Some vital tips to control cracking with Reinforce Concrete Slab on Ground

Steel reinforcing bars and welded wire reinforcement are used to check crack width in nonstructural slabs-on-ground.

Most slabs-on-ground are unreinforced or minimally reinforced for crack-width control. If steel reinforcement is arranged in the upper or top portion of the slab thickness, it restricts the widths of random cracks resulting from concrete shrinkage and temperature restraints, subbase settlement, applied loads or other issues. This type of reinforcement is normally defined as shrinkage and temperature reinforcement.

Shrinkage and temperature reinforcement is not same as structural reinforcement. Structural reinforcement is generally arranged in the bottom section of the slab thickness to enhance the load bearing strength of the slab. Most structural slabs-on-ground contain both top and bottom layers of reinforcement for managing crack-widths and improving load capacities. Due to constructability issues and costs regarding two layers of reinforcement, structural slabs-on-ground are not regularly used as nonstructural slabs.

The basics: Steel reinforcing bars and welded wire reinforcement can’t control cracking. Reinforcement mainly remains inoperative unless the concrete cracks. Once cracking happens, it gets activated and manages crack widths by limiting the expansion of crack.

When the slabs are provided on greater quality sub bases with uniform support and include low shrinkage concrete with joints perfectly installed at a gapping of 15 feet or less, reinforcement is normally is not required. Seemingly, there exist few random or out-of-joint cracking. In case of random cracks, they should remain moderately tight due to the restricted joint spacing and low concrete shrinkage thus future serviceability or maintenance issues will be reduced.

If slabs are arranged on difficult sub bases with risks of non-uniform support or comprise of medium to high shrinkage concrete or joint spacing surpassing 15 feet, then reinforcement should be provided to control the widths of cracks. Since crack widths expand and become about 35 mils (0.035 inches), the effectiveness of load transfer via aggregate interlock is reduced and differential vertical movements over cracks or slab "rocking" can happen.

Due to this, crack edges remain uncover and edge spalling takes place, particularly when the slab is uncovered to wheeled traffic and especially hard-wheeled lift trucks. As soon as spalling begins, crack widths at the surface get expanded and slab deterioration along cracks is raised considerably.

When contraction joints are inappropriate and not installed, shrinkage and temperature reinforcement is necessary. This design approach is sometimes called as continuously reinforced or joint-less slabs and produces several closely spaced (3 to 6 feet) fine cracks all through the slab.

Crack control options: Normally, the cracks in slabs-on-ground are controlled with the following ways -

1) check the location of cracking by installing contraction joints (does not control crack widths) or 2) Installation of reinforcement (does not control crack location).

With Option 1, we can know where to crack in the slab and widths of contraction joints or cracks in the joints are mostly managed by the joint spacing and concrete shrinkage. When joint spacings and concrete shrinkage are raised, joint widths also expand. Similar to cracks, when joint widths turn out to be about 35 mils, the effectiveness of the aggregate interlock to transmit loads and resist differential vertical movements across joints is considerably decreased. Because of this, several load-transfer devices like steel dowels, plates or continuous reinforcement through contraction joints are used to keep positive load transfer and control differential vertical movements across joints.

With Option 2, the slabs are allowed to crack indiscriminately but crack widths are controlled through steel reinforcing bars or welded wire reinforcement. Normally, contraction joints are not installed with this option rather cracking happens indiscriminately that develop several, tightly held together cracks.

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Published By
Rajib Dey
www.constructioncost.co
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Details about structural and non structural defects in buildings

Concrete has diversified nature. It casts in place by including or excluding reinforcement. It is also precast or pre-stressed to attain necessary strength. For this purpose, there should be adequate knowledge on the behavior and constituents based on which the concrete is produced.

There should not be any type of laxity in any of its phase like placement, design & maintenance as these can create deterioration and resist concrete to accomplish its proposed functions. Given below, some vital factors which can weaken the quality of concrete:

1. Accidental loading
2. Chemical reaction like sulfate attack, alkali carbonate reactions, alkali silica reactions etc
3. Erosion of steel reinforcement
4. Inferior construction detailing

5. Erosion
6. Freezing and Thawing
7. Shrinkage
8. Settlement
9. Fire and weathering

Flaws in Building Design: Due to deficient structural design, the concrete is uncovered to flexural and shearing stresses and as a result spalling and cracking of concrete are developed. Any sudden modification in cross section of any member can result in raising the stress concentration in that member that leads to cracking of concrete.

