Anatomy of Cracks in RC Members

Dr. C S Viswanatha, Chairman, Civil-Aid (A Bureau Veritas group Company)

Cracks in RC members are an indication of distress in the members due to several possible reasons. The anatomy of these, throws light on the distress intensity, the seriousness of the problem, the possible corrective measures and several other related issues.

Classification of Cracks

From visual inspection point, Cracks can be more specifically understood based on three factors namely, the configuration, the width and the depth. Formation and appearance of Cracks may be longitudinal, transverse, vertical, diagonal or random.  They may range in size from less than 0.1 mm (fine) to between 1 and 2 mm (medium) to over 2 mm (wide).

Cracks on a broader scale of understanding can also be classified as:

• Micro Cracks.
• Macro Cracks.
• Structural Cracks.
• Non- Structural Cracks.
• Stable Cracks.
• Unstable / Dynamic Cracks.
• Harmful cracks.
• Harmless Cracks.   

Visual Inspection

The most important and primary inspection of cracks is done visually. The configuration, extent, location, whether scattered or localized, orientation etc. are some of the features, which could be identified with careful visual examination of the concrete.

Origin of Cracks - Cracks might develop due to any of the following individual reasons or in combination.

• Structural deficiency resulting from design deficiency or construction deficiency.
• Deterioration of concrete or corrosion of rebars due to poor construction or inappropriate selection of constituent materials or poor maintenance.
• Temperature and shrinkage effects.
• Overloading of members.
• Settlement of supports.
• Natural hazards; etc.

Interpretation of Cracks

Most buildings crack at some time during their service life. The appearance of cracks is a symptom of distress within the fabric of the building. Often the cracking is of little consequence and once it is established that it is static, simple repair by filling or re-pointing is all that is required. However a crack may be the first sign of a serious defect, which may affect the serviceability or the stability of the building.

The appearance of cracks can also affect the value of the building, its insurability, the saleability and can be the subject of litigation. Therefore correctly diagnosing the cause of the cracks is essential. However it is far from a simple task and is often a subjective exercise. The implications of an incorrect assessment can lead to expensive and unnecessary remedial work. 

So how does the professional adviser decide if the cracks are significant? Providing information regarding reasons & classifications of cracks including their prevention is not enough. Whatever one may do, cracks might still appear. How to interpret these cracks, what types of investigations should be carried out and what corrective action should be done? Answers to these are important for maintaining the service life of the structure and deciding when the structure component has outlived its life.

This article attempts to interpret the cracks based on their appearance, pattern, characteristics and effects.

I. CRACKS IN BEAMS

1.0 Flexure cracks in beam-Figure 1

2.0 Flexure cracks in beam-Figure 2

3.0 Top Flexure cracks (Cantilever Fixed End) - Figure 3

4.0 Diagonal tension cracks in beam (or shear cracks)-Figure 4

5.0 Shear - flexure cracks in beam-Figure 5

6.0 Torsional crack in beam – Figure 6

7.0 Cracks due to abrupt curtailment of bars in beam – Figure 7 

8.0 Bond Slip cracks in beam - Figure 8

9. Disturbance cracks in beam - Figure 9

10. Construction joint crack in beam – Figure 10

II. CRACKS IN COLUMNS

1.0 Horizontal crack in column - Figure 11

2.0 Diagonal crack in column - Figure 12

3.0 Corrosion bond crack in column - Figure 13

4.0 Splitting cracks in column - Figure 14

5.0 Tension crack in Tie beam - Figure 15

•Occurs in tension members when capacity is exceeded or when concrete quality is poor or when design deficiency exists.

•Uniform width throughout and parallel to each other.

•Occurs over the whole length.

•Affects structurally soundness.

•Needs to be attended to (But not an emergency).

III. CRACKS IN SLABS

Generally cracks in slabs are not of any serious nature. Not much of concern from safety point of view, except when they are associated with excessive deflections. They need to be attended to only from psychological performance and durability considerations, but not an emergency.

Figure 16 Flexure Crack in one-way slab-bottom face

Figure 17 Flexure Crack in Two-way slab- bottom face

Top Flexure Cracks in Slab - Figure 18

Top Flexure Cracks In Slab may occur due to inadequate negative reinforcement OR Reinforcements not being continuous across the support.

Plastic Shrinkage Cracks on Slab Top- Figure 19

Plastic Shrinkage Cracks may appear randomly over the surface due to rapid early drying, low bleeding and quick surface evaporation. Time of their appearance may be from about 30 minutes to about six hours of pouring of concrete.

