Development of slip-form construction

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Development of slip-form construction

NATIONAL RESEARCH COUNCIL OF CANADA DIVISION OF BUILDING RESEARCH

DEVELOPMENt OF SLIP-FORM CONSTRUCTION by

A.W. _Smith

Report No. 131 of the

Division of Building Researoh

Ottawa February 1958

PREFACE

As a result of some inquiries addressed to the Construotion Section of the Division of Building Researoh a study was made of the use of sliding formwork or "slip-torm" in oivil engineering oonstruotion.

In aooordanoe with usual praotioe, this took the form of a literature searoh. It was found that the method had been developed extensively overseas, even though it

is often regarded as a peouliarly North American development. Since the method has now been used in Europe as

an aid to the reduotion of house costs, by applioation to the construotion of large reinforced oonorete residential buildings, the method is attaining an importanoe tar beyond

that given by its normal use for the construotion

ot

struotures suoh as grain elevators.

Aooordingly, it was deoided that this brief reoord would be issued b1 the Division in the hope that it may be

of service to others who are interested in this speoial oonstruction teohnique.

Ottawa

Februa17

1958

Robert F. Legget Direotor

DEVELOPMENT OF SLIP-FORM CON8TRUCTI6!

by

A.W. Smith

This method of oonorete oonstruotion was developed in Canada about 50 years ago, from a teohnique devised by the late Mr. J.S. Metoa1f in 1900 for building a silo of four small bins. This silo is still in use, and the plans for its oonstruotion have been preserved. For the last 30 years, the resultant

improved method has been used for the oonstruotion of reinforoed oonorete -grain elevator bins, or silos, to the exo1usion of all 'others. Today, following speoia1 developments in Soandinavian

oountries, it is used in the oonstruotion of mUlti-story

apartment blooks and warehouses, bridge piers, ohimneys, reservoirs, and for lining oanals and mineshafts, and bas also been used in the oonstruotion of single walls.

The method itself has apparently never been proteoted by patent, and any which may possibly have been taken out will have long since expired (1).,

Basically, this method involves the oontinuous p1aoing of ooncrete in a shallow mould having the same plan as the building

to be oonstructed. This rigid mould, or "slip-form" as it is called, forms the working deok which is jacked slowly upwards at a controlled rate until the required elevation is reached.

The main advantages can be listed as: (i) quicker construotion;

(ii) less formwork;

(iii) less scaffolding;

(iv) easier working conditions; (v) greater salvage;

(vi) better concrete;

(vii) a joint1ess, watertight structure.

In this country a minimum building height of approximately 50 feet

(4

storys) is required to balance the extra labour cost and supervision against the saving in formwork and construction time (2), but familiarity with the method may justify a considerable lowering of this figure.

2

-CONSTRUCTION

A basic requirement for the slip-form method is the uniform and consistent supply of materials from mixing plant to working deck. For this reason, a stand-by plant is desirable, and it is important to have sufficient storage space for all materials at the si te (3).

Originally it was thought advisable to work two 10-hour

shifts per day, but the modern tendency is to do as much as possible at ground level before concrete placing commences and then to work continuously in three 8-hour shifts (4).

Construction starts with accurate assembly of the sliding formwork, including the jacking system and working deck, directly on top of the foundation or floor slab. The jaoks, mounted on vertical steel rods embedded in the centre of the green concrete walls, support the sliding moulds and working deck which can be raised as a single unit by their upward movement on the jackrods. Initially, the forms are filled with concrete to almost their full depth and raised only 1/8 inoh at a time, so that at the end of the first shift, usually timed for early morning, there may be

6

inches of concrete showing and about

3

feet still inside the form. In the next shift the rate is increased to

3

inches per hour, then up to

6

inches per hour at the end of the shift, and from then on between

6

and 12 inches per hour for the remaining distance (2). When this is done, the concrete walls emerge from below the slip-form mould with their initial set and in an ideal condition for

finishing from a simple hanging scaffold attached to the form itself. When roof level is reached the working deck becomes the lower part of the roof-slab formwork, and when this slab is placed the slip-forms are dismantled. At this stage the interior beams and floor slabs are added, if this was not done on the way up.

MIXING AND PLANT

As noted, adequate storage of materials at the site, and a uniform, continuous supply of conorete to the working deck, are the two main considerations of this slip-form method.

Weather conditions, because of their influence on the rate of hardening of the concrete below the forms, usually determine the rate of vertical movement of the forms. This maximum rate of move-ment determines, in turn, the size of the mixer, the type of hoist

tower necessary to get the materials to the working deok, and the jaoking procedure (1).

