A. elements B. cementitious C. material D. applications
E. regulator F. solid G. e-glass H. laboratory I. lightweight
J. properties K. integrated L. environment
1. GFRC is an engineered concrete that has numerous …… in concrete products including ornamental structures, fountains, domes.
2. Glass fiber reinforced concrete composites contain high strength glass fibers that are surrounded by a …… medium.
3. Both the fibers and the …… maintain their natural individual chemical characteristics.
4. Glass fibers can be …… into the matrix either in constant or irregular lengths.
5. The glass fibers are the main …… that carry the load.
6. The most common type of glass fibers used for general applications is …….
7. All these properties are combined with the fact that GFRC looks like …… concrete.
8. GFRC …… can change depending upon the design of mix, fiber content, and the techniques used for manufacture.
9. GFRC has been tested in the …… and also in the actual installations, and can be anticipated to survive as long as pre-cast concrete.
10. The concrete takes the role of a thermal …… while exposed to fire and protects the materials from the flame heat.
UNIT 10 When Stress Is Good:
Pre- and Post-Tensioned Concrete
Read, translate the following text and be ready to fulfil the tests:
Portland cement concrete is a strong, dependable and relatively inexpensive construction material. Until it is stressed under tension, that is. It typically exhibits only about 15% of its equivalent compressive strength, which can spell catastrophe for structural applications. The first use of material most similar to modern concrete was by Roman engineers and was known as opus caementicium. The Pantheon still stands today as the largest un-reinforced concrete dome in the world. Presumably they also discovered very quickly that the material failed catastrophically in attempts to use it in tensile structural applications.
Pre-tensioning
In pre-tensioned concrete tensile elements such as cables, ribbons, or rods are clamped under calculated tensile stress. Concrete is cast around these elements and allowed to cure. When fully hardened the clamps are released and the stress is transferred within the rigid concrete. Only in this case the tensile elements are not holding the structure together, rather they act to place the concrete into a state of compressive stress prior to load application. When a load is applied within the design limit, the concrete structural piece will never see tensile stress of sufficient magnitude to cause failure.
Post-tensioning
Unbonded post-tensioning is accomplished by coating the tensile elements with lubricant and covering them with extruded polymer sleeves. These are then arranged within the concrete molding form with the ends of the tensile elements attached to anchors placed at the perimeter of the casting. Since the tensile elements are free to move within the casting, when curing is complete they can be pulled into tension and clamped to the embedded anchors at the edges of the piece. Once again the concrete is placed into a preloaded compressive state of stress by the tensioned elements. Unlike pre-tensioning, however, the post-tensioned elements can catastrophically pull out of the casting if the anchors fail or unskilled repairs are attempted. However they can also be adjusted to compensate for changes in loading or field conditions. Finally, bonded post-tensioning utilizes conduits placed into the concrete molding form. The tensioning elements are threaded through the conduits before the concrete is cast. Once curing is complete, the elements are pulled into tension and clamped or wedged at the conduit openings to hold the applied tensile stress. These elements are then grouted into place at the openings to permanently bond them with the structural casting.
Rise of Precast Concrete
First used in 1928, prestressed concrete has led to an explosion of applications in buildings, bridges, slabs, foundations, and roadways, and has driven development of the precast concrete industry. Although the cost is typically greater that standard reinforced concrete the increase in structural performance can more than offset the additional expense. Lighter, stronger, longer lasting, and requiring fewer joints, prestressed concrete construction materials have shown how good stress can be.
