High density concrete (HDC) consists of concrete with a density higher than normal 2300 to 2550 kg/m³ and is used for special purposes such as radiation shielding, counter weights, ballasts, safe walls and safe roofs.
The aim and objective of this assignment is to:
Outline the constituent materials required for high density concrete.
Outline the production, placing and compaction procedures for high density concrete.
Outline the quality control procedures required for high density concrete.
Discuss an application for high density concrete and highlighting any problems encountered.
3. Constituent materials for high density concrete. All constitute materials, cement, admixtures, and water used in high density concrete should conform to the standards as for normal density concrete, but the aggregates is different and may require special consideration during handling, batching, mixing, transporting and placing. (ACI 304R-00, 2000)
3.1 Aggregates. High density concrete (HDC), densities is achieved by using high-density aggregates usually iron oxides and depends on its intended use, physical and chemical properties, availability and cost. Concrete densities of 3700 to 3800 kg/m³, is obtainable utilizing aggregates with particle relative densities of at least 4500 kg/m³ and for concrete densities of 4800 kg/m³ aggregates should have a particle relative density of at least 6000 kg/m³. (J. Goodman, 2009)
Aggregate grading for HDC rarely conform to normal aggregate standards as the aggregate are rarely made for concrete production, this should be accepted by potential users as it is not detrimental to the production of HDC. The most important part of HDC aggregates is that the grading should be constant and that a workable grading limit be agreed on with the supplier, to base the mix design on.
The chemical properties of all high-density aggregates should be evaluated before use, chemical reactivity and particularly in high alkaline environments as found in cement pastes, long term durability such as alkali-aggregate reactivity, sulphate and chloride attack and other impurities. The supplier should be able to provide a full chemical analysis of all potential materials and alternative materials to be used. With proper technical evaluation and planning, different types of aggregates can be combined at the supplier yard to produce a concrete with predicted, specified density. Cost factor are very important as these aggregates are usually more expensive than normal aggregates, it can also be that the material may only be available in relative short supply. The cost of HDC increases with density but not necessary in direct proportion. (E. Miller, 2003)
Aggregates should be selected for their shape (cubical or rounded and free of flat or elongated, maximum density, workability, and cost. Particle relative density of the fine aggregate should be similar to that of the coarse aggregate as lower density fine aggregate can lead to segregation of the coarse aggregate through the mortar. (J. Goodman, 2009)
The grading of the coarse aggregate should be uniformly graded between 10 and 40mm, as per limits in Table 1. The grading of the fine aggregate should be within the limit as shown in Table 1 as this would help to prevent segregation. (ACI 304R-00, 2000)
Sieve sizes mm
Grading 1 for 37.5 mm maximum size aggregates % passing
Grading 2 for 19.0 mm maximum size aggregates % passing
Two types of aggregates can be used for high density concrete namely:
Natural occurring aggregates. Iron ores of various types have been used all around the globe for many years (outside the U.K), in the UK imported iron ore is used. (E. Miller, 2003) Using some natural occurring aggregates concrete densities of approximately 4000 kg/m³ can be obtained. (J. Goodman, 2009)
Different types of natural high-density aggregates which can be used for high density concrete (HDC) available in South Africa and the UK refer to Table 2.
Man-made aggregates. To achieve concrete densities above 4000 kg/m³ man-made or synthetic material such as ferrosilicon slag, steel or lead shot can be used. (J. Goodman, 2009)
For types of man-made aggregates which can be used for high density concrete (HDC) available in South Africa and the UK refer to Table 2.
