Watertight concrete tanks are important to specifiers and regulators. The customer's level of expectation is that the product will perform as expected. But occasionally, a leak is noticed at the seam or joint of the product. Why is this occurring, and how can it be fixed?
When a water retaining structure is leaking, the first response is to find fault with the sealing material. While this cannot be ruled out, there are several other potential causes that are contributing to the leak rather than a failure of the sealant. Sealants manufactured to recognized standards such as ASTM C990, have been designed and performance tested to withstand hydrostatic forces present in over 99% of precast concrete products.
PLACEMENT
The first issue that should be mentioned is placement. There is not an exact rule of thumb that works for every type of casting. The placement is dependant on the type of joint, the size of the joint, the weight of the casting, and the volume or size of sealant. Based upon testing, I have found that there are two rules of thumb that I follow. The first is to place the most volume of sealant within the annular space of the joint. The annular space is the intentional void between the angled surfaces of the opposing sections of the joint. Filling the annular space provides the best seal and allows the highest amount of compression. The second rule of thumb is to place the sealant in the joint nearest the hydrostatic force. If this is a water containment structure, then sealant should be placed nearest the inside of the joint.
Placing the sealant always means that at least two ends of the preformed sealant must be joined together. This should be done in a manner that best forms a continuous bead. The sealant should never be stretched. It is best to avoid overlapping and side-by-side placement. The ends should be cut at 45 degree angles, and then worked together by hand to form a continuous piece.
In a square or rectangular structure, the sealant should never be started on a corner. Because three point are all that are truely ever in one plane, leaks are often found in corners. It is best to begin and end the sealant a minimum of 12" from a corner.
The final point to consider is that squeeze out is not always good. Squeeze out is where sealant material is extruded out of the joint. Material that squeezes out of the joint is material that is not being used to fill the joint. With squeeze out, the purchaser does not know if there is a 1" band of sealant in the joint, or a 4" band. A wider band of sealant in the joint will always produce the best seal.
VOLUME
Creating a watertight joint is as much about using the correct volume of sealant material as it is putting it in the right place. Volume is defined as the area in the crossection of the sealant material. Volume is determined by multiplying the height by the width. For instance, a sealant with the crossectional sizes of 0.60H x 0.75W has a volume of 0.45 in. sq. To find the area needed, there are two methods. One is to lay several strips of sealing material across the joint surface, using baby powder to prevent adhesion, and then seting the next section onto the first section. by cutting off the ends of the sealant outside of the joint, and then rolling the compressed material into a roll, the required crossectional volume can be calculated. Use the highest volume of the pieces to determine the proper size.
When a water retaining structure is leaking, the first response is to find fault with the sealing material. While this cannot be ruled out, there are several other potential causes that are contributing to the leak rather than a failure of the sealant. Sealants manufactured to recognized standards such as ASTM C990, have been designed and performance tested to withstand hydrostatic forces present in over 99% of precast concrete products.
PLACEMENT
The first issue that should be mentioned is placement. There is not an exact rule of thumb that works for every type of casting. The placement is dependant on the type of joint, the size of the joint, the weight of the casting, and the volume or size of sealant. Based upon testing, I have found that there are two rules of thumb that I follow. The first is to place the most volume of sealant within the annular space of the joint. The annular space is the intentional void between the angled surfaces of the opposing sections of the joint. Filling the annular space provides the best seal and allows the highest amount of compression. The second rule of thumb is to place the sealant in the joint nearest the hydrostatic force. If this is a water containment structure, then sealant should be placed nearest the inside of the joint.
Placing the sealant always means that at least two ends of the preformed sealant must be joined together. This should be done in a manner that best forms a continuous bead. The sealant should never be stretched. It is best to avoid overlapping and side-by-side placement. The ends should be cut at 45 degree angles, and then worked together by hand to form a continuous piece.
In a square or rectangular structure, the sealant should never be started on a corner. Because three point are all that are truely ever in one plane, leaks are often found in corners. It is best to begin and end the sealant a minimum of 12" from a corner.
The final point to consider is that squeeze out is not always good. Squeeze out is where sealant material is extruded out of the joint. Material that squeezes out of the joint is material that is not being used to fill the joint. With squeeze out, the purchaser does not know if there is a 1" band of sealant in the joint, or a 4" band. A wider band of sealant in the joint will always produce the best seal.
VOLUME
Creating a watertight joint is as much about using the correct volume of sealant material as it is putting it in the right place. Volume is defined as the area in the crossection of the sealant material. Volume is determined by multiplying the height by the width. For instance, a sealant with the crossectional sizes of 0.60H x 0.75W has a volume of 0.45 in. sq. To find the area needed, there are two methods. One is to lay several strips of sealing material across the joint surface, using baby powder to prevent adhesion, and then seting the next section onto the first section. by cutting off the ends of the sealant outside of the joint, and then rolling the compressed material into a roll, the required crossectional volume can be calculated. Use the highest volume of the pieces to determine the proper size.
It should be noted that wider is not better. A common misconception held by many is that a wider pice of sealant is better than a narrow piece. This is a false assumption. Actually, it is best to have a taller piece of sealant and allow the sealant to compress as much as possible. Compression of at least 50% is preferred by most sealant manufacturers. As the sealant is compressed, it gets wider and thinner. At some point, the sealant will not compress any further. This is a point where the resistance force is eaqual to the force applied (weight) over the area of sealant. Time and temperature can affect this.
ADHESION
The final point I will touch on in this blog is adhesion. Sealant has to stic to the concrete to be most affective. Most sealant materials will naturally stick to clean, dry concrete. But if the joint contains dirt, rocks, concrete debris, and dust, the sealant will not stick. The joint surface should be clean and dry, and any dust that is present should be revoved. Primers can improve adhesion in one of two ways: either create a good mating surface imbedded into the concrete surface, or by creating a sticky surface for the sealant to be applied to. Also, most concrete has some level of porosity.
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