PCC Pavements 1. Explain the difference between pumping and faulting in a PCC pa
ID: 1713731 • Letter: P
Question
PCC Pavements
1. Explain the difference between pumping and faulting in a PCC pavement.
2. What are the key differences between ASR and “D” cracking? How would you avoid such problems?
3. Using the Concrete Pavement Design and Construction Practices Technical Digest, provide a recommendation for a concrete pavement type (or types) for the following cases: a. ESAL 2x10^6 , A-3 subgrade, dry climate, base material fairly erodible, joints to be sealed. b. ESAL 5x10^6 , A-6 subgrade, wet climate, treated base material, with installed edge drains. c. ESAL 16x10^6 , A-7 subgrade, dry climate, with full sub-drainage system with permeable base.
4. Explain the difference between the following conditions: frost heave soils; swelling soils; collapsing soils;
5. Estimate the modulus of subgrade reaction for the following subgrade soils: a. Clayey sandy gravel with a dry density of 130 lb/ft^3 . b. Plastic clay with a CBR of 5.
6. A normal PCC has a compressive strength of 5500 psi at 28 days, estimate its modulus of rupture.
7. What is PCC bleeding and honeycombing?
8. Your calculated saw-cutting window for a PCC pavement is 6 hours. Discuss some of the factors that will make you change this to a lower or higher number of hours.
9. Sketch the principal activities in PCC pavement construction (in logical sequence):
Explanation / Answer
1 . Basic thing to understand between faulting and pumping is that faulting is effect of pumping as faulting or what we say difference in elevation of pavement usually in PCC pavements is caused by cracks and undulations caused by pumping .
Pumping can be defined as comming out of material from below the pavement slab at the point of cracks or joints mostly due infiltration of water below the surface of pavement.
If pumping is controlled faulting will not occur .
2. The key difference between ASR cracking ( Alkali silica reactions) and D cracking (durability cracking) is that ASR cracking occurs due to presence of excess alkali material in cement due to which pH of cement is raised and which ultimately results in expansion of pavements which is undesirable and cause cracks whiles as D Cracking occurs due to degradation of aggregate used in concrete by continuous freeze thaw cycles due to which water start accumulating below the pavement surface and cause wearing of pavements hence effects overall durability of concrete pavement .
Second difference is Doing cracking occurs below the surface and is not visible untill it comes on surface hence it is hard to detect on the contrary ASR cracks can be detected by expansion of pavement surface .
There are various methods by which these problems in pavements can be prevented
ASR cracking can be prevented by The use of pozzolans in the concrete mix as a partial cement replacement can reduce the likelihood of ASR occurring as they reduce the alkalinity of the pore fluid.
D cracking can be prevented by the addition of air within concrete to allow water to easily expand and contract during freeze-thaw cycles without adding strain to the aggregate in the concrete
Other options are selecting aggregates to be used that perform better in freeze-thaw cycles or reducing the particle size of the aggregates to strengthen the concrete
7. PCC Bleeding : it is defined as accumulation of asphalt binder on the surface of pavement mostly in cold weather .
What happens actually is during hot weather conditions newly placed asphalt binder expands and fills the aggregate voids and during cold weather pavement contracts hence asphalt comes out on surface and it is referred as bleeding of PCC.
Honeycombing: Honey comb refers to voids in concrete caused by the mortar not filling the spaces between the coarse aggregate particles. It occurs due to wrong water cement ratio due to which concrete can't hold aggregates properly and honeybee nests are formed at places where cement is not reached hence voids remain unfilled and aggregates and rainforcement remain exposed to corrosion which ultimately effects strength of structural safety and stability.
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