7. PERFORMANCE
7.3. Layer JOINT Strength performance
7.3.2. Direct tensile strength
2. 70 to 90% bonded layer joints – satisfactory performance, comparable to many CVC gravity dams.
3. 50 to 70% bonded layer joints – less than satisfactory performance, lesser quality than typical for CVC dams.
4. Less than 50% bonded layer joints – unsatisfactory performance, often accompanied with seepage through layer joints.
Table 7.2 presents details of the eight RCC dams with the best reported average in-situ vertical static direct tensile strength across joints as of 2012. As can be seen the average tensile strengths vary from 1.30 to over 2.0 MPa and the compressive strengths from approximately 20 MPa to circa 40 MPa (Dolen & Dunstan, 2012).
Table 7.2
Data from the ten RCC dams with the best-reported in-situ vertical direct tensile strength across joints Dam Direct tensile strength across joints
(MPa)
Compressive strength (MPa)
@ 91 days @ 365 days @ 91 days @ 365 days
Shapai 2.05 28.3
Platanovryssi 1.77 29.6
Beni Haroun 1.53 22.8
Pirris 1.70 22.0
Olivenhain 1.54 21.9
Daguangba 1.32 19.3
Xekaman 1 1.42 24.7
Mianhuatan 1.40 33.3
U. Stillwater 1.40 38.5
Changuinola 1 1.26 1.40 24.3
Direct Tensile strength of “Hot Joints”
For successive layer placement before the initial setting time of the receiving layer, the direct tensile strength of hot joints is almost the same as parent RCC. This is not any different from the conventional mass concrete performance of layer joints within a layer of mass concrete. The only differences observed in hot joint performance are those joints damaged by rainfall, or exposed to extreme drying conditions, or wind during placement.
Direct tensile tests of cores from Upper Stillwater Dam, USA at 13 years’ age showed no difference in the direct tensile strength of layer joints compared to parent RCC strength (Dolen, 2003).
This included hot, warm, cold joints with vacuum cleaning as the only treatment and super-cold joints that were cleaned with high pressure water blasting and onto which a richer starter RCC mixture was applied (no bedding mix). The average of more than 200 core tests from two dam projects in Vietnam using the primarily hot and cold joint construction techniques and similar cementitious materials are shown in Table 7.4. The primary difference is Project A had significantly more super-cold joints than Project B (Ha et al, 2015). The strengths of hot joints averaged between 83 to 98% of the equivalent parent RCC strengths.
There is a large body of evidence to indicate that hot joints are the optimum condition for direct tensile performance, ranging from approximately 80 to 100% of the parent RCC strength.
Direct Tensile Strength of “Warm Joints”
Identifying warm joints by setting time is difficult due to the difference between ambient and laboratory conditions and thus the performance of warm joints is also difficult to assess. The initial setting times of in-situ RCC may be appreciably less than measured in the laboratory in warmer climates and more in colder climates. At Yeywa Dam, the direct tensile strength of hot and warm joints was approximately 1.5 and 1.0 MPa, respectively, a 33% decrease (Dolen & Dunstan, 2012). Tests from full- scale trials listed in Table 7.3 indicate that the strengths of hot and cold joints were almost equal to parent RCC strength, while the strengths of warm joints without a bedding mix were approximately 71%
of parent RCC strength. The strengths of warm joints treated with a coating of grout after surface preparation were approximately 90% of parent RCC strengths. Although the RCC mixtures at Upper Stillwater Dam were naturally retarded, the layer joints included a significant number of warm joints between 24 and 36 hours old. In this case, little or no difference in strength was evident between hot,
Table 7.3
Direct tensile strength of drilled cores showing different layer surface treatment methods.
* AC 14 – 14 day accelerated cure; 7 days standard + 7 days at elevated temperature.
RCC mixtures with a high percentage (more than 60%) of flyash and using a set retarder admixture may have a very long initial setting time, from 20 to more than 24 hours. The time duration of the warm joint thus varies with the dosage of retarder and ambient temperature.