Read and memorize the active vocabulary to the text:
Active Vocabulary
1. dependable надежный
2. tension растягивающее усилие, растяжение
3. to exhibit выставлять; проявлять качества
4. presumably предположительно
5. to fail выходить из строя, терпеть неудачу
6. ribbon лента, полоса
7. rod стержень, прут, брус
8. cast литье, отливка
9. release разъединение, высвобождение
10. to transfer перемещать, переносить
11.clamp зажим, скоба, хомут
12. unbonded не сцепленный с оболочкой
13. to accomplish выполнять, осуществлять
14. anchor анкер, связь
15. preloaded предустановленный
16. post-tensioning напряжение стальной арматуры и сжатие бетона после его заливки
17.to thread протягивать, пронизывать
18. curing отверждение
19. embedded встроенный, включенный
20. wedge клин
21. conduit канал, проток, труба, желоб
22.explosion взрыв, вспышка, быстрый рост
CONTROL TESTS
TEST I
Decide whether these statements are true or false:
1. a). In pre-tensioned concrete tensile elements such as cables, ribbons, or rods are covered with extruded polymer sleeves.
b). In pre-tensioned concrete tensile elements such as cables, ribbons, or rods are placed into a state of compressive stress.
c). In pre-tensioned concrete tensile elements such as cables, ribbons, or rods are clamped under calculated tensile stress.
2. a). When fully hardened the clamps are releasedand then arranged within the concrete molding form.
b). When fully hardened the clamps are releasedand the stress is transferred within the rigid concrete.
c). When fully hardened the clamps are releasedand then load is applied within the design limit.
3. a). Unbonded post-tensioning is accomplished by coating the tensile elements with lubricant and covering them with extruded polymer sleeves.
b). Unbonded post-tensioning is accomplished by attaching the tensile elements to anchorsplaced at the perimeter of the casting.
c). Unbonded post-tensioning is accomplished by pulling the tensile elements into tension and clamping to the embedded anchors.
4. a). The post-tensioned elements can catastrophically pull out of the casting if the tensile elements are free to move within the casting.
b). The post-tensioned elements can catastrophically pull out of the casting if the anchors fail or unskilled repairs are attempted.
c). The post-tensioned elements can catastrophically pull out of the casting if the tensile elements are not holding the structure together.
5. a). The tensioning elements are threaded through the conduits, placed into the concrete molding form.
b). The tensioning elements are threaded through the conduits after the concrete is cast.
c). The tensioning elements are threadedthrough the conduits before the concrete is cast.
TEST II
Match words in columns A and B (synonym match):
A B
1. relatively a). normally b). totally
c). fully d). comparatively
2. to spell a). to fulfil b). to mean
c). to prevent d). to bear
3. attempt a). effort b). strain
c). force d). manner
4. prior to a). till b). ago
c). in advance of d). apart
5. to harden a). to cast b). to solidify
c). to join d). to weld
6. sufficient a). ample b). rigid
c). resistant d). effective
7. magnitude a). development b). technique
c). difference d). dimension
8. to cause a). to consist b). to induce
c). to produce d). to fabricate
9. to extrude a). to utilize b). to conform
c). to expel d). to support
10. to offset a). to compensate b). to finish
c). to locate d). to specify
TEST III
Select the best possible answer out of the choices from the list to the statements (multiple choice):
1. Portland cement concrete is a strong, dependable ….
a. material, which can be used in tensile structural applications.
b. material, which can pull out of the casting if the anchors fail or unskilled repairs are attempted.
c. and relatively inexpensive construction material.
d. material, which can be used under calculated tensile stress.
2. When concrete is fully hardened the clamps are releasedand ….
a. the concrete is placed into a preloadedcompressive state.
b. a load is applied within the design limit.
c. the tensile elements are coated with lubricant.
d. the stress is transferredwithin the rigid concrete.
3. Since the tensile elements are free to move within the casting, when curingis complete, ….
a. they can be placed into the concrete molding form.
b. they can be pulled into tension and clamped to the embedded anchors.
c. they are holding the structure together.
d. they can be adjusted to compensate for changes in loading or field conditions.
4. When a load is applied within the design limit, ….
a. it is possible to place the concrete into the molding form.
b. the concrete structural piece will never see tensile stress of sufficient magnitude to cause failure.
c. theconcrete can be used in tensile structural applications.
d. stress is transferredwithin the rigid concrete.
5. Concrete is castaround the tensile elements and ….
a. put under calculated tensile stress.
b. allowed to cure.
c. covered with extruded polymer sleeves.
d. placed into a state of compressive stress.
TEST IV