Thermal Cracks in Slabs- Figure 20

Variations in the temperature of concrete cause it to expand and contract. Significant differences in temperature between the outer and inner regions can cause concrete to crack.

Cracks Due To Foundation Settlement-Figure 21

Due to differential settlement of any loaded column, inclined cracks develop at support region in the immediate horizontal members. Soil stabilization by any appropriate means is the solution.

Corrective Measures

Corrective measures taken up are essentially measures to bring back the member to original level of normalcy. This can be through:

• Providing additional reinforcements to counteract the deficiency.
• Further increasing the concrete cross-section along with additional reinforcements to counteract the deficiency. (Popularly known as encasement)
• Bonding steel plates or carbon laminates to original concrete surface appropriately.
• Sealing the cracks by pressure grouting with low viscosity monomers or epoxies or cement grouts.
• Prestressing members externally by appropriate means.
• Wrapping with glass / carbon fibre wraps appropriately.
• Substitution techniques.
• Polymerization techniques, etc.,
• Any one of the above or in combination is resorted to generally.

Concluding Remarks

The individual cracks in Reinforced Concrete members are unique by themselves. They are versatile diagnostic devices, clearly indicating the specific problems in the concerned Reinforced Concrete members. In addition, they also indicate the degree of seriousness of the problem. The cracks throw light on the discipline in the associated structural designs and construction quality. The corrective measures to be implemented are clearly indicated by the cracks themselves.

Crack Measuring Devices - NEW PAGE

Most buildings crack at some time during their service life. Usually the cracks are of little consequence but they may be the first indication of a serious defect. Monitoring the changes in crack width across a crack helps determine the cause of cracking and decide what remedial work should be specified.

Measuring Magnifier

Measuring magnifier from Pepleroptics with its magnification factor of 8x, can be utilised just for linear- or width measurement with an 0.1 mm graduation and 15 mm measuring length, which is supplemented by a 0.005″ graduation with a measuring length of 0.6″. Measurement accuracy is 0.0025″. Naturally both scales can be used interchangeably in the same magnifier. 

Demec Gauge

The DEMEC crack gauge comprises of an invar main beam with two conical locating points, one of which is fixed while the other point pivots on a knife edge. To measure the strain or crack propagation, the crack gauge is manually held over two stainless steel discs adhered to the structure under test. Typically, a disc is adhered on the material surface on either side of a crack in the surface.

The conical locating points in the gauge are seated into a precision drilled bore in the centre of each locating disc. The resultant displacement of the pivoting point is then indicated by the gauge. Thermal expansion and contraction within the instrument is inherently insignificant by design.

Digital display device

Avongard supply a range of products for monitoring the movement of cracks and can also provide in-house Courses on Crack Diagnosis of Low-Rise Buildings. Avongard are the originators and global suppliers of gauges for the monitoring of cracks in buildings and civil engineering structures.

CrackScope from German Instruments

The CrackScope CS-100 from German Instruments can be used for accurate measurement of the width of surface opening cracks as well as measurement of the depth of surface holes or irregularities. The CrackScope is a small size, lightweight and conveniently portable microscope with a 25× magnification. It has a built-in scale for crack-width measurement and another scale on the focusing adjustment ring for depth indication.

The built-in 3-mm scale has a least division of 0.05 mm, allowing the width of cracks to be estimated within ±0.025 mm. Depth measurement is achieved by focusing at the bottom of a depression and then focusing at the perimeter of the depression. By reading the scale engraved on the focusing ring and the needle of the lens barrel, depths can be measured with an accuracy of ±0.05 mm.

Lomaro Microscope from ELE

Specifically designed for measuring crack width in concrete, this high definition microscope operates via an adjustable light source provided by high power batteries. With its 40x magnification factor, this instrument is a pocket sized measuring device weighing about 550 gms.

References:

1. Marcel Cyr-InterNACHI
2. Presentation by-Jagadanand on cracks-in-buildings
3. Presentation by-Iangibb on Control of Cracks
4. Visual Inspection of Concrete by Nick Gromicko and Kenton Shepard
5. Causes & Cures for Cracking of Concrete Barriers- Centre for Structural Durability A Michigan DOT Centre of Excellence.
6. Prediction of Cracking in RCC- Nicholas J. Carino Structures Division and James R. Clifton Building Materials Division, National Institute of Standards and Technology, Gaithersburg,