Common Site Problems During Masonry Construction

Masonry structure is easy to design and construct, but various site issues may occur such as incorrect mix proportions, use of unauthorized admixtures, sulphate attack, freeze and thaw cycles and aesthetic failures.

Common Site Problems During Masonry Construction

1. Incorrect Mix Proportions

Incorrect mortar mix proportions are mostly the use of less quantity of binder materials than the required amount. This problem is reported to be come up when materials are mixed at the construction site.

Commonly, applicable codes and construction documents emphasize on measuring binder material either by weight or volume, but this measure is mostly ignored when material blending is carried out on project site.

Sometimes, number of shovels is used as a measuring technique, but this practice is not accurate and lead to incorrect mix proportions. This problem cannot be tackled unless all mix constituents are accurately measure.

2. Use of Unauthorized Admixtures

This problem is the reduction of mortar quality due to the addition of air entraining admixture that take the form of domestic detergent or washing up liquids. It is likely that the strength of mortar is compromised especially in bond due to excessive air entraining.

The main motivation of adding air entrain admixture is that it improves mortar plastic properties, and mortar utilization will be substantially eased. This problem is usually encountered when materials are mixed on site.

3. Sulphate Attack

Sulphate attack occurs as result of the reaction between tricalcium present in Portland cement and soluble sulphate which may come from various sources for instance masonry units and grounds.

The reaction is expansive and the size of ettringite, which is produced because of the reaction, is greater than of the reacted materials. Consequently, spalling, degradation and finally failure could occur.

Therefore, it is necessary to take suitable measures to prevent sulphate attack. This may be achieved through preventing sulphate to reach the mortar.

It is reported that, the number of masonry units contain large amount of sulphate are reducing constantly. So, sulphates in the ground would be the major problem and the contact between ground and mortar need to be prevented. This can be achieved by practicing correct detailing of damp poof courses and related details.

Regarding exceptional situations, where sulphates occur in atmosphere and masonry units, proper measure can be considered to decrease the possibility of sulphate attack.

For example, proper designing of coping and overhangs to avoid saturations will be offer great assistance, in addition to the provision of air entraining agent.

4. Freeze and Thaw Cycles

When masonry elements saturated with water and subject to cycles of freezing and thawing, the masonry member may suffer degradation and subsequent failure.

There are certain strategies used to protect masonry members from the effect of freezing and thawing for example introduction of suitable damp proof courses and copings.

Another effective technique is to use air entraining agent in mortar which proven to be substantially advantageous. That is why most applicable codes specify the use of air entraining agent to protect masonry construction form both freezing and thawing cycles and sulfate attacks.

Fig.: Effect of freezing and thawing cycles on masonry member

5. Aesthetic Failures

Aesthetic of masonry members does not affect its load carrying capacity but it is crucial and need to be considered during design and construction.

The formation of widespread bloom or efflorescence is not acceptable by the majority of clients. Therefore, it is required to take necessary action to prevent it for example covering newly constructed masonry at the end of each day to prevent saturation, otherwise masonry member will develop disfiguring stain and hence aesthetic appearance will be compromised.

Another aesthetic problem will arise when hydrated Portland cement is not protected since it remains highly soluble and saturation will cause leaching of calcareous solution from the material.

After water evaporated from this solution, solid material will set on the surface of the unit and joints. Added to that, the leaching of calcareous solution may lead to horizontal white stain formation.

Various Types of Joints in Concrete Construction

Joints in concrete building construction are construction joints, expansion joints, contraction joints and isolation joints. They prevent cracking of concrete.

Types of Joints in Concrete Constructions

Types of joints in concrete constructions are:

1. Construction Joints
2. Expansion Joints
3. Contraction Joints
4. Isolation Joints

Construction Joints in Concrete

Construction joints are placed in a concrete slab to define the extent of the individual placements, generally in conformity with a predetermined joint layout.

Construction joints must be designed in order to allow displacements between both sides of the slab but, at the same time, they have to transfer flexural stresses produced in the slab by external loads.