The vertical rise may increase to nearly 20 feet per day under ideal conditions

(5)

but the over-all average on a normal

3

-Storage space is reduced to a minumum by having the concrete materials delivered to the site in a dry, ready-mixed form. This also has advantages in freezing weather. Some contractors prefer a battery of small mixers to a single large one as a safeguard against breakdown and to give a faster, more even distribution to the working deck, although it may complicate any heating system necessary in cold weather.

Usually the mixer discharges directly into a skip bucket on a tower hoist and the concrete is carried up to a climbing hopper at the working deck level. From this temporary storage place the concrete is discharged through a quick-acting valve and inclined spout into small two-wheeled tip carts and, from these, into the walls themselves.

Single tower structures may use a single concrete receiving tray which revolves round a centre pin at the working deck level.

The hoist tower is sometimes erected to its full height before placing begins and the guys are transferred later to the new wall

(3).

In most cases it contains the man hoist or ladder system providing access to the working deck, as well as the electricity,

water supply, and climbing job crane to raise the steel reinforcement. In hot or windy weather, canvas screens may be needed to

prevent excessive drying out of the green concrete below the slip-forms

(3).

In very cold weather steam is introduced behind tarpaulins or similar coverings to promote the rapid curing of the concrete.

In this way, and by using enclosed scales and steam heated bins, it is possible to continue placing in sub-zero weather at the usual rate of

15

feet per day

(5).

Early curing is important because it controls the vertical movement of the forms.

The sliding action limits the wall thickness to a minimum of 6 inches. Below this value the weight of green ooncrete in the form does not balance the upward friction foroe assooiated with the slip-form movement, and holes may result due to sticking. A minimum 6-in. thiokness is usually necessary in any case to provide adequate cover for the reinforoement.

If placing must be stopped for any length of time, the forms must be jaoked clear of the wall until the top is 20 inches below deck level. This gives sufficient clearanoe between the staves and conorete to prevent stioking and distortion when the movement is started again. The applioation of grout to the top of the wall is advisable before work resumes.

Pumping and vibrations of any sort should be avoided on the working deck itself as this is also liable to oause distortion of

4

-CONCRETE

A fairly dry mix is required and, although ordinary portland cement is most cornmonlyused, there are advantages to be gained by air-entrained and high early strength cements,

depending on the weather conditions. Generally in winter a high early strength cement is used, and tropical climates often require a slow setting cement (1, 3). A 28-day strength of 3500-4000 Ib is commonly required.

FORMWORK (1, 3)

The slip-forms in grain elevator construction are usually made up of (1" x 4ft

) wooden staves of B.C. pine soaked in linseed

oil and between 3 and 4 feet long, placed vertically with a 1/16-to 1/8-in. gap between each 1/16-to allow for swelling. Three-quarter-inch plywood is used in some cases, and the outside section is often made 6 inches higher to protect scaffold workmen from spilt conorete.

The outside ring of staves is usually plumbed vertical, but the inside one is given a 1/4-in. taper to assist the sliding action, with the nominal wall thickness given by the spacing of

the two forms about half-way down.

The two rows of waling which ring the staves at the top and 12 inohes from the bottom, are usually a composite arrangement of three 2 in. x

6

in. planks and form the connection to the yoke.

This yoke, often a 290 Ib wooden frame but preferably of pressed steel (125 1b), straddles the top of the wall and contains the jack itself. It supports the form with built-in lift rods of 3/4-inch steel passing through the wa1ings, and its vertical legs extend down along the forms to aid alignment. An aluminum yoke, which would reduce the weight of the moving platform considerably, has been investigated (3).

The staves are diagonally braced on each side of the yoke to prevent sagging of the form between these jacking positions.

In large cells additional bracing is required across the centre portion from wall to wall. Two rolled steel joists have been used, but a ｬｩｾｨｴ･ｲ timber truss is preferred.

A I-inch decking is placed on top of 2 in. x

6

in. floor joists at l2-inch centres which rest, in turn, on top of the bracing and it is this integral system that supplies the rigidity so

necessary for slip-form work.

Window frames and door lintels can be placed in the space between the forms and held there as the slip-forms move past. In a similar manner wooden blocks can be made to leave ｫ･ｾ｡ｹｳ and chases for the interior beams and floors to be added later

(6).

It is

:5

-even possible to block out an entire wall section and place the columns alone. Timber bracing in the wall space between storys is used to support the soffit forms for the beam spanning such openings. This bracing is usually placed to give support to the

jacking rods at 2-foot intervals, and prevent them from buckling under load.

The placing is usually controlled to enable this blocking-off to be done during the day because it is essentially a manual operation and night lighting may be difficult.

At roof level, the forms are temporarily locked to the walls by driving waxed spikes through the green concrete just

below the upper ring of wales. Removal of the yokes is simplified by providing large washers at the end of the lift rods supporting the waling.

