What are Cement Substitutes in Concrete Mix?

The materials which are used in concrete mix to improve some properties such as longevity and strength substituting cement to some degree are called cement substitutes. The reason for use cement substitutes are as follows:

a. Cement substitutes are used to replace cement which generates green house gas in its production process.

b. Cement substitutes provides a way for reusing of byproducts from different industrial processes like slag and many others.

c. The materials used as cement substitutes conserve energy as compared to cement as they require less energy for repurposing.

Slag cement from ruby and scan cement of Heidelberg Cement Group

d. And at last environmental advantages are achieved using cement substitutes.

e. The materials used as cement substitutes conserve energy as compared to cement as they require less energy for repurposing.

f. And at last environmental advantages are achieved using cement substitutes.

The scenario of United States is, it imports 20% of national cement requirement. Thus the use of cement substitutes saves economy and energy to a great deal.

Cement substitutes are generally suggested to use in large scale projects than a small residential construction project as most of the contractors are not familiar with cement substitutes and special care are required to use these.

Thee important things to be keep in mind that the cement substitutes are not aggregate substitutes and additives. The example can make clear say, ground scrap rubber, concrete additives (like air-entrainment agent or plasticizer), ground glass are not cement substitutes.

The most common cement substitutes are:

a. Silica fume or microsilica
b. Flyash
c. Slag

Function of cement substitutes in concrete:

The use of slag or class F flyash can control alkali-silica reaction. Cement substitutes render less permeable concrete and give protection to the reinforcing steel against corrosion. The use of slag as cement substitutes of cement can limit sulfate attack from waste water and sea water.

What is adjustable steel column?

Adjustable steel column is a steel post that is designed to connect mechanically to roof to support it. The post supports a system of beams to provide additional stiffness that are required to withstand earthquake, wind loads or other sources of lateral displacement loads. The posts are usually hollow and an adjustable mechanism is attached to each post through providing threads. The adjustable system facilitates to adjust the column height when required. 

These columns are normally provided in basement. This column can be assembled as a single-piece or multiple-parts depending on the manufacturer’s production sizes and facilities. 

Some requirements for adjustable steel columns according to International Residential Code (IRC):

1. The post diameter should not be less than 75 mm irrespective of structural design to support the loads. Both structural design and IRC minimum requirement have to satisfy in determining post diameter.

The post must be protected by corrosion-inhibitive paint. The both outer and inner surfaces of steel column have to paint with corrosion-inhibitive paint. If steel is treated with corrosion resistant coating or corrosion-resistant steel are used this requirement does not bear any importance.

Alteration of groundwater table: Impacts on foundation

We everyone know the term buoyant effect of water. This phenomenon is important in ground condition alteration. It is evident that intergranular pressure is increased with the removing of buoyant effect of water. This can be achieved by lowering the water table. In many practical cases this is not feasible or sometimes it is used as temporary solution. If somehow ground water table are altered, the immediate increase in intergranular pressure is obtained by 

γ_w x z_w

Where z_w is the change in ground water elevation.

Now think that water table alteration is possible, feasible and effective. Then another problem arises with alteration limit avoiding adjacent property. The water table cannot be altered exactly within the limits of desired property. Thus the change in effective pressure also occurs beneath adjacent properties of which foundation are designed considering existing ground water condition. The resulting impacts are cracked pavement or floor as well as cracked building.

It is worth mentioning that ground water table can be raised producing the same problem of adverse impact on adjacent properties and requires careful understanding about the impact before achieving the raising.

Thus any alteration of ground water table results some kind and degree of effect on the environment. So it is necessary to take permission from the local environmental authorities to adopt such action.

How are Foundations Installed Displacing Soil?

Foundations are generally installed below ground surface from the view of geotechnical and environmental considerations. Foundation depth should be designed such that it remains below:

1. Organic top soil

2. Muck or peat

3. Unconsolidated materials like garbage bumps or sanitary landfill etc.

4. Freezing thawing influencing depth

5. And below other soils that are influenced highly with the moisture fluctuation like expansive black cotton soil

In the previous couple of posts I have discussed many problems of foundation on black cotton soil  as   well   other  expansive soil  and  also  their  remedies  and distribution of such soils in united states and India. In this post I shall discuss displaced soil effects. 

As foundations are placed below ground level, it displace some soil mass. When spread footing are used the displaced amount are summation of the footing weight and the column that rested on footing. Where basement is required for parking facilities, the basement slab is generally rested on top of the footing pad directly. Sometimes holes are excavated through the slab to provide footing and column and then the holes are backfilled to the ground surface. 

