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LUSAKA WATER SEWERAGE COMPANY
Terms of Reference for Construction Services
(Example)
Table of Contents
1.Objectives of the Lusaka Water and Sewerage Company
2.Tasks of the Contractor
3.Construction Procedures
4.Inspection of Construction
5.Concreting and Form Work
6.Surface Finish
7.Laying of Pipes
8.Timeframe, Reporting and Budget
9.Design Objectives and General Recommendations
LUSAKA WATER SEWERAGE COMPANY
Terms of Reference for Construction Services
1.Objectives of the Lusaka Water and Sewerage Company
The objectives the Lusaka Water and Sewerage Company (LWSC) intends to achieve it the low income urban areas found within its service area are:
- To improve public health and hygiene in all the low-income urban areas by increasing their access to safe water and proper sanitation.
- To ensure that water supply to low income areas is sustainable. This implies achieving a balance between the social dimension of water supply (making safe water accessible for all residents), the financial and commercial dimension (covering costs) and the quality of the infrastructure.
2.Tasks of the Contractor
Objective: To efficiently and effectively carryout construction works of the Project.
Specifically the Contractor shall carryout all the construction works according to acceptable standard of construction, some of which have been specified in this Terms of reference (ToR) document.
3.Construction Procedures
The Contractor’s role in a Project is fundamentally to build to the estimated cost, to the specified quality and within the agreed time. The Contractor must organise the Project to achieve its completion within this period.
a)Implications of delays
Delays cause additional costs, perhaps through ineffective resource utilisation or increased overheads. Delays are seen as lack of achievement on site.
b)Payments
The Contractor shall perform the services and carry out his obligations hereunder with all due diligence, efficiency quality and economy. The Contractor shall always act, in respect of any matter relating to this Contract as faithfully and shall at all times support and safeguard the Client’s legitimate interests in all the construction works.
Payment to the Contractor shall thus only be effected upon the certification that all the construction works are completed according to the laid down standards and the works shall be certified complete and satisfactory by the Technical Consultant and the Trust Fund Engineer.
c)Concrete Mix
The Contractor shall conform to the concrete and mortal strengths required in the design of the super structure for concrete and mortal mix ratios respectively.
Concrete should be of the relevant quality; this depends on both its constituent materials and mix proportions. There is a need to avoid some constituent materials which may cause durability problems to mix with the concrete. All the mixing of the concrete aggregates shall be done using standard tins for the purpose of achieving the required strength of the concrete works as per design specification. This shall apply for all the Trust Fund supported Projects and the Contractor shall make available the tins on site before the commencement of the any construction works.
d)Durability
To produce a durable structure requires the integration of all aspects of standards relating to materials and construction. The environmental conditions to which the concrete will be exposed should be a factor in the determination of the final quality of concrete to be produced of which the Contractor shall define in the beginning.
e)Control of Cover
The Contractor shall exhibit good workmanship to ensure that the reinforcement is properly placed and that the specified cover is obtained.
- 5 mm on bars up to and including 12mm size.
- 10 mm on bars over 12mm up to and including 25mm size.
- 15 mm on bars over 25mm size.
f)Curing of Concrete
The Contractor shall ensure good workmanship, particularly during the curing process of concrete which shall be well controlled. The Contractor shall ensure that no loss of moisture from the concrete during this process.
The Contractor shall allow for curing during which the surface has to be keptmoist for at least 14 days. This process of curing and protection should start immediately after the compaction of the concrete to protect it from:
a)Premature drying out, particularly by solar radiation and wind.
b)Leaching out by rain and flowing water.
c)Rapid cooling during the first few days after placing.
d)High internal thermal gradients.
e)Vibration and impact which may disrupt the concrete and interfere with its bond to the reinforcement.
g)Curing Methods
The Contractor shall use the available methods of curing such as:
a)Maintaining formwork in place.
b)Covering the surface with an impermeable material such as polyethylene, this should be well sealed and fastened.
c)Spraying the surface with an efficient curing membrane.
d)Covering the surface with a damp absorbent material.
e)By continuous or frequent applications of water to the surface, avoiding alternate wetting and drying and the application of cold water to warm concrete surfaces.
NOTE Methods d) and e) should not be used when there is the possibility of freezing conditions.
4.Inspection of Construction
The LWSC engineer shall carryout inspection procedures to ensure that the construction is in accordance with the design. This will cover materials, records, workmanship and construction.
