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properties
Properties of Materials ch 6
| Question | Answer |
|---|---|
| PCC is made from what two raw ingredients? | Calcareous material, which is a calcium oxide: limestone,chalk, oyster shells Argillaceous material, which is a combination of silica and alumina: clay, shale, blast furnace slag |
| clinker | raw materials are heated in a kiln changing it into cement clinker, it is then ground to a fine powder and gypsum is added to regulate the setting time... Thus cement is produced |
| Why does silica cause damage to PCC? | there are alkalis in the PCC that chemically react with the silica and cause disintegrationof concrete |
| fineness of PCC | hydration starts at the surface of cement particles, the finer the cement particles, the larger the surface area and the faster the hydration; therefore finer material results in faster strength development and a greater initial heat of hydration Finenes |
| blaine air permeability apparatus | used to indirectly measure the fineness of PCC |
| Wagner turbidimeter | determines the surface area by measuring the rate of sedimentation of cement suspended in kerosene. The finer the cement particles the slower the sedimentation |
| Specific gravity of PCC | Needed for mixture proportioning calculations Specific gravity without voids iS about 3.15 density including void between particles varies considerably depending on how it is handled in stored.For example, vibration during transportation of bulk cemen |
| hydration of PCC | chemical reaction between the cement particles and water. The process occurs through two mechanisms: through solution and topochemical |
| through solution hydration of PCC | 1 dissolution of anhydrous compounds into constituents 2 formation of hydrates in solution 3 precipitation of hydrates from the super saturated solution |
| gypsum | used to slow down the rate of alluminate hydration |
| structure development in cement paste | 1 initial CSH phase 2 forming of gels 3 initial set - development of weak skeleton 4 final set- development of rigid skeleton 5 hardening |
| evaluation of hydration progress | 1 the heat of hydration 2 the amount of calcium hydroxide in the paste developed due to hydration 3 the specific gravity of the paste 4 the amount of chemically combined water 5 the amount of unhydrated cement paste using xray quantitative analysi |
| voids in hydrated cement | affect concrete strength durability and volume stability |
| interlayer hydration space | occurs between the layers in the CSH. too small to affect the strength can affect the porosity of the paste. water in these voids can evaporate in low humidity conditions resulting in considerable shrinkage |
| capillary voids | results when the hydrated cement paste has a lower bulk specific gravity than the cement particles the size of capillary voids depends on the initial ratio of water to cement paste. |
| voids in hydrated cement | air can be trapped in the cement paste during mixing. trapped air reduces drinks an d increases permeability. well distributed minute air bubbles can greatly increase the dress bility of the cement paste. |
| properties of hydrated cement | setting- refers to the stiffening of the cement paste or the change from a plastic State to a solid state soundness- refers to its ability to retain its volume after setting compressive strength of mortar- measured by preparing 50 mm cubes, and subjecti |
| setting: 2 levels | initial set, and final set. these are arbitrary, based on measurements by either the vicat apparatus or the Gilmore needles |
| vicat test | requires the sample of cement paste be prepared, using the amount of water required for normal consistency according to a specified procedure. the 1 millimeter diameter needle is allowed to penetrate the paste for 30 seconds, the amount of penetration is |
| Gillmore test | requires that a sample of cement paste of normal consistency be prepared. a pat with a flat top is molded, and the initial Gillmore needle is applied lightly to the surface. the application process is repeated until the pat bears the force of the needle w |
| initial set time | must allow for handling and placing the concrete before stiffening. False set may occur in which the cement stiffens within a few minutes of being mixed, without the evolution of much heat |
| water-cement-ratio | weight of water/weight of cement higher ratio leads to lower compressive strength |
| Types of PCC | I - normal II- moderate sulfate resistance III- high early strength IV- low heat of hydration V- high sulfate resistance |