If the unit weight of the soil=γ_s
 

And unit weight of concrete= γ_c

Column reaction=P

Allowable bearing pressure=q_a

Footing depth=D

Thickness of footing=D_c

Then existing pressure before excavation and foundation installation

= γ_sxD

Now induced soil pressure increment due to column reaction,p=P/B² = q₁

Pressure increment due to displace soil=( γ_c- γ_s) xD_c= q₂

Net increment of pressure= q_n =q₁ +q₂ ≤q_a

In case of steel columns to avoid corrosion concrete pedestals is provided up to ground and in case of concrete columns, at the footing level it is attached to footing with dowels.

If the floor slab is rested directly on ground, careful selection of backfill soil and compaction method is required. The backfill soil that is placed around the basement wall should have free draining property and where required a perimeter drainage system is installed to control hydrostatic pressure.

What is admixture? How it Works?

Admixtures are a mix of several chemicals or single chemical that are added to mortar, concrete or grout during mixing or in wet condition just after mixing or even after hardened mix to have a concrete of desired of properties apart from natural concrete.

There have some standard that are used to define and evaluating the quality of admixture. New European Standard EN 934 was developed in 2002 which covers almost all type of admixture. The current part 2, out of recently developed five parts, covers concrete.

Parts 1-3 of BS 5075 also deals with concrete admixture and BS 4887 covers admixture for mortars. ASTM C494 is the most precisely specified standard that are widely used in USA and many other countries of the world as a reference for defining, using and selection of admixture.

Mechanism of working admixtures follows one or more of the following actions:

• Chemical influence on the hydration process of cement, Changing the rate of stiffening either by accelerating or retarding the rate of chemical reaction in one or more cementing phase.

• Achieving better dispersion of particles producing adsorption on cement surfaces. This action results plasticizing or superplasticizing.

• Achieving increment of air entrainment by influencing the surface tension of water.

• Increasing mix cohesion or plastic viscosity influencing the water rheology.

• To introduce specific properties, like increasing corrosion protection of steel embedded with concrete or water repellence, using special chemical over the hardened surface of concrete.

Enlarged Tip Piles for Granular Soils

Enlargement of pile tip increase bearing capacity of pile. This technique is generally used in granular soils. This technique can be performed in several ways. One type of enlargement is achieved by driving a tube at the bottom with a concrete plug to the desired depth. When is added the concrete plug is forced out into the soil. After base is completed the tube is withdrawn. But this withdrawal continues while the concrete expands out at the tip of the tube. This process forms a cast-in-situ (CIS) concrete shaft.

Alternately, corrugated shell casing or a pipe is bottom-driven into the base and the tube withdrawn. This forms an annular space between pile and soil. This annular space either filled with granular filler material or else closes onto the shell. This piling work is then completed as a cast-in-place (CIP) concrete pile. In these either CIP or CIS piling techniques, the shaft is provided with steel reinforcement as requirement of the design that consider structural capacity of pile  (both axial capacity of pile and lateral capacity of pile).

In the above figure phase (1) to (3) depict the boring of gain desire depth and using expandable wings to make enlarge base and then concreting to fill the base. The phase (4) shows that the withdrawal of boring system and  the phase (5) shows lowering of cast in place pile to touch the base cast previously.

The another alternative are achieved by attaching to pile shaft a reinforced concrete base having shape like a frustum of a cone. This is very common to use a corrugated shell shaft or a pipe having thin-wall and enlarged tip base being mandrel driven to bear in generally granular subsoils. As a cast-in-place pile the shaft is completed and steel reinforcement is provided as explained earlier. Enlarge-tip base in the above explained precast form can be used with solid shafts like timber piles and it can be of sizes of wide range.

Automatic Vicat Apparatus for Determination of Setting time of Cement

Cement is the bonding component of concrete which binds the inert filler materials of concrete mass. In this post we will discuss about Vicat Apparatus which is a simple equipment for testing cement. Everyone in civil engineering specially in material engineering are known about this equipment. So why this Apparatus are in discussion this is the question. we will discuss a automated apparatus manufactured by Geotechnical Testing Equipment.

The Automatic Vicat Apparatus is designed and manufactured using the most recent and sophisticated technology, it is used for the initial  and final setting time determination of cements or mortar pastes. The apparatus is manufactured with anticorrosion components to be used in places with humidity up to 90% and 20°C. 

The entire test is made in a fully automatic way and gives a very precise and repeatable result with controlled temperature as required by EN Specifications.

The results are printed on the incorporated printer and this eliminates the manual operations of installing and zeroing the paper graph on the drum.

The use of the appliance is extremely simplified by the guiding menu that is available in different languages.

CM 0123 

Automatic Vicat Apparatus Complete with EN and ASTM Initial and Final needles, Consistency Plunger, 1 x EN and ASTM Mould and Glass Plate. 