In Norway, floors have been placed after the wall shell has been completed, and using the working platform as it was

lowered to the ground. In more recent years each floor has been cast on fixed forms when the upward moving wall form is some

4

1/2 feet above the floor level (2).

The total area of the working deck has varied from a few hundred feet to a few acres, and is only limited by settlement considerations. Where settlement is anticipated, particularly on a long nar-r-ow structure such as a string of grain silos; tho

building may be constructed in two or three sections with a 3-foot gap bet.:een each. Insufficient floor space may necessitate a

seeond floor, known as the pouring floor, some 8 feet above the

working deck, or possibly a rigid balcony ringing smaller structures.

JACKS AND JACKING

Originally, a screw jack with a 1/2-inch pitch was used for raising the slip-form, but recent years have seen the development of hydraulic, pneumatio and electrical jaok systems.

Each system requires the jack to olimb a I-inch diameter

jacking rod embedded in the green conorete walls. Recent improvements make it possible to join these rods with steel screw plugs instead of 6-inch sleeves, and make them wholly recoverable by surrounding them in a thin steel tubing

(7).

These rods are usually

8

to 16 feet long and are initially placed in the wall in random lengths, or, at least in two distinct lengths, so that the weight of the slip-form can be taken on one half while the other half is being joined. These rods are spaced

7

to

8

feet apart but this depends on the number used and loads to be taken.

6

-(i) Screw Jacks These usually have been a 2-inch diameter tube type wfth a 172-inch pitch and a spring-loaded jaw clutch, but more recently a sleeve clutch and ball-thrust bearing have been used (1).

A spe otaL adaption (Thomsen - Simplex) (5) uses a ratchet type of jack climbing a special rack 18 inch long which is held in place on the jack-rod by an eccentric dog. After climbing tho full length, the rack is raised another 18 inches and the operation

repeated •. Set screws have been used (instead of eccentric dogs) to fasten these climbing racks (or sleeves) to the jack-rods.

Although these manually operated jack systems are mechanically simple, one man would find it difficult to handle more than 15 jacks, and since a big job may use 500 to 600 jacks, a large labour force is necessary. Balanced against this large labour force is the

higher installation and maintenance cost of the power-operated jacks Which, however, require only one operator.

(ii) Hydraulic Jacks (8) Developed in Sweden, and used there almost exclusively for-all slip-form work today, this type claims increased speeds, uniform and simultaneous lifting, ease of operation and, of course, reduction of labour force.

A central motor builds up pressure, which forces a lower set of jaws to grip the jack-rod, and then causes a cylinder with a set of upper jaws to rise, bringing the slip-form with it and compressing a return spring. When the pump stops the pressure

drops and the upper jaws prevent the weight of the form from pulling back the jack. The compressed return spring raises the bottom jaws to the start position ready for another cycle.

The pump builds up equal pressure in all lines to each jack, and gives a uniform I-inch rise every cycle (one direction only). Manufacturers of some jacks claim a vertical rise of 20 inches per hour.

Lifting capacity at 1,400 psi is

6

tons for large jacks and

3

for small.

Hydraulic systems may need special fittings built in at ground level to enable quick cutting of hydraulic lines when a section of formwork is left behind for roofing at a lower level. If the form junction itself is pre-cut on the ground, this lower roof may be severed in less than 15 minutes without disturbing the continuous pour

(9).

Initial levelling is most important, and the alignment is subsequently checked from marks on the jack-rods or a vertical scale built into the hoist tower or finished wall, and also from plumb lines on reels mounted on the climbing forms and having ground level references. A vertical tolerance of 1/2 inch in 50 feet is a common figure.

7

-Automatic levelling may be obtained with the use of a combination hydraulic-electric system.

(iii) Electric Jacks and Automatic Levelling

(3)

In this system each jack is mechanically operated by a

172

lip motor mounted on the yoke. Each motor has a "brain box" containing a float valve and mercury switch and connected to a central "master tank" by rubber hose.

The form is raised by elevating the master tank between 1 and 2 inches, waiting five minutes for the water level in each float cylinder at each jack to reach the level in the tank, and then starting the motor that operates the jacks. Each jack has a lifting capacity of ｬｾｯ tons.

On reaching the desired level the motors are automatically switched off by mercury switches although not necessarily all

together. The stopping of the last jack motor means the working deck is automatically level. Again it is most important that the working deck be perfectly level before jacking commences.

(iv) Air Jacks (10) Although no details are available there is a record

of

ｾＴｯ air jacks operated by two men allowing the placing of 24,000 yds in less than three weeks at an 18 foot per day average.

STEEL

.

