guide to consolidation of concrete in congested areas

ACI 309.3R-92 . Guide to Consolidation of Concrete in Congested Areas (Reapproved 1997) Reported by ACI Committee 309 Dan A. Bonikowsky* Neil A. Cumming Timothy P. Dolen Jerome H. Ford Joseph J. Fratianni Mikael P. J. Olsen, Chairman Steven H. Geb Gary R. Mass Richard E. Miller Jr. Roger A. Minnich H. Celik Ozyildirim *Subcommittee chairman. Originating committee chairman. *Subcommittee members. Steven A. Ragan actively contributed to the development of this document and served as chairman of the editorial committee. This guide is primarily directed toward architects/engineers and con- structors. It describes various situations where design requirements result in highly congested forms that impede consolidation of con- crete. Techniques to overcome these difficulties are presented. The guide also identifies for constructors various difficult placing and consolidation conditions and proposes solutions such as special pro- cedures and mix proportions. In addition, the guide alerts construc- tors to review design drawings closely where congested areas are ex- pected to insure that appropriate allowances have been included in their bids. 3.4 - Mix proportioning 3 .5-Concrete placing methods 3.6-Construction considerations 3.7-Tunnel linings Keywords: admixtures; concrete construction; consolidation; embedment; formwork (construction); mix proportioning; parting agents: placing; pre- placed aggregate concrete; reinforced concrete; reinforcing steel; splicing; structural design; surface defects; tunnel linings. Chapter 4-Consequences of congested areas in concrete construction, pg. 309.3R-4 4. l-Honeycombed concrete 4.2-Reduced density 4.3-Increased cleaning costs 4.4 Increased formwork costs 4.5-Increased placing costs CONTENTS Chapter l-Introduction, pg. 309.3R-1 Chapter 2-Criteria for designation as a congested area, pg. 3093R-2 Chapter 5-Recommended practices, pg. 309.3R-6 5.1 -Design considerations 5.2-Construction considerations 5.3-Summary 2.l-Reinforcing steel 2.2-Embedments and boxouts 2.3-Formwork 2.4-Definitions Chapter 6-References, pg. 309.3R-9 6.1 -Specified and/or recommended references 6.2-Cited reference Chapter 3-Factors contributing to congestion problems, pg. 309.3R-3 CHAPTER 1 INTRODUCTION Many concrete structures such as those with seismic provisions, post-tensioning, and high-strength concrete are difficult to consolidate because of congested areas within the formwork. This congestion can result in 3.l-Reinforcing steel arrangement 3.2-Embedded parts/boxouts 3.3-Formwork ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in designing, plan- ning, executing, or inspecting construction and in preparing specifications. Reference to these documents shall not be made in the Project Documents. If items found in these documents are desired to be part of the Project Documents, they should be phrased in mandatory language and incorporated into the Project Documents. Sandor Popovics Thomas J. Reading Donald L. Schlegel Bradley K. Violetta ACI 309.3R-92 became effective December 1,1992. Copyright 0 1992, American Concrete Institute. AU 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 any elec- tronic or mechanical device, printed, written, or oral, or recording for sound or use in any knowledge retrieval system or device, unless permission is obtained in writing from the copyright proprietors. 309.3R-1 ACI COMMlTTEE REPORT Fig. 1 -Dense reinforcing steel Fig. 2-Dense reinforcing steel structural inadequacy and time-consuming and expen- sive remedial work. Various techniques have been employed to alleviate this type of problem. This document presents an over- view of the factors contributing to the problem, the consequences of inappropriate concrete procedures in these areas, and recommended practices to minimize the problem. As a prerequisite to successful concreting in con- gested areas or difficult placing conditions, it is impor- tant that architects/engineers become more aware of how their designs will be constructed, and that con- structors become more aware of special procedures and necessary precautions. Most importantly, communica- tion between the architect/engineer and constructor is essential to insure that the design details, construction materials, and procedures are compatible. Fig. 3-Closely spaced embedments CHAPTER 22-CRITERIA FOR DESIGNATION AS A CONGESTED AREA Congested areas are those in which the reinforcing steel, embedments, bboxouts, prestress ducts and an- chorages, or configurations and form shape make con- crete placement and consolidation difficult to achieve. To obtain the desired placement results and degree of consolidation, access for inspection and consolidation, special concrete mixtures, special formwork, additional consolidation effort, and specific placing methods are frequently used. 2.1-Reinforcing steel Congestion causes problems when the clear spacing between reinforcing bars or between a bar and the form is less than 1 l 1 / 3 times the maximum size of coarse ag- gregate used in the concrete mixture. This condition is more likely to occur at splices and bends in reinforce- ment and at beam-column connections. Sometimes, congestion is caused by multiple layers of reinforce- ment in which the bars in the lower layers are not di- rectly below those in the upper layers, as shown in Fig. 1 and 2. See AACI 117 for tolerances for concrete con- struction and materials. 