Assessment of engineering properties of locally available light weight aggregates used in concrete
Assessment of engineering properties of locally available light weight aggregates used in concrete
Zain ul Abidin1, Bashir Alam1, Salman Afzal2
1Department of Civil Engineering, University of Engineering & Technology, Peshawar, Pakistan
2Department of Civil Engineering, GIST Peshawar, Pakistan
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Abstract:In this research study the engineering properties of locally available light weight aggregate, i.e. bloated slate aggregate in this study,were studied when utilized as a replacement for the conventional aggregates in concrete in order to achieve numerous engineering properties of resulting lightweight concrete.Physical properties of light weight aggregates, such as water absorption, specific gravity and loss angles abrasion values, were initiallycalculated and evaluated to check the feasibility of the local light weight aggregate for use as aggregate. Trial mixes of concrete batches were prepared and various engineering properties were determined during the plastic state and hardened state. These properties included workability, initial setting time, final setting time and compaction. Hardened concrete specimens were subjected to uniaxial compressive testing for compressive strength determination. The results of both the plastic state testing and hardened concrete specimen indicated that the bloated slate aggregates can be used as light weight aggregate when used for preparing lightweight concrete structures. The study showed that the bloated slate aggregate when used in concrete mix imparted lower slump values, lower setting times as compared to the normal weight concrete mixes. Similarly, the uniaxial compressive testing indicated that the concrete cylinders prepared in bloated slate aggregate possesses lower compressive strength values as compared to the normal weight concrete cylinders. Different trial mixes were prepared and analyzed for uniaxial compressive testing along with the analysis of mode of failure under the application of loading.
Keywords:Bloated slate aggregates, light weight aggregate, compressive strength
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
1.Introduction:
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
Structural lightweight aggregate concrete is an important and versatile material in modern construction. It has many and varied applications including multistory building frames and floors, bridges, offshore oil platforms, and Prestressed or precast elements of all types [1]. Many architects, engineers, and contractors recognize the inherent economies and advantages offered by this material, as evident by the various impressive lightweight concrete structures found today throughout the world [2].
Lightweight concrete has strengths comparable to normal weight concrete, yet is typically 25% to 35% lighter [3].Structural lightweight concrete offers design flexibility and substantial cost savings by providing less dead load, improved seismic structural response, longer spans, better fire ratings, and thinner sections, decreased story height, smaller size structural members, less reinforcing steel, and lower foundation costs [4].
Lightweight concrete precast elements offer reduced transportation and placement costs [5]. Rotary kiln process is generally used to process the natural or by- product material to make them a light weight material [6]. Some times in rotary kiln process the material are discharged and cooled after which they are crushed up to required gradation of aggregate size [7]. The resultant materials tend to be cubical or angular in shape having porous nature of structure. [8]. The bulk density of bloated slate aggregate is varying normally from 20 to 25 lb/ft3 depending on the nature of deposits of parent rock [9]. Their compressive strenght after 28days are varying from 600 psi to 1500 psi [9].
2. Materials and Methods:
2.1. Occurrence of Bloated slate:
The slate selected in this research study was obtained from precambrian Manki formation, as depicted in Figure 1 [9]. The Manki formation consists of thick sequence of Slate and Phyllite slate with some intercalations of quartzite and lime stone. Dolerite intrusions and quartz veins are found locally. It is bedded and fractured jointed and is extensively developed in the northern portion of Attock-Cherat range, west of Indus river [10].
A developed slate cleavage is prominent near Manki, Ziarat kaka sahib, Attock and Khairabad [10]. Manganese dendrite and pyrite cubes are present in abundance in lower portions of the slates of Manki formation. Limestone occurs in pockets. Another important feature is the presence of quartz veins usually prominent near the northern part of Manki Sharif.[11]
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
2.2. Physical characteristics of bloated slate aggregate (course aggregate):
Slate is a metamorphic laminated clay rock available in huge quantities in Pakistan, was crushed up to ½ indown (12.5mm) and ensured proper gradation. These slates were brought to the Pakistan Council of Scientific and Industrial Research (PCSIR) laboratory and were bloated by rotary Killen method for its bloating involving 1100oC temperature under control environment.
Specific Gravity (Dry) = 1.508
Specific Gravity (Saturated Surface Dry) = 1.601
Absorption (Saturated Surface Dry) = 6.186%
Los Angeles Abrasion = 42%
2.3. Preparation of trial mix for obtaining optimum mix
Trial mixes were prepared to obtain optimum mix. Different ratios were divided in to two sets; first of all ratio of 1:4:8 and 1:3:6 was initially used. The basis for this initial selection is the locally used construction language for concrete works.
