The Physics Behind the Lessons Learned in a Full-Size Stand-Alone Blindside Waterproofing Mockup
Abstract
Burying a pre-applied waterproofing system between an earth retention system and newly installed concrete, never to be seen again, creates a difficult scenario whereby designers, manufacturers and contractors are left to wonder just how well their system will perform. This paper will discuss a building that included two stories of below-grade construction that is located at the corner of a project site close to the intersection of two streets. The placement of the building created a situation with tight site constraints which required an earth retention system and a blindside waterproofing application. To add to the complexities of this specific application, the contractor proposed to use structural shotcrete to create the foundation walls. The contractor recommended the use of a proprietary pre-applied blindside membrane based on the history of working with the specific manufacturer's products on several other successful projects in the past. In order to better understand the product's capabilities and limitations, an evaluation process that included the assembly and forensic deconstruction of a full-size structural shotcrete mockup was performed.
There are three areas of concern often posed when evaluating pre-applied waterproofing membranes:
Does the system bond to the concrete?
Will the system survive the installation of the structural shotcrete?
Can the system resist lateral water migration?
From the lessons learned during the assembly and deconstruction of the mockup, we will review the findings and use the scientific method with applied physics to evaluate some of the original assumptions and to potentially validate the findings.
Learning Objectives
Develop an understanding of a pre-applied waterproofing system within a structural shotcrete wall assembly.
Develop an understanding of the physics and importance of bonding a pre-applied waterproofing system to a structural shotcrete wall assembly.
Develop an understanding of the physics and importance for the survivability of a pre-applied waterproofing system in a structural shotcrete wall assembly.
Develop an understanding of the physics and importance to resist lateral water migration of a pre-applied waterproofing system in a structural shotcrete wall assembly.
Introduction
Burying a pre-applied (blindside) waterproofing system between an earth retention system and newly installed concrete, never to be seen again, creates a difficult scenario where designers, manufacturers and contractors are left to wonder just how well their system will perform. The systemic issue with blindside waterproofing is that its application to concrete is never seen by the installer, hence the application name being "blindside." Manufacturers of these systems typically use the term pre-applied, which describes when the system is installed, before the concrete. The use of the word blindside or pre-applied are synonymous and will be used throughout this paper to have the same meaning.
The systemic issue of blindside waterproofing creates an application that is risky by nature, then begs the question of "why use it?" While it is preferred to avoid blindside waterproofing applications when possible, at times, the project specific requirements may require its inevitable use. The primary reason blindside waterproofing is used is typically the limitation to fully excavate a site. These limitations could be due to the size of the building as it relates to the lot size (zero lot size buildings) or obstructions to the site (adjacent roadways, or buildings).
On a recent new university laboratory building, the project specific conditions included a site that was limited in space for full excavation, which required the use of an earth retention system and the use of blindside waterproofing to address the two stories of below grade construction. The project team developed and performed an evaluation process that included a full-size stand-alone blindside waterproofing mockup for a structural shotcrete wall.
The lessons learned from the stand-alone mockup included the following:
Observation of the other trade's impact.
Resolving project specific details such as:
tie-wire "striker pin" penetrations
overspray of shotcrete onto adjacent exposed waterproofing
the impact of shotcrete on the membrane
de-tensioning of tiebacks
securement of waterproofing at end of a pour/shotcrete
The bond of the waterproofing to the shotcrete by destructive evaluation.
These lessons learned were only possible as the mockup was able to be viewed from the backside, which is not possible on the actual final in-situ construction. The temporary shoring wall of the mockup was able to be deconstructed in order to observe and evaluate the waterproofing membrane. There were three surprising observations that would have not been possible without deconstructing the mockup/shoring wall:
Significant influence the shotcrete overspray affected the bond between lifts of the concrete placement.
Large areas of moderately consolidated shotcrete were able to be discovered.
The tenacious bond of the blindside membrane to the area of well consolidated shotcrete.
The stand-alone mockup allowed the project team to understand the demands placed on the waterproofing system, the demands on the waterproofing installers, the demands of the shotcrete technicians, provided insight into improving the installation processes on the project, and gave the project team a better understanding of the obstacles inherent in structural shotcrete blindside waterproofing installations. For more information about the lessons learned specific to the stand-alone blindside mockup and the process of how the evaluation of the system occurred, refer to the article that can be found in the Interface magazine published in August 2019 titled, "Full-Scale Stand-Alone Blindside Waterproofing Mock-up and the Lessons Learned."
Constructing a stand-alone full-scale mock-up provided not only the opportunity to assess the viability of the system selected for the project, but also provided the valid criteria for judgement (were we asking the right questions). To further understand these lessons learned, one has to move beyond the observations made during the mockup and begin asking the questions of why they are so (why does the system work). Understanding the physics that occur during the construction of a blindside waterproofing wall may provide valuable insight for future designs of these systems. There are three benchmarks which pre-applied/blindside waterproofing systems have traditionally been measured against:
Does the system bond to the concrete?
Will the system survive the installation of the shotcrete?
Can the system resist lateral water migration?
An important aspect to understand is that the lessons learned, and the research directed at those lessons are system specific and were intended to assist in validating if the system was viable for this specific project. This is not to say that discoveries are not applicable to other system designs, only that the research did not test other systems at this time. The system selected and researched is composed of a Backing film, Rubberized asphalt, & Geotextile composite membrane (BRG).