2.2-Embedments and boxouts Embedments consist of items such as plumbing, prestress hardware, ducts, connection inserts, and an- chorages for handling devices that are cast into the concrete (see Fig. 3). Boxouts are used to form open- ings, keyways, or pockets in the concrete. When these items restrict the placement and consolidation of the concrete, they cause congestion. The spacing between embedments, boxouts, and the form must be at least 1% times the nominal maximum size of the coarse ag- gregate to avoid this problem. Frequently, these items cause congestion because the concrete cannot be placed and consolidated easily underneath them (see Fig. 4). The architect/engineer must be alert to such condi- tions, and construction procedures must provide for proper placement and consolidation of concrete on the undersides of these embedments. 2.2.1 Embedments may be anchors, weld plates, me- GUIDE TO CONSOLIDATION OF CONCRETE chanical and electrical boxes, threaded inserts, or other devices used to attach or incorporate items after form- work is removed. 2.2.2 Sleeves are openings that generally go all the way through a wall or slab to allow piping or other penetrations to pass. Formwork for these openings is often completely removed prior to piping placement. 2.2.3 Boxouts are either removable or stay-in-place. Removable boxouts are similar to sleeves but generally are much larger, as in door and window boxouts or hatch boxouts in slabs. Most removable boxout forms can be adapted readily for placement and vibration tubes, as described in Section 5.1.2. If these removable forms are not provided, the constructor must move the concrete horizontally under the formed boxout, which usually results in segregation of the concrete, adds ad- ditional stress to the boxout form, allows the buildup of frictional resistance along the formwork, and slows down the entire placement. Without placement and vi- bration tubes, the degree of consolidation is generally reduced under a boxout. Boxouts often are designed with open bottoms. Stay-in-place boxout forms, such as hollow metal door and window frames, often require bracing and do not allow placement and vibration tubes to be cut through them. This increases the chances of voids or incomplete consolidation. 2.2.4 Formwork accessories, architectural items such as cast-in numbers and letters, form liners, rustication, chamfers, and keyways can cause simple to complex consolidation problems in one manner or another. This is especially true of horizontal rustication or keyways in a wall. 2.3- Formwork The surface texture, shape, type, and orientation of the formwork may restrict concrete placement. Consid- eration needs to be given to form release agents that are compatible with form texture, particularly if intricate shapes are to be cast into the concrete at the formed surface. These release agents may also serve to some- what reduce the frictional resistance between the plastic concrete and the form, thereby improving the ease of removing entrapped air. The forms must also be de- signed for easy removal. Used or poorly oiled wood forms are more likely to hinder consolidation than steel or plastic-lined forms. The frictional resistance of a wood form impedes the flow of concrete and can create difficulty when used in conjunction with congested embedments. 2.4-Definitions 2.4.1 External form tie rods-External form tie rods are installed on the outside of narrow wall forms in the longitudinal direction. The tie rods are attached to the bulkhead walers at the ends of the wall. 2.4.2 Lie-flat hose-Lie-flat hose is a very pliable polyvinyl chloride reinforced discharge hose, typically Fig.4 Stacked boxouts purchased in 4- or 5-m. (100- or 125-mm) diameter by 300-ft (92-m) long rolls. 2.4.3 Side ports-Side ports are temporary openings in the form on one side of narrow walls. The purpose of the side ports is to allow insertion and extraction of vibrators and to observe consolidation of the concrete. 2.4.4 Slide valve-A slide valve is a short piece of steel pipe with a slide plate mounted in it. The pipe is the same diameter as the concrete discharge hose and the other end is bolted to the form. The purpose is to allow pumping of concrete through the open slide valve to completely fill a form to the underside of a horizon- tal structural steel beam. When the form is full, the slide plate is closed, preventing the concrete from seep- ing back through the valve. 2.4.5 Steel reinforced hose-Steel reinforced hose is a rubber concrete-discharge hose reinforced with strands of steel wire between the tube and outer cover. CHAPTER 3-FACTORS CONTRIBUTING TO CONGESTION PROBLEMS 3.1 -Reinforcing steel arrangement The reinforcing steel arrangement must take into ac- count the factors that contribute to congestion. Seismic and strength design requirements often result in a rein- forcing steel layout that inhibits access for preplace- ment cleanup and concrete placement and consolida- tion. Recommended practices are described in Chapter 5. 