Why are Casing and Drill Mud Required to Drill Deep Foundation?

The drilling is done to reach hard/stiff stratum to safely transfer loads from superstructure through deep foundation. During drilling there some problems and we have to overcome these problems.

Drilling may be done in three types of soil that can encounter frequent problem:

-soft clay

-sandy soil

-gravelly soil

During drilling hole into soft clays, there may have probability of caving. This is due to tendency of soil to squeeze in and subsequent closing of the hole. Thus disturbing the drilling process and redrilling or removing of caved soil is required. The shape of the hole also disturbed. 

The main purpose of drilling is to provide concrete and steel piles or other deep supporting members, may not be possible.

To avoid this situation, we use casing to support the hole in position and hold the shape and to keep the hole open. This casing has to be driven before commencing of excavation.

In case of sandy soil or gravelly soil, the caving phenomenon is also common. Now consider drilled shaft; excavating holes for drilled shaft, to avoid caving, we can drill in two methods:

a. Using casing as the drilling of holes progresses

b. Using drill mud

The usual drilling mud is slurry made of bentonite and water. We have published many posts about bentonite slurry; so we are not discussing it elaborately.

Influence of Temperature and Water Quality on Performance of Air Entraining Admixture in Concrete

Dear reader perhaps we have read our post “quality of water for concrete mix”. There we have discussed about biological and chemical content of water that may impair concrete properties.

Sometime water may be hard or they may be contaminated. The contamination that are due to detergent from industry or house hold water may affect air content in concrete. Even a little contamination due to detergent may affect air entrainment. In case of hard water, there may have also reduction in air content.

There have influence of temperature in performance of air entraining admixture too. When temperature of concrete mix is increased air content is decreased. With an increase in air temperature from (70⁰F-100⁰F), the air content decrease up to 25%.

The opposite influence is observed, when temperature is dropped from (70⁰F~40⁰F), the air content increase up to 40%.

From the above discussion we can conclude that the application of air entraining agent must be adjusted when temperature of concrete is changed.

Influence of Aggregate on Air Entrainment in Concrete

Dear reader we have discussed about influence of cement on air content in concrete. Here we will discuss about influence of aggregate on air content provided by a given dosage of air entraining admixtures.

The main application of air entraining agent is to cope with destruction of freezing and thawing impact on concrete. Here we will discuss both influence of fine aggregate and coarse aggregate on air content.

Fine aggregate:

-When finer fraction of passing no.100 sieve is increased the air content is decreased.

-The fine aggregates that pass through no.10 sieve but retained on no.30 sieve and no.50 sieve, can increase air content with their increased percentage. 

-some types of clay can produce difficulties in entraining air

Coarse aggregate:

-Dust content in coarse aggregate can decrease air content for a certain dosage of air entraining agent.

-crushed stone produce less air content than concrete produced by gravel.

Does Mixer Performance Influence on Air Entrainment of Concrete?

Dear reader we are discussing here influence of concrete mixer on air entrainment; in our upcoming post we will discuss about influence of aggregate and cement on performance of air entraining admixtures. The mixture we use may be

-Stationary mixer

-Paving mixer

-Transit mixer

The usual maintenance work of mixer is to clean it after mixing is completed so that no residue of hardened or semi-hardened concrete remains on surface of drum and also on the blades. If proper cleaning is not done the blade may be coated by hardened concrete, sometimes the blades may also be worn out.

Such worn out and or coated blades may reduce air entrainment in concrete for a given mixer.

Some factors that influence air content related to concrete mixer are:

a. Air content is increased when a mixer runs loaded with ingredient lesser than its capacity

b. Air content is decreased when a mixer is loaded exceeding its capacity.

c. It is very difficult to develop air entrainment in drum mixer of capacity of small loads

It is observed that air entrainment increases throughout first 70 revolutions of mixer after that they remain constant for short duration and after that a decreasing trend is observed.

