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Early performance of structural concrete slabs treated with a
hydrophobic admixture
Early performance of structural concrete slabs treated
with a hydrophobic admixture
Cusson, D.; Mitchell, L.; Glazer, R.; Hoogeveen, T.
NRCC-48142
A version of this document is published in / Une version de ce document se trouve dans :
CONCREEP 7 Workshop on Creep, Shrinkage and Durability of Concrete
and Concrete Structures, Nantes, France, Sept. 12-14, 2005, pp. 257-262
Concreep7 Workshop on Creep, Shrinkage and Durability of Concrete and Concrete Structures, Nantes, France, 12-14 September 2005, pp. 257-262.
Early performance of structural concrete
slabs treated with a hydrophobic admixture
Daniel Cusson – Lyndon Mitchell – Rock Glazer – Ted Hoogeveen
National Research Council Canada, Institute for Research in Construction1200 Montreal Road, Building M-20, Ottawa, Canada, K1A 0R6
Daniel.Cusson@nrc-cnrc.gc.ca; Lyndon.Mitchell@nrc-cnrc.gc.ca;
Rock.Glazer@nrc-cnrc.gc.ca; Ted.Hoogeveen@nrc-cnrc.gc.ca
ABSTRACT: This paper presents the early results of the performance of reinforced concrete slabs of a parking garage. Part of the garage was rebuilt with a hydrophobic concrete to reduce shrinkage and delay corrosion of the embedded reinforcement. A total of a four structural slab sections and two ramps were instrumented with embedded strain gauges, relative humidity/temperature sensors, and reference electrodes. The early results show that interactions with air-entraining agents seemed to reduce their effectiveness and freeze-thaw resistance. After 140 days, both the control and hydrophobic concretes had similar drying and shrinkage and low tensile stresses.
KEYWORDS: concrete slab; restrained shrinkage, remote monitoring, hydrophobic admixture
1. Introduction
Deterioration of concrete structures due to shrinkage cracking and reinforcement corrosion is a widespread problem around the world and a continual challenge for their owners (Cusson et al. 2000, 2004). The Laurier-Taché parking garage in Gatineau (Québec, Canada) – owned by Public Works and Government Services Canada (PWGSC) – is undergoing a $35 million retrofit. Part of the rebuilt structural concrete slabs and ramps of the parking garage were made with a hydrophobic concrete to reduce shrinkage and reinforcement corrosion. The National Research Council (NRC) was mandated by PWGSC to evaluate the field performance of these slabs and ramps by remote monitoring with embedded sensors.
The structural performance of the test slabs and ramps with respect to shrinkage and cracking is being evaluated, with the early results presented in this paper. The long-term performance of the test slabs and ramps, particularly corrosion and durability will also be investigated. The confidence generated by continuous monitoring and evaluation of concrete structures using durable concrete technologies will encourage their continued specification. The long-term objective of this project is to demonstrate the durability of concrete structures built or repaired with innovative technologies. Such structures should exhibit extended service lives with reduced maintenance costs.
2 Concreep7
2. Experimental program
2.1. Materials
The hydrophobic admixture is based on stearate technology. It works by modifying the surface tension properties of concrete pores, which leads to a reduction in shrinkage and permeability. The concrete had a water-cement ratio of 0.4, a cement content of 350 kg/m3, a hydrophobic admixture dosage of 30 L/m3 and a design strength of 35 MPa at 28 days.
2.2. Preliminary laboratory investigation
Before the fieldwork began, a series of large-scale concrete mixes were made in the laboratory to test different properties on fresh and hardened concrete samples. The properties evaluated included: porosity, drying shrinkage, compressive strength, and freeze-thaw resistance, as well as corrosion resistance. During the fieldwork, fresh concrete samples were also taken during placement of the test sections in the garage to obtain in-place concrete properties.
2.3. Parking garage structure and test sections
The Laurier-Taché parking garage is a large underground heated concrete structure. It is subjected to the same loading and environmental conditions as most parking facilities in Canada, including high-volume traffic loads and de-icing salt contamination. Suffering from severe reinforcement corrosion, concrete cracking and delamination, the travel surfaces were rebuilt in the fall of 2004 and later. For this research project, four 150-mm thick sections of structural concrete slabs (Nos. 1 to 4) and two 250-mm thick concrete ramps (Nos. 5 and 6) were fully instrumented to evaluate their performance with regard to corrosion resistance and durability (Figure 1). Two of these sections were built using concrete containing the hydrophobic admixture and the two others were made of normal concrete and used as controls. Both ramps were built with the hydrophobic concrete; neither had a wearing membrane installed. In an effort to minimize restrained shrinkage, the concrete of the test slabs was placed in square sections using a checkerboard pattern with a 4-m spacing between control joints. A larger checkerboard pattern was used for the control slabs with an 8-m spacing between control joints. An epoxy-based mortar was applied later on the surface of the control slabs to seal and protect them from moisture and wearing. A minimum of 7 days of wet curing was specified. Thus, two slab design options are being compared in this study, rather than the effectiveness of the hydrophobic admixture alone.
Two reinforced concrete prisms were built on site (150 mm high, 300 mm wide and 1000 mm long). The prisms were under the same conditions as the structural concrete slabs, except that the prisms were totally unrestrained.
