Arab Construction World – January 2009
Concrete is the world’s most commonly utilized building material. It is used in residential and commercial construction, bridges and roadways, water containment, sidewalks, driveways, and a myriad of other applications.
The reason for concrete’s popularity? It is versatile and flexible enough for use in many forms, including precast, cast-in-place, and shotcrete. Concrete is economical and structures can be built quickly, reducing construction timelines and labor costs. Concrete is durable and highly resistant to fire, explosion, and impact. It is also energy-efficient, sustainable, and recyclable. While concrete provides numerous benefits, it also presents challenges that must be managed to ensure quality results. One of the greatest challenges in working with concrete is cracking. Concrete cracks generally occur for one of three reasons: changes in volume due to drying shrinkage, direct stress due to applied loads, or flexural stress due to bending. Whenever the stress within concrete to pull apart exceeds the strength of the concrete to hold itself together, the concrete will crack.
Controlling Cracking With Concrete Joints
While concrete cracks cannot be prevented entirely, they can be controlled and minimized through proper jointing practices. A concrete joint is essentially a pre-planned crack. Properly spaced joints in a concrete wall, slab, or pavement help to accommodate shrinkage and contraction and prevent unsightly rand m cracking.
Esse itially, there are three types of concrete joints: crack-inducement joints, isolation joints, and construction joints.
Crack-Inducement Joints
Also known as control joints or contraction joint, crack-inducement joints create a plane of weakness and induce straight-line cracking at controlled, pre-se ected locations. Formed using a saw, hand-finishing tool, or pre-molded strip; crack-inducement joints extend to a depth of roughly one-quarter of the concrete’s thickness. These joints are commonly used in sidewalks, driveways, pavements, floors, and walls.
Isolation Joints
Isolation joints, also known as expansion joints, separate concrete slabs from structural elements such as walls, footings, or columns, and permit movement between the abutting faces of the slab and the structural elements. Intended to be dynamic, isolation joints prevent the restraint of movement that can lead to cracking. Isolation joints are commonly used to separate driveways and patios from sidewalks, garage slabs, or stairs. They extend the full depth of the slab and include a flexible, pre-formed joint filler.
Construction Joints
Construction joints are those that occur at the end of a day’s work. Also known as cold joints, construction joints provide stopping places during construction and separate sections of concrete, such as slabs or walls that are placed at different times. Although a true construction joint permits neither horizontal nor vertical movement, construction joints may align with, and function as, control or isolation joints.
The Importance of Waterproofing Joints
While joints play a vital role in fortifying concrete structures, they represent the most vulnerable part of the structure from a waterproofing perspective. Without an effective joint waterproofing system (known as a waterstop system), it is not a matter of if a joint will leak, but when.
A leaking concrete joint is more than simply a costly inconvenience. In the case of residential or commercial structures, incoming water or moisture can lead to the growth of mold or fungus. A leak in a concrete water tank can contaminate potable water or facilitate the escape of waterborne contaminants into the surrounding environment. Incoming water or contaminants can corrode steel reinforcement, jeopardize structural safety, and shorten the lifetime of a concrete structure.
Dynamic or moving joints such as isolation joints are waterproofed by virtue of the flexible, pre-formed joint filler that is inserted in the joint. With construction joints, however, an effective waterstop system must be used. Traditionally, waterstop systems have relied on physical barriers to block water from penetrating through joints. In the past quarter-century, however, more advanced and economical systems and materials have been developed to provide easier and more efficient application and longer-lasting protection.
Types of Concrete Waterstop Systems
PVC Waterstops
PVC waterstops are flat strips of high-quality PVC that are embedded into both sides of a construction joint to provide a physical barrier. Specially shaped to create a better bond with concrete, PVC waterstops are available in a variety of thicknesses, widths, and sizes.
Unlike hydrophilic waterstop systems such as bentonite or urethane, PVC waterstop can be installed during rainy or wet conditions. Some brands feature chemical- or oil-resistant properties.
Moreover, relative to other waterstop systems such as bentonite, PVC water-stops have a long useful life. PVC waterstop systems present two key challenges. Since they must be embedded and maintained in an upright position during concrete pours, they are time-consuming to install. PVC can easily become damaged during a concrete pour, and it is virtually impossible to know whether damage has occurred until the joint begins leaking. For best results, PVC waterstops should be installed by skilled trade people.
While PVC itself is highly affordable, the need for experienced, specialized labor results in installation costs of approximately US$5 per linear foot.
Bentonite
Bentonite is a swellable clay waterproofing compound that is glued or nailed in strips into construction joints. Known as a hydrophilic waterstop system, bentonite expands up to sixteen times its dry volume when it is exposed to water, forming a compression seal in concrete joints. Bentonite’s ability to swell enables it to fill small cracks and voids in concrete, preventing the ingress of water around joints. Since clay is a natural material, bentonite is a popular waterstop choice for potable water applications.
