Lightweight Insulating Concrete – RCI, Inc.

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Lightweight Insulating Concrete
As a Choice Re roofing System
                By Hubert T. Dudley, Ray Wetherholt, Zen Szewczyk, and Herb Cannon


    ightweight insulating concrete has been used as a roofing
        cases, a minimum of one inch. This design innovation resulted in
      substrate for more than 60 years. It is a combination of
       a lightweight insulating concrete system that is viableeven
cement, air, water, and an air-entrained aggregate. The aggregate     advantageousfor use in reroofing applications.
provides improved surface characteristics, including a smoother           Lightweight insulating concrete systems are placed using
finish, and a reduction in cracking. When aggregate is not            mixing and pumping equipment. An average of 8,000 square feet
included, the lightweight insulating concrete is referred to as cel   of lightweight can be installed in one day on reroofing projects.
lular concrete. Both types--aggregate lightweight insulating          A higher coverage rate averaging 15,000 square feet per day can
concrete and cellular concrete--provide similar solutions to          be achieved on new construction projects.
common reroofing problems.                                                On a typical reroofing application, a lightweight insulating
    The typical density range for lightweight insulating concrete     concrete system can be installed directly over the existing roof.
is 22-38 pcf. (Lightweight insulating concrete should not be con      Following a structural review and repair of the existing roof and
fused with lightweight structural concrete, which has a minimum       flashing to a watertight condition, the installation of a light
density of 50 pcf.) Table 1 shows the typical range of physical       weight insulating concrete system begins with placement of a 1/8
properties for materials used in insulating concrete systems.         inch minimum slurry coat of lightweight insulating concrete.
                                                                                                        The slurry corrects any substrate
                                                                                                        irregularities or low spots, and
                                                                                                        bonds the polystyrene to the
                                                                                                        substrate. Type I expanded
                                                                                                        polystyrene board, designed with
                                                                                                        a pattern of holes, is placed into the
                                                                                                        slurry in a stair-step configuration
                                                                                                        to create the needed slope-to-drain
                                                                                                        contour and achieve the required
                                                                                                        insulation values. A top fill of
                                                                                                        insulating concrete encapsulates
                                                                                                        the EPS board, bonding it to
                                                                                                        the substrate. The pour is
                                                                                                        screeded to a smooth, planar,
                                                                                                        monolithic surface suitable for
                                                                                                        membrane application.
                                                                                                            Lightweight insulating con
                                                                                                        crete systems are used effectively
                                                                                                        to solve common reroofing and
                                                                                                        recover problems in applications
                                                                                                        where bottom side venting is
Table 1                                                                                                 not available, including:
    Originally, lightweight insulating concrete systems were               Providing positive in-place slope-to-drain
installed with a varying thickness of concrete to provide slope.           Correcting existing surface irregularities
In the early 1970s, a constant thickness of polystyrene was                Avoiding disruptive tear-off noise and debris
incorporated into the system to provide increased insulation. In           Avoiding asphalt fumes that accompany hot-mopped,
the late 1970s, the system concept evolved to include stair-                  tapered board systems
stepped thicknesses of polystyrene. This created a slope-to-drain
                                                                           Creating potential initial cost savings due to labor savings,
contour that effectively removes water from the roof's surface,
                                                                              and long-term life cycle cost benefits
reduces weight, and increases insulation values. Additionally,
maximizing the use of polystyrene allows a significant reduction
                                                                           Encapsulating/covering roofing membranes that cannot
in the thickness of lightweight insulating concrete to, in some               be disturbed

4  Interface                                                                                                                    July 2001
                                                                       deck with a gravel-surfaced, hot asphalt built-up roof that
                                                                       included four plies of 15# asbestos felt. Located in a park-like
                                                                       setting surrounded by tall fir trees, the roof was difficult to access
                                                                       from the sides. The original surface was virtually flat, and the
                                                                       drains were located in a geometry that did not allow a standard
                                                                       tapered system to be economically feasible.
                                                                            Like most hospitals, Eastside required that the noise and
                                                                       fume smell be minimized. Slope had to be created for drainage,
                                                                       and the hospital wanted a system that would provide increased
                                                                       insulation value, if possible. With lightweight insulating con
                                                                       crete, we could meet all of these objectives.
                                                                            The old roof was removed down to the existing insulating
                                                                       concrete and allowed to air dry for a few hours. An upside-down
Photo 1--Lightweight Insulating Concrete system crosssection           cap sheet was nailed down using Zono-tite fasteners and was
To illustrate the viability of lightweight insulating concrete sys     covered with a heavyweight SBS-modified base sheet torch
tems as a problem-solving tool in such applications, the follow        adhered to the upside-down cap sheet. This provided a tempo
ing case studies of LWIC systems on reroofing and recover              rary roof and allowed for removal of an entire roof area before
projects are provided.                                                 the lightweight insulating concrete subcontractor performed his
                                                                       portion of the work.
Ray Wetherholt                                                              Roof specification and slopes were laid out initially on the
Wetherholt and Associates                                              design drawings for bidding purposes. Once the contractor was
    Let me start by explaining that, initially, I was not a believer   selected, the slopes were reviewed and modified as needed to
in lightweight insulating concrete, but began using it when I          accommodate doorways and embedded reglet heights.
became involved in several projects. In those projects, it was the          Overflow scupper openings were too low and had to be mod
best option for the architect and construction team to get slope       ified to allow for placement of the flanged, stainless steel sleeve.
to drain and avoid relocation of drains and complicated tapered             The lightweight insulating concrete subcontractor completed
system details.                                                        the 20,000 square foot job in a few days. The system was
    Lightweight insulating concrete can be another tool in the         allowed to cure for three to seven days, and an inverted cap
toolbox of the professional roofing consultant.                        sheet and new SBS-modified two-ply system were installed over
    The three projects I would like to review are located in the       the lightweight insulating concrete.
Seattle, Washington, metropolitan area. Each was unique and                 The lightweight insulating concrete contractor was able to
had different requirements. The product used on all three pro          set up his tractor trailer rig in the parking lot, batch on the
jects was NVS ("Non-vented System," marketed by W.R. Grace)            ground, and pump the material six stories to the roof. The pro
lightweight insulating concrete.                                       ject required extensive coordination among the hospital staff,
                                                                       facility managers, and the contractor. The project was completed
                                                                       eight years ago, and the roof is still performing well.
Eastside Hospital with GravelSurface, Hot Asphalt Roof
    The existing roof of the Eastside Hospital was lightweight
insulating concrete over a cast-in-place concrete

