Controlled deterioration of non-reinforced concrete anchors

The present invention provides a cement formulation that will degrade in a controlled

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2005291008 25 Jul 2011

The present applicants have discovered that the degradation of the binder in a cement can be accelerated somewhat (with respect to standard compositions) by using a cement composition with sufficient limestone filler and high water-to-cement ratio (w/c) to make the resulting concrete very open for diffusion of sulphates.

The present applicants have further discovered that concrete formulations which include calcium sulphate in the form of either anhydrite (CaSO₄), hemihydrate (CaSO₄’l/2H;O) or gypsum (CaSO₄*2H:O) as an additive, as well as sufficient limestone filler, experience a greatly accelerated rate of degradation. Such concrete will be stable as long as it is stored dry and will only require fresh water to start the Thaumasite formation. Furthermore, the reaction takes place uniformly throughout the concrete cross-section and an even crumbling is likely to occur.

According to the invention, there is provided a Portland cement formulation comprising calcium silicate, the formulation additionally comprising calcium carbonate and a source of sulphate.

The calcium carbonate may represent 10 to 50 wt % of the formulation and may be in the form

I 5 of limestone, chalk or calcite. The sulphate may represent 6 to 50 wt % of the formulation and may be in the form of a metal sulphate such as calcium sulphate. The composition of the cement is such that items formed from it will undergo disintegration as a result of a chemical reaction between the calcium silicate, the calcium carbonate and the source of sulphate, in the presence of water, to produce thaumasite. The particles in the cement which react to fonn Thaumasite will preferably be small (e.g.

less than 1 mm in diameter) in order to allow the reaction to progress at an appropriate rate.

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The preferred form of calcium sulphate is anhydrite (CaSO₄). Anhydrite is better for workability, in particularly if it is nearly “dead burnt” for delayed reactivity.

A preferred cement formulation is obtained when the calcium carbonate 5 and source of sulphate are present in amounts which give rise to a molar ratio of SO₄²7CO₃²' of between 0.2 and 3.0. Particularly, the calcium carbonate and source of sulphate may be present in a stoichiometric ratio with respect to

Thaumasite.

The cement formulation may additionally comprise calcium hydroxide. 10 The calcium hydroxide may represent 2 to 40 wt % of the formulation. Preferably, the cement formulation contains no additives which would not decompose into components occurring naturally in the environment, and no organic admixtures.

The main components of seawater are in decreasing order; 18,980 ppm 15 chloride (CT), 10,561 ppm sodium (Na⁺), 2,650 ppm sulphate (SO4²'), 1,272 ppm magnesium (Mg²⁺), 400 ppm calcium (Ca²⁺), 380 ppm potassium (K⁴), 140 ppm carbonate (CO3²'), 65 ppm bromide (Br), 13 ppm strontium (Sr) and up to 7 ppm silica (SiO₂). Seawater is in principle saturated with respect to calcium carbonate and is essential for crustaceans, mussels etc in building protective shells. For this reason seawater has pH on the basic side (around 8).

Thaumasite, Ca₃Si(OH)₆(CO₃)(SO₄)»12H₂, can be said to consist of 27.02% calcium oxide (CaO), 9.65% silica (SiO₂), 43.40% water (H₂O), 7.07% carbon dioxide (CO₂) and 12.86% sulphur trioxide (SO₃) although it is a calcium salt of silicate, carbonate and sulphate. Thaumasite occurs naturally, and transparent crystals are for instance found in the N’Chwaning Mine, Kalahari Manganese Field, Northern Cape Province, South Africa. Another site is the Bjelke Mine near Areskutan, Jamtland, Sweden.

Standard industrial concrete formulations include organic admixtures such as plasticizers, which improve the workability of the concrete and

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According to a further aspect of the invention, there is provided a 5 Portland cement formulation as described in any of the preceding aspects, which is mixed with an aggregate, optionally being a light weight aggregate, preferably with a particle size of less than 50 mm. The aggregate may optionally be any of the following: filler, sand, limestone with particle size greater than 1 mm or gravel.

The invention also extends to an anchor for releasably tethering a submarine device at the seabed, made substantially from a formulation which will allow the anchor to disintegrate as a result of a chemical reaction between the calcium silicate, the calcium carbonate and the source of sulphate, in the presence of water, to produce thaumasite. The anchor optionally includes a handle for the attachment of a release mechanism, which is preferably made of wood, leather or any other natural and environmentally non-polluting material suitable for the purpose. Alternatively, there may be a central hole for a central release mechanism.

