aci 309.2r-15: guide to identification and control of visible ......aci 309.2r-15 reported by aci...

Guide to Identification and Control of Visible Surface Effects of Consolidation on Formed Concrete SurfacesReported by ACI Committee 309

AC

I 30

9.2R

-15

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First PrintingFebruary 2015

ISBN: 978-1-942727-03-3

Guide to Identification and Control of Visible Surface Effects of Consolidation on Formed Concrete Surfaces

Copyright by the American Concrete Institute, Farmington Hills, MI. All rights reserved. This material may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of ACI.

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This document provides guidelines for identifying and controlling visible effects on the surface of concrete as it relates to consoli-dation on precast or cast-in-place-formed concrete surfaces. A perfectly homogenous and blemish-free concrete element is diffi-cult, if not impossible, to achieve. This document, therefore, does not define an acceptable level of quality, as this should be defined in the contract documents.

This guide explores the direct and indirect cause-and-effect rela-tionships, as well as other factors, concerning concrete surface appearance. Photographs are included in this document to illus-trate typical concrete surface finish effects that are a departure from absolute perfection. Negative surface effects in concrete can be minimized by proper planning during the design and specifica-tion stages. Significant consolidation factors that minimize unde-sirable concrete negative surface effects are also discussed.

Keywords: bugholes; consistency; consolidation; construction joints; discoloration; form offset; formwork (construction); layer lines; mixture proportioning; plastic settlement cracking; preplaced-aggregate concrete; quality control; sand streaking; surface air voids; surface defects; vibration; workability.

CONTENTS

CHAPTER 1—INTRODUCTION AND SCOPE, p. 21.1—Introduction, p. 21.2—Scope, p. 2

CHAPTER 2—DEFINITIONS, p. 22.1—Definitions, p. 2

CHAPTER 3—FACTORS CAUSING NEGATIVE SURFACE EFFECTS, p. 2

3.1—General causes of negative surface effects, p. 23.2—Design considerations of structural members, p. 23.3—Specifications, p. 43.4—Forms, p. 43.5—Properties of fresh concrete, p. 63.6—Placement, p. 73.7—Consolidation, p. 73.8—Special construction conditions, p. 7

CHAPTER 4—NEGATIVE SURFACE EFFECTS, p. 84.1—Honeycomb, p. 84.2—Air voids in formed surfaces, p. 84.3—Formstreaking, p. 84.4—Aggregate transparency, p. 84.5—Color variation, p. 84.6—Sand streaking, p. 94.7—Layer lines, p. 9

Patrick F. O’Brien Jr., Chair

ACI 309.2R-15

Guide to Identification and Control of Visible Surface Effects of Consolidation on Formed Concrete

Surfaces

Reported by ACI Committee 309

Timothy P. DolenChiara F. FerrarisJohn F. Gibbons*

Glenn A. Heimbruch

Vincent E. Hunt*

Gary R. MassRichard E. MillerLarry D. Olson

H. Celik OzyildirimSteven A. Ragan

Bradley K. Violetta

Consulting MembersJerome H. Ford

Donald L. Schlegel*Revision Committee members.

ACI Committee Reports, Guides, and Commentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.

Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.

ACI 309.2R-15 supersedes ACI 309.2R-98 and was adopted and published February 2015.

Copyright © 2015, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.

1



4.8—Form offsets, p. 94.9—Plastic settlement cracking, p. 9

CHAPTER 5—MINIMIZING NEGATIVE SURFACE EFFECTS, p. 9

CHAPTER 6—CONCLUSION, p. 10

CHAPTER 7—REFERENCES, p. 10Cited references, p. 10

CHAPTER 1—INTRODUCTION AND SCOPE

1.1—IntroductionThis guide is a reference source for specifiers, design engi-

neers, architects, contractors, and other professionals who work with concrete surface finish of formed surfaces. The ability to identify or categorize negative surface effects is the first step in detecting the root cause of them. The goal of this guide is to differentiate between various negative surface effects to improve the concrete process and subse-quent concrete quality. The cause of negative concrete surface effects is sometimes not correctly diagnosed. For example, air voids are usually attributed to lack of vibration in circumstances where the correct source of the imperfec-tion is ill-prepared formwork or improper selection of the form-release agent. With misdiagnosis, negative surface effects are likely to occur again because appropriate correc-tive actions have not been identified and taken.

