Experimental study of Permeability of Hardened Concrete

This is a experimental study of Permeability of Hardened Concrete. This study is also aimed to create a relationship between permeability and also ultimate compression strength using different types of brick chips (in terminology of absorption and strength) in different ratio.  The goal of this research work should be to, observe the permeability regarding brick aggregate and correlate that with different variables which in turn affect the permeability from the concrete. Finally in order to compare it with gemstone chips concrete.

Background

The durability of a concrete is closely related to its permeability. The permeability dictates the rate at which aggressive agents can penetrate to attack the concrete and the steel reinforcement. Concretes can be as impermeable as latex modified concretes.

A concrete structure is considered to be of adequate durability if it performs in accordance with its intended level of functionality and serviceability over an expected or predicted life cycle. Durable concrete must have the ability to withstand the potentially deteriorative conditions to which it can reasonably be expected to be exposed.

In time when significant amount of funds in the transportation industry are needed toward repairing and rehabilitating current concrete infrastructure, it has become critical that engineers emphasize durability and long term performance in all new construction project. Guidelines needed to be developed to ensure that new materials, technologies, test methods, and construction practices can be utilized to produce high quality, long-lasting concrete structures. Though it is in practice to use stone chips to produce low permeable concrete, but there is no institutional attempt to distinguish the behavior or effect of locally found brick aggregates on permeability of concrete. To observe the effect of used aggregate on penetration of water under pressure in concrete, several sets of test were performed.

Aim and objective:

Brick chips aggregate is widely used all over Bangladesh for construction purpose. But there is no extensive data to identify the permeability of concrete casted with brick chips aggregates. The aim of this research work is to, observe the permeability of brick aggregate and correlate it with different variables which affect the permeability of the concrete.

This study is also aimed to create a relationship between permeability and ultimate compressive strength using different types of brick chips (in terms of absorption and strength) at different ratio. Finally to compare it with stone chips concrete.

This experiment helps to develop performance based specification. If a certain level of permeability is required, specifications will provide a maximum allowable permeability value and strength. Using specified performance based specifications; engineers can measure the concrete permeability of strength and variety that it meets the requirement.

Therefore, the aim of this study is to achieve the followings

• To identify the behavior of concrete under water pressure for various types of aggregates.
• To establish a relationship between the strength and penetrability of concrete.

Methodology:

The study is completed in three phases. Each phase comprises four batches of concrete casting. In the first phase, concrete cubes and cylinders were casted with first-class bricks in four different batches. Variations of these batches were marked by different mix ratios, fine aggregates and admixture used.

In the second phase, third-class bricks were used in all types of casting. This was also completed by four batches of casting.

Finally, in the third phase, stone chips were used as coarse aggregate. Concrete cubes and cylinders of standard size were casted and cured accordingly in all cases. Cylinders were used for compressive strength test and cubes were used for permeability test. AT 315 was used to determine the impermeability of concrete to water according to EN 12390-8 “Depth of Penetration of Water under Pressure”. And for strength test “Universal Testing Machine” was used.

The outline of the experimental program can be explained as follows

Phase – 1 testing program:

Phase 1 involves the casting of 4 batch concrete with very high class brick-chips of different ratio.

Batch-1: Cement : Sand (50% Medium sand, 50% coarse sand): Brick chips = 1:2:4, and w/c=0.50.

Batch-2:  Cement : Sand (100% coarse sand): Brick chips = 1:2:4  and w/c = 0.50.

Batch-3:  Cement : Sand (100% coarse sand): Brick chips = 1:1.5:3 and w/c = 0.50.

Batch-4: Cement : Sand (100% coarse sand): Brick chips and 1% Admixture = 1:1.5:3 and w/c = 0.40.

Each Batch consists of 3 cubes for penetration/permeability test and 3 cylinders for compressive strength test.

Phase – 2 testing program:

Phase-2 involve the casting of 4 batch concrete with low quality Brick chips (absorption 22%) of different ratio as same as phase-1.

Phase – 3 testing program:

Phase-3 involves the casting of 4 Batch concrete with stone chips of different ratio as same as phase-1.

Cement, Fine Aggregate, Coarse Aggregates were tested according to ASTM standards.