| mixing water | any potable water is acceptable. some nonpotable water types should not be used average compressive strength of mortar cube made with nonpotable water should not be less than 90% of the mortar cube made With potable water after seven days the set time |
| concrete wash water disposal | waste water from ready mixed concrete operations is a hazardous substance (contains caustic soda and potash) disposal is regulated by the Environmental Protection Agency (EPA) the high Ph makes it corrosive conventional practices for disposing include |
| wash water reuse system | chemical stabilizing admixtures can be used. concrete properties are not significantly affected by the use of stabilized wash water |
| admixtures for concrete | 1 air entrainers 2 water reducers 3 retarders 4 hydration controller admixtures 5 accelerators 6 specialty admixtures |
| Four major reasons for using admixtures | 1 to reduce the cost of concrete construction 2 to achieve certain properties in concrete more effectively than by other means 3 to ensure quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions |
| air entrainers | produce tiny air bubbles in the hardened concrete to provide space for water to expand upon freezing. water expands 9 percent upon freezing, and causes internal stresses in the concrete |
| 3 mechanisms contribute to the development of internal stresses in concrete (air entrainers) | 1 critical saturation- freezing water expands in volume by 9%, if the percent saturation exceeds 91.7 %, volume increase generates stress in the concrete 2 hydraulic pressure- freezing water draws unfrozen water to it. the unfrozen water moving throughou |
| water reducers | developed to gain workability and at the same time maintain quality. different levels of effectiveness: conventional, mid-range, high-range |
| effects of adding a water reducer | 1 adding a water reducer without altering the other quantities in the mix increases the slump 2 the strength of the mix can be increased by using the water reducer by lowering the quantity of water and keeping the cement content constant 3 the cost of t |
| superplasticizer | high ranged water reducer. can be used when: 1 a low water cement ratio is beneficial ( high strength concrete, early strength gain, and reduced prosity 2 placing send sections 3 place in concrete around tightly spaced reinforcing steel 4 placing ceme |
| retarders | used to delay the initial set of concrete 1 offsetting the effect of hot weather 2 allowing for unusual placement, or long haul distances 3 provides time for special finishes can reduce the strength of concrete at early stages |
| hydration control admixtures | has the ability to stop and reactivate the hydration process of concrete. two parts: a stabilizer and an activator stabilizer completely stops the hydration of the cementing materials for up to 72 hours, adding the activator to the stabilized concrete r |
| accelerators | used to develop early strength of concrete at a faster rate than that developed in normal concrete. ultimate strength of high early strength concrete is about the same as that of normal concrete. 1 reduces the amount of time before finishing operations |
| use of calcium chloride as an accelerator | the PC recommend against using calcium chloride under the following conditions 1 concrete is prestressed 2 concrete contains embedded aluminum such as conduit, especially if the aluminum is in contact with steel 3 concrete is subjected to alkali-aggreg |
| alternatives to the use of calcium chloride as in accelerator | 1 using high early strength (type III) cement 2 increasing cement content 3 curing at higher temperatures 4 using non calcium chloride accelerators such as triethanolamine, sodium thiocyanate, calcium for mate, or calcium nitrate |
| specialty admixtures | engineers should just be aware of their existance. examples: work ability agents, corrosion inhibitors, damp-proofing agents, permeability-reducing agents, fungicidal, germicidal, and insecticidal, pumping aids, bonding agents, grouting agents, gas-formin |
| fly ash | by product of the coal industry. increases work ability of fresh concrete. extends the hydration process, allowing greater strength development and reduced porosity |
| slag cement | made from iron blast furnace slag, hydrates and sets in a similar to PCC |
| silica fume | bypropuct of the production of silicon metal or ferrosilicon alloys. gives high strength, and very durable, can also reduce concrete corrosion induced by deicing salts |
| natural pozzolans | in finely divided form in the presence of moisture it reacts chemically with calcium hydroxide at normal temperatures to form compounds having cementious properties. |
Created by:
mbreeden
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