Properties and Durability of Aggregate

PROGRESSIVE CONCRETE AND MORTAR DETERIORATION AS MEASURED BY COMPUTER‑CONTROLLED MULTIPLE SONIC PULSE METHOD

 PETER P. HUDEC AND WEI WANG  Geology Department, University of Windsor, Windsor, Ont. CANADA

Abstract

Various sonic methods have long been used to monitor the deterioration of concrete.  Sonic method is based on the impedance of sound velocity in the specimen by the cracks formed during the deterioration process. 

As part of an on‑going study of alkali reactivity, sonic pulse velocity measuring equipment was developed which is unique and suitable for small specimens.  The equipment is computer controlled ‑ the timing of pulse velocity, and  the accumulation, processing, and storing of data is fully automated.  The sample of mortar is placed under constant load, and a 30 microsecond sonic pulse is generated and timed.  One hundred pulses are measured; the velocity data is statistically cleaned, and the resultant mean velocity is stored on floppy disk media.  Spreadsheet template is then used for further processing and comparison of data.  Three specimens exposed to the same conditions test are used for comparison and control, and the results are averaged. 

The sonic results have been used to monitor the progress of alkali reactivity, freeze‑thaw deterioration, and uniaxial compressive strength changes;  good correlations were found to exists between these parameters and the sonic measurements.  Changes in sonic velocity rather than absolute velocity were used.  The velocity decreases as the sample is subjected to either freeze-thaw or alkali reactivity.  The aggregate type used in the mortar has a major influence both on absolute and on changes in the velocity.  Although the correlations were significant, the sonic method is not as reliable as direct testing of the specimen; salt and mineral crystallization in the pores is thought to affect the velocity.

Keywords:  Sonic pulse velocity, Alkali reactivity, Freeze1thaw durability, Statistical treatment, Computer spreadsheet, Strength, Mortar, Aggregate

 Hudec, Peter P. and Wang, Wei, 1990, Progressive concrete and mortar deterioration as measured by computer-controlled multiple sonic pulse method, Fifth Intern. Conf. on Durability of Building Materials and Components, Baker, J.M et al, eds., Chapman & Hall, New York, pp.599-606.

 1.  Introduction

 While conducting research into aggregate, mortar and concrete durability under freeze1thaw and alkali reactivity conditions, it was thought that some form of monitoring of the specimen condition as it goes through the various tests may be useful.  The available sonic equipment was not suitable for the small size samples used in the research, and new equipment had to be designed.  The availability of an older generation of microcomputers that could be dedicated to instrument control and monitoring prompted the design of equipment that could, with the help of the computer, accurately measure the velocity of the sonic pulses in the specimen.  By comparing the velocities at different stages of the experiment, it was hoped that the progress of the deterioration could be monitored.

 2.  Instrument and software design and description

The pulse velocity equipment (Figure 1) consists of two transducers: one serves as a transmitter of the pulse, the other as a receiver.  Both transducers are controlled by a Radio Shack Model 4 microcomputer via a 'black box' interface.  The computer clock performs the timing functions:  initiates the pulse, and times its passage through the specimen.  Software was written to both control the pulse frequency and the number of pulses, to retrieve the measured times, and to perform statistical cleaning and averaging of the results.

    One hundred pulses are sent and measured.  During acquisition, the data is temporarily stored in a RAM array.  The one hundred time intervals are averaged, and those exceeding one standard deviation either side of the mean are discarded.  New mean is calculated, and the data is stored on a floppy disk for later transfer to a spreadsheet on an IBM compatible for further analysis.  Three cores from the same sample are used in analysis. 

 3.  Sample Preparation and Description

 The specimens used were 25mm (1 inch) diameter mortar cores containing various type of aggregates being tested for their alkali reactivity and the effect of various chemical treatments on the reactivity.  The alkali reactive aggregates were chert from Putnam area, greywacke from Sudbury area, and carbonate from Ottawa area, all in Ontario. 

    Mortars containing different aggregate and different additives were cast into a 12x10x8 cm block, and quick-cured in water at 80^oC.  The mortar mix proportions and aggregate gradations were according to ASTM C-227 specifications.  Three cores of 25mm diameter and approximately 65mm long were cut from each block.  The ends were squared, ground, and 'dimpled' (for alkali expansion measurements). 

 4.  Sonic Pulse Velocity equipment evaluation and calibration 

    The samples are held in the instrument by linearly applied compressive force.  The effect of the compressive force and the repeatability of the measurements were determined by measuring the identical sample 30 separate times, each involving re-mounting of the sample in the instrument.  These results, and the effect of the aggregate type are given in Figure 2.  The results show that the loading force has a significant influence on the time of passage of the sonic pulse through the specimen.  As the load increases, the time decreases.  The relationship is exponential - small changes in light loading produce the largest differences, and as the load is increased, the differences grow smaller.  As a consequence of this testing, a load of 15lbs, carefully applied and measured by a torque wrench, was used throughout the experiment.

    The aggregate type also had a major influence on the sonic pulse velocity.  Since the mortar preparation was similar and parallel (in terms of additives), the only variable was the aggregate type.  This suggests that sonic methods are most useful for relative comparisons rather than absolute measurement of, for instance, strength or durability parameters.  Thus, absolute velocities, although somewhat indicative, are probably not as useful as relative change in velocity, expressed as percent of the initial or starting velocity.  Consequently, all results reported are as percent change in velocity.