Conclusion:

For construction practice, there should be specifications to ensure the quality of the work. Evolution of performance-based specification could be a appropriate purpose of this study work.

The first phase of the testing program reported herein, referred to as Phase 1 throughout the report, involved the testing of several sets of hardened concrete prepared from first class brick chips using different mix ratios, admixtures. The same tests were performed for stone and third class brick chips accordingly. As, brick chips are available for construction-work all over our country, the behavior of this type of aggregate under water pressure is vital.

Future work on this study will involve the effect of salt water intrusion on concrete structures of coastal areas of the country.

LITERATURE REVIEW

General

Permeation of water and consequent corrosion is one of the main causes of damage to hydraulic concrete structures. Under hydraulic pressure, concrete will be permeated and corroded by water, resulting in damage to the hardened cement paste. Permeability is thus one of the most important properties of hydraulic concrete structures.

Found on several ways, the water is the most important fluid on nature. Among its properties, is noticeable the capacity to penetrate in small pores or cracks, and the capacity of dissolve a large amount of substances.

Several researches refer and attest the great importance of the water molecule on the concrete structure, especially on the first ages, caused by the cement hydration and consequent hardness of the concrete. However, the presence of water after the hardness of the concrete and after the reduction, or the ceasing of the hydration reactions, may cause the deterioration of the concrete or of the steel bar present on the structure. The water take action as a direct agent (lixiviation) or transporting noxious substances, such as chloride ions, sulfate ions and acid, or components that can activate and propel many chemical reactions that speed up the degradation process of the matrix, proportioning this way a substantial reduction of the durability and the use life of the concrete and reinforced concrete structures.

Some authors emphasize that the permeability of the water is the most important factor to esteem the durability under the most diverse conditions of service of a structure. Therefore concrete must be projected and manufactured for the environment to which it goes to be displayed, because the permeability is related to the porosity that varies in accordance to the composition of the concrete, its factor water cement, its age and even though with its form of launching. In this paper, will be evaluated permeability and the compressive strength of the concrete with different compositions, water cement factor and ages, making possible to generate correlation curves, suggesting a standard of reference and analysis of the permeability in function of some variable of the concrete.

Permeability of concrete

The permeability of concrete is usually expressed as the highest water pressure that the concrete can be subjected to without incurring seepage under a specified test procedure, or the height of permeating water in the concrete specimen under a given water pressure at a specified time. From observations it is found that the permeability of the concrete decreased with time, eventually reaching a stable value.

The permeability of concrete plays an important role in durability because it controls the rate of entry of moisture that may that may contain aggressive chemicals and the movement of water during heating or freezing. The w/c ratio also increases the strength which may resist the concrete from cracking and thus permeation of water is checked.

Hardened cement paste is composed of particles connected over only a small fraction of their total surface. For this reason, a part of the water is within the field of the solid phase, i.e. it is adsorbed. This water has a high viscosity but is, nevertheless, mobile and takes part in the flow. The permeability of concrete is not a simple function of its porosity, but depends also on the size, distribution, shape, tortuosity, and continuity of the pores. Thus, although the cement gel has a porosity of 28 per cent, its permeability is only about 7 x 10^-16 m/s. This is due to the extremely fine texture of hardened cement paste: the pores and the solid particles are very small and numerous, whereas, in rocks, the pores, though fewer in number, are much larger and lead to a higher permeability. For the same reason, -water can flow more easily through the capillary pores than through the much smaller gel pores: the cement paste as a whole is 20 to 100 times more permeable than the gel itself. It follows that the permeability of hardened cement paste is controlled by its capillary porosity. The permeability of cement paste varies with the progress of hydration. In a fresh paste, the flow of water is controlled by the size, shape, and concentration of the original cement particles. With the progress of hydration, the permeability decreases rapidly because the gross volume of gel (including the gel pores) is approximately 2.1 times the volume of the unhydrated cement. Permeability of concrete can be classified as follows

Necessity of permeability study

A low permeability concrete generally possesses high strength and is resistant to the ingress of water and salt solutions. The reinforcing steel in concrete structures begins to corrode earlier and corrodes faster when the surrounding concrete is porous because chloride, oxygen, and moisture can more easily reach the steel. Measuring permeability helps detect durability problems and allows timely and cost-effective protection of the concrete structure.

