Plastic Shrinkage Cracks in Concrete and Its Prevention

Plastic Shrinkage Cracks in Concrete and Its Prevention

Cracking caused by plastic shrinkage in concrete occurs most commonly on the exposed surfaces of freshly placed floors and slabs or other elements with large surface areas when they are subjected to a very rapid loss of moisture caused by low humidity and wind or high temperature or both.

Plastic shrinkage usually occurs prior to final finishing, before curing starts. When moisture evaporates from the surface of freshly placed concrete faster than it is placed by curing water, the surface concrete shrinks.

Due to the restraint provided by the concrete on the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks that are usually not short and run in all directions.

In most cases, these cracks are wide at the surface. They range from a few millimeters to many meters in length and are spaced from a few centimeters to as much as 3 m apart.

Preventing Plastic Shrinkage Cracks in Concrete

Plastic shrinkage cracks may extend the full depth of elevated structural slabs. Since cracking because of plastic shrinkage is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete.

There are many methods and techniques to prevent this type of crack in case of rapid loss of moisture due to hot weather and dry winds.

These methods include the use of fog nozzles to saturate the air above the surface and using plastic sheeting to cover the surface between the final finishing operations.

In many cases, during construction it is preferable to use wind breakers to reduce the wind velocity; sunshades to reduce the surface temperature are also helpful.

Additionally, it is good practice to schedule flat work after the walls have been erected.

Types of Cracks in Concrete due to Moisture Change

Types of cracks in concrete due to moisture change is initial shrinkage cracks, plastic shrinkage, plastic settlement and initial expansion of concrete. These types of cracks occurs due to moisture change in concrete which are discussed in detail.

Building materials such as concrete, mortar, bricks, wood etc are porous in nature and expand by absorbing moisture from atmosphere and shrink on drying. These changes in building materials are cyclic in nature and are caused during moisture changes.

But building materials undergo irreversible changes due to change in moisture content during its initial conditions. These initial changes cause shrinkage or expansion of materials.

For example, irreversible  movement  in  materials  are shrinkage  of  cement  and  lime based  materials  on  initial  drying  i.e.  initial shrinkage/plastic shrinkage  and expansion of burnt clay bricks and other clay products on removal from kilns i.e. initial expansion.

Types of Cracks in Concrete due to Moisture Change

Initial Shrinkage in Concrete and Masonry

Initial  shrinkage of building materials which is partly  irreversible, occurs in all building materials that are cement / lime based such as  concrete, mortar, masonry units,  masonry and  plaster  etc. This initial shrinkage is one of the main causes of cracking in building structure.

Initial shrinkage as the name suggests occurs only once in the lifetime of concrete and mortar, i.e. during the time of construction when the moisture dries out during setting process. Initial shrinkage is most significant cause for the cracks in structures.

The effect of initial shrinkage in concrete and mortar depends on following factors:

Cement content – Shrinkage of concrete and mortar increases with richness of mix.

Water content – Increase in water quantity used in the mix increases the shrinkage.

Maximum size, grading and quality of aggregate –With increase in the maximum size of aggregate with good grading, the water-cement ratio decreases for the same workability requirement of concrete. The use of less water reduces the initial shrinkage of concrete due to reduction in porosity.

For example, the shrinkage of cement sand mortar is 2 – 3 times that of cement concrete using 20mm maximum size of aggregate and 3 – 4 times that of cement concrete using 40mm maximum size of aggregate.

Curing of concrete and masonry – Proper curing from the start of initial setting to at least 7 to 10 days helps in reducing initial shrinkages. The moisture provided through curing helps concrete and masonry to expand and thus, when they dry up, the final shrinkage is less.

Surface area of aggregates -Surface area of concrete increases with increase in fine aggregates and this requires large water quantity for the required workability. With increase in water quantity, the shrinkage of concrete and masonry increases when they dry up.

Chemical composition of cement – Shrinkage is less for the cement having greater proportion of tricalcium silicate and lower proportion of alkalis i.e. rapid hardening cement has greater shrinkage than ordinary portland cement.

Temperature of  fresh  concrete  and  relative  humidity  of  surroundings  – With  reduction  in  the  ambient  temperature  the  requirement  of  water  for  the same  slump/workability  is  reduced  with  subsequent  reduction  in

Concreting  done  in  mild  winter  have  much  less  cracking  tendency  than  the concreting done in hot summer months.

In cement concrete 1/3rd of the shrinkage take place in the first 10 days, ½ within one month and remaining ½ within a year time. Therefore, shrinkage cracks in concrete continues to occur and widens up to a year period.

Plastic shrinkage of concrete

Cracks appearing on the surface of concrete before setting of concrete is due to plastic shrinkage of concrete. The cause of shrinkage cracks in concrete is due to settlement of heavy aggregates at the bottom of concrete and rise of water to the surface. This process continues till the concrete has set and it is called as bleeding of concrete.

