Construction joints, also referred to as “cold-joints,” occur when one section of concrete is placed and cured or partially cured before the adjacent concrete placement occurs. This occurs frequently in concrete structures at locations including
● Transitions between horizontal and vertical components
● When formwork is insufficient to finish the structure in one placement, such as long lengths of wall area
● Where design elements require a change in form design
● When concrete placement is stopped, for schedule reasons or end of workday
In most of these cases a joint is not actually formed; the cold or construction joint reference refers to the area of concrete structures where two different concrete placements have occurred (properties of concrete preventing it from forming an excellent bond to itself and the previously placed concrete). In addition, control joints are added to a poured-in-place concrete structure to control cracking that occurs from shrinkage in large placements. Control joints are typically recommended for installation at no more than 30 ft apart. The joints are typically the weakest points of the concrete structural components, but not subject to movement other than structural settlement.
Below-grade conditions present conditions that make it very likely that water, which is present under hydrostatic pressure, will infiltrate through these construction joints. To prevent this from occurring, waterstops are commonly specified for installation at every construction joint on concrete work below-grade. The capability of waterstops to prevent infiltration at these weak points in the structure is critical to successful waterproofing of below-grade structures, so their importance should never be underestimated.
Waterstops are used for waterproofing protection on a variety of below-grade concrete structures including
● Water treatment facilities
● Sewage treatment structures
● Water reservoirs
● Locks and dams
● Basement wall and floors
● Parking structures
● Marine structures
Waterstops are premanufactured joint fillers of numerous types, sizes, and shapes.
Waterstops are available in a variety of compositions including
● Polyvinyl chloride (PVC)
● Neoprene rubber
● Thermoplastic rubber
● Hydrophilic (modified chlorophene)
● Bentonite clay
● Asphalt plastic
The first three, PVC and rubber types, are manufactured exclusively for use in poured-in-placed concrete structural elements. The remaining three, while mainly used for concrete installations, can be used with other building materials such as concrete block and are also excellent where installations involve metal protrusions in or adjacent to the construction joint. Manufacturers also make waterstops that are resistant to chemicals and adverse groundwater conditions. A summary of the properties of the various type water-stop is shown in Table 2.1. As with many products, manufacturers have begun making systems that approach “idiot-proof” installations.
TABLE 2.1 Comparison of Various Waterstop Types
PVC waterstops have long been the standard within the construction industry. They are provided in a variety of shapes and sizes for every situation to be encountered, as shown in Fig. 2.16.
|FIGURE 2.16 Typical PVC waterstops and their properties.|
PVC waterstops with the dumbbell shape in the middle are used for installation where actual movement is expected in the substrate, typically not thermal movement but structural movement. Figure 2.17 shows an expansion joint installation with the bulb portion of the waterstop left exposed to permit movement. However, waterproofing applications require the joint to be filled with a properly designed sealant joint to permit a waterproofing below-grade membrane to run continuously over the joint.
|FIGURE 2.17 Use of PVC waterstop in expansion joint.|
The problem with PVC waterstops is their susceptibility to improper installation (99% principle) or damage during the concrete placement. The waterstop must be held in place properly during the first half of the concrete placement. This is accomplished by a variety of methods as shown in Figs. 2.18 and 2.19. This situation is not idiot-proof and must be carefully monitored for quality control to ensure that the waterstop remains positioned during both halves of the concrete placement activities. Far too often, the PVC waterstop ends up folded over, preventing it from functioning properly. In addition, workers installing the reinforcing bars will often burn, puncture, or cut the waterstop.
|FIGURE 2.18 Placement and securing of waterstops at|
|FIGURE 2.19 Placement of waterstop for first|
half of concrete placement.
In striving to make waterstops idiot-proof, manufacturers have created several alternatives to the PVC standard including many hydrophilic derivatives. These systems, along with the bentonite and asphalt plastic, are used mainly for control joints and not provided for expansion joints. These systems are simple to install, and do not have to be installed in both sections of concrete placements. The material is adhered directly to the edge of the first concrete placement in preparation for the second placement of concrete. Note this detailing in Fig. 2.20 and in the photograph of the installed product, Fig. 2.21.
|FIGURE 2.20 Typical installations of hydrophilic or similar waterstop materials|
|FIGURE 2.21 Installed asphaltic waterstop.|
The materials generally expand after being wetted by the water contained in the concrete mixture. This swelling action enables the materials to fill the voids within the joint to form a watertight construction joint. Since these products expand in the presence of water, they must not be wetted prematurely.
This requires that the second concrete placement take place almost immediately after the waterstop placement, otherwise the joint might expand if exposed to rain or dew. The asphalt plastic is not susceptible to moisture like bentonite or hydrophilic materials, but their limited elastomeric capabilities might prevent the complete sealing of the joint if some areas are not bonded properly.
The materials are easily installed in a variety of positions for properly detailing watertight joints below-grade as shown in Fig. 2.22. None is meant for exposure to the elements and must be completely covered by the concrete placement. As such, they present limited expansion capabilities for the substrate. When an expan-
sion waterstop material is required, the PVC or rubber types are required.
|FIGURE 2.22 Several recommended uses of hydrophilic waterstop.|
Waterstop size is determined by the expected head of water pressure to be encountered at the joint. Table 2.2 summaries the recommended waterstop and minimum depth of embedment into the concrete sub- strate for various head pressures. Actual site conditions vary, and these measurements should be used only as approximations.
TABLE 2.2 Suggested Waterstop Sizing for General Conditions
Waterstop manufacturers will recommend actual joint design when actual job conditions are submitted for review.