Concrete deterioration can be due to adverse mechanical, physical, or chemical causes, as mentioned earlier. It is often the case where one or more deteriorative mechanisms are at work by the time a problem is identified. In fact, in terms of deterioration of concrete due to physical or chemical causes, the mobility of fluids or gases through the concrete are nearly always involved. The overall susceptibility, or penetrability of a concrete structure, especially when compounded by additional environmental or exposure challenges, is the key to its ultimate serviceability and durability. Low porosity/ permeability/ penetrability of concrete to moisture and gas is the first line off defense against: frost damage, acid attack, sulfate attack, corrosion of steel embedment’s and reinforcements, carbonation, alkali-aggregate reaction, and efflorescence to name a few of the most prominent concrete ailments.

The permeability of concrete can be measured by determining the rate of flow of moisture through concrete. Since the porosity of concrete resides in the paste, the permeability of concrete should be cement aggregate interface. It should be noted that the flow of water through concrete is of interest in construction aside from consideration of durability. Impermeable concrete is required for water retaining structures and construction below grade.

Water does not easily move through the very small gel pores and that permeability is controlled by an interconnecting network of capillary pores. As hydration proceeds, the capillary network becomes increasingly tortuous as interconnected pores are blocked by formation of C-S-H. This is accompanied by a continuous decrease in permeability coefficient and the time at which complete discontinuity of capillary pores occurs is a function of the w/c ratio. In concrete with a w/c ratio greater than 0.70, complete discontinuity of capillary pores can never be achieved, even with continuous moist curing the concrete will have relatively high permeability.

If the paste are allowed to dry and then rewetted, permeability coefficient is higher. This may be due to change in pore size distribution, that occur on shrinkage and which allow capillary pores to become partially interconnected again.

The effect is more marked in concrete since cracking at the paste aggregate interface will create further opportunities for water flow. Even in saturate concrete, permeability will be increased by imperfect consolidation or excessive segregation of materials, which can create bleeding channels within the paste.

Parameters of permeability

Effects of mix proportions and curing conditions:

It is considered the most important factor for durability. It can be noticed that higher permeability is usually caused by higher porosity .Therefore, a proper curing, sufficient cement, proper compaction and suitable concrete cover could provide a low permeability concrete.

w/c ratio:

Water cement ratio is another important factor which affects the strength of concrete and thus influences its resistance to cracking from the internal stress that may be generated by adverse reactions. The value of the coefficient of permeability decreases very substantially with a decrease in the water/cement ratio over the range of water/cement ratios of 0.75 to 0.26, the coefficient decreases by up to 4 orders of magnitude, and over the range of 0.75 to 0.45, by 2 orders of magnitude(A. M. Neville, 1996).

Many aspects of concrete durability are improved by reducing the permeability of concrete. The ACI 318 Building Code addresses an exposure condition for “concrete intended to have a low permeability when exposed to water” by requiring a maximum w/cm of 0.50 and a minimum specified strength of 4000 psi. This recognizes that a lower water-cement ratio is important to control the permeability of concrete.

Relative paste volume:

It must be considered that one parameter of w/cm by itself does not assure the compliance with this requirement will not adversely effect other properties of concrete. Mixtures with a lower paste (water + cementitious material) content and will likely have different performance than the mixture with the higher paste volume.

Available Performance Test for Evaluating Permeability:

European Standard Test:

The test performed to evaluate the permeability of concrete is based on European standard. According to the availability of the testing machine, the test standard was set. This standard specifies a method to determine the depth of penetration of water under pressure in hardened concrete which has been water cured.

This European Standard has been prepared by Technical Committee CEN/TC 104 “Concrete (performance, production, and placing and compliance criteria)”, the secretariat of which is held by DIN.

This European Standard gives the status of a national standard, either by publication of an identical text or by endorsement, at the latest by April 2001, and conflicting national standards had been withdrawn at the latest by December 2003.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.

This standard is one of a series concerned with testing concrete. The standard has been restricted to tests on specimens cured in water requirement in the original draft ISO Standard for the average depth of penetration to be estimated has been omitted.

