Better Utilization of Ultimate Strength Gain of Concrete with Pozzolanic Materials for Sustainable Development of Construction Works in Hong Kong

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1 Better Utilization of Ultimate Strength Gain of Concrete with Pozzolanic Materials for Sustainable Development of Construction Works in Hong Kong Research Team: Ir Prof Albert KH Kwan Ir Prof YL Wong Ir Dr Simon HF Wong Ir Dr Jaime SK Yeung

2 Research Initiative More and more use of Cement Supplementary Materials (SCMs) in concrete in past decades Slower strength development trend of concretes with SCMs Same acceptance criteria for 28-day strength for concrete with and without SCMs Strength increment of concrete with SCMs after 28-day is ignored

3 Research Initiative Higher cementitious content for concretes with SCMs to cater for same acceptance criteria for cube strength as concrete without SCMs higher cementitious contents lead to higher carbon footprint, higher heat of hydration, larger shrinkage, etc. and higher cost

4 Research Objectives Develop and give recommendation for revising the acceptance criteria of cube strengths for concretes incorporating SCMs to better utilize their ultimate strength gain for sustainable development

5 Research Methodology Collect production data for cube strengths at ages after 28-day (42, 56 & 90 days) from 8 local ready mixed concrete companies covering various % of SCM replacements and sources of materials Conduct laboratory trials to compare with the results obtained in actual production and to supplement data for concrete grades less frequently used in actual production Determine the average strength increment of concretes incorporating SCMs after 28 days Develop and give recommendations for practically adoptable acceptance criteria for cube strengths of concrete incorporating SCMs

6 Test Plan: Plant Production Data Table 1: Plant production data provided by Companies A to G Companies A B C D E F G No. of sets of cube results Age tested (days) 28, 56 28, 56 28, 56 28, 56 28, 42, 56, 90 28, 56 28, 42, 56, 90 Concrete Grades involved 40, 45, 50, 60 45, 60, 80 40, 45, 60 40, 45, 60 20, 30, 35, 40, 45, 50, 55, 60 40, 45, 50, 60, 80 20, 30, 40, 45 Water/binder ratios involved 0.33 to to to to to to to 0.65 SCM used PFA PFA PFA PFA PFA PFA GGBS % of SCM used 20, 25, 30, 34, 35, 40 25, 31, 35, 40 25, 35 25, to 40 25, 35, 40 40, 60

7 Test Plan: Laboratory Trials Table 2: Laboratory trial for PFA concrete mix Grade PFA content Slump 100mm 150mm 200mm 100mm Cubes 40D/20 45D/20 50D/20 60D/20 25% yes yes yes 35% yes yes yes 25% yes yes yes 35% yes yes yes 25% yes yes yes 35% yes yes yes 25% yes yes yes 35% yes yes yes 2 nos. x 7/28/42/56/90 days

8 Test Plan: laboratory trials Table 3: Laboratory trial for GGBS concrete mix Grade GGBS content Slump 100mm 150mm 200mm 100mm Cubes 40D/20 45D/20 50D/20 60D/20 35% yes yes yes 65% yes yes yes 35% yes yes yes 65% yes yes yes 35% yes yes yes 65% yes yes yes 35% yes yes yes 65% yes yes yes 2 nos. x 7/28/42/56/90 days

9 Test Results: Plant Production Data

10 Figure 1 % Increment in compressive strength from 28 days to 56 days of concrete with 25% to 40% PFA replacement

11 Table 4: Average % increment in compressive strength for 25% to 40% PFA concrete from 28 days (28D) to 56 days (56D) w/binder ratio % of PFA No. of samples Av. % increment in compressive strength from 28D to 56D Av. % increment in compressive strength from 28D to 56D w.r.t. % of PFA % % % % % % 25% % 11.5% % % % % % % % 10.3% % % 35% % 13.2% % % % 10.6%

12 Test Results Figure 2 % Increment in compressive strength from 28 days to 56 days / 90 days of concrete with 40% and 60% GGBS replacement

13 Research findings from Plant Production Data: OPC/PFA Concrete Data collected from 6 ready mixed concrete suppliers with different mix designs and sources of materials % Increment in compressive strength of PFA concrete from 28 days to 56 days is not sensitive to PFA sources % Increment not dependent on the w/b ratios and concrete grades Most 25% PFA mixes: % Increment from 28 days to 56 days fell steadily in the range of 10% to 11% Bored pile concrete with 35% PFA and w/b ratio of 0.35 and 0.36 have the % increment of around 15% (prolonged workability retention and stiffening time)

