Fibre-Reinforced Plastic (FRP) is an expensive method to repair concrete spalling. The purpose of this report is to determine the reasons of concrete spalling and method of investigation of spalling concrete. Moreover, according to the reasons of concrete spalling, define the critical reason of it and come out a program to check ponderance of spalling in a certain area.
In Singapore, many HDB flats will occur concrete spalling. All the age of the flats is about 15 years and above. Some times, the concrete spalling problems will occur again and again in a short term. That will brings inconvenient to the lessee because waste their time to supervise the job and extra money to repair. Contractors may also be suffered because one unit maybe needs to go several times in a short term.
The authors attached to Capstone Engineering Pte Ltd for 2 weeks to observe the procedures of survey and repair of concrete spalling. The contractor was using the traditional method which is Patch Repair Method to repair concrete spalling. Patch Repair Method is a common use method to repair the concrete spalling in the market but the only difference is each contractor using different concrete mixer and the quality of the jobs. The procedure of Patch Repair Method is shown as below.
a. Remove the defective concrete or (spalling concrete) area with hammer and chisel, or other approved pneumatic/electric tool (impact energy not grater than 1Nm) to a depth not exceeding 25mm as directed by the Superintrnding Officer's representative.
b. Remove rust from rebar using wire brush or other approved means. If he cross-sectional area of rebar is reduced by more than 15%, additional rebar with proper lapping must be added.
c. All sagged rebar shall be tied up at 250mm c/c to provide adequate concrete cover.
d. Apply the concrete surface with 1 coat of approved Lanko 751 (bonding agent for concrete surface) to ensure good adhesion with the repair material.
e. Apply Polymer Modified Cementitious Mortar in layer of no more than 20mm each and allow 4 hours between layers. The bonding agent is required between successive layers.
f. Paint the patched surface to match the adjacent surface.
Patch Repair Method gives inconvenient to the lessee because of the dust will make the whole unit dusty and waste time to clean the unit and the falling of concrete may also be concerned to the lessee due to damage their furniture.
Using FRP can shorten the repair period but it is very expensive so that it is only for the houses that have very serious concrete spalling problem to use. Nowadays still does not have any equipment or program to come out results to tell how serious the spalling problem is. If the most serious of spalling problems of the houses and large area of ceiling that has high probability occur concrete spalling can be detected, contractor and HDB officer may choose FRP to repair the problems to cut down the times to go to same unit and reduce inconvenient to lessee.
FRP is the best method to repair concrete spalling but it also very expensive to lessee, contractor and HDB Authority. From the research of the critical reason of spalling concrete, the authors created software which is able to give the contractor and HDB officer a rough picture of critical concrete spalling area and decide choosing between FRP and Patch Repair Method for a unit.
1.4 Literature Review
Kim Anh T Vu and Mark G Stewart (2001) have been reported that carbonation and corrosion steel are the main factors causing concrete spalling. However high moisture content, inadequate concrete cover and poor quality concrete are the primary causes of reinforcement corrosion and associated structural deterioration of reinforced concrete (RC) structures subjected to aggressive environments. Corrosion products are expansive, leading to the formation of the tensile stresses in concrete and therefore subsequent longitudinal cracking and spalling of the concrete cover. Probabilities of the cracking and spalling of the concrete are calculated herein using a structural deterioration life-cycle reliability model for typical RC bridge decks exposed to aggressive environments. Experimental testing results are used in the reliability analysis for more accurate prediction of the time spalling (crack width of 0.5mm) given that the time to first cracking is known (crack width of about 0.05mm)
Lawrence Martin & John Purkiss (1992) presented that corrosion occurs when the pH values of the concrete reduces from its original high value of around 13 during placing to below 9 at a later stage as shown in figure 2.1. This change in pH is known as a loss in passivity and due to two possible causes: (a) carbonation, and (b) Chloride attack. In addition, Hans. Bohni was also proved that the high alkalinity of the concrete pore water (PH 12.5) leads to a passive layer forming on the steel that reduces the corrosion attack to the negligible values. As long as this passive layer is sustained, corrosion will not occur. Corrosion of the reinforcement is one of the main durability problems of the concrete structures. The corrosion induced by two main factors: the carbonation of the concrete cover and the penetration of chlorides providing from marine atmosphere of from chemicals in the contact with concrete.
In August 2004, Bernard Erlin and William Hime highlighted that Carbonation reactions cause the high pH of the concrete (12.4 and higher) to dramatically drop to about 8.5 to 9. This can occur so fast that in minutes the paste at the surface is very thinly, but completely, carbonated. The water released by the chemical reactions continues both the formation of carbonic acid and the carbonation process. When carbonation begins, it is almost self-sustaining because of that released of water. But it soon becomes because of the increasing difficulty for carbon dioxide to penetrate into the depth of the concrete. Carbonation occurs at the highest rates at relative humidity from about 40 to 70 percent. Near 0 or 100 percent, there is little or no carbonation. Nonetheless, carbonation is usually deep into concrete arid regions in spite of a limited water supply because of condensation on the concrete surface and because of released of water when the cement hydrates carbonate.
