaction. The reasons behind this are:

a. To avoid premature drying of concrete, especially by wind and solar radiation.

b. To avoid leaching out from concrete by flowing water and rain.

c. To prevent rapid cooling of concrete during few days immediate after pacing.

d. To prevent high thermal gradient within concrete

Diffusion of Oxygen through Concrete

Dear reader we have learnt about diffusion through concrete. We are primarily interested on oxygen and carbon dioxide diffusion. We have already learnt diffusion mechanism. Any type of movement of fluids either in gaseous or liquid state, there must have pore system in concrete and depending on their internal connection system, the movement of fluid is varied.

We know intrusion of oxygen accelerates deterioration of metal reinforcement embedded within concrete and our aim is to reduce diffusion as small as possible. A term diffusion coefficient has already been introduced in our last post.

Pore system of concrete is noticed to change with time and which more prominent during hydration of cement paste. As diffusion is a function of interconnected pore system, diffusion co-efficient is also changed with time.

It is also noticed that diffusion through water filled space is significantly slower than that in air-filled space. In case of oxygen diffusion, it is obvious that moist curing significantly affect diffusion of oxygen through concrete. Diffusion coefficient of concrete is observed to reduce to great extent (up to a factor around 6) under prolong moist curing.

Moisture condition in the concrete has also significant influence on diffusion as water in pore also reduce diffusion rate. So a well cured concrete having less pores, pores with discontinuous system or water in pores, as a whole a good quality concrete have less susceptibility to diffusion. In our next post we will learn about effect of humidity on diffusion of fluid through concrete.

How do Fluid Ingress in to Concrete by Diffusion?

We have learnt in last post about difference between different processes of fluid ingression into concrete. Diffusion is a process in which vapor or gases is transported through concrete due to differential concentration or concentration gradient but not due to pressure gradient.

Diffusion may permit liquid vapor and gas to transport into concrete mass. When we are concerned about gases, our headache is for

-carbon dioxide

-oxygen

Other gas can be diffused but maximum harm is done by those two gases.

When water is considered notable amount of aggressive ions like chlorides and sulfates can migrate by diffused into pore water. We will discuss about ion ingression into pore water in concrete elaborately in our next posts.

Carbon dioxide do the harm with carbonation of the hydrated cement paste and oxygen take part in the corrosion of steel embedded in reinforced concrete.

There have a relation between diffusivity coefficient and molar mass of gas; this is diffusivity coefficient is inversely proportional to square root of molar mass of respective gas. So using this relation, we can determine diffusivity coefficient of an unknown gas from a known value of another gas (determined by experiments); say carbon dioxide diffuses 1.17 times (theoretically) slower than oxygen due to difference in molar mass.

Pointing Out of Earthquake Sources

For evaluation of earthquake hazard, every probable sources of earthquake activity, for a desired region or site, have to be identified and their capability to produce strong ground motion, in future, is also determined.

Some works for determining seismic sources is required and clues provided by nature are examined carefully; of these, some are obscure but others are obvious.

Now-a-days seismographs and their network all over the world facilitate inspection and subsequent interpretation of earthquake with great convenient. These networks now offer us to record occurrences of large earthquakes by hundreds of remote or closely set up seismograph, which could not be imagined in 1990s and remained undetected (instrumentally) anywhere in the world.

Within hours following parameters can be determined;

a. Earthquake magnitude

b. Location of rupture surface

c. Even source parameter

This development is relatively recent and in many regions, past record after strong ground motion is not available or have very few data. So if there have no strong ground motion record (instrumentally), for a desired region, doesn’t provide assurance that there were no such strong ground motion and in the future also there will not such ground motion.

So when instrumental earthquake record is not available, other clues are integrated related to earthquake activity. The clues may be

a. Geologic evidence

b. Tectonic evidence

c. Pre-instrumental seismicity

What are the Difference Among Permeability, Diffusion and Sorption within Concrete?

Dear reader fluids travels through concrete not only by permeability but there have also other mechanisms. Here we will discuss about the process which facilitates easy movement of fluids into concrete. We have already had few ideas about affect of ingressed fluid and it becomes now very important to distinguish among processes of ingression of fluids.

The process involved is

a. Permeability

b. Diffusion

c. Sorption

We have discussed about permeability of concrete in our last post and previous posts. In our next post we will learn about mathematical expression for determining permeability.

In case of permeability, important parameter is pressure i.e. fluid flows due to difference in pressure between two regions with in concrete whereas diffusion depends on concentration. In diffusion process, fluid moves due to difference in concentration. The property that defines diffusion is called diffusivity of concrete. Diffusion may associated with gases; they can move through

-water filled space

-air filled space

But the rate of diffusion of gas differs greatly depending on the space through which it passes. Say diffusion through water filled space is slower than through air filled space by (104-105) times.

Now we will learn about another process sorption; this process allows fluid to move by capillary movement in pores of concrete exposed to ambient medium. It should keep in mind that sorption is valid when concrete is in partially dry or saturated states as capillary suction can only take under this condition. So we should not worry about a concrete suction of fully dry or saturated condition.

Surface Moisture of Aggregate for Concrete

It is now clear to us that if we do not expect any moisture to move from or to aggregate, the pores in it must be filled with water. But in some cases surface moisture may remain in aggregate which may contribute to water in concrete mix. These surface moisture also sometimes results volume change of aggregate mass.

So as discussed in previous posts, the basic/ideal state of aggregate is saturated and surface dry (SSD). Now how surface moisture is introduced to aggregate?

There may have two causes

-natural

-Man made

The natural causes are moisture introduction by many natural process like rain, dew etc. when aggregates remain exposed to rain they collects significant amount of moisture on their surface after being saturated. This surface moisture remains for long period except on those on the surface of stockpile, we are considering.

This phenomenon is very common for fine aggregate. Surface moisture is often called free moisture. Obviously this surface moisture should be considered in mix proportioning by allowing them for in calculation for quantities in batching.

As discussed above, fine aggregate can retain free moisture of greater than 10 percent. This value is quite negligible in case of coarse aggregate; about 1 percent. This surface moisture is expressed as a percentage of SSD mass of aggregate. Dear reader we will discuss about bulking of sand mass of aggregate. Dear reader we will discuss about bulking of sand due to moisture content and its effect on concrete properties.