콘크리트 (Concrete)

콘크리트 구성 - 결합재 ; 골재사이의 간극을 채우고, 굳지않은 콘크리트의 유동성에 영향을 줌 - 골재 ; 경화한 콘크리트 품질에 영향을 줌

Introduction Concrete as a Structural Material CONCRETE (콘크리트) Coarse Aggregate (굵은 골재, 자갈) Fine Aggregate (잔골재, 모래) Mortar (모르타르) Cement paste (시멘트 풀) : Cement+Water CONCRETE STRUCTURES MATRIX + FILLER + FIBER Cement Paste Polymer Aggregate Mineral Powder Reinforcing steel Prestressing steel Fiber (Metal, Ceramic, Chemical)

Cohesiveness Cohesiveness is how well concrete HOLDS TOGETHER when plastic. Cohesiveness is affected by: THE AGGREGATE GRADING Graded Aggregate means that there is a range of size of Aggregates, from large rocks to small sands. Well-graded Aggregates give a more cohesive mix, too much coarse Aggregate gives a boney mix. WATER CONTENT A mix that has too much water will not be cohesive and may separate and bleed. Curing is keeping concrete damp for a period, to allow it to reach maximum strength. Longer curing will give more durable concrete. WEATHER Warmer weather will cause concrete to have a higher early strength. TYPE OF CEMENT Different types of cement will affect concrete properties: ie how quickly or slowly concrete gains strength. THE WATER TO CEMENT RATIO Too much water and not enough cement means concrete will be weaker and less durable. The water to cement ratio (W/C) is the weight of the water divided by the weight of cement.

ii) Workability: Workability means how easy it is to: Place Handle Compact and Finish a concrete mix. Concrete that is stiff or dry may be difficult to Handle and, if not constructed properly, will not be as strong or durable when finally hardened. A slump test can be used to measure the workability of concrete. In design mix the worker should keep the following points in mind to achieve the desired degree of workability. The amount of cement paste Distance of transport Strength and Durability Method of placing Loss of slump

The amount of cement paste: The cement paste is the soft or liquid part of the concrete mix. The more paste mixed with the coarse and fine Aggregates, the more workable a mix. The aggregate grading: Well-graded, smooth, rounded aggregates improve the workability of a mix. To make a more workable mix: Add more CEMENT PASTE. Use WELL GRADED aggregates. Admixtures: Use an ADMIXTURE. Never try to make a mixture more workable by just adding more water because this lowers the strength and durability of concrete.

iii) Strength: iv) Durability: Well made concrete is a naturally strong and durable material. It is DENSE, reasonably WATERTIGHT, able to resist changes in TEMPERATURE, as well as wear and tear from WEATHERING. Strength and Durability are affected by the density of the concrete. Denser concrete is more watertight (or less permeable). Concrete durability INCREASES with strength. Well made concrete is very important to protect the steel in reinforced concrete. v) Modulus of Elasticity: This property is used in all design, but it is seldom determined by test, and almost never as a regular routine test. For important projects, it is best to secure this information at least once, during the tests on the trial batches at the various curing ages. An accurate value will be useful in prescribing camber or avoiding unusual deflections. An exact value of Ec is invaluable for long- span, thin-shell construction, where deflections can be large and must be predicted accurately for proper construction and timing removal of forms.

vi) Volume change: Expansion and shrinkage Concrete has a very low coefficient of thermal expansion.  However if no provision is made for expansion very large forces can be created, causing cracks in parts of the structure not capable of withstanding the force or the repeated cycles of expansion and contraction. As concrete matures it continues to shrink, due to the ongoing reaction taking place in the material, although the rate of shrinkage falls relatively quickly and keeps reducing over time (for all practical purposes concrete is usually considered to not shrink any further after 30 years). The relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface. Because concrete is continuously shrinking for years after it is initially placed, it is generally accepted that under thermal loading it will never expand to it's originally-placed volume.

Concrete states: Concrete has three different states: Plastic Setting Hardening In each state it has different properties. 1) Plastic State: When the concrete is first mixed it is like 'bread dough'. It is soft and can be worked or moulded into different shapes. In this state concrete is called PLASTIC. Concrete is plastic during placing and compaction. The most important properties of plastic concrete are workability and cohesiveness. A worker will sink into plastic concrete. 2) Setting State: Concrete then begins to stiffen. The stiffening of concrete, when it is no longer soft, is called SETTING. Setting takes place after compaction and during finishing. Concrete that is sloppy or wet may be easy to place but will be more difficult to finish. A worker leaves footprints in setting concrete. 3) Hardening State : After concrete has set it begins to gain strength and harden. The properties of hardened concrete are strength and durability. Hardened concrete will have no footprints on it if walked on.

