Good Practices

Here are some good practices that should be followed to prevent likely defects from happening in future.

1. Site Organization Layout
2. Material Handling
3. Transportation
4. Delivery
5. Use of Cranage and Hoists
6. Unloading
7. Lifting and Handling on Site
8. Handling
9. Stacking
10. Stackyard Preparation
11. Storage
12. Setting Out
13. Damage Control
14. Safety in Crane Operation

Site Organization Layout

A. Temporary Storage

The temporary storage area designed to be adequate to permit easy access and ready for handling of the stored precast components. It shall have a clean, hard, level and well-drained surface to permit well-organised storage, and to prevent warpage, bowing, chipping, cracking, discolouration, staining or soiling of the precast components. No precast components shall be placed on the ground. They shall be stored with due protection from staining or physical damage.

B. Handling

The precast concrete components shall be handled with proper handling technique and transported in appropriate manner in order to avoid excessive stresses or damages. Precast components shall be lifted only at appropriate points shown on the organization layout.

Material Handling

A. General

It is essential that precast units be correctly handled during delivery, storage on site and erection. It is in the handling of units that the greatest damage and waste occurs. Improper handling may cause damage which is difficult or impossible to repair, or total failure, or perhaps a serious accident.

The risk is further increased where insufficient attention is paid to the scheduling of deliveries and the storage of units on site in the order in which they are to be used. Lack of planning at this stage may well delay the contract and cause unnecessary congestion on site, as well as requiring unnecessary carnage time due to double handling; additional handling thus increases the risk of damage unnecessarily.

B. Supply and Delivery

Close co-operation between site and supplier is required to ensure that units are delivered at the proper time and in the correct order. Demands, which result in units being delivered to site too far in advance, break into the manufacturing time available to the supplier and result in costly and unnecessary storage space, handling and protection on site. By means of a well-planned and mutually agreed delivery schedule, units can be moved from the supplier’s yard to the site and into a structure with the minimum amount of handling and delay.

Units should be marked clearly in a way it is understood by the site personnel. This is especially important where there are only slight visible differences between units or where outwardly similar or identical units incorporate variations in reinforcement, prestressing and other detailing.

Transportation

Usually during transportation, it is advisable for the contractor to establish contact with the local police so that suitable off-site parking places or vehicle routings can be arranged; delay in delivery which result from traffic congestion can upset the sequence of delivery and cause serious delay of the construction programme.

Careful attention to delivery arrangements is particularly important where local condition restrict constructional operations to a fixed time.

Transport regulations set out conditions of axle loadings and these demand care in positioning certain components. For the more critical units a drawing should be prepared which indicates the position and level of the pads, which actually come into contact with and support of the structural units. Vehicles should be filled with frames and supports appropriate to the load, and wither steel, timber or concrete frames should be provided for particular shapes of components.

Delivery

Where there is some projection, the load should be arranged in such a way that the driver has a clear view of the projection from his cab and, of course, the statutory arrangements regarding advice of journey and provision of assistance to the driver must be observed.Long units are frequently moved on turntable vehicles used in conjunction with multi-wheeled bogies. All routes must be surveyed for features such as projecting buildings, telegraph lines and natural obstacles likely to be the cause of accident; special care should be taken over road cambers and bridges, both from the point of view of the balance of the load and clearances.

Even the simplest loads will need inspection to ensure that any possibilities of the load moving in transit is ruled our and the driver should check the load regularly in the course of a long journey.

A few simple precautions can save endless trouble and expenses:

1. The flat body of the vehicle should be always swept clear of concrete particles.
2. Projections should be visible from the cab.
3. Clean, regular shaped battens should be used in stacking.
4. The driver must check the load at the time of acceptance and throughout the journey.
5. The driver should be instructed in protecting loads from possible damage due to chains and shackles.
6. The load should rest on properly designed stillages.

However, there are a few points which need to be pay attention to during the material handling stages as mentioned below:-

Delivery – Position of Loads

Vast majority of precast units are designed to sustain loads resulting from handling loads in one direction only. They will be delivered in the right position and in all subsequent handling and storage must be in that same position, otherwise they may be damaged beyond repair. Any damage may well be invisible and only become apparent when the unit begins to fail under working load. Correct orientation of the unit is critical and appropriate face should be clearly marked ‘top’ and great care taken to ensure that the units are not turned during handling.The aim of the precaster is to devise a system, which required the minimum amount of handling of precast units between manufacture and delivery. Also every time components are handled there is the danger of damage to the unit; lifting operations require time and labour, which could be applied to a more productive effort.

