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Catégorie :Category: mViewer GX Creator Lua TI-Nspire
Auteur Author: angywangy
Type : Classeur 3.6
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HW#9 CE 453/553 Properties and Performance of Concrete

1. Water is being considered as the primary agent of both creation and destruction of concrete.
Explain why
2. What chemical reactions are generally involved in sulfate attack in concrete? What is the
manifestation of these reactions?
3. Briefly explain external, internal, and thaumasite forms of sulfate attacks in concrete. State the
necessary conditions for each of the reactions to occur.
4. What are the common forms of sulfate ions present in natural and industrial environment? Write
the chemical reactions involved in each case during the sulfate attack.
5. We have discussed two kinds of alkali-silica reactions. What are they? Explain the necessary
conditions for those two kinds of ASR reactions.
6. Corrosion of steel in concrete is an electrochemical process- Explain the basic conditions necessary
for the electrochemical reaction to proceed in concrete.
7. With respect to corrosion of steel in concrete, explain the significance of the following terms:
carbonation of concrete, passivity of steel and electrical resistivity of concrete.
8. Briefly describes the measures that should be considered for the control of corrosion of embedded
steel in concrete.



1. Water is being considered as the primary agent of both creation and destruction of concrete.
Explain why.
Water is a major constituent of concrete since it is needed for hydration of cement paste.
The w/c ratio decides many of the properties of the concrete including strength and durability. If the
optimum w/c ratio is maintained, water strength the concrete by reacting with cement. Water
dissolves many compounds including CH in it and provides an alkaline medium for the concrete. This
retards the corrosion of steel reinforcement embedded in concrete.
On the other hand, water also plays a significant role in terms of durability aspects like freeze-
thaw, alkali-aggregate reactions, corrosion, sulfate attck and ettringite formation. Rate of
deterioration is affected by the type and the concentration of ions present in pore solution like
sulfates, chlorides and acidic compounds. Ettringite and products of ASR swell by absorbing water
and this is the main reason for the crack propagation. During the freezing of concrete the free water
inside expands in volume leading concrete to crack. Therefore, water is considered as the primary
agent of both creation and destruction of concrete.

2. What chemical reactions are generally involved in sulfate attack in concrete? What is the
manifestation of these reactions?

C3A.CH.H18+2CH+3S+11HC3A.3CS.H32 (Ettringite)
C3A.CŜ.H18 (Monosulfate) + 2CH + 2S + 12H  C3A.3CS.H32 (Ettringite)

Ettringite is an expansive mineral. The formation of ettringite in hardened concrete will cause (1)
expansion and cracking (2) weaken the concrete by reducing the stiffness and strength, (3) and
eventually the transformation of concrete into a mushy or non-cohesive mass are some
manifestations.

3. Briefly explain external, internal, and thaumasite forms of sulfate attacks in concrete. State the
necessary conditions for each of the reactions to occur.
 External sulfate attack
Higher concentrations of magnesium, sodium, and potassium sulfates in groundwater degrade concrete
as a result of chemical reactions between hydrated portland cement and sulfate ions. Ettringite is formed
as a result and it swells by absorbing water causing cracks in concrete. Cracks increase the permeability
in concrete and accelerate the process of deterioration.

 Delayed ettringite formation / internal sulfate attack
This occurs when either a gypsum-contaminated aggregate or a cement containing unusually high sulfate
content has been used in the concrete production is being heat cured for temperatures above 700C. Since
ettringite is not stable above 70c temperature, it decomposes to monosulfate hydrate form. The sulfate
ions released by the decomposition of ettringite are adsorbed by calcium-silicate hydrate. Later with the
desorption of sulfates ettringites reform and can cause cracking.

 Thaumasite sulfate attack
This requires a source of sulfate and limestone in concrete. Typically the concrete should be exposed to
a temperature lower than 15 0C. Thaumasite (CaSiO3ꞏCaCO3ꞏCaSO4ꞏ15H2O) forms under these
conditions. The silica needed for the formation of thaumsite will be derived from C-S-H, the strength
giving C-S-H collapse and cause weakening of the concrete

4. What are the common forms of sulfate ions present in natural and industrial environment? Write
the chemical reactions involved in each case during the sulfate attack.
Na2SO4 + Ca(OH)2 + 2H2O  CaSO4.2H2O + 2NaOH
MgSO4 + Ca(OH)2 + 2H2O  CaSO4.2H2O + Mg(OH)2
MgSO4 + C-S-H + H2O  CaSO4.2H2O + Mg(OH)2 + SiO2. H2O
5. We have discussed two kinds of alkali-silica reactions. What are they? Explain the necessary
conditions for those two kinds of ASR reactions.