This guide includes a summary of direct and indirect causes of negative surface effects in concrete surface finishes, along with photographs to illustrate them. The most serious effects resulting from ineffective consolidation procedures are also reviewed. They include honeycomb, cold joints, and exces-sive surface voids. A detailed description of these occur-rences and their causes are provided. Of equal importance is the employment of properly trained and motivated supervi-sory and nonsupervisory construction personnel to achieve the intended concrete finishes and surface textures. Extreme negative surface effects do not always conform to the accept-able limits required by contract documents and might be considered defective work. Methods for minimizing surface effects are also discussed.

1.2—ScopeThis guide does not define an acceptable level of quality,

as this should be determined by the parties involved with the project. A perfectly formed concrete surface, uniformly smooth or deeply textured and essentially free of negative surface effects and color variation, is impossible to attain. Repairs to concrete surfaces are costly and difficult. The best repair work will not be as good as an original prop-erly finished surface. Every effort should be made before and during construction to minimize repairs by establishing and maintaining quality concrete operations and adhering to acceptable consolidation procedures for producing formed concrete work. Concrete construction procedures and project costs do not always provide the conditions necessary

to consistently obtain perfectly homogenous concrete free of all negative surface effects. Several negative surface effects discussed in this guide are tolerable and inherent in concrete production. Other potential causes of such negative surface effects may exist beyond those listed in this report. It is the responsibility of the specifier to indicate in the contract docu-ments what constitutes acceptable and unacceptable nega-tive surface effects for the various surfaces to be produced under the terms of a given contract. Surface tolerance speci-fications can be found in ACI 347.3R-13, Table 3.1.

To achieve any concrete finish, the designer and contractor should use the most appropriate materials and design and construction practices to minimize negative surface effects and keep them within acceptable limits. This guide should not be used as a standard for surface finishes, but rather as a guide for the identification of surface effects and their causes. Because concrete consolidation is considered an established field, current research is limited.

CHAPTER 2—DEFINITIONS

2.1—DefinitionsACI provides a comprehensive list of definitions through

an online resource, “ACI Concrete Terminology,” http://www.concrete.org/tools/concreteterminology.aspx.

radius of influence—plan-view-area that a vibrator is able to produce sufficient impulses to consolidate concrete.

CHAPTER 3—FACTORS CAUSING NEGATIVE SURFACE EFFECTS

3.1—General causes of negative surface effectsTable 3.1 presents the primary causes of surface condi-

tions that factor into the resulting negative surface effects for the following factors: member design, formwork, construc-tion conditions, concrete properties, placement, and consoli-dation. Examples of common negative surface effects are illustrated in Fig. 3.1a through 3.1i. The causes of negative surface effects on formed concrete surfaces are described in Table 3.1.

3.2—Design considerations of structural membersCommon problems requiring consideration during design

and planning are congested reinforcement—in particular, splices, narrow sections, or complex form configurations. Conditions that require closed top forming, embedments, and battered forms also require consideration during design and planning. Figure 3.2 features a dense matrix of pipe inserts and illustrates the importance of having a consolida-tion plan well in advance of production.

To produce properly consolidated concrete with the desired appearance, placement and consolidation of concrete should be understood. The designer should have working knowl-edge of the concrete placement process. The designer and constructor should communicate during early phases of the concreting process. Early recognition of problem areas will allow enough time to take remedial measures, such as stag-gering splices, grouping reinforcing steel, modifying stirrup

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2 CONTROL OF VISIBLE SURFACE EFFECTS OF CONSOLIDATION ON FORMED CONCRETE SURFACES (ACI 309.2R-15)



Table 3.1—Causes of negative surface effects on formed concrete surfacesOccurrence: honeycombDescription: stony area with air voids; lacking of fines

Design of members Forms Construction conditions Properties of fresh concrete Placement Consolidation• Highly congested

reinforcement• Narrow form geometry• Reinforcement splices

• Leaking at joints• Severe grout loss

• Premature setting• Reinforcement too close

to forms• Lack of vibration access• Congestion due to

splices

• Insufficient fines• Low workability• Early stiffening• Excessive mixing• Oversized aggregate

• Excessive free-fall• Excessive concrete

lifts• Drop chute omitted• Tremie too small• Segregation due to

horizontal movement

• Vibrator too small• Incorrect frequency and amplitude

• Short immersion duration• Excessing vibrator spacing• Inadequate penetration• Insufficient number of vibrators