3.1.1 Splices-The density of reinforcing steel result- ing from current design procedures often makes it dif- ficult to provide continuity of reinforcing bars by the traditional method of lap splices. The various methods 309.3R-4 ACI COMMITTEE REPORT of splicing reinforcing bars, as discussed in Section 5.1.1.1, need to be considered by the architect/engi- neer . 3.2-Embedded parts/boxouts There is increasing use of embedments and boxouts to incorporate piping and electrical and mechanical systems into placements. The use of embedments and boxouts, in conjunction with dense reinforcement, of- ten results in congestion that inhibits acceptable plac- ing and consolidation practices. 3.2.1 Tolerances for placement of concrete around embedments and boxouts should be considered at the design stage. Frequently, mechanical and electrical em- bedments are located adjacent to doors and windows. These areas usually require additional reinforcing due to stress concentrations around the boxout. The core area in buildings is another example where additional reinforcement, embedments, and boxouts cause con- gestion. 3.3- Formwork The design of formwork can contribute significantly to congestion in placements if the design does not take into account other factors, e.g., location of embed- ments and boxouts, reinforcing steel arrangements, placing equipment, and form-tie spacing. The design should consider the number, location, and size of form-tie rods; location of embedments and blockouts; location of trunks or concrete hose; height of forms; and possible use of side ports. In narrow, congested walls, external form-tie rods should be con- sidered. Reduced spacing of wales leads to an increased number of form ties, resulting in added congestion. In- creased spacing of load-bearing members with higher capacity ties and form sheathing can ease congestion. 3.3.1 More concentrated vibration may be needed in congested areas. Since this may result in increased hy- drostatic head during placement, this should be taken into account in the formwork design. 3.4-Mix proportioning The advantages of a large maximum size aggregate concrete can quickly be lost if the mix proportioning does not take into account the congestion existing in the proposed placement. The use of modified mix proportions with smaller maximum size aggregate is becoming a necessary tool to achieve proper consolidation in certain congested areas of a placement. The modified mixture may also include admixtures, increased cement content, and fly ash. The modified mixture need only replace the original mix proportions in the zones of extreme congestion, e.g., around multiple embedments, boxouts, or dense reinforcement configurations. 3.5-Concrete placing methods The constructor must assess whether traditional con- crete placing methods will be adequate in congested ar- eas. The conditions of the placement must be consid- ered in selecting the best method for getting the con- crete to its final consolidated state (see ACI 304R). 3.6-Construction considerations Design considerations should include construction methods and should not be solely limited to the re- quirements in the design code and specifications. The design of heavily congested areas can have serious im- pact on quality, construction costs, and constructabil- ity. Best results are achieved when the architect/engineer works closely with the constructor to insure that the in- tent of the design can be met under field conditions. 3.7-Tunnel linings The concrete lining of tunnels is a difficult operation due to the logistics of concrete transportation and lim- ited access for concrete placement and consolidation. Congestion can be caused by temporary support mem- bers, reinforcing steel requirements, and grouting pipes. Heavily reinforced concrete tunnel linings have become more common in the 1980s. Best results are obtained with a plastic concrete mix- ture that has been proportioned to flow readily along form sidewalls, yet remain cohesive. Ample openings of sufficient size must be provided in the formwork for access by workers to consolidate concrete with immer- sion-type vibrators and for inspection as the work pro- gresses. Larger reinforcing bars at increased spacing is preferred to smaller, more closely spaced bars to pro- vide maximum access. Where heavily reinforced sec- tions are essential, the concrete lining thickness should be increased to allow room behind the form for work- ers. The cost of the additional concrete volume due to increased thickness often can be offset by a higher quality lining. In general, 14 to 16 in. (356 to 406 mm) clear distance is required between the reinforcement and ground excavation lines. Allowance must be made for temporary steel sup- ports that may interfere with access. The placement of concrete in heavily reinforced sections can also be im- proved by bundling reinforcing bars into groups of two or three bars to increase spacing. When encasing per- manent steel plate liners in underground work, it is es- sential to provide adequate concrete thickness for ac- cess by workers during concreting. CHAPTER 