Photo 2--Eastside Hospital

July 2001                                                                                                                 Interface  5
                                                                                                      An inverted cap sheet was attached
                                                                                                  over the lightweight insulating concrete
                                                                                                  using NVS fasteners, and a torch-grade,
                                                                                                  heavyweight SBS-modified bitumen
                                                                                                  membrane was applied. The roof has
                                                                                                  been performing successfully since the
                                                                                                  project's completion in 1997.
                                                                                                  Medical Clinic with Old Hot Asphalt
                                                                                                  Builtup Roof
                                                                                                      In 1991, the roof of the medical
                                                                                                  clinic needed slope for drainage, and
                                                                                                  the geometry worked for a tapered
                                                                                                  insulation system over the structural
                                                                                                  concrete roof deck. The roof was orig
                                                                                                  inally designed as a plaza deck area
                                                                                                  but was never used as such. The bid
                                                                                                  documents specified a foam adhesive
                                                                                                  to secure the insulation with an alter
                                                                                                  nate for use of hot asphalt. The roof
                                                                                                  system specified was polyisocyanurate
                                                                                                  board over the concrete deck, 3/4-inch
                                                                                                  perlite, a mopped fiberglass base
                                                                                                  sheet, and a 2-ply heavyweight SBS-
                                                                                                  modified bitumen system.
                                                                                                      The owner and the low bid con-
Photo 3--Lutheran Bible Institute

Flat Roof Church with Coal Tar Pitch Roof
     The existing roof on this church was flat, grav
el-surfaced, and had complicated drainage geome
try. Several "consultants" (including church
members and roofers) examined the roof and pro
posed adding drains, recovering the existing roof,
and spraying foam and a coating. Contractors were
worried that coal tar pitch would be considered
hazardous waste and associated disposal costs
would be incurred.
     The addition of drains through the structural
concrete deck was not economically feasible
because the underside of the roof deck is exposed
to the church interior. The Seattle weather, togeth
er with the dearth of qualified SPF contractors,
took away the option of SPF, leading us to consider
lightweight insulating concrete. A structural engi      Photo 4--Medical clinic.
neer was retained to review the building structure. He determined       tractor reviewed the roof system to identify possible cost savings
that after the loose gravel from the coal tar roof was removed,         and options to avoid hot asphalt fumes, which would be particu
lightweight insulating concrete would add a minimal, acceptable         larly problematic on a medical facility. The contractor proposed
amount of weight to the structure.                                      a lightweight insulating concrete system combined with an APP-
     On this project, the underlying roof was left in place, and        modified membrane torched down over a nailed base sheet. The
lightweight insulating concrete was installed over it. The system       system met the owner's budget requirements and eliminated
included EPS board placed in a stair-step configuration to create       unacceptable hot asphalt fumes. Later, the owner successfully
an effective slope-to-drain contour. Edges were raised to accom         used the same system on the rest of the building's 10-story roofs.
modate the added thickness. In most cases, curbs were raised to         Examination of the roofs in 1999 found them in good condition.
accommodate the insulation thickness as needed. On most of the
roof, 1/4-inch per foot slope for drainage was obtained. In a few
locations, the slope was limited by the chapel's steep copper roof.