Occurrence: air surface voidsDescription: small individual holes of irregular size and shape ranging up to 1 in. (25 mm) in diameter

Design of members Forms Construction conditions Properties of fresh concrete Placement Consolidation• Battered or interfering

construct• Form face

impermeable• Poor wetting

properties• Formwork too flexible• Improper form-

release agent• Improperly applying

form-release agent

• Excessive release agent• High concrete

temperature

• Low FM of fine aggregate• Fine aggregate with a high

FM• Low workability• Excessive cement or

pozzolan• Particle degradation• Excessive sand• High air content

• Too slow• Inadequate

pumping rate• Undersized bucket

• Amplitude too large• External vibration

inadequate• Head of vibrator partially

immersed

Occurrence: form streakingDescription: fine aggregate or coarse aggregate textured areas lacking cement and usually with a dark color on adjacent surface

Design of members Forms Construction conditions Properties of fresh concrete Placement ConsolidationNA • Leaking at joints or

tie holes caused by loose hardware

• Usually caused by horizontal concrete movement

• Excess water or high slump • Improper timing between placing and vibrating

• Excessive amplitude or frequency

Occurrence: aggregate transparencyDescription: dark or light areas of similar size and shape to that of the coarse aggregate; distinct mottled appearance

Design of members Forms Construction conditions Properties of fresh concrete Placement ConsolidationNA • Too flexible

• High-density surface finish

NA • Low fine-aggregate content• Gap-graded aggregate• Dry or porous aggregate• Excessive coarse aggregate• Excessive slump with

lightweight concrete

• Too rapid • Excessive external vibration

• Overvibration of light-weight concrete

Occurrence: color variationDescription: variations in color of the surface; visible within a few hours after removing the formwork

Design of members Forms Construction conditions Properties of fresh concrete Placement Consolidation• Heavy reinforcement

close to forms• Variation in absorp-

tive capacity of surface

• Reaction with form face

• Chemical reaction with release agents

• Leakage of forms at tie holes

NA • Non-uniform color of materials

• Inconsistent grading• Variation in proportions• Incomplete mixing• Calcium chloride can cause

darker color• Slump too high

• Segregation due toexcessive slump

• Vibrator too close to forms• Variable vibration next to

forms

Occurrence: sand streakingDescription: variation in color or shade due to separation of fine particles

Design of members Forms Construction conditions Properties of fresh concrete Placement ConsolidationNA • Form leakage

• Excess water at bottom of form forced up along form face by hydraulic pressure

• Low-viscosity mixtures • Lean oversanded mixtures• Low-viscosity mixtures

deficient in fines

• Too rapid for type of mixture

• Low-viscosity mixtures deficient in fines

• Excessive vibration• Excessive amplitude• Overmanipulation

Occurrence: layer linesDescription: dark colored zones between concrete layers

Design of members Forms Construction conditions Properties of fresh concrete Placement Consolidation• Highly congested

reinforcementNA • Insufficient planning

• High temperature• Wet mixture with tendency

to bleed• Slow placement • Lack of vibration

• Failure to penetrate into previous layer

Occurrence: form offsetsDescription: abrupt to gradual surface irregularities

Design of members Forms Construction conditions Properties of fresh concrete Placement Consolidation• Construction joint at

change in direction of formwork

• Inadequate formwork design for placement rate

• Poor form anchorage and inadequate bulkheads

NA NA NA


CONTROL OF VISIBLE SURFACE EFFECTS OF CONSOLIDATION ON FORMED CONCRETE SURFACES (ACI 309.2R-15) 3



spacing, increasing the section size, and selecting locations of horizontal construction joints. When unfavorable condi-tions exist that could contribute to substandard surfaces, one or more of the following actions should be taken:

(a) Redesign the member size(b) Redesign the reinforcing steel, possibly using fewer

larger bars with equal area(c) Provide adequate access for consolidation at horizontal

construction joints(d) Modify mixture proportions(e) Use mockup tests to develop a procedure(f) Alert the contractor to critical conditions

3.3—SpecificationsAcceptable specifications for concrete and its construction

are essential to ensure proper construction practices. Prac-tical and workable specifications allowing for unusual and complex job conditions are required.