4-CONSEQUENCES OF CONGESTED AREAS IN CONCRETE CONSTRUCTION 4.1 -Honeycombed concrete Honeycombed concrete can occur in congested areas due to the inability of vibrators to consolidate the con- crete around and through the congestion and out to the form face. There are several primary reasons for hon- eycombed concrete. l The nominal maximum size aggregate may be too large to pass through the clearances provided, result- GUIDE TO CONSOLIDATION OF CONCRETE 309.3%5 Fig. 5-Side ports in wall form ing in bridging of aggregate particles and blockage of flow. A harsh mix may also cause bridging and thus block flow. The densely placed reinforcing steel or embedded parts may prevent access for the vibrator to complete consolidation in these congested areas. Extension of vertical reinforcement above the form- work in heavily congested forms can restrict the lateral movement of workers. This restriction of movement can lead to operator fatigue and result in incomplete consolidation of the concrete. 4.2-Reduced density Proper density of in-place concrete is dependent upon adequate consolidation. Incomplete consolida- tion will lead to excessive amounts of entrapped air. This entrapped air causes reduced strength and in- creased permeability and can also decrease bond of concrete to the reinforcement. Large or numerous embedded parts can result in un- der-consolidation on the undersides of these parts, cre- ating air pockets. Unless corrective action is taken, ad- equate consolidation may not be achieved. 4.3-Increased cleaning costs Congestion within forms can lead to significant ad- ditional costs for clearing the formed space of debris. Construction materials left behind during form build- ing; reinforcing steel installation; and setting of em- bedded parts, boxouts, cableways, and pipes create se- rious cleaning problems. Debris in the bottom of the placement area cannot be blown across from one end to the other due to block- age by the reinforcement; therefore, cleaning costs are increased due to the need to clear the form in several isolated cells. The time required for hand removal of debris is substantially increased because workers must continuously climb in and out to cover the total area of the placement. Fig. 6-Lie-flat hose viewed through side port 4.4-Increased formwork costs Congested placements can lead to additional form- work costs when form design changes are required to minimize consolidation problems. It may be necessary to increase the form-tie spacing to reduce the number of form ties passing through a form. Stronger form faces, walers, and strongbacks are required to accomplish this. In short narrow walls, bulkhead ties may need to be placed outside the form to prevent the longitudinal form ties from interfering with concrete consolidation. It also may be necessary to install side ports, as shown in Fig. 5, for observation and consolidation purposes. If lie-flat hose is used for placement, it can be conveniently cut off and removed through the side ports (see Fig.6). As the concrete reaches the level of the side ports, the ports are closed and secured by bolt- ing or nailing them to the main form walers. Congested areas within forms may require that embedded parts be supported from a framework spanning the top of the formwork. This reduces the need to install stiffeners and positioning supports in an already congested form. 4.5-Increased placing costs Congested regions of reinforcing steel, primarily due to increased seismic requirements, have resulted in steadily increasing concrete placing costs. Placing methods are being modified due to increased form congestion and the reduction of clearances avail- able to get concrete to its final location. The use of cranes and buckets in conjunction with hoppers and trunks is often not possible due to the space restrictions in forms. Placing methods for con- crete are now frequently planned as independent oper- ations to avoid using crane time for such activities as placement of forms, reinforcement, and embedded parts. Concrete pumps are available with boom lengths ex- ceeding 200 ft (60 m). Concrete also can be pumped through stationary pipelines hundreds of feet long and then placed with a placing boom at the end of the line. The constructor can attach steel reinforced rubber 309.3R-6 ACI COMMITTEE REPORT Fig. 7-Lie-flat hose coupled to concrete line Fig. 8-Placement and vibration tubes: Large blockout within a wall with pipes through the formed blockout to permit access for concrete placement and vibration hose up to 5 in. (125 mm) in diameter and 30 ft (9 m) long to the end of the pump boom to get concrete to the point of deposition. Fragile lie-flat hose is often re- quired at the end of the rubber hose to get past extreme congestion (see Fig. 7). Where it is not possible for the vibrator operator to insert the vibrator all the way to the bottom of wall forms, the constructor should install side ports in the form to allow lowering the vibrators through these ports. CHAPTER 5-RECOMMENDED PRACTICES 5.1 -Design considerations 5.1.1 Reinforcing steel arrangement-Arrangement of reinforcing steel should provide enough space to al- low concrete placement into the form. The architect/ engineer may have to increase the member size over that required. by the design calculations so that suffi- cient room is provided for placement. In extreme cases, it may be necessary for the ar- rangement to include accessways through the reinforc- ing steel. 