Deflection is considered as one of the significant part in structural design. If there exist any issue in its consideration throughout design, that can produce cracking of concrete. Insufficient arrangement of drainage and expansion joints throughout the design also leads to deterioration and spalling of concrete.

Flaws During Construction: Flaws throughout building construction vary from inappropriate mixing, placing and curing of concrete. Detachment of shoring & formwork can also produces cracks in concrete.

When extra water is provided in concrete to enhance the workability of concrete, the water cement ratio is raised significantly and it can reduce the strength of concrete. Inappropriate alignment of formwork produces corrosion in concrete.

Structural Defects in Building Construction - The following structural defects are found in buildings:

1. Cracks in foundation (substructure)
2. Cracks in floors and slabs (superstructure)
3. Cracks in Walls (superstructure)

These above defects are occurred due to the following factors:

1. Inappropriate soil analysis
2. Inappropriate site selection
3. Application of defective materials
4. Inferior work

These structural issues can be resolved with perfect design and planning.

Non Structural Defects in Building Construction - The following non structural defects are common in buildings:

1. Defects in brick work
2. Dampness in old structures
3. Defects in plaster works

So, it is found that minimum design and construction defects lead to minor cracking or spalling which can weaken the concrete and result in collapsing of the structure. To get rid of these issues, proper care and attention should be taken in designing, detailing and construction of concrete structure.

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Published By
Rajib Dey
www.constructioncost.co
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How to get rid of common errors in concrete floor slab construction

To get rid of common errors in concrete floor slab construction, various steps like perfect base preparation, mix design, placement, finishing, and curing should be taken.

Typical concrete floor slab thickness in residential construction should be 4 inches. In case the concrete has to sustain extreme loads irregularly, the thickness should be five to six inches; as for instance, motor homes or garbage trucks.

To arrange the base, the ground level should be dug according to the perfect depth to provide for the slab thickness. Take out all organic material and large hard objects like stones and tree roots to a depth of minimum 4 inches. If it is required to develop the grade, apply gravel or sandy soil, and compress the final base with a vibratory plate or equivalent device.

The edge is built up with any straight material that can be fixed into position. If case of constant non-availability of straight lumber, plastic or metal forms should be used. Fix a string line with grade stakes or batter boards to provide a square, level reference prior to place the formwork.

For the concrete mix, it should satisfy the compressive strength requirements (normally 3000 pounds per square inch) devoid of measures that lead to extreme shrinkage. The shrinkage and cracking are increased because of the existence of water, a plasticizer should be used to attain required slump.

Also provide fibers to check plastic shrinkage cracking. Greater strength and entrained air are necessary for exterior slabs susceptible to freezing weather or deicing chemicals.

It is suggested not to add water at the jobsite more than 1 to 2 gallons per cubic yard. In case, extra slump is essential, take advice from the mixer truck driver concerning the quantity of water to be included devoid of taking the concrete out of specification.

Spread the concrete accross the slab area as close to its final position as possible, and then rake it into exact location. Hardens low-slump mixes manually with a vibrator or apply a vibratory screed. Complete with the least force and strokes of the float required to attain a smooth surface.

Develop control joints no farther apart than 24 to 30 times the slab thickness and at no time exceeding 15 feet along both the width and length of the slab by pushing a 1-inch deep grooving tool into the surface.

To keep Joint spacing more than 15 feet, different types of load transfer devices should be used which range from dowels or dowel plates. For slabs with long joint spacing or no joints, steel reinforcement will be suitable.

It will raise the chances for random cracking, but will maintain cracks firmly to keep up superior structural performance.

The curing process should be initiated when the finished surface can counter damage. The concrete should not be enabled to freeze or dry out. Arrange a curing compound over the surface, or apply proper moist curing.

In case of freezing, the slab should be wrapped with an insulator, like insulating blankets or a 4-inch-thick layer of straw that is weighted down so that it can’t blow away. Unless the concrete attains a strength of minimum 500 psi, put the insulator in place. It normally happens within a few days.

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Published By
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www.constructioncost.co
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Reason for damaging & collapsing of concrete buildings

A reinforced concrete building gets damaged and collapses due to several reasons such as sliding of roofs, falling of walls, crushing of columns, short column effects, diagonal cracking, foundation sinking and tilting etc.