Other prevalent systems that were not evaluated in this research but may find similar results include: High Density Polyethylene (HDPE) & Shielded Acrylic Adhesive, Thermoplastic Olefin & Reactive Butyl Alloy, and Expansive Compound (bentonite clay or hydrophilic polymer gel) in Geotextile containment. For description purposes the whole assembly is considered the membrane.
Learning Objective #1: Develop an understanding of a pre-applied waterproofing system within a structural shotcrete wall assembly.
A blindside wall is constructed from the outside in, beginning with an earth retention system. There are several methods on how to retain the earth. One method utilizes soldier piles and lagging that consist of a steel H-piles encased in concrete with wood timber lagging boards retaining the earth. A molded sheet drainage panel was installed onto the lagging and then followed by the installation of the waterproofing membrane. The reinforcing steel for the concrete wall is then installed. In some cases, the height of the wall may dictate the need for tie-wire anchors to restrict movement of the reinforcing steel as the shotcrete is placed. It is recommended to install the tie-wire anchors prior to the reinforcing steel to allow an opportunity to detail the waterproofing around each penetration without being obstructed by reinforcing steel. The final step of the wall construction is the placement of the shotcrete.
In general terms, most blindside constructed walls are similar. Some designs may incorporate more items or achieve the designed components differently. For example, the design of the earth retention system is highly dependent on the conditions of the soils and the depth of the excavation may require a more robust design. The system components previously described were used on the full-size mockup and noted in a generic fashion. To provide a better understanding of the system, each component of the wall is described in greater detail below.
Earth Retention System
Soldier piles are placed into the soil for the appropriate depth depending on design conditions. Some soil conditions require the piles to be stabilized with concrete. As excavation of the soil begins the unexcavated soil is retained with wood timber lagging boards. The lagging boards are placed from the top down to the full depth of the excavation, by placing each lagging board behind the inboard facing H-pile flange. Additional lateral support will be implemented with the use of a tensioned tieback anchor. The goal of this component is obvious - to provide additional support for the lagging wall which ultimately retains the soil. The tiebacks can either be abandoned in place or they are removed, or de-tensioned, once the foundation is completed. Some project constraints require the removal of the earth retention system and if not accounted for they can cause damage to a below-grade waterproofing system. However, it can be overcome in the design of the wall assembly. For the example project included in this paper, the earth retention system remained in place.
Drainage System
A molded-sheet drainage panel is often secured to the earth retention system prior to installation of the waterproofing. Many of the typical panels consist of a plastic dimple board core faced on one side with a geotextile fabric and the other side with a polyethylene sheet. The fabric face is placed against the earth retention system to assist in filtering soil fines to help prevent clogging of the below grade drainage system. This drainage layer is the first line of defense against water infiltration into the building. The drainage panel collects water and with gravity directs the water down to the footing. The water is collected into a collector box, which is similar to the molded-sheet drainage panel, but thicker, creating a higher capacity to collect the water. Due to the nature of the lagging system a drain tile has to be installed inboard of the foundation wall rather than outboard. PVC piping is used to drain the water from the collector box through the foundation wall and into the drain tile system that sits below the basement slab. Clean outs are typically provided inside the basement in the event of a clog in the drain tile. The drainage layer can remove a significant amount of water below grade before it reaches the waterproofing membrane.
Waterproofing Membrane
A sheet applied membrane is commonly used for blindside waterproofing applications. The BRG system selected for the example project is a composite laminated membrane comprised of 4 mil cross laminated HDPE backer, 65 mils of rubberized asphalt, and non-woven polyester geotextile. This particular BRG system is secured through the molded-sheet drainage panel to the wood lagging of the earth retention system. Fasteners are placed at the seams and at the top of the wall assembly. Each fastener is covered by the subsequent sheet within the seam which provides a continuous plane of membrane that does not include penetrations. Though penetrations should be avoided, they will occur and require detailing. Typically, the detailing will use a fluid-applied product in combination with a sheet applied detail strip. Each sheet applied manufacturer will vary on the required method penetrations are detailed.
The waterproofing layer is the component in the wall assembly that creates a plane where water should not pass through. ASTM D 1079 - Standard Terminology Relating to Roofing and Waterproofing defines waterproofing as "treatment of a surface or structure to prevent the passage of water in its liquid phase under hydrostatic pressure." Hydrostatic pressure can be caused by ground water or perched water that is trapped above a layer of clay. For the new laboratory building, the two-story basement was above the ground water level. Permanent ground water is not the only source for water infiltration into a basement. Rain events are also sources for water to enter a building below grade. Though rare, utility leaks below grade could also become a source of water that infiltrates into a building. Given the use of the two-story below grade basement lab space, the risk of water infiltration below-grade could be detrimental to the function of the building. The waterproofing layer acts as the last line of defense to protect the below grade portion of the building from water intrusion.
