Precast pre-stressed concrete has historically been used for a unlimited Project in construction industry in a way to develop countries. In last few decades its popularity has increased especially in transportation industry.
It offers a lot of important advantages in compare to traditional cast-in place concrete. Advantages which includes saving and repairing construction time, lower maintenance, rehabilitation, lower impact on environment and most importantly total construction cost.
CONCEPT OF PRECAST PRE-STRESSED CONCRETE PAVEMENT
As Engineers compare the cost between precast concrete pavement (PCP) and Cast-in place concrete pavement (CCP), they should also compare the design and construction technique behind it.
For the construction of precast concrete pavements, full panel replacements or maintenance and construction of new pavement can be done on severely cracked panels, punched out panels, deteriorated joints and prepared sub-base.
How the Precast pre-stressed concrete pavement work? lets look at the concept of its construction. includes pre-stressing the steel reinforcement bars in both transverse and longitudinal directions in the form of pre-tension and post-tension.
The pre-tension process is performed in the transverse direction, which would be perpendicular to the direction of the traffic flow, before casting the concrete in the panel form. The post- tension process is performed in the longitudinal direction after installed the slabs. Full depth panels are used by precast pre-stressed concrete pavement concepts.
For a highway that has heavy traffic most of the time, PPCS can be the best solution to replace full depth panels because these slabs already received the designed strength before installation.
For both construction and maintenance of the pavement, bedding should be kept as thin as possible why because the more thicker bedding causes more weaker support. Support condition of the precast concrete pavement (PCP) will be far better than cast-in place concrete pavement (CCP).
MATERIAL USED
The materials used in the construction of precast concrete pavement should be of good quality and should be easily placed, graded and compacted within required time limit.
In this technique the materials which will be used in the base are dense-graded, granular base or lean concrete. The settled base may be used and if any unsettled base is found it will be levelled to its past level utilizing compactors.
If the current base does not serve the necessities of the precast concrete pavement another base will be constructed. A slight layer of finely screened granular material or sand may be used to give a level surface to set the board.
A granular subbase might be reworked, compacted and graded. Extra bedding materials will be given if necessary to keep up the camber. The bedding materials for the base, which will be utilized, are sand and cementitious grout.
HOW PRECAST CONCRETE PAVEMENT CHEAPER
In order to analyze the cost of the construction of the precast concrete pavement Structural engineers needs to calculate overall cost which includes, Construction cost, maintenance and rehabilitation cost and user cost like vehicle operating cost, used delay cost etc.
COST CONSIDERATIONS
The design Engineers need to compare the cost of precast pavement to the cost of comparable pavement alternatives that can be installed in the same amount of time and still provide similar life expectancy.
COMAPRING THE MATERIAL
Material of both the pavement should be durable and long lasting but we should compare in terms of the cost of precast pavement with that of conventionally cast concrete pavement if the times required to install the two materials are not factored into the equation.
COMPARING LIFE CYCLE COST OF ALTERNATIVE MATERIALS
It is important to compare the life cycle rather than comparing only initial cost. Depending upon mix design, the rate of placement, weather conditions when finishing and curing, and the climate in which they are used. cast-in-place concretes is as variable as the types of concretes used for that purpose. Some fast-track formulations last only five years or less while others provide 10 years or more of service. When comparing these materials to precast concrete pavement, which has a life expectancy of 40 years or more.
CONCLUSION
A case study has been carried out with two project Precast concrete pavement (PCP) and Cast-in place Concrete Pavement (CCP), both project have design with same axial load factor for a one mile of length and comparison is carried out between the design alternative of the life cycle cost and initial construction cost.
The initial construction cost for PPCP is higher than the traditional method, because of the high cost associated of using cranes to place panels. It can be determined that maintenance cost for precast concrete pavement is less than the traditional onsite method. Furthermore, The Life cycle Cost of precast concrete pavement for both alternatives are less than the traditional pavement methods.
The required Time to construct traditional pavement (78 days) which is way higher as compare to PPCS (35 days) because cast-in place pavement needs some additional time in field to gain the required strength.
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