International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Optimizing Opening of PCC Pavements Using Integrated Seismic Lab and Field TestsDeren Yuan, The University of Texas at El Paso, El Paso, TX, USA 79968
Soheil Nazarian, The University of Texas at El Paso, El Paso, TX, USA 79968
Nenad Gucunski, Rutgers University, Piscataway, NJ 08854
The use of stress wave technique combined with maturity concept to monitor and predict the strength gain of Portland cement concrete is investigated. Stress wave velocities, from which the dynamic modulus of elasticity is calculated, can be related to the strength parameters and static modulus obtained from standard testing on the molded specimens and drilled cores. When these results are combined with the maturity parameters the predictive power is significantly improved. In this investigation, the lab tests on molded specimens and cores were carried out with the simplified free-free resonant column method and the field tests on concrete slabs or other structures were performed with a hand-held device called the portable seismic pavement analyzer. Experimental studies have been on low, medium and high-strength concrete mixes under different curing regimes and conditions. Based on these experiments, relationships between the dynamic modulus and the strength parameters as well as the maturity are proposed. The technique has shown to be a rapid, simple and economic means for optimizing concrete mix design, quality control/quality assurance of concrete construction and determining the time required before a repaired or newly constructed structure is ready for use.
The current state of practice in quality assurance-quality control (QA/QC) of earthwork for transportation and infrastructure projects is mostly concerned with the constructability and durability of the materials. To implement performance-based specifications or warranty-based construction, the current practice has to be supplemented by methods that will provide continuity between the design, laboratory and construction. In this paper, we have attempted to demonstrate simple test methods that can be added to the existing procedures to address such concerns.
A comprehensive QA/QC procedure has to incorporate several inter-related items. First, the major parameters considered in this process should be closely correlated to the parameters used in the design. Pre-construction laboratory tests for determining the suitability of a material should also be, to a large extent, based on the selected design parameters. In addition, the acceptable values for the selected parameters should be based on the same laboratory tests. Finally, the quality control during construction has to ensure that the acceptable levels are achieved.
Procedures to measure the modulus of different flexible pavement layers in the laboratory and during construction are briefly discussed. By taking advantage of the existing specimen preparation techniques used for determining optimum maximum density, the optimum modulus is determined. An NDT device used to measure the modulus in situ is also included. Test protocols for developing a project-specific QA/QC plan are proposed.
MATURITY AND SEISMIC CONCEPTS
The strength of a given concrete mixture is a function of its age and temperature history (a.k.a. maturity). Maturity measurement in the field consists primarily of monitoring the internal temperature of the concrete with respect to time. The temperature is monitored by attaching thermocouple wires inserted into the fresh concrete to a maturity meter (basically a temperature-measuring device). The temperature history obtained from the maturity-meter is used to calculate the maturity index such as time-temperature factor (TTF). The TTF at age t is related to strength by a strength-maturity curve. The following expression is used to calculate the maturity (Malhotra and Carino, 1991):
where Dt = time interval between consecutive measurements, Ta = average concrete temperature during time interval, Dt, and To = datum temperature (typically -10°C).
Seismic methods rely on generation and detection of elastic waves within a medium and measuring the velocity of propagation of these waves. The measured velocity can be converted to modulus based on theory of elasticity. Seismic tests can be carried out in the laboratory as well as in the field. The free-free resonant column test of specimens is particularly suitable for measuring the seismic modulus of concrete in the laboratory. When a cylindrical specimen is subjected to an impulse load at one end, seismic energy over a large range of frequencies will propagate within the specimen (see Figure 1). Depending on the dimensions and the stiffness of the specimen, energy associated with one or more frequencies is trapped and resonate as they propagate within the specimen. The goal with this test is to determine these resonant frequencies. Since the dimensions of the specimen are known, if one can determine the resonant frequencies, one can readily determine the modulus of the specimen using principles of wave propagation in a solid rod (Richart et al., 1970). As shown in Figure 2, resonant frequencies appear as peaks in a so-called amplitude spectrum. Two peaks are evident, one corresponding to the longitudinal propagation of waves in the specimen, and the other corresponding to the shear mode of vibration. Once the longitudinal resonant frequency, fL, and the length of the specimen, L, are known, laboratory Young's modulus, Elab, can be found from the following relation
where r is mass density. Poisson's ratio, u, is determined from
with fS being the shear resonant frequency and CL/D being a correction factor when the length-to-diameter ratio differs from 2.
The Portable Seismic Pavement Analyzer (PSPA) is used to estimate the in-place seismic modulus of a PCC slab. The PSPA (see Figure 3) consists of two transducers and a source
packaged into a hand-portable system. The device is operable from a computer. The major mechanical components of the PSPA sensor unit as depicted in Figure 4 are a near and a far accelerometers, and an electric source. The data collected with the PSPA can be processed to determine the modulus of the slab directly through the high frequency surface waves (USW) method (Baker et al., 1995).
The modulus of the slab, Efield, can be determined from surface wave velocity of the slab, Vph using
The wavelength at which the phase velocity is not constant anymore is closely related to the thickness of the top layer. It can be shown theoretically that the laboratory and field moduli, Elab and Efield, are related through Poisson's ratio2, u. The relationship is in the form of
Alexander (1996) and Ramaieh et al. (2001) demonstrated that, the velocities measured with the PSPA and free-free resonant column tests are highly correlated. They also concluded that the repeatability of the tests was better than those carried out by traditional strength tests.