6  Interface                                                                                                                 July 2001
Photo 5--Eastdale School

Zen Szewczyk
Eastdale School, Ontario, Canada
    You would think after performing thousands of condition
reports and preparing the same amount of design drawings and
specifications, one would rarely come across a situation that pre
sents a unique problem. Well, after 17 years, it happened.
    Our firm was called in by a school district to evaluate the
extent of deterioration of the roof assembly on a building that
was over 30 years old. The roof was the original four-ply asphalt
BUR. The owner was not experiencing many leaks. However,
due to the age of the roof, the owner thought it might be pru
dent to perform some preventive maintenance to extend the life
of the roof indefinitely. The owner thought it also prudent to
retrofit the roof with additional insulation, as it was believed the
roof did not have good thermal resistance.
    We arrived on site to find the roof completely under water,
so our initial investigation was limited to some wading around
and interior investigation. One thing we noted was that there
were relatively few roof drains. The existing deck varied from
area to area and consisted of poured-in-place concrete, metal
deck, wood deck, and concrete slab. The underside was plaster
ceiling. What little waterstaining existed was limited to around
openings. Some time later, when the roof surface was dry, we
returned to complete our evaluation. Infrared scans, core-cuts,
and probes all indicated saturation above all decks. The continu
ity of the decks and a good vapor barrier had been hiding a
secret for some time. Very little preventive maintenance had ever
been performed on this roof.
    Roof replacement should be easy! Remove and replace.
Drainage calculations showed that the size of the rainwater lead
ers was adequate to get all the water off the roof; however, they
were few and far between. The distance between drains was as
much as 125 feet. The idea of adding more drains was thwarted
by an inaccessible ceiling space, fire-walls, and a fire-resistant
mineral fiber no longer used in building construction.
    Random core samples confirmed that the existing poured-in
place concrete deck was very uneven with deviations up to two
inches. Introducing rigid tapered insulation would have yielded

July 2001                                                              Interface  7
an irregular surface with unsatisfactory results. Furthermore, in     Herb Cannon
order to achieve any success with rigid insulation, it would          The Cannon Group
require a slope of at least 1/4-inch per foot. The insulation thick
ness would have been from 3 inches at the drain to 19 inches at       Terminal Market at Hunts Point
the high point. The costs were rising.                                     In 1994, the 35-year-old roof of the Terminal Market at
     It didn't take long to conclude that the optimum solution was    Hunts Point, New York, sustained severe damage due to high
lightweight insulating concrete. The existing roof was removed        winds. The roof consisted of a poured gypsum deck, three-ply
to the deck. After deck repairs, a new vapor barrier was mopped       built-up roof, polyisocyanurate insulation, and a mechanically
to the deck to serve as a temporary roof. Lightweight insulating      fastened PVC membrane. An emergency removal of the PVC
concrete was pumped onto the roof surface where it would find         membrane and polyisocyanurate insulation was necessary.
its level. The stair-stepped polystyrene would be installed at a      Removal was followed by the installation of an SBS-modified
1/16-inch per foot slope. The top coat was installed and finished     bitumen sheet to secure the interior of the building. Then the
smooth. After four to seven days, fastener withdrawal tests were      building owners, the New York City Economic Development
conducted to determine if the fastener held a minimum 40              Corporation and the Hunts Point Terminal Produce Cooperative
pound load. The venting sheet was fastened to the concrete sur        Association, Inc., began considering a full redesign for the facili
face and the base sheet was installed. Then a 24-hour rainstorm       ty's 750,000-square foot roof. There were several circumstances
arrived. To everyone's delight, the water was draining off the        unique to Hunts Point that had to be considered when evaluat
roof like never before. Even after the cap sheet was installed, the   ing possible roof systems:
thickness of the overlaps had little effect on the ability of the           The location of the facility, on the bay across from LaGuardia
roof to shed water. The project was completed to the satisfaction              Airport, meant that the roof would continually be subjected
of the manufacturer, contractor, consultant, and the building                  to high winds.
owner--all within budget.                                                   The roofs of the 75-foot wide x 200-foot long bays did
                                                                               not have adequate slope.
                                                                                             The three-ply, built-up roof contained
                                                                                                material that, per New York State regu
                                                                                                lations, either had to be removed or
                                                                                             The market, which sees approximately $4
                                                                                                billion of produce business per year and
                                                                                                operates 24 hours a day, 7 days a week,
                                                                                                could not afford to be shut down for
                                                                                            Lightweight insulating concrete offered a
                                                                                        solution for each of these concerns:
                                                                                             The finished monolithic deck, with its
                                                                                               encapsulated insulation board, is resis
                                                                                               tant to high winds.
                                                                                             The lightweight insulating concrete sys
                                                                                               tem solved the ponding water problem
                                                                                               by creating proper slope-to-drain.
                                                                                             It was determined in a structural review
                                                                                               that the lightweight insulating concrete
                                                                                               system could be poured directly over
                                                                                               the existing assembly. Pouring over the
                                                                                               old built-up roof encapsulated the felts.
                                                                                               This was acceptable according to New York
                                                                                               State regulations in lieu of abatement.
                                                                                             By eliminating the need for tear-off, the
                                                                                               market was allowed to remain in operation
                                                                                               during the entire project. The lightweight
                                                                                               insulating concrete pour took an average
                                                                                               of three days per bay.
                                                                                            Following the lightweight insulating con
                                                                                        crete pour, the project was finished with a base
                                                                                        sheet mechanically fastened to an 1-90 wind
Photo 6--Terminal Market at Hunts Point                                                 uplift fastening pattern. A two-ply, torch-applied

8  Interface                                                                                                                  July 2001
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