Footings are one of the most stressed members on concrete elements as it carries the entire building on it. Footings should be inspected strictly to ensure that the bearing surface is good enough to sustain the weight over it. Inspection of footing must be thoroughly conducted to ensure compliance with the drawing before it can be poured with concrete. To prevent reinspection which would delay the completion, it is advisable to have a checklist prior to inspecting a footing.
These are the four major points to consider when inspecting a footing.
Step 1: Check the depth and dimension
This is one of the most crucial part of footing inspection. Make sure that the depth from the natural ground line and its dimension is as per plan. Footing holds the entire structure. And the depth plays an important role on the carrying column’s slenderness ratio.
Furthermore, the depth of footing varies depending on the nature of soil beneath it. For sandy soil, footings normally have deeper and bigger columns compared to rocky soil. The soil or earth underneath should be thoroughly compacted with respect to the structural specification to prevent excessive settlement. You wouldn’t want your structure to be the next Leaning Tower of Pisa!
As Quality or Civil Engineer, you should have knowledge about how to do a compaction test although you are not the one who would personally conduct the test you should have few technical know how. The compaction test should have been done before laying gravel or pouring the blinding concrete.
Quality Engineer Tip:
• If the actual footing depth is lower than the formwork, it is advisable to fix marking either nail on the inner side of the formwork or depth sounding such as steel rod with marking.
• Don’t use any cut piece block beneath reinforcement as its strength is lower than the specified concrete strength of the footing.
Step 2: Check the Reinforcing Bars Sizes, Spacing, and Grades
Being the bone of the structure, reinforcements are important. Check the sizes of the reinforcing bars, the grades of steel, and its spacing if it conforms to the structural plan and specifications. Check as well the codes. Normally for footing exposed to earth, the minimum distance to the reinforcing bars is 75mm, but it varies depending on the designer. Knowing the distance from the earth exposed to the reinforcing bars is important to prevent seepage of water which may later cause rusting of reinforcement, diminishing its structural integrity.
Check as well the thickness of gravel bed underneath the footing and its size, if you are using a gravel bed beneath the footing. Gravel bed is important for balancing the distribution of loads from the structure to the soil beneath. Normally, ¾ sized gravel are used. But for tanks like sewage treatment plants footings’ G-1 gravel is used. Check the structural specifications as this information should be stated there. In some countries like Dubai, blinding is popularly used beneath the footing.
Quality Engineer Tip:
• Make sure that materials such as steel bar reinforcement and concrete spacers (plastic or concrete) are approved. Don’t ever use any unapproved material on site.
• Concrete cover shall be maintained, thickness as per drawing. It is to ensure steel bar reinforcement will not be penetrated with water as may create rust on steel and may cause failure to the foundation or footing.
Step 3: Check the Credibility of the Formworks and Gridline Locations
As formworks hold the wet concrete, check the credibility and the bracing of the formworks. See to it that its dimension and depth is as per design. To prevent spillage, the formworks accessories should be properly tightened. Any change of dimension would mean a lot to its integrity. It would be okay if it made the footing bigger, but if it became smaller then that is another story.
In addition, double check if the footing formworks are placed on the right gridline. It is advisable that a survey team checks the formworks prior to concrete pouring. Any deviations on the property line would mean serious legal obligations. You wouldn’t want to have any property line problems with your neighbors. Else, you would trim down a footing (of your built house) because you exceeded a lot area which is not yours, and that is not good.
Also, see to it that soil beside the footing excavation do not erode or it would mix up with the concrete. Foreign materials like this should be prevented by placing slope protection.
Quality Engineer Tip:
• Avoid using of old formwork as it may create honeycombs on the surface of concrete. Fowrmwork must be watertight so as not bleed water from concrete and may cause segregation. Old formwork may absorb water from concrete and again cause a honeycomb.
• Carpenter must be well experienced enough to do the shuttering.
Step 4: Check the Designed Concrete Strength, Tools and Manpower Needed and Volume Required, Check the Weather Update Too!
Prior to actual concreting, quality engineer or site engineer should check the tools, equipment, and manpower needed respectively. See to it you are using the correct concrete strength design. For a residential house, the concrete strength for footing is from 3,000 to 4,000 psi. Normally, engineers also calculate the volume of concrete required to check the availability of material on site to used (for site mixed concrete) or book to batching plants for ready mixed concrete.
Quality Engineer Tip:
• The design mix of concrete should be submitted for approval and trial mix shall be done based to the concrete design mix.
• Quality Engineer should ensure that tests of concrete to be done are: slump test, moisture, specific gravity, compressive test and durability test.
For ready-mixed concrete, check in advance the route of concrete trucks mixers. By experience, if the access roads are too steep, the trucks shouldn’t be loaded with its maximum capacity. Else, the truck might overturn or would cause spillage on the runway. For in-situ mix concrete, always check that premium cement-sand-aggregates mixtures are used. Footings are the most stressed member of the structures thus its quality shouldn’t be compromised.
During concreting, make sure that there will be no water on the soil. Should there be, it should be completely pumped out using water pumps. Always check the weather update prior to your concreting schedule. This should be available on the internet. This is crucial especially for footing on sloping area as it may cause water run-off ruining your freshly casted concrete footing. It is advisable to postpone footing concreting on heavy rain.
These data should help you check footing on your own. This may not be a complete guide but it should help you whether you have a professional engineer with you or not.
Eng. Noel,
kindly post the method of statement for cold concrete and gravel, your post really helpful in the construction practice.
thanks