Specifications should be sufficiently broad in scope to permit adjustments of mixture proportions and required batch adjustments to produce uniformly workable concrete that responds readily to vibration. Concrete could still vary due to changes in aggregate grading, ambient and concrete temperature, air content, and batch quantities, even when these changes are within specification limits. Mixture proportions might need adjustments to produce the desired concrete characteristics and to minimize consolidation prob-lems. The mixture, however, should be adjusted in accor-dance with ACI 211.1 to maintain the design intent and avoid additional problems, such as excessive cracking. Specifica-tions should include the desired surface finish (ACI 301). Specifications should also call for vibrators of proper size and characteristics, as recommended in ACI 309R. Small-diameter vibrators should be required to supplement larger-diameter vibrators where access is limited.

3.4—FormsSome negative surface effects are related to inadequate

formwork design and the selection of inappropriate form facing material. Examples are leakage at joints, improperly selected form facing material, excessive overload on previ-ously placed concrete (ACI 303R), inadequate anchorage, poorly braced and excessively flexible forms, improper use of release agents, and oversized and unsealed tie holes. Negative surface effects also result from damaged forms, inadequate cleaning, improper patching and repair of the forms, and use of the forms beyond the manufacturer’s design service life. More information regarding overuse of forms can be found in ACI 347R.

Smooth forms in combination with the correct selection of a form-release agent allow air voids at formed surfaces to move upward more freely. ACI 303R discusses the selection and application of release agents.

There is a tendency for laborers to apply several times the required amount of form-release agent to forms. This

Fig. 3.1b—Air surface voids.

Fig. 3.1a—Honeycomb.





can at times prevent optimal product from being produced because the excess form-release agent can create small air voids at the formed surface. The type of sprayer used in the application of the form-release agent is also important to surface appearance. For example, some dry resin-based

release agents used on steel forms will greatly increase the number of bugholes. An excessive amount of release agent collecting in the bottom of the form could result in discolor-ation of concrete and create weak areas. Inadequately cleaned forms, damaged forms, or those reused too many times can contribute significantly to the formation of negative surface

Fig. 3.1c—Formstreaking.

Fig. 3.1d—Aggregate transparency.

Fig. 3.1e—Color variation.

Fig. 3.1f—Sand streaking.





effects. With any of these conditions, the concrete surface may peel during form removal.

The finish should be observed while the form is stripped so that appropriate corrective measures, if needed, can be implemented promptly. Inward sloping forms have a tendency to trap or restrict the movement of entrapped air and bleed water to the surface, increasing the occurrence of surface voids. Information regarding form strength, design, and other form requirements are found in ACI 347R.

3.5—Properties of fresh concreteThe following factors should be considered to obtain a

concrete mixture with the desired composition, consistency, and workability to facilitate its placement and consolidation. The composition, consistency, workability, and temperature of fresh concrete have a significant bearing on the ease with

which a concrete mixture may be placed and consolidated. For architectural surface finishes, the effect of each ingre-dient in the mixture could require special consideration. Placing conditions should also be considered during mixture proportioning.

Mixture adjustments should be made to proportions to maintain workability when materials and field conditions change, provided that critical properties such as durability and strength are maintained.

The designer should ensure that strength levels, nominal maximum aggregate size, and slump requirements for different structural elements are met. Concrete ingredients should be evaluated and proportions selected well in advance of the concreting operation to achieve desired properties for fresh concrete. Sticky mixtures could occur if the fine aggre-gate grading in the No. 16 to 50 (1.18 mm to 300 μm) size range approaches the upper limits specified by ASTM C33/C33M, or if high cement contents are used. Some pozzolans could also cause mixtures to be more cohesive, restricting the passage of entrapped air and trapping air voids at the interface between the concrete and form. If fine aggregate contains the proper amount of materials in the No. 30 to 50 (600 to 300 μm) size range, little bleeding will occur in the resulting concrete and placement and consolidation of the concrete will be facilitated and surface effects minimized.

Fig. 3.1g—Layer lines.

Fig. 3.1h—Form offsets.

Fig. 3.1i—Plastic settlement cracking.

Fig 3.2––Dense matrix of pipe inserts.





Soft aggregates could degrade during mixing and handling and produce additional fines. In some instances, the fines could make the mixture more cohesive and increase diffi-culty in removal of entrapped air. This is particularly true at high cementitious materials contents. Additional fines could also significantly increase the water demand, resulting in lower strength, increased shrinkage, and crazing of as-cast formed surfaces. A list of deleterious fines can be found in ASTM C33/C33M. Experience indicates that concrete at a given consistency will generally flow more easily at lower temperatures than at higher ones.

