Effect of Parking Concrete Slabs on Durability

7/27/2019 Effect of Parking Concrete Slabs on Durability

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362.1R·28 ACI COMMITTEE REPORT

Fig 3 7 Typical sealant detail Mer construction joint incast in place structural slab

3 6 G uldellnes for selection of durability systems

for f loors and roofs3 6 1 Introduction The performance of parking structures

requires special attent ion to durability systems in all envi

ro nments . Selecting the right combination of protection

systems is not a prescriptive process. This section is provided

to assist the designer in selecting an appropriate combination

of pro tective measures .

Recommendations in the text differentiate between

parking levels exposed directly to the elements roof and

parking levels not exposed directly to the weather floor .

The recommendations re fer to general park ing area and

adjacent drive lane s. Isolated ramps, helixes, or access lanes

with concentrated traffic may require protection greater than

the minimum recommendations. Other physical or functionalcharacteristics that may justify exceeding these minimum

recommendations arc:

I. Multiple-usc structure;

2. Perimeter of park ing level exposed to the wea ther;

3. Occupied space direc tly below parking level ;

4. Heated space above or below parking level

5. Isolated single-lane entrance or ex it without alternate

acce ss; and

6. High traffic volumes.

Section 3.6.4 provides descriptions of these characteristics

and their effects on the durability system.

Not all available protection measures arc appropriate or

compatible. Tables 3.1 through 3.4 provided minimum recom

mended durability measures for different types of structural

systems and service env ironments.

3 6 2 Structural llsideratiolls Recommended durability

meas ures are dependent on the structural system chosen and

their service environmen t For example, slabs that tend to

crack thin slabs and nonprestressed slabs should be

provided with traffic grade membranes on the roo f in certain

conditions and on all floors in others. Measures to isolate and

protect internal metal elements in the slab will also vary from

system to sys tem.

3 6 3 Environmental considerations hapter I discusses

various exposure conditions and types of deterioration that

may occur. Figure 3.1 defies various exposure zones to

provide the designer with initial guidance to address each of

specific cracks that continue to leak at a later date with the

rout and seal method .

Another method of sealing cracks is the use of epoxy

injection. This process involves injecti ng epoxy into a crack

under pressure to fill the void and adhere the surfaces back

together. Th is approach should be consi dered if the crack is

static and it is desirable to restore the structural integrity of

the cracked section. A varia tion on this method is to gravity

feed a low-viscosity epoxy, methacrylate, or other polymer

into the crack to fill the void and make it watertight In either

case, it is important that the underlying cause of the crack be

determined and corrected prior to the repair, or a new crack

may develop to replace the one being fixed. Epoxy injection is

nor a recommended procedure for repairing moving cracks.

CONSTJitU T ON JOINT f AeONCRETE SL/o.8

SLA8 REINf ORCING

3 5 2 4 Construe/ion join s Construction jo ints are

created at predetermin ed locati ons where one concrete

placement is terminated and another is begun later. Depending

on structural design. joints may be monolithic that is. the

interfaces of the joint are soundly bonded to ensure complete

struc tural integrity of the slab , or function as isolation or

contractionjoints.

Construction joints arc often sources of leakage in parking

structures. Deicers allowed to penetrate through the joint may

result in corrosion of reinforcement or otherembedded metals.

Leakage at monolithic construction joi nts may be reduced

if, before the second cast ing, laitance is removed to promote

a positive bond.Following placement, monolithic construction jo ints

should be tooled and sealed. Sealing should be accomplished

by filling the construction jo int with an elastorneric sealant

refer to Fig. 3.7 . Monolithic construction joints are usually

restrained from movementbecause of the amount of reinforce

ment crossing the jo int.

Optimum configuration of the jo int sealant is dependent

on the amount of movement anticipated during the servic e

life of the structure refer to ACI 504R .

