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8/12/2019 CE-106, Mix design, types of Cracks and Transportation of concrete
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CE -106 Final Lecture
CE-106 Civil Engineering Materials & Concrete TechnologyUET Peshawar
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Cold Weather Concreting
Temperature Problems in Concreting
Source: Feldman Architecture, CA
Source: NAHB Building Systems Council
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Cold Weather Concreting
Problems
Concrete constituents (cement, water & aggregates) will get cold.
Cold temperature will effect
Hydration Setting time
Hardening
Strength development
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Cold Weather Concreting
Precautions/Solutions
Materials and Proportions (indirect way)
Set Accelerators
Type III cement
Use more cement (more heat generation) Mix Temperature
Heat aggregates
Replace some of the mixing water with hot water
Common Practice
Insulating formwork (keep heat inside)
Use blankets, heaters
Air shelters (small jobs)
Keep formwork for a longer period of time
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Hot Weather Concreting
Problems
High temperature (Rate of hydration increases & loss of water occur)
Form hydration products quickly
Loss of slump, time of set reduced Rapid Hydration
More mixing water required due to the loss of consistency
Rapid Setting time
Plastic Shrinkage: cracks due to quick evaporation of bleed water from
the surface
Ultimate strength is always lower
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Hot Weather Concreting
Solution
Selection of material and mix proportions.
Set retarders
Cement Type II, IP
Less cement
Air entrainment to control slump
Mineral admixtures
Use cooled water or ice
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Corrosion of Reinforcement
Corrosion of reinforcing steel and other embedded metals is
the leading cause of deterioration in concrete. When steel
corrodes, the resulting rust occupies a greater volume than the
steel. This expansion creates tensile stresses in the concrete,which can eventually cause cracking, delamination, and
spalling.
For corrosion to occur, these elements must be present:
There must be at least two metals (or two locations on a single metal) at
different energy levels an electrolyte
a metallic connection
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Corrosion of Reinforcement
Corrosion is an electrochemical process involving the flow of
charges (electrons and ions). At active sites on the bar, called
anodes, iron atoms lose electrons and move into the
surrounding concrete as ferrous ions. This process is called ahalf-cell oxidation reaction, or the anodic reaction, and is
represented as:
2Fe 2Fe2+ + 4e-
2H2O + O2 + 4e- 4OH-
2Fe2+ + 4OH- 2Fe(OH)2
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Corrosion of Reinforcement
The corrosion process can be illustrated using the following
figure:
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Corrosion of Reinforcement
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Types of Cracks in Concrete
What Causes Cracks in the Concrete Unexpected cracking of concrete is a frequent cause of complaints.
Cracking can be the result of one or a combination of factors, such as
drying shrinkage, thermal contraction, restraint (external or internal) to
shortening, subgrade settlement, and applied loads.
Cracking can be significantly reduced when the causes are taken into
account and preventative steps are utilized.
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Types of Cracks in Concrete
Crazing
Crazing is a pattern of fine cracks that do not penetrate much below
the surface and are usually a cosmetic problem only.
They are barely visible, except when the concrete is drying after the
surface has been wet.
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Types of Cracks in Concrete
Plastic Shrinkage Cracking
When water evaporates from the surface of freshly placed concrete
faster than it is replaced by bleed water, the surface concrete shrinks.
Due to the restraint provided by the concrete below the drying surface
layer, tensile stresses develop in the weak, stiffening plastic concrete,
resulting in shallow cracks of varying depth. These cracks are often
fairly wide at the surface
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Types of Cracks in Concrete
Drying Shrinkage
Because almost all concrete is mixed withmore water than is needed to hydrate thecement, much of the remaining water
evaporates, causing the concrete toshrink.
Restraint to shrinkage, provided by thesubgrade, reinforcement, or another partof the structure, causes tensile stresses todevelop in the hardened concrete.
Restraint to drying shrinkage is the mostcommon cause of concrete cracking.
Usually contraction (control) joints areplaced in concrete to predetermine thelocation of drying shrinkage cracks.
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Types of Cracks in Concrete
D-cracking
It is a form of freeze-thaw deterioration that has been observed in some
pavements after three or more years of service.
Due to the natural accumulation of water in the base and sub base of
pavements, the aggregate may eventually become saturated.