In a typical, double-wall construction, the vertical rein-forcement in the outer wall is I-inch jack-rods at 7-foot centres plus 1/2-inch steel at 2-foot centres and for the interior wall the I-inch jack-rods are sufficient in themselves

(3).

Some

designers rely on horizontal reinforcement alone. Horizontal rein-forcement is usually 5/8 inch at l2-inch centres although many prefer 1/2 inch at 8-inch centres. Hooked bars are used only in slabs and beams, and there is usually no time for tying as this delays the concreting. Bars can be bent at floor level and left projecting for welding to the floor slab, and the wooden inserts for forming the floor chases are usually tied to the jack-rOds. The jack-rods can also be marked to show the horizontal reinforcement placing.

The steel, already bent at ground level, is usually stored on or above the working deck to avoid delay.

ORGANIZATION

As previously stated this slip-form method lends itself ideally to projects that can be carefully planned and controlled.

Large savings of materials, time and labour can be effected, resulting in lower costs.

8

-The method can be used to assist other construction operations. In one case, the roof slab was placed at ground level and taken up as the working deck, and a steel factory roof franle was assembled on the ground, taken up as part of the working deck, and then lowered to the finished wall by the same jacking system. The working deck, after being used as the lower roof shuttering, has been lowered from the finished slab and has served as shuttering for each floor on the way down.

Floor beams and interior partitions can be placed on the way up, although this requires a larger, more experienced labour

force and takes somewhat longer.

CONCLUSION₄

Although this method of construction had its origin 1n Canada it owes its present highly adaptable form to European use. There it has been used to real advantage for almost every type of building with a regular floor plan. This flow area may range from a few hundred square feet to well over an acre, and the very real economy increases with heightL

Perhaps the most important aspect of its use in Canada, however, 1s the simultaneous production of a protective building

shell to allow finishing work inside in freezing weather. All the construction work is done at one level (that of the working deck) and, therefore, protecting the workmen from the weather is a

relatively simple matter. A simple steam cover below the slip-form produces the required concrete strength in a short time and also protects the new work from frost.

REFERENCES

2.

4.

6.

Broughton, H.H. Moving forms for concrete construction,

Reinforced Concrete Review, Vol. 2, No.7, Oct. 1951, p.407.

Sliding Formwork. Architectural Forum, Vol. 97, No.5, Nov. 1952, p.150.

Stout, D.F. and R.E. Wilde. Automatic jacks speed sliding form construction. Journal American Concrete Institute, Vol. 53, No.5, Jan. 1952, p.38l.

Continuous pour raises silo fast. Construction Methods and Equipment, Vol. 35, No.1, Jan 1953, p.64.

Sliding-forms for tall television relay building. Construction Methods and Equipment, Volo 32, No.2, Feb. 1950, p.68.

van Erp, J.W.T. Current reviews. Journal American Concrete Institute, Vol. 27, No.2, Oct. 1955 reviewing Cement Vol. 24, No.5, Jan. 1953t p.232•

9

-7. Hydraulic jacks make slip-forms more versatile. Construction Methods and Equipment, Vol. 35, No. Ｕｾ May 1953, p.64.

8. Something new in slip-forms. Construction Methods and Equipment, Vol. 32, No.1. Jan. 1950, p.78.

9. Slip-forms pour tall feed mill. Construction Methods and Equipment, Vol. 37, No.2, Feb. 1955, p.74.

10. Concrete forms raised by pneumatic jacks. Roads and Engineering Construction, Vol. ＳＴｾ April 1956, p.94.

Bibliographz

MacDonald, J.W. Slip form speedily erects grain building. Civil Engineering, Vol. 21, No.3, March 1951, p.39. Hunter, L.E. Construction with moving forms. Concrete and

Construction Engineering, Vol. 45, Nos. 3-8, March-Aug, 1950, p.75.

ｍ｣ｋｾＬ W.R. Reinforced concrete construction with continuously

moving forms. Engineering and Contract Record, Vol. 65, N0.3, March 1952.

Holt, Ralph. Slip forms reduce cost of tall bridge piers. Civil Engineering, Vol. 20, No. 12, 1950, p.39.

Slip-form lines deep shaft. Construction Methods and Equipment, Vol. 37, No.5, May 1955. p.10.

Robinson, W.J. and L.H. Tuthill. Better concrete in slope paving by use of slip-forms. Journal American Concrete Institute, Vol. 27, No.1, Sept. 1955, p.1.

Woodford, T.V.D. Slip-forms for concrete canal lining. Journal American Concrete Institute, Vol. 23, No.8, April 1952, p.637.

Climbing shuttering for danish 'point houses'. Builders Digest, Vol. 12, No.5, May 1952, p.163.