L I M EST O N E PRISMS - SH E A R ST R E N G T H ST UD Y

Transcription

1 L I M EST O N E PRISMS - SH E A R ST R E N G T H ST UD Y Holland, Nancy L. 1 ; Nichols, John M. 2 1 PhD, Associate Professor, Texas A&M University, Construction Science Department, nholland@tamu.edu 2 PhD, Associate Professor, Texas A&M University, Construction Science Department, jm-nichols@tamu.edu Limestone has been used as building material for millennia, with examples including the Narbonne Cathedral. Texas has approximately 2000 limestone quarries that produce sawn stone for use as a building envelope material. As the construction industry responds to the issue of green construction, it is expected that limestone block will form an increasing proportion of the external wall cladding material on all types of buildings. A test procedure is being developed to measure the shear capacity of limestone prisms. This paper outlines the basic elements of the procedure and compares the test protocol to the equivalent ASTM and Canadian standards. A set of shear results for limestone samples obtained from three different quarries in Texas are reported in this paper. Keywords: Limestone, shear tests, housing IN T R O DU C T I O N The need to develop a simplified and more feasible method for determining the shear characteristics of limestone was initiated as a result of a study of the structural analysis of the Narbonne Cathedral. A finite element model of the cathedral was developed; however, a full structural analysis was precluded due to the lack of data pertaining to an acceptable shear failure model due to sliding rather than a failure due to crushing. The ASTM Test Method for Diagonal Tension (Shear) in Masonry Assemblages, E519/E519M -10 (Demain and O'Rourke,2007)., requires that a 1.2 metre by 1.2 metre brick assemblage be constructed and tested in compression on the diagonal. This is accomplished by placing two opposite corners in a v shaped holder or shoe at the top and bottom of the diagonally oriented specimen and a compression load applied. There is no statement on either precision or bias testing due to the nature of the materials. The ASTM method requires the use of large heavy duty equipment for the transport and testing of samples that is beyond the capability of most laboratories. With the increasing growth of a limestone industry in Texas and across the nation there is a need for a accessible shear test method for use by material suppliers and designers. Therefore, the authors have undertaken the process of developing a test method using small assemblages that are within the testing capabilities of most laboratories. This paper describes the findings of the second iteration using the experimental test method.

2 L I T E R A T UR E R E V I E W Holland, Paul and Nichols (Holland, Paul, and Nichols,2010) coenced an investigation into the shear properties of limestone masonry as part of larger body of research into the design and construction of the Narbonne Cathedral. Narbonne Cathedral, located in the southeastern corner of France close to the Italian border. The cathedral has been studied extensively by Paul and others for the last thirty years (Nichols,2010; Nichols, Paul, and Nichols,2010; Nichols, Paul, and Nichols,2011; Paul,1991). An understanding of the response of the walls to the applied earthquake loads is a key element in determining the structural capacity of a large unreinforced masonry building, such as Narbonne Cathedral. One of the critical failure mechanisms in large masonry church buildings is sliding of the wall and other elements in earthquakes, as 1989 Newcastle earthquake (Nichols,1999). The previous research showed that a linear equation could be established from the experimental work relating the shear stress to the vertical axial stress applied to the shear blocks. Equation 1 below shows this relationship f f v ac (1) Where the axial compression stress is f ac (MPa) and the nominal shear stress is f v (MPa). The initial study was completed on coercially available Texas limestone. T EST M E T H O D O L O G Y The test methodology was developed for the initial experimental work. The test methodology is: An axial load test rig was developed from a compression unit that could be loaded into an Instron DX 600 Universal Testing Machine (Instron Corporation,2005) as shown in Figure 1. The axial load test rig uses a small Enerpac Ram to apply a pressure to a square steel plate as shown in Figure 1. Figure 1: Test set-up