Alkali-silica reaction (ASR) Alkali-carbonate reaction (ACR)

Needs siliceous aggregates, including granite Needs carbonaceous aggregates like
and quartz limestone, dolomite with a clay content,
or insoluble residue content, in the range
of 5% to 25%

Occurs under sufficient moisture and high pH Having a Calcite-to-dolomite ratio of
approximately 1:1

High pH promotes the hydrolysis of silica. The High dolomite volume and small size of the
reactive silica reacts with alkali hydroxides discrete dolomite crystals (rhombs)
to form alkali silica gel which absorbs suspended in a clay matrix
water and expands.


6. Corrosion of steel in concrete is an electrochemical process. Explain the basic conditions necessary
for the electrochemical reaction to proceed in concrete.
Electrochemical process are characterized by the electrical charge transfer at the electrodes, chemical
reactions occurring in the electrolyte, both at the electrode interfaces and the bulk solution and flow
of electrons in an electrical circuit between the electrodes. Requirements for corrosion are:
 Galvanic couple
Anodic and cathode areas with different potentials need to be present. Anode is the surface where the
metal is dissolved into the electrolyte (oxidation)
Fe  Fe2+ + 2e- (metal dissolution).
Cathode is the surface where electrolyte chemicals react by absorbing electrons (reduction)
O2 + 2H2O + 4e-  4OH- (oxygen reduction).
Due to these two electrochemical reactions two electrodes form inside concrete with electron potentials.
 Electrical circuit
Metallic and electrolytic continuity between anode and cathode is the electrical circuit. The two
electrodes are connected by the electrolyte where the reactions occur and electronsand ions are
transmitted. The pore solution rich in different ions inside the concrete is the electrolyte which
completes the circuit.
 Moisture
Water is the medium or the solvent of most of the aqueous ions and it creates the electrolyte. Water is
also involved in the cathodic reaction. Therefore, water is a governing factor in corrosion.
 Oxygen
Oxygen gas dissolved in pore solution is needed to complete the cathodic reaction. Permeability of
concrete plays a major role in the corrosion where both moisture and atmospheric oxygen from
outside reaches the pore solution inside the concrete.
7. With respect to corrosion of steel in concrete, explain the significance of the following terms:
carbonation of concrete, passivity of steel and electrical resistivity of concrete.

Elements Description

CH present in concrete is depleted by reacting with atmospheric carbon
dioxide and this reduces the alkaline nature of the concrete. This results in
Carbonation of concrete
the increase in acidic nature. The passive iron-oxide film will be unstable
and will be removed from the embedded steel leading to corrosion of steel.

Steel is usually covered by a thin impermeable iron-oxide film which is
strongly adherent to the steel surface in an alkaline environment, thus
Passivity of steel
making the steel passive to corrosion. The passivity of steel has to be
removed for the corrosion to occur.

Once the passivity of the embedded steel is destroyed, it is the electrical
Electrical resistivity of resistivity and the availability of oxygen that control the rate of corrosion.
concrete Significant corrosion is not observed as long as the electrical resistivity of
concrete is above 50 to 70 × 103 Ω⋅cm.


8. Briefly describes the measures that should be considered for the control of corrosion of embedded
steel in concrete.
 Use of concrete-mixture parameters to ensure low permeability so that no external water can
penetrate inside. e.g., low water-cement ratio, adequate cement content, control of aggregate size
and grading, and use of SCMs.
 Optimum concrete mixtures to withstand various physical-chemical causes such as thermal
expansions, shrinkage, creep, freeze-thaw cycle, sulfate attack and alkali-aggregate expansion.
 For the corrosion protection, limit the permissible choride content.
 Use of corrosion inhibitors like calcium nitrite.
 Proper consolidatio...

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