ce-106, mix design, types of cracks and transportation of concrete

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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Problems

Concrete constituents (cement, water & aggregates) will get cold.

Cold temperature will effect

Hydration Setting time

Hardening

Strength development


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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 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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The corrosion process can be illustrated using the following

figure:


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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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The following graph can also be used for determining the required

volume of coarse aggregates.


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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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9. Adjustment of Moisture Content


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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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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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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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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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6. Calculation of Cement Content

Calculation of cement content. Based on steps 4 and 5, the required

cement content is:

=

. = 700


Using Table 4 for the fineness modulus of the fine aggregate of 2.70


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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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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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ce-106, mix design, types of cracks and transportation of concrete

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