Use of Cranage and Hoists

The driver is well placed to control the load and has a clear view of operations on the workshop floor.

Unloading

At the time of unloading at site it is suggested that a skilled banksman carries out the unloading operations of slinging and directing crane movement. The driver can unchain and unbolt units and sweep away small stones and dust but it is essential that the actual off-loading is skillfully handled.

The crane hook must be correctly positioned, the brothers or slings properly adjusted and the lift made such as to avoid slewing of the load, which may damage the unit or vehicle.

Some panel units need to be carefully supported as they leave the ‘A’ frame support and the bottom of panels must be guided clear of the stillages to avoid nib or corner damages.

Articulated vehicles can present problems on site particularly where there is any difficulty of access or where road conditions are bad. As they are without driven wheels, the trailers of articulated vehicles can quickly become bogged down and cause delay to subsequent operations, both with regard to transport and erection.

Lifting and Handling on Site

Therefore, it is vitally important that the suitable lifting tackle, such as spreader frames, be provided so that units are properly slung and ready for landing in the correct positions. Serious damages can be caused as a result if units hanging or striking against each other during handling, when a number of units are suspended from slings or chains.

Damage to corners and particularly to bearing areas of beams and panels can be extremely dangerous, every care should therefore be taken, when slinging or landing units, prevent corners and corbels from striking other units or becoming damaged by unintentional point or impact loadings.

A qualified person must adequately test any special fabricated lifting rig. Proper regard for safety requirements is essential. Lifting and handling plant should be adequate in all respects, properly maintained and regularly inspected at the beginning of the contract and at specified intervals. All chains, slings and other lifting tackle should be inspected in the yard or works between contacts.

Hand signals used for lifting, lowering, slewing and so on must be thoroughly understood by all concerned. Particular care should also be taken to minimize the effects of wind when cladding panels and other large flat units are being lifted since a sudden gust of wind can cause damage or a serious accident if the panels are not adequately steadied during lifting, by a bracing rope or line.

Handling

Mechanical Handling of Precast Units

The basic requirement of any scheme for the mechanical handling of materials, products or components is that vertical and lateral movements of material should be minimized within the requirements of the manufacturing process.

It should be reduced to a minimum amount of effort required by the operatives engaged in the manufacturing process, and the method used should be capable of being operated or supervised by semi-skilled operatives.

Mechanical handling arrangements should be considered as an integral part of the manufacturing process, and ideally handling and movement operations should be designed into the overall manufacturing operation.

Handling in all directions requires careful consideration in lateral transfer between machines, lifting, lowering, and to a large extent, turning.

Double handling and subsequent looping of the line of process should be avoided. The process should be as simple as possible.

The number of loads, the actual mass of individual loads and the desirability of travel at particular stages during manufacture should be taken note during the planning of the handling.

The followings should be taken into consideration during the decision on suitable handling and transportation arrangement.

• Direction of travel
• Size and mass of units
• Size available handling space
• Frequent of movement
• Desirable rate of movement and distance to be covered
• Whether fragile or fully matured units are being handled
• Means of support during handling

The first consideration is the direction of travel of goods or materials that require to be elevated or lowered, as this may lead to special problems. If the arrangements call for units to be turned or specially oriented relative to the workplace, then extra, possibly more expensive, equipment will need to be installed.

Horizontal travel at the working level requires excessive space. The cost of some sort of mechanical arrangement foe elevating or lowering goods may well be recovered by saving on expensive working space. Headroom is an important factor and the actual space taken by the handling equipment, whether it is a crane or a monorail hoist, must be taken into consideration.

Sometimes calculations may need to be carried out by a competent person to ensure that the structure is capable of withstanding both static and moving loads. Care should also be taken to ensure that vibration and dynamic loadings, caused by the braking and sway of loads, do not cause damage to the structural frame for the building.

Stacking

Stacking and storage areas should be arranged so that access causes a minimum amount of interference to site operations, but at the same time is convenient for the actual erection operations so avoiding unnecessary handling of units. If space permits, the stacking area should be far enough away from the access roads to avoid splashing and staining; Units should, wherever possible, be stored well away from fuel stores and other obvious sources of staining.

Stack Yard Preparation

Properly prepared foundations and roads are essential for stacks and a planned approach to the provision of these facilities can result in their achievement with the minimum of cost.