As long as  the  rate  of  evaporation  is  lower  than  the  rate  of  bleeding,  there  is  a continuous  layer  of  water  at  the  surface  known  as  “water  sheen”,  and shrinkage  does  not  occur.  When  the concrete  surface  looses  water  faster than  the  bleeding  action  bring  it  to  the  top,  shrinkage  of  top  layer  takes place, and since the concrete in plastic state can’t resist any tension, cracks develops on the surface. These cracks are common in slabs.



The extent of plastic shrinkage in concrete depends on:

• Temperature of concrete,
• Exposure to the heat from sun radiation,
• Relative humidity of ambient air and velocity of wind.

Plastic settlement cracks

Plastic settlement cracks occurs on concrete surface due to settlement of large aggregates. In the case of reinforced concrete, sometimes the settlement of aggregates is obstructed by reinforcement. These obstructions break the back of concrete above them forming the voids under their belly.

Plastic settlement cracks are normally observed:

• Over form work tie bolts, or over reinforcement near the top of section.
• In narrow  column  and  walls  due  to  obstruction  to  sedimentation  by resulting arching action of concrete due to narrow passage.
• At change of depth of section.

Initial expansion of concrete:

When the clay bricks are fired during manufacturing, due to high temperature not  only  the  intermolecular  water  but  also  water  that  forms  a  part  of molecular structure of clay is driven out. After burning, as the temperature of the bricks falls down, the moisture hungry bricks starts absorbing moisture from  the  environment  and  undergoes  gradual  expansion,  bulk  of  this expansion is irreversible.

For the practical purpose it is considered that this initial expansion ceases after first three months.

Use  of  such  bricks  before  cessation  of  initial  expansion  in  brickwork,  will cause irreversible expansion and may lead to cracking in masonry.

ESTIMATE BID PRICE-TENDER COST OF CONSTRUCTION PROJECT

The Estimation of bid price or tender cost of a construction project is calculated as the sum of the total cost of construction and mark-up price. Total cost of construction project consists of direct cost and indirect costs for the construction works.

Direct cost of construction project includes the cost of materials, labors, plants and equipment while the indirect costs are associated with expenses on personnel recruitment and their training, research and development required for the construction works.

Thus a direct cost is associated with the activities performed at construction site, i.e. if the activities are not carried out, direct costs are not incurred. In case of indirect costs, they are non-traceable based on activity. Indirect costs may have incurred even if the construction works / activities have not been performed.

Thus, following formulas can be used to calculate the cost of construction projects:

Direct Cost = Labor Cost + Plants & Equipment Cost + Crew Cost + Materials Cost + Subcontractor Cost

Indirect Cost = Project Overheads + Common Plants & Equipment Cost + Common Workmen Cost

Mark-up Cost = Profit + Contingency + General Overheads + Allowance for Risks

Construction Project Cost = Direct Cost + Indirect Cost + Mark-up Cost

Estimation of Direct Cost of Construction Project:

Direct cost of construction project consists of cost of materials, labors, plants and equipment, subcontractors and crew costs. Following are the guidelines for estimating components of direct cost:

Cost of Material:

Cost of materials includes cost of all construction materials which will go into the building or the construction work. These materials include cement, aggregates, reinforcement steel etc. The cost of material consists of actual cost of the material at the source, their packing and handling charges, transportation, excise duty, sales tax, entry tax etc. on the actual rate of material.

Therefore, the cost of material = Actual cost of material at the source + packing and handling charges + transportation cost + taxes

Cost of Labor:

The cost of labor is the payment made to labor for works done at construction site on daily or weekly or monthly basis. The cost of labor includes the basic wage paid, bonus payments, cost for labor welfare schemes such as pension, insurance, health benefits, notice pay, housing, food, retrenchment, mobilization and demobilization costs.

The cost of labor are influenced by local laws. Thus they vary from place to place and also depend on skill levels of the labors.

Cost of Plants & Equipment:

Cost of plants & equipment covers the cost incurred towards owning and operating the equipment for various construction activities.

The cost of plants and equipment includes cost of owning and operating, mobilization cost, cost of fuel, oil and lubricants, cost of repair and maintenance of equipment, cost of spare parts, cost of operators, drivers, statutory expenses, personnel, carrier expenses and demobilization costs.

Subcontractor Costs:

In a large construction project, the main contractor has one or many sub-contractors under them for various construction works or activities or specialized works such as waterproofing, wood works, aluminium works, anti-termite treatments etc.

The cost associated with specialized construction work or any work done by a subcontractor is called as subcontractor costs.

The main contractor gets the quotation from subcontractors for the proposed construction work to be done by them. Then the cost of works to be completed by subcontractor is considered as subcontractor costs.

Crew Cost:

A crew is a group of men (workers, supervisors etc.) and machineries which are assigned a particular type of construction activity such as excavation, piping, welding, woodwork etc.

The cost associated with these crew to complete the given work is taken as the crew cost. This cost can also be taken directly instead of calculating cost of labor, equipment for such works.

For example, excavation crew consists of 2-3 workers, excavator machine, dumpers etc. By knowing the capacity of this crew to complete the work in a time frame and cost of the crew per unit time, we can compute the cost of excavation directly without calculating the cost of labor, equipments separately.