Where disputes arise on the quality of mortal and concrete, tests shall be made on concrete in accordance with the relevant standards; this testing shall conform to BS 5328.
5.Concreting and Form Work
a)Concreting in Hot Weather
The Contractor shall ensure that precautionary measures are taken into during concreting during hot weather to avoid the loss of moisture and/or rapid stiffening of the concrete which prevents its proper compaction. High temperatures and loss of moisture after compaction can cause thermal and plastic cracking and a reduction in strength and durability.
The temperature of the concrete can be reduced by cooling the water and aggregate; the temperature of the cement is less important although the use of warm cement is best avoided.
Particular attention should be given to placing, compacting and protecting the concrete as soon as possible after mixing. To prevent moisture loss from the concrete, the curing of surfaces not protected by forms should commence immediately after compaction. If the surface is not to be marked, it is best to provide the initial curing with sheets of impervious material, preferably pigmented to reflect radiation, supported away from the surface and fastened at the edges to prevent droughts.
b)Formwork
Design and Construction
When designing the formwork, the Contractor should take account of safety and quality of the surface finish required.
The formwork should be sufficiently rigid and tight to prevent loss of grout or mortar from the fresh concrete. Formwork and its supports should maintain their correct position and be to correct shape and profile so that the final concrete structure is within the limits of the dimensional tolerances specified. They should be designed to withstand the worst combination of self-weight, reinforcement weight, wet concrete weight, concrete pressure, and construction and wind loads, together with all incidental dynamic effects caused by placing, vibrating and compacting of the concrete.
The formwork should be capable of being dismantled and removed from the cast concrete without shock, disturbance or damage. Where necessary, the arrangement should be such that the soffit form, properly supported on props, can be retained in position for such period as may be required to achieve the required strength and satisfy the specification. If the component is to be pre-stressed whilst still resting on the soffit form, provision should be made to allow for elastic deformation and any variation in mass distribution.
Cleaning and Treatment of Forms
Rubbish should be removed from the interior of the forms before the concrete is placed. The faces of the forms in contact with the concrete should be clean and treated with a suitable release agent, where applicable.
Release agents should be applied so as to provide a thin uniform coating to the forms without contaminating the reinforcement.
The formwork will be removed as soon as possible after concrete has set, to avoid thermal cracking. The formwork should be removed without shock, as the sudden removal of wedges is equivalent to an impact load on the partially hardened concrete. Materials and plant should not be placed on any new construction in such a manner as to cause damage.
6.Surface Finish
a)Type of Finish
A wide variety of finishes can be produced. Surfaces cast against forms may be left as cast, e.g. plain or profiled, the initial surface may be removed, e.g. by tooling or sandblasting, or the concrete may be covered, e.g. by paint or tiles; a combination of these techniques may be adopted, e.g. a ribbed profile with bush-hammered ribs.
Upper surfaces not cast against forms may be trowelled smooth or profiled, e.g. by tamping; the initial surface may be removed, e.g. by spraying, or it may be covered, e.g. by a screed or plastics floor finish. When selecting the type of finish, consideration should be given to the ease of producing a finish of the required quality, the viewing distance and the change of appearance with time.
In the case of external surfaces, account should be taken of the weather pattern at the particular location, any impurities in the air and the effect of the shape of the structure upon the flow of water across its surface. Such considerations will often preclude the specification of surfaces of uniform colour as these are very difficult to produce and deteriorate with time, particularly if exposed to the weather. Unacceptable deflection may be defined as deflection that is harmful for the member concerned.
b)Quality of Finish
The Contractor shall ensure a high quality finish which will be easy to clean and visually pleasing; the Contractor shall be required to use experienced personnel conversant with the production of finishes and close collaboration with the site.
The finish will be usually obtained by the use of proper formwork or moulds of timber, plywood, plastics, concrete or steel. Small blemishes caused by entrapped air or water may be expected, but the surface should be free from voids, honeycombing and other blemishes.
The Contractor can also obtain good finish by the use of high quality concrete and formwork. The concrete should be thoroughly compacted and all surfaces should be true, with clean arises. Only very minor surface blemishes should occur. The surfaces should then be improved by carefully removing all fins and other Projections, thoroughly washing down, and then filling the most noticeable surface blemishes with a cement and fine aggregate paste to match the colour of the original concrete. The release agent should be carefully chosen to ensure that the concrete surface will not be stained or discoloured. After the concrete has been properly cured, the face should be rubbed down, where necessary, to produce a smooth and even surface.
c)Production
The quality of a surface depends on the constituents and proportions of the concrete mix, the efficiency of mixing, the handling and compaction of the concrete and its curing. The characteristics of the formwork and the release agent may also be of critical importance. The Contractor must ensure that all requirements that are needed to contribute towards the achievement of producing the required type or quality of finish should be put in place.
d)Inspection and Making Good
The surface of the concrete should be inspected for defects and for conformity to the surface finish specified and where appropriate, for comparison with approved sample finishes.