In addition, (Bertolini et al 2004) have shown that concrete spalling can be prolonged if there is sufficient nominal cover. As shown in Figure 1.1, as the depth of nominal cover increases, the time required for corrosion to occur increases exponentially. As shown in Figure 1.1, a nominal cover of 25mm would set the initiation time of corrosion to 100 years.
Once corrosion begins, cracks caused by the buildup of bulky corrosion products develop in he surrounding concrete. The buildup of pressure eventually causes spalling of the concrete and exposure of the reinforcing steel (Litvan G.G., 2001). This process is illustrated in figure 2.
Carbonation is one of the main factors reduce the concrete lifespan. Concrete strength, concrete cover, moisture content and workmanship are the main factors affect carbonation depth in a concrete. If carbonation depth reach the steel rebar level will easily let moisture penetrate into concrete and causing steel rusting and lead to concrete spalling.
Carbonation reactions cause the high pH of the concrete (12.4 and higher) to dramatically drop to about 8.5 to 9. This can occur so fast that in minutes the paste at the surface is very thinly, but completely, carbonated. The water released by the chemical reactions continues both the formation of carbonic acid and the carbonation process. When carbonation begins, it is almost self-sustaining because of that released of water. But it soon becomes because of the increasing difficulty for carbon dioxide to penetrate into the depth of the concrete (Erlin B., Hime W. 2004).
As shown in Figure 2.1, the corrosion rate of steel is a function of the alkalinity of its environment. At high pH, the tendency of steel to corrode is negligible; the steel is "passive". Cured concrete contains large amounts of lime which render the concrete strongly alkaline, having a pH of between 12 and 13. Thus, steel embedded in concrete normally does not rust. If, however, the lime in the concrete is leached out or neutralized by carbonation (the transformation of the lime into calcium carbonate through a reaction with carbon dioxide), the passive state of the steel is terminated and corrosion will occur. (Litvan G.G. , 2001)
The depth of carbonated concrete also depends on the year of cast the concrete. That is because the concrete expose under the air and the get reaction with carbon dioxide and moisture. Refer to figure 2.2 shows that the older the year of concrete, the more the depth of carbonation will be.
The concrete strength will also affect the depth of carbonation. The higher the concrete strength, the lower the depth of carbonation will be.
2.1.1 Concrete Cover
As figure 2.3 shows that increasing the thickness of concrete cover over the steel lengthens the diffusion path and thus lowers the rate at which oxygen migrate towards the steel (Litvan G.G., 2001). Figure 2.1 is recommended that a minimum concrete cover of 58 mm be placed over the top steel and that minimum of 20 mm cover the bottom bars.
2.1.2 Concrete Strength
Before 1990, concrete mixing plant located at the site. The proportion of the concrete normally was not the same every time. Because of the concrete strength is not the same and normally was less than the required concrete strength, the speed of diffusion of carbonation was very fast. In figure.2.4, there is a very good correlation between the rate of carbonation and the cube compressive strength. That shows that the lower the cube compressive strength (concrete strength) is, the higher the rate of carbonation will be.
After a period of years, the higher the probability of concrete failure due to carbonation will be. In Singapore, many HDB flats is more than 15 years and the concrete strength the authors measure at field is about 20 MPa, the probability is about 0.5. The result is quite high for occurring concrete spalling. That also proves that more and more HDB houses occur spalling problems in these years and some of he houses are not the first time occur this problem.
Carbonation occurs at the highest rates at relative humidities from about 40% to 70%. Near 0 or 100 percent, there is little or no carbonation (Erlin B., Hime W. 2004). Humidity in Singapore is about 70% to 80% as shown in figure 2.5 which is much higher than 70%, so, moisture content is not one of the main factor causing concrete spalling but it need to be considered also. Authors can not find any research to show what the impact causing concrete spalling when the moisture reach certain level. Therefore the percentage of moisture content causing concrete spalling is quite low compare to concrete strength and concrete cover.
Before 1990, the Singapore residents started move to city. During that period, flats' productivity is very high but the quality was not good. Most of the officer at the site did not have any certificate but have experience. They construct the building base on their experience. Most of the worker did not have proper training for the construction technical. If the cast-in-situ concrete did not be sufficient compacted, 1% of air void reduce 5% of strength.
3.Method of Investigation
3.1 Electromagnetic Cover Measurement
The function of the electromagnetic cover measurement is to confirm the correct location and cover reinforcing bars, quality control checks. It also inspection of the concrete members for the reinforcing bar locations and sizes, for which records are not available.