3.2 Cements. All cements conforming to SANS 50197 as for conventional concrete can be used for HDC if it produces the required physical properties. If alkali-reactive constituents are present in the aggregates cements with low alkali contents or a suitable blend of cements and cement extenders should be used. (J. Goodman, 2009) Low-alkali cement not having more that 0.6 % Na2O-eq may be used with a potentially alkali-reactive aggregate such as lead shot. (B. Oberholster, 2009)
The following extenders: Ground granulated blast furnace slag (GGBS), fly ash (FA) and Condensed silica fume (CFS) have lower densities than Ordinary Portland Cement (OPC) and can be used if they do not reduce the density of the concrete below specified limits, (J. Goodman, 2009) Eric Miller (2003) however states “The cement can be OPC or a blend of this with either Ground granulated blast furnace slag, Pozzolanic fly ash; the latter which will not reduce the density of the concrete but may enhance the otherwise poor workability”. Fly ash blends and lower water contents can successfully be used to increase the density of concretes. When lad shot is used High alumina cement (HAC) must be used, (J. Goodman, 2009) as it reduces the chemical reactivity of lead in high alkaline conditions. (E. Miller, 2003)
When using alkali-reactive aggregate (lead shot) deleterious expansion can be prevented when using high-alkali cements by adding extenders complying with SANS 1491 by using a minimum percentage of the following extenders by total mass of cement:
At least 40% Slag (S) by mass
At least 15% CSF by mass
At least 20% FA by mass (B. Oberholster, 2009)
Barytes for example tend to have a very coarse grading, the cohesivenesss of the mix can be imoproved by adding additional cementitious material such as PFGA/GGBS. The cementitious content can be higher than required for normal strength or w/c design criteria. Adding additional cementitious material to modify the fines content of the mix may, seem expensive but in HDC it is the cheapest ingredient used and relative easy to include. (E. Miller, 2003)
3.3 Admixtures. Adding admixtures can minimise segregation, bleeding, both of which can be problematic with HDC. The durability can be improved with a low free water/cement ratio (w/c) especially so in structural grade HDC. Using super plasticisers is recommended which in conjunction the cement type may marginally increase concrete density. Super plasticisers are beneficial in reducing water to minimise bleeding and maintain a cohesive mix that has minimum segregation. (E. Miller, 2003)
The following admixture can be used in HDC:
Air-entraining admixtures as it controls bleeding and settlement improve workability and assist in obtaining more homogenous concrete but will reduce the density of the concrete. Water-reducing admixtures as it will increase concrete density by reducing the amount of water in the mix, water is the material used in HDC with the lowest density. Shrinkage-reducing admixtures as it ensure dense, crack-free concrete used for radiation-shielding concrete. (J. Goodman, 2009)
When lead shot or lead containing aggregates are to be used, the following should be considered when selecting a chemical admixture. The alkali content of chemical admixtures for concrete used in South Africa is between about 0.2 and 7%. In many instances the contribution to the alkali content of the pore solution is significant and cannot be ignored. One of the widely used super plasticisers has a Na2O-eq of about 5% and if added at 1% of cement to concrete with a cement content of 350 kg/m³ it will ad 0.17 kg/m³ Na2O-eq to the concrete. Lithium based compounds can be used with lead shot to control the alkali reactivity. (B. Oberholster, 2009)
The effect of admixtures should be established under field conditions, preferably by testing, to determine if the admixtures achieve the required results or totally unsuitability. (J. Goodman, 2009)
Admixtures may only be used in liquid form and batched in solution in the mixing water. Admixtures shall comply with the requirements of ASTM C 94 or AASHTO M I94; air entraining agents shall comply with the requirements of ASTM C 260 or AASHTO M 154. Admixtures shall not contain any chlorides.
3.4 Water. Water shall be clean and free from detrimental concentrations of acids, alkalis, salts, sugar and other organic or chemical substances that could impair the durability and strength of concrete or imbedded steel. (COLTO, 1998)
4. Production, placing, compaction and quality control of high density concrete. 4.1. Production. Standard batching procedures can be used for naturally occurring aggregates such as Barytes and Magnetite with volume modifications to allow for the increase in aggregate relative density. These, and occasionally other types, are sometimes required in significant volumes to justify large-scale batching through conventional plant. It is sometimes necessary to use smaller drum mixers approximately 0.5m³ and even smaller for lead shot. (E. Miller, 2003)
Normal batching (if sufficiently accurate) and handling equipment can be used but great care should be taken not to overload the equipment as HDC is much denser. It is imported to clean all equipment used to minimize cross contamination. (J. Goodman, 2009)
To aid production aggregates can be purchased from some suppliers pre-weighted in ‘dump sacks’ or other containers to suit the size of the batching plant, the additional cost being more than the offset by an almost nil wastage, and providing additional protection from external contamination. Iron or lead shot can also be purchased in 25 and 12.5 kg bags respectively but this tends to make batching very tedious. The use of dump sacks for supplying relatively small amounts of aggregates has obvious advantages as suppliers have the flexibility to blend precise quantities of fine and coarse aggregates of different types to offer tailor-made premixed all-in blends as well as tailor-made densities.