The invention also extends to a method of tethering a submarine device at the seabed, which comprises: forming an anchor by mixing a cement or concrete formulation as described in any of the above aspects, respectively, with water, allowing the mixture to harden to form a finished anchor, attaching the submarine device to the anchor, and deploying the anchor and submarine device at a required location at the seabed. This method may be combined with the further steps of releasing the submarine device from the anchor and allowing the anchor to disintegrate as a result of a chemical reaction between the calcium silicate, the calcium carbonate and the source of sulphate, in the presence of water, to produce thaumasite.

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The present invention can be put into practice in various ways, some of which will now be described in the following set of example compositions, with reference to the accompanying drawings, in which:

Figure 1 is a plot of compressive strength at age 28 days versus water5 to-cement ratio for concrete;

Figure 2 is a plot of compressive strength evolution for concrete as a function of time and limestone (LS) addition;

Figure 3 is a plot of compressive strength evolution for concrete as a function of time and addition of limestone (LS)/anhydrite (ratio stoichiometric with respect to Thaumasite)

The aspect of the invention extending to an anchor made substantially from a formulation as described may be put into practice in various ways, an example of which is described below with reference to the accompanying drawings, in which:

Figure 4 is a side view of the anchor;

Figure 5 is a section on A-A of Fig. 4;

Figure 6 is a plan view of the anchor;

Figure 7 is a view of the top surface of the anchor;

Figure 8 is a view of the underside of the anchor.

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The following materials were used in trial concrete mixing.

Cement: Norcem Rapid Portland Cement (Industry cement), laboratory cement “IN5”

Limestone: 8 plastic bags of Verdalskalk Calcium carbonate, approximately 5 200 kg

Anhydrite: 1 bucket of Anhydrite, approximately 80 kg

Aggregate:

big bag of Norstone sand 0-8 mm, approximately 300 kg big bags of Verdalskalk, limestone 8-16 mm crushed stone, approximately

300 kg bags of Froseth sand 0.4 mm, approximately 50 kg

Laboratory concretes

Proposed laboratory mixes to make concrete cubes and beams are shown in Table 1. The reference concrete is the one used by Spenncon Verdal AS today. Spenncon has previously produced concrete elements approximately 1,000 x 1,000 x 90 mm for EMGS. The composition of the other laboratory recipes is with increasing limestone filler content, ending up with a stoichiometric concrete composition that deteriorates the binder totally. The limestone content is increased in steps of 20% and the cement + limestone filler

A + anhydrite mass is kept constant to 410 kg/m concrete.

The concrete density is proposed equal for all the mixes. The water/cement (w/c) ratios are increasing from 0.45 to 0.81 and thereby the porosity increases as well.

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Table 1: Nominal concrete composition, kg/m³

<td> Mix No</td><td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td><td> 7</td><td> 8</td><td> 9</td>

<td> % Limestone</td><td> 0</td><td> 20</td><td> 20</td><td> 40</td><td> 40</td><td> 60</td><td> 60</td><td> 80</td><td> 80</td>

<td> Rapid cement</td><td> 410</td><td> 342</td><td> 342</td><td> 293</td><td> 293</td><td> 256</td><td> 256</td><td> 228</td><td> 228</td>

<td> Water, free</td><td> 185</td><td> 185</td><td> 185</td><td> 185</td><td> 185</td><td> 185</td><td> 185</td><td> 185</td><td> 185</td>

<td> Water/cement- ratio</td><td> 0.45</td><td> 0.54</td><td> 0.54</td><td> 0.63</td><td> 0.63</td><td> 0.72</td><td> 0.72</td><td> 0.81</td><td> 0.81</td>

<td> Limestone filler</td><td> 0</td><td> 68</td><td> 68</td><td> 117</td><td> 117</td><td> 154</td><td> 154</td><td> 182</td><td> 182</td>

<td> Anhydrite</td><td> 0</td><td> 0</td><td> 86</td><td> 0</td><td> 147</td><td> 0</td><td> 192</td><td> 0</td><td> 228</td>