3.6—PlacementConcrete should be placed at a rate that is fast enough to

avoid layer lines but also slow enough to avoid segrega-tion. Once the coarse aggregate is separated from the mortar by poor handling and placement practice, it is impossible to work the mortar back into the voids and restore a dense mass by vibration. Segregation causes honeycomb. Spat-tered mortar on the form produces color variations and poor surface texture. Placing concrete too slowly could cause loss of workability or produce layer lines or cold joints (Fig. 3.6) due to improper consolidation. The rate of placement and internal vibration factors, such as intensity and spacing, should be selected to minimize entrapped air in the concrete.

In general, it is easier to consolidate layers of concrete that are 12 in. (305 mm) thick or less; however, thicker lift layers can be used in some instances. Where mixtures of dry or stiff consistencies are required, the placement rate should be slower to permit adequate consolidation to avoid bugholes and honeycombing.

Technological advancement in formwork, concrete mixtures, and consolidation equipment make it possible to produce a high-quality concrete surface appearance using much deeper lifts than in the past. The maximum place-ment rate should not exceed the falsework designer’s recommendation.

3.7—ConsolidationConcrete consists of coarse aggregate particles in a matrix

of mortar, and irregularly distributed pockets of entrapped air. If the concrete is air entrained, an additional, evenly distributed system of entrained air bubbles is present. The volume of entrapped air in unconsolidated concrete may vary from approximately 5 to 20 percent, depending on workability of the mixture, size and shape of the form, amount of reinforcing steel, and method of depositing the concrete. The purpose of consolidation is to remove as much of this entrapped air as practical.

Vibration is the most common method of consolidation. It causes rapid movement of the concrete mixture particles, briefly liquefying the mixture and reducing the internal fric-tion. When vibrated, concrete becomes fluid and, through the action of gravity, seeks a lower level and denser condition as entrapped air rises to the surface and is expelled. It compacts laterally against the form and around the reinforcing steel. In practice, vibration is normally continued until the entire placement acquires a uniform appearance and its surface just

starts to glisten or large bubbles cease to appear. A film of cement paste should be discernible between the concrete and forms. These visual indicators are not necessarily an accu-rate indication of good consolidation. ACI 309R provides guidance on judging the adequacy of vibration.

Undervibration is far more common than overvibration, and could be caused by the following:

(a) Use of an undersized, underpowered, or poorly main-tained vibrator

(b) Excessive or haphazard spacing of vibrator insertions(c) Inadequate vibration during each insertion(d) Failure of the vibrator to penetrate into the preceding

layer(e) Vibrator in the wrong position relative to the formCommon negative surface effects resulting from undervi-

bration are honeycomb, excessive entrapped air voids, and layer lines.

Overvibration could occur if prolonged vibration is continued for several times more than the recommended time period. Overvibration, generally caused by the use of oversized equipment, improper procedures, high slump, or improperly proportioned mixtures, could result in segrega-tion, excessive form deflection, sand streaking, and form damage. Backstrom et al. (1958) found that air content of concrete is decreased by increasing periods of vibration, but little effect is noted on spacing factor of air-entrained concrete. In concrete of nominal 6.5 percent air, the air content dropped from 6.7 to 1.2 after 2, 6, 12, 20, 30, and 60 seconds of vibration, but the number of cycles to 25 percent loss in air volume and spacing factor was unchanged. The consequences of overvibration are minimized if a well-proportioned mixture with proper slump is used. The behavior of fresh concrete during vibration is discussed in ACI 309.1R.

3.8—Special construction conditionsRegardless of how carefully a concrete finish is speci-

fied, the resultant quality depends on careful construction site organization and well-trained and skilled personnel.

Fig. 3.6—Cold joints.





Competent supervision is essential to assure construction forces properly handle and assemble the forms and methodi-cally place and consolidate the concrete. Supervisors should be alert to unfavorable conditions during the installation of forms and reinforcement, and immediately bring these condi-tions to the designer’s attention. The designer should also locate horizontal construction joints at points of maximum access for placement and consolidation exits. The contractor should review the joint layout as detailed in ACI 301 and submit a revised joint plan that is favorable for construction. Combining lifts could restrict access for proper consolida-tion and increase the likelihood of surface effects.

