4 Mechanical Behavior of Concrete Layer Concrete material is sensitive to the change of humidity and temperature…온도깊이온도깊이
5 Concrete MaterialConcrete = cement (Lime + Silica) water + air + aggregatePaste = cement + waterMortar = Paste + fine aggregate
6 Components of Cement Material Aluminates (알루미늄산염)C3A5 ~ 15% 차지Can cause premature stiffeningSulfates (황산염)CSH23~5%Reduces the chance of premature stiffeningSilicates (규산염)C3S (50~70%)C2S (15~30%)초기 및 장기 강도에 영향
7 Hydration (수화현상) 시멘트 혼합물에서의 수화 현상은 시멘트가 물과 결합 시 생기는 반응 혼합 초기에 Ettringite 생성(C-A-S-H)수화열 급속히 감소초기 강도 발현 준비초기강도 증가Silicate ReactionC-S-H혼합 초기에수화열이 급속히 발생:C3A + H2O = CAH양생전 초기 강도 발현수화열은 sulfate에 의해 흡수양생전 초기강도 억제초기 수축 발현콘크리트 인장강도 발생 – 균열 발생가능성본격적인 양생 시작 & 구조물 역할
8 Analysis of Concrete Material Workability refers to the consistency, mobility, andcompactibility of fresh concrete.Workability depends on:properties of aggregate and cementmixture methodwater contentequipmentconstruction condition
9 Slump Test Measures Consistency Use for QC Recommended Values in Design Tables(b) Normal Slump (c) Shear Slumpor Larger Collapse Slump
10 Slump TestThe slump test is a means of assessing the consistency of fresh concrete. It is used, indirectly, as a means of checking that the correct amount of water has been added to the mix.The steel slump cone is placed on a solid, impermeable, level base and filled with the fresh concrete in three equal layers. Each layer is rodded 25 times to ensure compaction. The third layer is finished off level with the top of the cone. The cone is carefully lifted up, leaving a heap of concrete that settles or ‘slumps’ slightly. The upturned slump cone is placed on the base to act as a reference, and the difference in level between its top and the top of the concrete is measured and recorded to the nearest 5 mm to give the slump of the concrete.
11 Slump TestWhen the cone is removed, the slump may take one of three forms. 1) In a true slump the concrete simply subsides, keepingmore or less to shape.2) In a shear slump the top portion of the concrete shearsoff and slips sideways.3) In a collapse slump the concrete collapses completely.Only a true slump is of any use in the test. If a shear or collapse slump is achieved, a fresh sample should be taken and the test repeated. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which the flow test (see separate entry) is more appropriate.
12 Slump Test Poor consistency is indicated by: Shear Slump = Lack of plasticity & cohesionCollapse Slump = Extremely wet or harshImprove by changing proportionsadding cement or fine aggregate to improve cohesionadjusting water content
13 Concrete Material Property: Compressive Strength (KS F 2405) The compressive strength of concrete is the most common performance measure used by the engineer in designing buildings and other structures.The compressive strength is calculated from the failure load divided by the cross-sectional area. (Three 150*300 mm cylindrical samples are tested)Compressive strength can vary from 17 MPa for residential concrete, 28 MPa and higher in commercial structures, and even higher than 70 MPa for certain applications.For concrete pavement application, it is generally required to have 28 MPa of concrete compressive strength.For concrete ballast application (i.e. RHEDA 2000), it is generally required to have 50 MPa of concrete compressive strength.
14 Concrete Material Property: Compressive Strength
15 Concrete Material Property: Compressive Strength
16 Concrete Material Property: Compressive Strength
17 Concrete Flexural Strength =(Modulus of Rupture, 휨강도) Test MethodFlexural test (ASTM C78) or KS F (단순보 중앙점 하중법) or KS F (단순보의 3등분점 하중법)Where,The unit for strength is MPa
18 Concrete Elastic Modulus The most common method to measure modulus is the secant modulus (ASTM C 469),Empirical equation:W is the density of concrete in pcf, Sc is modulus of rupture
19 Concrete Shrinkage and Creep (KS F 2424) 소성수축 (plastic shrinkage):양생 전 물 증발에 의해 일어남화학적 수축 (chemical shrinkage):수화현상에 의해 물의 감소로 일어남건조수축 (drying shrinkage): 양생 후 환경적 변화 (습도, 온도)에 의해 일어남
23 Overview of Concrete Material 이론적으로, 최대 강도는 수화현상에 필요한 물의 양이있으면 발현 될 수 있으나, 실제 낮은 물-시멘트 비는workability에 악영향을 미침도로 시공에 있어서 동결융해에 대한 내구성을 확보하기 위해서 흔히 air-entrained concrete를 사용At a given w/c ratio,AirStrengthw/c ratioStrength
25 Air-entraining Admixtures These admixtures acts as surfactants at the air-water interface and work to trap air in individual bubbles. The bubbles must be stabilized within the hardened paste. The tiny bubbles act as reservoirs to accommodate water expelled from the capillary pores during freezing action. Air entrainment will increase workability (especially if fly ash is used) and reduce bleeding action. As long as the air content is below 6%, little loss of strength is experienced. Although there is some concern over compatibility in mixtures containing multiple admixtures, a greater concern exists relative to dosage rates.