January 2005

PERFORMANCE OF CONCRETE DECKS: A CASE STUDY OF DURABILITY, PERFORMANCE, EVALUATION AND REPAIR

James S. Cohen, PE CEng Eur Ing
James Cohen Consulting, PC

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The concrete wearing surface to a parking deck forming the roof of the outpatient and emergency surgery areas at a major community hospital deteriorated within seven years after construction rendering it unserviceable. Investigations into the cracking, delamination and disintegration of the concrete included material sampling and testing, impact echo surveys, probes, delamination surveys and elevation surveys. The deterioration was found to be due to a combined lack of slope at the supporting slab to promote drainage to the central drains, lack of a drainage layer beneath the wearing course to enable drainage and a ready source of water entry into the insulation layer between the wearing course and supporting slab. This was exacerbated by poor quality concrete, poor jointing details, lack of shrinkage/temperature reinforcement and inadequate drainage holes in the central drains. The concrete wearing course, insulation and waterproofing layers were consequently removed and replaced with a new sloped drainage surface topped by waterproofing, drainage composite, insulation and light weight air entrained concrete reinforced with epoxy coated deformed bars. Subsequent to the reconstruction of the deck a review of the construction records and correspondence was also undertaken to cross reference the original design details to the actual construction.

1. INTRODUCTION

During 1981-82 a leading community hospital underwent a major expansion. This comprised partial demolition of the original structure and construction of new facilities including a multistory addition and below grade outpatient clinic. The outpatient clinic, which included surgery areas, was roofed by a parking deck which had a top wearing course surface level with the surrounding ground.

Problems associated with the new expansion joint between the multistory addition and parking deck/outpatient clinic arose within five months after pouring the concrete wearing course. The problem was not resolved for four years, despite multiple repairs, including local replacement of the concrete wearing surface at the expansion joint and joint materials.

Cracks were first reported in the concrete wearing surface in an area remote from the expansion joint within three years of concrete placement. The cracking commenced to spread over the majority of the surface, despite attempts at local replacement of the concrete.

The wearing surface to the parking deck continued to deteriorate, with extensive map cracking, scaling and delamination of the upper surface. The accelerating rate of deterioration necessitated the temporary repair of the more severely affected areas of the wearing surface before the onset of winter during 1991. In 1991-92 site investigations into the cause of the cracking and means to repair the damage were undertaken. This culminated in the complete replacement of the concrete wearing surface, including underlying insulation, waterproofing layers and central drains.

2. BACKGROUND

The parking deck forms the central area to a hospital following an expansion program commenced in 1981 and largely completed in 1984. The hospital is located in an area of the United States having relatively flat terrain and is subject to both frequent storms in the Spring and Summer months and sub-freezing temperatures during the winter.

The parking deck has approximate dimensions of 160 feet (48.8 m) by 180 feet (54.9 m). It is surrounded on the north, east and west sides by the hospital. Vehicular access to and from the parking deck is from

a road to the south of the hospital. A U-shaped perimeter, approximately 20 feet (6.1 m) wide, along the hospital walls is devoted to pedestrian use, as is the adjoining short portion of sidewalk located at the northwest corner of the parking lot. The construction of these areas is similar to the car parking area. Figure 1 shows a plan outline of the hospital following the expansion. Figure 2 is a photograph of the parking deck shortly after construction.

The original design of the wearing surface, shown in Figure 3, included a lightweight concrete "fill" of varying thickness lightly reinforced with steel mesh. This was to overlay a 1� (38 mm) to 2 inch (51 mm) porous fill of � to � inch (6.4 to 12.7 mm) diameter rounded gravel placed over a 2 inch (51 mm) thick rigid closed-cell expanded polystyrene insulation board. A protective asphalt impregnated and coated fiberboard was placed beneath the insulation and on top of a liquid-applied rubberized asphalt membrane compound. The asphalt membrane was applied directly to the top of the concrete structural slab. The wearing course concrete specifications were later altered to a 3000 psi (20.7 Mpa) normal weight concrete. The gravel appears to have been deleted from the drawings at a later date.