Advantages of High-Range Water Reducing Admixtures in Concrete Batching

High range water reducing admixtures are often termed as superplasticizers. We have published many posts about concrete superplasticizers. They differ from conventional water-reducing admixtures by reduction of water requirements at very rate. There have another factor; high range water reducers must not produce any side effects on concrete properties at both plastic and hardened condition.

The side effect may be, as example, excessive retardation. With changing dosages of high range water reducers, can be used to produce various form of concrete. You will be surprised to know that they can reduce water requirements up to 30% and sometimes more.

Here we are providing some advantages of high range water reducing admixtures in a list according to National Ready Mixed Concrete Association in brief. In our upcoming post we will discuss them elaborately.

Advantages of application of high range water reducing admixtures in concrete:

a. The reduce water requirement significantly

b. They also reduce consumption of cement in concrete production

c. They as usual reduce workability with very low water-cement ratio

d. Being more workable they reduce placement effort i.e. less energy is required to compact and sometimes self compacting concrete may be produced depending on the purposes of concreting operation.

e. They ensure effectiveness of cement in the mix

f. With application of these, concrete will gain strength earlier

g. They not only effect early strength but also effect on long term strength by showing an increment.

h. Readily reduce permeability.

What are the Functions of Admixture Dispensers in Concrete Batching?

Dear reader here we will discuss about liquid admixtures for concrete. We have already had few ideas about admixture dispensers. When we will discuss about admixture dispensers, we have to cover total process of admixtures storage at production plant to providing admixture to batch prior discharging of concrete. So this operation of admixtures dispensers includes four separate functions as discussed below:

a. Transportation of admixtures by dispenser from storage at plant to the batch location.

b. Measurement of quantity of admixtures is done with dispensers to determine accurate application of admixtures.

c. Verification of dispensed volume with dispensers for accurate batching.

d. Injection of required amount of admixtures onto or into the batch.

The functions discussed above are basic functions of admixture dispensers. But in practical cases some functions may be done in combination of two or three functions. As an example:

Consider function (b) and (c) i.e. the measurement of admixture quantity and verification of volume may be combined. This is done to have reliable result to avoid inaccurate or false batching in respect of admixture injection. This also ensures optimum application of admixture to concrete batch to provide economy.

Dear reader we will discuss throughout our upcoming posts about

–various method of dispensing

-specific method for producing specific concrete

-in actual practice limitation of dispensers

-accuracy of injection of dispenser

The points noted above are very important as successful application of chemical admixtures (in respect of accurate measure and correct injection) is the key factor for producing designed concrete to be served for desired purposes.

Do Deicers Disintegrate Concrete?

Does deicers result corrosion to concrete? The answer is no. so you are thinking why we are discussing this topic as they produce no corrosion. We are missing few words that are “right use” of deicers. Here the frequently used term rock salts used as deicers are nothing but  sodium chloride .

Two most popular deicers are

–sodium chlorides

-calcium chlorides

This fact should be noted that when both are in dry condition, they do nothing harm to concrete surface. These deicers work by forming brine coming in contact with moisture.

These brines penetrate through ice toward its surface, spread out and melt ice breaking bond between it and ice. The melting is further accelerated by the heat (exothermic reaction) generation.

We were and still now are too much dependent on deicers. We were looking for all ice to melt with deicers. This is the main misconception about deicers application.

The appropriate expectation from deicers is to break bond between concrete and ice and then by manually or other mechanical means we have to remove remaining snow or ice. In case of manual removing we can use plow or shovel.

We are suggesting you to use optimum deicers. Do you know why?

The explanation is:

Biggest enemy of concrete is freezing and thawing cycles. We know that the density of water is greater than ice. So ice gets expanded resulting expansion pressure in concrete pore which may even bust concrete apart resulting spalling.

The excess deicers left over concrete results speed up of freezing and thawing cycles. This accelerates this phenomena and also increases proximity of concrete to damage. So instruction provided in package of deicers that you should mechanically remove snow or ice when they loose bond form concrete. So one should always keep an eye to find out salt residue.

