civil techniques

Causes of Cracks in concrete structures

abbu riyaz 1 May, 2023

Causes of Cracks in concrete structures

There are many Causes of Cracks in concrete structures, they are

Thermal movement
The permeability of concrete.
Corrosion of Reinforcement
Creep
Moisture Movement
Structural design and specifications
Poor Construction practices.
Improper maintenance

1.Thermal movement

All materials expand on heating and contract on cooling. The thermal movement depends on temperature variations, dimensions, a coefficient of thermal expansion and physical properties of materials. The external exposed to direct solar radiation and the roof subject to thermal variation is liable to cracking.

Remedial Measures:

Thermal joints avoid by expansion joints, control joints and slip joints.

2.The Permeability of concrete

Concrete permeability controls by factors like water-cement ratio, water hydration, curing, air voids, micro-cracks and continues exposure to thermal variations.

3.Corrosion of Reinforcement

Concrete provides protection to reinforcing steel. Steel will not corrode as concrete around it is impervious. And does not allow moisture or chlorides to penetrate within the coverage area. A properly designed and constructed concrete is initially water-tight. The reinforcement steel protects by a physical barrier of the concrete cover having low permeability and high density.

Remedial Measures:

Increase the concrete cover with proper materials like plastic, concrete, readymade cover blocks, etc., instead of using stones and wood pieces. Increased concrete cover over the reinforcing bar reduces corrosion.

use concrete with low permeability.

4.Creep

Concrete subjected to loading exhibits a slow time-dependent deformation known as creep. Creep increases with increase in water and cement content, water-cement ratio and temperature. It decreases with increase in humidity of surrounding atmosphere and age of material at the time of loading. Use of admixtures and pozzolanas in concrete increases creep. Amount of creep in steel increases with rising in temperature.

5.Moisture Movement

Shrinkage occurs in all building materials that are cement/lime based. Heavy aggregate concrete shows less shrinkage than lightweight aggregate concrete.

Materials with pores in the structure in the form of intermolecular space expand on absorbing moisture and shrink on drying. These movements are cyclic in nature and causes by increase or decrease in inter pore pressure with moisture changes.

Remedial Measures:

Shrinkage cracks in masonry minimize by avoiding the use of rich cement mortar in masonry.

Plastering of masonry after proper curing.

6.Structural design and specifications

Cracks in concrete forms due to poor structural design and specifications. At the starting of design first, consider the environmental conditions existing around the building site. It is important to determine the type of foundations, the type of concrete materials use in concrete. And the grade of concrete depending on chemicals present in groundwater and subsoil. Closely spaced of reinforcement steel bars due to inadequate detailing causes segregation.

precautions while designing

specification for concrete materials and concrete.

Proper quality and thickness of concrete cover around the reinforcement steel.

proper reinforcement layout and detailing

7.Poor Construction practices

Construction should design and supervise only by civil engineers or persons having proper knowledge and experience in construction practices. People without construction knowledge are engaging nowadays to supervise construction works and to carry out construction works.

Lack of good construction practices due to ignorance, carelessness, greed or negligence.

For a healthy building, it is necessary to ensure good quality materials selection and good construction practices.

Some of the main causes for poor construction practices

Using poor quality materials
improper selection of materials
cheap and nonstandard materials
improper concrete mix
Inadequate control on various steps of concrete products such as batching, mixing, transporting, placing, finishing and curing
Inadequate quality control and supervision causing large voids(honeycomb)
Improper construction joints between
Addition of excess water in concrete and mortar mixes.
Poor quality of plumbing and sanitation materials and practices.

8.improper Maintenance

Buildings should maintain at regular interval of time properly to avoid damages. Attend Leakages as earliest as possible before corrosion of steel inside concrete starts. The external painting of the building helps in protecting the building against moisture and other chemical attacks. Necessary water-proofing and protective coating on reinforcement steel or concrete.

civil techniques

PILE TYPES

abbu riyaz 1 May, 2023

PILE TYPES BASED ON CONSTRUCTION METHOD

Three pile types of pile foundations according to their construction methods:

Driven piles,
Cast-in-situ piles, and
Driven and cast-in-situ piles.

1. Driven Pile Foundations:

It is of concrete, steel or timber. These piles are prefabricating before placing at the construction site. If drive piles are of concrete, they are precast. These piles are drive using a pile hammer.

When piles drive into the granular soils, they displace the equal volume of soil. This helps in compaction of soil around the sides of piles and results in the densification of soil. The piles which compact the soil adjacent to it is compaction pile. This compaction of the soil increases its bearing capacity.

Saturated silty soils and cohesive soils have poor drainage capability. So these soils are not compacts when piles are drill through it. The water is to be a drain on the soil to be compact. Stresses develop adjacent to the piles have to be borne by pore water only. This results in an increase in pore water pressure and a decrease in bearing capacity of the soil.