Construction of the wearing surface was in 20 foot (6.1 m) square bays separated by � inch (12.7 mm) joints with compressible water resistant fillers and sealant. The minimum slope of the wearing surface was inch per foot (1:100). However, the concrete wearing surface is located above a flat reinforced concrete slab which serves as a roof to the underlying offices.

Documents indicated that the designed minimum overall thickness of the total overburden to the reinforced concrete roof, including waterproofing, drainage medium (if any), insulating and wearing surface, was only 4 inches (101.6 mm) at the drain boundaries and deck/ground edge. This was confirmed during the investigation. Actual thickness of the wearing course therefore varied from 1� inches (38.1 mm) at the drains and deck edge to over 12 inches (304.8 mm) along the expansion joint. Deterioration of the wearing surface was first noted a few years after completion of the parking deck. It was also reported that various unsuccessful efforts had been made to arrest the deterioration, including patching and sealing of the deck surface.

3. INVESTIGATION AND INVESTIGATION RESULTS

At the time of the first visit to the hospital in 1991, the concrete surface was observed to have undergone significant cracking and localized disintegration. Water was observed to be bubbling up through the numerous cracks in the wearing course at significant distances away from the central drains, despite several preceding days of dry weather. Cracks in the 20 x 20 foot (6.1 x 6.1 m) square concrete panels included longitudinal cracks through panel centers, corner cracks, edge cracks and map cracking. Particularly with regard to map cracking, a grey gel outlining the cracks was observed at many locations.

The first stage of the investigation included removal of samples of the concrete wearing surface and waterproofing membrane for petrographic and other testing. The second stage of the investigation included condition and elevation surveys, design verification and analysis checks of the parking lot support structure. Recorded data taken during the survey included relative levels of the wearing surface, thickness measurements, and general locations of cracks, delaminations, spalling and other miscellaneous defects. Among the items included in the scope of this second stage investigation was a determination of the construction thickness of the concrete wearing surface, using impact echo testing. The weather during and immediately preceding the field surveys was sunny and dry.

It was not possible to measure crack widths as these were generally obscured by the grey gel. This gel was later determined during the petrographic analysis to be formed by dissolved and then deposited lime transported by upward migration of water through cracks in the concrete.

Areas of potential concrete delamination were determined using a simple chain drag method, whereby a short length of heavy chain is dragged across the concrete surface. However, it was found that the readings reflected areas of ponding water beneath the wearing course rather than delamination. Areas of delamination would normally be expected to be discontinuous across expansion joints, especially where adjacent panels are composed of concrete from different pours. The original readings were, however, continuous across individual concrete panel expansion joints. Following the clearance of mineral and other deposits in the central trench drains, and the ensuing dry weather, the areas of "delamination" which had been marked were reduced from the original readings.

Thickness measurements were taken via impact echo instrumentation to minimize disruption to the parking deck and hospital activities. With the impact echo technique, a short stress pulse introduced into the concrete wearing surface by striking the surface with a steel ball, modal hammer, spring loaded impact or other suitable instrument. The waves produced by the impact reflect from internal discontinuities and boundaries. An accelerometer or other transducer used in acoustic emission testing and ultrasonic testing with adequate frequency response is mounted on the surface of the structure, close to the impact point, to measure the time lag of the signals. By knowing the propagation velocity (via calibration at known thickness locations) these time lags are analyzed with a Fast Fourier Transform analyzer to determine the depth of discontinuity.

To calibrate the equipment, as well as to provide confidence in the results, a limited number of holes were drilled through the wearing surface to physically measure thicknesses. Polynomial curves relating the test results to these physical readings were generated to determine a best-fit curve with which to calculate thicknesses from the raw results. The calculated thicknesses were accurate to within one-half inch when readings were not taken in the vicinity of delaminations, concrete disintegration or embedded foreign material.