Germans Water permeability Test (GWT):

The equipment used in this tests is named Germans Water permeability Test (GWT) made by German Instruments A/S. According to the Instruction and Maintenance Manual, the GWT can be used for tasting of micro cracking and porosities of the concrete surface, the “skin-concrete”, on-site. Also, the test system is applied for testing of joints and the integrity of waterproofing membrane by performing testing before and after membrane is applied. In Figure 2.1 its the equipment GWT. (Instruction and Maintenance Manual of GWT, 1993)

Fig: GWT machine

Borehole method:

A method for measuring the water permeability in a concrete structure comprising:

1. Pressure sealing a selected length of a borehole in a concrete structure
2. Injecting an aqueous medium into the selected length of borehole
3. Applying sufficient pressure to the injected aqueous medium and for a time necessary to produce a substantially steady  body ,  measurable flow of the measurable flow of the aqueous medium into the concrete structure from the selected length of borehole
4. Measuring the steady state flow of aqueous medium , and
5. Computing from the steady state flow measurement the water permeability of the concrete structure.

The apparatus employing the method is disclosed here. Fig is schematic drawings of an embodiment of a flow measurement means in accordance with this method.

Fig: Borehole Test Method

Apparatus Used For the Performed Test:

AT 315 is used to determine the impermeability of concrete to water according to EN 12390-8 “Depth of Penetration of Water under Pressure”.

The apparatus is to be connected to a normal air compressor capable of ensuring at least 6 bar (max. 12 bar) of compressed air continuously and equipped with dehumidifier and oil filter.

Connection is then be made to the laboratory water supply and to a drainage system.

Work taps/valves can be turned to two positions: open and closed.

Water supply tap is used for injecting water in the circuit and Water discharge tap is used for draining water from the circuit.

There is also a Air discharge tap which is used for depressurizing circuit.

Pressure regulation of hydraulic circuit is pneumatically controlled by means of a precision valve with hand-wheel: when turned anticlockwise, pressure decreases and, vice versa, when turned clockwise pressure increases.

The pneumatic circuit is equipped with a 0-10 bar manometer to indicate pressure regulated by the Hand-wheel valve. This circuit is also equipped with a safety valve calibrated to open at approx. 6 (bar).

The three test bays are usually used simultaneously; should they need to be used individually, so as to run various tests along with others already being run, it is a good idea to fill the apparatus with the water necessary for usage of the three test bays.

The air-water interface comprises three burettes arranged in the upper part of the apparatus: relevant graduated scales enables absorption to be measured.

The burettes should not be filled completely to prevent water overflowing into the air circuit.

Performed test Procedure:

This standard specifies a method for determining the depth of penetration of water under pressure in hardened concrete which has been water cured.

Water is applied under pressure to the surface of hardened concrete. The specimen is then split and the depth of penetration of the water front is measured.

The test specimen, for example, concrete cubes, is placed in any suitable equipment in such a manner that the water pressure can act on the test area and the pressure applied can be continuously indicated. An example of a test arrangement is shown in Figure 1.2.

It is preferable that the apparatus should allow the other surfaces of the test specimen to be observed. The water pressure may be applied to the surface of the test specimen either from the bottom, or the top.

A necessary seal shall be made of rubber or other similar material. The dimension of a test area is approximately half of the length of the edge or diameter of the test surface.

The specimen shall be cubic, cylindrical or prismatic of length of edge, or diameter, not less than 150 mm.

Figure: Arrangement of test specimen

Key

1 Packing piece

2 Sealing ring

3 Scewed on plate

4 Screw-threaded rod

5 Water under pressure

6 Screwed on plate

Aggregates used in concrete structures:

Aggregates are the important constituents in concrete either plain or reinforced. Aggregates were considered as chemically inert materials, but now it has been recognized that some of the aggregates are chemically active and also certain aggregates exhibit chemical bond at the interface of the aggregates and paste.

 Coarse aggregates:

It is the aggregates whose particles completely pass through 20 mm B.S sieve and which are entirely retained on 4.75 mm B.S sieve.

Fine aggregate:

Aggregates whose sizes are 4.75mm and less is considered as fine aggregates.