Batching times will take longer per cubic metre, due to smaller unit batch volumes, but individual batch mixing times should be similar to those for standard aggregates concretes. Care must be taken with Barytes to assess potential particle break down, since this material can be weak and fragile. Conversely mixing times may need to be extended for materials such as Fergran, which has a very rough and irregular shape. It is advisable to pre-grout mixers when small quantities are produced. (E. Miller, 2003)
Special care should be taken to prevent loss of fines, contamination, segregation, aggregate breakage when shipped, handled and stored. The aggregate should be kept as dry as possible. Aggregate container should be marked properly containing the mass and type of the aggregate. (J. Goodman, 2009)
4.2 Placing. Conventional placement methods may be used for HDC, provided that the mixture is workable and the forms are relatively free of embedded items. Such concrete, however, presents special problems due to the tendency of the high-density aggregates particles to segregate. Segregation is greatest where the aggregates are not uniform in grading or density, the mixture contains excessive water, or the slump is excessive. The slump for high-density aggregates mixtures should normally be between 40 and 75 mm.
Placement of conventionally mixed HDC is subjected to the same consideration of quality control as normal concrete, except that it is far more susceptible to variations in quality due to improper handling. HDC is particularly subject to segregation which results in variations of strength and density.
HDC will not flow in a form and must be placed in each discrete area and compacted in place with minimum vibration. Under no circumstances should an attempt be made to move HDC with vibration equipment. Concrete should be placed in layers not more that 300 mm thick. If an excessive amount of grout builds up on the surface, it should be removed while the concrete is still in plastic. When segregation cannot be avoided or when embedded items or restrictions prohibit conventional placement, the pre-place aggregate method may be used.
Pre-placed aggregate construction consists of erecting formwork, pre-placing coarse aggregate in the forms, and mixing and placing high-density grout using the same procedure as those employed with normal grout.
Pressures exerted on formwork by HDC are greater than those exerted by normal concrete and formwork must be designed accordingly. Pressure on vertical forms can be reduced by placing concrete in slow rising lifts. (J. Goodman, 2009)
If possible the use of long, rigid chutes or drop pipes should be avoided. If HDC concrete is placed in narrow forms or through restricted areas, it is advisable to use short, flexible drop chutes that can collapse and restrict the fall. (ACI 304R-00, 2000)
4.3 Compaction. Compaction is of great importance when using HDC due to the special purposes i.e. radiation shielding it is used for which requires maximum density, and the concrete should be free of segregation and voids. Compacting HDC is more difficult to compact than normal concrete and extra special care should be taken to ensure proper compaction. Both internal vibration and external vibration are used. However care should be taken when using high-density aggregates some are friable and can easily break down. Poker vibrators insert points should be spaced closer to ensure that HDC are compacted properly.
Normal frequencies vibrators as well as high frequency (180 Hz) can be used successfully satisfactory. Higher frequencies vibrators must be used for shorter periods, to reduce the danger of segregation. Over-vibration causes segregation of the high-density aggregates by settlement meaning the large aggregate migrate to the bottom of the element casted. (J. Goodman, 2009)
The area radius of action of vibrators used in HDC is less effective. This result in HDC not being compacted properly and greater care should be exercise to insure proper compaction of the concrete. Excessive grout can be lifted to the lift surface top due to vibration and re-vibration, as to remove entrapped air and establish aggregate-to-aggregate contact. The lift surface grout should be removed while the concrete is still fresh. (ACI 304R-00, 2000)
4.4 Quality Control. HDC requires careful planning at design stage, tender assessment and pre-production stages of a contract as well as during actual production on-site. Cost can be very high compared with standard concrete as volumes are often too small to be successfully produced in conventional ready-mixed or site-mixing concrete plants. Workability can be difficult to assess, hence the need for trial mixes which should be attended by site personnel including those actually placing the material. A higher degree of quality control is needed, with fresh densities being frequently measured prior to mixes being placed. This requires appropriate equipment that is not usually on site. Full method statements and quality plans for all stages, including materials procurement, planning, production, placing and testing, should be prepared and approved prior to starting of work.
Because of the need to satisfy special requirements, testing and quality control are extremely important. In many cases, testing of the structure, including the removal of cores is not permissible. Care must be taken to ensure that good concreting techniques are used.