Black Cotton Soil of Deccan Lava Plateau

In India Black cotton soil spread over Maharashtra, western parts of Madhya Pradesh, parts of Andhra Pradesh, parts of Gujarat, some parts of Tamil Nadu and Deccan Lava Plateau. Among these the black cotton soils of Deccan Lava Plateau are discussed here. 

Most of black-cotton soils of India are spread across the Deccan Lava Plateau, the Malwa Plateau, and interior Gujarat, where there is both moderate rainfall and underlying basaltic rock. Because of their high clay content, black soils develop wide cracks during the dry season, but their iron-rich granular structure makes them resistant to wind and water erosion. They are also highly moisture-retentive, thus responding well to irrigation.

The soil in the Deccan plateau is made up of black basalt soil. This type of soil is rich in humus. In addition to iron content they contain fairly high quantities of lime, magnesia and alumina.

Such texture and composition of soil are formed due to volcanic action of Deccan region. These igneous rocks break down in to the black soils rendering fertility to them. They commonly known as the black cotton soil because it is best suited for the cultivation of cotton.

What is Anchor Bolt?

The factored base shear in column, in normal loading conditions, is resisted by friction between plate and support of it. The introduction of anchor bolts is to provide addition capacity against shear that may occur due to many unfavorable conditions.

Function of anchor bolts:

1.To stabilize column preventing uplift induced by moments that generated by lateral load(earthquake and wind) or any other sources.

2.  To keep the column in position during erection process.

Types of Anchore bolt:

a. Drilled-in bolts 

b. Cast-inplace bolts

The application of these types of anchor bolts are :

a. Drilled-in bolts

Drilled-in anchor bolts are not frequently used. In this process bolts are installed after the concrete is set.

b. Cast-inplace bolts

Cast-inplace anchor bolts are placed before setting of concrete. This ancor bolt may be bolts, threaded rods having nuts or hooked bars.

Hooked bars:

Among these anchor bolts the hooked bars are suitable for base plates that are loaded axially.

-

Bolts:

Bolts can be used either to anchor base plate subjected to moments or to anchor axially loaded base plate.

Threaded rod’s:

Both axially loaded base plates and base plates subjected to moments and be anchored by threaded rod’s with nuts. This system of anchoring are required when the size and length obtained from design calculation for the specific design exceed those of standard size bolts. This threaded nut system fails when stress in the rod is reached to tensile capacities.

How to Determine Structural Shaft Resistance of Micropile?

The internal capacity of a micropile frequently governs its overall design, because of its small cross sectional area and the large resistance provided by the grout/ground bond due to the construction techniques.

The reinforcement steel is the element that carries most of the load. However the load is resisted by both the steel and the grout. It is important to take into account this composite action to optimize the internal pile design. The use of steel pipe or casing as reinforcement elements has become more popular, especially when requiring minimal deflections or supporting lateral loads.

In practice, the design compressive stress in the steel reinforcements is limited to 50% of the yield strength. The pile capacity is normally derived from the allowable structural capacity of the reinforcements in the preliminary design.
 
Other components, such as the grout and additional reinforcement bars can be included to enhance the allowable structural capacity. However extra care is needed to ensure its effectiveness during construction.

The grout commonly consists of cement and water, with water-cement ratios between 0.40 and 0.55. The minimum ratio is set by the requirement that the grout should be fluid enough to allow efficient pumping and injection. The maximum ratio is imposed results because an excessive amount of water would cause bleeding, low strength, increased shrinkage and poor durability. Fine sands can be added to the mix to reduce costs. Sand cement ratios are limited to 3, but they rarely exceed 1.5. Admixtures are added to modify grout properties: prevent shrinkage, reduce water content, maintain pumpability, accelerate or retard setting, and, prevent bleeding.

Defective Piles and Corrective Measures

In previous posts we discussed about defective piles. Now we discuss what to do for defective pile. Defective pile associated with injurious splitting, steel deformation, brooming and splintering of wood, or concrete spalling and crushing. A pile which is not positioned properly will not be allowed to repositioning applying forces or other mechanism. The contractors are responsible for corrective measures for defective pile at their own expanses including coercions of driving pile below the designed cut-off level, wrong positioning of piles and damaging of pile due to  improper  or  wrong driving. The following methods are used to for correction of defective piles:

(a) The piles shall be withdrawn and replaced by new and, if necessary, longer piles, or

(b) Replacement piles shall be driven adjacent to defective or low piles, or

(c) The piles shall be spliced or built up, as otherwise provided herein, or a sufficient portion of the footing extended to properly embed the piles. Timber piles shall not be spliced without specific permission of the Consultant. All piles, pushed up by the driving of adjacent piles or by any other cause, shall be driven down again.

In case of application of hammer of grater energy to obtain required penetration and for achievement of designed bearing capacity, as accepted by the consultant, the contractor cannot claim extra work.