The aim of the study is to investigate the effect of the replacement of Portland limestone cement (CEM II/A-L) with ground granulated blast furnace slag (GGBS) at different levels in concrete for exposure class XF after 200 cycles of freezing and thawing. This work is a part of an ongoing project developed in the materials laboratory of LAB Casey Concrete, Co Wexford, in co-operation with Warsaw University of Technology, Poland.
Concrete is degraded by cycles of freezing and thawing. The impact of 200 cycles of freezing and thawing on the durability of concrete was examined by replacing CEM II/A-L with GGBS at 30%, 50% and 70%. A control mix at 0% GGBS was also cast. The w/c = 0.45 was consistent throughout the trials and mixes where considered with and without air-entraining agent (AEA). All concrete mixtures were controlled to have a similar slump by using a dosage of superplasticiser. Concrete was assessed for water absorption, tensile strength, loss of compressive strength and loss of mass.
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Testing air in the fresh concrete by using air metre[/caption]
Performance advantages were achieved by replacing CEM II/A-L with GGBS in the range of 30%-50%. Such advantages are increased 28 and late strength compressive and tensile strength, reduction in water absorption, reduction in mass loss and strength loss after 200 freeze/thaw cycles.
The level of compressive strength of concrete samples not aerated and aerated subjected to freezing and thawing, control samples and samples after 120 days of ripening in curing tank water is significant. Determination of mass loss of samples subjected to 200 cycles of freezing and thawing in all mixes resulted in compressive strength ≤20% and in mass loss ≤5%, which conforms to the Polish standard PN-B-06250 requirement (this represents severe winter conditions).
The air void structure of the concrete was studied using the Air Void Analyser, which determined whether the differences in the composition of concrete mix would influence the concrete durability with a cycle of freezing and thawing. The results indicate that all concrete mixes with air entrainment showed better freezing and thawing resistance than without air entrainment.
The study demonstrated the freeze-thaw resistance of CEM II/A-L and GGBS combinations at 30%, 50% and 70% after 200 cycles of freezing and thawing resulting in durable concrete for XF exposure class.
Background to the project
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Digital device for freezing concrete cube[/caption]
The durability of concrete is assessed by measuring its resistance to three destructive influences within a specified time. The factors that cause damage to and destruction of concrete can be divided into the following groups (Neville, 2000):
- Physical causes include the effects of high temperatures and effects related to the different coefficients of thermal expansion of the aggregate and the hardened cement paste; cyclical freezing and thawing; or water contained in the pores of the capillary freezing and exerting pressure on the surrounding wall of the pores (leading to micro cracking);
- Chemical causes include aggressive ions, soft water, acids and salt water;
- Mechanical causes include stroke, overload, abrasion and vibration.
The process of degradation of concrete due to freezing and thawing is explained by Richardson (2002), who found that the problem occurs due to the extension of the pores of the cement paste frozen in the ice. Richardson indicates the expansion of the water ice is about 8%, while Micah Hale
et al (2008) state that at about 9%, hydraulic pressure is created inside the concrete when there is no space to extend the ice.
Shang
et al (2009) determine that the size of the hydraulic pressure depends on the permeability grout, degree of saturation and the rate of freezing.
Richardson (2002) found that the resulting network of pores and micro-cracks magnification allows passage of a greater quantity of water during thawing, leading to a further accumulation of ice, and thus the cumulative effect of degradation in the concrete. The extension of the ice in capillary pores, which is mainly responsible for the degradation of the concrete, is also caused by the osmotic pressure.
Frost resistance may be shaped by application: aeration grout, reduce capillary pores by reducing the water-cement ratio, respectively, selected aggregates and cement and mineral additives (Rusin, 2002). The concrete used in construction, in addition to being able to resist freezing and thawing cycles, must also be resistant to the effects of de-icing. This resistance is significantly dependent on the type of cement.
Cement that would ensure durability of concrete is characterised by:
- Increased resistance to aggressive chemical agents;
- A low or moderate heat of hydration;
- Constancy of volume;
- Prolonged bonding time.
PN-EN 197-1: 2002 identifies five types of cement depending on the content of granulated blast furnace slag. Sulphate-blast furnace cement HSR is mainly used in environmental exposure classes XA1, XA2 and XA3. Not found in the literature inconclusive regarding the behaviour of cement concrete with steel in exposure classes XF1 ÷ XF4.
Polish construction practice commonly used methods to determine the frost resistance of concrete should be the method described in PN-88 / B-06250 ‘Concrete ordinary’ (the usual method).
According to the standards evaluating the resistance of concrete to frost, it is assumed that:
- The loss in mass of the samples before frozen in relation to mass of the same samples after frozen may not exceed 5%;
- The decrease in compressive strength of the samples frozen in relation to the strength of the samples not frozen (witnesses) may not exceed 20%.