The density of HDC is important, if not more so at times, than strength and a suitable testing regime should be adopted to continually monitor this property using fresh density tests on site before concrete is placed. Cubes and cylinder tests are similar to normal concrete. Generally 100 mm and 150 mm cubes can be very heavy and are of significant cost. Hence additional technical staff will be required to undertake the increased level of quality control necessary both at the mixing plant and on-site. A fresh density test can be evaluated using a fine aggregate density pot. The normal 0.01 m³ pots will be very heavy – up to 90 kg for lead shot concrete. Test methods are the same as for standard density products though additional tamping may be required to fully compact mixes containing poorly shaped aggregates. (E. Miller, 2003)
The properties of freshly mixed HDC needs to be tested and should include the following: density, temperature, slump and air content. Specification limit for should be specified for all the important properties. (J. Goodman, 2009)
When HDC is required it is advisable that the design engineer issues an addendum specification to the standard concrete section as some aspects of its use are fundamentally different from standard concrete production.
Points for consideration:
Aggregates Source limitation and delivery;
Appropriate physical, chemical and petrological properties.
Low free w/c;
Mandatory use of super plasticisers;
Full scale laboratory trial mixes to confirm fresh and hardened properties including fresh density.
Designers should consider the implication of non-compliant mixes, particularly with regard to marginal densities. This may be importuned and some statistical safety margins may need to be included on a basis similar to those used for compression strengths. A final point to consider is the potential expense of remedial work as some grades of HDC, particularly those with iron aggregate, will be extremely difficult to remove. (E. Miller, 2003)
Table 3: Summary of typical properties of high-density concretes.
Concrete type/ Property
Chilcon coarse natural sand
Use of Spectrophotometry in Enzyme Kinetics
An enzyme catalyses the conversion of a substrate to a product. The rate of the catalysed reaction or the activity of the enzyme can be determined by measuring either the decrease in substrate concentration or the increase in product concentration as a function of the reaction time. The development of an enzymatic reaction is monitored by the alteration in absorbance as a function of time – due to the differing absorbance values of the substrate and product. As absorbance changes correspond linearly with the changes in concentration, these changes can be calculated directly from the absorbance data (when provided with the absorption coefficients of the reacting species).
If there is no significant difference in absorbance values of the substrate and product, colour derivatives can be synthesized with chromophoric reporter groups. As such, a ‘silent’ enzyme reaction with a colourless product can be linked with another enzyme reaction that uses the product of the initial enzymatic reaction for a conversion that initiates a change in absorbance. The introduction of an NADH-consuming or NADH-producing reaction is often employed as changes in NADH concentration are easily followed by the strong change in absorbance at 340 nm.
One of the first enzyme determinations to gain pervasive acquiescence in clinical medicine was the measurement of alkaline phosphatase activity and it is still widely used today. Alkaline phosphatase catalyses the cleavage of ester bonds in phosphoric acid (Figure 1).
Human serum alkaline phosphatase can hydrolyse natural phosphates found in bone, kidney and intestines. P-nitrophenylphosphate can be hydrolysed by this enzyme to yield the alcohol p-nitrophenol, which absorbs strongly in alkali at 405nm. Two problems encountered in many past and currently used alkaline phosphatase methods are (1) the long incubation times required due to the extremely low concentration of alkaline phosphatase in serum; and (2) phosphate inhibition either from endogenous orthophosphate in the sample or from that released during the hydrolysis of the substrate. Therefore a new method should minimize the influence of these two factors, and ideally, any spectrophotometric procedure should be based upon the measurement of a highly absorbing reaction product at a wavelength where the substrate has negligible absorbance. Fortunately, hydrolysis of the colourless substrate p-nitrophenolphosphate in alkaline solution, to form the highly coloured yellow product p-nitrophenolate ion, fulfils these requirements.
In this experiment, the kinetics of hydrolysis of P-nitrophenolphosphate by alkaline phosphatase will be measured.
The activity of these enzymes can be measured by following the liberation of phosphate or of the other product released by hydrolysis. The assay can be simplified by using a substrate whose phosphate-free product is highly coloured. In this experiment, we will utilize p-nitrophenol phosphate as the substrate, which upon hydrolysis releases phosphate to generate p-nitrophenol under alkaline conditions. P-nitrophenol has a high molar absorptivity at 405 nm (e405 = 18.8 x 103 M-1cm-1).
The purpose of this research endeavour was to determine the use of enzymes in biological systems, to use a spectrophotometer to assess the kinetics of the enzyme alkaline phosphatase, as well as to analyse the effects of incubation time, enzyme concentration and also to determine the optimal pH at which alkaline phosphatase acts.
The success of such a method depends upon (1) the use of a suitable analytical procedure for the precise determination of small amounts of reducing sugar, and (2) a careful standardisation of the technique for mixing enzyme and substrate, withdrawing samples, and arresting the reaction in these samples.