Shotcrete Wall
The final component of this assembly is the structural shotcrete wall which includes the reinforcing steel. In lieu of a shotcrete application, cast-in-place concrete can also be used. The lab building used structural shotcrete and the research presented in this paper was based on that application. However, similar lessons learned and learning objectives could also be addressed for cast-in-place concrete. After the placement of the concrete footing, reinforcing steel is placed in a rigid grid system. Loading requirements will affect the size of reinforcing steel as well as the placement or spacing of the reinforcing steel. Depending on the size of the wall, some installations will require the use of tie-wire anchors to minimize the reverberations of the reinforcing steel during the placement of shotcrete. The tie-wire anchors are installed prior to the reinforcing steel to allow the waterproofing contractor the opportunity to detail each anchor. In the case of the example project, the anchors were placed approximately in a 4'x4' grid creating several hundred penetrations in the waterproofing membrane. One of the successes of the stand-alone mockup proved that this detail needs to be easily repeatable providing a secure anchor with the appropriate depth for a fluid-applied product to be applied. By minimizing the reverberation of the reinforcing steel, the shotcrete application will have a greater success of appropriate cover around the reinforcing steel as well as minimizing shadowing of the shotcrete. Shadowing is the result of voids behind reinforcing steel, typically the result of a shotcrete application occurring directly perpendicular to the reinforcing steel, as well as the movement of the reinforcing steel during the shotcrete application.
Following the placement of reinforcing steel is the shotcrete application. Before the shooting of shotcrete begins, one last review of the waterproofing membrane is conducted, and damaged waterproofing membrane is repaired. At approximately 80 mph, concrete is shot out of a nozzle against the waterproofing membrane. Once the depth of shotcrete is achieved, the excess material is cut away and recycled. The interior surface of the concrete is then completed with a trowel finish. Though shadowing can be detrimental to the success of the waterproofing membrane, it also can become problematic for the structural wall. The quality of shotcrete and its application through a nozzle-man can be validated with a certification process by the American Concrete Institute and the use of core samples through a stand-alone mockup.
The completed wall assembly has multiple components, installed by different trades, and each component being relied upon for the success of the entire assembly. The system is not complete until the entire wall assembly is installed and tied into the above-grade wall. An exposed wall (or post-applied) waterproofing system is similar in that it too has multiple trades and components. However, the advantage of an exposed wall is that the waterproofed structure can be observed prior to backfill. Additionally, exposed walls do not require the numerous tie-back anchors and tie-wire striker pins that penetrate the blindside membrane. Defects or damage to a waterproofing membrane can be observed and corrected. Aside from backfill, the waterproofed structure is complete. In a blindside waterproofing application this final observation of the waterproofing is not possible. The waterproofing installer does not apply the waterproofing to the completed structure; rather the shotcrete (or cast-in-place concrete) is applied to the waterproofing membrane.
Learning Objective #2: Develop an understanding of the physics and importance of bonding a pre-applied waterproofing system to a structural shotcrete wall assembly.
In an Interface article by Justin Henshell and Paul Buccellato titled "Below-Grade Blindside Waterproofing Membrane Systems: A State-of-the-Art Report," the authors outline blindside waterproofing membranes into two broad categories: "Attached" and "Nonattached." The selected BRG system used on the lab building is considered an attached, or bonded membrane. Bonded membranes limit lateral migration of water between the membrane and the concrete substrate. The technology used to create bonded membranes are such that the manufacturers designed the system to be bonded. Data sheets for bonded membranes publish peel adhesion strength values which describe this function for designers and demonstrate its importance of the membrane performance. As soils settle over time, or the earth retention systems degrade, a bonded membrane will remain in place where as an unbonded membrane may fall away from the structure. The integrity of the bond of a membrane is a beneficial attribute which can contribute to the long-term performance of the structure creating a composite that combines both attributes of each component (within the membrane and the concrete combined).
BRG systems typically achieve a bond to the concrete or shotcrete in two ways - mechanically with the geotextile and adhesively with the internal compound. The inherent nature of blindside applications creates a scenario where the shotcrete or concrete is applied to the waterproofing membrane. The success of the waterproofing membrane bond is directly related to the successful installation of the concrete and the capabilities of the concrete installers. Observations made during the review of the full-scale stand-alone mockup revealed the issues of shotcrete overspray. The waterproofing membrane was well bonded to the shotcrete overspray. However, the overspray did not bond well to the next lift of shotcrete. The integrity of the waterproofing system is directly related to the integrity of the waterproofing bond. Shotcrete or concrete that is not well consolidated will not bond to the waterproofing membrane. Poor consolidation can be a cause of the mix design or improper mixing of the concrete. This can create concrete that is too dry which can become very difficult to consolidate. Shadowing, previously discussed, is also a form of poor consolidation which can result in voids behind reinforcing steel. Poor consolidation is not the fault of the waterproofing installer, yet its impact on the successful installation is paramount.
The bond of the waterproofing membrane can become disbonded if the membrane doesn't account for shrinkage cracks that occur in concrete. For exposed waterproofing applications, the waterproofing installer has the opportunity to allow the cracks to develop and provide an appropriate detail to address the cracks. Bonded blindside waterproofing membranes require the ability to bridge cracks as the concrete is applied to the waterproofing membrane. BRG systems bridge shrinkage cracks by dispersing the energy caused by the shrinkage through adequate thickness of the elastomer compound. The adequate thickness of the elastomer compound disperses the energy within the membrane at the location of the shrinkage crack. Without adequate thickness of the elastomer compound the shrinkage crack energy will be transferred to other components within the system.