Construction joints must allow horizontal displacement right-angled to the joint surface that is normally caused by thermal and shrinkage movement. At the same time they must not allow vertical or rotational displacements. Fig.1 summarizes which displacement must be allowed or not allowed by a construction joint.

Fig.2: Types of Construction Joints in Concrete Structures

Expansion joints in Concrete

The concrete is subjected to volume change due to many reasons. So we have to cater for this by way of joint to relieve the stress. Expansion is a function of length. The building longer than 45m are generally provided with one or more expansion joint. In india recommended c/c spacing is 30m. The joints are formed by providing a gap between the building parts.

Contraction Joints in Concrete

A contraction joint is a sawed, formed, or tooled groove in a concrete slab that creates a weakened vertical plane. It regulates the location of the cracking caused by dimensional changes in the slab.

Unregulated cracks can grow and result in an unacceptably rough surface as well as water infiltration into the base, subbase and subgrade, which can enable other types of pavement distress.

Contraction joints are the most common type of joint in concrete pavements, thus the generic term “joint” generally refers to a contraction joint. Contraction joints are chiefly defined by their spacing and their method of load transfer. They are generally between 1/4 – 1/3 the depth of the slab and typically spaced every 3.1 – 15 m

Isolation Joints in Concrete

Joints that isolate the slab from a wall, column or drainpipe

Isolation joints have one very simple purpose—they completely isolate the slab from something else. That something else can be a wall or a column or a drain pipe. Here are a few things to consider with isolation joints:

Walls and columns, which are on their own footings that are deeper than the slab subgrade, are not going to move the same way a slab does as it shrinks or expands from drying or temperature changes or as the subgrade compresses a little.

Even wooden columns should be isolated from the slab.

If slabs are connected to walls or columns or pipes, as they contract or settle there will be restraint, which usually cracks the slab—although it could also damage pipes (standpipes or floor drains).

Expansion joints are virtually never needed with interior slabs, because the concrete doesn’t expand that much—it never gets that hot.

Expansion joints in concrete pavement are also seldom needed, since the contraction joints open enough (from drying shrinkage) to account for temperature expansion. The exception might be where a pavement or parking lot are next to a bridge or building—then we simply use a slightly wider isolation joint (maybe ¾ inch instead of ½ inch).

Blowups, from expansion of concrete due to hot weather and sun, are more commonly caused by contraction joints that are not sealed and that then fill up with non-compressible materials (rocks, dirt). They can also be due to very long unjointed sections.

Very long unjointed sections can expand enough from the hot sun to cause blowups, but this is rare.

Isolation joints are formed by placing preformed joint material next to the column or wall or standpipe prior to pouring the slab. Isolation joint material is typically asphalt-impregnated fiberboard, although plastic, cork, rubber, and neoprene are also available.

Isolation joint material should go all the way through the slab, starting at the subbase, but should not extend above the top.

For a cleaner looking isolation joint, the top part of the preformed filler can be cut off and the space filled with elastomeric sealant. Some proprietary joints come with removable caps to form this sealant reservoir.

Joint materials range from inexpensive asphalt-impregnated fiberboard to cork to closed cell neoprene. Cork can expand and contract with the joint, does not extrude, and seals out water.

Scott Whitelam with APS Cork says that the required performance is what determines the choice of joint materials. How much motion is expect, exposure to salts or chemicals, and the value of the structure would all come into play—and of course the cost.

Polyethylene foam isolation joint material comes in various colors. C2 Products

At columns, contraction joints should approach from all four directions ending at the isolation joint, which should have a circular or a diamond shaped configuration around the column. For an I-beam type steel column, a pinwheel configuration can work.

Always place the slab concrete first and do not install the isolation joint material and fill around the column until the column is carrying its full dead load.

Types of Sealants Used for Joints in Buildings -Properties, Uses, Working

Sealant is a material which is used to seal the joints between materials such as concrete, glass, aluminum, masonry wall etc. In general joints are provided in the structures to prevent the damage produced by stresses.

Properties of Good Sealant

Different types of sealants with good properties are available. The basic properties of a good sealant should be as follows.

• The sealant should have good bond with building materials.
• The sealant should be soft.
• It should be flexible.
• It should not affected by the weather changes.
• It should strong against stress and stress relief cycle.