Procedure for the study determines that a sample-witness stored in water at 18 +/- 2°C, for the same period of time in which the test samples are subjected to freeze-thaw cycles. The result is a continuous increase in the strength of witnesses, while in samples frozen the process is slowed down.
Materials and preparation of samples
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Concrete air metre, concrete cubes and moulds for concrete[/caption]
For the preparation of concrete mixes were used:
- Portland cement CEM-IIA-L 42.5N – Noel O'Brien Cement – according to EN 197-1.
- Addition of ground granulated blast furnace slag (GGBS) - ECOCEM Dublin- accordance with EN 15167-1: 2006, was used in amounts of 0%, 30%, 50%, 70%.
- Crushed Aggregates- Casey's Quarry-by PL-EN 12620: 2004 fraction 4 / 16mm and 16 / 31.5mm,
- Natural sand fraction 0 / 4mm – Casey’s Sandpit
- Mixing water – water comply with PN-EN 1008: 2003
- Admixtures: according to Standard EN-EN 934-2: 1999 was used: S.Plasticizer Viscocrete 10 from Sika Ireland Ltd, used in an amount of 0.80% to the value of the cement.
- Centrament Air 202 from MC –Building Chemicals
The properties of concrete mixes with W/C = 0.45 are shown in Tables (1) and (2).
| Table (1): Not Aerated Concrete |
| The properties of the concrete mix |
| W/C = 0.45 |
| Cement content % |
100% Cem-IIA-L |
30% GGBS |
50% GGBS |
70% GGBS |
| Slump [mm] |
100 |
105 |
105 |
110 |
| Air content [%] |
0.9 |
1.1 |
1.3 |
1.6 |
| Cure in water [⁰C ] |
+20 |
+20 |
+20 |
+20 |
|
| Table (2): Aerated Concrete |
| The properties of the concrete mix |
| W/C = 0.45 |
| Cement content % |
100% Cem-IIA-L |
30% GGBS |
50% GGBS |
70% GGBS |
| Slump [mm] |
110 |
105 |
115 |
115 |
| Air content [%] |
4.6 |
5.1 |
5.2 |
5.5 |
| Cure in water [⁰C ] |
+20 |
+20 |
+20 |
+20 |
|
Absorption test: The absorption test results of concrete samples are shown in Graph (1).
The absorption of concrete samples results showed after 120 days, that the samples containing GGBS are less water absorption rate (1.8 ÷2.4 %) than the samples 100% CEMII/A-L were the absorption rate is (2.9%).
Compressive strength: The test results of compressive strength for concrete samples after periods of 120 days ripening in curing tank water in 20⁰c are shown in Graph (2).
The results have shown that the increase in compressive strength of not aerated concrete samples containing GGBS after 120 days was significantly higher than other of not-aerated concrete made with 100% CEM II/A-L, which was (47.6N).
The highest compressive strength in the concrete mixes was the samples containing 30% GGBS (52.4N) and 50% GGBS (53.6N).
The results have shown that,the increase in compressive strength of aerated concrete samples containing GGBS after 120 days was significantly higher than other of aerated concrete made with 100% CEM II/A-L which was (41.4N). The highest compressive strength in the concrete mixes was the samples containing 30% GGBS (44.3N) and 50% GGBS (43.8N).
Tensile strength: The test results of tensile strength for concrete samples after periods of 120 days ripening in curing tank water in 20⁰C are shown in Graph (3).
The results have shown that the increase in tensile strength of not-aerated concrete samples containing GGBS after 120 days was (7.1 ÷ 7.9N), significantly higher than other not-aerated concrete made with 100% CEM II/A-L, which was (6.8N).
The highest tensile strength in the concrete mixes was the samples containing 50% GGBS (7.9N).
The results have shown that the increase in tensile strength of aerated concrete samples containing GGBS after 120 days was (6.6 ÷ 6.8N), significantly higher than other air-entrained concrete made with 100% CEM II/A-L, which was (6.2N). The highest tensile strength in the concrete mixes was the samples containing 30% GGBS (6.8N).
Durability test
Durability test results for not-entrained and entrained concrete samples after 200 cycles freezing and thawing: (1 cycle= 4 hours at -20◦c in the freezer and 4 hours at + 20◦c in the curing tank water).
Due to compressive strength: The test results of (control and after 200 cycles) compressive strength concrete samples are shown in the Graph (4).
The results have shown that the increase in compressive strength for not-aerated concrete samples containing GGBS, which was measured after periods of ripening in curing tank water as a control sample, is significant.