Formed concrete surfaces under box-outs and battered forms require special considerations for placement. The mixture might require adjustments to produce a readily-flowable concrete that is capable of filling the formed area. Self-consolidating concrete is a useful solution to certain applications. For large surface areas, it might be neces-sary to cut holes in a battered form to provide access for vibrating the concrete. With thin layers and careful vibra-tion, air bubbles can be drawn up the side of the form. Large mass-concrete sections placed in irregularly shaped forms might have negative surface effects due to non-uniform or widely spaced locations for tremies, pipes, or chutes. Poorly planned and executed placement procedures could cause concrete to build up in piles. This promotes segregation, cold joints, layer lines, honeycomb, and subsidence cracks. To obtain acceptable results, placing methods should be well planned and well supervised.

CHAPTER 4—NEGATIVE SURFACE EFFECTS

4.1—HoneycombHoneycomb (Fig. 3.1a) is a condition of irregular voids

due to failure of the mortar to effectively fill the spaces between coarse aggregate particles. Where bridging of the coarse aggregate particles or stiffness of the mixture is a cause of honeycomb, vibration may assist in overcoming the bridging by increasing the flowability of the concrete. Factors contributing to honeycombing include congested reinforcement, segregation resulting in insufficient paste content and improper fine aggregate to total aggregate ratio, improper placing techniques, rapid stiffening of hot concrete, difficult construction conditions, and insufficient vibration effort. Changes in construction practices and mixture proportions to improve workability, and the use of water-reducing admixtures to increase slump, could assist in reducing or preventing honeycombing.

4.2—Air voids in formed surfacesSurface air voids, commonly called bugholes, (Fig. 3.1b)

are isolated cavities that result from entrapment of air bubbles in the surface of formed concrete during placement and consolidation. Air voids on vertical faces are more likely to occur in sticky or stiff concrete mixtures of low work-ability that might have an excessive fine aggregate content, entrapped air, or both. Vibrators with excessive amplitude or an incomplete insertion of the vibrator head could result

in an increased quantity of air voids. Excess water normally manifests itself in other textural effects such as bleeding channels or sand streaks on vertical formed surfaces. Bleed water voids can form at the top of a column and on battered formed surfaces. Surface voids can be minimized by the procedures discussed in Chapter 5.

4.3—FormstreakingFormstreaking (Fig. 3.1c), caused by mortar leaking

through form joints and tie holes, can be aggravated by over-vibration from vibrators that are too powerful or by using forms that vibrate excessively during consolidation. Placing excessively wet or high-slump concrete mixtures will result in more mortar washing out through tie holes and loose-fitting forms. Special care is occasionally required when high-range water-reducing admixtures are used, as they tend to increase leakage at form joints.

4.4—Aggregate transparencyAggregate transparency (Fig. 3.1d) is a condition charac-

terized by a mottled appearance on the surface that results from deficiencies in mortar. Causes include concrete mixtures with low fine aggregate content, dry or porous aggregates, or high slump with lightweight and normalweight aggregates. Also, high-density or glossy form surfaces can cause aggre-gate transparency. This discoloration is most apparent on smooth, off-the-mold finishes. Using light-colored aggre-gates and white cement can also reduce the effect.

Possible causes of aggregate transparency include:a) Low sand content in the concreteb) Forms that are too flexible or of non-uniform stiffnessc) Prolonged external vibration or use of vibrators with

too much amplituded) Dry or highly absorptive coarse aggregatee) Inadequate curing

4.5—Color variationColor variation (Fig. 3.1e) can occur within a placement if

the concrete is not uniform or is incompletely mixed. Vibra-tors inserted too closely to the form can cause color variation by marring the form surface (Fig. 4.5). External vibration used haphazardly may also cause color variation. Further-

Fig. 4.5––Color variation.





more, color variations may result from nonuniform form absorption (Fig. 4.5), nonuniform application of the release agent, or both.

4.6—Sand streakingSand streaking (Fig. 3.1f) is a streak of exposed fine

aggregate in the surface of formed concrete caused by heavy bleeding along the form. Sand streaking is frequently results from the use of harsh, low-viscosity mixtures, particularly those deficient in the No. 50 to 100 (300 to 150 μm) and smaller sizes. Streaking tendencies increase when the ratio of fine aggregate to cementitious material increases, such as in lean mixtures. Although the characteristics of portland cement and pozzolans, when used, have some influence on bleeding, the grading of fine aggregate is of greater impor-tance. Sand streaking is controlled with tight forms, proper mixture proportioning, and well-graded fine aggregate to minimize bleeding. Streaking is aggravated by excessive vibration, over-manipulation of the vibrator, the use of a vibrator with excessive amplitude, or excess water at the bottom of the form that is forced up along the form face by hydraulic pressure.

