Increasing Bond between Steel-Concrete Interface with Gas Forming Admixtures

Dear reader we are introducing a new type of concrete admixtures, gas forming admixture. These admixtures are used to increase void content generated by gas. The purposes of this inclusion of admixtures are to generate or release gas bubbles during and just after placement in fresh concrete mix and continue this till cement paste to set.

These admixtures retain its initial volume forming gas voids. In higher inclusion rates it will render lightweight concrete. Normal application of admixture to entrain air is to serve as resistant to freezing and thawing. But this admixture we are discussing will not make it freezing and thawing resistant.

The most common material used as gas forming admixture is metallic aluminum powder. We will discuss about the materials that form gas in fresh mix in next post.

Now come to the point enhancing bond between steel-concrete interfaces. Every admixture should be applied in right concentration, right composition and right time application.

Here also controlled dosages and right timing produce controlled formation of gases and this controlled gas release results a small expansion of mix (fresh mix). Now if we can restrain this expansion, we will able to derive greater bond strength which will produce improved bonding to steel interface and this is achieved without much reduction of compressive strength.

Prevention of Plastic Shrinkage Cracking in Concrete

We have published many posts about cracking in concrete members. Of this plastic shrinkage cracking is generated by the adverse affect of weather. Here we will try to summarize some preventive measure to control such cracking.

While concreting in adverse weather condition which leads to moisture losses at very rapid rates, plastic shrinkage cracks is generated. The factors that influence the moisture loss are follows:

a. Air temperature

b. Concrete temperature

c. Relative humidity

d. Wind velocity

These factors have to be considered at concrete surface and surroundings. The combination of these factors can accelerated the rate of evaporation at surfaces; these may be either in cold or hot weather.

Now prevention of plastic shrinkage cracking means preventing rapid moisture loss. Moisture loss, as discussed above, due to hot weather and dry winds. We have publish some useful information regarding hot weather concreting according to ACI 305R. Here we are listing the measures to reduce moisture loss:

a. Application of fog nozzles; these method saturates the air surrounding (above) finish surface.

b. Application of sheeting over concrete; most common is plastic sheeting. This method also retards moisture loss. Sheets cover the surface between consecutive finishing operation.

c. Wind breaks; these reduce the velocity of wind; thus reducing moisture loss due to dry wind. The flat work is scheduled in such way that erected wind breaks start working.

d. Sunshades; this also reduce the temperature of surface.

Besides this scheduling placement or starting concreting operation at night, is sometimes advised, which works good without external preventive measures discussed above. This is advised during hot day time with low humidity in windy weather.

We have witnessed concreting in Burj khalifa at night to avoid unfavorable weather of Dubai. You can visit “special feature of concrete in Burj Dubai” for more information.

ASTM Classification of Organic Soil Based on Organic Content

Dear reader throughout previous posts we have learnt that organic soil is a problem soil which is subjected to excessive foundation settlement with anaerobic reaction happened inherently in them. We have discussed about peat in our previous post. Here in this post we will discuss about distinct method of soil classification unlikely to organic clay, organic silt, and peat.

Various approaches are adopted to classify organic soil. These approaches depend on various purposes to be served by classification say sometimes liquid limit is the controlling criteria and sometimes organic content is the controlling criteria.

In many respects such soils exhibits behaviors that don’t match with that of traditional soil mechanics. There have still controversial issue related to classification approaches derived for various purposes.

Now we will discuss about a method of classification of organic soils based on their organic contents, sometimes termed as OC (Edil,1997). The classification is as follows:

1. Organic content leads to inorganic soil, in these types of soils there have less percentage of or moderate percentages of organic content, but still they behave like inorganic soil. They are as follows:

a. Organic content less than 5%; such soil have little influence on soil behavior and considered as inorganic 

b. Organic content remains between (6-20)%; these soils’ behaviors are slightly influenced by organic content. But still they behave like inorganic soil.

Example: organic clays, organic silts.

2. Organic content leads to soil behaviors like organic soil. Here organic contents control the properties of soils. These soils are as follows:

a. Organic contents exist between (21-74)%. In such concentrations of organic particles they control over soil properties but still conventional soil mechanics is applicable. 

Example: silty organic soil, clayey organic soils.

b. Organic content in excess of 75%; in such condition conventional soil mechanics cannot be applied specially at lower stress level.

Example: peat

How is Concrete Exposed to Chlorides?

Concrete corrosion may be taken place due to many causes and corrosion contributors. The main contributor that results corrosion to steel reinforcement in reinforced concrete member is chlorides. Presence of chlorides in it may cause severe corrosion.

Now in this post we will find out, how concrete comes in contact with chlorides. These are as follows:

a. Exposing it to saline water

b. Exposure like brackish water

c. When it is exposed to soils that contain significant salinity(saline soils). The chlorides penetrates through it to steel by diffusion process

d. We all know about deicing solutions which contain chlorides. This deicing solution can penetrate through pores or cracks remain in concrete.

e. From admixtures or other constituents of concrete that contain chlorides.

When corrosion is started by such chloride concentration, it is very hard to control corrosion. Some admixtures can slow down or reduce such corrosion. These potential of admixtures to inhibit corrosion will be discussed in the next post.

How can Alkali-Aggregate Reaction of Concrete be Controlled with Admixtures?

Dear reader we have discussed about concrete cracking due to alkali-aggregate reaction. The product of this reaction is alkali-silica gel which is formed at location of reactive silica and takes its place in weak planes and surfaces of aggregate particles. The consequence of this is destroying bonds between aggregate particles and hydrated paste around them.