5.1.1.1 Reinforcement splicing methods-Until the late 197Os, most reinforcing steel arrangements pro- vided for lapping reinforcing steel bars without causing undue congestion problems. More stringent seismic re- quirements have resulted in a dramatic increase in the amount of reinforcing steel per unit area, especially at beam and column connections. Lapping the bars would cause such severe congestion that space between bars would almost disappear, re- quiring a change to splicing. Sometimes this congestion problem associated with splicing can be solved by mechanically connecting the reinforcing bars, as described by ACI 439.3R. In spe- cial cases, the reinforcing bars may be spliced by welded connections, provided that proper welding pro- cedures are used considering the metallurgy of the re- inforcing steels being joined. However, with either a mechanical or welded connection, there will be some localized increase in the reinforcement diameter, which should be considered in detailing clearances and bar spacing. 5.1.2 Embedded parts/boxouts-Embedment, sleeve, and boxout configuration should consider reinforcing details, concrete mix proportions, and especially the nominal maximum size of aggregate. If possible, em- bedments should be spread out. Void forms should be used to eliminate form pene- trations, but if they are large [more than 2 ft (0.6 m) in either direction], a placement and vibration tube (see Fig. 8) should be provided. Boxouts that remain in place should have tolerances to allow them to be shifted and placement and vibra- tion tubes should be provided. Boxouts that are to be removed and exceed 2 ft (0.6 m) in either direction also should provide placement and vibration tubes. In situations where the boxout spans from one form face to the other, access should be provided through the bottom of the boxout. As the concrete reaches the bot- tom of the boxout, the access can be closed off with a preformed insert, which is then bolted to the boxout form. 5.1.3 Placing-The constructor must assess whether his traditional placing methods will be adequate for the job. Bidding merely on the total volume, average placement size, and known project access conditions can result in reduced profit margins. The constructor must review reinforcing steel, embedment, and form- work drawings to tailor the placing methods to suit the conditions. The constructor may need to request changes in the design of the placement or formwork to obtain a quality product at a reasonable cost. Increased cooperation between the architect/engi- neer and constructor prior to beginning work will facil- itate quality construction. Prebid and preconcreting meetings to discuss all phases of the concrete work are encouraged. 5.2-Construction considerations 5.2.1 Use of admixtures-Proper placement of con- crete in congested areas usually requires the concrete to have flowing characteristics. Flowing concrete is gen- erally considered to have a slump of 7% in. (190 mm) or more, while remaining cohesive without excessive bleeding or segregation (ACI 309R). The use of such material permits placement and consolidation in areas where less workable concrete mixtures cannot be prop- erly placed and consolidated due to lack of mobility and vibrator access. Flowing concrete is commonly used in congested ar- eas where the member itself is unusual in shape or size or a large amount of reinforcement is present. Since producing flowing concrete only by adding ex- tra water results in lower quality concrete, such con- crete should be obtained through the use of chemical admixtures. Admixtures used to achieve flowing con- crete should meet the requirements of ASTM C 494 and ASTM C 1017. Commonly used materials for produc- ing flowing concrete include: 1. High-range water-reducing admixtures (superplas- ticizers), ASTM C 494, Types F or G. 2. A combination of high-range water-reducing ad- mixtures plus a water-reducing and retarding admix- ture, ASTM C 494, Type D, or water-reducing and ac- celerating admixture, ASTM C 494, Type E. 3. High dosages of a water-reducing normal-setting admixture, ASTM C 494, Type A, plus a water-reduc- ing and accelerating admixture, ASTM C 494, Type E. Where flowing concrete is required, trial mixtures should be tested with materials representative of those to be used in the project and under the environmental conditions expected on the project. Trial mixtures should be made using the initial slump resulting from the maximum allowable specified water-cement ratio. Chemical admixture dosages can be varied to achieve the desired slump range. If necessary, the initial slumps can be reduced by lowering the water-cement ratio and thus improving the hardened properties of the con- crete. Excessive retardation and loss of air content should be avoided. 