Then with freezing and thawing cycles, cracking of the concrete starts in
the saturated aggregate at the bottom of the slab and progresses
upward until it reaches the wearing surface.
D-cracking usually starts near pavement joints.
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Types of Cracks in Concrete
Alkali-Aggregate Reaction
Alkali-aggregate reactivity is a type of concrete deterioration that
occurs when the active mineral constituents of some aggregates react
with the alkali hydroxides in the concrete.
Indications of the presence of alkali-aggregate reactivity may be a
network of cracks, closed or spalling joints, or displacement of different
portions of a structure.
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Types of Cracks in Concrete
Thermal cracks
Temperature rise (especially significantin mass concrete) results from the heatof hydration of cementitious materials.
As the interior concrete increases intemperature and expands, the surfaceconcrete may be cooling andcontracting.
This causes tensile stresses that mayresult in thermal cracks at the surface if
the temperature differential betweenthe surface and center is too great.
The width and depth of cracksdepends upon the temperaturedifferential, physical properties of theconcrete, and the reinforcing steel.
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Types of Cracks in Concrete
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Types of Cracks in Concrete
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Repairing Techniques of Cracks
Objective of Crack Repairing
Crack repair could be done to accomplish one or more of the following
objectives:
Restore and Increase strength.
Restore and Increase stiffness
Improve functional performance.
Provide water tightness.
Improve appearance of the concrete surface.
Improve durability.
Prevent development of corrosion in steel.
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Repairing Techniques of Cracks
Choice of Repairing Technique
Depending on the nature of damage, one or more repair methods may
be selected, for example, tensile strength may be restored across a
crack by injecting it with epoxy or other high strength bonding agent.
However, it may be necessary to provide additional strength by adding
reinforcement or using post-tensioning. Epoxy injection alone can be used
to restore flexural stiffness if further cracking is not anticipated.
Conditions Assessment
Information Gathering
Field Survey
Field Tests
Laboratory Tests
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Repairing Techniques of Cracks
Sealing of Cracks Sealing of cracks as stand alone repair should be used in conditions where structural
repair is not necessary.
Isolated cracks whether extending through the concrete section or partially into it,should be sealed at the concrete surfaces.
For this a slot of approx. 25mm wide should be saw cut upto 10mm deep along thecrack keeping crack at the center of the slot.
The concrete should be chiseled out from between the two saw cut edges andconcrete should be further undercut beyond the 10mm depth up to say 20mm depthso that the base width is slightly greater than the surface width.
After the slot is thoroughly cleaned, soaked with water for 10 hrs. and surface dried,
a bond coat/ primer coat, of an approximate latex bonding compound should beapplied.
Once the primer becomes tacky, high strength polymer modified cementitious mortarshould be filled in the slot, properly tamped and surface finished.
Curing compound should be applied as soon as surface becomes touch dry. 7 dayswet curing should be done by covering with wet Hessian and polythene sheet.
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Repairing Techniques of Cracks
Epoxy Injection
Cracks as narrow as 0.3mm can be bonded by the injection of epoxysuccessfully in buildings, bridges and other concrete structures.
However, unless the cause of the cracking has been corrected, it will
probably recur near the original crack. If the cause of the crack cannot be removed and it is not causing
reduction in strength of the structure, then either the crack could besealed with flexible sealant thus treating it as a joint or establish a jointthat will accommodate the movement and then the crack should begrouted with epoxy.
With the exception of certain moisture tolerant epoxies, this technique isnot applicable if the cracks are actively leaking and cannot be driedout.
Epoxy injection requires a high degree of skill for satisfactory execution,and the ambient temperature may limit application of the technique.
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Repairing Techniques of Cracks
Drilling and Plugging through Crack
One of the approximate methods would be to drill holes normal to
cracks, fill them with a suitable epoxy or epoxy-mortar formulation and
then place reinforcement bars (of predetermined sizes and lengths) in
them to stitch across the cracks.
The bars may be placed in the clean holes prior to filling the epoxy (so
as to save loss of epoxy) but then great care is needed not to entrap
any air.
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Repairing Techniques of Cracks
Stitching
Stitching involves drilling holes on both sides of the crack and grouting in
U-shaped metal units with short legs (staples or stitching dogs) that span
the crack as shown in Figure.
