Concrete Damage

Damage on concrete structure after construction may happen during the construction or building operational process. Damage happened during the construction will trigger damage on the operation process. The damage may be caused by:
1. Planning error
2. Implementation error
3. Inappropriate use
4. External causes, such as earthquake, fire, landslide, tsunami, etc.

Types of concrete damage are cracks, voids, spalling, scaling, erosion, drumminess, chemical causes (sulphate penetrations, salt, chloride acid, etc) and physical causes (volume change due to concrete fluctuation, difference of settlement, temperature change, fire, overloading, earthquake, cyclical load, fatigue, etc).

Crack is a damage commonly found within concrete structure, where there is a separation between long and narrow concrete masses. Fracture on concrete structure is happened before or after ossification (plastic condition). Crack may also happened in the early stage, when concrete not ossified yet, concrete mortar still in plastic condition and cannot hold maximum load. Kind of cracks like this is caused by solidification due to cold weather (in the region with cold weather), shrinkage, and settlement.

Shrinkage crack is happened due to uneven concrete shrinkage among its parts. It is commonly found in floor plate. There is mass movement within concrete of plastic condition, water with lighter masses will move upwards. Water will be trapped on the lower surface of aggregate or big-sized frame, yielding water bags. After concrete ossified and water slowly evaporated, water bags become empty or vacant. If it receives load, this part will potentially create early fracture.

Complex crack will be created after concrete solidified, which is caused by physical reason, chemical, thermal, or overloading. Crack created after construction on the element of beam structure is usually termed with structural fracture. Wide, length, location and orientation of the crack can indicate its type and causes, thus it can be quickly and appropriately anticipated.

Voids are deep holes or hollows that usually caused by:

1. Condensation during improper implementation thus mortar cannot fill hollows among aggregates.

2. Frame concrete that extremely dense.

3. Leakages on begisting which make water or cement pasta come out.

4. Water excess in the mixture.

5. Improper gradation of the mixture.

Kind of voids are honey combing, sand streaking, bugholes, and form scabbing.

Spalling is extrication of concrete surface in form of small fragments or chunks. This kind of damage is caused by frame corrosion, overloading, fire, etc. when there is corrosion on frame concrete, its volume will be doubled and triggers tension on concrete around the frame. If this tension exceeds concrete capacity surround it, then it will cause fracture followed by spalling. In the condition of fire, spalling is caused by expansion difference between aggregate and mortar that working in contradictive manner. Scalling is exfoliation of some parts of concrete surface due to solidification and liquefaction, which happened in cold area..

Erosion is extrication of small / soft grain from concrete surface due to abrasion; if the process is dry then it is called dusting. Abrasion happened due to wave is called water erosion.

Drumminess is a term used for concrete with low quality. It is called drumminess because when part of concrete surface is bumped into a hammer, it yields a high-pitched sound like a drum. Drumminess can be caused by corrosion on concrete frame, improper patching (material used for patching cannot properly patched on the concrete).

Source: Types and Causes of Damage on Concrete Structure (Andreas Triwiyono), irc.nrc-cnrc.gc.ca/pubs/ctus/8_e.html, www.phys.tue.nl/nfcmr/damage-crys.html, www.wsdot.wa.gov/Projects/US2/Eb...otos.htm, www.concrete.org/Troubleshooting,

Post-fire Assessment of Building

The great number of building fire in Indonesia had inspired the author to share a little thing about post-fire assessment of building.
Fire happened on buildings was caused by various reasons, such as electrical failure, uncontrolled gas or primus stove, explosion from chemical materials, etc. fire also happened because it was intentionally burned, for example by a mass of people in a riot.
Of course after the fire was overcome, there are many questions both from the owner and user of building, journalist or people that directly or indirectly suffered its impact. These questions for example, what is the cause of fire? How much the lost suffered? Is the structure burned can be used? If can, what kind of rehabilitation or strengthening is needed? How much is the cost needed to do rehabilitation or strengthening the structure?
To answer these questions, there will be an assessment needed. In this case, the author only share about assessment needed in order to know the suitability or strength of the building remained after fire.
The first assessment method taken is field survey inspection. It is purposed to know the situation of location (building structure burned), observation can be based on color change on concrete surface, whether there are cracks on concrete surface or not, whether there is plastic deformation of structure’s element or not. All above aspect are dedicated to detect the highest temperature, strength and stiffness of the structure. The highest temperature and duration of fire if it is accurately estimated can help us determinate the degradation of structure’s material post-fire. Field survey is also purposed to obtain secondary data, such as built drawing, material test (concrete and steel), function of every floor and room, and gain information from witnesses in the location, such as how long the fire took place and what was the cause.
This inspection must be documented, picture taking is performed in general and in detail (close up) if needed.
From this early inspection, then a report is arranged containing visual description of structure’s condition after fire. In this report, there is also recommendation included for the owner about next assessment, such as emergency solution that must be realized, is it a repairing or strengthening needed on path of the structure.
This report can be a final report if the assessor is sure enough (of course he/she must be a well experienced forensic engineer) only with visual inspection about the condition of structure, is the structure if still strong enough or must be collapse because of severe damage or is this no need an additional handling because the structure will be collapse by itself.
The second step taken is collecting primary data through sample collection and field or laboratory test. This data collection can be taken through non-destructive test and destructive test.
Non-destructive test is a method to obtain data of the structure without damage it, for example estimation of concrete strength using Schmidt Hammer instrument to know / detect the location of steel. Steel diameter, distance between steels and thickness concrete cover using rebar detector instrument; to measure width of small size crack, we can use microcrackmeter, this instrument has an accuracy to 0.01 mm; to predict the concrete strength, elasticity modulus of the concrete, fracture, porosity (cavity or corrosion), we use UPV (ultrasonic pulse velocity) tool; chemical test is included too in order to estimate the highest temperature experienced, such as using phenalphtalein test (PP-test) or CaO-free test.
Destructive test is a method of data collection by breaking structure’s material for collecting sample. For example, taking concrete sample by core drill method (diameter of 10 cm) and core case (diameter of 5 cm). this sample then picked to the laboratory for pull-strength test.
If the building’s owner gives permission, then we can directly perform full scale of load test on parts of the most severe structure until twice of the load plan and record sag response happened in several critical spots in order to estimate strength, stiffness, stability and elastic’s response limit remained. This load can be taken using water. When the structure can not hold the water, then it be broken.
Next step taken is performing re-analysis of the structure accorded to the material’s property after fire and the latest code. From this re-analysis, we will know which structure’s element that must be repaired or stiffened.
Based on damage level experienced and capacity available, according to the damage level experienced, capacity that must be available, and steps previously explained, technique or method of repairing or strengthening can be determined.


Sources : (teknik forensik-bambang suhendro, analisis degradasi kekuatan struktur beton bertulang pasca kebakaran-bambang suhendro,monitoring dan instrumentasi-priyosulistyo,material and technology for retrofiting-andreas triwiyono,kebijakan dan sistematika menghadapi kasus kebakaran-hamid shahab)