It is essential to protect structures from ingress of foreign matter such as dirt, dust, water, chemicals and organisms. Depending on the requirement of each joint there are various types of sealing solution available in the market. In this article some of the very commonly used joint sealants in civil construction are discussed
One of the most essential materials for making the building envelope waterproof is the joint sealant. Joints are there invariably in all civil construction. According to material of construction, there are different kinds of joints. Some are only for joining, some are for accommodating the movements. Movements are mainly because of thermal gradients caused by temperature variations. There are also movements due to seismic activity, settlement, shrinkage, creep and dynamic loads, temperature variations due to processes etc. All joints which are formed due to above reasons must be properly sealed, monitored and maintained.
There are mainly two types viz. Liquid applied and Pre-formed. Varieties of polymers are used to manufacture liquid applied joint sealants. These include polysulfide, polyurethane, modified polyurethanes, silyl terminated polyether’s, SPUR’s, polymer modified bitumen, flexible epoxy, silicones and butyl rubber based materials are now being used.
Joints in concreteThere are different types of joints in concrete. As there is always a shrinkage involved while using cement concrete, this factor must be considered while concrete placement. Concrete cannot be poured in thick sections or longer spans, for two reasons. First reason is the heat generated in the concrete due to hydration of cement, which causes differential thermal gradient between the core and the surface. As the mass increases this gradient also increases. As per standard practices this gradient shall be as close as or less than 200C. This limits the placement volume. Other reason is that, most of the times continuous pours are not possible or placement may get interrupted due to unavoidable delays. Apart from this longer and wider sections may crack due to drying, chemical, autogenous shrinkage, creep or thermal expansion and contractions. Expansion joints, control or contraction joints, isolation joints and detailing joints are some of the type of joints in buildings.
Expansion joints are provided to allow movement in a structure caused by thermal or other factors such as wind loading. Control joints are provided for expected cracking due to factors such as settlement, drying shrinkage or separation in building materials after construction. Internal control joints are typically non-moving while in the exterior they also accommodate movement. External control joints require more design work.
Isolation joints are placed at junctures where changes in material require isolation for any differential movement between two different materials. Typical example is window frame perimeter abutting façade materials.
Detailing joints are designed as a part of waterproofing system. They are vital to impart water tightness at structure details such as pipe penetrations and changes in planes prior to application of primary water proofing compounds. There are some thumb rules for design of joints. They are:• Joint size no smaller than 6 mm• Generally limit the joint size in buildings to 25 mm• Joint opening shall be kept at a minimum of four times anticipated movement.
Sealant design shall take following rules in consideration while deciding width and depth.• Minimum material thickness shall be 6 mm• Up to 12 mm wide joints shall have Depth (D) = Width (W) Wider joints and traffic joints shall have D = W/2• Three way adhesions is not allowed so use of backer Rod or Bond breaker tape is a must• While using PE closed foam backer rod, it shall be 25% more in diameter than the joint size• Porous surfaces generally require primer to avoid passage of air / impurities to sealant in wet stage and for proper adhesion.
In liquid applied sealants there are mainly two types, viz. single component and two components. Single components PU based sealants such as KEM JOINT PU 100 work on a principle of curing by reaction with atmospheric moisture. Polysulfide based sealants such as KEM Joint GG and KEM Joint PG, have two component systems. These cure because of chemical reaction between the hardener and base components.
The illustrations contained herein provide an insight into application and mixing methods.
Standards, Specifications and GuidelinesFollowing is the list of some of the important specifications and guidelines which may help in understanding the joint design essentials and sealant specifications.• BS 4254 – Specification for two part polysulphide based sealant• BS 5212 – Cold applied joint sealant systems for concrete pavements• ASTM C 920 – Standard Specification for Elastomeric Joint Sealants• ASTM C 834 – Standard Specification for Latex Sealants• ISO 11600 – Building Construction – Jointing Products – Classification and requirements for sealants• ACI 224.3R – Joints in concrete construction• BS 6093- Design of joints and jointing in building construction• ACI 504R – Guide to sealing joints in concrete structures. Both ACI 224.3R and BS 6093 code of practice give guideline for design of joints• ACI 224.3 Section 1.2 guides in designing of joints based on characteristics such as resistance, configuration, formation, location, type of structure, and function • Resistance: Tied or reinforced, doweled, non doweled, plain.Configuration: Butt, lap, tongue, and groove.Formation: Sawed, hand – formed, tooled, grooved & insert formed.Location: Transverse, longitudinal, vertical & horizontal.Type of Structure: Bridge, pavement, slab – on – grade & building.Function: Construction, contraction, expansion, isolation & hinge.Example: Tied, tongue and groove, hand – tooled, longitudinal pavement construction joint.
Some other considerations which are important are discussed below. Sealants are classified as elastic, elasto-plastic or plastic according to their response to movement.
Movement Accommodation Factor (MAF) indicates the range of movement between maximum compression & maximum extension that the sealant can accommodate. This is generally expressed as percentage. While designing a joint, the specification joint gap width should be derived from all deviation to which the joint is subjected. However, minimum joint gap widths are required for the satisfactory performance of sealants in butt joints. These are generally calculated based on the MAF provided by the sealant manufacturer and the Total Relevant Movement (TRM) of components at the joint.
The stresses induced in a sealant in a joint subject to movement are more critical in plain butt joint than in a lap joint. The latter may accept up to twice as much movement as a butt joint using the same compound and joint dimensions. The joint seal shape is important for elastic and elastoplastic sealants when they are used in joints subject significant movement. It has been establish that for elastic sealant optimum performance is obtained at ratios of about 2:1 width:depth and that ratio should generally not be less than 1:1, subject to minimum depth of 5 mm.