3 The shear samples were assembled from three cut stone segments, arranged so that the area of the centre stone is approximately the same as the combined area of the two outer stones as shown in Figure 2. The approximate sizes of the outside units were 22 by 76 by 130 and the centre units were centre unit 36 by 76 by 130 Figure 2: Limestone blocks arranged for testing An overlap between the blocks of approximately 100 was maintained during the construction of the three unit assemblage. Each three unit test assemblage was assembled using a 1:3 Portland cement to sand mortar and allowed to cure for 28 days under a covered walkway. Each completed and cured assemblage was placed into the compression test rig, a Masonite sheet was used on the wall side of the compression unit and a 7 plywood sheet was used on the ram side of the compression unit. Three axial compression pressures were selected for the test sequence with readings at the Enerpac ram gauge of 5.52, 9.65 and MPa respectively. The resultant axial stress applied to the limestone units ranged from 1 to 4 MPa The test rig containing the limestone test assemblage was then placed into the Instron with 7 plywood packing plates on the top and the bottom of the limestone test assemblage to provide padding against the steel plattens. The results for the experiments were recorded on the standard Instron Partner software (Instron Corporation,2005).

4 A completed assemblage is shown in Figure 3 prior to testing in the Instron DX600. Figure 3: Limestone blocks compression unit assembled in the Instron D X 600 prior to testing for shear L I M EST O N E SA MPL ES A ND PR OPE R T I ES Three types of limestone were obtained from a Texas Limestone Company. The three types of limestone are designated: Light, which is a finely grained, appearing to have a high sand content. Grey, which is a finely grained, appearing to have a high clay content. Shell, which is a coarse grained sand limestone with significant voids caused by the presence of sea creatures during the formation. Figure 4 shows the three limestone types used for the compression testing. Figure 4: Limestone Blocks, Shell, G rey and Light Used for Compression Testing.

5 Table 1 contains the measurements for the cut limestone blocks designated as Shell. The results of the compression test yields an average compression stress of 12.7 ± 3.8 MPa, with a characteristic strength of 6.4 MPa. The coefficient of variation of the tests was 30%. Table 1: Shell Cut-block Limestone Dimensions and Failure Compressive Stress Width Breadth A rea Force Number 2 k N Average ± standard deviation Compression Stress MPa 82.3 ± ± ± ± ± 3.8 Table 2 contains the measurements the measurements for the cut limestone blocks designated as Grey. The results of the compression test yields an average compression stress of 41.9 ± 4.1 MPa, with a characteristic strength of 35.4 MPa. The coefficient of variation of the tests was 10%. Table 2: G rey Cut-block Limestone Dimensions and Failure Compressive Stress Width Breadth A rea Force Number 2 k N Average ± standard deviation Compression Stress MPa 78.2 ± ± ± ± ± 4.1 Table 3 contains the measurements the measurements for the cut limestone blocks designated as Light. The results of the compression test yields an average compression stress of 20.0 ± 1.5 MPa, which yields a characteristic strength of 17.5 MPa. The coefficient of variation of the tests was 7.5 %. Table 3: Light Cut-block Limestone Dimensions and Failure Compressive Stress Width Breadth A rea Force Number 2 k N Average ± standard deviation Compression Stress MPa 76 ± ± ± ± ± 1.5 The Shell Limestone was the weakest of the tree and had a high variability due to the voids that occurred during rock formation, due to shell and other life forms. The Grey limestone appears to have been laid down with significant clay content and was the strongest material. The Light limestone appears to have been laid down with high sand content and was the second strongest material.

6 R ESU L TS O F SH E A R T EST IN G The four types of failure mechanisms observed were: Bond failure at the interface of the mortar and the limestone blocks, a number of the light test assemblages failed before testing took place. Crushing of the centre block due to the applied load After bond failure, a sliding mechanism was observed in the samples Only a single sample exhibited failure in the mortar. Figure 5 and 6 depict two of the failure modes for the samples, mortar interface failure and inner mortar failure which was only recorded for one sample. Figure 5: Shell Limestone Blocks failure mode as mortar interface failure Figure 6: Light Limestone Blocks failure mode as mortar failure

7 Table 4 presents a suary of the average shear stress applied to the samples and the applied axial compression load for the three limestone materials and the original limestone tests from the previous research for comparison purposes. Table 4: Results for Shear Tests Number Type Axial Centre Shear Compression Block Stress Stress Stress (MPa) (MPa) (MPa) Coent * 1 Shell Shell Shell Shell Shell Shell Shell Shell Shell G rey G rey G rey G rey G rey G rey G rey G rey G rey Light Light Light Light Light ** 24 Light Light Light Light Light O riginal Tests O riginal Tests O riginal Tests O riginal Tests O riginal Tests O riginal Tests O riginal Tests * Number of bonded faces at start of the test ** Crushing failure of centre block The linear regression analysis of the shear results yielded an increasing shear stress with increasing axial compression stress levels. The results have distinct two bounds on the force to displacement curve; the first is the failure of the mechanical bond between the mortar and the limestone blocks. The force to displacement graph up to this failure point tended to be linear, when allowance is made for settling of the plywood packing. The blocks tended to