Different studies and report also available on the reduction of alkali-aggregate reaction under application of pozzolanas(Stanton 1950). The materials and admixtures that are used to reduce expansion potential of concrete due to ARS(alkali-silica reaction) are listed below:

 a. Soluble lithium salt

b. Soluble barium salt

c. Some types of air-entraining admixtures

d. Some types of water-reducing admixture

e. Some retarding admixtures

It is observed in laboratory that 1% inclusion of lithium salts results significant reduction of expansion. The same result is also found with inclusion of (2-7) % of some barium salts.

Now, have there a molar ratio that nullifies alkali-silica reaction?

 

Yes, there have a particular molar ratio that can totally suppress expansion due to such reaction. It was reported (Stark, Ong 1993) that when molar ratio reaches 0.67 the expansion is totally suppressed. Inclusion 1% lithium salts and slight higher concentration of barium salts in concrete results significant reduction of expansion due to alkali-aggregate reaction.

Rapid Concreting with Flowing Concrete

In the previous post we have discussed about flowing concrete and admixtures generally used in flowing concrete. Here we will learn about the mobility rendered by flowing concrete.

In our many construction operation associated with concrete work, there have many constraints like

1. Relative inaccessibility to the job site

2. In case of deep member, difficulties in compaction,

3. In concrete member that is congested with closely spaced reinforcing bar

4. In case of concrete member of unusual shape etc.

We frequently face reinforcement congestion problem in heavily loaded section like joints in beam-column connection, column strip in flat plate or mat foundation and many other situations.

Thus we need a concrete of maximum horizontal coverage from any discharge point. This requirement of high placement rate can be fulfilled by flowing concrete.

Such inaccessibility may be height of reaching concrete. In case of concrete at elevated place and also supplying concrete through pump to any less accessible site, such concrete is essential. Due to its high mobility it provides economical solution in decreasing pumping pressure, simultaneously it increase distance and rate of pumping.

This concrete is specially useful where rapid concreting rate or maximum placement volume per day is required. It also renders early strength properties to concrete as it has low water/cement ratio. The early strength may be useful in early post-tensioning and for early stripping requirements.

What is Flowing Concrete? How to Make It Flow?

Now-a-days we are constructing structures in such inaccessible place that the in-situ concrete production, in many cases, is impossible. We have witnessed the concreting in Burj Khalifa at extreme elevation. Powerful concrete pump lifted this concrete to the desired elevation.

Now our question is, does powerful pump is enough to make reached concrete such elevation, produced at ground or at plant or at any suitable location? The answer is no. we have published numerous post about concrete pumping, blockage, factor effecting efficient pumping and some guideline to make concrete pumpable.

Here we will learn about flowing concrete. This concrete can be defined as follows:

ASTM C1017: The concrete that maintain its cohesive nature with a slump of more than 7.5 in, …..

This concrete does not subject to segregation, retardation at usual rate or suffers excessive bleeding, in one word it offers a cohesive mix at even such large slump. Now we can increase slump with increased water/cement ratio. But it renders an extremely inferior quality concrete with all negative impact of segregation which is not expected in our regular concreting practice. 

So, we will use plasticizing admixtures to obtain this concrete. These admixtures are normally termed as HRWRA which must conform to ASTM C 494.

In normal practice plasticizers can be added in two ways:

a) Concrete is transported to job site having (2-3) in. initial slump and plasticizers are added at job site to reach desired slump of 8” or more.

b) Plasticizers can be added at plant to furnish placement slump then delivered to job site.

But the concrete having plasticizers lose slump at a rapid rate than that of concrete having same slump but not produced with plasticizers.

Rig Categories in Deep Foundation Work

Dear reader in the very before post we have learned about pile rigs, its applicability and components of modern pile rig. Erection safety of pile rig was also discussed in this blog. Here we will learn about categories of pile rig.

Pile rig is a special machine that helps construction of deep foundation by drilling through soft soil to even rock. In accomplishing such job, they use various types of drilling equipment and many advance tools that can now ensure accurate drilling through the soil layers to be penetrated. 

A deep foundation transfer structural loads to the deep strong strata. There have two components of deep foundation; one is deep portion that transfer load to deep stronger soil and other is embedment of deep portion at cut-off level or sometimes ground level that collects structural loads and transfer it through the above mentioned deep portion.

Depending on purposes severed on the basis of energy developed, drilling depth and served pile diameter we can conclude three categories.

They are:

a. Small sized:

1. Engine power: 180 Kw

2. Torque:(60-100)KN.m

3. Drilling dia: (0.5-1.2)m

4. Total quality:40t

b. Middle sized:

1. Engine power: (125-200)Kw

2. Torque:(120-180)KN.m

3. Drilling dia: (0.8-1.8)m

4. Total quality:(42-65)t

c. Large sized:

1. Engine power: 300Kw

2. Torque: 240 KN.m

3. Drilling dia: (1-2.5)m

4. Total quality:100t

Biological Activity and Engineering Properties of Organic Soils

We know that organic soil is a problematic soil. We have already learned about the relation between foundation problem and organic soils. In organic soils there have large organic content which requires special attention to the soil. Especially when organic content increased greater than 75%, the soil has to be treated as special soil which shows behavior that hardly comply with our traditional soil specially at low stresses level.

Organic contents usually changes to new form by biological break down. The organic changes occur in contents below water table at a relative very slow rate. This changing in form is called decomposition. But in some cases, this decomposition of organic content may be associated with an accelerated rate and leads to problems to foundation.

The biological activity, taken place in this soil is measured by the analyzing production of gas during their decomposition process. The rate of production of gas indicates the decomposition rate of them. This decomposition significantly influences engineering properties of soil. The shear strength, settlement criteria and stability of an organic deposit are highly affected by this biological action.

Considering above degradation reaction, engineers usually do not recommended this soil for founding medium of structures or considered suitable as subgrade of embankment.