3.2 Rebound Hammer Test
The main application of rebound hammer test is determination of the concrete uniformity and strength development monitoring, which is possible if appropriate correlation graphs are available. Rebound Hammer is shown in figure 3.2. The value get from rebound hammer is not very accurate to the actual concrete strength. The laboratory test, they did at least 30 concrete cubes to find out the concrete strength by using rebound hammer but in the field on flats, we only can only get few data from one slab. Therefore, we can use the average value of rebound hammer after correction and key into the Decision Making Tool.
3.3 Moisture Meter Test
Moisture meter is used to measured the percentage of water or moisture content (MC) on the concrete surface. Furthermore moisture meter can also measure the relative humidity, temperature and dew point of the floor being installed.
4. Filed Study
The Patch Repair Method is common used in concrete spalling repair work. It is inconvenience to lessees because they need to spent whole day to stay at home and supervise the workers. However, this problem may occur in a short period. So this is another trouble to lessee.
During the authors' attachment period, they observed the concrete cover of some houses that occurred concrete spalling areas were less than 15mm by using Electromagnetic Cover Measurement as shown in figure 4.1. Normally, this problem was appeared of the houses more than 20 years. They also used Rebound Hammer to check the concrete strength. The concrete spalling area is shown in figure 4.2, figure 4.3 and figure 4.4.
The three figures shown as below are the plan view of the concrete spalling areas. The shape created by thin line represents the shapes of the concrete spalling area and the thick lines represent the data box of each grid. The data at top part of the data boxes is concrete cover and the lower part is concrete strength.
5. FRP Decision Making Tool
5.1 Method of Using Decision Making Tool
The authors created the FRP Decision Making Tool for determine ponderance of concrete spalling is based on four reasons which are concrete cover, concrete strength, depth of carbonation and moisture content. Concrete cover and concrete strength is the main reasons causing concrete spalling so that in the program the authors created, the ratio of that two reasons highest among other reasons.
The steps for using the FRP Decision Making Tool are shown as below.
a. Key in the row's and column's grid line on sheet 1 and press the "Run" button
b. Key in the value on each cell for concrete cover, concrete strength, depth of carbonation and moisture content.
c. Press "Result" button to show the ratio on Sheet 2. Sheet 2 not only show each ratio of the area but also the color on each cell to let user can be more clear about the ponderance of spalling in each area at field. Decision also will be given in Sheet 2 to decide using FRP or traditional method to repair.
d. Press "Reset" to reset all the grid and value in Sheet 1 and Sheet 2.
Sheet 3 is the reference of the program in Sheet 1 and Sheet 2. User can check the ratio of each factor individually by key in the value in the cells in Sheet 3.
Three set of data shown in figure 4.5, 4.6 and 4.7 be collected on field and key into the Decision Making Tool and come out the value to show the condition of that concrete spalling area. If there is any part of data did not get during the inspection, user can lift blank on the grid of the Decision Making Tool. The value of the grid of factor will become 0. The example below is the first set of data key in to the Decision Making Tool and the value of the probability of concrete spalling in each grid as shown in figure 5.2. There have some grids at the right corner of figure 5.1 were left blank that is because there did not have that part of value so that the result in Sheet 2 of the Decision Making Tool is blue color and the value is 0. the moisture content in this example was assumed to be 80%.
The orange color appear in the result grid in figure 5.2 means the concrete of that area will spall in a short period or can be observed clearly at field. The blue color and green grids mean that area is quite good but the reinforcement may start to expand already. The contour line shown in figure 5.3 is similar to the actual condition on the field. The spalling occurred near the X-axis of the surveyed area and the worse condition in the contour line is near to the X-axis and also at the center of that area. So that the Decision Making Tool can be reliable to make a decision for the concrete spalling area.
Insufficient concrete cover cause carbonated depth of concrete cover reach the reinforcement and let the moisture corrode reinforcement. Corroded reinforcement will expand two times its volume and cause concrete spalling.
In the program the authors created, the highest ratio to define the concrete spalling ratio is concrete cover, follow by concrete strength, depth of carbonation and moisture content. The overall ratio can be defined as a critical of concrete spalling area to be repair.
The program can let the user to make a decision between using Fiber Reinforced Plastic (FRP) and Patch Repair Method base on the ratio of concrete spalling. If there has many areas have low ratio, user can decide to use the latter method to repair but if there has many areas have high ratio user can decide to use FRP to repair the concrete spalling area.
The ratio of each factor is estimate by the authors. There do not have any proof to prove the ratio is correct and need have another research to prove the ratio of each factor. The authors were base on the factors causing concrete spalling to define the ratio of each factor.