As with other hydrophilic waterstop systems, bentonite must be kept dry prior to installation and concrete must be poured immediately after the bentonite is applied. Exposure to rain or moisture can cause bentonite to expand prematurely, damaging joints and weakening concrete as it cures. As with PVC waterstops, bentonite can be displaced or damaged during subsequent concrete pours and extra care must be taken to ensure an effective application.
Another consideration with bentonite systems is that the clay eventually dries out and deteriorates. Bentonite water-stops have a finite number of wet-anddry cycles, making them less suitable for extreme weather conditions or environments that frequently become wet.
While bentonite is relatively economical, application can be labor intensive. A bentonite system will cost approximately US $3.75 per installed linear foot.
Urethane Waterstops
Similar to bentonite, urethane waterstops are spongy, hydrophilic compounds that, when exposed to water, swell up to 350 percent of their original volume, forming a compression seal in concrete joints. Urethane waterstops can be applied in strips or with a caulking gun, which can help to reduce application time.
As with other hydrophilic waterstop systems, exposure to moisture can lead to premature hydration and expansion of urethane waterstops, which can crack concrete or cause blowouts. Once applied, urethane waterstops must be allowed to cure for 24 hours before concrete is poured, and keeping the applied urethane dry in the interim is essential.
Like bentonite and other physical barrier systems, urethane waterstops should only be installed by qualified professionals. Special care must be taken to ensure proper placement and avoid damaging or displacing the urethane during concrete pours. Similar to other swellable waterstop systems, urethane waterstops will eventually dry, crack and deteriorate. Depending on the type of urethane used, a urethane waterstop system will cost US$7 to US$15 per installed linear foot.
Metallic Waterstops
Made of steel, copper, bronze or lead, metallic waterstops are embedded in concrete across joints to form a continuous, fluid-tight barrier. Metallic waterstops are used in specialized applications such as in dams or heavy construction projects where additional strength is required, or where exposure to chemicals or extreme temperatures could damage conventional physical waterstops.
Metallic barriers must be embedded and maintained in an upright position across the joint while pouring concrete to ensure they do not fall over or become damaged during pours. Skilled labor is required and the material and labor costs for these waterstop systems are extremely high.
Crystalline Waterstops
Crystalline waterstop systems utilize advanced integral crystalline waterproofing (ICW) technology to block the movement of water through concrete joints. When applied to concrete, ICW chemicals cause microscopic crystals to grow, permanently sealing the spaces between concrete particles and blocking the movement of water. In a crystalline waterstop system, a cementitious mixture containing highly concentrated ICW chemicals is applied to the joint site before a new wall is poured.
Crystalline waterstops are growing dramatically in popularity because they offer several crucial advantages over other systems. They are quick and easy to install and do not require skilled labor. Premium crystalline waterstop systems have the ability to self-seal small cracks. When micro-cracks form in crystalline-treated concrete, incoming water causes additional ICW crystals to grow, filling the crack and maintaining a watertight seal. Some crystalline waterstops combine two levels of protection: a virtually indestructible physical barrier and a crystalline chemical barrier. (Many crystalline waterstop manufacturers, such as The Kryton Group of Companies, also produce waterproofing systems for walls and slabs, offering one-manufacturer accountability for an entire structure.)
Unlike some other waterstop systems, crystalline technology lasts the lifetime of the concrete structure, and unlike the vast majority of waterstops, crystalline technology can be used to retrofit areas where no waterstop system was installed, or where the installed system has become damaged or deteriorated over time.
Crystalline waterstops are highly affordable and can cost up to 50 percent less than bentonite or PVC waterstops. An average crystalline waterstop system will cost about US$2.50 per linear foot, installed.
Choosing the Right Waterstop System
With so many different types of water-stop systems available, it is important to choose the one that is right for each concrete construction project. Since costs and installation times vary widely between systems, budget and construction timeline should be key considerations. Consider also what is at stake if the waterstop fails: if a leak jeopardizes zero-tolerance areas such as electronics or computer rooms, you may want to choose a waterstop system that is less likely to become damaged during concrete pours. Ask about the life expectancy of the system you are considering: some have finite lifecycles while others last the lifetime of the structure.
The best way to protect a concrete structure and achieve an effective, watertight seal is to combine a reliable waterstop system with an effective concrete water-proofing system. Your concrete supplier can provide assistance in the concrete waterproofing system that is best for your project.
Author
Founded in 1973, The Kryton Group of Companies develops, manufactures and markets a wide range of products designed to waterproof, repair, and protect concrete structures. Kryton has grown to become a multi-million dollar company with offices, agents and manufacturing facilities around the world. For more information about Kryton and its products, visit www,kryton.com.
Sources
• Concrete in Practice 6: What, Why & How? Joints in Concrete Slabs, National Ready-Mixed Concrete Association, 1989.
• Design and Control of Concrete Mixtures, Steven H. Kosmatka, Beatrix Kerkhoff, William C. Panarese, Norman F. MacLeod and Richard J. McGrath, Cement Association of Canada and the Portland Cement Association, 2002.