4.7—Layer linesLayer lines (Fig. 3.1g) are dark horizontal lines on formed

surfaces that indicate the boundary between concrete place-ments. Layer lines are caused by premature stiffening or insufficient consolidation of the previous layer of concrete due to lack of penetration of the vibrator into that layer, or the use of a mortar bonding layer between placements.

An extreme example of layer lines is cold joints. This situ-ation can often be avoided by contingency planning, backup equipment, working to keep the concrete surface alive, the use of retarding admixtures, and working to penetrate vibrator into lower lifts.

4.8—Form offsetsForm offsets (Fig. 3.1h) are usually caused by inadequate

stiffness or anchorage of the forms. They can be aggravated by an excessive rate of placement, an excessively powerful vibrator, or both.

4.9—Plastic settlement crackingPlastic settlement cracking (Fig. 3.1i) results from the

development of tension as the concrete settles just after time of initial setting. Cracks are caused because the upper concrete bridges between the forms while the lower concrete settles. These cracks may occur when there is an insufficient interval between placing the concrete in the columns and placing the concrete for the slabs or beams. They may also occur adjacent to blockouts or over reinforcing bars with shallow cover.

To prevent plastic settlement cracking, the concrete can be revibrated. Revibration is most effective when completed at the latest time at which the vibrator head will readily pene-trate the concrete under its own weight. Subsidence cracking over reinforcing bars can be controlled by increasing

concrete cover during the design phase and by using low-slump concrete that is well consolidated.

CHAPTER 5—MINIMIZING NEGATIVE SURFACE EFFECTS

A number of studies (Shilstone 1977; Stamenkovic 1973; Samuelson 1970; Reading 1972) have determined how to achieve better consolidation to improve surface appearance. Recent formal studies have been limited. To minimize the negative concrete surface effects, the following practices should be followed:

a) The time the vibrator is in the concrete should be of sufficient duration for proper consolidation.

b) Vibrator insertions should be properly spaced (1.5 times the radius of influence) and overlapped and the vibrator removed slowly.

c) Each concrete layer should be consolidated from the bottom upward.

d) The time the vibrator is in the concrete may be extended when using impermeable forms that permit air trapped at the form surface to escape through joints.

e) Inward sloping forms and other complex design details should be avoided.

f) Depth of placement layers should be limited.g) Vibrator should penetrate the previous layer.h) Tightening devices and gaskets to prevent leakage at

form joints should be provided as needed.i) Placing ports should be designed into forms as needed.Where practical, bugholes can be minimized by the use of

a 2-1/2 in. (65 mm) diameter vibrator of high frequency with medium to low amplitude. The vibrator should be immersed in the concrete around the perimeter of the form without damaging the form. Where reinforcement is placed near the form wall, the vibrator should be inserted inside the rein-forcement. Care should be taken to ensure that the vibrator has a sufficient radius of action to liquefy the concrete at the form.

Form vibration may be used to supplement the internal vibration. Doing so, however, could cause a major increase in form pressure. An alternate procedure is to use a high-frequency, low-amplitude form vibrator. Vibration proce-dures should be evaluated at the beginning of a project to determine the vibration time for each type of vibrator for a given mixture. Guidance on the selection of appropriate vibration amplitudes, frequencies, and equipment is given in ACI 309R.

In areas where surface air voids are most prevalent, revi-bration may be used to reduce them. Revibration is more effective if it is completed at the latest possible time at which the vibrator head will readily penetrate the concrete under its own weight. Greater benefits are obtained with higher-slump concrete mixtures, especially in the upper portion of a place-ment where excessive entrapped air voids are most preva-lent. This practice, however, could increase the laitance, requiring removal from the horizontal construction joints and could create color nonuniformity.

Other measures, such as altering mixture proportions, using high-range water-reducing admixtures, and using





smaller nominal maximum size aggregate to improve work-ability should also be considered as methods of minimizing surface effects, provided that design requirements are met. These measures have often been successful, particularly when trying to consolidate concrete in congested areas. Further guidance is found in ACI 309R.