When pull testing the BRG system, the bond failure results can occur in three ways. The first would be adhesive failure in which the elastomer stays intact and debonds from the concrete. The second would be cohesive failure in which the elastomer is pulled apart and remains adhered to the concrete. The third would be a substrate failure in which disbondment occurs within the concrete. The compound component of the BRG system is an elastomer. When a pulling force is applied the elastomer will stretch into an hourglass-like shape. A successful test is demonstrated when a cohesive and adhesive failure of the membrane occurs. A cohesive failure of the concrete would be considered a failed test. The application of blindside waterproofing places the successful bond at the application of shotcrete and similarly with cast concrete. Based on multiple bond tests, the root cause of a failed test are the results of poor consolidation of the concrete. Consistent consolidation within shotcrete is difficult to achieve resulting in areas of well bonded waterproofing and areas where the concrete has cohesive failure represented by not being bonded. Regarding crack bridging capabilities, the BRG system accounts for both bonded and disbonded areas and performs similarly.
As part of the full-size stand-alone mockup, a separate small scale 4'x4' mockup was constructed. The small-scale mockup was a backup in the event the destructive removal of the lagging boards on the backside of the full-size mockup compromised the waterproofing bond. The small-scale mockup provided an opportunity to verify the bond of the shotcrete to the waterproofing membrane. A cohesive failure was observed as remnants of the geotextile fabric remained in the concrete mockup after pull testing was performed.
Other samples conducted as part of the research for this paper used job site mixed concrete and bag mixed concrete. The pull test for the job site mixed concrete revealed remnants of the compound and geotextile on the concrete sample demonstrating cohesive failure of the waterproofing membrane after completion of pull testing. Pull testing for the bag mixed concrete revealed poor bonding of the membrane. After the membrane was removed, the revealed surface of the concrete felt sandy and is believed to be the results of poor consolidation. Repeating the bond test for the bag mixed concrete with considerably more water allowing better consolidation revealed similar results to the job site mixed concrete.
Destructive evaluation of the full-scale stand-alone mockup provided the opportunity to observe debonded areas of shotcrete. Areas of poor consolidated shotcrete were observed on the full-scale stand-alone mockup as a result of shotcrete overspray between lifts. The shotcrete overspray bonded successfully to the BRG system. The second lift of shotcrete revealed to be poorly consolidated around the shotcrete overspray.
Manufacturers of bonded membranes conduct ASTM D 903 (Standard Test Method for Peel or Stripping Strength of Adhesive Bonds) to failure and publish the performance value on the data sheets. The force exhibited through ASTM D 903 may never occur on a membrane during its service life. However, the results represent the benchmark for a particular system to perform. The industry has not provided a recommended value for blindside membranes. However, ASTM D 7832 - Standard Guide for Performance Attributes of Waterproofing Membranes Applied to Below-Grade Walls / Vertical Surfaces (Enclosing Interior Spaces) establishes minimum values for exposed membranes. A benchmark for minimal attributes for blindside membranes may prove to be valuable. However, for structural shotcrete applications a guide for in-field testing methods to demonstrate successful installations may be more valuable.
Learning Objective #3: Develop an understanding of the physics and importance for the survivability of a pre-applied waterproofing system in a structural shotcrete wall assembly.
As stated previously, a pre-applied system is not complete until the concrete is placed, and the pre-applied system is the substrate for the structure that is not intended to be exposed again. Considering the harsh environment that a pre-applied system is exposed to before, during, and after the placement of the concrete, being a viable waterproofing for the building is a very difficult task. When utilizing shotcrete as the concrete placement methodology, the task becomes even more challenging considering the concrete will be shot at 80 mph at the waterproofing system. Survivability for all phases of construction is critical for producing a watertight building with a pre-applied waterproofing system.
Survivability Prior to Placement
The first phase of survivability is during the assembly of the components necessary to produce a structurally stable wall which includes tiebacks, anchors, and rebar. After surviving the installation, the membrane needs to then survive the elements of the environment until the concrete can be placed. In the example project, the waterproofing system was applied against a wood lagging wall that had integral wall stabilizing tiebacks. Note, in this case, some of the tiebacks were required to be de-tensioned after completion of the structural wall. The BRG system was installed on top of a molded-sheet drainage panel with an integral geotextile filter fabric that was applied over the lagging wall.
Along with providing a drain-plane for rainwater runoff, the molded-sheet drainage panel provides a unitizing effect for gaps, voids, and undulations of the shoring, reducing the opportunity for puncture damage, and dispersing point loads during placement of the shotcrete.
As indicated, another important component that contributes to the structural integrity and strength of a concrete wall is the reinforcing. In shotcrete applications, the reinforcing cage must be stabilized to reduce the oscillation during placement of the shotcrete which can create voids in the concrete. The cage is stabilized with tie-wires tied to "striker pins" (tie-wire anchors). Setting the striker pins into the shoring requires the pins to penetrate the waterproofing membrane. Sealing penetrations is a common task, but the pins typically used in non-waterproof situations posed some challenges. Initially, traditional pins were used and then the tie-wires were installed and tightened. During the tightening of the tie-wires in the mock-up, it was observed that striker pins bent under strain and damaged the membrane. The resolution was to use a pin that would not bend and could be detailed with the appropriate amount of sealant.
The survivability prior to placement of the concrete was driven more by the lessons learned and the coordination of the trades to not damage the waterproofing system than by the durability of the system. In comparison, the survivability of the waterproofing during and after the placement of concrete is directly related to the robustness of the system.
