May 17, 2018
By Alireza Biparva
Proper waterproofing of construction joints plays a vital role in fortifying below-grade concrete structures. Construction joints are stopping places—caused by non-continuous concrete pour—and a plane of weakness. Hence, they represent the most vulnerable part of the structure from a waterproofing perspective.
Without an effective joint waterproofing system known as a waterstop, it is not a matter of if the joint will leak, but rather, when it will leak. Leakage or dampness not only impacts the serviceability of the structure, but also causes major deterioration such as corrosion of reinforcing steel in concrete. Water penetration is a global problem as it reduces the service life of concrete structures. It is responsible for more than 80 per cent of damage to reinforced concrete facilities, thereby continuing to rack up the repair costs for owners and developers.
One of the technologies used to waterproofing joints is a polyvinyl chloride (PVC) waterstop, also known as the traditional “dumbbell” due to its shape. These plastic sheets are placed across the joint before the concrete is poured to create a physical barrier for blocking water penetration. This type of waterstop relies on the ribs in the design to prevent water from passing through the joint. The PVC waterstop is great for blocking water, and is relatively inexpensive, but has limitations. Improper compaction around the waterstop creates a pathway for water—PVC waterstop can bend while concrete is poured, forming a tunnel and area where water can infiltrate. This does not mean the waterstop is not working; an installation deficiency or concrete compaction is causing leakages. To make matters worse, it is virtually impossible to recognize the problem until the joint begins to leak, which is too late. Additionally, when PVC is considered as a single barrier, water can seep in until it reaches the waterstop. As you can see in Figure 1, the rebar on the water side of the joint is exposed to the water and will corrode.
These difficulties have opened the door for other waterstop systems such as hydrophilic swelling strips and crystalline waterstop coating.
The swelling waterstop is a synthetic rubber strip designed for waterproofing construction joints. Synthetic rubber swelling strips act as a physical barrier and can swell up to 1000 per cent of original volume to seal the construction joint and stop water flow. Its ability to swell and block water intrusion, not only under hydrostatic conditions, but also when facing salt or contaminated water, sets it apart from other products. This is important considering the state of the moisture in below grade and marine environments.
Crystalline waterstop coating is a powder mixed with water to a slurry consistency, and brushed on the joint area. It utilizes advanced integral crystalline waterproofing (ICW) technology to block the movement of water through concrete joints and acts as a chemical barrier. The crystalline waterstop coating exhibits a hydrophilic property, meaning, the special chemicals in the coating react on exposure to water, allowing millions of long, needle-shaped crystals to grow deep into the concrete mass. These crystals permanently block and prevent the passage of water through capillary pores, micro-cracks, and joints, thereby making the joint waterproof. As long as moisture remains present, crystals continue to grow throughout the concrete. Once the moisture content is reduced, the crystalline chemicals lie dormant until another dose of water causes the chemical reaction to begin again.
Crystalline waterstop coating can be used in conjunction with a swelling waterstop at all static concrete-to-concrete joints where water penetration is a concern. This is called a double protection system. Crystalline waterstop coating as single protection can also be employed as a dampproofing treatment when the joint is subjected to water with low or no hydrostatic pressure.
Since crystallization takes some time to occur, corrosion of rebar on the wet side is a concern in both single and double protection systems as it is with the PVC system (Figure 2). However, testing has demonstrated the coating not only acts as a chemical barrier to prevent water penetration, but also protects the rebar.
To investigate the effects of crystalline coating, a joint research study was conducted by a concrete and waterproofing solutions manufacturer and a University of British Columbia co-op student. This study was performed in two phases at the Kryton Research Centre, Vancouver, in December 2016. In the first phase, half-cell potential measurement, as well as visual inspection, was used to analyze the corrosion mitigation of coated steel reinforcing bars embedded in concrete. In the second phase, in parallel to the corrosion tests, a modified pull-out test was employed to assess the bonding of the materials with concrete and steel.
Phase 1: Corrosion
Overall, when the coating is applied at the joint, the interface and bottom section of rebar are coated with crystalline coating. Hence, when water penetrates through cracks, it contacts the coated rebar instead of the rebar directly. Thus, in this phase, the corrosion resistance of reinforcement steel with crystalline waterstop coating was compared with uncoated rebar. The coating was applied to some samples. After curing, the coated and uncoated samples were directly immersed in a sodium chloride (NaCl) solution for a certain period. This setup simulates a marine environment.
At the end of each testing period, a visual inspection of the specimens was conducted to compare the level of corrosion. Additionally, a half-cell potential measurement device was employed to monitor the corrosion process of the specimens in accordance with ASTM C876-09, Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete. Further, the potential readings were analyzed in accordance to the International Union of Laboratories and Experts in Construction Materials, Systems, and Structures’ (RILEM) book TC-154, EMC: Electrochemical Technique for Measuring Metallic Corrosion.
After monitoring the samples for 28 days, half-cell potential measurements showed crystalline coating material directly applied on the rebar surface, dramatically delayed the corrosion activity compared to the control (uncoated) samples. These results were also confirmed by a visual inspection (Figure 3).
Phase 2: Bonding
Previous studies have shown any coating on the reinforcement bars in concrete structures could affect the ultimate bond strength of the rebar. Therefore, as part of this research project, it was critical to assess the impact of crystalline coatings on the bond strength of embedded rebar with surrounding concrete. One of the most commonly used test methods for assessing the ultimate bond strength of embedded rebar is the pull-out test. According to RILEM book RC 6, Bond test for reinforcement steel, five samples are needed to make the comparison in this test.
Therefore, 10, 10M black steel rebars were cut into 152 mm (6 in.) pieces. After cleaning the bars, crystalline coating was brushed onto five rebars. The coated bars were then cured for hardening. After 24 hours of initial curing, the coatings had sufficiently hardened. The bars were then placed into cylindrical-shaped moulds. A batch of concrete was prepared in accordance with ASTM C192, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory, and poured into the cylindrical moulds. Filled concrete moulds were transferred into the curing room, covered with plastic tenting, and sprayed with water every day for 28 days. After 28 days of curing, the specimens were subjected to a modified pull-out test using a compression machine.
The results show the crystalline coating did not reduce the bond strength of the reinforcement steel bar with its surrounding concrete, but increased this ultimate bond strength by 3.85 per cent. The control (untreated rebar) had a bond strength of 5.501 MPa (797.8 psi) while the treated sample had a bond strength of 5.717 MPa (829.18 psi).
Based on the complexity of new buildings, limitations of traditional methods, and the goal to make more sustainable structures, advanced waterproofing solutions, especially for high-risk areas such as construction joints, seem necessary. Crystalline coating and swelling waterstop can provide a reliable barrier and prevent water penetration through the joint during its service life. The solution not only prevents water penetration, but also has the potential to reduce the corrosion of rebar in concrete without any negative effects on the ultimate bond strength.
Alireza Biparva, M.A.Sc., LEED GA, is technical manager and concrete specialist at Kryton International. He has more than 10 years of experience in the field of concrete permeability. Biparva oversees a variety of research projects, focusing primarily on concrete permeability studies and the development of innovative products and testing methods for the concrete, waterproofing, and construction industries. He is an active member of the American Concrete Institute (ACI). Biparva has published several research papers in international journals and conferences on concrete permeability, waterproofing, durability, and sustainability. He can be reached at firstname.lastname@example.org.
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