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Difference between Control Joint and Expansion Joint?

Table of Contents

• Control Joint in Concrete
  • Need of Control joint in Concrete
  • Location of Contraction Joint
• Expansion Joint in Concrete
  • • Need of Expansion Joint in Concrete
    • Characteristics of Expansion Joints
    • Types of Expansion Joint

Control Joint in Concrete

Control joints in concrete are provided at regular interval to from a weak plane, so that cracks are formed at the joints but not in undesired places. Control joints are provided in concrete pavements, slabs, walls, floors, dams, canal linings, bridge, retaining walls etc.

When concrete is placed, due to shrinkage, creep and thermal movement concrete tends to reduce in size due to which small cracks are formed in the concrete at weak zone.

Fig 1: Cracks formed due to shrinkage of concrete.

Need of Control joint in Concrete

Concrete tends to shrink or reduce in size when it starts hardening. This shrinkage of concrete creates tensile stresses in the concrete which develops the minute cracks at the weak plane.

These cracks are restricted and prevent the formation of large cracks due to the presence of reinforcement in the concrete. But if its unreinforced concrete, the small cracks tends to develop into a large cracks at irregular interval. To prevent such cracks, control joints must be installed at appropriate intervals. It is also recommended to install these joints in reinforced concrete too.

Fig 2: Forming of vertical contraction joint.

Location of Contraction Joint

Generally these joints are pre-defined in the drawings given by designer or architect. If not defined, they will be in a regular pattern or be an integral part of the architectural features. Control joints form a convenient point at which to stop concrete work at the end of the day. Control joints should never be formed in the middle of a bay.

Control joint is placed at the location of highest concentration of tensile stresses resulting from shrinkage are expected:

• At abrupt changes of cross-section; and
• In long walls, slabs.

Expansion Joint in Concrete

Expansion joints are placed in concrete to prevent expansive cracks formed due to temperature change. Concrete undergoes expansion due to high temperature when in a confined boundary which leads to cracks. Expansion joints are provided in slabs, pavements, buildings, bridges, sidewalks, railway tracks, piping systems, ships, and other structures.

Fig 3: Cross section of expansion joint

Need of Expansion Joint in Concrete

Concrete is not an elastic substance, and therefore it does not bend or stretch without failure. However, concrete moves during expansion and shrinkage, due to which the structural elements shift slightly.

To prevent harmful effects due to concrete movement, several expansion joints are incorporated in concrete construction, including foundations, walls, roof expansion joints, and paving slabs.

These joints need to be carefully designed, located, and installed. If a slab is positioned continuously on surfaces exceeding one face, an expansion joint will be necessary to reduce stresses. Concrete sealer may be used for the filling of gaps produced by cracks.

Characteristics of Expansion Joints

1. Expansion joints permits thermal contraction and expansion without inducing stresses into the elements.
2. An expansion joint is designed to absorb safely the expansion and contraction of several construction materials, absorb vibrations, and permit soil movements due to earthquakes or ground settlement.
3. The expansion joints are normally located between sections of bridges, paving slabs, railway tracks, and piping systems.
4. The expansion joints are incorporated to endure the stresses.
5. An expansion joint is simply a disconnection between segments of the same materials.
6. In the concrete block construction, the expansion joints are expressed as control joints.

Fig 4: Expansion Joint in pavement

Types of Expansion Joint

Based on the location of joint, expansion joints are divided into following types,

1. Bridge expansion joints
2. Masonry Expansion Joint
3. Railway Expansion Joints
4. Pipe Expansion Joints

Based on the type of material used in making of joint, expansion joints are further classified into following types,

1.  Rubber expansion joint
2. Fabric expansion joint
3. Metal expansion joint
4. Toroidal expansion joint
5. Gimbal expansion joint
6. Universal expansion joint
7. In-line expansion joint
8. Refractory lined expansion joint

  

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• How To Guide
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Short Column Effect in Multi-Storey Buildings

Short column posses high stiffness that attracts most of the force acting on the multi-story building. Inadequate design of short column to sustain these forces will result in damage and shear failure. This behaviour of the short column under excessive force is called as short column effect.

Special attention is required for short columns and hence recommended to avoid in building design especially for earthquake-prone areas. The effect of a short column is disastrous as they undergo brittle shear failure. The phenomenon of short column effect, its results and remedies are explained below.

Short Column Effect

In certain building either due to the ground conditions or due to the requirement of intermediate beam there requires the construction of short columns as shown in figure-1 below. Various structural configurations also demand the need for short columns.

Fig.1. Short Columns Used in Different Building Conditions( Alqatamin,Talposi)

The short columns are stiffer compared to long columns and hence most of the earthquake forces acting on the building are taken by the short columns (Figure-2). More the stiffness of any structural element, less will be its flexibility to undergo any sort of deformation. Earthquake design of a building is more efficient through the flexible design of the structure.

Fig.2. Short Columns Attracts more shear force compared to long columns( Alqatamin,Talposi)

More is the stiffness, more will be the force required to bring any deformation. If the short columns are not designed to take such large force, damage is caused to the column and hence the structure is affected. This is called as short column effect.