Elevations of the parking deck surface were also taken at the five locations per panel corresponding to the impact echo test locations. From the survey readings and thickness measurements, calculations were made of the slopes of both the top and bottom surfaces of the wearing surface panels.

Material test results indicated that, although the cement, sand and aggregate contents were adequate, the water content was slightly high, air bubble characteristics were poor and the compressive strengths were highly variable. Compressive cylinder strength test results of the seven year old concrete varied from very low (17.0 MPa) to adequate (37.3 MPa). The presence of ettringite within the lower portions of the cores indicated that the concrete had been saturated for a significant period. Microfractures within the cement paste indicating classic freeze thaw damage to the concrete matrix also tended to be present primarily in the lower portions of the cores.

The wide range of strength values suggests poor quality control during the concrete pours. Low strengths were reported to the designers during construction but were not rejected, as they considered the wearing course to be a "fill" with minimal strength and durability requirements.

The waterproofing membrane was in general compliance with the original specifications. No evidence of water penetration had, in any case, been reported through the membrane. Significantly, no drainage layer was found above or below the insulation.

Most upper surface elevation measurements indicated a slope towards the parking lot drains in excess of one per hundred, which corresponds roughly the minimum surface slope which is recommended by the American Concrete Institute (ACI) Committee 330. Soffit readings, generated from top surface impact echo data and calculated thicknesses, indicated slopes significantly less than this, often down to the limit of accuracy of the measurement and calculation methods, suggesting nominally tilted concrete panels and a flat structural support slab. These readings were later confirmed by visual observation during demolition.

At the commencement of the survey the drains were fully blocked with what appeared to be calcium deposits. The design had called for modular trench drains with a provision for seepage holes only. The weep holes which were provided were frequently blocked. These were cleared during the investigation as part of the hospital's normal maintenance program. The blocked drains prevented drainage of the underlying surface to the parking lot allowing water to build up within the insulation layer at significant distances from the drains. The wearing course and insulation were thus frequently under high hydrostatic pressures.

Carbonation penetration was minimal indicating good finishing practice. Although chloride contents near the surface (25 mm) of the concrete wearing surface were high, particularly away from the perimeter of the parking area, the wearing surface was effectively free of embedded steel and therefore was not considered to be significant with regard to the concrete deterioration.

A nominal wire reinforcement mesh was provided, presumably to control shrinkage and thermal cracking. However, the specification for the mesh was apparently based upon the average anticipated thickness of the wearing course and was therefore inadequate for the majority of the deck. Additionally, no provision was given for satisfactory placement or inspection of the mesh, which would be easily bent and/or displaced from foot traffic. Hence, the mesh was consistently placed near or within the underlying sand layer, rendering the mesh totally ineffective for either strength or crack control purposes. Early age cracking due to shrinkage and temperature effects consequently occurred.

It was concluded that the deterioration was primarily due to freezing of entrapped water. Early age temperature and shrinkage cracking of the wearing course was uncontrolled due to the absence of the requisite reinforcement and was exacerbated by the poor concrete quality. The water, trapped under pressure, saturated the temperature and shrinkage cracks, transverse and corner cracks primarily due to traffic and dominant longitudinal cracks resulting from early age cracking exacerbated by traffic loading. Disintegration of concrete and enlargement of the cracks quickly ensued during periods of cold weather. The deterioration was progressive and affected the majority of the wearing surface within a relatively brief time period.

Tilting of individual 20-ft (6.1 m) square bays panels was most likely also due to winter freezing of entrapped water in the insulation layer. Water under pressure from the weight of the wearing course concrete saturated the concrete. It is also opined that during winter months this saturated water would tend to accumulate and freeze within the concrete near the exposed surface of the wearing course further delaminating the concrete.