Estimate Indirect Cost of Construction Project:

The indirect costs are also called as overheads. A construction project has many activities to be complete. Some costs are incurred in a construction project which are not associated with only one activity and they can be distributed to many activities. Thus these costs are not considered as direct costs and considered separately as indirect costs.

For example, for a material cost, the indirect cost can be purchase cost, inventory carrying cost etc.

The indirect cost includes project overheads, common workmen, common plants and equipment costs.

Project Overheads Cost:

Project overhead cost consists of following in a construction project:

i) Salaries and benefits – This includes the salaries and benefits to staffs including engineers, supervisors and other administrative posted at construction site. The gross salaries of these are considered for this cost. Thus, salaries and benefits to staffs shall be considered in bid price estimation calculation.

ii) Insurance cost – A construction project possesses many types of safety and health hazards. Thus insurance for life and health and provided to all staff and workmen involved in the construction project.

iii) Financing cost – A large construction project required a large amount of money to mobilize and carryout the construction activities. For this, the loans are arranged from various banks which attracts interest. Also costs are incurred during bidding process such as bid bond, performance bond, advance bond and payment guaranty, retention guaranty, letter of credit, establishment charges, work insurance, fines and penalties etc. So, these costs are appropriately considered while estimating bid price or tender cost of a construction project.

iv) Conveyance cost, Travel and transfer Cost – Many times equipments, staffs and workmen need to move from one place to another for various purposes which incurs a cost known as conveyance cost. This is also considered while estimation of bid price of a construction project.

v) Temporary Site Installation Cost – Temporary site installation such as site office, workshops, stores, fencing, warehouses and storage areas, sanitary, first aid, furniture and fittings, computers and other electronics, temporary roads, car parking and even residential units and messing facilities are provided at construction site. The cost associated with these temporary site installations are also considered in bidding price calculation of construction project.

vi) Clients and Consultants Requirements – Many times a contractor has to provide facilities to client and consultants such as office staffs, office space, furniture, telephone etc. The cost associated with these facilities also have to be taken into tender cost calculation of construction project.

vii) Utilities Cost – The cost associated with utilities such as water, power etc. for office and residential areas are to be borne by the contractor and shall be considered for bidding price estimation calculation.

viii) Taxes and Duties – Taxes such as contract tax, excise duties, custom duties, income tax, sales tax, service tax, value added tax etc. are different type of taxes which may be application for a construction project. Thus cost of these taxes are considered appropriately for bidding price calculation wherever applicable.

ix) Miscellaneous Expenses – There can be other expense other than mentioned above such as administrative expenses, field expenses, stationary, documentation, communication, tools and consumables, safety and protective equipments etc. The cost associated with these miscellaneous expenses are considered for tender price estimation calculation.

Common Plant & Equipment and Common Workmen Cost:

Some plants and equipments and construction workers are associated with many construction activities. They are grouped and their cost is calculated separately and included in the tender cost – bid price estimation calculation of a construction project.

Types of cracks in Concrete Structures

Types of cracks in Concrete Structures

Types of cracks in concrete structures are structural cracks and non-structural cracks.

Structural Cracks in Concrete

Structural cracks are those which result from incorrect design, faulty construction or overloading and these may endanger the safety of a building and their inmates.

Non Structural Cracks in Concrete

Non Structural cracks occur mostly due to internally induced stresses in building materials. These cracks normally do not endanger the safety but may look unsightly, create an impression of faulty work or give a feeling of instability.

Cracks reappear over repaired surface as rust scales were not removed

Defects in Concrete

Concrete defects can be broadly classified into two categories :

Macro Defects

If these defects are present, concrete has low strength and will rapidly deteriorate due to easy ingress of water and other chemicals. Invariably, structure will require repairs within a few years of its construction. Causes will have to be analysed and defects removed before doing any additional protective treatment.

Often, waterproofing of concrete slabs is carried out superficially and it fails to give the desired benefit because the defective concrete below this waterproofing layer has not been treated to seal the macro/micro defects which existed within the concrete slab.

The main causes of these defects are generally due to inadequacies in design and / or construction practices.

Micro Defects

These defects are not visible to the naked eye. They are usually very fine voids caused by large capillary pores resulting from the use of low grades (strength) of concrete with high water to cement ratio.

They could also occur due to addition of excess water or high water to cement ratio of concrete mix. Fine cracks are generally present in concrete and can occur due to various reasons. They do not pose a serious threat to concrete deterioration initially as they are generally not deep and are discontinuous.

With lapse of time due to variations in temperatures, changes in weather conditions, changes in loading conditions they increase in depth, length and width and combine with other fine cracks to create continuous passage for moisture, chlorides, sulphates and other chemicals from the environment to enter and start corrosion of steel in concrete and other deleterious reactions.

Corrosion of steel and spalling of concrete due to ingress of moisture

To conclude, macro defects and micro defects in concrete are both harmful to the health of buildings and can cause deterioration of concrete depending on the extent of their presence, environmental conditions around the building and maintenance done during its life cycle.

However macro defects by virtue of being larger can cause faster deterioration and more damage to the structure than the micro defects.