This article is co-authored by my friend and colleague Jason Walters. Jason is the Team Leader for USAR SC-Regional Response Team 4. We're discussing this topic at All Hazards Contemplations.

Introduction

FEMA’s USAR system, basic firefighting texts, and other fire-rescue references describe how to recognize and respond to a variety of structural collapse situations. These collapse types are specific to structures with rigid components. Freshly-poured concrete isn’t rigid, and collapses involving wet concrete create a unique set of circumstances not described in typical structural collapse references.

Collapses have traditionally been classified in four categories. These are the Lean-To Collapse, the V - Collapse, the Pancake Collapse and the Cantilever Collapse.(1) Some USAR documents now describe an additional collapse type – the A-Frame Collapse.(2, 3) The A-Frame Collapse is also known as a Tent Collapse. An A-Frame Collapse is essentially two back-to-back Lean-To collapses that share a common wall or other upright structural component.

There is another collapse type that has recently been identified. This collapse type involves concrete that is still wet. We call it the Bathtub Collapse. Unlike cured concrete, wet concrete does is not solid and when freshly poured, it does not form slabs and or give off dust. Wet concrete runs to the lowest point available, then collects like water in a bathtub. Bathtub collapses have some things in common with other collapse types, but there are several significant differences. The most important are the difficulty in stabilizing a collapse involving wet concrete, handling concrete that does not stay in one place, and the relatively short time it takes for the wet concrete to harden.

Typical Bathtub Collapse

Concrete Weight

Wet concrete is slightly heavier than a corresponding volume of dry concrete. When concrete cures, some of the water evaporates, but much of the water stays in the concrete. Water binds chemically to the solids in the concrete, and thus concrete retains much of the water weight when it cures. Concrete loses some weight as it cures, but surprisingly, that weight loss is relatively small.

The rule of thumb for the weight of a cubic foot of wet concrete with aggregate mix is 4000 lbs/yard3, or approximately 162 lbs/ft.3. The rule of thumb for the weight of a cubic foot of dry concrete with aggregate mix is 3700 lbs/yard3 or 150 lbs/ ft3. (4) The bottom line is that all concrete is heavy. Remember, the primary difference between wet concrete and dry concrete – wet concrete flows to the lowest point and then collects there.

A factor that construction personnel may not take into account is that once a concrete slab is poured, water, wet burlap, or other wet material is often left on the concrete surface to assist in insulating and hydrating the concrete as it cures. This water adds additional weight that may not be considered in the design of the shoring system that supports the pour. If that additional water weight is not accounted for in the shoring system, then a collapse is more likely.

Building Construction Factors

Virtually any type of building construction may be involved in a bathtub collapse. Bathtub collapses usually occur when construction personnel pour a concrete floor at an elevation above the lowest structural level. Bathtub collapses occur in one of three basic configurations. The first is when the collapse rests on the ground or on a slab at grade level. The second bathtub collapse type involves collapses above grade level. The third type is a bathtub collapse into a basement or other below-grade area. Bathtub collapses will most commonly occur at or below grade. Bathtub collapses that begin above the second floor are rare, as the collapse of an upper floor often causes a progressive pancake collapse that destroys the entire structure.
Basic bathtub collapse strategies are based on grade-level collapses. Above-grade and below-grade bathtub collapses involve the same basic strategy as a grade-level collapse, with a few additional considerations.

Construction Process Factors

The collapse of a concrete floor during or immediately after a pour may be due to one or more of the following factors:

• Inadequate shoring beneath the pour
• Wall-floor structural connector failure
• Shoring material failure
• Excessive amount of concrete poured
• Excessive pour concentration
• Failure of walls, beams, or other supporting structural materials

The Bathtub Collapse Sequence

Steel span drops with the outside edges supported, forming a rough bathtub shapeWet concrete runs to the center of the bathtub
Wet concrete runs out of small openings in the edges of the bathtub. These may be quickly blocked due to the heavy concrete viscosity or obstructions outside the bathtub. If small openings are blocked, the concrete in the bathtub will form a larger and deeper pool. This will make size up and extrication more difficult.
Concrete forms a thicker but smaller diameter puddle than the original pour
Rebar, Q-decking or other steel sheeting, and shoring materials are twisted and mixed into the wet concrete

Supporting beams and damaged overhead structural materials may create widowmakers
Supporting beams may fall into the bathtub prior to or during the rescue operation

Size-Up and Strategy

Size-up should be completed in accordance with standard structural collapse protocols. This should include the situation, potential entrapment problems, specific hazards, and a 360-degree look at the structure. When possible, include an elevated look at the collapse. An aerial ladder or nearby building may be used as an elevated observation post. When size-up is complete, Command should develop the Incident Action Plan (IAP) goals, communicate the IAP to all responders, make tactical assignments, and ensure that the personnel accountability system is fully implemented.