<td> Ardal 0-8 nun sand</td><td> 885</td><td> 885</td><td> 840</td><td> 885</td><td> 810</td><td> 885</td><td> 790</td><td> 885</td><td> 770</td>

<td> Freseth 0-3 nun sand</td><td> 40</td><td> 40</td><td> 40</td><td> 40</td><td> 38</td><td> 40</td><td> 38</td><td> 40</td><td> 37</td>

<td> Verdalskalk 8-16 mm gravel</td><td> 880</td><td> 880</td><td> 840</td><td> 880</td><td> 810</td><td> 880</td><td> 785</td><td> 880</td><td> 770</td>

<td> Density</td><td> 2400</td><td> 2400</td><td> 2401</td><td> 2400</td><td> 2400</td><td> 2400</td><td> 2400</td><td> 2400</td><td> 2400</td>

<td> Cement + limestone</td><td> 410</td><td> 410</td><td> 410</td><td> 410</td><td> 410</td><td> 410</td><td> 410</td><td> 410</td><td> 410</td>

From each mix 100 mm cubes and 100 x 100 x 400 mm prisms were made. The concrete was demoulded after 20 hours and placed in water at 20°C until 7 days age.

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Laboratory procedures

Concrete for documentation of properties was mixed in a 60 litre forced action mixer. Each concrete was mixed in two batches to achieve a total volume of 120 litres.

The mixing was carried out according to the following procedure:

1. 1 min mixing of dry materials

2. addition of mixing water during 1 min mixing

3. addition of excess mixing water to get a slump of approximately

200 mm

4. 2 min rest

5. 2 min mixing

Fresh concrete properties for each mix were determined according to EN 12350, part 2 (slump), part 6 (density) and part 7 (air content).

Compressive strength was determined on 100 mm cubes according to 15 EN 12390 part 3.

Curing regimes

After 7 days the specimens were stored at three temperature regimes:

1. In laboratory fresh water at 20°C

2. In sea water 5°-9°C

3. In concentrated seawater (5 times natural concentration) in laboratory at

5°C

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Testing schedule

Three cubes were tested for compressive strength after demoulding at 24 hours. Three cubes were tested for compressive strength after 7 days in fresh water of 20°C. The other test specimens were placed in hardening regime 2 and 3 for later testing. The testing schedule from 1 month after mixing for each mix is shown in Table 2 (the number indicates number of cubes or prisms subjected for testing).

Table 2: Testing schedule for all mixes

<td> Testing after mixing</td><td colspan="3"> 1 month</td><td colspan="2"> 2 months</td><td colspan="2"> 3 months</td><td colspan="2"> 4 months</td><td colspan="2"> 5 months</td><td colspan="2"> 1 year</td>

<td> Bending strength 5°C</td><td> 2</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td> Compressive strength 5°C</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td> 3</td><td></td><td></td><td></td>

<td> Hardening conditions</td><td> A</td><td> B</td><td> C</td><td> B</td><td> C</td><td> B</td><td> C</td><td> B</td><td> C</td><td> B</td><td> C</td><td> B</td><td> C</td>

A - Laboratory fresh water at 20°C

B - Seawater 5-9°C

C - Concentrated (5 times) seawater to increase the deterioration, 5°C

Results

Fresh Concrete

The real compositions of the 9 mixes are shown in Table 3. The workability was measured by standard slump measure according to EN 12350WO 2006/038018

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2. The density and air content was measured according to EN 12350-6 and EN 12350-7, respectively.

The density and air content was measured according to EN 12350-6 and EN 12350-7, respectively.

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Table 3: Real composition and fresh concrete results, (surface dry aggregates)

<td colspan="2"> Concrete Mix No</td><td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td><td> 7</td><td> 8</td><td> 9</td>

<td rowspan="7"> Kg perm³ concrete</td><td> Industry cement</td><td> 400</td><td> 334</td><td> 277</td><td> 286</td><td> 212</td><td> 250</td><td> 170</td><td> 223</td><td> 143</td>

<td> Calcium Carbonate</td><td> 0</td><td> 67</td><td> 55</td><td> 114</td><td> 85</td><td> 150</td><td> 102</td><td> 179</td><td> 114</td>

<td> Anhydrite</td><td> 0</td><td> 0</td><td> 70</td><td> 0</td><td> 107</td><td> 0</td><td> 129</td><td> 0</td><td> 144</td>