Methods:
First trial was selected using local field terminology in which ratio used was 1:4:8(1part cement: 4parts sand: 8parts coarse aggregate) and 1:3:6(1part cement: 3parts sand: 6parts coarse aggregate) by weight as initial trial to have some basic information. Due to high absorption of bloated slate aggregate it was thoroughly wetted and mixed with sand and cement but the mix was not properly mixed, due to which the cement sand mortar was prepared and then the wet aggregate were mixed with the paste. Slump test was carried out for different water cement ratios and was recorded. Concrete cylinders were prepared with the workability needed for construction of slabs. After 24 hours as the cylinders was removed from moulds, they showed a rough surface. A total of six cylinders were prepared and kept in water tank for curing.
At 7 days, 14 days and at 28 days one cylinder from each mix was tested for compressive strength of bloated slate concrete.
After analyzing the data the mix was made richer than the above ratios and two sets of cylinders were prepared and for that particular water cement ratio, slump criteria was the controlling factor. The w/c ratio for which the desired slump achieved was used for preparing cylinders for compressive strength testing. These sets having the ratios of 1:2:4 (1part cement: 2 parts sand: 4parts coarse aggregate) and 1:1.5:3 by weight.
Cylinders were prepared in the same way as discussed above. Cylinders were kept in water tank and from each mix they were tested after 7, 14 and 28days for compressive strength. After analyzing the data it was revealed that the ratio between 1:2:4 and 1:1.5:3 should be adopted as optimum mix.
More cylinders were prepared for the ratio of 1:1.7:3.5 by weight and tested after 7,14 and 28 days respectively after analyzing the data, as it was selected as optimum mix.
3.Results and Discussion
3.1. Behavior of bloated slate aggregate:
Bloated slate aggregates, which were crushed down to the required gradation after bloating, have porous nature due to expansion. They have very rough surface and harsh to work with. The moisture absorption of 6% shows its high porosity due to which it is difficult to mix it directly with dried matrix.
If it is mixed in dry conditions with cement plus aggregate and then water is applied, they do not allow proper mixing and the workability of the mix is much reduced due high absorption of concrete. They make the concrete harden in short time. So to avoid this, the aggregate should be applied with water according to its absorption and then they should be added with the water cement ratio. The obtained slate aggregate from the source was found covered slightly with Mica due to which its weight was higher than water. The unit weight (oven dried) was observed to be 94 lb/ft3 which is lower than normal aggregate. The Los Angeles abrasion test showed results relatively better than they were expected as the aggregates can be broken by a slight hit onto the ground.
The aggregates have less abrasion resistance to loading as compared to normal weight aggregate. It is very difficult to achieve the level of workability with normally used water to cement ratio which is 0.5 due to roughness of the aggregate texture and porous nature.
When the aggregate is broken into pieces before bloating, it has less porosity and has smooth surface which helps in achieving a level of workability. But after bloating if it is broken into pieces then it possess angular and harsh structure.
3.2. Water cement ratio and workability relation
Trial batches were prepared to obtain the optimum mix to be used in the construction of light weight aggregate concrete members. Different trials of water to cement ratio were conducted and results analyzed as depicted in Figure 2 and Figure 3.
It is indicative from Figure 2 that at 0.5 water/cement ratio the mix is very harsh and very difficult to produce slump and at 0.72 to 0.8 it gives some reasonable value of slump. Above these values the cement-sand mortar sets due to their heavier weight and the aggregates are bonded with each other due to the slurry in the pores of the aggregate.This serves as reinforcement in connecting the aggregate with each other.
In other words the slump is achieved when some balanced amount of aggregates and mortar setting more than the mortar. This behavior is opposite to that normal weight aggregatemixes in which the aggregate sets which is termed as segregation.
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
Figure 2: Relationship between w/c(x-axis) and slump (y-axis) for 1:2:4 concrete
Figure 3: Relationship between w/c ratio (x-axis) and slump (y-axis) for 1:1.7:3.5 concrete
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
3.3. Setting time of bloated slate aggregate concrete
The setting time of the bloated slate aggregate concrete is almost half as compared to normal weight aggregate due to its nature. The final setting time is also affected in this. Concrete sets within 30 to 40 minutes and also final setting time is reduced in this case up to 8 to 9 hours. The setting time is affected by quantity of aggregate and cement.
3.4. Compaction of bloated slate aggregate concrete
It is very difficult to obtain smooth surface from Bloated slate aggregate concrete by application of vibrator as during vibration the aggregate comes to upper portion and the cement paste sits due to which the finished surface is not achieved although by rodding it give better results.
3.5. Compressive strength of bloated slate aggregate concrete
Compressive strength of bloated slate aggregate is summarized in Table 1 for different mixes in order to obtain optimum mix to be used in the construction hollow core light weight aggregated slab panels. For various mixes 28 days compressive strength varies from 380psi to 1450psi.
