A. elements B. cause C. to crack D. subjected
E. technology F. manufacturing G. advantages
H. reinforcing I. producing J. material K. formwork L. tension
1. Post-tensioning is simply a method of …… pre-stressed concrete, masonry, and other structural elements.
2. Compression is a force that squeezesor crushes, and …… is a force that pulls something apart.
3. When a concrete floor slab is …… to forces, it flexes and bends.
4. The bending and flexing create high tensile forces that can …… the concrete floor slab to crack.
5. Adding post-tensioned reinforcement instead of rebar alone combines the action of reinforcing the tension zones with the …… of compressing the concrete slab.
6. Post-tensioning is done onsite by installing post-tensioning tendons within the concrete …… in a manner similar to installing rebar.
7. Post-tensioning is simply a method of producing pre-stressed concrete, masonry, and other structural …….
8. As a building ……, concrete is very strong in compression but relatively weak in tension.
9. The post-tensioning …… was introduced to the United States in the 1950s.
10. Because pre-tensioning requires specially designed casting beds, it is used generally in the precast …… process.
UNIT 12 Why Does Concrete Crack?
Concrete Defects in Civil Engineering
Cracks are one of the most common defects in concrete. This article will discuss the main reasons for concrete cracking and how concrete surface cracking can be avoided. Various types of cracks are quite common in civil construction. Most of the time cracks could be avoided by using proper concrete mix proportions. Following are the four major reasons why cracks develop in concrete:
• Excess water quantity: Most often time Portland cement is used for construction. Portland cement, being a hydraulic type of cement, needs water for developing strength. Also, water increases the flowability of the cement concrete and thus increases the ease of its use. But when the concrete dries and hardens, the excess water from the concrete evaporates and the concrete shrinks. As a result of that, cracks develop in the concrete.
• Too dry weather: After the wet concrete mix is spread, it takes a few weeks to complete the whole hydration process. This hydration process gives strength to the concrete but also needs sufficient water. If the concrete does not get sufficient water for hydration, it tends to develop cracks.
• Incorrect selection of cement: Cement comes in different grades as per the strength requirement of the concrete. In case you have selected wrong or weak cement, your concrete won’t have enough strength and will develop cracks.
• Control Joints: Control joints are planned cracks. Control joints give room for shrinkage and thus control joints help in avoiding random cracks. Typically a 4” thick concrete slab should have the concrete joints around 8-12 feet apart.