<td> Ardal sand 0-8 mm</td><td> 856</td><td> 861</td><td> 866</td><td> 865</td><td> 870</td><td> 863</td><td> 863</td><td> 867</td><td> 862</td>

<td> Fr0seth sand 04 mm</td><td> 39</td><td> 39</td><td> 38</td><td> 38</td><td> 38</td><td> 38</td><td> 38</td><td> 38</td><td> 38</td>

<td> Verdalskalk 816 mm</td><td> 856</td><td> 861</td><td> 861</td><td> 860</td><td> 865</td><td> 858</td><td> 858</td><td> 862</td><td> 857</td>

<td> Free water</td><td> 219</td><td> 211</td><td> 208</td><td> 209</td><td> 201</td><td> 208</td><td> 208</td><td> 204</td><td> 207</td>

<td colspan="2"> Water/binder-ratio</td><td> .55</td><td> .631</td><td> .750</td><td> .731</td><td> .950</td><td> .835</td><td> 1.22</td><td> .915</td><td> 1.45</td>

<td colspan="2"> Slump, batch 1, mm</td><td> 195</td><td> 180</td><td> 200</td><td> 190</td><td> 190</td><td> 190</td><td> 210</td><td> 200</td><td> 210</td>

<td colspan="2"> Slump, batch 2, mm</td><td> 205</td><td> 190</td><td> 200</td><td> 200</td><td> 200</td><td> 190</td><td> 210</td><td> 200</td><td> 210</td>

<td colspan="2"> Air content batch 1, %</td><td> 1.2</td><td> 1.3</td><td> 1.2</td><td> 1.2</td><td> 1.4</td><td> 1.3</td><td> 1.0</td><td> 1.3</td><td> 0.9</td>

<td colspan="2"> Air content batch 2, %</td><td> 1.2</td><td> 1.2</td><td> 1.3</td><td> 1.3</td><td> 1.3</td><td> 1.2</td><td> 0.9</td><td> 1.3</td><td> 0.9</td>

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<td> Density batch 1, kg/m3</td><td> 2370</td><td> 2370</td><td> 2375</td><td> 2375</td><td> 2375</td><td> 2365</td><td> 2365</td><td> 2370</td><td> 2370</td>

<td> Density batch 2, kg/m³</td><td> 2365</td><td> 2375</td><td> 2375</td><td> 2370</td><td> 2380</td><td> 2370</td><td> 2370</td><td> 2375</td><td> 2360</td>

Hardened concrete (in fresh water)

Eight 100 mm cubes were cast for determination of compressive strength according to EN 12390-3 at ages, 1, 7 and 28 days. The results are listed in

Table 4.

Table 4: Testing results after hardening in air and fresh water

<td> Concrete Mix No</td><td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td><td> 7</td><td> 8</td><td> 9</td>

<td> Compressive strength MPa after 1 day in air at 20°C</td><td> 30.8</td><td> 25.1</td><td> 16.2</td><td> 19.0</td><td> 12.4</td><td> 16.1</td><td> 7.2</td><td> 13.2</td><td> 5.2</td>

<td> Compressive strength MPa after 7 days in water, 20° C</td><td> 42.0</td><td> 37.1</td><td> 27.3</td><td> 33.1</td><td> 22.6</td><td> 27.9</td><td> 15.9</td><td> 23.6</td><td> 13.1</td>

<td> Compressive strength MPa after 28 days in water, 20° C</td><td> 48.1</td><td> 44.4</td><td> 33.7</td><td> 38.7</td><td> 27.3</td><td> 33.1</td><td> 20.2</td><td> 27.8</td><td> 16.1</td>

<td> Compressive strength MPa after 28 days in water, 5°C</td><td> 46.5</td><td> 43.2</td><td> 31.5</td><td> 36.9</td><td> 26.6</td><td> 31.6</td><td> 18.9</td><td> 26.7</td><td> 15.1</td>

<td> Flexural strength, MPa after 28 days in water, 5°C</td><td> 5.9</td><td> 5.2</td><td> 3.5</td><td> 5.2</td><td> 3.4</td><td> 4.0</td><td> 2.7</td><td> 4.0</td><td> 2.5</td>

Comments: Grey shades mark results for concrete with anhydrite

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Hardened concrete in sea water

The compressive and flexural strength of concrete cured in both fresh and sea water are given in Table 5. Grey shades mark results for concrete with anhydride. indicates disintegrated concrete.