8 slide at a constant stress once the mechanical bond was broken on one side of the assemblage. This test series was made with a lime rich mortar, whilst this series was manufactured with cement based mortar. This change may have contributed to the higher coefficients of variability observed in this second series of experiments Shell Limestone 4.50 Original Tests Grey Limestone Light Limestone Shell Limestone Original tests Grey Limestone y = x R² = Shear Stress (MPa) Light Limestone y = x R² = y = x R² = y = x R² = Compression Stress (MPa) Figure 7: Shear Stress plotted against the Compression Stress

9 Equation 2 below shows this relationship, where m, b are the slope of the equation and the intercept on the Y axis. fv mf ac b Equation 3 below shows this relationship for the data presented in Table 5. f f v ac (2) (3) Table 5: Linear Regression Results for Shear Stress against Compression Stress Description Intercept a Slope b Regression Coefficient Shell (MPa) G rey (MPa) Light (MPa) O riginal (MPa) Mean (MPa) Standard Deviation (MPa) C O V (5) C O N C L USI O NS The compressive stress of the three types of limestone tested in descending order were 41.9, 20.0 and 12.7 MPa for the Grey, Light, and Shell cut blocks respectively. The shear stress results mirrored the same order as the compression results, with the Grey Light and Shell capacity, 1.5, 1.09 and MPa respectively. The shear capacity of the specimens from the initial tests set of 2010 was 0.67 MPa. The textural nature of the 2010 specimens was fine grained sand without voids. The linear regression analysis of the shear results yielded an increasing shear stress with increasing axial compression stress levels. The results have distinct two bounds on the force to displacement curve; the first is the failure of the mechanical bond between the mortar and the limestone blocks. The force to displacement graph up to this failure point tended to be linear, when allowance is made for settling of the plywood packing. The blocks tended to slide at a constant stress once the mechanical bond was broken on one side of the assemblage. This test series was made with a lime rich mortar, whilst this series was manufactured with cement based mortar. This change may have contributed to the higher coefficients of variability observed in this second series of experiments. Further research work is required on the test procedure due to the observed variability of the test results as shown by the high coefficients of variation. The test procedure requires further investigation to determine the probable causes of the high variability and means of correcting. The areas of research to be investigated to reduce the variability are in the preparation of the sample assemblages for squareness, the optimal mortar design for the various categories of limestone, the definition of limestone categories with respect to material composition and further definition of the test procedure.

10 A C K N O W L E D G E M E N TS The authors wish to acknowledge the assistance of C. Tedrick at the Architectural Ranch for assistance in specimen manufacture and testing. Limestone was obtained from the Texas Quarries. R E F E R E N C ES Demain, E. D., and J. O'Rourke. Geometric Folding Algorithms: Linkages, Origami, Polyhedra. Cambridge, Cambridge University Press, Holland, N., V. L. Paul, and J. M. Nichols. "An Experimental Investigation of the Shear Properties of Limestone Masonry." In 8th International 2010 in Dresden, Dresden, IMS, Instron Corporation. Instron Model Dx Series Static Hydraulic Universal Testing Machine. Norwood, MA, Instron, Nichols, A. B. "Narbonne Cathedral." In 8th International, Dresden, IMS, Nichols, A. B., V. L. Paul, and J. M. Nichols. "The Intent of the Buttresses of Narbonne Cathedral." In 8th International 2010 in Dresden, Dresden, IMS, Nichols, A. B., V. L. Paul, and J. M. Nichols. "Vaulting of Narbonne Cathedral." In Eleventh North American, edited by A Schultz., Minneapolis, MN, The Masonry Society, Nichols, J. M. "The Assessment and Repairs of Certain Structures after the Newcastle Earthquake." Masonry International 13, no. 1, 1999, pp Paul, V. "The Projecting Triforium at Narbonne Cathedral: Meaning, Structure and Form?" Gesta 30, no. 1, 1991, pp