CHAPTER 6—CONCLUSIONNegative surface effects in formed concrete surfaces can

result in an unnecessary increase of project cost if correc-tive actions are needed. To keep these effects within speci-fied limits, an awareness of their causes and their cures is essential. The causes of these effects may lie in initial design concepts, specification, materials selection, proportioning, placement, consolidation, or workmanship. The services of a specialist in concrete and concrete construction may be used to assist in obtaining concrete surfaces conforming to higher quality. The execution of the work by well-trained work crews under competent supervision will ensure a concrete surface meeting the requirements of the owner or designer.

CHAPTER 7—REFERENCESCommittee documents are listed first by document number

and year of publication followed by authored documents listed alphabetically.

American Concrete InstituteACI 211.1-91(09)—Standard Practice for Selecting

Proportions for Normal, Heavyweight, and Mass ConcreteACI 301-10—Specifications for Structural Concrete

ACI 303R-12—Guide to Cast-In-Place Architectural Concrete Practice

ACI 309R-05—Guide for Consolidation of ConcreteACI 309.1R-08—Report on Behavior of Fresh Concrete

During VibrationACI 347R-04—Guide to Formwork for ConcreteACI 347.3R-13—Guide to Formed Concrete Surfaces

ASTM InternationalASTM C33/C33M-13—Standard Specification for

Concrete Aggregates

Cited referencesBackstrom, J. E.; Burrows, R. W.; Mielenz, R. C.; and

Wolkodorff, V. E., 1958, “Origin, Evolution and Effects of the Air Void System in Concrete, Part 3—Influence of Water-Cement Ratio and Compaction,” ACI Journal Proceedings, V. 55, No. 9, Sept., pp. 359-375.

Reading, T. J., 1972, “The Bug Hole Problem,” ACI Journal Proceedings, V. 69, No. 3, Mar., pp. 165-177.

Samuelson, P., 1970, “Voids in Concrete Surfaces,” ACI Journal Proceedings, V. 67, No. 22, Nov., pp. 868-874.

Shilstone, J. M., 1977, “Surface Blemishes in Formed Concrete,” Proceedings, RILEM/ASTM/CIB Symposium on Performance Evaluation of External Vertical Surfaces of Buildings (Otaniemi, Espoo, Aug.-Sept.), Technical Research Centre of Finland, Espoo, Finland, pp. 3-7.

Stamenkovic, H., 1973, “Surface Voids Can Be Controlled,” Concrete Construction, V. 18, No. 12, Dec., pp. 597-598.





As ACI begins its second century of advancing concrete knowledge, its original chartered purpose remains “to provide a comradeship in finding the best ways to do concrete work of all kinds and in spreading knowledge.” In keeping with this purpose, ACI supports the following activities:

· Technical committees that produce consensus reports, guides, specifications, and codes.

· Spring and fall conventions to facilitate the work of its committees.

· Educational seminars that disseminate reliable information on concrete.

· Certification programs for personnel employed within the concrete industry.

· Student programs such as scholarships, internships, and competitions.

· Sponsoring and co-sponsoring international conferences and symposia.

· Formal coordination with several international concrete related societies.

· Periodicals: the ACI Structural Journal, Materials Journal, and Concrete International.

Benefits of membership include a subscription to Concrete International and to an ACI Journal. ACI members receive discounts of up to 40% on all ACI products and services, including documents, seminars and convention registration fees.

As a member of ACI, you join thousands of practitioners and professionals worldwide who share a commitment to maintain the highest industry standards for concrete technology, construction, and practices. In addition, ACI chapters provide opportunities for interaction of professionals and practitioners at a local level.

American Concrete Institute38800 Country Club DriveFarmington Hills, MI 48331Phone: +1.248.848.3700Fax: +1.248.848.3701

www.concrete.org



38800 Country Club Drive

Farmington Hills, MI 48331 USA

+1.248.848.3700

www.concrete.org

The American Concrete Institute (ACI) is a leading authority and resource

worldwide for the development and distribution of consensus-based

standards and technical resources, educational programs, and certifications

for individuals and organizations involved in concrete design, construction,

and materials, who share a commitment to pursuing the best use of concrete.

Individuals interested in the activities of ACI are encouraged to explore the

ACI website for membership opportunities, committee activities, and a wide

variety of concrete resources. As a volunteer member-driven organization,

ACI invites partnerships and welcomes all concrete professionals who wish to

be part of a respected, connected, social group that provides an opportunity

for professional growth, networking and enjoyment.

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