3.5.2.5 Cracks racks in concrete occur for a variety

of reasons . Because cracks are a source of mois ture and

chlo ride intrusion into the concr ete , sealing them is an

important issue.

The re arc several common methods of scaling cracks . The

effectiveness of a given method is dependent upon the

underlying cause and behavior of the crack. Refer to ACT

224R and ACI 224.IR for a more complete discussion of the

causes of cracki ng. Many cracks in parki ng structures are

subject to dynamic movement due to temperature change.

Effective repairs for these cracks must be able to withstand

ongoing movement in the crack. 1he most effective method to

accomplish this repair is to provide a groove of approximately

1/2 in. wide by 1/2 in. deep sealed with an elastomeric sealant.

For small c racks approx imately 0.0 15 in. or less that

show little or no movement, treatment with a penetrating

silane or sucxane sealer may render the crack hydrophobic.

One course o f action to repair multiple smallc racks is to treat

the surface area with a silane or suoxane sealer, then retreat

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DeSIGN OF DURA lE PARKING STRUCTURES

Fig. .7- Scaling of floor surface:

Concrete Bridgl Du b for conversion factors expressing

chloride content.

Corrosion can occur in uncracked concrete due to chloride

ions. moisture, and oxygen permeating into the concrete

(refer to Section3.3.2). However, corrosionofreinforcement is

generally more severe and begins earlier at cracks and places

where water can easily penetrate. Information on corrosion

of metals in concrete is available in AO 222R. Corros ion

of Metals in Concrete

1.4.1.2 Bonded prestressing sud - The corrosion of

prestressing strand in pretensioned double-tees and inverted

lee-beams used in parking structures has normally occurredwhere there is a breach in the scaled joints and where

brackish water reaches the bottoms of members.

Corrosion of grouted prestressing steel has occurred where

the grout did not encase the wires, bar, or strand within a

grout duct, and moisture or chlorides gained access to the

open void.

1.4.1.3 Unbonded prestressing fflel- Most cases of

corrosion of unbonded prestressing steel in parking structures

have involved either natural saltwater or deicer salt exposure

to loosely sheathed systems with inadequate amounts of

grease. Other areas most susceptible to corrosion include

poorly grouted stressing end anchorages. intermediate

stressing points at construction joints. and regions of insuffi

cient concrete cover.

1.4.1.4 Other I mIHddl d n l tals orroded electrical

conduits have been observed in structures exposed to deicer

salts. Likewise. uncoated aluminum has been observed 10

corrode in concrete containing chloride and particularly

where the aluminum has been in contact with the steel

reinforcement. Embedded metals of all kinds should be

specifically reviewed for their durability and function.

1.4.2 Freezing and thawing damagl S caling of concrete

is a deterioration observed in parking structures exposed to a

freezing and thawing environment. Cyclic freezing and

deicer scaling is discussed extensively in ACI 201.2R.

Guide to Durable Concrete. The phenomenon usually

begins withthelossof thinflakes at the surface.Asdeteriorationprogresses. coarse aggregates may be exposed. In advanced

Fig. .S Spalling of beam SOff i l beside ll ahng isolanonjo int.

stages, the surface may progress from an exposed aggregate

appearance to that of rubble. Frequently. with prolonged

water saturation and repeated freeze-thaw cycles, theconcrete will develop fine cracks paralleling the exposed

surface.The presence of deicers willacceleratethisdeterioration

f jg 1.7) .

The addition of air entrainment is the most effective

method of increasing the resistance of concrete to damage

due to freezing and thawing. The entrained air-void size and

spacingin the concrete is also important (refer to ACt 345R).

Severe abrasion accelerates the deterioration of concrete

undergoing scaling. Good drainage (pitch of surface 10

drains) diminishes the severity of freezing and thawing

exposure by reducing the moisture content of the concrete .