 5.  The testing sequence

 The usual sequence of testing involved:

               1.  Determination of initial velocity (in some cases)

               2.  Accelerated alkali reactivity testing

                           (measurement of velocity in middle and end of test)

               3.  Freeze-thaw testing

                           (measurement of velocity at the beginning and end of test, and in

                                       some at every cycle of the 5-cycle test)

               4.  Uniaxial compressive strength testing

6.  Test results and discussion

 6.1  The effect of alkali reactivity on sonic pulse velocity

 An accelerated alkali reactivity (AR) testing in 80^oC 1N NaOH solution was done on all the samples.  Sonic velocity was determined before, during, and after the test.  The results are shown in Figure 3.  Two aggregate types are shown: Putnam chert and Sudbury greywacke.  Chert is known as a fast reacting aggregate, whereas the greywacke reacts more slowly.  This is shown both by the AR expansion rate (um/day), and by the differences in the change of sonic velocity. The progress of the alkali reactivity is shown by the 10 day and 22 day sonic measurements.  The results are an average of 11 samples of chert and 9 samples of greywacke mortars.

   As expected, the velocity decreases as alkali reaction proceeds.  Sonic velocity measurements confirm that internal cracking due to AR slows the sonic pulse.  The cracking is the results of expansion of silica gel formed during the AR process.  

   Figure 4 shows the relationship between the velocity decrease and both the AR expansion and the terminal uniaxial strength of the Spratt carbonate mortar.  The strength of the mortar decreases with the velocity decrease; at the same time, the expansion measured during AR is shown increasing.  This indicates that the parameters measured have the expected trends. 

 6.2  The results of Freeze-Thaw cycles on sonic pulse velocity

 Freezing and thawing tests were run on the mortar cores that were first exposed to AR.  While this may give different results compared to freeze-thaw tests run on fresh cores, relative comparisons can be drawn.  In any case, concrete and mortar exposed in nature undergoes both alkali reactivity and freezing and thawing.  AR probably aids in increased freeze-thaw deterioration.  Sonic measurements serve to monitor the degree of deterioration.

    Freezing and thawing tests were done by saturating the cores in a 3% NaCl solution, and then freezing the cores in air in a closed container on a sponge saturated with the chloride solution.  This was found to be a severe, and reproducible test for aggregates, mortars, and concrete in prior experiments.  The freeze-thaw loss was determined by weighing the amount of spalled and scaled material from the cores, and calculating this as a percent of the original weight.  The sonic pulse velocity was also measured at the beginning, during, and at the end of the cycles.  Velocity was also measured on the dry and saturated cores both before and after the freeze-thaw experiment.  It was noted that the velocity was always substantially higher in the wet state than in the dry state.  Sound travels well through water; in addition, water in the mortar pores probably sets up thixotropically, adding to the rigidity of the system.

    Figure 5 illustrates the results obtained when the percent of spalled and scaled material was compared to the decrease in the sonic pulse velocity.  The relationship is not a strong one, but a general trend can be observed that as the velocity decreases, the freeze-thaw loss increases.  The freeze-thaw loss in this case is principally a surface phenomenon, not affecting significantly the body of the mortar.  The pulse travels through the body, and thus is not affected.

    It was also observed that when the spalled amount and the remaining core mass were added together, the resulting mass was usually greater than the original mass of the core.  Obviously, salt was crystallizing within the pores of the mortar, which would also provide bridging and rigidity to increase the sonic velocity.

 7.  Velocity and change in velocity relationship

The absolute pulse velocity is affected by the aggregate type.  However, if this is removed as a variable, the absolute velocity has an influence on subsequent velocity decrease.  Figures 6 and 7 illustrate this point.  In both the carbonate and greywacke aggregate mortars, the initial velocity determines the percent decrease in the velocity as the sample goes through its various tests.  This can be explained by the relationship of the velocity to compressive strength - the higher the strength, the higher the velocity.  Higher strength mortars resist the degradation during AR and freeze-thaw better than lower strength mortars.  In this instance, the variability of strength is due to various admixtures.

 8.  Summary and conclusions

The summary of the properties measured is given in Figure 8.  The properties are expressed as arithmetic means.  The figure shows that the mortars with the silica-rich aggregate behave similarly, whereas the carbonate containing mortars have somewhat different relationships among the means of different properties.  In particular, the relationship of the velocity change to the freeze-thaw loss is reversed for the carbonate mortar.  This emphasizes the observation that the test results tend to be aggregate specific, and some caution must be taken in using the results as means of determining the quality and durability of the aggregate.

    The sonic pulse velocity measurements were shown to be sensitive to the aggregate type present in the mortar.  Although only three aggregate types were used, it is probable that this applies to others.  In addition to aggregate type, admixtures, proportions, method of preparation and curing all have an influence on the internal structure of concrete and mortar, and will influence the velocity of sound waves.  Sonic methods are probably best suited for relative comparisons rather than as means of evaluation.

    The sonic pulse method described in the report is well suited to following the progress of alkali reaction, and can supplement expansion measurements.  Other sonic methods will probably work equally well.  The procedure developed here is quick, and the equipment relatively inexpensive.  Any computer, with proper electronic interface, can be used.  The total cost of equipment other than the computer is estimated at less than $250.00.  This does not include the development or assembly time.

 9.  Acknowledgement

 The work described in this paper was supported by a research grant from the National Research and Engineering Council of Canada (NSERC).