Topsoil and vegetation should be cleared mechanically and edge form installed to contain dumped materials from rejected units, broken units and waste concrete from washing and cleaning. Large units placed within the area can be broken up at some convenient time and the final slab reinforced with offcuts and scrap steel. These materials when suitably compacted and provided with a layer of concrete, applied as a surcharge and tamped into position providing an excellent running surface for wheeled vehicles and standing area for units and stillages.

Consideration must be given to drainage such that falls are installed to sumps of soak ways.

The area should be simple to maintain and its surface kept clean to protect the ground beneath from water penetration and to reduce settlement. Where exposed aggregate work is concerned the concrete yard surface will reduce the tendency for a stained area to form where clay and mud become splashed onto units during heavy rain.

Stacking racks and stillages require careful attention to ensure that safe equipment is provided at minimum cost.

Concrete should be used wherever possible and any steel connection or pins required should be coated with plastics to prevent rust transmission onto the units in the stack.

The design of stacking frames for units stored vertically demands considerable thought to avoid the tendency for progressive failure of the stack. If the racks are to be made in concrete then they should be as solid concrete walls, if steel is used they should be effectively tied and braced.

A design engineer should be enlisted to calculate the necessary sectional sizes of members.

Space must be left to allow for minor error in the placing of panels into the rack and a provision should be made for each component to be individually wedged or clipped into place by an operative working from a safe position above the unit. When long panels are racked vertically it is advisable to provide a support at each end to endure that panels do not distort or twist during a long period of storage.

All items of handling equipment are designed to sustain loads applied in a given direction and while they are usually more than adequate for this purpose, use in applications other than those specified will inevitably result in damage, whether it involves the equipment itself or the unit being handled.

The safe working load for any given loading arrangement must be indelibly marked on the equipment and, wherever possible, positive stops arranged to eliminate the possibility of misapplication or incorrect loading.

Each time the unit is landed or turned it is likely to be damaged, strained or distorted. Any period of time during which the unit is allowed to stay in a strained or distorted situation may result in permanent damage. Units which are tuned into a position such that excessive strain is developed may fail.

A recognized handling arrangement should be established for each type of unit and the appropriate equipment should be available for use in turning and movement.

Provided care is taken in the use of clean batters, lined and leveled into place the best results will be achieved.

For successful handling and stacking to be achieved with the minimum of damage it is essential that a constant appraisal be made of the suitability of the approved methods. Examination should be carried out on the stillages and stacking frames to ensure that settlement has not spoiled their efficiency. Inspection of the non-standard units in stock should be made to avoid damage resulting from ad hoc or temporary arrangements such as occurs when a non-standard component fails to span between supports especially designed to support a typical unit, or when a splayed surface fails to take full and continuous support from a particular part of stacking frame.

Units stacked in what are nominally vertical stacking racks, but which are out of plumb due to badly placed backings or wedges, can become permanently distorted, or in ‘wind’ where bearers settle or move units, become overstressed and damaged, additional stresses may become imposed on adjacent components in stacks. Units in stacks that bear on stillages which settle may overturn and, quite apart from the damage to the units in the overturned stack, there may be disturbance of adjacent stacks with the possibility of accident of operatives working in the vicinity of the stack yard.

Timber bearers resting in badly drained ground or on concrete that is continually moist tend to rot or become slimy, with the result that the friction between the bearer and the ground slab fails. As a result, panels can either slide or the bottom of members, which nest into stacks can move causing accidents to workmen and damage to units. High stacking, due to delays in delivery, can impose massive loadings on the lower members in the stacks, and where sandwich units are concerned, local high spots can cause concentration of loads in a particular area of a panel or slab unit causing damage to the sandwich.

Where possible, deliveries made on the basis of first unit into stock first unit onto site, present the best and most consistent result, although in cases of difficulty it will prove helpful if, for example, the complete flight of units for a gable end, is set out and matched prior to delivery.

Storage

Level area, free from settlement under the anticipated weight of the stack, should be provided and made ready in advance. Timber battens, of a size sufficient to facilitate the fitting of slings or other handling equipment, should be located at the bearing or lifting points of the units; where heavy units are involved it may be necessary to set the ground level battens on pads of lean or rough concrete to prevent differential settlement.

Special care should be taken to make sure that levels are accurate and the battens do not produce leadings in excess of the design ones. As such loadings can easily cause failure in storage. Ensure that battens are clean and touch the concrete only where required for adequate support. Oily, greasy, creosoted or resinous timber battens or even damp timber batten can produce local variations incurring and therefore in surface colour, can cause permanent marks and stains on the surface of the units.