1.4_3 Cracking and l l n pt lll tTcltio l Cracking of

concrete exists in many forms. Some common types are:

mtcrocracking. partial depth cracks in the top of members.

and through-slab cracks. Observations of parking structures

suggest that corrosion will occur earlier and is much more

likely at wide cracks than at uacracked or finely cracked

areas. For information on resistance 10 cracking. refer 10

Section 3.5.2.5.

In addition to abetting corrosion. water penetration through

the slab is undesirable. When substantial amounts of water

penetrate completely through the slab at cracks and jo ints.

corrosion and freeze-thaw damage of the sides or bouoms of

underlying members may occur. Damage to ribs, jo ists, webs,

beams, columns, heavily loaded joints, and bearings is more

critical to structural integrity than damage to the slab because

these elements support larger tributary areas. Severe damageto a beam at an isolation joint is shown in Fig. 1.8.



CAUSES EVALUATION AND REPAIR OF CRACKS 224.1R·17

monomer or resin can bepoured onto the surface and spread

with brooms, rollers, or squeegees. The material should be

worked back and forth over the cracks to obtain maximum

filling because the monomer or resin recedes slowly into thecracks . The use of this methodon elevated slabs will require

sealing of the cracks on the bottom of the slab to contain

material from leaking through the crack. Excess material

should be broomed off the surface to prevent slick, shining

areas after curing . If surface friction is important, sand

should be broadcast over the surface before the monomer or

resin cures.

If the cracks contain significant amounts of silt, moisture,

or other contaminants, the sealant cannot fill them. Water

blasting followed by a drying time may be effective in

cleaning and preparing these cracks. Cores may be taken to

verify crack filling and the depth of penetration measured.

Cau tion should be employed to avoid cutting existingreinforcement during the coring process. Cores can be tested

to give an indication of the effectiveness of the repair

method. The accuracy of the results may be limited,

howeve r, as a function of the crack orientation or due to the

presence of reinforcing steel in the core. For some polymers,

the failure crack will occur outside the repaired crack.

3 e G routlng3.8.1 Portland cement grouting W ide cracks, particularly

in gravitydams (Warner 20 ) and thick concrete walls,may

be repaired by fill ing with portland-cement grout. This

method is effective in stopping water leaks, but it will not

structurally bond crackedsections. The procedure consists ofcleaning the concrete along the crack; installing built-up

seats (grout nipples) at intervals astride the crack (to provide

a pressure-tight connection with the injection apparatus);

sealing the crack between the seats with a cement paint,

sealant, or grout; flushing the crack to clean it and test the

seal; and then grouting the whole area. Grout mixtures may

contain cement and water or cement plus sand and water,

depending on the width of the crack. The wlcm however,

should be kept as low as practical to maximize the strength

and minimize shrinkage. Water reducers or other admixtures

may be used to improve the properties of the grout. For small

volumes, a manual injection gun may be used; for larger

volumes, a pump may be used . After the crack is filled , the

pressure should be maintained for several minutes to ensure

good penetration.

3.8.2 Chemical grouting hemical grouts, such as

urethanes and acrylomides , arc activated by catalysts or

water to form a gel, a solid precipitate, or foam that will fill

void spacewithin concrete. The materials are primarily used

for scaling cracks from water penetration. Bondstrengths are

typically low, so structural repairs are not madewhh chemical

grouts. Cracks in concrete as narrow as 0.002 in. (0.05 mm)

have been filled with chemical grout.

The advantages of chemical grouts include applicability in

moist environments (excess moisture available). wide limits

of control of gel time, and thei r ability to be applied in vel)

fine fractures. Disadvantages are the high degree of skillneeded for satisfactory usc, and lack of strength.

3 DrypacklngDrypacking is the hand placement of a low water content

mortar followed by tamping or ramming of the mortar into

place, producing intimate contact between the mortarandtheexisting concrete (U.S. Bureau of Reclamation 1975; C rack

Repair Method: Drypacking 1985). Because of the low w/cm

of the material. there is little shrinkage. and the patch

remains tight and can have good quality with respect to

durability, strength, and watertightness.