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Table 5: Testing results after hardening in air, fresh water and sea water

<td> Concrete Mix No</td><td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td><td> 7</td><td> 8</td><td> 9</td>

<td> Compressive strength MPa after 1 day in air at 20°C</td><td> 30.8</td><td> 25.1</td><td> 16.2</td><td> 19.0</td><td> 12.4</td><td> 16.1</td><td> 7.2</td><td> 13.2</td><td> 5.2</td>

<td> Compressive strength MPa after 7 days in fresh water, 20° C</td><td> 42.0</td><td> 37.1</td><td> 27.3</td><td> 33.1</td><td> 22.6</td><td> 27.9</td><td> 15.9</td><td> 23.6</td><td> 13.1</td>

<td> Compressive strength MPa after 28 days in freshwater, 5° C</td><td> 46.5</td><td> 43.2</td><td> 31.5</td><td> 36.9</td><td> 26.6</td><td> 31.6</td><td> 18.9</td><td> 26.7</td><td> 15.1</td>

<td> Compressive strength MPa 2 months in seawater, 5°C</td><td> 45.7</td><td> 43.7</td><td> 30.5</td><td> 37.4</td><td> 22.9</td><td> 31.8</td><td> 9.9</td><td> 27.1</td><td> 6.4</td>

<td> Compressive strength MPa 3 months in sea water, 5°C</td><td> 48.4</td><td> 45.2</td><td> 24.3</td><td> 37.0</td><td> 12.8</td><td> 31.8</td><td> 1.1</td><td> 27.8</td><td> -</td>

<td> Compressive strength MPa 4 months in sea water, 5°C</td><td> 49.9</td><td> 45.3</td><td> 12.1</td><td> 37.8</td><td> 6.4</td><td> 34.0</td><td> -</td><td> 28.1</td><td> -</td>

<td> Compressive strength MPa 5 months in sea water, 5°C</td><td> 49.5</td><td></td><td></td><td></td><td></td><td></td><td> -</td><td></td><td> -</td>

<td> Compressive strength MPa 12 months in sea water 5°C</td><td></td><td></td><td></td><td></td><td></td><td></td><td> -</td><td></td><td> -</td>

<td> Flexural strength MPa 28 days in freshwater, 5°C</td><td> 5.9</td><td> 5.2</td><td> 3.5</td><td> 5.2</td><td> 3.4</td><td> 4.0</td><td> 2.7</td><td> 4.0</td><td> 2.5</td>

<td> Flexural strength MPa 28 days in seawater, 5°C</td><td> 6.5</td><td> 5.6</td><td> 3.6</td><td> 5.1</td><td> 3.5</td><td> 4.6</td><td> 2.7</td><td> 3.9</td><td> 2.3</td>

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<td> Flexural strength MPa 2 months in seawater, 5°C</td><td> 6.3</td><td> 5.8</td><td> 3.7</td><td> 5.5</td><td> 3.1</td><td> 5.0</td><td> 1.8</td><td> 4.2</td><td> 1.2</td>

Hardened concrete in concentrated sea water (salt water)

Compressive strength of concrete stored in concentrated (5x) sea water is given 5 in Table 6.

Table 6: Testing results after hardening in air, fresh water and salt water

<td> Concrete Mix No</td><td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td><td> 7</td><td> 8</td><td> 9</td>

<td> Compressive strength MPa after 1 day in air at 20°C</td><td> 30.8</td><td> 25.1</td><td> 16.2</td><td> 19.0</td><td> 12.4</td><td> 16.1</td><td> 7.2</td><td> 13.2</td><td> 5.2</td>

<td> Compressive strength MPa after 7 days in fresh water, 20° C</td><td> 42.0</td><td> 37.1</td><td> 27.3</td><td> 33.1</td><td> 22.6</td><td> 27.9</td><td> 15.9</td><td> 23.6</td><td> 13.1</td>

<td> Compressive strength MPa after 28 days in freshwater, 5° C</td><td> 46.5</td><td> 43.2</td><td> 31.5</td><td> 36.9</td><td> 26.6</td><td> 31.6</td><td> 18.9</td><td> 26.7</td><td> 15.1</td>