Precast units should be stacked and stored in such a way that sorting and double handling is kept to and absolute minimum. Make sure that the first units required are the most accessible and they should be stacked correctly handed so that turning is not required when the units are removed from the stack prior to erection, especially when dealing with long units such as beams. As that introduces not only unnecessary crane movements but increases the risk of damages, especially on a congested site.

Other damages which may occur during storage are projecting steel which is exposed to the weather for any length of time will rust, and the rust stains may well spread to exposed concrete faces; this can be prevented by painting the projecting steel with cement paste applied by brush or gloved hand.

Setting Out

The line and level arrangements must be under the control of an experienced engineer, capable of appreciating the design requirements of fixity and stability. Lining and leveling are generally carried out by means of a theodolite and dumpy level.

Levelling process – Ideally adjustable bolts or bearing nuts should be provided to mate with bearing plates cast into the adjacent units. Shimming is often used as a means of levelling and although this is a relatively cheap method, it can be prove dangerous in cases of limited bearing or where massive joints call for excessive number so shims with the likelihood of instability during erection.

Insufficient area of shim can result in spalling or failure of either of the bearing components, particularly where large components are being installed.

Bearing surfaces must be clean, square and of adequate dimension and it is essential that connections made at corbels and wall arises should be examined. In the event of the slightest doubt as to adequacy the bearing components should receive support from props and runners until a permanent connections has been made and concreted.

It is essential that the correct lifting equipment be used to avoid torque, torsion and eccentric loading on all components. Slabs can fail so easily when three out of four loops are concentric and the remaining loop is out of place. Similarly, long units lifted with short brothers tend to be put into a condition where lateral wind or bending is induced into the unit.

Communications between all parties involved in the erection process must be maintained either by signal, telephone or radio and operations must always be under the control of one skilled erection supervisor.

For speed of erection, and satisfactory results in joints, connections and waterproofing it is essential that the units delivered to site should have been subject to rigorous quality control during manufacture at the works.

The site must be able to rely on both a continuous supply of components and a service in the matter of plant, equipment and ancillary supplies, it must be remembered that often these services and suppliers are as important as the precast components comprising the frame or the structure.

Damage Control

Clearly, from all aspects of economic production the number of times a unit needs to be handled must be restricted, and arrangements should be made by the provision of equipment and training to take account of such equipment as is necessary to ensure that the minimum amount of damage is caused to the green concrete units.

Damaged units, which involve expensive repair works cause considerable upsets in production. Also a damaged unit will almost certainly require special treatment in routing from the shop additional handling and supervisory time. It is always difficult to assess the extent of damage within newly cast units, and this ultimately brings about prolonged arguments with regard to responsibility. Careful attention to the techniques employed in all stages will reduce the costs incurred as a result of damage, and reduce engineer and management time in decisions as to acceptability.
Savings can only be achieved if good understanding exists amongst the production personnel of the principles governing the handling process. For example, if supervisors do not understand the basic principles of reinforced concrete design they will not be qualified to ensure that units are correctly handle.

Repairs to Damaged Units

Minor damage to units can be successfully repaired on site provided the proper methods are used. Simple load tests can be carried out on repaired structural units while all major repairs should be discussed with the appropriate authority, so that testing procedures may be agreed upon.

Sand-cement mortar or finishing material must never be used for structural repairs; all such work should be carried out with concrete of a quality equivalent to that of the manufactured unit and all defective or damaged concrete should be cut away and surface prepared to receive the new concrete. If required additional reinforcement should be inserted. The repair material should be placed, compacted and adequately cured in a well-made temporary mould or form. A person with the necessary engineering training should supervise all stages of the repair.

Repairs to exposed concrete surfaces, especially those with exposed aggregate or textured faces, require a high level of skill if the final job is not to be obvious and unattractive. Repair work of this nature is highly specialized and it is advisable to consult the supplier who may be able to provide a specialist with the necessary skill and experience.

Safety In Crane Operation

In every construction project, it is important that sufficient load carrying capacity is available within the operating radius to pick up and deposit the loads.  Generally, the tower crane should covers 100% of the required building area.

For details, please refer to the LTA Code of Practice for Railway Protection.

To ensure quality in the construction, the following points need to be observed:

• Gaps between formworks must be checked to prevent grout loss.
• Formwork must be checked for Verticality and Alignment.
• Ensure that formworks are propped and braced sufficiently to achieve correct sizes, shape and alignment.
• To ensure that the reduced levels are checked and marked before concreting of slab top.
• To ensure sufficient cover is provided for reinforcement