Effects of Aggregate Fineness and Added Calcium Hydroxide on Portland Cement Mortar Crack Propagation
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EFFECTS OF AGGREGATE FINENESS AND ADDED CALCIUM HYDROXIDE ON PORTLAND CEMENT MORTAR CRACK PROPAGATION C.H. DETRICHE, J.P. OLLIVIER AND S.A. RAMODA Laboratory of Civil Engineering,
I.N.S.A.,
U.P.S.,
Toulouse,
France
ABSTRACT The resistance of a coherent granular material, such as mortar or concrete, to crack propagation cannot be appreciated without taking into account the heterogeneity of the material. Heterogeneity results from the presence of aggregates and from the local modifications of the composition and of the behavior introduced by these aggregates in the volumes of intergranular paste situated close to their interface, the "aureoles de transition." It is possible to modify the characteristics of these zones to develop the global response of the material.
INTRODUCTION The resistance to crack propagation of mortar or concrete depends on the characteristics of the cement paste and aggregates as well as on those of the bond between paste and aggregates grains. lt is a well-known fact that the "aureoles de transition" are the weakest points of concretes. Their low intrinsic mechanical strength are due to specific characteristics such as high porosity and oriented growth of large CH crystals with their c axes roughly perpendicular to the aggregates [1]. Hence it may be assumed that those zones have also the weakest intrinsic resistance to crack propagation. This paper specifies the role played by the "aureoles de transition" in the overall resistance of mortar to crack propagation. EXPERIMENTAL PROCEDURE The specific energy of fracture in mode I (or the energy release rate GF) as well as the critical stress intensity factor Kc are determined from a three-point bend test on notched beams. The test is conducted according to the procedure recommended by RILEM 50-FMC, using a controlled deformation speed up to the complete fracture of the specimen [2]. The mortars are made with CPJ 45 R portland cement and crushed marble sand. The volume concentration of cement is defined by -, the volume ratio of cement to cement plus water. The volume concentration of sand is defined by a, the volume ratio of sand to sand, cement and water. The mortars are cured at 20°C and 90% relative humidity for 28 days before testing.
RESULTS Effect of the Sand Grading Three sands were used. The maximum grain size dmax was constant and less than 3.15 mm. The apparent specific areas (calculated from the grading 2 curves) were equal to 100, 44, and 19 cm /g respectively. Values of Kc and GF are plotted versus the specific surface area in In the Fig. 1 the concentrations a and I are constant and in Figs. 1 and 2. the Fig. 2 the consistency of the mortar and a are constant.
Mat. Res. Soc. Symp. Proc. Vol. 85. ' 1987 Materials Research Society
158
I
AF13Kc MNm0
5
1
0o
NF m-1 100
0.5
so a=0.5 ym0.4 dmaxs=lt15mm
0
1
50
I-cm2g-1 100
The higher values of Kc and GF are always obtained for the sand containing the coarser grains. This effect is more evident with the mortars prepared with a constant consistency. It is attributed to
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