<td> Compressive strength MPa 28 days in salt water, 5°C</td><td> 44.7</td><td> 41.7</td><td> 29.2</td><td> 33.9</td><td> 23.6</td><td> 30.1</td><td> 15.5</td><td> 25.1</td><td> 13.2</td>

<td> Compressive strength MPa 2 months in salt water, 5°C</td><td> 47.2</td><td> 42.5</td><td> 20.7</td><td> 35.5</td><td> 21.8</td><td> 30.1</td><td> 12.7</td><td> 25.5</td><td> 9.3</td>

<td> Compressive strength MPa 3 months in salt water, 5°C</td><td> 45.9</td><td> 42.5</td><td> 17.6</td><td> 34.9</td><td> 10.8</td><td> 28.1</td><td> 4.1</td><td> 26.4</td><td> 2.0</td>

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<td> Compressive strength MPa 4 months in salt water, 5°C</td><td> 44.3</td><td> 41.4</td><td> 14.4</td><td> 34.8</td><td> 7.4</td><td> 26.0</td><td> 13/2</td><td> 25.8</td><td> 16/2</td>

<td> Compressive strength MPa 12 months in salt water, 5°C</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td> Flexural strength MPa 28 days in freshwater, 5°C</td><td> 5.9</td><td> 5.2</td><td> 3.5</td><td> 5.2</td><td> 3.4</td><td> 4.0</td><td> 2.7</td><td> 4.0</td><td> 2.5</td>

<td> Flexural strength MPa 28 days in c. salt water, 5°C</td><td> 6.5</td><td> 6.0</td><td> 3.4</td><td> 5.1</td><td> 3.2</td><td> 4.7</td><td> 2.7</td><td> 4.0</td><td> 2.3</td>

<td> Flexural strength MPa 2 months in salt water, 5°C</td><td> 5.8</td><td> 6.2</td><td> 3.6</td><td> 5.3</td><td> 3.0</td><td> 5.0</td><td> 2.1</td><td> 4.7</td><td> 1.5</td>

Comments: Grey shades mark results for concrete with anhydrite

Discussion

Concrete cured in fresh water

The development of compressive strength after 28 days curing versus 5 water/cement ratio is shown in Fig. 1 and compared with results from Norcem. The concrete with limestone filler as well as the concrete with limestone filler and gypsum are close to the reference mixes.

Concrete cured in sea water

All the concrete cubes (and prisms) were stored their moulds in 10 laboratory conditions the first day and thereafter hardened in fresh water up to seven days. Then the prisms were placed in seawater (5°C to 9°C) and tested 1, 2, 3, 4, 5 and 12 months after casting. Fig. 2 shows the results with limestone filler only while Fig. 3 depicts the result with limestone filler and anhydrite in stoechiometric ratio with respect to Thaumasite formation.

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Addition of limestone as the only additive has not yet (within the test period) given any significant deterioration, even for the most permeable and porous concrete.

The addition of both limestone filler and anhydrite to the concrete has 5 caused increasing disintegration with increasing amount of additives. Concrete specimens with 60 and 80% limestone filler of cement weight were totally destroyed after 3 months.

Table 7: Strength development after hardening in freshwater and sea water

<td> Concrete Mix No</td><td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td><td> 7</td><td> 8</td><td> 9</td>

<td> Compressive strength MPa 7 days in fresh water, 20° C</td><td> 42.0</td><td> 37.1</td><td> 27.3</td><td> 33.1</td><td> 22.6</td><td> 27.9</td><td> 15.9</td><td> 23.6</td><td> 13.1</td>

<td> Compressive strength MPa 28 days in freshwater, 5° C</td><td> 46.5</td><td> 43.2</td><td> 31.5</td><td> 36.9</td><td> 26.6</td><td> 31.6</td><td> 18.9</td><td> 26.7</td><td> 15.1</td>

<td> Compressive strength MPa 28 days in salt water, 5°C</td><td> 45.7</td><td> 41.1</td><td> 30.5</td><td> 35.8</td><td> 25.1</td><td> 30.1</td><td> 17.8</td><td> 25.8</td><td> 13.6</td>

<td> Strength sea water 28 days/Strength fresh water 28 days</td><td> 0.98</td><td> 0.95</td><td> 0.97</td><td> 0.97</td><td> 0.94</td><td> 0.95</td><td> 0.94</td><td> 0.97</td><td> 0.90</td>