2. Cast-in-situ Pile Foundations:

Cast-in-situ piles are the concrete pile. These piles construct by drilling holes in the ground to the required depth. And then filling the hole with concrete. Reinforcements utilize in the concrete as per the requirements. These piles are of small diameter compare to drill piers.

Cast-in-situ piles are straight bore piles or with one or more bulbs at intervals are cast. The piles with one or more bulbs are under-reamed piles.

3. Driven and Cast-in-situ Piles

Driven and cast-in-situ piles have the advantages of both driven and cast-in-situ piles. The procedure of installing a driven and cast-in-situ pile is as follows:

A steel shell of the diameter of a pile driven into the ground with the aid of a mandrel inserting into the shell. After driving the shell, the mandrel removes and concrete pours into the shell.

The shell is of corrugate and reinforces thin sheet steel or pipes. The piles of this type are shell type piles.

The shell-less type formes by withdrawing the shell while the concrete places. In both the types, the bottom of the shell is close to a conical tip which separates from the shell. By driving the concrete out of the shell an enlarge bulb form in both the types of piles. Th shells left in place and the tube concretes. This type of pile is very much use in piling over water.

civil techniques

RCC Column Construction Methods

abbu riyaz 1 May, 2023

RCC Column Construction Methods consist of various steps and procedures to follow in the construction of columns at the site.

RCC Column Construction Methods

Layouts
Reinforcement work
Formwork
Pouring of concrete

1. Layout

Layouts proceed by surveying instrument. After marking the grid lines, the location of columns Identifies and marks.

2. Reinforcement work

Checklist for Column reinforcement works on site:

Check the numbers and diameter of vertical bars
The spacing between vertical bars
Development length which depends on the diameter of the bar
Lapping in alternate bars to come at the same height and not coming inside beam or slab.
Lapping l/3 or 2l/3 of column
The spacing between stirrups
The hook bends properly at right angles
Check the stirrups corner

3. Formwork

Columns carry a load of the structure so it is critical. It is important to align column for verticality so that load transmits properly. The column shuttering should strong enough to take the pressure of fresh concrete and remain in position during concreting.

Guidelines on column shuttering

Holding the concrete shuttering firmly in place and for proper alignment, a concrete pad is cast before fixing the shuttering. The starter cures for a day or two so that it is hard enough to fix the shuttering around it.
Column box or shuttering for columns is made of plywood sheets or steel sheets fabricators with adequate stiffeners.
Thin films of oil/grease applies to the inner surface of shuttering to easily remove from a column. After the concrete hardens.
Shuttering is properly aligns with its verticality and diagonals are checks to ensure accuracy in dimensions.
Formwork thoroughly supports with props size before pouring the concrete. So that it does not move horizontally or vertically during concreting.
Gaps in shuttering joints to seal with plaster or a piece of wood to prevent leakage an of a slurry.
Appropriate space to provide in the inner face of the shutter and reinforcement by fixing cover blocks.
It is preferable to remove shutters after 24 hrs of casting. If need to remove earlier, it should not remove within sixteen hours.
The removal of shutters is handling gently without jerks so that edges of columns will safe without damages.
supports of column shuttering to prevent from moving during concreting.
Diagonals of the shuttering to check to ensure dimensional accuracy.

4. Concreting of RCC Column

Concreting of an RCC column is by two methods

Manually
With the help of machine or pump

Points to follow during and after concreting:

For less quantity machine mix concrete adopts and for larger quantity, ready-mix concrete (RMC) orders.
Approval of placing concrete whether by pump or manually from the client.
Concrete pours up to slab bottom; the remaining column gets concrete during pouring of slab & beam.
Mechanical vibrator to compulsorily use in the column but excess vibration cause segregation. Each layer is to compact thoroughly.
Target slump to be 160 mm.
The height of pouring concrete should not exceed 1.5 m.
Avoid construction joint in the column.
check proper cover.
The temperature should be below 30 degrees while pouring concrete.
After the pouring and vibrating, the horizontality and verticality of the column to check.

civil techniques

RCC Column Types

abbu riyaz 30 Apr, 2023

RCC Column Types are base on its shape, length, and forces. A column is a vertical member which takes the complete load of the beam, slabs, and the entire structure. The floor and another area of the building are modifying as per the requirement of the client.

The size of the columns, the quantity of cement sand and aggregate to mix, the number of steel bars to place, the spacing between the stirrups are mention in the structural drawing. It designs as per the actual load on the column and considering the factor of safety.

A column is a vertical member which effectively takes a load of compression. A column is a compression member as load acts along its longitudinal axis. Bending moment occur due to wind earthquake or accidental loads.

Column transfers the load of the structure of slabs beams above to below, and finally, the load transfer to the soil.

RCC Column Types

Types of RCC Column based on Shapes

Circular
Square or rectangular

1. circular

The circular column is for exposed outside for good architecture view.

2. Square or rectangular

The Square or rectangular columns are traditionally adopting for many structures.