5.2.2 Use of modified mixtures (ACI 211.1 and 211.2)-Normally, architects/engineers will specify the largest nominal maximum size aggregate mixtures that are readily available and can be consolidated by con- ventional placing methods. However, the need to meet stringent seismic requirements has led architects/engi- neers to make provisions in the specifications to use smaller maximum size aggregate for some placements or portions of placements. The arch itect/engineer should consider this substitution based on the degree of congestion of reinforcing steel or embedded parts. As an example, when the concrete is specified with a nominal maximum size aggregate of 1 l /2 in. (40 mm), the architect/engineer may allow for substitution of a portion of the concrete (in practice about 20 to 30 per- cent) with %-in. (20-mm) nominal maximum size ag- gregate (Bonikowsky). Where the design mixture specifies nominal maxi- mum size aggregate of 3/4 in. (20 mm) for extremely congested areas, the architect/engineer ‘may allow sub- stitution of a portion of the placement with nominal maximum sized aggregate of 1/2 in. (13 mm). When the maximum aggregate size of a specified mix is reduced, the mix has to be modified by adjusting the water and cement content to maintain the water-cement ratio and design strength. Some specifications also allow the ad- dition of fly ash to enhance workability. Typically, an addition of fly ash equal to 5 percent of the cement weight will provide excellent lubrication. At times, up to 30 percent is allowed. 5.2.3 Formwork-Formwork design should be based on full hydrostatic head conditions wherever practical. Form-tie locations need to be considered when choos- ing a form system and are often fixed in liquid head forms. Full hydrostatic head forms often have large ties [l-in. (25-mm) diameter or greater] and require place- ment, pockets and cleanouts. Bulkhead design should also consider full hydrostatic head. If longitudinal ties or special corner ties are required, external ties should be considered. Formwork accessories such as rustica- tion, chamfers, and keyways should be considered in reinforcement details, mix proportions, and placement, as well as consolidation. In general, formwork design should follow the prac- tices and guidelines presented in ACI 303R and ACI 347R. Careful consideration should be given to areas of congested reinforcement or other embedments. In ar- eas of heavy congestion, concentrated vibration is likely to occur that can increase the hydrostatic pressure on the forms. When ‘needed, the spacing of load-bearing members should be increased and combined with higher capacity ties and sheathing. The use of external tie rods in narrow congested walls also can help reduce the con- gestion. When vertical access to the forms from the top is limited and internal chutes cannot be used, side ports should be incorporated to allow for the placement of the concrete and consolidation by internal vibration. Battered form faces or counterforts generally result in areas of poor consolidation due to the problems of placement, vibrator access, and restricted air migration during vibration. The additional concrete required for a vertical rather than sloped face may be highly cost- effective if required repair of the formed surface is sig- nificantly reduced. Corbels and haunches generally are areas of conges- tion. Similarity of shape and position can reduce form- work costs. 5.2.4 Consolidation methods Congestion is forcing architects/engineers to take into consideration the con- struction aspects of placing and consolidating quality concrete. Some reinforcing steel arrangements are in- corporating openings to provide access for cleaning, placing, and consolidation. Fig. 5 shows designed-in accesses in a heavily reinforced wall section. All aspects of the consolidation operation in con- gested forms should be well planned prior to start of the concrete placement. Smaller size vibrators may be used in the lower areas within the forms when a high- 309.3R-8 ACI COMMITTEE REPORT Fig. 9-Slide valves for pressure pumping of narrow congested walls to the underside of horizontal struc- tural steel beams range water-reducing admixture is used with a modi- fied concrete mixture as described in Section 5.2.2. When it is reasonable to return to the normal concrete mixture using larger maximum size aggregate, bigger and more effective vibrators [typically up to 3 in. (75 mm) in diameter] should be used. When access into the form by the placing crew is limited due to reinforcing steel, additional vibrators should be lowered down through the upper reinforcing mat from the top of the placement. This practice will reduce the tendency of operators to try to throw the vi- brators horizontally past interferences and will encour- age them to operate vibrators in a nearly vertical posi- tion. If the constructor is using pneumatic vibrators, he should insure a good supply of compressed air with headers located near the form. Oilers should be mounted on each line coming off the air header. He should also provide sufficient spare vibrators in the event of a vibrator breakdown. Adequate power should also be provided for electric vibrators. In congested narrow wall forms, it may be necessary to place side ports in one of the wall forms. The