Survivability During Placement
The placement of concrete into a form lined with a pre-applied waterproofing system can be a challenging situation, but the placement of shotcrete can create a much more dynamic and challenging scenario. When evaluating the survivability of a waterproofing system, it is critical to assess the individual components and the system as a whole. The application of shotcrete applies forces that can compromise the system's watertightness by puncturing the membrane or opening the seams. After the mock was completed on the example project, the lagging was removed from the backside. The waterproofing was then evaluated by coring holes through the wall and by removing the waterproofing from the concrete where possible. Once we applied the lessons learned the BRG system was found to have survived the installation of the shotcrete and function as intended.
The lagging wall was removed to expose the BRG system. The mockup provided the opportunity to observe striker pins, a side lap, adhesion to well consolidated shotcrete and poorly consolidated shotcrete via the core holes, and the overall functionality of the system. The striker pin was well sealed. The side lap was well sealed and fully bonded. The consolidated shotcrete was fully bonded to the membrane and well-integrated into the wall. The poorly consolidated shotcrete was partially bonded to the membrane and substrate failure of the shotcrete was observed.
Dynamic Forces Applied to the BRG System
To best understand the dynamic forces in applying shotcrete, a research report "A Study of the Dynamics of Shotcrete Formwork" by Michael David can be compared to the results against the physics used in the design principles of a BRG system.
The illustration from Standard Practices for Shotcrete used in the "A Study of the Dynamics of Shotcrete Formworks" depicts the method used to apply shotcrete. An important aspect to note is that the shotcrete is applied in a circular motion. During the process, the installer will have starting points and moments of lag where there will be higher concentration of impact. These focal points are areas of high abrasion and deflection of the shoring. As the waterproofing is pre-applied, the dynamic forces documented in Mr. David's work are applied directly at the system causing significant stress.
The best way to grasp the concentration of force generated during the installation of shotcrete is by reviewing two and three-dimensional color simulation imaging that maps the deflection and acceleration of the formwork. These are static images from Mr. David's report depicting the concentration points. It is important to understand that each focal point not only is a point of concentrated abrasion on the membrane but is a pivot point that is applying stress on the surrounding system.
When viewing the application process of the shotcrete on the stand-alone mockup, it demonstrated that the focal points were not just areas of concentrated pressure, but revealed an oscillating effect vibrating the entire mockup. Not only were the contact points areas of high abrasion for the membrane, but an epicenter of shock that also stressed other components of the assembly such as the laps and penetrations. Observing the forces that were applied to the system provided the necessary insights to understand how the waterproofing system managed forces.
BRG's are typically a composite membrane. The semi-saturated geotextile provides a working surface that is trafficable (for horizontal applications) and can be handled by the installers. It can serve as a protection course to shield the membrane from impact damage and UV exposure. It also provides a mechanical bond to the concrete. The rubberized asphalt is the core waterproofing element, provides adhesion to the concrete and laps, and disperses energy. The HDPE backing film provides abrasion resistance and adds tensile strength to the membrane. All three components together create the overall durability of the membrane.
Physics can be used to develop an understanding of why and how the BRG system performed in this specific mockup and in general. Ultimate tensile strength, often shortened to tensile strength, is the capacity of a material or structure to withstand loads tending to elongate. The benefits of tensile strength cannot be realized unless applied to match the design intent of the system. For example, if the system is designed to resist the movement, high tensile strength is required, and elongation is less relevant. If the system is designed to accommodate movement, lower tensile strength is needed, and elongation becomes more important. The calculation of tensile strength for ASTM D412 (Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers-Tension) is based on the energy exerted and the thickness of the material. The ultimate energy required to resist a load will vary relative to the thickness of the material. For example, two samples of HDPE varying in thickness made from the same polymer will have the same tensile strength, but in application, the force required to elongate the two will differ.
Elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied. Modulus is important to how energy is applied to the components attached to the elastomer. Less energy is required to elongate a low modulus elastomer than a high modulus elastomer. Thus, less energy is applied to other components of the assembly with a low modulus elastomer than with a high modulus elastomer with the same elongation and thickness.
BRG systems are designed to dissipate energy and accommodate movement compared to resisting the forces. This is a critical aspect for survivability in pre-applied waterproofing applications, but is even more important in the installation of shotcrete as highlighted above. As previously indicated, the non-woven geotextile is a protection course for the waterproofing element. Shotcrete is projected directly at the membrane at up to 80 mph. The geotextile absorbs the initial impact and shields the rest of the assembly from sharp edges of the aggregate. The thick layer of low modulus elastomeric rubberized asphalt then disperses the energy that is imparted onto the HDPE backing film. BRG composite materials do well at absorbing energy and not conducting it to the rest of the system (e.g. pipe penetrations, seams, etc.). In comparison, systems that utilize a high tensile strength HDPE membrane with a thin layer of elastomeric adhesive, energy is distributed in the form of shock, vibration, and movement throughout the system. Though the membrane may survive, the surrounding components (e.g. pipe penetrations, seams, etc.) of the system receive much of the energy and movement which may adversely affect the survivability of the system as a whole.
As previously discussed, the entire waterproofing system must survive the placement of concrete and a vital part of the system is the laps. The first two key forces developed in the application of shotcrete are shock and vibration. The first blast of shotcrete generates an impact shock. The continuing application of the shotcrete creates an oscillating effect which sends vibrations out into the surrounding system. BRG systems address these forces in two ways - the membrane absorbs energy and the low modulus rubberized asphalt at the lap absorbs energy. As the rubberized asphalt is thick and low modulus, less energy from the shock and vibration on the left side of the lap is imparted to the right side of the lap. For comparison, other more rigid systems distribute the vast majority of energy in the form of shock and vibration into the lap with little buffering.