4. CONCLUSIONS AND RECOMMENDATIONS

It is essential to permit water penetrating through a deck wearing course to drain out of the system. An illustration of the recommended deck design as described by ASTM C898 (and C981) deck design guides, reprinted with permission, is given in Figure 4 (copyright ASTM). Items of particular relevance are the presence of the free-draining drainage course, the � inch (12.7 mm) minimum diameter weep holes at 1� inch (38.1 mm)maximum spacing at the bottom of the drainage course and the vertical face of the drain, shown to be free of horizontal projections.

The importance of adequate weep holes cannot be overstated. The entrapment of water within the insulation layer of the parking deck investigated can be partially attributed to the lack of adequate drainage through the body of the trench drain. The visible bubbling of water through the cracks in the wearing course substantiates this interpretation and, additionally, indicates the large hydrostatic pressures which are generated from the overburden and live loads, forcing the water through the concrete.

ACI report ACI325.1R Design of Concrete Overlays for Pavements, published in 1967, specifies a minimum separated overlay thickness of 3� inches (88.9 mm). Although a wearing course to a parking deck is not, strictly, a pavement, their recommended minimum design thickness would reasonably be expected to be directly applicable to the wearing surface.

The report by ACI Committee 330 on Parking Lots, published in 1987, also provides guidance for the design of the wearing surface including recommended thicknesses for the wearing surface as a function of intended use, concrete strength and subgrade stiffness. The insulation, waterproofing, protection board, drainage course and any other materials between the wearing course and roof slab act as the subbase for the parking deck. Moduli of elasticity can be determined directly from the manufacturer, by tests or published data.

Depending upon the anticipated truck traffic, most public concrete wearing courses overlaying parking decks supported by concrete slabs require minimum thicknesses of between 3 � and 5 inches (88.9 and 127 mm). Due allowance needs to be given to the effects of poorly consolidated fill material, gaps, protection board and waterproofing. It is also important to note that the compressive strength is specified to provide necessary durability to environmental effects, abrasion and, where applicable, deicing salts. A more direct means of ensuring proper durability would be to specify minimum cement and maximum water content and maximum permeability.

Even when the underlying slab is adequately waterproofed, and when reasonable precautions have been taken to prevent penetration of water through the wearing surface to the waterproofed roof surface, it is still recommended that both the exposed and underlying surfaces are sloped towards the drains. The benefits of adequate drainage include: (1) reduced freeze-thaw damage (2) contaminants tend to be carried off the surface with the runoff water, (3) water penetration through the slab is reduced, and (4) the likelihood of surface ice-buildup is reduced. To remove the free water from the area to nearby drains, most references recommend a minimum slope of inch per foot (1:100) for the underlying drainage course, whether or not the wearing surface is intended to be closed jointed (surface drained) or open jointed (permeable surface).

It is also essential to include, in addition to minimum slopes to both wearing course and underlying structural slab, a drainage course, in recognition that water may infiltrate below the wearing course. A path must be provided above the protection board to carry this water to the drains. Trapped water will ultimately lead to a damaged structure, waterproofing and consequent water penetration into the building.

REFERENCES

[1] ACI Committee 325, Design of Concrete Overlays for Pavements (ACI325.1R) (American Concrete Institute, Detroit, 1967)

[2] ACI Committee 330, Guide for Design and Construction of Concrete Parking Lots (330R-87) (American Concrete Institute, Detroit, 1987)

[3] ASTM Committee C24, High Solids Content, Cold Liquid-Applied Elastomeric Waterproofing Membrane With Separate Wearing Course (C898-89)(American Society for Testing and Materials, Philadelphia, 1989)

[4] ASTM Committee C24, Standard Guide for Design of Built-Up Bituminous Membrane Waterproofing Systems for Building Decks (C981-89) (American Society for Testing and Materials, Philadelphia, 1989)

[5] Laura E. Gish, Editor, ASTM STP 1084, Building Deck Waterproofing (American Society for Testing and Materials, Philadelphia, 1990)

[6] David C. Monroe, Waterproofing Principles, The Construction Specifier (Dec 1990)

[7] Ruggiero and Rutila, Plaza Waterproofing Design Fundamentals The Construction Specifier (Jan 1991)