Voids concrete

Figure 6.4 Chloride threshold for corrosion initiation as a function of content of interfacial voids (concretes made with different types of cement OPC = ordinary Portland cement, SRPC = sulfate-resistant Portland cement) [14]...

Localization of void sliding ducts in the concrete of a storage silo... 

It also requires two- sided aceess to the structural element in question. The degree of contrast between homogenius concrete and concrete with voids will not decrease linearly with increasing thickness, and the maximum practical thickness of concrete elements which can be studied for small voids using film radiography is of course limited, but sufficient for most civil engineering applications. 

Similar behaviour occurs when trying to locate voids in concrete cast behind steel plates, e g. the steel liner in nuclear containment walls. Our own experience has shown that in the case of a steel liner (encast at depth 250 mm) the reflected compression waves are dominant regardless of the condition of the concrete behind the plate. 

The methods that are based on the reflection of compression waves will generally not give information about the concrete which lies deeper than the most shallow large planar defect (crack or void ). 

A problem obviously exists in trying to characterise anomalies in concrete due to the limitations of the individual techniques. Even a simple problem such as measurement of concrete thickness can result in misleading data if complementary measurements are not made In Fig. 7 and 8 the results of Impact Echo and SASW on concrete slabs are shown. The lE-result indicates a reflecting boundary at a depth corresponding to a frequency of transient stress wave reflection of 5.2 KHz. This is equivalent to a depth of 530 mm for a compression wave speed (Cp) of 3000 m/s, or 706 mm if Cp = 4000 m/s. Does the reflection come from a crack, void or back-side of a wall, and what is the true Cp ... 

Air-Entrainment Agents. Materials that are used to improve the abiUty of concrete to resist damage from freezing are generally known as air-entrainment agents. These surfactant admixtures (see Surfactants) produce a foam which persists in the mixed concrete, and serves to entrain many small spherical air voids that measure from 10 to 250 p.m in diameter. The air voids alleviate internal stresses in the concrete that may occur when the pore solution freezes. In practice, up to 10% air by volume may be entrained in concrete placed in severe environments. 

The best way to do this is to grade the aggregate so that it packs weii. If particies of equai size are shaken down, they pack to a reiative density of about 60%. The density is increased if smaiier particies are mixed in they fiii the spaces between the bigger ones. A good combination is a 60-40 mixture of sand and gravei. The denser packing heips to fiii the voids in the concrete, which are bad for obvious reasons they weaken it, and they aiiow water to penetrate (which, if it freezes, wiii cause cracking). 

This is an effective treatment on eroded and weak surfaces but will not open up the sub-surface voids or pockets or provide a surface profile on dense concrete. 

Concrete structures should be constructed in accordance with BS 5337 or BS 8110 as appropriate. The concrete must be properly cured under shuttering, or a curing membrane, for seven days followed by at least 28 days open curing prior to the commencement of lining. Surfaces have to be free from voids and projections and are often screeded to give a satisfactory finish. 

Asphalt concrete is properly proportioned to resist the potentially damaging effects in the road. Asphalt concrete paving mixtures should be evaluated for the following properties stability, flow, air voids, stripping resistance, resilient modulus, compacted density, and unit weight. Table 4.18 provides a list of standard laboratory tests that are presently used to evaluate the mix design or expected performance of fresh and hardened asphalt concrete. 

Splitting tensile strength of cylindrical concrete specimens Microscopic determination of parameters of the air-void system in hardened concrete... 

Air content of freshly mixed concrete by the pressure method Air content of freshly mixed concrete by the volumetric method Unit weight, yield, and air content of concrete Specific gravity, absorption, and voids in hardened concrete Resistance of concrete to rapid freezing and thawing Scaling resistance of concrete surfaces exposed to deicing chemicals... 

Concrete masonry is simply the name given to the cement building blocks employed in the construction of many homes, and it is simply a precast block of cement, usually with lots of voids. Precast concrete is concrete that is cast and hardened before it is taken to the site of construction. Concrete sewer pipe, wall panels, beams, grinders, and spillways are all examples of precast cements. 

During the mixing of concrete, the folding action of the mixing sequence causes air voids to be formed in the system, which in normal concrete would be reduced by the mechanical forces used in placing the concrete, leaving... 

Mix no. Properties of fresh concrete Air-Void determinations on hardened concrete ... 

Water-cement ratio (by wt) Nature of mix Air content (%) Paste content (%) Voids in concrete (%) Specific surface area (mm) Void spacing factor (mm)... 

In air-entrained superplasticized concrete, the commonly accepted minimum value of the spacing factor of the air void system (0.2 mm) to provide adequate freeze-thaw protection is usually exceeded [68-71]. 

Despite the fact that the minimum spacing factor is exceeded, the concrete freeze-thaw resistance of the concrete does not appear to be adversely affected [71]. Table 2.7 clearly shows how the presence of an SNF superplasticizer increases the spacing factor of the air void system at each of the three water-cement levels evaluated [71]. 

Fig. 3.25 Change in void size distribution on vibration of air-entrained concrete (Johansen). 

The chemical materials used to produce dampproofers are able to form a thin hydrophobic layer within the pores and voids and on the surfaces of the concrete in one of three ways (1) reaction with cement hydration products ... 

Dampproofed concrete, therefore, should not show significant uptake of surface water in conditions of rain and wind up to about 100 km h T In fact, on prolonged exposure, some wetting does occur, because of defects in the hydrophobic coating, and the presence of larger voids in the concrete, up to perhaps 1 mm wide these are due to incomplete compaction, or to the nature of the concrete in the case of blocks. 

The presence of an excessive number of voids in the concrete [38] or porosity due to poor compaction or a deficiency of fine aggregate [39] leads to an increase in the amount of reinforcement corrosion. 

The mechanism of action by which silane and siloxanes reduce expansion has been attributed to water repellence and air entrainment. Phosphate addition or coatings may interfere with the dissolution of silica gel and the formation of gel. It is also possible that phosphate reduces the osmotic potential and the swelling pressure in the gel. The manner in which air entrainment reduced expansion was attributed to the accommodation of alkali-silica gel in the air void system. For example, it was found that air-entrained concrete with 4% air voids could reduce AAR expansion by 40% [23]. 

Typical uses include the production of non-dispersible underwater concrete and reduction of the accumulation of bleed water in mass concrete placed in deep forms. Consequently, AWAs are useful in mass concrete work because they prevent the formation of laitance on the surface of the concrete and thereby reduce the excessive cleaning between successive lifts. The admixtures also reduce the voids formed under horizontal reinforcing bars. Therefore, bond to steel increases and potential corrosion problems are reduced. The admixtures are also used in conjunction with WRAs in oil-well cementing grouts to reduce pipeline friction and rapid water loss and grouting of pre- and post-tensioned concrete ducts [47]. New valves and control devices under development in Europe and Japan used in conjunction with AWA will likely advance the field on underwater concrete. 

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