Early performance of structural concrete slabs 3
Test sections ydrophobic
concrete
Regular concrete (except at ramps)
1 2 3 4 Ramps 6 5 H
Figure 1. Plan view of the parking garage floor
2.4. Sensors and data logging system
More than 100 sensors were installed in six floor sections of the parking garage. As shown in Figure 2, each test section included the following set of sensors: (i) four reinforcing bars instrumented with two weldable strain gauges (WSG) were used to measure the shrinkage and thermal strains in both orthogonal directions near the top and bottom of the slab; (ii) two embedment strain gauges (ESG) were placed at mid-height in both directions; (iii) three relative humidity and temperature (RH/T) sensors were installed at different heights in the slab to assess the profile of moisture; and (iv) four manganese dioxide (MnO2) reference electrodes (RE) were positioned near the top and bottom reinforcement in both directions to monitor their corrosion potential. In addition, an RH/T sensor and a CO2 sensor were installed in the ambient indoor environment of the parking garage near the test sections.
The data acquisition system selected for the field study consisted of a standalone micro logger and multiplexers. The system can store both data and programs in non-volatile memory. It was equipped with a modem and a data line to permit remote communication with the host computer at NRC. This system was selected for its capability to support numerous types of sensors, on-line data manipulation and statistical functions. The data acquisition system was installed at the ceiling level and attached to the steel conduits protecting the sensor cables. The remote monitoring started in November 2004 prior to the placement of the concrete slabs.
4 Concreep7
RE
WSG
RH/T ESG
Figure 2. Typical instrumentation setup in one test section
3. Analysis of results
3.1. Preliminary laboratory results
The use of the hydrophobic admixture had minimal effect on compressive strength, shrinkage and slump compared to a control concrete made without the admixture. The hydrophobic admixture, however, did release ammonia gases during mixing, which can be a health issue if not properly guarded against.
Care must be taken when designing concrete mixes with this type of product, as there are potential conflicting chemical interactions with air-entraining agents. These interactions can lead to lower than intended porosity. Initial investigations revealed that adding the recommended dose of air-entraining agent to the concrete mix had little or no effect on freeze-thaw resistance. It is possible that the hydrophobic admixture attracted the hydrophobic tail of the air-entraining agent, rendering it ineffective. Further study is required before any firm conclusions can be drawn.
In the field, the control concrete had slumps from 100 to 120 mm, an air content of 5.2 %, and compressive strengths from 45 to 54 MPa at 28 days. The hydrophobic concrete had slumps from 110 to 140 mm, air contents from 3.9 to 4.3 %, and compressive strengths from 36 to 40 MPa at 28 days. Both the control and the hydrophobic concretes passed the ASTM C666 Test on freeze-thaw resistance.
Early performance of structural concrete slabs 5
3.2. Early field performance results
Figure 3 presents the RH measured in two test slabs (e.g. Section 2 for hydrophobic concrete and Section 3 for control concrete). After 140 days, both concretes had dried gradually to an RH near 80%. The top (T) and bottom (B) sensors, located at the reinforcement levels (40 mm from slab surfaces) indicated slightly higher drying than the middle sensor (M), as expected. No significant differences in drying were observed between the two concretes after 140 days.
Figure 4 shows the total strain (i.e. thermal & shrinkage) measured by the embedded strain gauges (ESG) in Sections 2 and 3 and in the unrestrained reinforced concrete prisms. After 140 days, both concretes had shrunk gradually to a strain close to 200 µe. For each concrete placed in the slabs, the strain in the long span direction was lower than that in the short span direction, which is a direct consequence of the structural degree of restraint being higher in the long direction (45-50% vs. 15-20% at 140 days, respectively). Moreover, the strains measured in the prisms were larger than those measured in the corresponding garage slabs since shrinkage was not restrained in the prisms. The tensile stress that developed in the concrete slabs is not significant, since the difference in shrinkage between the partially restrained slabs and the unrestrained prisms is small. After 140 days of monitoring, both design options gave satisfactory performance in keeping restrained shrinkage and the resulting tensile stresses low.
70 80 90 100 0 28 56 84 112 140 RH (%) 70 80 90 100 0 28 56 84 112 140
Time after setting (days)
RH (%) Control concrete T B M Hydrophobic concrete T B M
6 Concreep7 -400 -200 0 200 0 28 56 84 112 140 Strain ( µ e) -400 -200 0 200 0 28 56 84 112 140
Time after setting (days)
Strain (
µ
e)
Control concrete
Hydrophobic concrete Long span direction
Short span direction
Free prism in lab
Long span direction Short span direction
Free prism in lab
Figure 4. Total strain measured in the test sections
5. Summary and conclusions
The field performance of concrete slabs and ramps rebuilt with a hydrophobic concrete is being evaluated. From the early results, these conclusions can be drawn: 1. The use of the hydrophobic admixture had minimal effect on slump, shrinkage
and compressive strength. Interactions with air-entraining agents seemed to reduce air-entraining effectiveness and freeze-thaw resistance.
2. Drying and shrinkage occurred similarly in the control and hydrophobic concretes, with an RH close to 80% and a total strain near 200 µe at 140 days. Both slab designs provided low restrained shrinkage and low concrete stresses. Acknowledgments
The authors would like to thank Mr. Allan Wiseman of PWGSC for providing his expertise and funding, and Mr. Glendon Pye of NRC for his technical assistance. 6. References
Cusson, D., Hoogeveen, T., Qian, S. (2004): "Field performance monitoring and evaluation of concrete repair systems on a highway bridge," 5th Structural Specialty Conference of the Canadian Society for Civil Engineering, Saskatoon, June, 10 pp.
Cusson, D., Repette, W. (2000): “Early-age cracking in reconstructed concrete bridge barrier walls,” ACI Materials Journal, 97(4), 438-446.
