PRECAST SEWER MANHOLESCONSTRUCTION QUALITY CONTROL

Imtiaz Ur Rehman (B.Sc. Civil Engineer)

MANHOLE CONSTRUCTIONPRELIMINARY WORKS

APPROVALSShop Drawing ApprovalEquipment ApprovalPrecast Yard ApprovalMaterials Approval PRE-POUR INSPECTIONSRebarFormworkOpeningsWater stopHDPE LiningLifting Hook

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APPROVALS SHOP DRAWINGS APPROVALS• The shop drawings are got approved & issued for construction

(IFC). Precast elements are casted as per these approved drawings. Equipment / Tools & Plants Approval• Form Work• Vibratory Equipment

Precast Yard• Approved Site location of Precast Yard• Leveled, Impervious and Equipped with lifting facility• Concrete Curing Facility

APPROVALS• Transportation Facility• Sun shade & other welfare facilities

Materials Approvals• Concrete• Steel Reinforcement• Water Stop, Rebar Cover Spacers• HDPE Lining Material• Curing Agent

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PRE-POUR INSPECTIONS Rebar inspection (lap length,

spacing, bar dia, cover etc) Formwork Inspection (water

tight, strong enough to hold concrete, easy to remove

Openings Inspection (opening dia, opening location etc)

Water Stop Inspection (size, location, joint etc.)

HDPE Lining (size, location, joint etc)

Lifting Hook (capable of lifting weight of manhole including factor of safety)

SAFETY PRECAUTIONS ARE TAKEN INTO CONSIDERATION FOR ALL STAGES OF CONSTRUCTION

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MANHOLE CONSTRUCTION IN PROCESS INSPECTION POST-PROCESS INSPECTION

IN PROCESS INSPECTION Grade of concrete Cleaning Temperature of concrete Slump & Time of Batch Vibration/ compaction Crushing Strength POST-PROCESS INSPECTIONS Removal of Formwork Repair Works Iron Steps Curing Bitumen Coating Final Inspection through MIR

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IN-PROCESS INSPECTION GRADE OF CONCRETE (25 MPa Class Concrete-SRC/Type V cement. Concrete Specimen previously tested for

water tightness as per ACI 350.1) CLEANING The surface receiving concrete must be clean and free from dust, rubbish etc. TEMPERATURE OF CONCRETE

Not above 32 Degree Centigrade. Batching Plants have chillers to produce chilled water used for concrete production. More over, high temperature can be controlled by provide shade to aggregate or sprinkle water on aggregate.

SLUMP & TIME OF BATCH OF CONCRETE • Site Slump =7.5 with tolerance of 2.5 cm• Concrete to be delivered at site in 30 minute and to be poured within next 30

minutes. Number of drum revolutions of concrete truck not to exceed 300 during travel time.

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IN-PROCESS INSPECTION VIBRATION & COMPACTION OF CONCRETE Optimum vibration and compaction of concrete is assured

in order to get smooth compacted surface and to avoid segregation of the ingredients of the concrete.

CRUSHING STRENGTH OF CONCRETESpecimen cylinders of concrete are casted and tested for 7

days and 28 days age strength of concrete. For precast manholes, 25 MPa is required 28 days strength. On 7 days age 70-75% of strength is usually obtained and on 28 days age, 95-120% strength is achieved.

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POST-PROCESS INSPECTION REMOVAL OF FORMWORK Usually after 24 hours of concrete pouring, formwork

removal is carried out. REPAIR WORKS

If minor holes in concrete are observed, they can be repaired using 1:1 Cement Sand rich mortar. But if the pores are larger or reinforcement is exposed, the job done is straightaway rejected.

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POST-PROCESS INSPECTION FIXING OF IRON STEPS Iron steps coated with 2 layers of PE are driven into the

freshly exposed concrete as per drawing locations using hammer.

CURING OF CONCRETE7 days curing of concrete is done either by curing

compound or by sprinkling water after wrapping it in hessian cloth. However, there are several other methods of satisfactory curing of concrete e.g. ponding, steam curing, using moist sand etc.

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POST-PROCESS INSPECTION BITUMEN COATING After curing is complete, the manholes are applied with

two coats of bitumen epoxy paint. The second coat is applied after complete drying of first coat.

FINAL INSPECTION THROUGH MIRThe prepared manholes are finally inspected through

submission of MIR. After the manholes are approved, they are fit for use at site.

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