양질의 콘크리트 (Good Uniform Concrete)

콘크리트의 장단점 장 점 단 점 ․크기, 모양에 제한이 없다. ․재료의 확보 및 운반 용이 ․내구성, 내화성, 내진성이 좋다. ․강도 조절이 자유롭다. ․시공시 특별한 숙련공이 필요없다. ․유지비가 거의 들지 않으며, 경제적이다. ․역할적인 결점은 다른 재료를 사용하여 보충할 수 있다. ․자중에 비해 강도가 작다. ․압축강도에 비하여 인장강도와 휨강도가 작다. ․건조, 수축에 의한 균열 ․개수나 파괴 곤란. 재사용 곤란 ․경화시 시간이 많이 소요→시공일수 길다.

Properties of fresh concrete (굳지않은 콘크리트의 성질) ■ 작업단계 : mixing → transporting → placing → compacting → finishing 비비기 운반 치기 다지기 마무리 ㄱ) 재료의 균일한 분포 ㄴ) 완전한 다짐 ■ Fresh concrete의 성질 a) workability (워커빌리티) : ㄱ) consistency (반죽질기) ㄴ) mobility(유동성) ㄷ) compactibility(다짐성) b) stability (안정성) : ㄱ) segregation(골재분리) ㄴ) bleeding(블리딩) Concrete has three different states: Plastic (소성) Setting (응결) Hardening (경화) c) Cohesiveness(응집성) is how well concrete HOLDS TOGETHER when plastic.

Workability ● 정의 ; 반죽질기에 따른 시공난이의 정도 및 재료의 분리에 저항하는 정도를 나타내는 굳지않은 콘크리트의 성질 효과 ; 완전한 다짐효과를 준다. ● workability를 구성하는 성질 a) Consistency(반죽질기) : 수량(水量)의 다소에 따른 반죽의 질고 된 정도를 나타내는 성질 b) Mobility(유동성) : concrete가 거푸집이나 mould(몰드)에 잘 들어가 채워질 수 있는 정도 c) Compactibility(다짐성) d) Plasticity(성형성) : 거푸집에 쉽게 다져 넣을 수 있고, 거푸집을 제거하면 천천히 형상이 변하기는 하나 허물어지거나 재료가 분리하거나 하는 일이 없는 굳지않은 콘크리트의 성질 e) Finishability(피니셔빌리티) : 굵은골재의 최대치수, 잔골재율, 잔골재의 입도, 반죽질기에 따른 마무리하기 쉬운 정도

● Workability에 영향을 주는 인자 (시멘트, 단위수량, 잔골재, 굵은골재, 혼화재료, 온도) b) ambient conditions ; temperature, humidity, wind velocity c) mixing 과 compaction 사이의 시간 d) 운반 및 다짐 방법 e) 거푸집의 크기, 형상, 표면조직 f) 철근의 양과 치수 ● Workability의 측정 a) slump 시험(slump test) 1913년, 미국, chapman 개발 ∙ slump값이 3 ～ 18cm의 경우 적당 ∙ 다짐봉 ψ16mm, L = 50cm 강봉 workability test of concrete Slump test Kelly ball test K slump test Vee bee consistometer test Flow table test.

a) Slump Test (슬럼프 시험)

b) Vee bee consistometer Test Procedure for Vee bee Consistometer: Insert the Slump cone in the attached cylindrical container. Fill the concrete sample in the cone in four layers; each layer should be one-quarter of the height of slump cone and tamped with tamping rod for 25 strokes each. After tamping the top layer, struck off the concrete with a trowel making the cone correctly filled. 2. After filling the concrete in the slump cone, the transparent glass disc attached to a swivel arm is moved and placed on the top surface of the concrete and Note the reading from the graduated scale as an “initial reading” 3. And then remove the cone slowly in the vertical direction and wait until the concrete settles in the new position. 4. Again move the swivel arm and place it on the lowered concrete for the final reading. Note it as “final reading”    5. Now, switch on the vibrator and start the stopwatch. The vibrations are continued until the concrete is wholly remoulded i.e, the surfaces become horizontal and the whole concrete surface adheres uniformly to the transparent disc. The time required for complete remoulding in seconds is recorded. 6. Note the time taken by concrete to become horizontal (remoulding) as Vee bee Secs. The consistency of concrete is measured as Vee bee Secs. You can refer the below video for more details:                                                                                                  

Workability Procedure for Kelly ball test: c) Kelly ball test (구관입시험) 지름 6인치의 주철제 반구(半球)에 눈금이 매겨진 막대를 붙인 것이며, 전체 중량은 30 파운드. 이것을 흙손으로 끝손질한 콘크리트면 위에 조용히 놓고, 콘크리트 속으로 들어가는 깊이를 구하여 컨시스턴시의 척도로 한다. 콘크리트의 깊이가 15㎝ 이상, 면적의 최소 치수가 30㎝ 이상의 경우에 사용 Procedure for Kelly ball test: Freshly mixed concrete (test sample) is poured into a container up to a depth of 20cm. Once the container is filled with the concrete, the top surface is leveled and struck off. The Kelly ball setup is kept on concrete as shown below by holding the handle of hemisphere such that the frame touches the surface of the concrete. Ensure that the setup is kept at minimum 23cm away from the container ends. (Place at the middle portion of the container) 4. Now release the handle and allow the ball to penetrate through the concrete. Once the ball is released, the depth of penetration is immediately shown in the graduate scale to the nearest 6mm. 5. Note down the depth of penetration from the attached graduated scale. 6. Repeat the same experiment for three times at different portions in the container and average the value. Average Value of the reading = Slump value