Drypack can be used for filling narrow slots cut for the

repair of dormant cracks .The use of drypack is not advisable

for filling or repai ring active cracks.

Before a crack is repaired by drypacking, theportionadjacent

to the surface should be widened to a slot about 1 in. (25 mm)

wide and I in. (25 mm) deep. The slot should be undercut so

that the base width is slightly greater than the surface width.

After the slot is thoroughly cleaned and dried, a hond coat,consisting of cement slurry or equal quantities of cement and

fine sand mixed with water to a fluid paste consistency, or an

appropriate latex bonding compound (ASTM C 1059),

should be applied. Placing of the drypack mortar should

begin immediately. The mortar consists of one part cement,

one to three parts sand passing a No. 16 (1.18 mm) sieve, and

just enough water so that the mortar will stick together when

molded into a ball by hand.

the patch must match the color of the surrounding

concrete, a blend of gray portland cement and white portland

cement may be used. Normally, about 3 while cement is

adequate, but the precise proportions can be determined only

by trial.To minimize shrinkage in place, the mortar should stand

for 1/2 hour after mixing, and then be remixed before use.

The mortar should be placed in layers about 3/8 in. (10 mm)

thick. Each layer should be thoroughly compacted over the

surface using a blunt stick or hammer and each underlying

layer scratched to facilitate bonding with the next layer.

There need be no time delays between layers.

The repair should be cured by using either water or a curing

compound. The simplest method of moist curing is to support a

strip of foldedwet burlap along the length of the crack.

3 1o Ctack arrestDuring construction of massive concrete struc tures, cracks

due to surface cooling or other causes may develop and

propagate into new concrete as construction progresses.

Such cracks may he arrested by blocking the crack and

spreading the tensile stress over a larger area (U.S. Army

Corps of Engineers 1945).

A piece of bond-breaking membrane or a grid of steel mat

may be placed over the crack as concreting continues . A

semicircular pipe placed over the crack may also be used

(Fig. 3.7). A description of installation procedures for semi

circular pipes used during the construction of a massive

concrete structure follows:

I The semicircular pipe is made by splitting an 8 in .

(200 mm), 16 gauge pipe and bending it to a semicirc ular

section with an appro ximately 3 in . (75 mm) flange oneach side;



GUIDETODUR LE CONCRETE 201.2R-5

When consolidating concrete, the duration of vibration

can have a significant effect on the air content of concrete,

with the effect being influenced by the initial slump of the

concre te Kosmatka et aI. 2002). Increasing the time ofvibration leads to a reduc tion in the overall air content.

Consolida tion, however, is normally necessary to obtai n the

strength and durability properties required over the service

life of the concrete structure. Provided that the concrete is

proportioned and vibra ted properly, internal vibration will

mainly remove the larger airvoids, so that the air-void spacingfactor will not be affected adversely. Placement and consoli

dation methods should be carefully evaluated to ensure thatthe desired fresh concrete properties are attained ACI 309R).

Good consolidation is a prerequisite for obtaining low

permeability , which is critical for making concre te resistant

to weathering and most agents of deterioration. Low pcrmc

ability can be achieved more readily with concrete mixturesdesigned for good workability and placeability. ACI 211.1

and 21I,2 provide guidance for selecting mixture proportions,

andACI 309R provides guidance for consolidating concrete.

3 5 B leedlngOnce concrete is in place, but before the time of initlal

setting, particles denser than water su ch as cement and

aggregates) may settle due to the force of gravity. displacing

part of the mixing water which may reach the top surface as

bleed water. This condition is more pronounced in mixtures

with excessive water content. Also, non-air-entrained

concrete mixtures will exhibitmore bleeding than air-entrained

mixtures of similar composition. It is the difference in densitybetweenwater and other components in themixture thatcauses

bleeding to occur. Just as entrapped air moves to the surface

Lankard 1995) during vibration, waterwill be displaced due

to settling of the solid components of the concrete.