One of the biggest difficulties with placement of RCC on warm joints is the tendency to over- broom the surfaces, resulting in a large volume of surface mortar that must be removed before the surface can be finally cleaned. Quite often, the time-consuming debris removal leads to the development of a cold joint.
There is some indication that early-age warm joints subject to some form of brushing are susceptible to a decrease in direct tensile strengths when no bedding mix is applied.
Direct Tensile Strength of “Cold Joints”
The direct tensile strength of cold joints has often been found to be comparable to either hot joints or parent concrete for both HCRCC and LCRCC mixtures, provided the layer surface has a high- quality “exposed-aggregate” finish (implying exposure of coarse aggregate) and is clean. The results of core tests from a test section and from Lai Chau Dam in Vietnam are shown in Table 7.4 using similar mixture proportions and the same flyash source showed a cold joints tensile strength range from approximately 83 to 98% of that of the parent RCC cores and approximately 89 to 105% of that of hot joints. Test results from two low cementitious RCC dams in the USA also showed that strengths on cold joints with bedding concrete were 90 to 95% of the strength of hot joints.
Tests on LCRCC mixtures (Loaded VeBe time ~ 45 sec or higher) showed that both the percentage of bonded layer joints and the direct tensile strength of cold joints were significantly improved by using a bedding concrete or mortar.
There is a large body of evidence to indicate that cold joints having an exposed (coarse) aggregate finish have excellent direct tensile performance, ranging from approximately 80 to 100% of the parent RCC strength. Bedding mixes (mortar, grout and concrete) have been used on practically all LCRCC and MCRRC dams and on some HCRCC dams.
Results of Direct Tensile Testing of RCC Cylinders and Cores at 90 days Age
Percent of Parent (core) Parent Tensile Strength - MPa Layer Joint Tensile Strength - MPa
Layer No.
Cylinder Core
Layer No.
Test Cores Test Age
(Days) Test Age (Days)
AC 14* 90 105 Layer Type 105
L1 1.32 1.07 1.13 L1-L2 Warm – No Grout 0.97 86
L2 1.29 1.06 1.11 L2-L3 Hot 1.32 119
L3 1.20 0.87 1.34 L3-L4 Cold - Mortar 1.29 96
L4 1.15 0.88 1.22 L4-L5 Hot 1.18 97
L5 1.22 0.99 1.29 L5-L6 Warm - Grout 1.24 96
L6 1.41 1.01 0.97 Average Layer Joint
Average 1.3 1.0 1.18 L1-L5 All 1.2 102
Average All Core Layer Joints 1.17
Table 7.4 .
Results of “hot” and “cold” layer joints from two Vietnamese RCC projects using similar placement methodology (150 mm diameter cores).
Sample
Project A Project B
Average strength (MPa)
Percent Cylinder
Percent Core
Average strength (MPa)
Percent Cylinder
Percent Core Properties of laboratory cast cylinders
[number of tests]
Cylinder compressive strength – 365 days
23.3
[390] 100 132 21.3 100 117
Cylinder direct tensile strength- 365 days
1.5
[40] 100 109 1.39 6.5 103
Properties of drilled cores Core compressive
strength
17.6
[120] 76 100
18.2
[307] 85* 100
Core direct tensile strength (parent/un- jointed)
1.38
[110] 92*
7.8**
100
1.35
[340] 97*
100 7.4**
Core direct tensile strength – “hot” joints
1.29
[110] 86*
7.3**
93 1.24 89*
92 6.9**
Core direct tensile strength – (MMF ~ 600 ⁰C-hr)
1.28
[~150] 92* 95
7.0**
Core direct tensile strength – (MMF ~ 800 ⁰C-hr)
1.23
[~150] 88*
91 6.8**
Core direct tensile strength – “super cold”
joints
1.15
[48] 77*
6.5**
83
1.32
[6] 95*
98 7.3**
* Percentage of cylinder compressive or cylinder direct tensile strength.
**Percentage of core compressive strength or core parent direct tensile strength.