Workability d) flow test(흐름시험) : flow table과 mould(윗지름 17.1cm, 밑지름 25.4cm, 높이 12.7cm) 이용 2층 각회 다짐 15초 동안 15회 낙하후 펼쳐진 지름 측정 ** Slump Flow Test for Concrete : 콘크리트 슬럼프 흐름시험

Procedure of K Slump Test: e) K Slump Test Procedure of K Slump Test: Wet the tester and clean it with a cloth. 2. Raise the plastic tube let it sit on the pin support. 3. Take a container and pour some fresh concrete and level it. 4. Now insert the k slump tester vertically down until the disc floater rests at the surface of the concrete. Do not rotate while inserting or removing the K slump tester. 5. Wait for 60 seconds, lower the measuring rod slowly until it rests on the surface of the concrete that has entered the tube and read the slump value directly on the scale of the measuring rod

Stability ․ consistency, cohesiveness의 영향을 받는다 i) concrete 의 uniformity를 나타내는 성질 ii) 구성하는 성질 a) segregation(재료의 분리) b) bleeding iii) segregation ․ solid particle의 크기와 비중차이 ․ solid particle의 비표면적 ․ 모르타르의 양 a) 지나친 wet and dry b) deficiencies in sand 특히 finer 인 경우 c) 운반 도중의 심한 충격

Stability d) placing 시 : 고소(高所)에서 낙하 e) over - vibration 나타나는 결과 a. 외관상 ; ㄱ) blemishes (흠, 결함, defects) ㄴ) sand streaks ㄷ) porous layers ㄹ) honey combing b. 강도(强度)․내구성(耐久性) 등. ⅳ) bleeding : mix에서 물이 분리되는 현상 다지기로부터 cement paste가 경화할 때 까지 발생 ․ 원인 ; ㄱ) 입자의 크기와 비중차이 (시멘트, 골재입자 등이 침하→물이 상승) ㄴ) 물의 leakage․위치이동․상승 ․ 효과 ; ㄱ) 표면에 laitance 발생 ㄴ) 골재와 철근주위에 모여서 강도 저하 ㄷ) 물의 균등한 분포에 지장을 줌 ㄹ) 상부 concrete 다공질, 강도, 수밀성 및 내구성 저하

초기균열 i) 침하 수축균열 : 콘크리트 타설 후 표면 가까이에 있는 철근, 매설물 또는 입자가 큰 골재 등이 콘크리트의 침하를 국부적으로 방해하기 때문에 발생 ii) 프라스틱 수축균열 : 급속한 수분 증발에 의해 콘크리트 마무리면에 생기는 가늘고 얇은 균열

Hardened concrete(경화콘크리트)의 성질 ․Strength : 압축강도․전단강도․휨강도․인장강도․부착강도 ․Deformation : 단기 탄성변형․장기하중에 의한 장기변형 ․Shrinkage (건조수축) ․Durability (내구성) : freezing ＆ thawing weathering chemical attack internal chemical reaction

Strength(강도)

Strength(강도)

콘크리트의 탄성계수 mc가1,450~2,500 kg/m3 일 때 보통 골재를 사용할 때 mc=2,300 kg/m3 이므로,

철근의 탄성계수(Elastic Modulus of Reinforcements) : Reinforcing steel : Es=200,000 Mpa =200 GPa Prestressing steel : Eps=200,000 MPa =200 GPa Structural steel : Ess=210,000 MPa =210 GPa

Strength(강도)

Strength(강도)

Strength(강도)

전단강도 (shear strength), 剪斷强度

휨강도 (Flexure strength) or 파괴계수(Modulus of Rupture)

인장강도 (Tensile Strength)

인장강도 (Tensile Strength)

크리프 건조수축 일정응력이 장시간 계속하여 작용하고 있을 때 변형이 계속되는 현상 탄성변형률의 1.5 ~ 3배 변형률(ε) t0 t t∞ 하중작용 하중제거 영구변형 크리이프 회복 탄성회복 εci=탄성변형 εcc=크리이프 변형 건조수축 습기를 흡수하고 건조됨에 따라 콘크리트가 팽창, 수축 하며 이로 인하여 표면에 인장응력이 발생하여 균열이 진전되는 현상 최종 수축량 : 0.0002 ~ 0.0007 = 200 ~ 700 × 10-6

Deformation(변형)

Deformation(변형)