The third key force developed in the application of shotcrete is caused by the movement from deflection of the shoring. Point loading from the shotcrete application can create deflection of the shoring and the pre-applied system causing the assembly to be stretched. BRG systems address these forces in two ways - the membrane absorbs movement and the thick low modulus rubberized asphalt compresses in the seams. Movement can be dispersed by the thick low modulus rubberized asphalt core throughout the sheet. In addition, the compression in the seams is addressed by the thickness and modulus of the elastomer. The elastomer is able to elongate lengthwise but shrinks depth-wise. More rigid systems distribute the movement directly into the lap and there may be an insufficient amount of elastomer to accommodate the movement. Interestingly, ASTM D1876 Standard Test Method for Peel Resistance of Adhesives (T-Peel Test) is not a useful test to measure these effects. The installed waterproofing membrane does not receive a peel force applied at 180-degrees to the membrane surface. Two samples with the same film and compound with different thickness of compound may have similar ASTM D1876 results; however, the variance in elongation likely is not captured during the testing.
Survivability After Placement
From the research and evaluation of the full-size blindside mockup, the BRG system was found to meet the demands of the project and demonstrated durable attributes. However, the question still remains: will it remain watertight throughout the life of the building? Again, the concept of survivability must be based on both the membrane and the system. As described in Learning Objective #2, the system being bonded to the structure is an important attribute. Now the question becomes, how will the components (BRG and concrete wall) function together to provide a leak-free building? Interestingly, much of the characteristics applicable to the survivability of a BRG system during placement of the shotcrete can apply to the long-term survivability and watertightness.
Infinite elongation is an attribute that contributes to how the membrane manages movement and energy at future cracks. Infinite elongation is basically stretching nothing. Before a crack exists, there is infinitesimal material directly over the epicenter of the future crack. If the material directly bonded to the structure has greater adhesion to the structure than cohesion to itself, the surface material will rupture in proportion with the structure, creating a failure in the membrane. How then can a material pass ASTM C1305 - Standard Test Method for Crack Bridging Ability of Liquid-Applied Waterproofing Membrane? An elastomer either can address movement by having sufficient material to dissipate it or disbond from the structure to develop sufficient material to accommodate the movement. In a similar manner to how the impact of shotcrete is dissipated into a BRG membrane, the movement produced by a crack in the structure can be dissipated into the membrane. The movement from a crack into more rigid systems is directed into the rigid membrane.
Accommodation for movement at a crack within seams is similar to the method of managing movement due to the deflection of the shoring during the application of the shotcrete. The elastomer in the seams for BRG systems elongates lengthwise but shrinks depth-wise. The compression in the seams is addressed by the thickness and modulus of the elastomer. More rigid membrane systems typically distribute the movement directly into the lap and there may not be sufficient elastomer to accommodate the movement.
Deconstructing the full-size stand-alone mockup and studying the physics relative to the BRG system design with the project construction methods provides significant insight. Comparing the observations from the mockup with the physics of the material chosen aligns with the results. It is within reason to conclude that the BRG system should provide the long-term performance required by the project.
Learning Objective #4: Develop an understanding of the physics and importance to resist lateral water migration of a pre-applied waterproofing system in a structural shotcrete wall assembly.
Over the past decade, the term Lateral Water Migration (LWM) has become a driving phrase in the industry relative to pre-applied waterproofing systems. LWM is roughly understood to be the ability for water to flow along the interface of a waterproofing membrane and the adjacent substrate. As an example, we often consider the difference between a loose laid roof membrane and a fully adhered membrane. It is often assumed that the adhered membrane would resist water migrating laterally to areas of the roof other than the point of origin better than a loose laid membrane. Interestingly enough, it is not uncommon to find standing water within roofs with concrete decks, but there are little or no leaks inside the building. The quality of the concrete can have a significant impact on the relevance of LWM. Situations such as this example are a primary reason why there is a need to develop a clear definition of LWM in pre-applied waterproofing, perform a deeper insight into the physics of LWM in pre-applied waterproofing, and understand the importance of LWM in pre-applied waterproofing.
The following is a definition for Lateral Water Migration in pre-applied below-grade waterproofing for consideration in this paper: The transmission/transport of water along the interface between a bonded waterproof membrane and a water-resistant structure (one that does not allow water to pass but vapor will) where forces are applied which may include gravity, atmospheric, capillary, and/or head pressures.
Establishing the definition is an important step for this paper as the initial bench testing results of the BRG system prior to constructing the stand-alone mockup were sporadic and mimic the bonding observations in Learning Objective #2. Poor quality of the concrete can produce poor results of resistance to LWM. In an attempt to unify the results and potentially match the performance observed on the stand-alone mockup, Redi-mix concrete from a jobsite was used for specimens in the bench testing. Well consolidated bench specimens were produced to create a baseline and poorly consolidated specimens were also produced to replicate some of the issues observed at the stand-alone mockup.