Characteristics of coarse aggregates:

A good aggregate should not contain any deleterious material which may cause physical and chemical changes in the concrete. An aggregate should have clean, uncoated, properly shaped particles of strong, dense, durable mineral and rock materials. Some of the important characteristics of aggregate are given here.

Surface texture:

Surface texture is a physical property, the measure of which depends upon the relative degree to which particle surfaces are polished or dull, smooth or rough. Surface texture depends on hardness, grain size, pore structure, structure of the rock and degree to which forces acting on the particle surface.

Surface texture greatly affects the bond between particles and cement paste. An aggregate with rough surface has better bond than smooth surfaced particles. Similarly an aggregate with smooth surface but having surface pores is considered good for bond.

Porosity and absorption:

Some of the aggregates are porous and absorptive. Porosity and absorption of aggregates will affect the water/cement ratio and hence the workability of concrete. The porosity of aggregate will also affect the durability of concrete when the concrete is subjected to freezing and thawing and also when the concrete is subjected to chemically aggressive liquids.

The aggregates should have smaller porosity and absorption capacity. Absorption value is used to calculate the change in the mass of an aggregate due to water absorbed in the pore space within the constituent particles compared to that at the dry condition.

Increase in volume:

Fine aggregates (sands) increase in volume due to moisture content. Coarse aggregates do not increase in volume because of moisture content. But amount of moisture in coarse aggregates should be considered while determining the amount of water required during production of concrete.

Deleterious materials:

Iron pyrites, coal, mica, shale, clay, alkali, organic impurities are some of the materials whose presence in the aggregates is viewed as harmful. These materials should not be present in such quantity that may affect the strength and durability of the concrete. Deleterious materials cause following effects.

• They interfere with the hydration of cement
• They affect bond between cement paste and aggregates
• They reduce the strength and durability of cement concrete
• They modify the setting action and contribute to efflorescence

As a convention the total amount of deleterious materials in aggregates should not exceed 5%.

1^st class brick:                       

• Standard sized
• Uniform red or yellow colored
• Well burnt
• Regular shaped
• Uniformed textured
• No efflorescence
• No pebbles , gravels or organic matter
• AC < 20% , crushing strength > 1500 psi
• Emits metallic sounds , no finger nail impression

3^rd class brick:

• Soft and light red colored
• Shape , size not regular
• Under brunt (slightly over brunt is acceptable)
• Extensive efflorescence , non-uniform texture
• Ac>= 22% ,crushing strength < 1000 psi
• Emits dull / blunt sound ;
• Left finger nail scratch marks

Classification   based on fineness modulus:

Fine sand              FM=2.20~2.60      d< #16 sieve  =1/16 in sieve

Medium  sand      FM=2.60~2.90       d< #8 sieve    =1/8 in sieve

Coarse  sand         FM=2.90~3.20      d< #4 sieve     =1/4 in sieve

SUPERPLASTICIZER ADMIXTURE :

The use of superplasticizers (high range water reducer) has become a quite common practice. This class of water  reducers were  originally developed in Japan and Germany in the early 1960s; they were introduced in the  United States in the mid-1970s.

The capability of superplasticizers to reduce water requirements 12-25% without affecting the workability leads to production of high-strength concrete and lower permeability.

Experimental Program

General:

The experimental setups, tests performed, and test procedure for materials used and for permeability test will be discussed in this chapter which includes

• Controlling tests of materials used
• Compressive strength test of concrete cylinders
• Method of determining permeability.

Phase 1 testing program:                             

Phase 1 involves the casting of 4 batch concrete with very high class brick-chips of different ratio.

Materials:

The following sections provide information on materials used in casting in this phase

Coarse aggregates: first class brick chips were used as coarse aggregates. And these bricks were collected from local market.

Fine aggregates: local sand and Sylhet sand were used in first batch of concreting. Rest of the three batches used Sylhet sand only.

Cement: The concrete used for all type of casting was made using the ordinary Portland Cement Type I/II.

Ordinary portland cement:

It’s a product of an intimate mixture of correctly proportioned calcareous and argillaceous materials, obtained by the calcination of a very high temperature. The calcined product, called clinker, is then finely pulverized by grinding into a very fine powder. Finally its mixed with gypsum to obtain cement.

