- Open Access
Compression Behavior of Form Block Walls Corresponding to the Strength of Block and Grout Concrete
© The Author(s) 2014
- Received: 8 July 2014
- Accepted: 28 October 2014
- Published: 25 November 2014
This study aimed to present a reinforced concrete block system that reduces the flange thickness of the existing form block used in new buildings and optimizes the web form, and can thus capable of being used in the seismic retrofit of new and existing buildings. By conducting a compression test and finite element analysis based on the block and grouted concrete strength, it attempted to determine the compression capacity of the form block that can be used in new construction and seismic retrofit. As a result, the comparison of the strength equation from Architectural Institute of Japan to the prism compression test showed that the mortar coefficient of 0.55 was suitable instead of 0.75 recommended in the equation. The stress–strain relation of the block was proposed as a bi-linear model based on the compression test result of the single form block. Using the proposed model, finite element analysis was conducted on the prism specimens, and it was shown that the proposed model predicted the compression behavior of the form block appropriately.
- form block
- prism test
- finite element analysis
- grout concrete
- mortar coefficient
- compression behavior
In the recent seismic retrofit of school buildings in South Korea, whose construction is susceptible to damage from earthquakes, the most widely used retrofit method for frame buildings is installing a damper in the openings such as windows in tandem with the expansion of infill walls. Infill walls are usually cast-in-place concrete, but another possible construction method involves the use of reinforced block walls.
Compared to cast-in-place concrete, reinforced block walls have a somewhat lower structural capacity but offer excellent constructability, and thus, if they satisfy the required capacity, they can be recommended for use in a seismic-strengthening method. Usually, blocks used as reinforced block walls make it difficult to fill the hollow block zones with a sufficient amount of grouted concrete. Furthermore, their hollow areas are relatively small, and as such, the amount of grouted concrete is also small. In addition, bar arranging in construction is difficult, and thus, they provide little structural integrity. If the amount of grouted concrete increases, the blocks become thinner, so that when casting, their resistance to lateral pressure decreases, and they can be easily broken during delivery or construction. Therefore, the size of the hollow zones should be determined based on a comprehensive review of the constructability and strength of the blocks. The form block used for a new construction has wider hollow zones than the existing blocks. In other words, it increases the volumetric ratio of grouted concrete, which increases its strength and thus improves the structural capacity of the wall after the completion of the construction.
Accordingly, this study aimed to present a reinforced concrete block system that reduces the flange thickness of the existing form block used in new buildings and optimizes the web form, and can thus capable of being used in the seismic retrofit of new and existing buildings. By conducting a compression test and finite element analysis based on the block and grouted concrete strength, it attempted to determine the compression capacity of the form block that can be used in new construction and seismic retrofit.
2.1 Construction Process of the Reinforced Form Block Wall
For the design of the reinforced form block walls, the structural capacity of the walls against compression and shear force should be determined. In particular, the compression capacity of the walls whose hollow block zones are filled with grouted concrete allows the walls to function as braces on frames with the compression strut under the horizontal load, and therefore becomes the most important structural capacity. Therefore, for the design of the new construction of the form block and for seismic retrofit, it is essential to predict the appropriate compression structure of block walls.
2.2 Previous Researches
Masonry walls and concrete blocks have long been used in construction, and much research has been conducted on them in other countries. Recently, Jonaitis and Zavalis (2013) conducted a test to determine the compression behavior of an empty concrete block and of another block filled with grouted concrete. A fracture was found on the mortar joint of the empty concrete block, and as such, an increase in compression stress caused lateral deformation. Furthermore, it was discovered that the concrete block filled with grouted concrete showed a compression behavior similar to that of cast-in-place concrete. Shing et al. (1989) evaluated the fracture mechanism, ductility, and energy dissipation capacity of shear block walls through the cyclic loading test to determine their nonlinear behavior. Based on the test results, he reported that through seismic retrofit by using shear block walls, a resistance capacity to shear and ductility could be acquired to some degree. Zhai and Stewart (2010) presented a new theoretical equation with the material strength, types of live load, the ratio of dead load to live load, and the combination of the eccentricity and concentric load as parameters to establish the safety guideline for reinforced block walls in China, and verified the appropriateness of the equation through tests. In Japan, form blocks are widely used in new construction projects where only grouted concrete used to fill the hollow zones inside the blocks would be considered to provide sufficient strength, and blocks would be considered to function only as a form (Architecture Institute of Japan 2006).
As opposed to Japan, in South Korea, three-story or lower buildings are constructed with unreinforced block, and those higher than three-story buildings are usually frame building or wall-type apartment. Therefore, few new buildings are constructed with form block. Accordingly, little research has been conducted on the form block, and some studies focused on its use for the seismic retrofit of frame type building. Yun et al. (2005) conducted an experiment on the use of blocks manufactured with recycled aggregate as infill walls, which showed that if block walls made with recycled aggregate are used in the construction of infill walls, the initial stiffness and shear strength can be improved. Compared to the cast-in-place infill walls, however, the reinforcement with block walls made with recycled aggregate is less effective, and due to constructability issues, it was shown to be difficult to acquire a reliable reinforcement effect. Kim et al. (2004) established a concept of a reinforced masonry wall with stacked hollow form blocks with a bar arranged inside and filled with concrete, and conducted a test on the wall reinforced with form block walls in a concrete frame. The test results showed that such a concept could produce an excellent seismic retrofit effect, and when reviewed based on the equation for the shear strength of reinforced masonry walls in Japan Code, the shear strength of the wall was found to be relatively under-evaluated.