Concrete Corrosion in Relation to Ice Removal Agent

From the past, when people became used to apply deicers, it becomes common that these salts (deicers) results disintegration of surface. These salts are:

a. Sodium chloride

b. Calcium chloride

c. Sometimes both are applied

But this guidelines concluded some results that are based on laboratory and practical experiences with

a. Sodium chloride

b. Calcium chloride

c. Ethyl alcohol

d. Urea

It is also observed that these salts can accelerate reinforcement corrosion leading failure called concrete spalling.

Here we will address deicers as ice removal agents. Many performances of concrete have not yet been understood clearly. In this case also we have a fairly well understanding. However, it is ascertained that the mechanism of disintegration is primarily due to physical causes rather than the chemical causes. Dear reader in the next part of this topic we will learn about mechanism involved in disintegration of concrete under application of ice removal agents.

Two types of pressures are developed there which do the harm to concrete member. These pressures are:

A. Disruptive osmotic pressure

B. Hydraulic pressure

The pressures are developed mainly in paste, just like ordinary frost disintegration, but the degree of pressure is more severe.

These formations of pressures in the paste, is greatly influenced by concentration of ice removal agents. Here we are providing some observations concluded in the guide: ACI 201.2R; Guide to Durable Concrete.

Klieger and Verbeck in 1957 showed that when a concentration of moderate intensity is used in ponding concrete mass, a most severe scaling of concrete is observed. The moderate concentration means (3-4)%.

The above observations were conducted over four ice removal agents namely

a. Sodium chloride

b. Calcium chloride

c. Ethyl alcohol

d. Urea

Cady and Brown (1975) also found similar results as discussed in previous posts, i.e. intermediate concentration results most severe disintegration of surface.

Litvan in 1975 and 1976 also sought results consistent with other two observations. But he had added some extra conclusion which is ice removal agent results high degree of saturation in concrete. He furnished that this is the main cause of detrimental effect.

At a particular temperature of concrete , salt solution in ice removal agents produce a vapor pressure than that of water i.e. little or almost no drying is observed between wetting and cooling cycles. ASTM C672 provides determination procedure to assess resistance of a particular mix against scaling under application of ice removal agents. 

Ultimate Limit State Principle for Soil Reinforcement

Limit state principles for soil reinforcement are of two types. These are:

a. Ultimate limit state

b. Serviceability limit state

Here we will discuss about ultimate limit state. In the next post we will learn about serviceability limit state. Ultimate limit state:

This principles includes

-collapse

-similar other types of structural failure

This states are reached for certain modes of failure. This failure occur when restoring forces become less or equal to disturbing forces.

The collapse that are considered in limit state, should be opposed by sufficient safety margins. The safety margins are:

-Partial material factors

-Partial load factors

Partial factors assumed should be equal or greater than unity.

Disturbing forces are increased by using prescribed multiplication factors discussed above to derive design load.

Material factors, prescribed, are used to derive desire strength. Here restoring forces are decreased by dividing actual material strength by material factors.

It is believed that when design strength equal to or greater than design loads, sufficient safety margin is provided against the collapse assumed in ultimate limit states.

Objective and Type of Reinforcement Used to Soil Retaining Element

Here we will discuss about reinforcement provided in soil to retain and stable earth. Reinforcement that are used, generally, in soil may include following reinforcement types and providing techniques of them

-metallic strips

-metallic girds or meshes

-polymer strips

-polymer grids or meshes

-anchoring or multi-anchoring

-In-situ reinforcement and soil nailing

In case of anchoring and multi-anchoring, ground anchoring is not granted.

Reinforcement that are discussed above may be used in

a. Engineering fill

b. To have better load bearing capacity

c. To improve weak soils to perform better to support

–embankment

–resilient structures

d. Can be provided in natural ground to keep steeper slopes stable.