As indicated before, the combination of maturity and seismic methods complements one another quite readily. The calibration process for relating strength and maturity can be readily adapted for laboratory seismic testing. In fact, the same specimens can be used for both tests.
The seismic method has several advantages over the maturity one. In most current specifications, two thermocouples per 1000 yd3 of PCC poured are required, approximately one sample for every 375 ft of a 12-ft wide standard lane is considered. As such, any variability in the strength of concrete due to batching errors, construction, equipment-related problems or the curing process might not be found with the maturity tests. A proposed protocol that combines the two methodologies is illustrated using an example.
For compressive strength, a total of 15 standard 6 in. (diameter) by 12 in. (length) specimens are prepared. For flexural strength, a similar number of specimens but in the shape of standard beams is poured. It should be mentioned that if one would be interested in only establishing relationships between the seismic modulus and maturity only three specimens are necessary. During specimen preparation, thermocouples are inserted into 3 cylinders. The specimens are then cured in a water tank.
The protocol consists of four phases: maturity measurements, seismic modulus tests, strength tests, and development of the correlations. Each is discussed below.
I. Maturity Tests:
II. Seismic Tests:
III. Strength Tests:
IV. Development of Correlations
Typical variation in compressive strength from standard cylinders with maturity parameters is shown in Figure 5a. The time-temperature factor (TTF), as defined in Equation 1, is used to represent the maturity parameter in the figure. A good correlation is observed between the compressive strength and TTF as judged by a coefficient of determination (R2 value) of about 0.97. Three-point bending tests were also carried out on beams cured under standard conditions. The variation in flexural strength with TTF is shown in Figure 5b. The flexural strength is strongly (R2 = 0.96) correlated to the TTF.
The variation in seismic modulus with TTF on the cylinders, used for determining the compressive strength is shown in Figure 6. A high correlation between the seismic modulus and TTF is also obtained. Also included in the figure is the variation in the seismic modulus with TTF for the beams tested for flexural strength. Since the results from the cylinders and beams follow one another closely, the seismic modulus is practically independent of the shape of the specimen being tested (see Ramaiah et al., 2001, for a detailed statistical analysis). Figure 6 can be readily used to project the modulus of concrete as a function of time as typically done with the strength-maturity relationship.
In the next step, the seismic moduli measured at different times are related to the compressive and flexural strengths. For the sake of brevity, only the results from the compressive strength are shown in Figure 7. The compressive and (even though not shown) tensile as well as flexural strengths are highly correlated to the seismic modulus. These three relationships can in turn be used to predict the strength of materials as a function of seismic moduli measured in the field with the PSPA.
Three 6-in.-diameter specimens were cored after 7 days of curing and the other three after 28 days. The average compression strength of these cores at each age is plotted against the seismic modulus in Figure 7. Both data points lie fairly close to the trend line. This means that, by conducting a PSPA measurement and using the equation of the trend line, the strengths are estimated by an accuracy of better than 10% to 15%. Similarly, the results from the traditional maturity model are shown in Figure 8. As anticipated, the compressive strength is predicted reasonably well with an accuracy of better than 20% for the seven-day strength and 10% for 28-day strength.
It seems that it may be feasible to develop a preliminary unique relationship between the compressive strength and seismic modulus. If this is proven true in the long term, one can utilize this relationship more readily than those developed for the maturity. The results from a number of mixtures tested in this study are accumulated in Figure 9. All but one mixture is made out of limestone. The other is made of siliceous river gravel (marked as SRG). The SRG mixture demonstrates one pattern. However, the trends from all other mixtures follow a similar pattern. All these mixtures have one thing in common; they are made from the limestone aggregates from El Paso area. The global best-fit curve through all data points is shown in Figure 9. The R2 value of the global best-fit curve is about 0.89. This data trend indicates that it may be possible to develop a unique calibration curve for preliminary assessment of the concrete work in a given region. The variations in compressive strength with maturity parameter for the same mixtures are shown in Figure 10. Large deviation from the general trend is observed.
SUMMARY AND CONCLUSION
Timely opening of the roads to traffic is extremely important. However, if the traffic is allowed on the facility before the PCC has gained adequate strength, the pavement performance may be compromised. Even though the maturity concept can vastly contribute to that goal, seismic technology in conjunction with maturity testing seems to be more powerful.
The advantage of this procedure is that the same specimens used for laboratory calibration of the maturity data can be used for seismic calibration; however, instead of placing thermocouples at isolated places during construction, a portable device can be used to test a large number of points. In that way, the variability in the curing of concrete due to possible differences in the materials, curing procedures, workmanship and construction equipment can be measured and considered.
In this paper the preliminary protocol to be followed as well as the technical and operational feasibility of implementing this protocol is explored. The results are quite promising in assisting highway agency to optimize the quality of the concrete and accelerate the opening of the roads to public.
Texas Department of Transportation and the Federal Highway Administration provided the financial support for this study. The authors would like to express their sincere appreciation to Jim Hunt of the TxDOT Dallas District for ever-present support on this study.