We noticed a clear decrease in compressive strength in all sample mixes after 200 cycles. The results have shown that the increase in compressive strength for aerated concrete samples containing GGBS (39.8÷ 44.6N), which was measured after periods of ripening in curing tank water as a control sample, is significant.
The results of decrease in compressive strength concrete samples in [%] are shown in the Graph (5).
The results (after 200 cycles) showed less decrease in compressive strength in concrete samples with 100% CEMII/A-L (the decrease was 5.7%). There was more decrease in the samples containing GGBS, which was (8.1 ÷ 15.5%).
The results (after 200 cycles) showed less decrease in compressive strength in air-entrained concrete samples with 100% CEMII/A-L (the decrease was 4.1%) and also more decrease in the samples containing GGBS, which was (5.4 ÷ 9.3 %).
Due to loss of mass: The test results for loss in mass of concrete samples in (%) after 200 cycles are shown in Graph (6). The results have shown for not-aerated concrete samples after 200 cycles that the loss in mass was only (0.54%) for samples 100% CEMII/A-L. For samples containing GGBS, the loss of mass was between (0.79÷1.03%).
The results have shown for aerated concrete samples after 200 cycles that the loss in mass was only (0.12%) for samples 100% CEMII/A-L. For samples containing GGBS, the loss of mass was between (0.21÷0.58%).
Table (3) summaries the concrete samples test results after 200 cycles of freezing and thawing.
| W/C = 0.45 |
CEM II |
CEM II/GGB |
CEM II/GGBS |
CEM II/GGBS |
| Cement content % |
100 |
70/30 |
50/50 |
30/70 |
| Compressive strength of
Control samples
[N/mm2] (not aerated) |
47.7 |
52.9 |
54.2 |
46.6 |
| Compressive strength of
Control samples
[N/mm2] (aerated) |
41.4 |
44.6 |
44.9 |
39.8 |
| Compressive strength
After 200 cycles
[N/mm2] (not aerated) |
45.0 |
48.6 |
48.1 |
39.4 |
| Compressive strength
After 200 cycles
[N/mm2] (aerated) |
39.7 |
42.2 |
41.7 |
36.1 |
| Loss of compressive strength
After 200 cycles [%] (not aerated) |
5.7 |
8.1 |
11.3 |
15.5 |
| loss of Compressive strength
After 200 cycles [%] (aerated) |
4.1 |
5.4 |
7.1 |
9.3 |
| Loss in mass after test
frost resistance F 200 [%] (not aerated) |
0.54 |
0.79 |
0.87 |
1.03 |
| Loss in mass after test
frost resistance F 200 [%] (aerated) |
0.12 |
0.21 |
0.37 |
0.58 |
| Table (3): Summary concrete samples test result after 200 cycles freezing and thawing |
Conclusions of the study
Based on the survey and the results, it can be concluded that concretes made using 100% CEMII/A-L and concretes made with the addition of ground slag in quantities (30%, 50%, 70%) met the requirements of PN-88 / B-06250 for the degree of frost resistance F200.
The results from compressive strength and loss in mass indicate that all concrete mixes with air entrainment showed better freezing and thawing resistance than without air entrainment.
The air entrainment in concrete mixes was a rate of 4.6÷5.5% and this is in accordance with the requirements of BS EN 206-1, for the composition of concrete exposed to frost aggression (exposure class XF), where one of the conditions required is minimal degree of aeration concrete ≥4%.
Concrete made using the additive ground slag showed inferior resistance to frost compared to tests of concretes made from 100% CEMII/A-L.
The increase in concrete compressive strength after 120 days of ripening in curing tank water with added GGBS was significantly higher than other concretes made with cement 100% CEM II/A-L. The increase in control concrete compressive strength with 50% of GGBS was significantly higher than other concrete made with 100% CEM II/A-L (Graph 2 and 4) and Table 3).
-Determination of mass loss of samples subjected to 200 cycles of freezing the weight of the witnesses indicated that the requirements of PN-B-06250 (loss in weight ≤5%) were achieved in all tested concretes made with cement CEM II/A-L and with addition of ground slag (ggbs) in amounts (30%, 50%, 70%) (see Table 3 and Graph 6).
From experience, we can confirm that, if the level of compressive strength of concrete containing additives granulated blast furnace slag (GGBS) after frost resistance test meets the specific class of concrete compressive strength, the concrete samples did not show any cracks, and the loss of the mass is consistent with the standard, then we can conclude the concrete as resistant to frost.
Authors:
Prof Pawel Lukowski, head of the Department of Building Materials Engineering, Warsaw University of Technology
Dr Eng Joanna Julia Sokolowska, Department of Building Materials Engineering, Warsaw University of Technology
MSc Eng Ali Salih, technical and quality manager, Casey Concrete, Gorey, Co Wexford