It can be seen that the strength is affected very much by increasing cement content keeping the sand content constant up to a limit where the failure of aggregate take place. Further increasing the content though increases the strength but not very effectively as shown in Table 1, that the failure of aggregate occurs in the ratio of 1:2:4 and 1:1.5:3. This means that due to the increase in cement content increases a very little increment in strength of aggregate by occupying the pores with in the aggregate structure.
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
Table 1: Mix ratios v/s strength performance
Ratios / 7 days strength / 14 days strength / 28 days strength / Failure patterns1:3:6 / 505 psi / 650 psi / 712 psi / Failure of mortar
1:2:4 / 890 psi / 1012 psi / 1225 psi / Partial aggregate and bond failure
1:1.5:3 / 1010 psi / 1205 psi / 1422 psi / Aggregate
1:1.7:3.5 / 974 psi / 1105 psi / 1365 psi / Aggregate
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22
Assessment of engineering properties of locally available light weight aggregates used in concrete
The compressive strength of the bloated slate aggregate concrete is greater in first 14 days. While it has normal distribution, not too scattered which is contributed by the presence of sand as from previous studies on no fines bloated slate aggregate having variations in the strength of cylinders. By volume mix proportioning more strength may be obtained but then there is no considerable reduction in weight of light weight aggregate concrete. The ratio of 1:2:4 by weight of (cement, sand and aggregate) for normal concrete gives 3000 psi strength and the failure occurs by bond separation.
4.Conclusion and Recommendation:
Conclusions:
- Bloated slate when crushed before bloating has workability much more than when are crushed after bloating. As the optimum water to cement ratio in formercase was 0.55 while in the later it was 0.73.
- Every bloated slate aggregate concrete mix has its own optimum water/cement ratio below or above which there is no appreciable workability.
- The strength of bloated slate aggregate varies with cement contents.
- The bloated slate aggregate concrete has 25% less weight than that of normal weight aggregate concrete.
- The setting time of bloated slate aggregate concrete is almost half as compared to that of normal weight aggregate concrete.
- If mixture proportioning is done by weight for the lightweight aggregate concrete then the strength achieved is less but weight reduction is more in the case of volumetric batching.
- The failure of bloated slate concrete is abrupt as compared to that of normal weight aggregate.
Recommendations:
- More refined testingis required to study the behavior of concrete in which normal weight aggregate are replaced by bloated slate aggregates.
- Some of the bloated slates aggregates were covered by Mica due to which its weight was more than pure bloated slates. The investigation of pure bloated slates should also be carried out.
References:
[1] Newman and Owens, “Properties of lightweight concrete”. In Newman, J. and Choo, B.S. (eds.) Advanced Concrete Technology, Butterworth-Heinemann, Burlington, Vermont, USA, 2003.
[2] Ghafoori, N. and Dutta, S., (1995b), Laboratory investigation of compacted no-fines concrete for paving materials. Journal of materials in civil engineering, 7(3), pp 183-191.
[3] Sommerville, J., Craig, N. and Charles, A., (2011), No-fines concrete in the UK social housing stock: 50 years on. Structural Survey, 29(4), pp 294-302.
[4] Francis, A.M., (1965), Early concrete buildings in Britain. Concrete and Constructional Engineering, 60(2), pp 73-75.
[5]“Guide for Structural Lightweight Aggregate Concrete”. ACI 213R-87, American Concrete Institute, Detroit, Michigan. 1987.
[6] “Advantages of Structural Lightweight Aggregate Concrete”. Expanded Clay, Shale and Slate Institute,
[7] Hassan, A., 1970. Crushing strength of concrete made from locally expanded clay aggregate. West Pakistan. Engineering Congress, Lahore, 15, 39-49.
[8] Hassan, A., 1972. Pilot plant for expanded clay aggregate. Engineering News, 17, 43-51.
[9] Hussain, A., 1981. An introductory note on prospects and potential for lightweight aggregates in Pakistan. Geological Survey of Pakistan Unpublished Report, 4.
[10] Hussain, A., Chaudhry, M. A., Siddiqi, F. A., Zubair, M., 1983. Lightweight aggregate; A study of raw material in Pakistan, Islamabad-Peshawar region. Geological Survey of Pakistan Records, 66.
[11] Rubina Bilqees, Pirzada Naeem, Tazeem Khan and M. Muhammad Junaid,. “Engineering tests of aggregate from Lightweight Expanded Slate of Manki Formation” Journal of Himalayan Earth Sciences 44(2) (2011) 53-60
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 03, No. 01, January 2014, pp 19-22