Materials and Methods Construction of Standard p-Nitrophenol Graph
The standard consisted of a series of test tubes of final volume 11.1ml. Volumes 0.1mM p-nitrophenol 0, 0.5, 1.0, 1.5, 2.5, 3.5, 5.0, 7.0, 10.0 were added to each test tube. 0.02M NAOH sufficient enough to bring the final volume up to 11.1ml was added. Test tubes were mixed by inversion and quantified against the blank sample using absorbance determinations conducted on a spectrophotometer at 405nm. Standard curve preparation is both labour intensive and error prone, with quantitative accuracy being dependent on both the accuracy of standard quantification and the quality of standard curve construction. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC169985/
Effect of Incubation Time on Enzyme Activity
Buffered substrate (5.5mM p-nitrophenylphosphate in 0.05M glycine buffer containing 0.5mM MgCl2 at pH 9.5) was equilibrated at 370C for 10 minutes before being added to 10.1 ml 0.02M NaOH. Upon addition of the enzyme, the tube was capped, inverted and incubated at 370C. After 5, 10, 20, 30 and 40 minute intervals, 1.0ml of the reaction mixture was transferred to a tube containing NaOH and was measured spectrophotometrically at 405nm.
Effect of Enzyme Concentration on Substrate Hydrolysis
Varying concentrations of distilled water and enzyme solution were added to test tubes filled with buffered substrate solution and incubated at 370C for 30 minutes. After 30 minutes, 10.0ml NaOH was added to inactivate the enzyme and the absorbances were read at 405nm.
Effect of pH on Enzyme Activity
0.5ml of buffer (at varying pH) were added to 0.5ml of 11mM unbuffered substrate solution and incubated at 370C. After 5 minutes, 0.1ml of selected enzyme solution was added and reaction mixtures were left to incubate for 30 minutes. After the incubation period, 10.0ml NaOH was added to each test tube and the absorbance was read at 405nm.
Discussion Effect of incubation time on Enzyme Activity
The effects of different incubation times on the kinetics of alkaline phosphatase are summarized in Table 2. The rate of reaction for each time period was calculated and results were displayed in Table 3. Upon examination of results, it is clear that the rate of product generation slowed as time went on. The linear relationship is lost and a clear polynomial relationship is seen between incubation time and production of p-nitrophenol. During the initial 5 minutes, 5.57 umoles of p-nitrophenol was generated at a rate of 1.114 umol/ min, producing over 2-fold more product than in any other time-bracket. The rate of reaction of alkaline phosphatase decreased during the period 0-10 minutes to that of 0.223 umoles/ min. There is little variation in the amount of end-product created in the 10-40 minutes and the graph plateaus (SD=0.06). One possibility for the loss of linearity could be that the substrate concentration has decreased significantly after the first 5 minutes of the reaction, resulting in the slower reaction rate. Overall, the product concentration decreases as incubation time increases. This “falling-off” in rate is a common hinderance in enzyme kinetic studies and to overcome this, many measures are often only conducted in the initial period.
Effect of Enzyme concentration on Substrate hydrolysis
The effects of different enzyme concentrations on substrate hydrolysis of alkaline phosphatase are summarized in Table 4. A clear linear relationship can be observed from Figure 5. As increasing concentrations of alkaline phosphatase are added to the reaction mixture, the velocity of substrate hydrolysis increases exponentially. According to the Michaelis theory, the reaction velocity would be expected to be proportional to enzyme concentration so long as the concentration of enzyme remains small compared with that of the substrate. The relationship seen in Figure 5 is in close agreement with that predicted by the Michaelis theory. Additionally, Eadie ’s work highlighted the relationship between the level of activity of the enzyme (i.e. the reaction velocity with high substrate concentration) and amount of substrate concentration required for the reaction to occur. Therefore, in order to further the understanding of alkaline phosphatase kinetics, the author suggests the introduction of measures investigating the level of activity of alkaline phosphatase. In summary, the enzymatic hydrolytic power increases progressively during the period of increasing enzyme concentration.
Effect of pH on Enzyme Activity
The effects of pH on enzyme activity are summarised in Table 5. Figure 6 highlights the rate of product generated in varying alkali conditions. As stated before, alkaline phosphatase, as its name suggests, hydrolyses its substrate at alkali conditions. However, it is clear from Table 5 that the optimal pH for alkaline phosphatase to hydrolyse p-nitrophenolphosphate is 10.4, with 3.81 umol p-nitrophenol being generated. Decreased concentrations of product were observed for pH values lower and higher than 10.4.