When the water is mixed with cement, a series of chemical reaction takes place. As a result , the cement paste first sets and then hardens into stone like mass.

Calcareous materials: Compunds of Ca and Mg ; e.g  limestones , shell , dolomites etc

Argillaceous materials: Materials include of silica, alumina and iron of oxide such as clay and shale

Gypsum is added to delay the setting action of the cement for some time so that it may be properly mixed , applied and finished .Without gypsum, setting action of cement starts the moment the water is added to cement , thus giving no time for mixing, placing  and finishing. The setting time can be varied by suitably adjusting the percentage of gypsum.

Composition of Ordinary Portland cement:

Main three ingredients are Clay, silica and lime. Besides, cements contains small amounts of iron oxide, MgO, SO₃, Alkalies and other materials. Typical percentages are these constituents in a good OPC may be as follows:

Table: Chemical ingredient of Portland Cement

Water: tap water was used every time.

Gradation of aggregates:

This test is primarily used to determine the grading of materials proposed for use as aggregates or being used as aggregates. Gradation of aggregate means particle size distribution of the aggregates.

Grading determines the workability of the mix which controls segregation, bleeding, water cement ratio handling, placing and other characteristics of the mix. These factors also affect economy, strength, volume change and durability of hardened concrete.

Coarse aggregates were prepared from bricks. Then the samples were dried and weighted. Sieves were nested in order of decreasing size of opening from the top to bottom and the samples were placed on the two sieves. Then the sieves were agitated manually for 15 minutes. The weights of each size increment were measured by balance and the results are available in the appendix section of the report.

Conclusion and Recommendation

General:

This study involved an experimental investigation of permeability of hardened concrete and a performance based comparison between brick aggregates and stone aggregates to prevent permeability. The test program was divided into three phases and each phase consists of four batches of concrete casting. Phase 1 involved evaluating performance of first class bricks by introducing changes in mix proportion. Similarly phase 2 testing was done by third class bricks. Finally to mark a difference between brick aggregates and crushed stone aggregates, the third phase testing was done by stone chips. This chapter highlights the conclusions that were drawn from the test results obtained and the recommendations that were developed concerning the performance of used coarse aggregates.

Conclusion:

Based upon the tests conducted, the following conclusions are at the forefronts:

1. Several tests were performed in each phases of the study. From the obtained test results, it can be said that for a certain type of aggregate, use of lower water cement ratio and higher percentage of cement can definitely reduce the permeation of water.
2. Use of admixture for workability is a good approach to reduce water penetration through concrete.
3. The higher the strength achieved, the lower the permeability is obtained.
4. Permeability of concrete is greatly affected by the absorption of aggregate used. For the same % of cement used, permeability increases for higher absorption of aggregates.
5. For same w/c ratio, permeability increases with absorption.

And from stone aggregates to brick aggregates, permeability increases exponentially with absorption.

6. Presence of higher percentage of brick aggregates in concrete also affects the permeability of concrete. So for a impermeable concrete structure it’s effective to use small percentage of brick aggregates in total batching.
7. From results, it can be concluded that permeability of 3^rd class brick chips is 1.5 times higher than that of 1^st class brick chips. And 20~30 times higher than that of stone chips aggregate.
8. It is also remarkable that still after using admixture, permeability of 1^st class brick chips is almost 9 times higher than stone chips aggregate without admixture.
9. From obtained results, it is clear that, permeability of first class brick chips concrete is 10~20 times higher than that of stone chips of concrete. And use of admixture with brick aggregate can not even bring about a lower permeability of concrete than that of stone chips without admixture. So, while using brick chips, importance of the structure should be reevaluated.
10. Finally, for low permeable concrete, selecting proper materials and mix proportions, adequate consolidating the concrete during placement and providing proper and effective curing is of great importance.

Recommendation:

The recommendations are:

• There is no available experiment based specification for brick chips aggregate for our country, so a specification should be developed for construction of roads in coastal areas, bridges, and hydraulic structures.
• It is better to use stone chips aggregates for important structures.
• Further studies should be carried out to identify the effect of salt water intrusion in structures.