As for the design of masonry wall construction in the Korean Building Code (2009), the cement-to-sand ratio (1:2.0–3.0) for bearing mortar is codified instead of the design strength; it is known that the value is for developing the compression strength of 30–40 Mpa. In the case of the grout, as with the bearing mortar, cement-to-sand ratio is used, but the minimum strength is set to be 1.3 times more than the compression strength of the masonry unit. If it is satisfied, after 28-day curing, the compression strength of the masonry can be decided through the prism test or unit strength test of block.
In International Building Code (2012) of USA, also, the strength of bearing mortar is decided by the mix ratio of cement-to-sand. The grout design also is achieved by the cement-to-sand mix ratio and its minimum strength and slump flow are codified. The compression strength of the grout should be over the design compression strength of masonry after 28-day curing, between 13.79 and 34.47 MPa (in the case of the concrete block). The slump flow should be ranged from 610 to 762 mm. As with KBC, the compression strength of the masonry wall can be verified through either the prism test or the unit strength of masonry, and it should meet the scope of 10.34–27.58 MPa (in the case of the concrete block).
From the above, in the case of KBC and IBC, if the bearing mortar and grout are mixed based on the standard, and the compression strength is over the standard strength, either the result of the prism test or that of the masonry unit strength can be used so that in the end, the strength of the form bock wall is determined by the masonry unit. When the strength of the grout, however, is lower than the masonry unit strength, neither code does not clearly identify how the strength can be calculated.
On the other hand, Japan (JASS 7), which shows considerable use of the form block, which is often used in seismic retrofit, offers equations for the compression strength, elastic module, and shear module of block walls using the material test results of the bearing mortar, grout, and masonry unit.
Seismic retrofit using blocks needs sufficient explanation of the structural capacity of the concrete-filled block walls against compression, based on which the resistance capacity against lateral force should be determined. Accordingly, this study aimed to present a design method for form block walls with optimized web and flange dimensions by considering the constructability and structural capacity and by conducting a compression test and a prism compression test on a single block as part of the research on examining the efficiency of reinforcement against compression. Furthermore, it aimed to examine an analysis method with which to predict the compression behavior of form block walls using nonlinear finite element analysis.
3.1 Compression Test Layout
This block was developed to be used as an in-filled wall in seismic retrofit of frame structure. As a construction process shown in Fig. 1, at first, the bars for vertical continuity shall be vertically anchored in the beams and then the form blocks shall be laid on the beams. In order to have horizontal continuity, bars also shall be horizontally anchored to columns. Those bars shall be connected to other bars by lap splicing. All blocks shall be laid on bearing mortar. After laying three blocks vertically, grout concrete shall be poured into the void holding bars vertically.
Mixing design of form-block.
Designed compressive strength (MPa)
3.2 Compression Test Result
Strain variation of single form-block.
At ultimate state
Load, P bu (kN)
Stress*, f bu (MPa)
Displacement δ bu (mm)
Strain+, ε bu (%)
At elastic end
Displacement δ by (mm)
Strain+, ε by (%)
ε by /ε bu
4.1 Overview and Method
Dimension and material strengths of prism test specimens.
Width × length × height (mm × mm × mm)
f bu (Mpa)
f g (Mpa)
200 × 208 × 580
200 × 208 × 582
200 × 207 × 584
200 × 208 × 584
200 × 209 × 581
200 × 209 × 578
200 × 208 × 580
200 × 209 × 582
4.2 Prism Test Result
4.2.1 Load–Displacement Curve
4.2.2 Strain and Failure Shape
4.2.3 Compression Strength Evaluation of the Form Block
Comparison of prism test and calculation result.
Mortar factor e s
Ultimate load (kN)
Ultimate strength Fm,exp (MPa)
5.1 Overview and Modeling
The crack model of the prism specimens was the total strain crack model, which was used with the consideration of concrete cracks. In this analysis, the total strain crack model of the discrete crack model was used. And fixed crack model was applied.
5.2 Analysis Result of the Single Form Blocks
5.3 Analysis Result of the Prism Specimens
From the single form block, all the specimens showed bi-linear load–displacement relation. The strain ratio of the elastic limit to the strain at maximum strength increased according to the maximum compression strength of material. It reaches 2 % of the maximum compression strength of the block, and the total average value without considering the compression strength change of the block was about 80 % of the strain when the strain at the elastic limit was at the maximum strength.
The prism test result showed that when the grout concrete strength was lower than or similar to the strength of the block, the block strength governed the prism strength. When the strength of the grout concrete was considerably higher than that of the block, however, the grout concrete strength governed the prism strength. In the case of the failure shape, due to the compression deformation generated by the load increment as well as the tensile force on the block web due to the horizontal expansion of the grouted concrete poured into the hollow zone of the block, the lateral flanges of the form block failed.
The comparison of the strength equation from Architectural Institute of Japan to the prism compression test shows that the mortar coefficient is not variable value. If 0.75, which is usually implemented in the Japanese wall structure design standard, is used, the strength of the specimens is over evaluated by about 12 %. In this study, based on the test result, 0.55 was used as the mortar coefficient for safe design purpose. The calculation result based on this mortar coefficient was compatible with the test result.
Based on the compression test result of the single form block, the stress–strain relation of the block is proposed as a bi-linear model. The finite element analysis result using the proposed model showed that the stress flow and the load–displacement curve were very similar to those of the test result. Also, using the proposed model, finite element analysis was conducted on the prism specimens, and it was shown that the proposed model predicted the compression behavior of the form block appropriately.
The authors acknowledge the support provided by Korea Association of Industry, Academy, and Research Institute (KAIARI) as one of the 2012 international business cooperation and technology development projects.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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