14 Research findings from Plant Production Data: OPC/GGBS Concrete Only 1 concrete producer supplying the plant production data for GGBS concrete with one source based on the findings from the literature review, the increment percentage is not sensitive to the source of GGBS The % increment from 28 days to 56 days for 40% and 60% GGBS are 11% and 13% respectively (similar to 25% to 40% of PFA) % Increment from 28 days to 90 days for 40% and 60% GGBS contents are both 18%

15 Test Results: Laboratory Trials

16 Table 5: Grade 40D/20 (25% PFA) 40D/20 (35% PFA) 45D/20 (25% PFA) 45D/20 (35% PFA) 50D/20 (25% PFA) 50D/20 (35%PFA) Laboratory results for Compressive strength of OPC-PFA concrete % strength increment Average compressive strength (MPa) Designed slump (mm) 7-day 28- day 42- day 56- day 90- day 56-day/ 28-day 90-day/ 28-day % 23.9% % 17.7% % 23.7% % 22.1% % 22.1% % 27.6% % 16.5% % 15.4% % 21.4% % 16.7% % 14.8% % 16.6% % 10.8% % 17.2% % 20.4% % 14.3% 60D/20 (25% PFA) % 17.3% 60D/20 (35% PFA) % 18.6% Av. % increment in strength at 56D & 90D against 28D (25% PFA) 11.9% 18.2%

17 Research findings from Laboratory trials: OPC/PFA Concrete Five concrete producers participated in the laboratory trial 48 concrete mixes were tried with the sources of raw materials currently used in routine production the % increment from 28 days to 56 days for 25% PFA and 35% PFA are 11.5% and 13.2% respectively lab results and plant production results were basically in line % increments for both 25% and 35% PFA mixes do not directly relate to concrete grades or w/b ratio

18 Fig. 6 Grade Laboratory results for Compressive strength of OPC-GGBS concrete at different ages Designed slump (mm) Average compressive strength (MPa) % strength increment 7-day 28- day 42- day 56- day 90- day 56-day/ 28-day 90-day/ 28-day 40D/20 (35% GGBS) 40D/20 (60% GGBS) 40D/20 (35% GGBS) 40D/20 (60% GGBS) 50D/20 (35% GGBS) 50D/20 (60% GGBS) % 12.2% % 5.9% % 7.0% % 9.4% % 10.7% % 15.4% % 7.2% % 7.5% % 5.4% % 7.1% % 14.5% % 11.1% % 8.7% % 8.9% % 6.3% % 11.8% 60D/20 (35% GGBS) % 10.8% 60D/20 (60% GGBS) % 19.2% Av. % increment in strength at 56D & 90D against 28D (35% GGBS) 6.4% 8.2%

19 Research findings from Laboratory trials: OPC/GGBS Concrete Only one concrete producer participate in lab trials for OPC/GGBS concrete 8 concrete mixes tried with the sources of raw materials currently used in routine production The av. % increment for 35% and 60% GGBS concrete at 56 days and 90 days are 6.4%/8.2% and 9.6%/11.7% respectively the lab results (35% and 60%) and plant production results (40% and 60%) are basically in line the % increments for GGBS mixes in lab trials seem not being affected by the concrete grades or w/b ratios

20 CONCLUSION PFA and GGBS have demonstrated their potential abilities to have further development in concrete strength after the age of 28 days A 10% strength increment after 28 days up to 56 days can be conservatively assumed for concrete incorporating 25% PFA, 35% PFA and 60% GGBS The increment in strength after 28 days for concrete 35% GGBS seems not substantial enough for developing a separate compliance criterion Recommendation for new compliance criteria for cube compressive strength of concrete will only be given to concrete incorporating at least 25% PFA or 60% GGBS

21 ORIGINAL SPECIFICATION FOR ACCEPTANCE CRITERIA OF CONCRETE CUBE STRENGTH Clause 16.59: Testing: Compressive Strength of Concrete Two test cubes shall be made from each sample of concrete taken as stated in Clause Each pair of test cubes shall be tested to determine the compressive strength at 28 days.

22 ORIGINAL SPECIFICATION FOR ACCEPTANCE CRITERIA OF CONCRETE CUBE STRENGTH Clause 16.61: Compliance Criteria: Compressive strength of designed mix concrete The results of tests for compressive strength at 28 days of designed compressive strength mix concrete shall comply with the following requirements: Each test result shall not be less than the grade strength by more than the appropriate amount stated in Column A of Table 16.10, and, The average of any four consecutive test results, or the average of the first two or first three test results if less than four test results are available, shall exceed the grade strength by at least the appropriate amount stated in Column B of Table

23 ORIGINAL SPECIFICATION FOR ACCEPTANCE CRITERIA OF CONCRETE CUBE STRENGTH Table in G.S.: Compliance criteria for compressive strength of designed mix concrete (Table 16.10) Column A Column B Grade strength (MPa) Compliance criteria Maximum amount by which each test result may be below the grade strength (MPa) Minimum amount by which the average of any four consecutive test results shall be above the grade strength (MPa) 100 mm cubes 150 mm cubes 100 mm cubes 150 mm cubes 20 or greater C C Below 20 C

24 Recommendation 1 Change the Specification clauses to: Clause Two test cubes shall be made from each sample of concrete taken as stated in Clause Each pair of test cubes shall be tested to determine the compressive strength at 28 days except for designed mix concretes with at least 25% of PFA or 60% of GGBS in the total cementitious content, for which the test cubes shall be tested at 56 days. The compliance criteria for compressive strength of designed concrete as stipulated in Clause and Table can be remained unchanged.