Types of Sealants Used for Joints in Buildings – Properties and Uses

There are several types of sealants are:

• Silicone based sealants
• Urethane based sealants
• Acrylic based sealants
• Polysulphide based sealants

Out of the above sealants, Polysulphide sealants are more popular in construction world.

Polysulphide based Sealant

Polysulphide sealants are widely used because of good sealant properties. They are basically applied in cold conditions. Polysulphide sealants are available in two types of systems:

• Two-part system
• One-part system

Two-part system

This system of sealant contains two parts called base and accelerator. To prepare a sealant these both should be mixed. After mixing them both react chemically and forms thick paste. This paste should be used within 48 hours after mixing. After applying sealant it will take 8 days for full curing.

Two-part system Polysulphide sealant is available in two special forms namely, gun grade and pour grade. Gun grade is used for inclined joints, vertical joints and overhead joints while pour grade is used for horizontal joints.

One-part system

One-part system contains premixed sealant which can be directly used without any mixing. They are capable of absorbing moisture form the atmosphere and reaction occurs. In this case full curing of sealant will take 3 to 4 weeks.

Uses of Polysulphide based Sealants

Polysulphide based sealants are used in different areas of constructions as follows:

1. Building structures joints like basements, glazing frames, ceiling joints, floors, roofs, external walls, cladding, retaining walls etc.
2. Water retaining structures joints such as dams, reservoirs, canal linings, culverts etc.
3. Joints in bridges, roads, aerodromes etc.

Equipment for Polysulphide based Sealants Application

Sealant should be applied with proper equipment. The equipment should be as follows:

• Filling device
• Gun
• Mixer
• Spatula
• Backup material
• Bond breakers
• Masking tape

Filling Device

The mixed or prepared sealant should not be exposed to atmosphere for longer time. So, proper filling device is used and this can be attached directly to the gun for direct usage of sealant. It is well suitable for large scale works (30N or more sealant).

Gun

The gun is a device which include PVC made cartridges and nozzles to deliver the sealant. Using this gun with sealant can be easily placed in the joints in any position.

Mixer

Mixer is usually required for two-part sealant system. So, the base and accelerator should be mixed effectively.

Spatula

Spatula can be used as alternative for gun but it is suitable for small quantity works.

Along with equipment, some accessories are required for sealant which can improve its application.

Backup material

Back up material controls the depth of sealant in the joint.

Bond breakers

Bond breakers in the form a tape is made of PVC or metal or paper. Three face adhesion can be prevented by using bond breakers.

Masking tape

When the sealant is applied in the joints, the sides of joint may be damaged by spreading of sealant. To prevent this masking tape is provided on both sides of joint. Some time it may not be used if the skilled persons are working.

Working Conditions of Polysulphide based Sealants

1. Temperature (application and service)
2. Size of joint
3. Storage of sealant
4. Water resistance
5. Chemical resistance
6. Setting time and cure time
7. Movement
8. Durability

Temperature (application and service)

While applying sealant the temperature range should be 5o C to 50o C. And the sealant can work or service effectively in the temperature range of – 40o C to +80o C.

Size of joint

The width of joint should be 5 mm to 50 mm. the depth of sealant applied in the joint should be 5 mm for metal and glass structures and 10 mm for concrete and brick joints.

Storage of sealant

The mixed paste of two-part system sealant can be stored up to 12 months in dry and cool place in closed container.

Water resistance

After full curing the sealant will resist water and impermeable.

Chemical resistance

Chemical resistance of sealant is very great and they offer great resistance against oils, petrol, white spirit, fuels etc.

Setting time and curing time

The setting time and curing time will depends mainly on the temperature of that particular location. These times for different temperatures is given below.

Temperature (oC)	5	15	25	35
Setting time (hours)	72	36	18	8
Curing time	8 weeks	4 weeks	2 weeks	8 days

Movement

Movement of sealant after applying is 25% for butt joints and 50% lap joints.

Durability

In traffic surfaces such as roads, bridges the sealant can last up to 10 years while in other cases it can last up to 25 years.