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Repairing Techniques of Cracks
Stitching
Stitching should be used when tensile strength has to be restored back
across major cracks. Stitching a crack tends to stiffen the structure and
the stiffening may increase the overall structural restrain, causing the
concrete to crack elsewhere. Therefore, it is necessary that properinvestigation is done and if required, adjacent section or sections are
strengthened using technically designed reinforcing methods.
The procedure consists of drilling holes on both sides of the crack,
cleaning the holes and anchoring the legs of the staples in the holes, with
either a non-shrink cement grout or any epoxy resin-based bondingsystem
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Repairing Techniques of Cracks
External Prestressing
The flexural cracks in reinforced concrete can be arrested and
even corrected by the Post-tensioning method. It closes the cracks
by providing compression force to compensate for tensions and
adds a residual compression force. This method requires anchorageof the tie rods (or wires) to the anchoring device attached to the
beam
Cement Grouting
Wide cracks, particularly in mass concrete abutments/piers and
masonry substructures may be repaired by filling with portlandcement grout.
This method is effective in sealing the crack in concrete, but it will
not structurally bond cracked sections.
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Mix Design
Introduction
Mix design can be defined as the process of selecting suitable
ingredients of concrete and determining their relative quantities with the
purpose of producing an economical concrete which has certain minimum
properties, notably workability, strength and durability.
Any mix design procedure will provide a first approximation of the
proportions and must be checked by trial batches
Methods of Mix Design
American Method (ACI 211.1 19)
Commonly used method
British Method
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Mix Design
Basic Considerations
Economy
The material costs are most important in determining the relative costs of
different mixes.
The labor and equipment costs, except for special concretes, are generallyindependent for the mix design.
Since cement is more expensive than aggregate, it is clear that cement
content should be minimized.
Workability
A good mix design must be capable of being placed and compacted, withminimal bleeding and segregation, and be finishable.
Water requirements depend on the aggregate rather than the cement
characteristics.
Workability should be improved by redesigning the mortar faction rather
than simply adding more water.
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Mix Design
Basic Considerations
Strength and Durability
In general, the minimum compressive strength and a range of w/c ratios are
specified for a given concrete mix.
Possible requirements for resistance to freeze-thaw and chemical attack mustbe considered.
Therefore, a balance or compromise must be made between strength and
workability.
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ACI Mix Design Procedure
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ACI Mix Design Procedure
3. Maximum Aggregate Size
The largest maximum aggregate size that will conform to the following
limitations:
Maximum size should not be larger than 1/5 the minimum dimension of
structural members, 1/3 the thickness of a slab, or 3/4 the clearancebetween reinforcing rods and forms. These restrictions limit maximum
aggregate size to 1.5 inches, except in mass applications.
Current thought suggests that a reduced maximum aggregate size for a
given w/c ratio can achieve higher strengths. Also, in many areas, the
largest available sizes are 3/4 in. to 1 in
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ACI Mix Design Procedure
4. Estimation of Mixing Water and Air Content
An estimation of the amount of water required for air-entrained and
non-air-entrained concretes can be obtained from Table 2 (given below).
Approximate mixing water (lb./yd3) and air content for different slumps
and nominal maximum sizes of aggregates
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ACI Mix Design Procedure
4. Estimation of Mixing Water and Air Content
An estimation of the amount of water required for air-entrained and
non-air-entrained concretes can be determined from the graph as well.
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ACI Design Mix Procedure
5. Water/Cement Ratio
This component is governed by strength and durability requirements
Strength Without strength vs. w/c ratio data for a certain material, a
conservative estimate can be made for the accepted 28-day compressive
strength from Table 3. Durability If there are severe exposure conditions, such as freezing and
thawing, exposure to seawater, or sulfates, the w/c ratio requirements may
have to be adjusted.
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ACI Mix Design Procedure
6. Calculation of Cement Content
Once the water content and the w/c ratio is determined, the amount of
cement per unit volume of the concrete is found by dividing the
estimated water content by the w/c ratio.
weight of cement =
However, a minimum cement content is required to ensure good
finishability, workability, and strength.
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ACI Mix Design Procedure
7. Estimation of Coarse Aggregate Content
The percent of coarse aggregate to concrete for a given maximum size
and fineness modulus is given by Table 4.