The short column fails by shear failure by means of x-shaped cracking. The shear force created in short column will be 8 times the shear force created in long columns.

Fig.3. X Shaped cracking in short columns of a multi-storey building

Behaviour of Short Columns During Earthquakes

A building constructed on a sloped ground as shown in figure-1, under earthquake action is forced to move along with the floor. If both long and short columns are present, a majority of the force is attracted by the short columns and suffer more damage compared to long columns.

Short column effect is also observed in the columns that support loft slabs or mezzanine floors  (Figure-1). These elements are added between two existing floors.

Another case is the presence of short and long column on the same floor level as shown in figure-4 below. Here too the short column offers more resistance compared to long columns. Thus short columns are subjected to more force and damage.

Fig.4: Short columns and Long columns at same Floor level( Alqatamin,Talposi)

Remedies for Short Column Effect

1. The first possible solution is to avoid the use of a short column in the architectural design stage itself.

2. If short columns cannot be ignored, special design requirements are followed.As per ACI code, for those columns that have chances to undergo short column effect will require special confining reinforcement called the ductile reinforcement. The reinforcement provided must extend to the columns below and above by a certain amount as stated in standards.Figure-5 below shows the reinforcement detailing of the short column and long columns.

Fig.5: Reinforcement Details for Short Columns and Long Columns;( Alqatamin, Talposi)

3. In order to reduce the short column effect on an existing building, the short columns between the two floors are willed. The openings are closed by constructing a full height wall.

4. If wall building is not possible, the short columns have to be retrofitted by any existing methods. A quality structural engineer who has sufficient experience in this area have to implement this.

  

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Concrete Compressive Strength Variation with Time

The age of concrete structures have lots to do with its strength and durability properties. Understanding the strength-time relationship of concrete helps to know the effect of loading at a later age.

The different effects on concrete strength with age are explained in this section.

Table of Contents

• Variation of Concrete Strength with Time
• Rate of Strength Gain With Time
• Factors Affecting Long-term Compressive Strength of Concrete
  • 1. Water-Cement Ratio
  • 2. Curing Conditions
  • 3. Temperature
  • 4. Environmental Conditions

Variation of Concrete Strength with Time

As per studies and researches, the compressive strength of the concrete will increase with age. Most researches were conducted to study the 28th-day strength of concrete. But in reality, the strength at 28th day is less compared to the long-term strength that it can gain with age.

The concrete strength variation with age can be studied by different methods. The figure-1 below shows the strength variation of a concrete present at dry and wet condition. This graph is based on the study conducted by Baykof and Syglof (1976).

They found that, in dry conditions, after 1 year there is no increase in concrete strength, as shown in figure-1. On the other hand, the strength of specimens stored in a wet environment (at 15°C) is considerably increased.

Fig.1: Variation of concrete strength with time

Fig.2: Concrete compressive strength variation with time (Washa and Wendt (1989))

Rate of Strength Gain With Time

The process of continued hydration will increase the strength of concrete. If the environmental conditions to which the concrete is exposed facilitates the hydration, the strength is gained continuously with age. But this rate of hydration is fast at the early stages and delays later.

The compressive strength gained by concrete is thus measured at its 28th day after which the rate of strength is lowered. The compressive strength gained at later ages are tested by means of non-destructive tests.

The table-1 below shows the rate of strength gained from the first day to 28th day.

Table.1: Strength Gained by Concrete With Age

Age	Strength Gained (%)
1 day	16%
3 days	40%
7 days	65%
14 days	90%
28 days	99%

Proper curing conditions will help in preventing the escape of moisture that will facilitate strength gain reactions.The figure-3 below shows the variation of compressive strength with age for different curing conditions.

Fig.3. Compressive strength versus age for different curing environment ( Mamlouk & Zaniewski)

Factors Affecting Long-term Compressive Strength of Concrete

The achievement of concrete compressive strength in long term is different from early age strength gain. The different factors affecting the long-term compressive strength of concrete are:

1. Water-Cement Ratio

An adequate water-cement ratio is necessary to undergo hydration reactions at later ages. Hydration reactions improve the compressive strength of concrete.

Inadequate water content will leave a tremendous amount of pore before 28 days that will increase the chances of creep and shrinkage issue with time. This will affect the compressive strength of concrete adversely.

2. Curing Conditions

Proper curing conditions is a kind of preparation of concrete before letting it to service conditions. The extent of curing of concrete is performed based on the anticipated exposure conditions of the structures.

Properly cured and high-quality concrete is not affected by extreme conditions with age. Effective curing hence improves the concrete compressibility.

3. Temperature

Studies have shown that high temperature speeds up the hydration reaction, but the products gained won’t be uniform or of good quality. This can leave pores which affect the strength of concrete.

4. Environmental Conditions

Concrete structure with age is subjected to environmental conditions like rain, freezing and thawing, chemical attacks etc. An impermeable concrete can undergo moisture penetration, frequent freeze, and thaw that creates cracks in concrete.

Chemical attacks can corrode the reinforcement reducing the yield strength of reinforcement. All these can affect the concrete strength capacity.