Subject to the strength and durability of the concrete being unimpaired, the making good of surface defects may be permitted but the level of acceptance should be appropriate to the type and qualities of the finish specified and ensure satisfactory permanence and durability.
On permanently exposed surfaces great care is essential in selecting the materials and the mix proportions to ensure that the final colour of the faced area blends with the parent concrete in the finished structure. Voids can be filled with a fine mortar, while the concrete is still green or when it has hardened. Fine cracks can be filled by wiping a cement grout. Fins and other projections can be rubbed down.
7.Roof Design
a)Construction of the Roof Slab: Formwork
The design and construction of formwork should take account of safety and of the surface finish required. The formwork should be sufficiently rigid and tight to prevent loss of grout or mortar from the fresh concrete.
Formwork and its supports should maintain their correct position and be to the correct shape and profile so that the final concrete structure is within the limits of the dimensional tolerances specified. The formwork should withstand the worst combination of self-weight, reinforcement weight, wet concrete weight, concrete pressure, and construction and wind loads, together with all incidental dynamic effects caused by placing, vibrating and compacting of the concrete.
The formwork should be removed without shock, as the sudden removal of wedges is equivalent to an impact load on the partially hardened concrete. Materials and plant should not be placed on any new construction in such a manner as to cause damage.
Small blemishes caused by entrapped air or water may be expected, but the surface should be free from voids, honeycombing and other blemishes.
b)Construction of the Roof Slab: Concrete mix
The Contractor shall conform to the concrete strengths required in the design of the slab concrete. All the mixing of the concrete aggregate shall be done using standard tins for the purpose of achieving the actual mix of concrete. This shall apply for all the Trust Fund supported Projects and each Contractor shall make available the tin before the commencement of the any construction works.
c)Roof Slope
The kiosk roof slab shall have a tilting angle of 0.02 to allow for free draining of rain water.
8.Laying of Pipes
a)Laying of Pipes
The trench design and depth for pipe lying for PVC or HDPE pipes should take into account the combined loading of pressures especially imposed loads such as vehicles and also other natural phenomenon such as erosion of the sub-surface by the wind and water which may affect or expose the laid pipe in future.. A minimum soil cover of 75cm shall be provided for above the pipe surface. When a pipe crosses a roadway a minimum depth of 100cm of earth shall be provided above the pipe diameter. This shall apply to all the Water Supply Projects supported by the Trust Fund.
b)Trench Cutting
Before laying a pipeline, the preliminary work required should include pegging out, clearing and disposal of all debris from the route that could inflict damage on the pipe.
HDPE/AC/GI pipes shall be laid only in approved trenches with the correct depth. The trench shall be such that the pipe can be laid to the required alignment and at the required depth. A minimum soil cover of 75cm to 100cm shall be provided for above the pipe surface. When a pipe crosses a roadway a minimum depth of 100cm of earth shall be provided above the pipe diameter. The width of the trench should be kept to minimum considering the working space required and the site condition.
Whenever the soil condition at the bottom of the pit is poor, a suitable foundation for the pipe is to be made using other materials (in extreme cases).
If excavation is not possible to the minimum depth of 100cm as stated above, full factsshall be brought to the notice of the Peri-Urban Manager in writing giving details of location and reason for not able to excavate that particular portion to the required depth. Approval may be granted by the Manager in writing under certain genuine circumstances only. The decision of the Peri-urban Manager shall be final and binding.
The Contractor shall exercise due care to ensure that the HDPE pipe is not subjected to any damage or strain.
9.Timeframe, Reporting and Budget
The Contractor shall ensure that the Project is completed within the specified time schedule and budget. All payments to the Contractor shall be paid upon the satisfactory certification of the Project by the Engineer appointed by the LWSC
Abbreviations Used
AC:Asphaltic concrete
GI:Galvanised iron
HDPE:High density poly-ethylene
LWSC:Lusaka Water and Sewerage Company
PVC:Poly vinyl chloride
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Toolkit for Urban Water Supply Projects Module 6 ToR for Contractors