The value from the table multiplied by the dry-rodded unit weight (the
oven-dry (OD) weight of coarse aggregate required per cubic foot ofconcrete).
To convert from OD to saturated surface dry (SSD) weights, multiply by
[1 + absorption capacity (AC)].
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ACI Mix Design Procedure
7. Estimation of Coarse Aggregate Content
The following graph can also be used for determining the required
volume of coarse aggregates.
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ACI Mix Design Procedure
8. Estimation of Fine Aggregate Content
There are two standard methods to establish the fine aggregate content,
the mass method and the volume method. We will use the "volume"
method.
The volume of fine aggregates is found by subtracting the volume ofcement, water, air, and coarse aggregate from the total concrete
volume.
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ACI Mix Design Procedure
9. Adjustment of Moisture Content
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ACI Mix Design Procedure
10. Trial Batch
Using the proportions developed in the preceding steps, mix a trial
batch of concrete using only as much water as is needed to reach the
desired slump (but not exceeding the permissible w/c ratio).
The fresh concrete should be tested for slump, unit weight, yield, aircontent, and its tendencies to segregate, bleed, and finishing
characteristics. Also, hardened samples should be tested for compressive
and flexural strength.
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ACI Mix Design Example
Concrete is required for an exterior column located above
ground where substantial freezing and thawing may occur. The
28-day compressive strength should be 5,000 lb./in2. The
slump should be between 1 and 2 in. and the maximum
aggregate size should not exceed in.
The properties of the materials are as follows:
Cement : Type I, specific gravity = 3.15
Coarse Aggregate: Bulk specific gravity (SSD) = 2.70; absorption
capacity = 1%; dry-rodded unit weight = 100 lb./ft3
; surface moisture= 0%
Fine Aggregate: Bulk specific gravity (SSD) = 2.65; absorption
capacity = 1.3%; fineness modulus = 2.70; surface moisture = 3%
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ACI Mix Design Example
1. Required material information
Already given
2. Choice of Slump
The slump is given ( 1 to 2 inches)
3. Maximum Aggregate Size
Given: inches
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ACI Mix Design Example
4. Estimation of Mixing Water and Air Content
Since freezing and thawing is important, the concrete must be air-
entrained. From Table 2, the recommended air content is 6%; the water
requirement is 280 lb/yd3
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ACI Mix Design Example
5. Water Cement Ratio
Water/cement ratio. From Table3, the estimate for required w/c ratio to
give a 28-day strength of 5,000 psi.
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ACI Mix Design Example
6. Calculation of Cement Content
Calculation of cement content. Based on steps 4 and 5, the required
cement content is:
=
. = 700
7. Estimation of Coarse Aggregate Content
Using Table 4 for the fineness modulus of the fine aggregate of 2.70
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ACI Mix Design Example
7. Estimation of Coarse Aggregates (Continued)
The coarse aggregates will occupy: 0.63 27 ft3/yd3 = 17.01 ft3/yd3
The OD weight of coarse aggregates: 17.01 ft3/yd3 100 lb/ft3 =
1,701 ft3/yd3
8. Estimation of Fine Aggregates Content by the Absolute
Volume Method
Water: 280 lb/62.4 lb/ft3 = 4.49 ft3
Cement: 700 lb/(3.15 x 62.4 lb/ft3) = 3.56 ft3
Coarse Aggregate: 1701 lb/(2.70 x 62.4 lb/ft3
) = 10.10 ft3
Air: 6% x 27ft3/yd3 = 1.62 ft3
Total 19.77 ft3
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ACI Mix Design Example
8. Estimation of Fine Aggregates Content by the AbsoluteVolume Method (Continued)
Therefore, the fine aggregate must occupy a volume of:
= 27 ft3 19.77 ft3 = 7.23 ft3
The oven dry (OD) weight of the fine aggregate is:= 7.23 ft3 2.65 62.4 lb/ft3 = 1,196 lbs.
9. Adjustment for Moisture in Aggregates
Since the moisture level of the fine aggregate in our storage bins canvary, we will apply a simple rule to adjust the water required.
Decrease the amount of water required by surface moisture content of theweight of the fine aggregate.
Increase the amount of aggregate by the amount equal to the surfacemoisture. (Refer to Concrete Technology, Neville, 2nd Edition)
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Questions and Discussion