Types and Causes for Damage and Collapse of Concrete Buildings.

Given below, the details about the most common types of damages in reinforced concrete buildings:

1. Sliding of Roofs off the Supports: Where the beams are just supported on walls or columns, they are susceptible to slide if the severity of earthquake surpasses the frictional resistance and several times come out of the support and collapse, specifically when the bearing length is minor.

2. Collapsing of Infill Walls: The infill panel walls amid reinforced concrete columns overturn outer the framework when they are not firmly retained or secured with the frames.

3. Crushing of Column Ends and Virtual Hinging: When extreme shaking occurs, the column ends are susceptible to serious eccentric compressive stresses which compel the concrete to get crushed and broke down from the exterior surfaces. In frequent cycles, the damage proceeds interiors, consequently the effective section is shortened significantly. Both the column ends substantially function as pins and the entire framework falls down like a mechanism.

4. Short Column Effect: If infill walls having wide openings are joined to the columns, the sections of the columns to be deformed against lateral seismic loads turn out to be very short with reference to their normal height.

Such short columns develop into much harder as compared to other columns and pull greater shear forces under which they experience extreme diagonal tension which result in collapsing of the column.

5. Diagonal Cracking in the Columns: Columns are exposed to diagonal cracking resulting from large seismic shears occurred under extreme ground shaking. When the building also sustains the twisting action, the crack may change to a spiral form that decreases load bearing strength of the columns significantly.

6. Diagonal Cracking of Column Beam Joint: Several times, diagonal cracking happens through the intersection of the columns with the beams that considerably damages the strength of the frame.

7. Drawing Out of the Reinforcing Bars: Where the anchor length of the column bars or overlaps among the longitudinal bars are insufficient for producing full tensile strength of the bar, they are frequently drawn out because of tensions occurred in the column against reversal of stresses.

8. Collapse of Gable Frames: Reinforced concrete gable frames, frequently applied for school workshops, gymnasia and assembly halls, and cinema halls, may be expanded devoid of secondary resistance obtainable as soon as a joint fails. These are frequently found to fail and collapse if not properly designed and detailed.

9. Foundation Sinking and Tilting: Sinking or tilting of foundations of columns because of seismic shaking happens in loose soft soils and can result in extreme cracking of the superstructure and even fall down.



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How To Resist Cracking In Fresh Concrete

Usually, the contractor should take responsibility with all essential steps to track and resist cracking in fresh hydrating concrete, devoid of the size or volume of the pour. The steps should be approved by the Engineer and ensure that extreme surface crack width on hardened concrete measure instantly after the pour does not surpass 0.004 times the nominal cover of the primary reinforcement.

The contractor will be responsible for and offer sanctioned instrumentation for measuring the variation of internal temperature in large pours. The extreme concrete temperature at the point of delivery usually shall not surpass the lower of either 37 degree C, or 6 degree C beyond the existing shade temperature compliant with the approvals of ACI. The limiting internal temperature differential measured over the maximum faces of concrete mass shall not go above 25 degrees C at any time.

Curing of hardened concrete will be done according to the curing specification. Usually, the element surface is not chilled to disperse heat from the concrete. Curing methods, like the drenching of heated concrete elements uncovered to long and direct radiation, which produce temperature gradients inside the concrete mass, are not recommended for application.

For big pours, the contractor will be liable for and take additional provisions to lessen concrete temperature gradient as well as check the loss of surface moisture. Such steps are described below :-

 Maintaining all mix ingredients shaded where feasible to decrease their temperatures in         the stockpile

 Chilling of mixing water and/or substituting part or entire of the extra water with ice.

 Lessening the cement content with the application of admixtures (but not lower that is           essential for the stability)

 Applying a cement having a inferior heat of hydration

 Injecting liquid nitrogen once the concrete is blended

 Limiting the time amid mixing and assigning of the concrete to below 2 hours

 Delivering permitted surface insulation constantly over all uncovered surfaces to resist           draughts as well as keep identical temperature with the concrete mass

 Starting curing instantly once final tamping is done and carry on till the permitted surface     insulation system is completely prepared

 Providing shade to the concrete surface to resist heat obtained from direct radiation.

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Published By

Rajib Dey

www.constructioncost.co

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