RCC Column Types base on length

Short column
Long column

Where L is the height of the column, B is the width

1. Short column

If length/breadth ratio is lesser than or equal to twelve then it is a short column.

Floor height is approximately 3 m or 10 feet, L/B ratio is less than twelve, it’s a short column.

2. Long column

If length/breadth ratio is greater than twelve then it is a long column.

If the height of floor is more than 3 m or 10 feet, L/B ratio results in a long or short column.

Types of RCC Column based on moments

Biaxial column
Uniaxial columns

1. Biaxial column

Biaxial column designs for axial load and moment in two directions. In building corners, most columns are biaxial columns.

2. Uniaxial columns

Uniaxial columns design for axial load and moment in one direction. In the sides, a column is a uniaxial column.

Internal columns can be any of these.

civil techniques

Formwork

abbu riyaz 30 Apr, 2023

Formwork is a temporary mold to provide support to fresh concrete places in structural member until the concrete has set. This helps the structural member to gain sufficient strength to carry its self-load and load from other members.

Concrete formworks are for fresh concrete constructions such as walls, slabs, beams, columns, footings etc. Formworks requirements for different structural members are different and they are named based on the type of structural member.

There are many types base on its material, their use and the type of structural members. The core functioning of the formwork remains the same.

Types of Formwork Based on Structural Member:

Formwork uses in the construction of reinforced concrete foundations, columns, slabs, walls etc., and these are as follows:

Footing Forms – for the foundation
Column Forms – for RCC Column construction
Wall Forms – for RCC wall construction
Floor Forms – for construction of RCC Slabs

1. Footing Forms – for Foundation

The first step in any concrete construction starts with the construction of the foundation. Foundation can be for columns or walls. Basing the type of structural member, the shape, and size of footing designs. The size and shape depending on the type and dimension of the footing.

2. Column Forms – for Concrete Column Construction

Column forms subjects to lateral pressure because of their small cross-section, large heights and high rates of concrete placement. It is necessary to provide tight joints and strong tie support to the formwork.

As the sizes of concrete column increases, the stiffness of the formwork increases by increasing thickness of sheathing. Then vertical stiffeners add to prevent sheathing deflection.

3. Wall Forms – for RCC Wall Construction

Formwork for wall construction is subjecting to relatively lower lateral pressure than column forms due to their large cross-sectional area.

4. Floor Forms – for construction of RCC Slabs

Formwork for reinforcing concrete slabs depends on the type of slab construct. The floor slabs are structural slabs supports on a steel or concrete structural frame, or slab-on-grade.

civil techniques

Pile cap

abbu riyaz 30 Apr, 2023

A pile cap is a mass concrete structure contains more piles embeds in it. A group of pill drives into clay, soft, and loose soils to provide a strong, stable and suitable foundation.

The pile designs considering the punching shea around the heads of the piles and column base. It also designs for bending moment due to the transmission of loads from columns to the individual piles.

Pile cap Necessity

Pile is necessary when the bearing capacity of soil below the structure is insufficient for a spread footing. It transfers the load to deeper, firmer strata. Piles used where the soil particularly affects by seasonal changes, to transfer the load below the level of such influence.

The load support exceeds the bearing capacity of a single pile, a group of piles uses.

The group capped by a spread footing or a cap to distribute load to all piles in the group. Where there are a large number of closely space piles, provide individual caps. It is more economical to provide just one large cap, forming a piled raft.

SHAPES

The shape and plan dimensions of the pile cap depend on two factors.

Number of piles in the group and
The spacing between each pile.

The most common shapes are

1. Number of piles in the group

A group of piles accommodates in a pile cap.

Two pile

In two pile cap, two piles are embeds in one pile cap. The picture shows a two and one pile cap PCC arrangments for pile cap.

Three pile

The picture shows a typical three pile arrangement PCC for pile cap.

Three pile cap

The image shows a three pile cap concreting.

The shapes of the pile caps minimize the plan area for symmetrical pile arrangement about the load.

It is overhand the outer piles by at least 150mm and not excessive. And not more than the diameter of the pile diameter.

There are different shapes of pile caps according to the number of piles.

DEPTH

The overall depth provides sufficient bond length for the pile reinforcement and the column reinforcement.

The depth decides by the following criteria.

Punching shear.
Pile anchorage.
Shrinkage and swelling of the soil.
Frost attack.
Groundwater table.

The most important thing is a shear capacity of the pile which affects the selection of the pile depth.

SECONDARY REINFORCEMENT:

The secondary reinforcement provides to prevent the piles from splaying outwards from the pile cap.

This reinforcement provided at the bottom of the pile cap running around the longitudinal reinforcement. Projecting from the piles into the pile cap.
The direction of the secondary reinforcement is changing at the head of each pile.

The amount of secondary reinforcement changing its direction at the head of each pile. And is not less than 20% of the main tensile reinforcement and well bond.