To simulate the varied ways that water impinges upon a waterproofing system, two modified ASTM test methods were utilized using specimens constructed from jobsite concrete: Modified ASTM D5385 - Standard Test Method for Hydrostatic Pressure Resistance of Waterproofing Membranes and Modified ASTM D5957 - Standard Guide for Flood Testing Horizontal Waterproofing Installations. The modified ASTM D5385 test has been previously used for LWM testing of pre-applied waterproofing membranes against hydrostatic head pressure and the modified ASTM D5957 has been used to test the effects of water not under a hydrostatic pressure.
For LWM to be properly identified, there are four key components that need to be considered that were included in the definition: "transmission/transport of water along the interface," "waterproof membrane" in conjunction with "water-resistant substrate," and "where force is being applied." These aspects should be valid for evaluating most pre-applied systems but are applied specific to a BRG during the following testing.
Modified ASTM D5385
ASTM D5385 uses an apparatus to contain the specimen and impinge water under pressure against a membrane which is loose laid over a concrete substrate with a relief groove scored into the block substrate. The test evaluates the membrane's resistance to hydrostatic pressure. The modified version of the test includes casting concrete directly onto a waterproofing membrane. A pressure release port is included that touches the membrane on the interior side and passes through the concrete. A rupture is cut into the membrane on the water side. The test evaluates the ability of the assembly to resist water traveling between the membrane and the substrate from the rupture location to the port. The test specimens were cast of 3,000 psi concrete onto the BRG membrane at 80 degrees F and allowed to cure for 28 days. The specimens were 15.5 inches long by 8 inches wide and 6 inches deep. The pressure release port is 1/4 inch steel pipe and the rupture is a 1 inch in diameter hole in the membrane.
Traditionally, the specimen is orientated horizontally, but that method does not necessarily capture the effect of increasing head pressure on the membrane. For the tests performed for this paper, the specimen was turned vertically to allow for simulation of increasing head pressure.
Specimens were produced to simulate the variable consolidation observed at the stand-alone structural shotcrete mock-up. The ports were placed at varying distances from the rupture to pressure release port to test if there was an influence on the results.
Specimen I.D. - LWM Results:
5385-0-C: No LWM observed
5385-2-C: LWM observed at 15 psi
5385-2-PC: LWM not applicable
5385-6-PC: LWM observed at 25 psi
To set a baseline control, a well consolidated specimen (5385-0-C) was constructed without a pressure release port and tested for LWM. No LWM was observed and the dyed water impinged upon the specimen did not leave the area of the rupture.
5385-2-C
The rupture was set at the top of the specimen, 4 inches from the three adjacent sides of the specimen, and 2 inches above the pressure release port. Water was impinged upon the specimen up to 10 psi (equivalent to 23 feet of head pressure) for 15 minutes with no observed leakage from the pressure release port. The pressure was increased to 15 psi (equivalent to 35 feet of head pressure) and at approximately 3 minutes and 30 seconds into the test cycle, water was observed exiting the pressure release port. The test was stopped and the specimen was deconstructed. The path the water traveled was from the rupture to the port. The specimen did crack within the test apparatus across the port, but no water was observed exiting the crack.
5385-2-PC
The rupture was set at the top of the specimen, 4 inches from the three adjacent sides of the specimen, and 2 inches above the pressure release port. Water was impinged upon the specimen and water entered the specimen then exited the port without applying pressure to the apparatus. The natural head pressure created by filling the reservoir was enough to cause water to migrate into the voids within the concrete resulting in water migration. However, because the concrete is not considered water resistant by the definition presented in this paper, the results would not be classified as LWM.
5385-6-PC
The rupture was set at the top of the specimen, 4 inches from the three adjacent sides of the specimen, and 6 inches above the pressure release port. Water was impinged upon the specimen up to 10 psi (equivalent to 23 feet of head pressure) for 15 minutes with no observed leakage from the pressure release port. The pressure was increased to 15 psi (equivalent to 35 feet of head pressure) with no water being observed exiting the pressure release port and the cycle was completed, but seepage was observed along the top of the apparatus. The pressure was increased to 20 psi (equivalent to 46 feet of head pressure) with no water being observed exiting the pressure release port and the cycle was completed, but seepage continued to be observed along the top of the apparatus. The pressure was increased to 25 psi (equivalent to 58 feet of head pressure) with no water being observed exiting the pressure release port and the cycle was completed, but the seepage observed along the top of the apparatus had increased. The test was stopped and the specimen was deconstructed. The path the water traveled was from the rupture to the area of seepage around the top of the apparatus, but never reached the port. Under closer examination, the specimen was found to have poorly consolidated concrete in the area of seepage.
The specimens based on well consolidated concrete produced significant insights:
Water did not leave the rupture area without a pressure release.
If a pressure release is provided and LWM is observed, the water travels to the point of the pressure release.
It appears that, as the distance between the rupture and the pressure port increases, greater head pressure is required to produce LWM to that port.
If the pressure release is removed, LWM will likely not occur.
Unlike the well consolidated concrete, it would be difficult to eliminate external water intrusion from entering the assembly. The multiple passageways within the simulated poorly consolidated shotcrete are the reasons for the difficulty in eliminating the water flow if a rupture were to occur. Regardless of the system, resistance to LWM (by the definition presented in this paper) is not possible because of the natural capillary effect created with poorly consolidated concrete and post installation repairs are difficult to achieve.