Defective hardened concrete surfaces are frequen tly the

result of finishing fresh concrete while bleed water is present

on the surface . If bleed water is worked into the concre te

near thesurface byfinishing opera tions, the surface concrete

can suffe r from increased w m and decreased air conte nt.

Premature finishing of the concrete surfa ce while bleeding is

still active may result in an accumulation of bleed water

below the sealed surface that can result in subsequent spalling,

scaling, or delam ination. Problems such as craze cracks anddusting can occur, and abrasion resistance can be reduce d.

Scaling of the surface from freezing and thawing is a

common result of reduced air conte nt in the surface concrete .

3 6 Cracklng of fresh conc reteCracks can fonn in fresh concrete as a result of plastic

shrinkage, settlement, or temperature differential between

the exposed surface and the interior. Insulating blankets can

prevent tempe rature differential-induced cracking.

Plastic shrinkage cracking is characterized by a series of

short, discontinuous cracks that fonn before initial setting of

the concrete.Plasticshrinkage cracks may occur approximately

parallel to each other, but will typically occur in a random,

irregular pattern, Plastic shrinkage cracks may be only

superficial, or they may extend more deeply int o a slab,even

to its full depth .

Plastic shrinkage cracking is caused by rapid evaporation of

moisture from the surface. f th e loss of moisture at the surfaceis greater than the rate that moisture can be replenished by

bleed water or capil1aryaction, tensile stress will develop in the

surface region. This phenomenon is related to the evaporation

rate, which in tum is influenced by the combined effects of

humidity, wind, and concrete and air temperatures. Detailed

discussions of the mechanism and preventative measures can

be found in ACI 305R, 308R, and 302 l R.

Settlement cracks typically occur directly above reinforcing

bars and ex tend to the bar. Settlemen t cracking is caused by

subsidence of fresh concrete, which can occur due to

improper consolida tion or, more commonly, from bleedin g.

Because crack s act as a avenue for the ent ry of water,

deicing chemicals, carbon dioxide, and aggressive chemicals,

the ir presence may lead to a reduc tion in the durability of the

concrete . The extent of the influence on durability will be

dependent on the width and. more importantly, the depth, of

the cracks . Cracks less than 5 mm 0.2 in.) deep may have a

negligible effec t. Cracks of greater dep th, however, increase

the penetrat ion of carbon dioxide, water, and deicing

chemicals, increasing the likelih ood of re inforcement

corrosion, damage due tu free zing and thawing, and AAR .

For nonreinforccd concrete not exposed to freezing and

thawing or aggressive chemicals, plastic shrinkage cracks may

be of little significance, other than for aesthetic considerations.

Crack s in fresh concrete can appear to be closed during the

finishing operations. The closure, however, may be only a

thin skin over the surface of the crack, and the crack may

subsequently reopen. I f such cracki ng occurs, actions should

be taken to prevent the continued occurrence of cracks

forming in the fresh concrete.

3 7 Summary[he strcngth and durability of concrete can be significantly

impaired by the use of improper placement, finishi ng, and

curing techniques while concrete is in the unhardened state.

Appropriately proportioned concrete mixtures that are properly

placed, consolidated, finished, cured. tested, and inspecte d

will help to ensure that the desired durability characteristics

are achieved.

CHAPTER 4 FREEZINGAND THAWINGOF CONCRETE

4 1 lntroduct lonDeterioration of concre te exposed to freezing conditions

can occur when there is sufficient internal moisture present

that can free ze at the given exposure conditions. The source

of moisture can beeither internal water already in the pores

of concrete that is redistributed by thermodynamic conditions

to provide a high enough degree of saturation at the point of

freezing to cause damage ) or external water entering the

concrete from an external source,such asrainfall).Dry concrete

generally below approximately 75 to 80 internal relative

humidity) is normally immune to damage from freezing.

Effect of Parking Concrete Slabs on Durability

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