25 Recommendation 1 Advantage: Little change to G.S. by just revising the test date from 28 days to 56 days for designed mix concretes with at least 25% of PFA or 60% of GGBS Disadvantage: The completion date for the respective concrete works will be delayed for 28 days more thus leading to subsequent problems with contract requirements, payments, etc.

26 Recommendation 2 General percentage increment of 10% in strength after 28 days for concrete incorporating at least 25% PFA or 60% GGBS Considering a safety factor for the forecast of strength development, the increment in strength from 28 days to 56 days is predicted to be 5% only compressive strength of concrete at 56 days with at least 25% PFA or 60% GGBS is considered to be 1.05 x the compressive strength at 28 days

27 Recommendation 2 Revise Clause (5) to: For the purpose of assessing compliance of designed mix concrete as stated in Clauses and 16.62, the average of the two compressive strengths of the pair of test cubes shall be calculated and referred to as the test result. For designed mix concretes with at least 25% of PFA or at least 60% of GGBS in the total cementitious content, the test result can be multiplied by the factor of 1.05.

28 Recommendation 2 Advantage: no need to change test date at 28 days and therefore no change in the completion date of works, contract requirements, payment, etc. Disadvantage: the potential increment of concrete having at least 25% of PFA or 60% of GGBS after 28 days may be underestimated the change in the specification (Clause (5)) may also be difficult to understand and the implementation of the revised specification for the calculation of test results has to be adapted.

29 Recommendation 3 simply relax the requirement for the surplus in compressive strength by deducting 3 MPa from those requirements stated in C1, C2 and C3 for the average of four test results the minimum individual test result is not relaxed in order not to relax the lower bound of individual test result to avoid possibility of individual doubtful test result.

30 Recommendation 3 Table in the G.S. has to be revised as follow: Column A Column B Grade strength (MPa) Containing at least 25% PFA or at least 60% GGBS in total cementitious content Complian ce criteria Maximum amount by which each test result may be below the grade strength (MPa) Minimum amount by which the average of any four consecutive test results shall be above the grade strength (MPa) 20 or greater 100 mm cubes 150 mm cubes 100 mm cubes 150 mm cubes No C1a Yes C1b No C2a Yes C2b Below 20 N/A C

31 Recommendation 3 Advantage: Mildest change and easiest to follow Disadvantage: the potential increment of concrete having at least 25% of PFA or 60% of GGBS after 28 days may be underestimated

32 CONCLUSION & RECOMMENDATION FOR FURTHER STUDIES

33 CONCLUSION Substantial strength gain after the age of 28 days is observed for concrete incorporating SCMs Current compliance criteria for compressive strength of concrete in G.S. has not addressed the difference in strength development trends for concrete with and without SCMs Total cementitious contents of designed mix concretes incorporating SCMs are deliberately increased to cater for the same acceptance criteria and their substantial strength increments after 28 days are totally ignored

34 CONCLUSION Increased cementitious contents lead to increased heat of hydration, higher risk of thermal crack and increased carbon footprint Feasibility of revising the acceptance criteria in existing G.S. by taking into account the strength development of concrete incorporating SCMs after 28 days Three recommended approaches of revising the G.S. are given for discussion

35 RECOMMENDATION FOR FURTHER STUDIES Further studies on the development trends of concrete with lower % of SCMs If the G.S. is revised, overall effect of reduction in carbon footprint and quality performance shall be monitored and reviewed Being a starting point, recommendation given in this research study is relatively conservative. Studies for the possibility of further relaxation in the acceptance criteria is recommended to go for greener concrete

36 ACKNOWLEDGEMENTS Construction Industry Council Standing Committee for Concrete Technology Hong Kong Construction Materials Association Ready Mixed Concrete Committee Hong Kong Concrete Institute Ir Julian Lee (CIC) Dr Kate Chen (CIC) Ir KC Lam (SCCT) Ir HD Wong (SCCT) Ir Roy Hung (SCCT) Ir Prof CS Poon (HKCI) Ir Prof Adam Choy (HKCI)

37 END OF PRESENTATION