Inspection, Good Construction Practices, Workmanship and Codal Provisions
Inspection, Good Construction Practices, Workmanship and Codal Provisions
Inspection, Good Construction Practices, Workmanship and Codal Provisions
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GOOD CONCRETE
Concrete in the fresh state should be cohesive,so that it can be transported and placed without segregation. Many properties of concrete are related to its compressive strength: 1 Density 2 Impermeability 3 Impact-resistance 4 Abrasion-resistance
The compressive strength of concrete is dependent on W/Cratio of concrete. 28-day compressive W/C ratio strength of concrete 43-grade 53-grade (N/mm2) cement cement 30 0.47 0.55 35 0.42 0.50 40 0.37 0.45 45 0.35 0.41 50 0.30 0.37 55 0.28 0.33 60 0.32
QUALITY ASSURANCE
Qualily assurance in construction relates to:
1 Proper design. 2 Use of satisfactory materials and components. 3Proper workmanship in the execution of works and finally, 4 Proper care during the use of the structure including timely maintenance and repair..
The job of Q.C and Q.A would involve quality audit of both the inputs & outputs. INPUTS in the form of materials for concrete,WORKMANSHIP in all stages of batching,mixing,transportation,placing, compaction and curing & the related plant, machinery and equipment resulting in the form of concrete in place.
The Quality assurance Plan shall include: a)Test reports and manufacturer,s certificate for materials,concrete mix design details; b) Pour cards for site organization & clearance for concrete placement; c)Record of site inspection of workmanship,field tests; d) Non-conformance reports, change orders; e)Quality control charts& statistical analysis.
QUALITY CONTROL CHARTS ARE RECOMMENDED WHEREVER THE CONCRETE IS IN CONTINUOUS PRODUCTION OVER CONSIDERABLE PERIOD.
STANDARD DEVIATION
Standard Deviation is a good measure of Q.C. GOOD control means lower S.D (say 3 or 4 N/mm2). At least 30 test results should be used to calculate Standard Deviation. Each test result is average of 3 cube test results at 28 days.
CALCULATION OF S.D
S.D= Root mean square deviation X X/ X-X/ ( X-X/)2 1. 20 -1 1 2. 21 0 0 3. 22 21 1 1 n=5 4. 24 3 9 5. 18 -3 9 ---------20 S.D=[20/(5-1)]1/2 =2.27N/mm2
10 mm
4.75 mm
10-35
0-5
25-55
0-10
30-70
0-10
40-85
0-10
It is important to maintain the W/C at its correct value. For this,moisture content of coarse &fine aggregates should be determined as frequently as possible.
MIXING OF CONCRETE
The mixing time should be at least 2 minutes in a drum mixer. For more efficient mixers(in a batching & mixing plant or in an RMC plant),the mixing time may be about minute. (Manufacturers recommendation may be followed)
PLACING OF CONCRETE
The concrete shall be placed & compacted in its final position as early as possible & before initial setting of concrete. Care should be taken to avoid displacement of reinforcement or movement of formwork Max. free fall of concrete may be 1.5m.
CONCRETE ADMIXTURES
The dosages of retarders,plasticizers & superplasticizers shall be restricted to 0.5,1.0 and 2.0% respectively,by weight of cementitious materials,unless a higher value is agreed upon between the manufacturer & the constructer based on performance test.
COMPACTION OF CONCRETE
Concrte should be thoroughly compacted. Each 1% void can reduce the compressive strength of concrete by 5%. Over- vibration & under- vibration are harmful and should be avoided. Fresh concrete should be thoroughly compacted near the construction joints.
CURING OF CONCRETE
Exposed surfaces of concrete shall be kept continuously damp or wet condition by ponding or by covering with wet burlap,hessian etc.
For Concrete with blended cement or mineral admixtures(flyash or ggbs or silica fume), the curing period should be increased.
A white synthetic fiber mat with a white polyethylene backing that allows proper water content during the concrete curing process is being laid. They are lightweight and reusable.
CURING COMPOUNDS
Curing compounds may be used in case of scarcity of water or for highrise structures e.g bridge piers,RCC columns,chimneys and concrete roads. These compounds( wax-based or resinbased) are spray-applied on exposed concrete surfaces,as soon as possible after the concrete has set and when there is no water scene is visible on the surface.
CONCRETE COVER
Concrete cover should be adequate in concrete structures. Inadequate cover to reinforcements will result in corrosion of reinforcements.
20 30 45 50
Extreme
75
ENVIRONMENT
EXPOSURE CONDITIONS
Mild
Concrete surfaces protected against weather or aggressive conditions except those situated in coastal area
Moderate
Concrete surfaces sheltered from severe rain or freezing whilst wet, sheltered from saturated salt air in coastal area. Concrete exposed to condensation and rain. Concrete continuously under water. Concrete in contact or buried under nonaggressive soil/ground water
ENVIRONMENT
EXPOSURE CONDITIONS
Severe
Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immersed in sea water, and concrete exposed to coastal environments
ENVIRONMENT
EXPOSURE CONDITIONS
Very Severe
Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet. Concrete in contact with or buried under aggressive subsoil/ground water
Extreme
Surface of members in tidal zone. Members in direct contact with liquid/solid aggressive chemicals
ENVIRONMENT
EXPOSURE CONDITIONS
Abrasive
Concrete surfaces exposed to abrasive action e.g. machinery, metal tyres, etc. for this, specialist literature to be referred, for durability requirement
Slump required: 75-100 mm. Vacuum is applied through porous mats. (filter mat & suction mat) Applied vacuum is about 0.08MPa. About 20% of mixing water gets removed. Process takes about 15 to 20 minutes. Flyash or any other pozzolana should not be incorporated in the concrete mix. Usual floating,trowelling,brooming&curing are requred. Placed concrete bonds well with the old concrete(if any).