side ports are typically 2 ft (0.6 m) square with a spacing of 6 ft (1.8 m). The side ports are used to lower the vibra- tors into the form and to observe the concrete placing within the form. This is necessary to insure that bridg- ing of the concrete during placement has not occurred. It may also be possible to lower small-diameter vibra- tors between the outer layer of reinforcement and the form face, except in the case of architectural faces, where external form vibrators should be used. External form vibrators are discussed in ACI 309R, Chapter 5, and formwork considerations are discussed in SP-4, Chapter 5. Due to the increased time required for congested placements, it may be necessary to use high-range wa- ter-reducing and retarding admixtures or a high-range water-reducing admixture with extended slump reten- tion. Great care must be taken by the operator not to lodge or snag the vibrator within the placement be- cause it can become virtually impossible to extract. The constructor and inspector must be aware that it is a far lesser evil to overvibrate than to undervibrate due to the risk of honeycombed concrete, air pockets, and lack of density in congested areas. 5.2.5 Placing methods-Congested forms and diffi- cult placing conditions have resulted in drastic changes in placing methods. Concrete pumping or conveyors are used more frequently than crane and bucket under such conditions. The prime means of insuring good consoli- dation continues to be the ability to place concrete as close to its final position as possible. The majority of concrete placed in congested forms is placed by pump booms or placing booms using 4- or 5-in. (100- or 125-mm) diameter steel reinforced hose. To insure good pumpability, the architect/engineer is usually restricted to a maximum of l 1/2-in. (40-mm) nominal maximum size aggregate. The majority of concrete in congested forms can be placed by lowering the concrete hose through the rein- forcement to within 6 ft (1.8 m) of the surface, dis- charging the lift thickness, then raising and reinserting the hose at typically 10-ft (3-m) centers. In narrow wall forms where it is not possible to lower the concrete hose through the reinforcement, lie-flat hose coupled to the steel reinforced hose has been used successfully. The lie-flat hose is very pliable and can transfer concrete vertically through very narrow spaces. The hose is relatively inexpensive, making it economi- cal to cut off for removal from the placement if it be- comes caught on reinforcement or embedments (see Fig. 7). Where wall placements extend up to the underside of structural steel members or concrete beams, concrete should be placed under pressure through slide valves, as depicted in Fig. 9. When the form is full, the slide valve is closed and the line disconnected. After the concrete has set, the slide valve and supporting form are re- moved. The remaining concrete stub is removed by chipping and the wall is ground smooth. Pumping concrete from the bottom of the form can offer a solution to congestion in some instances. Flow- ing concrete is recommended for use with this method. The shape of the element has a great deal to do with whether or not the technique is viable. Rectangles, squares, and other polygons require special design of formwork because pressure concentrates at the corners of angles or point loading is developed. Circular unres- tricted structures lend themselves best to pumping from the bottom. Unrestricted means that the concrete must be unimpeded all the way around the inside of the col- umn and there are no baffles that restrict upward movement. If there is a vertical steel “H” section within the col- umn, the concrete will not pump if the concrete enters at a point perpendicular to one of the flanges of the “H.” If concrete is discharged directly into the web of GUIDE TO CONSOLIDATION OF CONCRETE 309.3R-9 the “H,” the concrete will pump. This effect is dimin- ished when at least 12 in. (300 mm) of concrete cover over the “H” is present. Successful pumping has been achieved with less than 4-in. (lOO-mm) cover if pumped into the web. When pumping from the bottom, there should be re- strictions on the number and size of embedments or boxouts and their position in the form. Also, if there are large numbers of dowels, the flow of concrete may be restricted, causing pump and/or form failures. At least a 4-in. (lOO-mm) clearance should be provided be- tween the embedment and the reinforcement or 4 in. (100 mm) free at the top of the placement below the structural steel or turned out reinforcement. Preplaced aggregate (PA) is another placing method that has been used effectively in congested areas. To produce concrete by the PA method, coarse aggregate is first placed in the prepared form. Then the voids in the preplaced aggregate are filled with a fluid grout consisting of cement, sand, water, and sometimes an admixture, which is pumped into the forms from the bottom through form inserts or pipes. Materials re- quirements, procedures, and properties are described in ACI 304.1R. The PA method has been used to advantage for placing concrete around congested reinforcement. Where the reinforcing steel and