Important strategy considerations include:

Define the building factors including construction type
Identify the most likely victim locations
Develop and communicate the IAP
Safety considerations
Shoring
Remove easily accessible victims
Make the rescue vs. recovery decision
Estimate the concrete cure time
Wet concrete removal methods

Bathtub Collapse Incident Management

Command should consider appointing at least a Safety Officer, a Liaison Officer, and a Rescue Group Supervisor for even a small bathtub collapse.(5) The Safety Officer can help isolate the scene and identify the primary hazards. The Liaison Officer can work with the construction company to determine how many workers are missing or known to be entrapped. The Liaison Officer should communicate with the construction supervisor, gather information, and keep construction personnel available to assist if needed. The Rescue Group Supervisor can concentrate on rescue tactics and needs and allow the Incident Commander to keep his/her attention focused on the overall incident strategy and safety.

Resources

Structural collapses typically require more resources than may seem likely during the early incident stages. It is important to have at least one engine company for water supply, one truck company for tools and an aerial device, a heavy rescue or USAR unit for tools and shoring materials, and additional manpower. A large law enforcement presence may be required to keep bystanders, construction personnel, or distraught relatives out of the collapsed structure. Additional construction personnel and heavy equipment such as cranes, front-end loaders, and other machinery may be useful in the rescue effort. If in doubt, call for additional resources early and often. Structural collapse rescue is hard work, and personnel may quickly become exhausted, especially in extremes of temperature and/or
precipitation.

Safety Considerations

One of the first priorities is to assign an Incident Safety Officer. This should be an officer who has a good basic knowledge of building construction, collapse types, USAR strategy and tactics, and common USAR safety problems. The Safety Officer should ensure that a safety zone is established. Collapse zones should be established to exclude responders from areas exposed to potential secondary collapse, particularly in areas beneath widowmakers. The Safety Officer should ensure that building utilities are shut down. Construction company generators and other power supplies should be shut down to reduce electrical hazards and atmospheric contaminants. Construction personnel should be kept on standby, as their generators may be useful power sources later in the incident.

Assessing the outside of the bathtub

The Safety Officer

A Safety Officer should be appointed early in the response. The Safety Officer should don the appropriate PPE and the Safety command vest. Once search and rescue operations begin, the Safety Officer should be located at an elevated observation point, if possible. Observing from an elevation gives the Safety Officer the ability to observe conditions in the bathtub as well as the condition of supporting walls, columns, and the stability of the surrounding structure. Most importantly, an elevated observation point gives the Safety Officer a better perspective on how rescue operations may change structural and personnel safety. For example, if wet concrete piles up against the base of a column that is already leaning, it may topple that pillar and cause an additional collapse. A properly-positioned Safety Officer will be able to anticipate this problem, advise Command, and ensure that the concrete flow is diverted prior to impinging on the damaged column.

Safety Officer’s view into the bathtub from an elevated observation point

Personal Protective Clothing

Standard USAR PPE is usually adequate for bathtub collapse operations. Lace-up safety boots are the most appropriate footwear. Wet concrete has a consistency very much like quicksand, and fire boots may be pulled off of firefighters who walk in it. Leather construction gloves, mechanics gloves, or extrication gloves are adequate for most hand protection, but medical exam gloves will be required for patient care.

Modified Turnout Gear Ensemble used for heavy cutting PPE

This concludes Part 1. Part 2 will discuss discusses tactical considerations, interior size-up, victim recovery, investigations, and incident termination.

References
(1) Goodson, Carl, et al
IFSTA Essentials of Firefighting, 5th Ed.
IFSTA, Stillwater, OK, pp 362-364

(2) English, Leslie, et al
NFPA 1670, Standard on Operations and Training for Technical Search and
Rescue, 2004 Ed.
NFPA, Batterymarch Park, MA, pp 25-27

(3) U.S. Army Corps of Engineers
US&R Structures Specialist Field Operations Guide, 3rd Ed.
U.S. Army Corps of Engineers Readiness Support Center, 2001, p VI-3

(4) http://corlissconcrete.com/Q&A.htm#4

(5) Jones, Jeff
NIMS Field Operations Guide, 1st Ed.
InforMed, Tigard, OR, pp 14-20

All photos courtesy of Hilton Head Island Fire & Rescue