<td> Flexural strength MPa 28 days in freshwater, 5°C</td><td> 5.9</td><td> 5.2</td><td> 3.5</td><td> 5.2</td><td> 3.4</td><td> 4.0</td><td> 2.7</td><td> 4.0</td><td> 2.5</td>

<td> Flexural strength MPa 28 days in sea water, 5°C</td><td> 6.5</td><td> 5.6</td><td> 3.6</td><td> 5.1</td><td> 3.5</td><td> 4.6</td><td> 2.7</td><td> 3.9</td><td> 2.3</td>

<td> Strength sea water 28 days/Strength fresh water 28 days</td><td> 1.10</td><td> 1.08</td><td> 1.03</td><td> 0.98</td><td> 1.03</td><td> 1.15</td><td> 1.00</td><td> 0.98</td><td> 0.92</td>

Comments: Grey shades mark results for concrete with anhydrite

WO 2006/038018

PCT/GB2005/003859

Compressive strength deterioration is less than 10% for all mixes during the three first weeks of exposure to sea water, which make them suitable as anchors for the seabed logging period.

The flexural strength was higher after three weeks exposure in sea 5 water, except for the cement with the highest amount of limestone filler and anhydrite.

Concrete cured in concentrated sea water

There was only a small difference in deterioration between sea water and concentrated (5x) sea water for mixes with anhydrite. Since the specimen without anhydrite in natural seawater did not deteriorate, it is difficult to say whether the concentrated seawater increases the deterioration rate or not, hut the compressive strength was somewhat lower after storage in concentrated sea water as seen by comparing results in Table 5 and 6.

Including limestone filler (i.e. calcium carbonate) in the recipe did not deteriorate the hardened concrete within 6 months after submersion in seawater.

To speed up the deterioration, tests were also performed by adding calcium carbonate and anhydrite in a stochiometric ratio with respect to Thaumasite that deteriorates the binder totally. These specimens disintegrated in contrast to the specimen without anhydrite.

Increasing the amount of additives decreased the 7 and 28 days compressive strength for concrete hardened in fresh water.

Preferred Physical Form

Referring to figures 4 to 8 of the drawings, the anchor comprises 25 a body 10 with legs 11. The presence of two inclined surfaces 14 located on each side of the anchor results in the rotation of the anchor during sinking. Further, there is a hole 12 suitable for attaching a central release mechanism

2005291008 07 Feb 2011 (not shown). Slots 13 are introduced in the body of the anchor, to stabilise the anchor during sinking.

Specific Preferred Embodiment

A recommended concrete composition with 40% limestone filler and a stoechiometric ratio of anhydrite with respect to Thaumasite was found to disintegrate in seawater after 4 months. The composition of the recommended concrete is:

<td> Ingredients in kg/m³</td><td> Producer</td><td> Recipe 1</td><td> Recipe 2</td>

<td> Rapid Portland Cement</td><td> Norcem</td><td> 210</td><td> 210</td>

<td> Free water</td><td></td><td> 195</td><td> 195</td>

<td> Limestone filler</td><td> Verdal Kalkverk</td><td> 85</td><td> 85</td>

<td> Anhydrite</td><td> Outocompu, Odda</td><td> 110</td><td> 110</td>

<td> Sand 0-8 mm</td><td> Norstone, Ardal</td><td> 880</td><td> 920</td>

<td> Sand 0-4 mm</td><td> Froseth, local deposite</td><td> 40</td><td> 0</td>

<td> Crushed stone 8-16 mm</td><td> Verdal Kalkverk</td><td> 880</td><td> 880</td>

The concrete contains no ingredients harmful to the marine environment it is meant to serve in. All ingredients are found in natural gravel, limestone and/or seawater. The concrete does not contain any organic admixtures. The anchor is characterised in that its physical form causes it to rotate during sinking by means of the presence of inclined surfaces located along each side of the anchor.

It will be understood that the term “comprise” and any of its derivatives (eg. comprises, comprising) as used in this specification is to be taken to be inclusive of features to which it refers, and is not meant to exclude the presence of any additional features unless otherwise stated or implied.

2005291008 07 Feb 2011

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

It will be appreciated by those skilled in the art that the invention is not restricted in its 5 use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications in its scope.

2005291008 25 Jul2011