Modified ASTM D5957
ASTM D5957 uses flooding to impinge water under moderate head pressure against a membrane to produce a visual leak. The test is intended to be used to evaluate a horizontal deck application but has been extended up onto wall flashings or sloped decks with constant water. The modified version of the test includes the same specimen types casting concrete directly onto a waterproofing membrane in the Modified ASTM D5385, with the pressure release port to the interior, and having a rupture cut into the membrane on the water side. The test evaluates the ability of the assembly to resist water traveling between the membrane and the concrete from the rupture to the port under water flow without head pressure. The test was run continuously for 24 hours.
Specimens were produced to simulate the varied concrete consolidation observed at the stand-alone structural shotcrete mock-up. The test and specimen simulate the typical application of a pre-applied waterproofing system that is not in a water-table nor in a hydrostatic pressure situation.
Specimen I.D. - LWM Results:
5957-0-C: No LWM observed
5957-0-PC: LWM not applicable
To set a baseline, the test was performed on a well consolidated specimen without a port (5957-0-C). The specimen was found to not have water exiting the area of the rupture and no LWM was observed.
5957-0-PC
With the simple application of flowing water, water entered specimen 5957-0-PC and exited the port. The surface tension of the water and porosity of the poorly consolidated concrete was sufficient to cause water to migrate into the voids within the concrete resulting in LWM. Since the concrete is not considered water resistant, the results would not be classified as LWM.
Unlike the well consolidated concrete, it will be difficult to eliminate external water intrusion from entering the assembly. The multiple passageways within the simulated shotcrete are the reasons for the difficulty in eliminating the water flow if a rupture were to occur. The concrete has a natural capillary effect and numerous venting ports. Regardless of the system, resistance to LWM is not possible and post installation repairs are difficult to achieve.
Based on the research and limited testing performed, it became apparent that in a shotcrete application, a common false positive of LWM is related to the concrete structure. Though water may migrate within the assembly, often it is not traveling between the contact point of the waterproofing and the structure. The water could potentially be transferring within the concrete itself causing the false positive as the structure is not water-resistant. In this case, the water migrating within the assembly is not considered Lateral Water Migration per the proposed definition. LWM is a desirable attribute of attached waterproofing membranes because it isolates water in the area of a rupture; however, because of the nature of shotcrete, LWM is limited to resisting water migration within the shotcrete wall. The actual occurrence of LWM within the BRG system for shotcrete applications is limited to isolated situations where there is a rupture in the system, well consolidated concrete, and elevated hydrostatic pressure.
Considering the observed survivability of BRG systems, the design of the system accommodates the physics of the shotcrete application and is no more difficult to correct. When evaluating any pre-applied waterproofing system's in-service performance, the evaluation should consider all of the physics involved and not solely rely on one attribute. A battery of tests needs to be developed that considers all of the factors required to produce repeatable long-term success with a pre-applied waterproofing system.
Conclusion
Full-scale blindside mockups have proved to be valuable, as most mockups do. Every building is unique with different variables that are learned on a project. A mockup provides an opportunity to set the stage and set the standard for quality for all stakeholders involved. The lessons learned from the deconstruction process of the mockup provided the platform for discussing the physics behind a blindside structural shotcrete wall. The in-depth research presented in this paper is not intended to resolve every issue for every system that is available. However, general conclusions can be made that may be applicable to many structural shotcrete basement walls, in that one could develop criteria to evaluate if a system is viable for their project specific conditions.
This study began with three benchmarks for blindside waterproofing applications and addressed four learning objectives. The first learning objective demonstrated how a blindside wall is constructed from the outside in, with the last component - shotcrete - applied to the waterproofing system. The second learning objective outlined critical flaws in a structural shotcrete wall providing insight into the importance of proper consolidation to achieve a quality bond to the BRG waterproofing system. The third learning objective described and evaluated the significant forces that a shotcrete application has on a blindside waterproofing membrane. The final learning objective evaluated the issues related to lateral water migration by considering the concrete substrate's inability to resist water intrusion if poorly executed.
Based on a combination of research and the lessons learned from the stand-alone mock-up, the Backing film, Rubberized asphalt, Geotextile composite membrane system appeared to be a viable waterproofing solution for the project. It is generally understood that waterproofing membranes should bond to the concrete or shotcrete substrate. Shotcrete applications have a greater impact on the system's success than any other components in the system or than cast concrete. As an industry, a shift in our design process should begin to evaluate blindside waterproofing systems by analyzing how the shotcrete is applied to the waterproofing membrane. The evaluation begins with the understanding that the pre-applied waterproofing membrane is the substrate that receives the concrete. The waterproofing system is not complete until the concrete is applied, and it is an important component for the overall system to function at its maximum performance. The goal to create a watertight system begins with a watertight substrate - the waterproofing membrane. The system is complete after the concrete is applied creating a composite system with the goal that it remains watertight at the completion of the concrete application.
References
[1] David Leslie, Jerry Carter, "Full-Scale Stand-Alone Blindside Waterproofing Mock-up and the Lessons Learned," IIBEC Interface, Technical Journal of International Institute of Building Enclosure Consultants, Volume XXXVII No. 7, August 2019.
[2] Justin Henshell and Paul Buccellato, "Below-Grade Blindside Waterproofing Membrane Systems: A State-of-the-Art Report," RCI Interface, May/June 2011.
[3] Michael David, "A Study of the Dynamics of Shotcrete Formwork," Thesis submission at the Massachusetts Institute of Technology, June 2010.