forms are already in place, grout pipes are inserted from the top or sides to the bottom of the space to be filled. Coarse aggregate is then dropped into place or shoved in from the sides, and assisted by rodding and/or blowing with the help of air lances. After the form has been completely filled with aggre- gate, the grout is pumped into the forms. Alterna- tively, the coarse aggregate may be placed in lifts as the reinforcement and forms are erected. Fig. 10 shows a portion of a boxout left in the side of a nuclear con- tainment structure 50 ft long by 35 ft wide by 6 ft thick (15.2 x 10.6 x 1.8 m). The reinforcement placed during initial construction was too congested to permit the use of vibrators, especially because the rear wall was a steel membrane that could not be cut to receive ports. The boxout was filled with PA concrete in 7-ft (2.1-m) lifts. The preplaced aggregate method provides three plac- ing advantages: 1. There is no time limit on placing the coarse aggre- gate. 2. Areas that do not contain aggregates due to bridg- ing are not critical because all spaces are filled with grout having approximately the same strength as the surrounding concrete. The PA method can signifi- cantly reduce the chances of honeycomb. 3. Continuous pumping of the grout eliminates cold joints. However, if pumping is interrupted for any rea- son, the effect of the cold joint that forms is negligible because coarse aggregate particles extending through the grout surface provide structural continuity across the interface between the two grout placements with a high probability that the negative effects of the cold joint can be minimized or eliminated. Fig. 10-Preplaced aggregate method: Close-up of congested reinforcement in blockout in side of a con- tainment structure. Grout inserts [l-in. (25mm) diam- eter pipes] are shown in right center (one uncapped) and near bottom (two, temporarily capped). Top of first lift of coarse aggregate [approximately 7 ft (2.1 m) deep] is visible at bottom of photo. Grout will be pumped to a few inches below surface of the coarse ag- gregate to provide a keyed joint with the succeeding lift Disadvantages of the PA method include the diffi- culty of isolating congested sections to be placed mon- olithically from less heavily reinforced concrete. PA concrete may be somewhat time-consuming and labor- intensive. 5.3-Summary Successful concreting under difficult conditions or in highly congested sections requires an effective combi- nation of design, placement, and consolidation tech- niques. While this document has presented a number of options that can be considered by architects/engineers and constructors, it must be recognized that each situ- ation may be unique. The architect/engineer and con- structor, in consultation with each other, must assess each situation and agree on the most appropriate ap- proach for the situation in question. However, it is of utmost importance that situations requiring special at- tention be identified with sufficient lead time to allow proper planning. CHAPTER 6-REFERENCES 6.1 -Specified and/or recommended references The documents of the various standards-producing organizations referred to in this document are listed with their serial designations. These publications ing organizations: may be obtainedfrom the follow- 309.3R-10 ACI COMMITTEE REPORT American Concrete Institute 304R P.O. Box 19150 Detroit, MI 48219 304.1R ASTM 1916 Race Street Philadelphia, PA 19103 309R Guide for Consolidation of Concrete 347R Guide to Formwork for Concrete 439.3RR Mechanical Connections of Reinforcing Bars SP-4 Formwork for Concrete American Concrete Institute ASTM c 494 117 211.1 211.2 212.3R 303R Standard Specifications for Tolerances for Concrete Construction and Materials Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete Standard Practice for Selecting Proportions for Structural Lightweight Concrete Chemical Admixtures for Concrete Guide to Cast-in-Place Architectural Concrete Practice c 1017 6.2-Cited reference Bonikowsky, Dan, “Consolidation of Concrete in Congested Ar- eas at Darlington NGS,"” Consolidation of Concrete, SP-96, Ameri- can Concrete Institute, Detroit, 1987, pp. 10-18. Guide for Measuring, Mixing, Transporting and Placing Concrete Guide for the Use of Preplaced Aggregate Concrete for Structural and Mass Concrete Applications Standard Specification for Chemical Admix- tures for Concrete Chemical Admixtures for Use in Producing Flowing Concrete ACI 309.3R-92 was submitted to letter ballot of the in accordance with ACI standardization procedures. committee andprocessed . placement of concrete in heavily reinforced sections can also be im- proved by bundling reinforcing bars into groups of two or three bars to increase spacing. When encasing per- manent steel plate liners. spacing of 6 ft (1.8 m). The side ports are used to lower the vibra- tors into the form and to observe the concrete placing within the form. This is necessary to insure that bridg- ing of the concrete. the concrete will not pump if the concrete enters at a point perpendicular to one of the flanges of the “H.” If concrete is discharged directly into the web of GUIDE TO CONSOLIDATION OF CONCRETE